Head-to-head comparison of [177Lu]Lu-FAP-2286 and [161Tb]Tb-FAP-2286 efficacy in a PDAC mouse model: Is there an added benefit of internal conversion and Auger electrons for FAP-TRT?

Head-to-head comparison of [177Lu]Lu-FAP-2286 and [161Tb]Tb-FAP-2286 efficacy in a PDAC mouse model: Is there an added benefit of internal conversion and Auger electrons for FAP-TRT? | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Head-to-head comparison of [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 efficacy in a PDAC mouse model: Is there an added benefit of internal conversion and Auger electrons for FAP-TRT? Circe D. van der Heide, Carolline M. Ntihabose, Mark Konijnenberg, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8237978/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 09 Jan, 2026 Read the published version in EJNMMI Research → Version 1 posted 4 You are reading this latest preprint version Abstract Background Terbium-161 (Tb-161) emits internal conversion and Auger electrons, in addition to beta-minus radiation, which might be of added benefit for targeted radionuclide therapy (TRT) compared to Lutetium-177 (Lu-177). We extensively compared Lu-177 and Tb-161 for fibroblast activation protein (FAP)-targeted TRT in a preclinical setting. To study this, FAP-2286 was labeled with Lu-177 and Tb-161 and characterized in vitro on FAP-expressing cells and ex vivo using patient tumor samples. Moreover, in vivo studies (i.e. biodistribution and efficacy) were performed using a clinically representative pancreatic ductal adenocarcinoma (PDAC) mouse model. Biodistribution was performed 1, 4, 24, and 48 h post injection of 5 MBq/500 pmol [ 177 Lu]Lu-FAP-2286 or [ 161 Tb]Tb-FAP-2286. Subsequently, animals were treated with 4×40 MBq/500 pmol [ 177 Lu]Lu-FAP-2286 or [ 161 Tb]Tb-FAP-2286 and with alternating doses of 2×40 MBq/500 pmol of each radiopharmaceutical. Results No difference in [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 uptake was observed in the cell models. In vivo studies did not show a survival benefit after 4×40 MBq/500 pmol [ 177 Lu]Lu-FAP-2286 or [ 161 Tb]Tb-FAP-2286, while Kaplan-Meier analyses demonstrated modestly prolonged survival after tandem therapy, in mice that first received [ 177 Lu]Lu-FAP-2286 followed by [ 161 Tb]Tb-FAP-2286. Dosimetry calculations based on autoradiography on patient tumor samples showed that even with lower binding, a higher absorbed dose to the tumor can be accomplished with [ 161 Tb]Tb-FAP-2286. Conclusions In our vitro and in vivo studies, [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 demonstrated similar behavior. In the applied PDAC mouse model, FAP-TRT showed limited therapeutic efficacy, with a modest response observed in the tandem therapy group that first received [ 177 Lu]Lu-FAP-2286, followed by [ 161 Tb]Tb-FAP-2286. Fibroblast activation protein (FAP) cancer-associated fibroblast (CAF) targeted radionuclide therapy (TRT) pancreatic ductal adenocarcinoma (PDAC) Terbium-161 Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Background Pancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer type with a poor 5-year survival rate of less than 10% [ 1 ]. PDAC is typically treatment resistant due to the presence of a dense tumor stroma. The most abundant cellular component of this tumor stroma is the cancer-associated fibroblast (CAF), which has been associated with therapy resistance and suppression of immune cells [ 2 ]. CAFs are characterized by the expression of fibroblast activation protein (FAP), which is only scarcely expressed in healthy tissues [ 3 ]. This cancer specific and abundant expression of stromal FAP in PDAC, and other solid cancers, makes it an attractive biomarker for anti-cancer interventions, including targeted radionuclide therapy (TRT) [ 4 , 5 ]. Currently, FAP-2286 is one of the most promising compounds for FAP-TRT, due to its relatively high tumor retention [ 6 ]. Accordingly, a large prospective clinical Phase I/II trial with [ 68 Ga]Ga/[ 177 Lu]Lu-FAP-2286 to determine the potential of FAP-targeted theranostics in PDAC and other cancer types is ongoing (NCT04939610). At present, [ 177 Lu]Lu-DOTATATE and [ 177 Lu]Lu-PSMA-617 are clinically approved for TRT of somatostatin receptor 2 (SSTR2)-expressing neuroendocrine tumors (NETs) and prostate specific membrane antigen (PSMA)-overexpressing metastatic castration-resistant prostate cancer (mCRPC), respectively [ 7 , 8 ]. Unfortunately, [ 177 Lu]Lu-DOTATATE and [ 177 Lu]Lu-PSMA-617 treatment efficacy can be limited, and a subset of patients do not respond to treatment despite prominent tumor uptake observed on pre-treatment scans. To enhance therapeutic efficacy, cytotoxic radionuclides other than lutetium-177 (Lu-177) are being studied. Among them is terbium-161 (Tb-161), which has gained considerable interest recently [ 9 ]. The decay characteristics of Tb-161 are similar to that of Lu-177 (Lu-177: Eβ − av = 133 keV, T 1/2 = 6.65 days vs. Tb-161: Eβ − av = 154 keV, T 1/2 = 6.91), but besides a slightly higher β − -energy, Tb-161 emits a significant amount of internal conversion electrons (CEs) and Auger electrons (AEs) [ 10 ]. These CEs and AEs have a higher linear energy transfer than the β − particles, and potentially have added therapeutic benefit, especially when in close proximity of the DNA [ 11 ]. Monte Carlo simulations have predicted an increased absorbed dose of Tb-161 over Lu-177 [ 12 , 13 ], and in line with this, preclinical studies demonstrated improved efficacy has been described for Tb-161 labeled PSMA- and SSTR2-targeting radiopharmaceuticals over the Lu-177 labeled counterparts. [ 13 , 14 ]. Based on these promising results, clinical trials evaluating 161 Tb-labeled radiopharmaceuticals for NET and mCRPC treatment are ongoing (e.g. NCT04833517, NCT05521412, NCT06343038, NCT05359146). Tb-161 can potentially also be more beneficial than Lu-177 for FAP-TRT. Presumably, FAP-TRT causes cancer cell death by irradiating the cancer cells indirectly via the FAP-expressing CAFs. However, especially in stroma-dense tumors such as PDAC, CAF elimination could also be a successful treatment strategy to reduce treatment resistance and potentially improving efficacy of other therapies. It remains to be determined whether the additional short-range CEs and AEs emitted by Tb-161 can induce greater CAF and/or cancer cell death compared to Lu-177. Accordingly, we performed in vitro and in vivo studies to compare [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286, aimed at evaluation of their efficacy and impact on tumor stroma density in a clinically representative PDAC mouse model after both mono- and tandem radionuclide therapy. Methods Radiolabeling and stability FAP-2286 (MedChem Express, Monmouth Junction NJ, USA) was labeled with either non-carrier added [ 177 Lu]LuCl 3 (Lu-177, PI Medical, Raamsdonksveer, The Netherlands) or non-carrier added [ 161 Tb]TbCl 3 (Tb-161, TerThera BV, Breda, The Netherlands) in a total volume of 140 µL. The radiochemical purity (RCP) was determined by high-performance liquid chromatography (HPLC). Stability was determined by analyzing the RCP over time after 2 and 24 h, by incubating 2.5 MBq radiolabeled FAP-2286 in 200 µL phosphate buffered saline (PBS) (Gibco, Breda, The Netherlands) or 100 µL commercially available mouse serum (Invitrogen, Carlsbad CA, USA) at 37°C. Details on these labeling and stability studies are described in the Supplementary Information. Cell culture Human fibrosarcoma cells transduced to express human FAP (HT1080-huFAP), kindly provided by Prof. Uwe Haberkorn (University of Heidelberg), and pancreatic stellate cells (PS-1), provided by Queen Mary University, were cultured as previously described [ 15 ]. The 19TT-F human breast CAFs were obtained from Prof. John Martens and were cultured in RPMI Glutamax© (Gibco). One day prior to in vitro assays, cells were seeded in 12-well plates (Sarstedt, Nümbrecht, Germany) to reach 80% confluence. For in vivo studies a cell line-derived xenograft model (CDX) was established using the T110299 cell line, derived from a Ptf1a WT/Cre Kras WT/LSL−G12D P53 LSL − R172H/fl mouse that spontaneously grows primary PDAC [ 16 ]. T110299 cells were cultured in DMEM Glutamax© containing pyruvate (Gibco). All cell culture media were supplemented with 10% fetal bovine serum (Gibco) and 100 UI/mL penicillin + 100 µg/mL streptomycin (Merck Life Science NV, Amsterdam, The Netherlands). In vitro competition binding and uptake The half-maximal inhibitory concentration (IC50) of the radiopharmaceuticals was determined on HT1080-huFAP cells using 1 nM [ 177 Lu]Lu-/[ 161 Tb]Tb -FAP-2286, incubated with increasing concentrations (10 − 12 to 10 − 6 M) of the FAP-inhibitor UAMC-1110 (Biosynth Ltd, Berkshire, United Kingdom) for 45 min. To determine radiopharmaceutical uptake, HT1080-huFAP, PS-1, and 19TT-F cells were incubated with 1 nM [ 177 Lu]Lu-/[ 161 Tb]Tb-FAP-2286 for 5-180 min, +/- 1 mM UAMC-1110 to determine specificity of uptake, as previously described [ 15 ]. The uptake is expressed as percentage added activity (%AA) per 200,000 cells. In vitro autoradiography Radiopharmaceutical binding to fresh frozen 10 µm slices of T110299 CDXs, human PDAC, and breast cancer (BC) tissue (n = 3/group), mounted onto Superfrost Plus™ microscope slides (VWR, Leuven, Belgium), was assessed as previously described [ 17 ]. In short, tissue slices were incubated with incubation buffer (167 mM Tris-HCL, 5 mM MgCl 2, 1% BSA) containing 1 nM [ 177 Lu]Lu-/[ 161 Tb]Tb-FAP-2286, +/- 1 mM UAMC-1110 to determine specificity of binding. Tissues were exposed to super-resolution phosphor screens (Perkin Elmer) for at least 24 h. Uptake was quantified as digital light units (DLU)/mm 2 and normalized to 1 µL standards. The data were expressed as % AA. In vivo studies For all in vivo studies, C57BL/6 male mice (Janvier, Le Genest-Saint-Isle, France) were subcutaneously inoculated with 50 µL HBSS (Gibco) containing 0.5×10 6 T110299 cells, with a tumor take of 100%. Details on inoculation and tumor growth, animal welfare, and animal numbers are provided in the Supplementary Information (Table S1 -2). In all studies, animals were monitored daily, and animal weight and tumor size was measured twice each week. Ex vivo biodistribution and ex vivo autoradiography On day 11 after inoculation of T110299 cells (tumor sizes; Lu-177: 431 ± 173 mm 3 and Tb-161: 315 ± 159 mm 3 (Fig. S1 )), animals received an intravenous injection (IV) in the tail vain of [ 177 Lu]Lu-/[ 161 Tb]Tb-FAP-2286 (5 MBq/500 pmol/200 µL), diluted in PBS containing 0.06 mg/mL Kolliphor® HS 15. At 1, 4, 24, and 48 h post injection (p.i.) (n = 5/group), blood was collected via orbital puncture under isoflurane/O 2 anesthesia, followed by cervical dislocation and collection of tumor and organs of interest (i.e., pancreas, liver, GI-tract (stomach, small intestine, cecum, colon), kidneys, lungs, heart, salivary glands, muscle, femur bone). A subset of animals (n = 4/group) received an injection of radiolabeled FAP-2286 + 17.5 nM UAMC-1110 following biodistribution studies 4 h p.i. to determine specificity of uptake. The collected materials were weighed and measured in a γ-counter. Data were expressed as percentage injected dose per gram of tissue (% ID/g). Half of the tumor was snap frozen in liquid nitrogen, sectioned (10 µm), mounted on Starforst glass slides, and exposed to a super-resolution phosphor screen to determine radiopharmaceutical binding. In vivo efficacy of mono and tandem treatment T110299 tumors were allowed to grow for seven days reaching a size of 89 ± 38 mm 3 . On day 7, 10, 14, and 17 after T110299 cell inoculation, mice received IV injections of 200 µL 40 MBq/500 pmol [ 177 Lu]Lu-/[ 161 Tb]Tb-FAP-2286 (n = 10/group), or vehicle solution (PBS + 0.06 mg/mL Kolliphor® HS 15) (n = 8). An additional group of animals (n = 2–3/group) were sacrificed 1 h after each IV injection, and tumors were harvested to measure radiopharmaceutical uptake and to assess potential changes in FAP-expression or stroma-to-cancer cell ratios over time. Immunohistochemistry (IHC) was performed on the excised tumors and a pathologist (MCD) scored the tumor tissues blindly to determine the percentage of tumor cells and stroma/CAFs of the total tumor area. Details on the IHC are described in the Supplementary Information. Next, tandem efficacy was determined in T110299 xenografted mice with an average tumor size of 219 ± 83 mm 3 on day seven post inoculation. Subsequently, mice received 40 MBq/500 pmol on day 8, 11, 15, and 18 post T110299 inoculation. Mice received 2×[ 177 Lu]Lu-FAP-2286 followed by 2×[ 161 Tb]Tb-FAP-2286, or vice versa (n = 13/group), or vehicle injections (n = 9) (Fig. S2 ). Dosimetry The biodistribution data of [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 were used to characterize the organ and tumor time-activity curves. These curves were fitted with mono-exponential curves to enable integration over time, leading to Time Integrated Activity (concentration) Coefficients ([TIAC]). Absorbed doses to the mouse organs and tumor xenograft per injected activity (IA) were calculated according to the modified MIRD equation: with r t the target organ or tumor, r s the source organ, m(r s ) the mass of the source organ and S(r t ←r s ) the S-value. Both the organ masses and the S-values were taken from the RADAR 25 g mouse phantom [ 18 ]. The tumor absorbed doses were calculated by using the sphere S-values from the Olinda dosimetry software [ 19 ]. The dosimetry was used to estimate the treatment response, taking the radiosensitivity of the tumor cells [ 20 ], their doubling time, and the kinetics of FAP-2286 and its dosimetry when labeled to Lu-177 or Tb-161, into account. Dosimetry in the PDAC and BC tissues was performed by inserting the autoradiography data as source distributions in a cylindrical tissue model set up in the Monte Carlo code MCNP (version 6.2). Autoradiography images were resampled to a pixel size of 85 µm and all data above 1800 DLU/mm 2 were used as source in the middle of a cylindrical tissue model of 3 mm thickness. The energy absorption inside an 85 x 85 x 3000 µm 3 voxel grid was tallied and transformed into S-values [in mGy/MBq.s]. The 161 Tb and 177 Lu beta-particle and low energy electron spectra of ICRP-107 were used. The number of histories used in the source distribution was set at 10 million to obtain statistical uncertainties below 5% in each voxel. Absorbed dose was subsequently calculated using ImageJ software (version 1.54g) by drawing and measuring the whole tissue slice or the tumor area in accordance with the pathologists findings in the corresponding H&E slice. Statistics Statistical analyses were performed using GraphPad Prism 9.0 (San Diego, California USA), and results were regarded statistically significant if p < 0.05. All experiments were performed at least in triplicate, unless stated otherwise. More detailed information is provided in the Supplementary Information. Results Stability in vitro The stability of [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 in labeling solution was high, and there was no significant difference between the radiopharmaceuticals, i.e. 97.6 ± 0.6% vs 95.2 ± 2.8% (p > 0.05), respectively, after 2 h incubation, and 93.4 ± 2.1% and 93.4 ± 2.6%, (p > 0.05), respectively, after 24 h incubation. In contrast, in mouse serum a decrease in stability was observed for both [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286. A significant difference in stability between the radiopharmaceuticals was observed after 2 h, but not after 24 h of incubation (i.e. 70.0 ± 0.5% vs 75.8 ± 1.0% p 0.05, respectively) (Table S3). In vitro competition binding and uptake There were no significant differences in either the IC50 (i.e. Lu-177: 2.9 ± 7.2 nM vs. Tb-161:1.5 ± 3.2 nM, p > 0.05, Fig. S3) or in the uptake of [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 evaluated in HT1080-huFAP and on PS-1 cells (p > 0.05) (Fig. 1 ). Radiopharmaceutical uptake occurred rapidly in both cell lines, but was 5–10 fold higher in HT1080-huFAP cells than in PS-1 cells. Moreover, radiopharmaceutical uptake was largely internalized by HT1080-huFAP cells, whereas in PS-1 cells it remained mainly membrane-bound. No differences in the internalization ratio was found between [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 in either cell line. Matching uptake of the two radiopharmaceuticals was further confirmed in 19TT-F cells (p > 0.05, Fig. S4). Ex vivo biodistribution Biodistribution of [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 in T110299-xenografted mice demonstrated tumor uptake of 1.85 ± 0.39% ID/g for [ 177 Lu]Lu-FAP-2286 and 1.39 ± 0.45% ID/g for [ 161 Tb]Tb-FAP-2286 at 1 h p.i. Radiopharmaceutical washout was already observed at 4 h p.i., with only 0.32 ± 0.10% ID/g [ 177 Lu]Lu-FAP-2286 and 0.23 ± 0.05% ID/g [ 161 Tb]Tb-FAP-2286 remaining at this time point, and further decreasing at 24 h and 48 h p.i. (Fig. 2 a-b, Table S4-5). Besides the tumor, the kidneys were the only organ with noticeable radiopharmaceutical uptake, i.e. 7.85 ± 1.96% ID/g at 1 h p.i. and 3.20 ± 0.68% ID/g at 4 h p.i. for [ 177 Lu]Lu-FAP-2286, and 6.09 ± 1.41% ID/g at 1 h p.i. and 3.84 ± 0.59% ID/g at 4 h p.i. for [ 161 Tb]Tb-FAP-2286 (Fig. 2 c-d, Table S4-5). The kidney uptake for [ 161 Tb]Tb-FAP-2286 was significantly lower at 1 h p.i. (p < 0.001), but significantly higher at 4 h p.i. (p < 0.001). Co-injection with UAMC-1110 did not demonstrate a significant decrease in [ 177 Lu]Lu-FAP-2286 or [ 161 Tb]Tb-FAP-2286 tumor uptake. Only kidney uptake was found to be significantly decreased compared to that of animals injected with only the radiopharmaceutical (Lu-177: 7.85 ± 1.96% vs. 3.20 ± 0.68% ID/g, and Tb-161: 3.84 ± 0.59% vs. 1.84 ± 0.18% ID/g, p < 0.0001) (Fig. S5). Ex vivo autoradiography on the excised tumors showed results in accordance with the ex vivo biodistribution. However, the ex vivo autoradiography did indicate a significantly lower signal in tumors from the animals co-injected with UAMC-1110 compared to the tumors from mice that only received radiolabeled FAP-2286 (i.e. Lu-177: 9206 ± 2507 vs. 23057 ± 3786 DLU/mm 2 , p < 0.01, and Tb-161: 5551 ± 418 vs. 15911 ± 3106 DLU/mm 2 , p < 0.01, respectively) (Fig. S6). Additionally, an in vitro autoradiography on treatment naïve T110299 tumors, demonstrated that co-incubation with 1000x excess of UAMC-1110 significantly blocked binding of [ 177 Lu]Lu-FAP-2286 (p < 0.05, Fig. S7a,c) and [ 161 Tb]Tb-FAP-2286 (p < 0.01, Fig. S7b,c). Dosimetry The organ and tumor dosimetry are presented in Table S6. Based on the dosimetry calculations 4×40 MBq/500 pmol IV injections were selected, corresponding to 4×1.5 Gy Lu-177 and 4×1.6 Gy Tb-161 (Fig. S8). In vivo efficacy of mono treatment Treatment with [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 in the T110299-xenografted mice showed no effect on tumor growth (Fig. 3 ). Radiopharmaceutical uptake in the tumors 1 h p.i. showed an uptake of 1-1.5% ID/g for both radiopharmaceuticals, which was in line with the biodistribution studies (Fig. 4 a). No significant changes in body weight were observed(Fig. S9a). IHC on tumors harvested 1 h p.i., demonstrated heterogeneity in stroma-to-cancer cell ratios and level of FAP-staining. Accordingly, no differential effect on stroma composition could be distinguished between vehicle, [ 177 Lu]Lu-FAP-2286, and [ 161 Tb]Tb-FAP-2286 treated mice. Stroma density pooled for all three groups indicated a significant increase in stroma density over time (Fig. 4 b, IV to IV2 p < 0.01; IV1 to IV3 p < 0.05). However, stroma density did not correlate with FAP-expression level, radiopharmaceutical uptake, tumor size, or tumor weight (Fig. S10). In vivo efficacy of tandem treatment To determine whether tandem therapy has additional value, T110299-xenografted mice were treated with 2×40 MBq/500 pmol [ 177 Lu]Lu-FAP-2286 followed by 2×40 MBq/500 pmol [ 161 Tb]Tb-FAP-2286, or vice versa. The Kaplan-Meier survival curve demonstrated a significant longer survival for animals that received 2×40 MBq/500 pmol [ 177 Lu]Lu-FAP-2286 followed by 2×40 MBq/500 pmol [ 161 Tb]Tb-FAP-2286 over those that received vehicle (Fig. 5 a). Tumor shrinkage was not observed in any of the groups (Fig. 5 b-e). In line with the previous treatment study, no significant changes in body weight were observed between the treatment arms (Fig. S9b). In vitro autoradiography and accompanying dosimetry Autoradiography on patient PDAC material demonstrated potent but heterogeneous binding of both [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 (Fig. 6 a). Although lower binding was observed for [ 161 Tb]Tb-FAP-2286 compared to [ 177 Lu]Lu-FAP-2286 the difference was not significant (Fig. 6 b, p > 0.05). Dosimetry calculations performed on the autoradiography data resulted in 1.35-fold higher absorbed dose in both the tumor area and to the whole tissue slice with [ 161 Tb]Tb-FAP-2286 compared to [ 177 Lu]Lu-FAP-2286 (Fig. 6 c). The same trends were observed for radiopharmaceutical binding to BC tissue samples (Fig. S11). Discussion Improved efficacy of Tb-161 over Lu-177 for SSTR2- and PSMA-TRT has been reported for the treatment of mCRPC [ 13 , 21 , 22 ] and NETs [ 14 , 23 ], respectively. Next to enhanced therapeutic efficacy, introducing Tb-161 as substitute or alongside Lu-177 can alleviate the growing supply pressure on Lu-177 [ 24 ]. Therefore, we aimed to compare Lu-177 and Tb-161 for FAP-TRT. To our knowledge, this is the first study characterizing and comparing [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286. The radiolabeling of FAP-2286 with both Lu-177 and Tb-161 was successful, and the radiopharmaceuticals remained equally stable in labeling solution and PBS. In contrast, their stability in mouse serum decreased to 70% and 75% after 2 h for Lu-177 and Tb-161, respectively, and further decreased drastically below 15% after 24 h. Nevertheless, due to rapid blood circulation in mice [ 25 ], the fast tumor uptake observed, and since > 70% of the radiopharmaceutical remained intact during the first 2 hours, the decrease in stable compound is not expected to have had a major effect on radiopharmaceutical uptake in our study. In vitro cell uptake studies demonstrated that the behavior of [¹⁷⁷Lu]Lu-FAP-2286 and [¹⁶¹Tb]Tb-FAP-2286 was similar regardless of the applied cell model. In line with this, in vitro autoradiography studies showed similar binding of the radiopharmaceuticals to cancer tissues. Thus, the behavior of FAP-2286 does not appear to be altered when radiolabeled with Tb-161 compared to radiolabeling with Lu-177. In contrast to other studies that used FAP-2286 in vivo in different models, we observed relatively low uptake of the radiopharmaceutical in the T110299 CDX [ 6 , 26 , 27 ]. This is most likely because, unlike models established with cancer cells (over)expressing FAP, the T110299 CDX depends on infiltration of murine FAP (muFAP)-expressing CAFs for FAP-2286 uptake [ 6 ]. The T110299 xenografted mouse model has previously been used successfully for evaluation of FAP-targeting interventions [ 28 , 29 ]; however, model-dependent uptake of FAP-2286 has been reported [ 26 ]. Additionally, the injected peptide mass might affect radiopharmaceutical uptake, as illustrated by various recent studies [ 30 , 31 ], yet there is no consensus on the optimal mass for FAP-2286 or other peptide-based FAP-targeting radiopharmaceuticals. Another explanation for the observed low tumor uptake might be the slightly lower affinity of the peptide for huFAP compared to muFAP (1.1 ± 0.5 nM vs. 4.7 ± 1.5 nM, respectively) [ 6 ]. FAP-2287, which is similar to FAP-2286 but has higher muFAP affinity [ 32 ], was recently developed and should be considered in future studies using a models with muFAP-expressing CAFs. Ex vivo biodistribution studies did not show significantly lower tumor uptake in mice co-injected with UAMC-1110 compared to those injected with [¹⁷⁷Lu]Lu-FAP-2286 or [¹⁶¹Tb]Tb-FAP-2286 alone. Although the injected 35-time excess of UAMC-1110 was insufficient to saturate muFAP and prevent radiolabeled FAP-2286 binding, we believe that the majority of radiolabeled FAP-2286 tumor uptake was still FAP-specific. First, using more sensitive ex vivo autoradiography on these same tumor, we observed significant effective blocking of FAP-2286. Second, in additional in vitro autoradiography studies with untreated PDAC299 tumors, a 1000× excess of UAMC-1110 did block binding of radiolabeled FAP-2286. Lastly, it has previously been demonstrated in vivo that FAP-2286 binding is FAP-specific, by using a higher excess (e.g. 60-fold) of an unlabeled FAP-targeting compound for blocking [ 26 ]. Thus, the UAMC-1110 concentration was likely too low to achieve complete blocking in our in vivo studies, yet evidence suggests that radiolabeled FAP-2286 binding is FAP-specific, which is further supported by its current evaluation in a large prospective clinical trial (NCT 04939610). Besides, mice receiving radiolabeled FAP-2286 co-injected with UAMC-1110 showed an unexpected significant decrease in kidney uptake compared to those injected with the radiopharmaceutical alone, even though kidneys do not express FAP. This may be due to altered clearance and reabsorption rates caused by the high concentration of UAMC-1110. Due to the low tumor uptake, dosimetry calculations predicted that a relatively high injected dose of 4×40 MBq/500 pmol would be needed to achieve up to 50% tumor shrinkage. Despite this high dose, no survival benefit was observed, apart from a minor growth delay in the tandem therapy arm that first received 2×[¹⁷⁷Lu]Lu-FAP-2286 followed by 2×[¹⁶¹Tb]Tb-FAP-2286. The dosimetry model predicted a marginal advantage for administering Lu-177 followed by Tb-161 compared to the reverse sequence; however, tumor control efficacy remained lower than predicted. This discrepancy is likely due to the model not accounting for all biological factors, including heterogeneity of FAP expression, indirect irradiation from CAFs to cancer cells, and differences in radiosensitivity between CAFs and cancer cells. Treated tumors were subjected to IHC evaluation to determine whether Lu-177 or Tb-161 had differential effects on stroma-to-cancer cell ratios or FAP expression levels. Unfortunately, due to high heterogeneity in stroma density, it was not possible to distinguish the effects of either radiopharmaceutical. Notably, tumor growth rates differed between the mono and tandem therapy studies, with vehicle-treated mice in the first treatment study reaching the humane endpoint later than those in the later tandem therapy study. This may be due to mice in the tandem therapy group being inoculated at 9 weeks old, compared to 6 weeks old in the mono therapy group. Other studies have shown that the age of CJ54BL/6 mice can impact the immune system development, which in turn affects tumor development and treatment response [ 33 , 34 ]. Additionally, since cells with a higher division rate are in general more radiosensitive [ 35 ], this difference in growth rate may partly explain the differential therapeutic response between the two studies. So far mixed responses to [¹⁷⁷Lu]Lu-FAP-2286 therapy have been reported in clinical studies [ 36 ] and, unfortunately, there is insufficient data on PDAC patients receiving FAP-TRT to directly relate our findings to that of the clinic. The negative outcome of FAP-TRT observed in our study, which used a histopathological representative model, is in contrast with previous promising results in another model [ 6 ], which suggests that FAP-TRT efficacy might be model dependent. This finding raises questions about the value of preclinical in vivo studies that use less clinically representative models, as well as about the overall potential of FAP-TRT in PDAC. It should be noted that the heterogeneity in FAP-expression and tumor growth, together with the low FAP-2286 uptake, were limitations specific for our study. To further confirm the value of FAP-TRT in stroma dense PDAC, and elucidate differences between Lu-177 and Tb-161, a model with a higher uptake and corresponding higher response rate is necessary in future studies. CDX models established using a cancer cell line with high FAP expression could be applied, although this would be less representative of patient histopathology and its translational value with regard to FAP-2286 uptake remains unclear. Moreover, a model without CAF infiltration would not allow for discerning effects on CAFs and cancer cells. In an attempt to gain insights in the clinical uptake of FAP-2286, [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 binding to patient PDAC (and BC) tissue was compared. No significant difference in overall tissue binding was observed between the two radiopharmaceuticals, but high intra- and inter-tumor heterogeneity was noted. Dosimetry calculations showed that the absorbed dose was markedly higher in tumor areas compared to the overall tissue slide, confirming that radionuclide treatment directed at CAFs could effectively irradiate malignant areas. Additionally, dosimetry on cancer tissues predicted that the absorbed dose of [¹⁶¹Tb]Tb-FAP-2286 would consistently be higher, even in areas where its binding was lower than that of [¹⁷⁷Lu]Lu-FAP-2286. However, further studies are needed to determine whether this higher dose, predominantly caused by CEs and AEs, would result in increased efficacy. Future studies using a larger sample set and potentially incorporating more complex 3D dosimetry in subsequent tumor slices will be valuable to better understand the extent to which tumor stroma density and FAP heterogeneity could serve as critical determinants of FAP-TRT efficacy. Conclusions Our in vitro data suggests that [ 177 Lu]Lu-FAP-2286 and [ 161 Tb]Tb-FAP-2286 can be used interchangeably for FAP-TRT. In vivo studies demonstrated that a modest tumor growth delay could be achieved using a tandem therapy strategy, yet the efficacy of FAP-TRT therapy was too limited to determine true differential effects between the two radiopharmaceuticals. A model with similar histopathological clinical resemblance, yet a higher response to FAP-TRT, will be necessary in future studies to discern differential effects of Lu-177 and Tb-161 on CAFs and cancer cells when applying FAP-TRT. Abbreviations PDAC pancreatic ductal adenocarcinoma TME tumor microenvironment FAP fibroblast activation protein CAF cancer-associated fibroblast TRT targeted radionuclide therapy Lu-177 Lutetium-177 Tb-161 Terbium-161 mCRPC metastatic SSTR2 somatostatin receptor 2 PSMA prostate specific membrane antigen CE conversion electron AE Auger electrons LET linear energy transfer DTPA diethylenetriaminepentaacetic acid HPLC high-performance liquid chromatography RCY radiochemical yield RCP radiochemical purity PBS phosphate buffered saline muFAP murine FAP huFAP human FAP p.i. post injection BC breast cancer DLU digital light units Declarations Ethics approval and consent to participate All in vivo studies were conducted according to the guidelines of the Declaration of Helsinki. Approval was granted by the Animal Welfare Committee of the Erasmus MC (CCD#: 2216075, SP21000146/31-12-2024 and SP2300186/31-12-2024), and was in accordance with European law. Clinical Trial number: not applicable. Consent for publication Not applicable. Availability of data and material All available data are described in the manuscript or are available in the Supplementary Information. Additional information can be obtained from the corresponding author upon reasonable request. Competing interests Not applicable. Funding This work was not supported by external funding. Authors’ contributions SUD and CDH came up with study concept and design. Radiochemistry analyses and radiolabeling were performed by CN, HM, YS, and EB. Material preparation, in vivo studies, data collection and analysis were performed by CDH, CR, DS, MCD. Dosimetry calculations and data analyses were performed by MK and CDH. The first draft of the manuscript was written by CDH and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Acknowledgements We thank TherThera BV for providing the Terbium-161 for the study. We are grateful to Prof. Jens Siveke and Dr. Marija Trajkovic-Arsic for providing us with the T110299 cells, and to Dr. Sanne van Lith for her support with model development. Competing interests None References Kleeff J, Korc M, Apte M, La Vecchia C, Johnson CD, Biankin AV, et al. Pancreatic cancer. Nat Reviews Disease Primers. 2016;2:16022. https://doi.org/10.1038/nrdp.2016.22 . Zhang T, Ren Y, Yang P, Wang J, Zhou H. Cancer-associated fibroblasts in pancreatic ductal adenocarcinoma. Cell Death Dis. 2022;13:897. https://doi.org/10.1038/s41419-022-05351-1 . Xin L, Gao J, Zheng Z, Chen Y, Lv S, Zhao Z, et al. Fibroblast Activation Protein-α as a Target in the Bench-to-Bedside Diagnosis and Treatment of Tumors: A Narrative Review. Front Oncol. 2021;11:648187. https://doi.org/10.3389/fonc.2021.648187 . Privé BM, Boussihmad MA, Timmermans B, van Gemert WA, Peters SMB, Derks YHW, et al. Fibroblast activation protein-targeted radionuclide therapy: background, opportunities, and challenges of first (pre)clinical studies. Eur J Nucl Med Mol Imaging. 2023;50:1906–18. https://doi.org/10.1007/s00259-023-06144-0 . Kratochwil C, Flechsig P, Lindner T, Abderrahim L, Altmann A, Mier W, et al. 68 Ga-FAPI PET/CT: Tracer Uptake in 28 Different Kinds of Cancer. J Nucl Med. 2019;60:801–5. 10.2967/jnumed.119.227967 . Zboralski D, Hoehne A, Bredenbeck A, Schumann A, Nguyen M, Schneider E, et al. Preclinical evaluation of FAP-2286 for fibroblast activation protein targeted radionuclide imaging and therapy. Eur J Nucl Med Mol Imaging. 2022;49:3651–67. https://doi.org/10.1007/s00259-022-05842-5 . Hennrich U, Eder M. [ 177 Lu]Lu-PSMA-617 (Pluvicto ™ ): The First FDA-Approved Radiotherapeutical for Treatment of Prostate Cancer. Pharmaceuticals (Basel). 2022;15. https://doi.org/10.3390/ph15101292 . Hennrich U, Kopka K. Lutathera(®): The First FDA- and EMA-Approved Radiopharmaceutical for Peptide Receptor Radionuclide Therapy. Pharmaceuticals (Basel). 2019;12. https://doi.org/10.3390/ph12030114 . Ruigrok EAM, van Weerden WM, Nonnekens J, de Jong M. The Future of PSMA-Targeted Radionuclide Therapy: An Overview of Recent Preclinical Research. Pharmaceutics. 2019;11. https://doi.org/10.3390/pharmaceutics11110560 . Alcocer-Ávila ME, Ferreira A, Quinto MA, Morgat C, Hindié E, Champion C. Radiation doses from 161Tb and 177Lu in single tumour cells and micrometastases. EJNMMI Phys. 2020;7:33. https://doi.org/10.1186/s40658-020-00301-2 . Ku A, Facca VJ, Cai Z, Reilly RM. Auger electrons for cancer therapy – a review. EJNMMI Radiopharmacy Chem. 2019;4:27. 10.1186/s41181-019-0075-2 . Hindié E, Zanotti-Fregonara P, Quinto MA, Morgat C, Champion C. Dose Deposits from 90 Y, 177 Lu, 111 In, and & 161 Tb in Micrometastases of Various Sizes: Implications for Radiopharmaceutical Therapy. J Nucl Med. 2016;57:759. https://doi.org/10.2967/jnumed.115.170423 . Müller C, Umbricht CA, Gracheva N, Tschan VJ, Pellegrini G, Bernhardt P, et al. Terbium-161 for PSMA-targeted radionuclide therapy of prostate cancer. Eur J Nucl Med Mol Imaging. 2019;46:1919–30. https://doi.org/10.1007/s00259-019-04345-0 . Borgna F, Haller S, Rodriguez JMM, Ginj M, Grundler PV, Zeevaart JR, et al. Combination of terbium-161 with somatostatin receptor antagonists—a potential paradigm shift for the treatment of neuroendocrine neoplasms. Eur J Nucl Med Mol Imaging. 2022;49:1113–26. https://doi.org/10.1007/s00259-021-05564-0 . van der Heide CD, Campeiro JD, Ruigrok EAM, van den Brink L, Ponnala S, Hillier SM, Dalm SU. In vitro and ex vivo evaluation of preclinical models for FAP-targeted theranostics: differences and relevance for radiotracer evaluation. EJNMMI Res. 2024;14:125. 10.1186/s13550-024-01191-6 . Hingorani SR, Wang L, Multani AS, Combs C, Deramaudt TB, Hruban RH, et al. Trp53R172H and KrasG12D cooperate to promote chromosomal instability and widely metastatic pancreatic ductal adenocarcinoma in mice. Cancer Cell. 2005;7:469–83. https://doi.org/10.1016/j.ccr.2005.04.023 . Verhoeven M, Ruigrok EAM, van Leenders G, van den Brink L, Balcioglu HE, van Weerden WM, Dalm SU. GRPR versus PSMA: expression profiles during prostate cancer progression demonstrate the added value of GRPR-targeting theranostic approaches. Front Oncol. 2023;13:1199432. https://doi.org/10.3389/fonc.2023.1199432 . Keenan MA, Stabin MG, Segars WP, Fernald MJ. RADAR Realistic Animal Model Series for Dose Assessment. J Nucl Med. 2010;51:471. https://doi.org/10.2967/jnumed.109.070532 . Stabin MG, Konijnenberg MW. Re-evaluation of absorbed fractions for photons and electrons in spheres of various sizes. J Nucl Med. 2000;41:149–60. Carlson DJ, Stewart RD, Li XA, Jennings K, Wang JZ, Guerrero M. Comparison of in vitro and in vivo alpha/beta ratios for prostate cancer. Phys Med Biol. 2004;49:4477–91. https://doi.org/10.1088/0031-9155/49/19/003 . Schaefer-Schuler A, Burgard C, Blickle A, Maus S, Petrescu C, Petto S, et al. [ 161 Tb]Tb-PSMA-617 radioligand therapy in patients with mCRPC: preliminary dosimetry results and intra-individual head-to-head comparison to [ 177 Lu]Lu-PSMA-617. Theranostics. 2024;14:1829–40. https://doi.org/10.7150/thno.92273 . Bernhardt P, Svensson J, Hemmingsson J, van der Meulen NP, Zeevaart JR, Konijnenberg MW, et al. Dosimetric Analysis of the Short-Ranged Particle Emitter (161)Tb for Radionuclide Therapy of Metastatic Prostate Cancer. Cancers (Basel). 2021;13. https://doi.org/10.3390/cancers13092011 . Baum RP, Singh A, Kulkarni HR, Bernhardt P, Rydén T, Schuchardt C, et al. First-in-Humans Application of (161)Tb: A Feasibility Study Using (161)Tb-DOTATOC. J Nucl Med. 2021;62:1391–7. https://doi.org/10.2967/jnumed.120.258376 . Dash A, Pillai MRA, Knapp FF. Production of 177Lu for Targeted Radionuclide Therapy: Available Options. Nuclear Med Mol Imaging. 2015;49:85–107. https://doi.org/10.1007/s13139-014-0315-z . Debbage PL, Griebel J, Ried M, Gneiting T, DeVries A, Hutzler P. Lectin Intravital Perfusion Studies in Tumor-bearing Mice: Micrometer-resolution, Wide-area Mapping of Microvascular Labeling, Distinguishing Efficiently and Inefficiently Perfused Microregions in the Tumor. J Histochem Cytochemistry. 1998;46:627–39. https://doi.org/10.1177/002215549804600508 . Millul J, Koepke L, Haridas GR, Sparrer KMJ, Mansi R, Fani M. Head-to-head comparison of different classes of FAP radioligands designed to increase tumor residence time: monomer, dimer, albumin binders, and small molecules vs peptides. Eur J Nucl Med Mol Imaging. 2023;50:3050–61. https://doi.org/10.1007/s00259-023-06272-7 . Wang R, Huang M, Wang W, Li M, Wang Y, Tian R. Preclinical Evaluation of 177 Lu-Labeled FAP-Targeted Peptide for Tumor Radiopharmaceutical Imaging and Therapy. J Nucl Med. 2025;66:250. https://doi.org/10.2967/jnumed.124.268689 . Dorst DN, Smeets EMM, Klein C, Frielink C, Geijs D, Trajkovic-Arsic M, et al. Fibroblast Activation Protein-Targeted Photodynamic Therapy of Cancer-Associated Fibroblasts in Murine Models for Pancreatic Ductal Adenocarcinoma. Mol Pharm. 2023;20:4319–30. https://doi.org/10.1021/acs.molpharmaceut.3c00453 . Smeets EMM, Dorst DN, Franssen GM, van Essen MS, Frielink C, Stommel MWJ, et al. Fibroblast Activation Protein-Targeting Minibody-IRDye700DX for Ablation of the Cancer-Associated Fibroblast with Photodynamic Therapy. Cells; 2023. Galbiati A, Bocci M, Neri D, Cazzamalli S. Effect of molar dose on the in vivo tissue biodistribution profile of FAP-targeted radioligand therapeutics. Eur J Nucl Med Mol Imaging. 2025;52:1399–405. https://doi.org/10.1007/s00259-024-06969-3 . Bilinska A, Ballal S, Bal C, Läppchen T, Pilatis E, Menéndez E, et al. Improved FAPI-radiopharmaceutical pharmacokinetics from the perspectives of a dose escalation study. Eur J Nucl Med Mol Imaging. 2025. https://doi.org/10.1007/s00259-025-07141-1 . Zboralski D, Osterkamp F, Christensen E, Bredenbeck A, Schumann A, Hoehne A, et al. Fibroblast activation protein targeted radiotherapy induces an immunogenic tumor microenvironment and enhances the efficacy of PD-1 immune checkpoint inhibition. Eur J Nucl Med Mol Imaging. 2023;50:2621–35. https://doi.org/10.1007/s00259-023-06211-6 . Xie J, Zhang J, Wu H, Tang X, Liu J, Cheng G, Li P. The influences of age on T lymphocyte subsets in C57BL/6 mice. Saudi J Biol Sci. 2017;24:108–13. https://doi.org/10.1016/j.sjbs.2016.09.002 . White JR, Gong H, Colaizy TT, Moreland JG, Flaherty H, McElroy SJ. Evaluation of hematologic variables in newborn C57/BL6 mice up to day 35. Vet Clin Pathol. 2016;45:87–95. https://doi.org/10.1111/vcp.12314 . Baskar R, Dai J, Wenlong N, Yeo R, Yeoh KW. Biological response of cancer cells to radiation treatment. Front Mol Biosci. 2014;1:24. 10.3389/fmolb.2014.00024 . McConathy JMY, Rodon J, Goenka AH, Moy RH, Morse S, Demange A, Aimone P, Hope TA.. 671P LuMIERE: A phase I/II study evaluating safety, dosimetry, and preliminary activity of [177Lu]Lu-FAP-2286 in patients with advanced solid tumors. Ann Oncol. 2024;35:S526. https://10.1016/j.annonc.2024.08.737 . Supplementary Files SupplementarymaterialsLuvsTbFINAL2EJNMMIRES.docx Cite Share Download PDF Status: Published Journal Publication published 09 Jan, 2026 Read the published version in EJNMMI Research → Version 1 posted Reviewers agreed at journal 17 Dec, 2025 Reviewers invited by journal 17 Dec, 2025 Editor assigned by journal 17 Dec, 2025 First submitted to journal 16 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8237978","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":561616619,"identity":"974431ee-8d3d-4998-aa7e-816238252968","order_by":0,"name":"Circe D. van der Heide","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Circe","middleName":"D. van der","lastName":"Heide","suffix":""},{"id":561616620,"identity":"1f34bdfa-7b2b-49e5-846c-c0a5eb6c93eb","order_by":1,"name":"Carolline M. Ntihabose","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Carolline","middleName":"M.","lastName":"Ntihabose","suffix":""},{"id":561616621,"identity":"2bd432eb-7ab4-47f6-91e7-ac6ee8461c76","order_by":2,"name":"Mark Konijnenberg","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Mark","middleName":"","lastName":"Konijnenberg","suffix":""},{"id":561616622,"identity":"d77a4825-86a9-4f3f-ba37-2a742becbea2","order_by":3,"name":"Hanyue Ma","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Hanyue","middleName":"","lastName":"Ma","suffix":""},{"id":561616623,"identity":"64d3f200-1aeb-43bc-879b-8a69919a671a","order_by":4,"name":"Debra Stuurman","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Debra","middleName":"","lastName":"Stuurman","suffix":""},{"id":561616624,"identity":"c90db11d-9355-4753-a55f-dd8ea59fe357","order_by":5,"name":"Corrina de Ridder","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Corrina","middleName":"","lastName":"de Ridder","suffix":""},{"id":561616625,"identity":"a9b4a1be-e77f-4049-9ce7-594ce086df7e","order_by":6,"name":"Yann Seimbille","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Yann","middleName":"","lastName":"Seimbille","suffix":""},{"id":561616626,"identity":"57999226-4734-4310-9f50-7749dfe063a3","order_by":7,"name":"Michail C. Doukas","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Michail","middleName":"C.","lastName":"Doukas","suffix":""},{"id":561616627,"identity":"2e631c8b-abbe-4022-831e-b913b92e7faf","order_by":8,"name":"Erik de Blois","email":"","orcid":"","institution":"Erasmus Medical Centre: Erasmus MC","correspondingAuthor":false,"prefix":"","firstName":"Erik","middleName":"","lastName":"de Blois","suffix":""},{"id":561616628,"identity":"9bf8820b-200c-43a5-870d-bb340813e9f8","order_by":9,"name":"Simone U. Dalm","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzUlEQVRIiWNgGAWjYHACA4YHDDbIAswNhLUkMKQhCzASpeUwCVrkG5g3fkjMOS9n3n786WaeP9vkGaQb8WsxOMBWLJG47baxzJkcs9u8bbcNG2QOEtDCwGMA0pI4gyGH7TZvw+0EBolEQg7jMf6RuO1c/Qz+589u8/whQgvDAR4zoC0HEiQkEsxu87ARocXgMFuZReK2ZMMZEm/Mbs4F+qWNoMPamzff+LjNTl6CP/3ZjTd/bsvzSyQfwO8wZnQBNvzqR8EoGAWjYBQQAwDpCkcTiq+1LwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-6393-6901","institution":"Erasmus MC","correspondingAuthor":true,"prefix":"","firstName":"Simone","middleName":"U.","lastName":"Dalm","suffix":""}],"badges":[],"createdAt":"2025-11-29 15:24:43","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8237978/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8237978/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s13550-026-01372-5","type":"published","date":"2026-01-09T15:59:31+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":98763073,"identity":"46a69231-a6d5-4836-a5b7-6a9cbd029f04","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"png","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1090892,"visible":true,"origin":"","legend":"","description":"","filename":"Fig1Uptakesovertime.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/d7cf43bedb6e304e77fd3770.png"},{"id":98779885,"identity":"c7099089-8edb-4da9-8fab-574d1b5ca764","added_by":"auto","created_at":"2025-12-22 12:30:52","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1276530,"visible":true,"origin":"","legend":"","description":"","filename":"Fig2BioD1hvs4h.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/8bfd01c2e9de48b9bd4ed636.png"},{"id":98763071,"identity":"506451a9-45b9-4381-8a77-da894e224135","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"png","order_by":3,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":686309,"visible":true,"origin":"","legend":"","description":"","filename":"Fig4CytotoxIV1IV4tumors.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/fcf2199ad57be55505146ce1.png"},{"id":98778442,"identity":"4f93a482-f082-4b83-96a9-e10b8c3ec5f1","added_by":"auto","created_at":"2025-12-22 12:29:15","extension":"png","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2389031,"visible":true,"origin":"","legend":"","description":"","filename":"Fig5Survivalcytotoxtandem.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/aa0c218ff6c2bb1323fd0e61.png"},{"id":98763084,"identity":"07f7a3d1-96fa-4b34-aaab-ddcb27a2bcab","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"png","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":15972343,"visible":true,"origin":"","legend":"","description":"","filename":"Fig6PDACdosimetryanduptakeARGx.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/66873450e3f234bd092f589c.png"},{"id":98780501,"identity":"98d2f643-2fb6-40be-b5e7-00b351341889","added_by":"auto","created_at":"2025-12-22 12:31:24","extension":"png","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1176591,"visible":true,"origin":"","legend":"","description":"","filename":"Figure3ExVivoBioD.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/0b0edaf66a77cb3638b65ff6.png"},{"id":98763078,"identity":"78863606-0ee2-423b-89f6-30bee86d8975","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"xml","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":15578,"visible":true,"origin":"","legend":"","description":"","filename":"ejreEJRED2500511.xml","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/842a44c253f556f837a505ef.xml"},{"id":98763090,"identity":"4f212902-d156-4b8e-9824-39eaa6d7e680","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"xml","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1413,"visible":true,"origin":"","legend":"","description":"","filename":"EJRED250051111756.go.xml","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/1b5a3d33e7be94e8a91530cc.xml"},{"id":98763094,"identity":"2ffe0c43-657d-4a72-b722-a39c24a66465","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"xml","order_by":9,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":976,"visible":true,"origin":"","legend":"","description":"","filename":"EJRED2500511Import.xml","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/172ac46199d2c7a5dacdf718.xml"},{"id":98779236,"identity":"25d6d376-67bb-404d-b35a-86a5ffc0d89c","added_by":"auto","created_at":"2025-12-22 12:30:05","extension":"xml","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":115208,"visible":true,"origin":"","legend":"","description":"","filename":"EJRED25005111enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/7b31fd2bfb2b69052063581d.xml"},{"id":98763109,"identity":"32780da4-bc03-41ee-ab3a-9701548c1f2c","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1090892,"visible":true,"origin":"","legend":"","description":"","filename":"Fig1Uptakesovertime.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/346a9b063bd6d2ff22559a0b.png"},{"id":98763092,"identity":"af8f054c-75ba-4628-8309-b38cca8c9d01","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1276530,"visible":true,"origin":"","legend":"","description":"","filename":"Fig2BioD1hvs4h.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/ac7fcd84c34efbdf6bb6ebe2.png"},{"id":98763103,"identity":"034a0a86-7ea7-4e82-9a5f-83cccd25f7d4","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":686309,"visible":true,"origin":"","legend":"","description":"","filename":"Fig4CytotoxIV1IV4tumors.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/1b1fd3d9913928c5045a1864.png"},{"id":98763110,"identity":"68e73527-8fc9-437d-8772-c658a54387f2","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2389031,"visible":true,"origin":"","legend":"","description":"","filename":"Fig5Survivalcytotoxtandem.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/02590859141cbc933b4a0f41.png"},{"id":98763099,"identity":"ac4de56c-c04b-4a59-9a2c-0826921aa0ca","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":15972343,"visible":true,"origin":"","legend":"","description":"","filename":"Fig6PDACdosimetryanduptakeARGx.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/5946cd4aee92514a22f05bd6.png"},{"id":98763120,"identity":"d0f3fec4-6cf2-465b-bef4-a5c0092351a2","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1176591,"visible":true,"origin":"","legend":"","description":"","filename":"Figure3ExVivoBioD.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/7ae2847ebb888575108c7918.png"},{"id":98763098,"identity":"5231fada-31a2-4813-99c5-16279718c209","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":3322,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/0d97e07fcddcc24bab77fc1b.png"},{"id":98777736,"identity":"1a87bc06-9a85-47b9-a4cc-815caef1ad47","added_by":"auto","created_at":"2025-12-22 12:28:24","extension":"png","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1090892,"visible":true,"origin":"","legend":"","description":"","filename":"Fig1Uptakesovertime.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/7657e3c37013b500437da2fc.png"},{"id":98763111,"identity":"518e81f6-848c-4d45-a2e2-def1c165ce3c","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1276530,"visible":true,"origin":"","legend":"","description":"","filename":"Fig2BioD1hvs4h.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/35854de6f18d3ac5941b577f.png"},{"id":98779428,"identity":"55ead84c-d135-4bbb-a808-78b50976691c","added_by":"auto","created_at":"2025-12-22 12:30:20","extension":"png","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2712117,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/db31ded0ff848785cf0addb3.png"},{"id":98763108,"identity":"c44399b8-7049-48b0-bdab-dc5cc1d85bdc","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":686309,"visible":true,"origin":"","legend":"","description":"","filename":"Fig4CytotoxIV1IV4tumors.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/8c3a556f4a07f28b698dfedb.png"},{"id":98780787,"identity":"3c9e09ce-2319-414b-8a50-eb86f1dfb94d","added_by":"auto","created_at":"2025-12-22 12:31:39","extension":"png","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2389031,"visible":true,"origin":"","legend":"","description":"","filename":"Fig5Survivalcytotoxtandem.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/5c97f46ed986c004870c0bc3.png"},{"id":98780315,"identity":"85743408-b761-48ba-aae9-4fde0a49f963","added_by":"auto","created_at":"2025-12-22 12:31:13","extension":"png","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":15972343,"visible":true,"origin":"","legend":"","description":"","filename":"Fig6PDACdosimetryanduptakeARGx.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/d081d757bd1a54100942e731.png"},{"id":98763106,"identity":"7f9dece5-fad4-43d3-b4a2-2f22384af18a","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":483333,"visible":true,"origin":"","legend":"","description":"","filename":"OnlineFig1Uptakesovertime.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/d58a0ebf84b6a3d452900724.png"},{"id":98763123,"identity":"cca6ceac-3f4c-4815-815a-1713cc01341b","added_by":"auto","created_at":"2025-12-22 10:02:57","extension":"png","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":530930,"visible":true,"origin":"","legend":"","description":"","filename":"OnlineFig2BioD1hvs4h.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/7956e979bc8cc648c2de92c9.png"},{"id":98763118,"identity":"ab3b6cdd-094d-40c3-b6dd-af950918fc91","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":28,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":268732,"visible":true,"origin":"","legend":"","description":"","filename":"OnlineFig4CytotoxIV1IV4tumors.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/381392d2d63be1483008ee1c.png"},{"id":98763088,"identity":"0a59d842-8c09-47d2-ac97-e7e5bf2f046b","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":612046,"visible":true,"origin":"","legend":"","description":"","filename":"OnlineFig5Survivalcytotoxtandem.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/0b99549fe7b0fe2a7aaa1e31.png"},{"id":98780863,"identity":"76e4488a-bb9d-4baf-b8e7-f27ca0732286","added_by":"auto","created_at":"2025-12-22 12:31:46","extension":"png","order_by":30,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2213551,"visible":true,"origin":"","legend":"","description":"","filename":"OnlineFig6PDACdosimetryanduptakeARGx.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/353f196c3cebc05784b076c4.png"},{"id":98780338,"identity":"728bdd06-1716-42c4-a85c-89e17815d85a","added_by":"auto","created_at":"2025-12-22 12:31:14","extension":"png","order_by":31,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":461874,"visible":true,"origin":"","legend":"","description":"","filename":"OnlineFigure3ExVivoBioD.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/adef30e074114f85332dcfd0.png"},{"id":98780810,"identity":"9423cc61-86db-4dc0-bd50-5d45ee69104b","added_by":"auto","created_at":"2025-12-22 12:31:42","extension":"png","order_by":32,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1784,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/d5d7752cac7650b68ac2d586.png"},{"id":98777515,"identity":"d91cf776-7139-4f55-8fcf-0a46823e8285","added_by":"auto","created_at":"2025-12-22 12:27:52","extension":"png","order_by":33,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":483333,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/9855145557864e3e33a8d484.png"},{"id":98763114,"identity":"dca3b3e2-de56-42c3-8ad2-3a023a4b241a","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":34,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":530930,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/16714b58ad7269173ca9b734.png"},{"id":98780860,"identity":"561db84a-fa8c-47bf-97d2-4cc23a22f9c7","added_by":"auto","created_at":"2025-12-22 12:31:46","extension":"png","order_by":35,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":732007,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/68a78ca6c8c945f24eabef32.png"},{"id":98763121,"identity":"0b157a2b-b089-48fe-8b66-4ff7f81cec58","added_by":"auto","created_at":"2025-12-22 10:02:57","extension":"png","order_by":36,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":268732,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/180a6c797890bf436563e2f4.png"},{"id":98780322,"identity":"40a27336-ce4a-4094-8193-b6be421fc069","added_by":"auto","created_at":"2025-12-22 12:31:13","extension":"png","order_by":37,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":612046,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/7c217f263ea6a50aa362d6b2.png"},{"id":98763125,"identity":"ebfb139b-67e4-4131-bfe9-a54226df299c","added_by":"auto","created_at":"2025-12-22 10:02:57","extension":"png","order_by":38,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2213551,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/9a9c69e2bea2ec7251a1820b.png"},{"id":98763126,"identity":"d495147a-b0b9-47a2-90f6-07432b5fa52d","added_by":"auto","created_at":"2025-12-22 10:02:57","extension":"xml","order_by":39,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":115421,"visible":true,"origin":"","legend":"","description":"","filename":"EJRED25005111structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/4e3adc84283718a89dac4e25.xml"},{"id":98763124,"identity":"8240f369-f1bc-4f5c-b3ff-5d625cbc2276","added_by":"auto","created_at":"2025-12-22 10:02:57","extension":"html","order_by":40,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":124650,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/51cd5c3e2abd4df115ec3c54.html"},{"id":98763080,"identity":"d43091d1-26cf-4b77-b1c2-ee66acecaa85","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":132905,"visible":true,"origin":"","legend":"\u003cp\u003eMembrane-bound and internalized [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 vs. [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 in (a) HT1080-huFAP (ns) cells and (b) PS-1 CAFs after 5-60 min of incubation (ns) (n=3). \u003cem\u003e%AA = percentage added activity, ns = non-significant\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/1d73f90ea088d750628394b5.png"},{"id":98763082,"identity":"777380a8-44cf-424d-a868-4c9faafabfb1","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":138233,"visible":true,"origin":"","legend":"\u003cp\u003eEx vivo biodistribution of 5 MBq/500 pmol (a) [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and (b) [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 at 1, 4, 24, and 48 h p.i. (n=4-5). Side-by-side comparison of organ uptake of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 at (c) 1 h and (D) 4 h p.i. \u003csup\u003e**** \u003c/sup\u003e\u003cem\u003ep\u003c/em\u003e \u0026lt;0.0001. \u003cem\u003e%ID/g = percentage injected dose per gram\u003c/em\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/f70c93f60e1d0c54f602646b.png"},{"id":98763074,"identity":"e91c02f4-76d2-4d1f-bf04-055c65fd75bd","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":169801,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Kaplan-Meijer curve of the survival (ns) and (b) tumor volume (ns) of mice treated with 4×40 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-/[\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 (n=10/group), or vehicle (n=8). (c-e) Tumor volumes of individual animals per treatment group. \u003cem\u003ens = non-significant\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/a4f6157c0664280f879f5b25.png"},{"id":98763085,"identity":"6ae8e754-7c1f-458e-a7d1-d064c3844808","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":80144,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Ex vivo tumor uptake of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 (ns) (n=3/time point) after each intravenous injection (IV) of the radiopharmaceuticals. (b) The relative stroma-to-cancer cell ratios after each IV injection (n=8/time point). \u003csup\u003e*\u003c/sup\u003e\u003cem\u003ep\u003c/em\u003e \u0026lt;0.05, \u003csup\u003e**\u003c/sup\u003e\u003cem\u003ep\u003c/em\u003e \u0026lt;0.01. \u003cem\u003e%ID/g = percentage injected dose per gram, ns = non-significant\u003c/em\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/43e5faef5786de8d3035d519.png"},{"id":98763107,"identity":"0325e7de-76ad-4412-8687-f0b44a2af7a5","added_by":"auto","created_at":"2025-12-22 10:02:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":172863,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Kaplan-Meijer curve of the survival and (b) tumor volume of mice treated with 2×40 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 (n=13) followed by 2×[\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 or vice versa (n=10/group), and vehicle treated mice (n=9). (c-e) Tumor volumes of individual animals per treatment group. \u003csup\u003e* \u003c/sup\u003e\u003cem\u003ep\u003c/em\u003e \u0026lt;0.05.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/35cec3da10cae6d7788bbede.png"},{"id":98763076,"identity":"eae02e61-e522-4300-af87-66eff3122599","added_by":"auto","created_at":"2025-12-22 10:02:55","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":361780,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Hematoxylin and eosin (H\u0026amp;E) staining of PDAC patient material (tumor area delineated in black) and in vitro autoradiography results after incubation with radiolabeled FAP-2286. (b) Quantification of the normalized radiopharmaceutical binding in three red encircled areas for both Lu-177 and Tb-161 (ns). (c) Calculated absorbed dose to the tumor area and to the total tissue slice in mGy/MBq.s. \u003cem\u003e, ns = non-significant\u003c/em\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/340266f08c321d8299cd85a9.png"},{"id":100070111,"identity":"7c2775d1-f81d-4c41-94c8-0e8c61962df6","added_by":"auto","created_at":"2026-01-12 16:16:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1750342,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/526b2d0d-5905-48cf-8de9-b82e30fc3859.pdf"},{"id":98779112,"identity":"28dc0c25-3177-4bd1-ac4b-16683ece3bda","added_by":"auto","created_at":"2025-12-22 12:29:58","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2509176,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementarymaterialsLuvsTbFINAL2EJNMMIRES.docx","url":"https://assets-eu.researchsquare.com/files/rs-8237978/v1/6c4123d1a8df010b48a66583.docx"}],"financialInterests":"","formattedTitle":"\u003cp\u003eHead-to-head comparison of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 efficacy in a PDAC mouse model: Is there an added benefit of internal conversion and Auger electrons for FAP-TRT?\u003c/p\u003e","fulltext":[{"header":"Background","content":"\u003cp\u003ePancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer type with a poor 5-year survival rate of less than 10% [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. PDAC is typically treatment resistant due to the presence of a dense tumor stroma. The most abundant cellular component of this tumor stroma is the cancer-associated fibroblast (CAF), which has been associated with therapy resistance and suppression of immune cells [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. CAFs are characterized by the expression of fibroblast activation protein (FAP), which is only scarcely expressed in healthy tissues [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. This cancer specific and abundant expression of stromal FAP in PDAC, and other solid cancers, makes it an attractive biomarker for anti-cancer interventions, including targeted radionuclide therapy (TRT) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Currently, FAP-2286 is one of the most promising compounds for FAP-TRT, due to its relatively high tumor retention [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Accordingly, a large prospective clinical Phase I/II trial with [\u003csup\u003e68\u003c/sup\u003eGa]Ga/[\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 to determine the potential of FAP-targeted theranostics in PDAC and other cancer types is ongoing (NCT04939610).\u003c/p\u003e \u003cp\u003eAt present, [\u003csup\u003e177\u003c/sup\u003eLu]Lu-DOTATATE and [\u003csup\u003e177\u003c/sup\u003eLu]Lu-PSMA-617 are clinically approved for TRT of somatostatin receptor 2 (SSTR2)-expressing neuroendocrine tumors (NETs) and prostate specific membrane antigen (PSMA)-overexpressing metastatic castration-resistant prostate cancer (mCRPC), respectively [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Unfortunately, [\u003csup\u003e177\u003c/sup\u003eLu]Lu-DOTATATE and [\u003csup\u003e177\u003c/sup\u003eLu]Lu-PSMA-617 treatment efficacy can be limited, and a subset of patients do not respond to treatment despite prominent tumor uptake observed on pre-treatment scans. To enhance therapeutic efficacy, cytotoxic radionuclides other than lutetium-177 (Lu-177) are being studied. Among them is terbium-161 (Tb-161), which has gained considerable interest recently [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The decay characteristics of Tb-161 are similar to that of Lu-177 (Lu-177: Eβ\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003csub\u003eav\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;133 keV, T\u003csub\u003e1/2\u003c/sub\u003e = 6.65 days vs. Tb-161: Eβ\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003csub\u003eav\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;154 keV, T\u003csub\u003e1/2\u003c/sub\u003e = 6.91), but besides a slightly higher β\u003csup\u003e\u0026minus;\u003c/sup\u003e-energy, Tb-161 emits a significant amount of internal conversion electrons (CEs) and Auger electrons (AEs) [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. These CEs and AEs have a higher linear energy transfer than the β\u003csup\u003e\u0026minus;\u003c/sup\u003e particles, and potentially have added therapeutic benefit, especially when in close proximity of the DNA [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Monte Carlo simulations have predicted an increased absorbed dose of Tb-161 over Lu-177 [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], and in line with this, preclinical studies demonstrated improved efficacy has been described for Tb-161 labeled PSMA- and SSTR2-targeting radiopharmaceuticals over the Lu-177 labeled counterparts. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Based on these promising results, clinical trials evaluating \u003csup\u003e161\u003c/sup\u003eTb-labeled radiopharmaceuticals for NET and mCRPC treatment are ongoing (e.g. NCT04833517, NCT05521412, NCT06343038, NCT05359146).\u003c/p\u003e \u003cp\u003eTb-161 can potentially also be more beneficial than Lu-177 for FAP-TRT. Presumably, FAP-TRT causes cancer cell death by irradiating the cancer cells indirectly via the FAP-expressing CAFs. However, especially in stroma-dense tumors such as PDAC, CAF elimination could also be a successful treatment strategy to reduce treatment resistance and potentially improving efficacy of other therapies. It remains to be determined whether the additional short-range CEs and AEs emitted by Tb-161 can induce greater CAF and/or cancer cell death compared to Lu-177. Accordingly, we performed in vitro and in vivo studies to compare [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286, aimed at evaluation of their efficacy and impact on tumor stroma density in a clinically representative PDAC mouse model after both mono- and tandem radionuclide therapy.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eRadiolabeling and stability\u003c/h2\u003e \u003cp\u003eFAP-2286 (MedChem Express, Monmouth Junction NJ, USA) was labeled with either non-carrier added [\u003csup\u003e177\u003c/sup\u003eLu]LuCl\u003csub\u003e3\u003c/sub\u003e (Lu-177, PI Medical, Raamsdonksveer, The Netherlands) or non-carrier added [\u003csup\u003e161\u003c/sup\u003eTb]TbCl\u003csub\u003e3\u003c/sub\u003e (Tb-161, TerThera BV, Breda, The Netherlands) in a total volume of 140 \u0026micro;L. The radiochemical purity (RCP) was determined by high-performance liquid chromatography (HPLC). Stability was determined by analyzing the RCP over time after 2 and 24 h, by incubating 2.5 MBq radiolabeled FAP-2286 in 200 \u0026micro;L phosphate buffered saline (PBS) (Gibco, Breda, The Netherlands) or 100 \u0026micro;L commercially available mouse serum (Invitrogen, Carlsbad CA, USA) at 37\u0026deg;C. Details on these labeling and stability studies are described in the Supplementary Information.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCell culture\u003c/h3\u003e\n\u003cp\u003eHuman fibrosarcoma cells transduced to express human FAP (HT1080-huFAP), kindly provided by Prof. Uwe Haberkorn (University of Heidelberg), and pancreatic stellate cells (PS-1), provided by Queen Mary University, were cultured as previously described [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The 19TT-F human breast CAFs were obtained from Prof. John Martens and were cultured in RPMI Glutamax\u0026copy; (Gibco). One day prior to in vitro assays, cells were seeded in 12-well plates (Sarstedt, N\u0026uuml;mbrecht, Germany) to reach 80% confluence. For in vivo studies a cell line-derived xenograft model (CDX) was established using the T110299 cell line, derived from a Ptf1a\u003csup\u003eWT/Cre\u003c/sup\u003e Kras\u003csup\u003eWT/LSL\u0026minus;G12D\u003c/sup\u003e P53\u003csup\u003eLSL\u0026thinsp;\u0026minus;\u0026thinsp;R172H/fl\u003c/sup\u003e mouse that spontaneously grows primary PDAC [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. T110299 cells were cultured in DMEM Glutamax\u0026copy; containing pyruvate (Gibco). All cell culture media were supplemented with 10% fetal bovine serum (Gibco) and 100 UI/mL penicillin\u0026thinsp;+\u0026thinsp;100 \u0026micro;g/mL streptomycin (Merck Life Science NV, Amsterdam, The Netherlands).\u003c/p\u003e\n\u003ch3\u003eIn vitro competition binding and uptake\u003c/h3\u003e\n\u003cp\u003eThe half-maximal inhibitory concentration (IC50) of the radiopharmaceuticals was determined on HT1080-huFAP cells using 1 nM [\u003csup\u003e177\u003c/sup\u003eLu]Lu-/[\u003csup\u003e161\u003c/sup\u003eTb]Tb -FAP-2286, incubated with increasing concentrations (10\u003csup\u003e\u0026minus;\u0026thinsp;12\u003c/sup\u003e to 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e M) of the FAP-inhibitor UAMC-1110 (Biosynth Ltd, Berkshire, United Kingdom) for 45 min. To determine radiopharmaceutical uptake, HT1080-huFAP, PS-1, and 19TT-F cells were incubated with 1 nM [\u003csup\u003e177\u003c/sup\u003eLu]Lu-/[\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 for 5-180 min, +/- 1 mM UAMC-1110 to determine specificity of uptake, as previously described [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The uptake is expressed as percentage added activity (%AA) per 200,000 cells.\u003c/p\u003e\n\u003ch3\u003eIn vitro autoradiography\u003c/h3\u003e\n\u003cp\u003eRadiopharmaceutical binding to fresh frozen 10 \u0026micro;m slices of T110299 CDXs, human PDAC, and breast cancer (BC) tissue (n\u0026thinsp;=\u0026thinsp;3/group), mounted onto Superfrost Plus\u0026trade; microscope slides (VWR, Leuven, Belgium), was assessed as previously described [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. In short, tissue slices were incubated with incubation buffer (167 mM Tris-HCL, 5 mM MgCl\u003csub\u003e2,\u003c/sub\u003e 1% BSA) containing 1 nM [\u003csup\u003e177\u003c/sup\u003eLu]Lu-/[\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286, +/- 1 mM UAMC-1110 to determine specificity of binding. Tissues were exposed to super-resolution phosphor screens (Perkin Elmer) for at least 24 h. Uptake was quantified as digital light units (DLU)/mm\u003csup\u003e2\u003c/sup\u003e and normalized to 1 \u0026micro;L standards. The data were expressed as % AA.\u003c/p\u003e\n\u003ch3\u003eIn vivo studies\u003c/h3\u003e\n\u003cp\u003eFor all in vivo studies, C57BL/6 male mice (Janvier, Le Genest-Saint-Isle, France) were subcutaneously inoculated with 50 \u0026micro;L HBSS (Gibco) containing 0.5\u0026times;10\u003csup\u003e6\u003c/sup\u003e T110299 cells, with a tumor take of 100%. Details on inoculation and tumor growth, animal welfare, and animal numbers are provided in the Supplementary Information (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e-2). In all studies, animals were monitored daily, and animal weight and tumor size was measured twice each week.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eEx vivo biodistribution and ex vivo autoradiography\u003c/h2\u003e \u003cp\u003eOn day 11 after inoculation of T110299 cells (tumor sizes; Lu-177: 431\u0026thinsp;\u0026plusmn;\u0026thinsp;173 mm\u003csup\u003e3\u003c/sup\u003e and Tb-161: 315\u0026thinsp;\u0026plusmn;\u0026thinsp;159 mm\u003csup\u003e3\u003c/sup\u003e (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e)), animals received an intravenous injection (IV) in the tail vain of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-/[\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 (5 MBq/500 pmol/200 \u0026micro;L), diluted in PBS containing 0.06 mg/mL Kolliphor\u0026reg; HS 15. At 1, 4, 24, and 48 h post injection (p.i.) (n\u0026thinsp;=\u0026thinsp;5/group), blood was collected via orbital puncture under isoflurane/O\u003csub\u003e2\u003c/sub\u003e anesthesia, followed by cervical dislocation and collection of tumor and organs of interest (i.e., pancreas, liver, GI-tract (stomach, small intestine, cecum, colon), kidneys, lungs, heart, salivary glands, muscle, femur bone). A subset of animals (n\u0026thinsp;=\u0026thinsp;4/group) received an injection of radiolabeled FAP-2286\u0026thinsp;+\u0026thinsp;17.5 nM UAMC-1110 following biodistribution studies 4 h p.i. to determine specificity of uptake. The collected materials were weighed and measured in a γ-counter. Data were expressed as percentage injected dose per gram of tissue (% ID/g). Half of the tumor was snap frozen in liquid nitrogen, sectioned (10 \u0026micro;m), mounted on Starforst glass slides, and exposed to a super-resolution phosphor screen to determine radiopharmaceutical binding.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eIn vivo efficacy of mono and tandem treatment\u003c/h3\u003e\n\u003cp\u003eT110299 tumors were allowed to grow for seven days reaching a size of 89\u0026thinsp;\u0026plusmn;\u0026thinsp;38 mm\u003csup\u003e3\u003c/sup\u003e. On day 7, 10, 14, and 17 after T110299 cell inoculation, mice received IV injections of 200 \u0026micro;L 40 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-/[\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 (n\u0026thinsp;=\u0026thinsp;10/group), or vehicle solution (PBS\u0026thinsp;+\u0026thinsp;0.06 mg/mL Kolliphor\u0026reg; HS 15) (n\u0026thinsp;=\u0026thinsp;8). An additional group of animals (n\u0026thinsp;=\u0026thinsp;2\u0026ndash;3/group) were sacrificed 1 h after each IV injection, and tumors were harvested to measure radiopharmaceutical uptake and to assess potential changes in FAP-expression or stroma-to-cancer cell ratios over time. Immunohistochemistry (IHC) was performed on the excised tumors and a pathologist (MCD) scored the tumor tissues blindly to determine the percentage of tumor cells and stroma/CAFs of the total tumor area. Details on the IHC are described in the Supplementary Information.\u003c/p\u003e \u003cp\u003eNext, tandem efficacy was determined in T110299 xenografted mice with an average tumor size of 219\u0026thinsp;\u0026plusmn;\u0026thinsp;83 mm\u003csup\u003e3\u003c/sup\u003e on day seven post inoculation. Subsequently, mice received 40 MBq/500 pmol on day 8, 11, 15, and 18 post T110299 inoculation. Mice received 2\u0026times;[\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 followed by 2\u0026times;[\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286, or vice versa (n\u0026thinsp;=\u0026thinsp;13/group), or vehicle injections (n\u0026thinsp;=\u0026thinsp;9) (Fig. \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003eDosimetry\u003c/h3\u003e\n\u003cp\u003eThe biodistribution data of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 were used to characterize the organ and tumor time-activity curves. These curves were fitted with mono-exponential curves to enable integration over time, leading to Time Integrated Activity (concentration) Coefficients ([TIAC]). Absorbed doses to the mouse organs and tumor xenograft per injected activity (IA) were calculated according to the modified MIRD equation:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"395\" height=\"55\"\u003e\u003c/p\u003e\u003cp\u003ewith \u003cem\u003er\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e the target organ or tumor, \u003cem\u003er\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e the source organ, \u003cem\u003em(r\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e)\u003c/em\u003e the mass of the source organ and \u003cem\u003eS(r\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u0026larr;r\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e)\u003c/em\u003e the S-value. Both the organ masses and the S-values were taken from the RADAR 25 g mouse phantom [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The tumor absorbed doses were calculated by using the sphere S-values from the Olinda dosimetry software [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The dosimetry was used to estimate the treatment response, taking the radiosensitivity of the tumor cells [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], their doubling time, and the kinetics of FAP-2286 and its dosimetry when labeled to Lu-177 or Tb-161, into account.\u003c/p\u003e \u003cp\u003eDosimetry in the PDAC and BC tissues was performed by inserting the autoradiography data as source distributions in a cylindrical tissue model set up in the Monte Carlo code MCNP (version 6.2). Autoradiography images were resampled to a pixel size of 85 \u0026micro;m and all data above 1800 DLU/mm\u003csup\u003e2\u003c/sup\u003e were used as source in the middle of a cylindrical tissue model of 3 mm thickness. The energy absorption inside an 85 x 85 x 3000 \u0026micro;m\u003csup\u003e3\u003c/sup\u003e voxel grid was tallied and transformed into S-values [in mGy/MBq.s]. The \u003csup\u003e161\u003c/sup\u003eTb and \u003csup\u003e177\u003c/sup\u003eLu beta-particle and low energy electron spectra of ICRP-107 were used. The number of histories used in the source distribution was set at 10\u0026nbsp;million to obtain statistical uncertainties below 5% in each voxel. Absorbed dose was subsequently calculated using ImageJ software (version 1.54g) by drawing and measuring the whole tissue slice or the tumor area in accordance with the pathologists findings in the corresponding H\u0026amp;E slice.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eStatistics\u003c/h2\u003e \u003cp\u003eStatistical analyses were performed using GraphPad Prism 9.0 (San Diego, California USA), and results were regarded statistically significant if p\u0026thinsp;\u0026lt;\u0026thinsp;0.05. All experiments were performed at least in triplicate, unless stated otherwise. More detailed information is provided in the Supplementary Information.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eStability in vitro\u003c/h2\u003e \u003cp\u003eThe stability of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 in labeling solution was high, and there was no significant difference between the radiopharmaceuticals, i.e. 97.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6% vs 95.2\u0026thinsp;\u0026plusmn;\u0026thinsp;2.8% (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05), respectively, after 2 h incubation, and 93.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1% and 93.4\u0026thinsp;\u0026plusmn;\u0026thinsp;2.6%, (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05), respectively, after 24 h incubation. In contrast, in mouse serum a decrease in stability was observed for both [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286. A significant difference in stability between the radiopharmaceuticals was observed after 2 h, but not after 24 h of incubation (i.e. 70.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5% vs 75.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0% p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; and 13.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5% vs 12.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5% p\u0026thinsp;\u0026gt;\u0026thinsp;0.05, respectively) (Table S3).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eIn vitro competition binding and uptake\u003c/h2\u003e \u003cp\u003eThere were no significant differences in either the IC50 (i.e. Lu-177: 2.9\u0026thinsp;\u0026plusmn;\u0026thinsp;7.2 nM vs. Tb-161:1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2 nM, p\u0026thinsp;\u0026gt;\u0026thinsp;0.05, Fig. S3) or in the uptake of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 evaluated in HT1080-huFAP and on PS-1 cells (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Radiopharmaceutical uptake occurred rapidly in both cell lines, but was 5\u0026ndash;10 fold higher in HT1080-huFAP cells than in PS-1 cells. Moreover, radiopharmaceutical uptake was largely internalized by HT1080-huFAP cells, whereas in PS-1 cells it remained mainly membrane-bound. No differences in the internalization ratio was found between [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 in either cell line. Matching uptake of the two radiopharmaceuticals was further confirmed in 19TT-F cells (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05, Fig. S4).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eEx vivo biodistribution\u003c/h2\u003e \u003cp\u003eBiodistribution of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 in T110299-xenografted mice demonstrated tumor uptake of 1.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.39% ID/g for [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and 1.39\u0026thinsp;\u0026plusmn;\u0026thinsp;0.45% ID/g for [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 at 1 h p.i. Radiopharmaceutical washout was already observed at 4 h p.i., with only 0.32\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10% ID/g [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and 0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05% ID/g [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 remaining at this time point, and further decreasing at 24 h and 48 h p.i. (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-b, Table S4-5). Besides the tumor, the kidneys were the only organ with noticeable radiopharmaceutical uptake, i.e. 7.85\u0026thinsp;\u0026plusmn;\u0026thinsp;1.96% ID/g at 1 h p.i. and 3.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.68% ID/g at 4 h p.i. for [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286, and 6.09\u0026thinsp;\u0026plusmn;\u0026thinsp;1.41% ID/g at 1 h p.i. and 3.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.59% ID/g at 4 h p.i. for [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec-d, Table S4-5). The kidney uptake for [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 was significantly lower at 1 h p.i. (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001), but significantly higher at 4 h p.i. (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001).\u003c/p\u003e \u003cp\u003eCo-injection with UAMC-1110 did not demonstrate a significant decrease in [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 or [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 tumor uptake. Only kidney uptake was found to be significantly decreased compared to that of animals injected with only the radiopharmaceutical (Lu-177: 7.85\u0026thinsp;\u0026plusmn;\u0026thinsp;1.96% vs. 3.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.68% ID/g, and Tb-161: 3.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.59% vs. 1.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.18% ID/g, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.0001) (Fig. S5).\u003c/p\u003e \u003cp\u003eEx vivo autoradiography on the excised tumors showed results in accordance with the ex vivo biodistribution. However, the ex vivo autoradiography did indicate a significantly lower signal in tumors from the animals co-injected with UAMC-1110 compared to the tumors from mice that only received radiolabeled FAP-2286 (i.e. Lu-177: 9206\u0026thinsp;\u0026plusmn;\u0026thinsp;2507 vs. 23057\u0026thinsp;\u0026plusmn;\u0026thinsp;3786 DLU/mm\u003csup\u003e2\u003c/sup\u003e, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01, and Tb-161: 5551\u0026thinsp;\u0026plusmn;\u0026thinsp;418 vs. 15911\u0026thinsp;\u0026plusmn;\u0026thinsp;3106 DLU/mm\u003csup\u003e2\u003c/sup\u003e, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01, respectively) (Fig. S6). Additionally, an in vitro autoradiography on treatment na\u0026iuml;ve T110299 tumors, demonstrated that co-incubation with 1000x excess of UAMC-1110 significantly blocked binding of [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, Fig. S7a,c) and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 (p\u0026thinsp;\u0026lt;\u0026thinsp;0.01, Fig. S7b,c).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eDosimetry\u003c/h2\u003e \u003cp\u003eThe organ and tumor dosimetry are presented in Table S6. Based on the dosimetry calculations 4\u0026times;40 MBq/500 pmol IV injections were selected, corresponding to 4\u0026times;1.5 Gy Lu-177 and 4\u0026times;1.6 Gy Tb-161 (Fig. S8).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e\u003cem\u003eIn vivo efficacy of mono treatment\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eTreatment with [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 in the T110299-xenografted mice showed no effect on tumor growth (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Radiopharmaceutical uptake in the tumors 1 h p.i. showed an uptake of 1-1.5% ID/g for both radiopharmaceuticals, which was in line with the biodistribution studies (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). No significant changes in body weight were observed(Fig. S9a). IHC on tumors harvested 1 h p.i., demonstrated heterogeneity in stroma-to-cancer cell ratios and level of FAP-staining. Accordingly, no differential effect on stroma composition could be distinguished between vehicle, [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286, and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 treated mice. Stroma density pooled for all three groups indicated a significant increase in stroma density over time (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, IV to IV2 p\u0026thinsp;\u0026lt;\u0026thinsp;0.01; IV1 to IV3 p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). However, stroma density did not correlate with FAP-expression level, radiopharmaceutical uptake, tumor size, or tumor weight (Fig. S10).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003eIn vivo efficacy of tandem treatment\u003c/h2\u003e \u003cp\u003eTo determine whether tandem therapy has additional value, T110299-xenografted mice were treated with 2\u0026times;40 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 followed by 2\u0026times;40 MBq/500 pmol [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286, or vice versa. The Kaplan-Meier survival curve demonstrated a significant longer survival for animals that received 2\u0026times;40 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 followed by 2\u0026times;40 MBq/500 pmol [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 over those that received vehicle (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Tumor shrinkage was not observed in any of the groups (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb-e). In line with the previous treatment study, no significant changes in body weight were observed between the treatment arms (Fig. S9b).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003eIn vitro autoradiography and accompanying dosimetry\u003c/h2\u003e \u003cp\u003eAutoradiography on patient PDAC material demonstrated potent but heterogeneous binding of both [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). Although lower binding was observed for [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 compared to [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 the difference was not significant (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb, p\u0026thinsp;\u0026gt;\u0026thinsp;0.05). Dosimetry calculations performed on the autoradiography data resulted in 1.35-fold higher absorbed dose in both the tumor area and to the whole tissue slice with [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 compared to [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec). The same trends were observed for radiopharmaceutical binding to BC tissue samples (Fig. S11).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eImproved efficacy of Tb-161 over Lu-177 for SSTR2- and PSMA-TRT has been reported for the treatment of mCRPC [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] and NETs [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], respectively. Next to enhanced therapeutic efficacy, introducing Tb-161 as substitute or alongside Lu-177 can alleviate the growing supply pressure on Lu-177 [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Therefore, we aimed to compare Lu-177 and Tb-161 for FAP-TRT. To our knowledge, this is the first study characterizing and comparing [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286.\u003c/p\u003e \u003cp\u003eThe radiolabeling of FAP-2286 with both Lu-177 and Tb-161 was successful, and the radiopharmaceuticals remained equally stable in labeling solution and PBS. In contrast, their stability in mouse serum decreased to 70% and 75% after 2 h for Lu-177 and Tb-161, respectively, and further decreased drastically below 15% after 24 h. Nevertheless, due to rapid blood circulation in mice [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], the fast tumor uptake observed, and since \u0026gt;\u0026thinsp;70% of the radiopharmaceutical remained intact during the first 2 hours, the decrease in stable compound is not expected to have had a major effect on radiopharmaceutical uptake in our study.\u003c/p\u003e \u003cp\u003eIn vitro cell uptake studies demonstrated that the behavior of [\u0026sup1;⁷⁷Lu]Lu-FAP-2286 and [\u0026sup1;⁶\u0026sup1;Tb]Tb-FAP-2286 was similar regardless of the applied cell model. In line with this, in vitro autoradiography studies showed similar binding of the radiopharmaceuticals to cancer tissues. Thus, the behavior of FAP-2286 does not appear to be altered when radiolabeled with Tb-161 compared to radiolabeling with Lu-177.\u003c/p\u003e \u003cp\u003eIn contrast to other studies that used FAP-2286 in vivo in different models, we observed relatively low uptake of the radiopharmaceutical in the T110299 CDX [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. This is most likely because, unlike models established with cancer cells (over)expressing FAP, the T110299 CDX depends on infiltration of murine FAP (muFAP)-expressing CAFs for FAP-2286 uptake [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The T110299 xenografted mouse model has previously been used successfully for evaluation of FAP-targeting interventions [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]; however, model-dependent uptake of FAP-2286 has been reported [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Additionally, the injected peptide mass might affect radiopharmaceutical uptake, as illustrated by various recent studies [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], yet there is no consensus on the optimal mass for FAP-2286 or other peptide-based FAP-targeting radiopharmaceuticals. Another explanation for the observed low tumor uptake might be the slightly lower affinity of the peptide for huFAP compared to muFAP (1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 nM vs. 4.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5 nM, respectively) [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. FAP-2287, which is similar to FAP-2286 but has higher muFAP affinity [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], was recently developed and should be considered in future studies using a models with muFAP-expressing CAFs.\u003c/p\u003e \u003cp\u003eEx vivo biodistribution studies did not show significantly lower tumor uptake in mice co-injected with UAMC-1110 compared to those injected with [\u0026sup1;⁷⁷Lu]Lu-FAP-2286 or [\u0026sup1;⁶\u0026sup1;Tb]Tb-FAP-2286 alone. Although the injected 35-time excess of UAMC-1110 was insufficient to saturate muFAP and prevent radiolabeled FAP-2286 binding, we believe that the majority of radiolabeled FAP-2286 tumor uptake was still FAP-specific. First, using more sensitive ex vivo autoradiography on these same tumor, we observed significant effective blocking of FAP-2286. Second, in additional in vitro autoradiography studies with untreated PDAC299 tumors, a 1000\u0026times; excess of UAMC-1110 did block binding of radiolabeled FAP-2286. Lastly, it has previously been demonstrated in vivo that FAP-2286 binding is FAP-specific, by using a higher excess (e.g. 60-fold) of an unlabeled FAP-targeting compound for blocking [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Thus, the UAMC-1110 concentration was likely too low to achieve complete blocking in our in vivo studies, yet evidence suggests that radiolabeled FAP-2286 binding is FAP-specific, which is further supported by its current evaluation in a large prospective clinical trial (NCT 04939610). Besides, mice receiving radiolabeled FAP-2286 co-injected with UAMC-1110 showed an unexpected significant decrease in kidney uptake compared to those injected with the radiopharmaceutical alone, even though kidneys do not express FAP. This may be due to altered clearance and reabsorption rates caused by the high concentration of UAMC-1110.\u003c/p\u003e \u003cp\u003eDue to the low tumor uptake, dosimetry calculations predicted that a relatively high injected dose of 4\u0026times;40 MBq/500 pmol would be needed to achieve up to 50% tumor shrinkage. Despite this high dose, no survival benefit was observed, apart from a minor growth delay in the tandem therapy arm that first received 2\u0026times;[\u0026sup1;⁷⁷Lu]Lu-FAP-2286 followed by 2\u0026times;[\u0026sup1;⁶\u0026sup1;Tb]Tb-FAP-2286. The dosimetry model predicted a marginal advantage for administering Lu-177 followed by Tb-161 compared to the reverse sequence; however, tumor control efficacy remained lower than predicted. This discrepancy is likely due to the model not accounting for all biological factors, including heterogeneity of FAP expression, indirect irradiation from CAFs to cancer cells, and differences in radiosensitivity between CAFs and cancer cells. Treated tumors were subjected to IHC evaluation to determine whether Lu-177 or Tb-161 had differential effects on stroma-to-cancer cell ratios or FAP expression levels. Unfortunately, due to high heterogeneity in stroma density, it was not possible to distinguish the effects of either radiopharmaceutical. Notably, tumor growth rates differed between the mono and tandem therapy studies, with vehicle-treated mice in the first treatment study reaching the humane endpoint later than those in the later tandem therapy study. This may be due to mice in the tandem therapy group being inoculated at 9 weeks old, compared to 6 weeks old in the mono therapy group. Other studies have shown that the age of CJ54BL/6 mice can impact the immune system development, which in turn affects tumor development and treatment response [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Additionally, since cells with a higher division rate are in general more radiosensitive [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], this difference in growth rate may partly explain the differential therapeutic response between the two studies.\u003c/p\u003e \u003cp\u003eSo far mixed responses to [\u0026sup1;⁷⁷Lu]Lu-FAP-2286 therapy have been reported in clinical studies [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e] and, unfortunately, there is insufficient data on PDAC patients receiving FAP-TRT to directly relate our findings to that of the clinic. The negative outcome of FAP-TRT observed in our study, which used a histopathological representative model, is in contrast with previous promising results in another model [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], which suggests that FAP-TRT efficacy might be model dependent. This finding raises questions about the value of preclinical in vivo studies that use less clinically representative models, as well as about the overall potential of FAP-TRT in PDAC. It should be noted that the heterogeneity in FAP-expression and tumor growth, together with the low FAP-2286 uptake, were limitations specific for our study. To further confirm the value of FAP-TRT in stroma dense PDAC, and elucidate differences between Lu-177 and Tb-161, a model with a higher uptake and corresponding higher response rate is necessary in future studies. CDX models established using a cancer cell line with high FAP expression could be applied, although this would be less representative of patient histopathology and its translational value with regard to FAP-2286 uptake remains unclear. Moreover, a model without CAF infiltration would not allow for discerning effects on CAFs and cancer cells.\u003c/p\u003e \u003cp\u003eIn an attempt to gain insights in the clinical uptake of FAP-2286, [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 binding to patient PDAC (and BC) tissue was compared. No significant difference in overall tissue binding was observed between the two radiopharmaceuticals, but high intra- and inter-tumor heterogeneity was noted. Dosimetry calculations showed that the absorbed dose was markedly higher in tumor areas compared to the overall tissue slide, confirming that radionuclide treatment directed at CAFs could effectively irradiate malignant areas. Additionally, dosimetry on cancer tissues predicted that the absorbed dose of [\u0026sup1;⁶\u0026sup1;Tb]Tb-FAP-2286 would consistently be higher, even in areas where its binding was lower than that of [\u0026sup1;⁷⁷Lu]Lu-FAP-2286. However, further studies are needed to determine whether this higher dose, predominantly caused by CEs and AEs, would result in increased efficacy. Future studies using a larger sample set and potentially incorporating more complex 3D dosimetry in subsequent tumor slices will be valuable to better understand the extent to which tumor stroma density and FAP heterogeneity could serve as critical determinants of FAP-TRT efficacy.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eOur in vitro data suggests that [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 can be used interchangeably for FAP-TRT. In vivo studies demonstrated that a modest tumor growth delay could be achieved using a tandem therapy strategy, yet the efficacy of FAP-TRT therapy was too limited to determine true differential effects between the two radiopharmaceuticals. A model with similar histopathological clinical resemblance, yet a higher response to FAP-TRT, will be necessary in future studies to discern differential effects of Lu-177 and Tb-161 on CAFs and cancer cells when applying FAP-TRT.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003ePDAC pancreatic ductal adenocarcinoma\u003c/p\u003e \u003cp\u003eTME tumor microenvironment\u003c/p\u003e \u003cp\u003eFAP fibroblast activation protein\u003c/p\u003e \u003cp\u003eCAF cancer-associated fibroblast\u003c/p\u003e \u003cp\u003eTRT targeted radionuclide therapy\u003c/p\u003e \u003cp\u003eLu-177 Lutetium-177\u003c/p\u003e \u003cp\u003eTb-161 Terbium-161\u003c/p\u003e \u003cp\u003emCRPC metastatic\u003c/p\u003e \u003cp\u003eSSTR2 somatostatin receptor 2\u003c/p\u003e \u003cp\u003ePSMA prostate specific membrane antigen\u003c/p\u003e \u003cp\u003eCE conversion electron\u003c/p\u003e \u003cp\u003eAE Auger electrons\u003c/p\u003e \u003cp\u003eLET linear energy transfer\u003c/p\u003e \u003cp\u003eDTPA diethylenetriaminepentaacetic acid\u003c/p\u003e \u003cp\u003eHPLC high-performance liquid chromatography\u003c/p\u003e \u003cp\u003eRCY radiochemical yield\u003c/p\u003e \u003cp\u003eRCP radiochemical purity\u003c/p\u003e \u003cp\u003ePBS phosphate buffered saline\u003c/p\u003e \u003cp\u003emuFAP murine FAP\u003c/p\u003e \u003cp\u003ehuFAP human FAP\u003c/p\u003e \u003cp\u003ep.i. post injection\u003c/p\u003e \u003cp\u003eBC breast cancer\u003c/p\u003e \u003cp\u003eDLU digital light units\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll in vivo studies were conducted according to the guidelines of the Declaration of Helsinki. Approval was granted by the Animal Welfare Committee of the Erasmus MC (CCD#: 2216075, SP21000146/31-12-2024 and SP2300186/31-12-2024), and was in accordance with European law. Clinical Trial number: not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll available data are described in the manuscript or are available in the Supplementary Information. Additional information can be obtained from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was not supported by external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSUD and CDH came up with study concept and design. Radiochemistry analyses and radiolabeling were performed by CN, HM, YS, and EB. Material preparation, in vivo studies, data collection and analysis were performed by CDH, CR, DS, MCD. Dosimetry calculations and data analyses were performed by MK and CDH. The first draft of the manuscript was written by CDH and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank TherThera BV for providing the Terbium-161 for the study. We are grateful to Prof. Jens Siveke and Dr. Marija Trajkovic-Arsic for providing us with the T110299 cells, and to Dr. Sanne van Lith for her support with model development.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNone\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKleeff J, Korc M, Apte M, La Vecchia C, Johnson CD, Biankin AV, et al. Pancreatic cancer. Nat Reviews Disease Primers. 2016;2:16022. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/nrdp.2016.22\u003c/span\u003e\u003cspan address=\"10.1038/nrdp.2016.22\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang T, Ren Y, Yang P, Wang J, Zhou H. Cancer-associated fibroblasts in pancreatic ductal adenocarcinoma. Cell Death Dis. 2022;13:897. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41419-022-05351-1\u003c/span\u003e\u003cspan address=\"10.1038/s41419-022-05351-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXin L, Gao J, Zheng Z, Chen Y, Lv S, Zhao Z, et al. Fibroblast Activation Protein-α as a Target in the Bench-to-Bedside Diagnosis and Treatment of Tumors: A Narrative Review. Front Oncol. 2021;11:648187. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3389/fonc.2021.648187\u003c/span\u003e\u003cspan address=\"10.3389/fonc.2021.648187\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePriv\u0026eacute; BM, Boussihmad MA, Timmermans B, van Gemert WA, Peters SMB, Derks YHW, et al. Fibroblast activation protein-targeted radionuclide therapy: background, opportunities, and challenges of first (pre)clinical studies. Eur J Nucl Med Mol Imaging. 2023;50:1906\u0026ndash;18. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-023-06144-0\u003c/span\u003e\u003cspan address=\"10.1007/s00259-023-06144-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKratochwil C, Flechsig P, Lindner T, Abderrahim L, Altmann A, Mier W, et al. \u003csup\u003e68\u003c/sup\u003eGa-FAPI PET/CT: Tracer Uptake in 28 Different Kinds of Cancer. J Nucl Med. 2019;60:801\u0026ndash;5. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2967/jnumed.119.227967\u003c/span\u003e\u003cspan address=\"10.2967/jnumed.119.227967\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZboralski D, Hoehne A, Bredenbeck A, Schumann A, Nguyen M, Schneider E, et al. Preclinical evaluation of FAP-2286 for fibroblast activation protein targeted radionuclide imaging and therapy. Eur J Nucl Med Mol Imaging. 2022;49:3651\u0026ndash;67. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-022-05842-5\u003c/span\u003e\u003cspan address=\"10.1007/s00259-022-05842-5\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHennrich U, Eder M. [\u003csup\u003e177\u003c/sup\u003eLu]Lu-PSMA-617 (Pluvicto\u003csup\u003e\u0026trade;\u003c/sup\u003e): The First FDA-Approved Radiotherapeutical for Treatment of Prostate Cancer. Pharmaceuticals (Basel). 2022;15. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/ph15101292\u003c/span\u003e\u003cspan address=\"10.3390/ph15101292\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHennrich U, Kopka K. Lutathera(\u0026reg;): The First FDA- and EMA-Approved Radiopharmaceutical for Peptide Receptor Radionuclide Therapy. Pharmaceuticals (Basel). 2019;12. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/ph12030114\u003c/span\u003e\u003cspan address=\"10.3390/ph12030114\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRuigrok EAM, van Weerden WM, Nonnekens J, de Jong M. The Future of PSMA-Targeted Radionuclide Therapy: An Overview of Recent Preclinical Research. Pharmaceutics. 2019;11. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/pharmaceutics11110560\u003c/span\u003e\u003cspan address=\"10.3390/pharmaceutics11110560\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlcocer-\u0026Aacute;vila ME, Ferreira A, Quinto MA, Morgat C, Hindi\u0026eacute; E, Champion C. Radiation doses from 161Tb and 177Lu in single tumour cells and micrometastases. EJNMMI Phys. 2020;7:33. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1186/s40658-020-00301-2\u003c/span\u003e\u003cspan address=\"10.1186/s40658-020-00301-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKu A, Facca VJ, Cai Z, Reilly RM. Auger electrons for cancer therapy \u0026ndash; a review. EJNMMI Radiopharmacy Chem. 2019;4:27. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1186/s41181-019-0075-2\u003c/span\u003e\u003cspan address=\"10.1186/s41181-019-0075-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHindi\u0026eacute; E, Zanotti-Fregonara P, Quinto MA, Morgat C, Champion C. Dose Deposits from \u003csup\u003e90\u003c/sup\u003eY, \u003csup\u003e177\u003c/sup\u003eLu, \u003csup\u003e111\u003c/sup\u003eIn, and \u0026amp; \u003csup\u003e161\u003c/sup\u003eTb in Micrometastases of Various Sizes: Implications for Radiopharmaceutical Therapy. J Nucl Med. 2016;57:759. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2967/jnumed.115.170423\u003c/span\u003e\u003cspan address=\"10.2967/jnumed.115.170423\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eM\u0026uuml;ller C, Umbricht CA, Gracheva N, Tschan VJ, Pellegrini G, Bernhardt P, et al. Terbium-161 for PSMA-targeted radionuclide therapy of prostate cancer. Eur J Nucl Med Mol Imaging. 2019;46:1919\u0026ndash;30. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-019-04345-0\u003c/span\u003e\u003cspan address=\"10.1007/s00259-019-04345-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBorgna F, Haller S, Rodriguez JMM, Ginj M, Grundler PV, Zeevaart JR, et al. Combination of terbium-161 with somatostatin receptor antagonists\u0026mdash;a potential paradigm shift for the treatment of neuroendocrine neoplasms. Eur J Nucl Med Mol Imaging. 2022;49:1113\u0026ndash;26. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-021-05564-0\u003c/span\u003e\u003cspan address=\"10.1007/s00259-021-05564-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003evan der Heide CD, Campeiro JD, Ruigrok EAM, van den Brink L, Ponnala S, Hillier SM, Dalm SU. In vitro and ex vivo evaluation of preclinical models for FAP-targeted theranostics: differences and relevance for radiotracer evaluation. EJNMMI Res. 2024;14:125. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1186/s13550-024-01191-6\u003c/span\u003e\u003cspan address=\"10.1186/s13550-024-01191-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHingorani SR, Wang L, Multani AS, Combs C, Deramaudt TB, Hruban RH, et al. Trp53R172H and KrasG12D cooperate to promote chromosomal instability and widely metastatic pancreatic ductal adenocarcinoma in mice. Cancer Cell. 2005;7:469\u0026ndash;83. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ccr.2005.04.023\u003c/span\u003e\u003cspan address=\"10.1016/j.ccr.2005.04.023\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVerhoeven M, Ruigrok EAM, van Leenders G, van den Brink L, Balcioglu HE, van Weerden WM, Dalm SU. GRPR versus PSMA: expression profiles during prostate cancer progression demonstrate the added value of GRPR-targeting theranostic approaches. Front Oncol. 2023;13:1199432. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3389/fonc.2023.1199432\u003c/span\u003e\u003cspan address=\"10.3389/fonc.2023.1199432\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKeenan MA, Stabin MG, Segars WP, Fernald MJ. RADAR Realistic Animal Model Series for Dose Assessment. J Nucl Med. 2010;51:471. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2967/jnumed.109.070532\u003c/span\u003e\u003cspan address=\"10.2967/jnumed.109.070532\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStabin MG, Konijnenberg MW. Re-evaluation of absorbed fractions for photons and electrons in spheres of various sizes. J Nucl Med. 2000;41:149\u0026ndash;60.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCarlson DJ, Stewart RD, Li XA, Jennings K, Wang JZ, Guerrero M. Comparison of in vitro and in vivo alpha/beta ratios for prostate cancer. Phys Med Biol. 2004;49:4477\u0026ndash;91. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1088/0031-9155/49/19/003\u003c/span\u003e\u003cspan address=\"10.1088/0031-9155/49/19/003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSchaefer-Schuler A, Burgard C, Blickle A, Maus S, Petrescu C, Petto S, et al. [\u003csup\u003e161\u003c/sup\u003eTb]Tb-PSMA-617 radioligand therapy in patients with mCRPC: preliminary dosimetry results and intra-individual head-to-head comparison to [\u003csup\u003e177\u003c/sup\u003eLu]Lu-PSMA-617. Theranostics. 2024;14:1829\u0026ndash;40. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.7150/thno.92273\u003c/span\u003e\u003cspan address=\"10.7150/thno.92273\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBernhardt P, Svensson J, Hemmingsson J, van der Meulen NP, Zeevaart JR, Konijnenberg MW, et al. Dosimetric Analysis of the Short-Ranged Particle Emitter (161)Tb for Radionuclide Therapy of Metastatic Prostate Cancer. Cancers (Basel). 2021;13. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/cancers13092011\u003c/span\u003e\u003cspan address=\"10.3390/cancers13092011\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBaum RP, Singh A, Kulkarni HR, Bernhardt P, Ryd\u0026eacute;n T, Schuchardt C, et al. First-in-Humans Application of (161)Tb: A Feasibility Study Using (161)Tb-DOTATOC. J Nucl Med. 2021;62:1391\u0026ndash;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2967/jnumed.120.258376\u003c/span\u003e\u003cspan address=\"10.2967/jnumed.120.258376\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDash A, Pillai MRA, Knapp FF. Production of 177Lu for Targeted Radionuclide Therapy: Available Options. Nuclear Med Mol Imaging. 2015;49:85\u0026ndash;107. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s13139-014-0315-z\u003c/span\u003e\u003cspan address=\"10.1007/s13139-014-0315-z\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDebbage PL, Griebel J, Ried M, Gneiting T, DeVries A, Hutzler P. Lectin Intravital Perfusion Studies in Tumor-bearing Mice: Micrometer-resolution, Wide-area Mapping of Microvascular Labeling, Distinguishing Efficiently and Inefficiently Perfused Microregions in the Tumor. J Histochem Cytochemistry. 1998;46:627\u0026ndash;39. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1177/002215549804600508\u003c/span\u003e\u003cspan address=\"10.1177/002215549804600508\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMillul J, Koepke L, Haridas GR, Sparrer KMJ, Mansi R, Fani M. Head-to-head comparison of different classes of FAP radioligands designed to increase tumor residence time: monomer, dimer, albumin binders, and small molecules vs peptides. Eur J Nucl Med Mol Imaging. 2023;50:3050\u0026ndash;61. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-023-06272-7\u003c/span\u003e\u003cspan address=\"10.1007/s00259-023-06272-7\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang R, Huang M, Wang W, Li M, Wang Y, Tian R. Preclinical Evaluation of \u003csup\u003e177\u003c/sup\u003eLu-Labeled FAP-Targeted Peptide for Tumor Radiopharmaceutical Imaging and Therapy. J Nucl Med. 2025;66:250. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.2967/jnumed.124.268689\u003c/span\u003e\u003cspan address=\"10.2967/jnumed.124.268689\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDorst DN, Smeets EMM, Klein C, Frielink C, Geijs D, Trajkovic-Arsic M, et al. Fibroblast Activation Protein-Targeted Photodynamic Therapy of Cancer-Associated Fibroblasts in Murine Models for Pancreatic Ductal Adenocarcinoma. Mol Pharm. 2023;20:4319\u0026ndash;30. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acs.molpharmaceut.3c00453\u003c/span\u003e\u003cspan address=\"10.1021/acs.molpharmaceut.3c00453\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSmeets EMM, Dorst DN, Franssen GM, van Essen MS, Frielink C, Stommel MWJ, et al. Fibroblast Activation Protein-Targeting Minibody-IRDye700DX for Ablation of the Cancer-Associated Fibroblast with Photodynamic Therapy. Cells; 2023.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGalbiati A, Bocci M, Neri D, Cazzamalli S. Effect of molar dose on the in vivo tissue biodistribution profile of FAP-targeted radioligand therapeutics. Eur J Nucl Med Mol Imaging. 2025;52:1399\u0026ndash;405. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-024-06969-3\u003c/span\u003e\u003cspan address=\"10.1007/s00259-024-06969-3\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBilinska A, Ballal S, Bal C, L\u0026auml;ppchen T, Pilatis E, Men\u0026eacute;ndez E, et al. Improved FAPI-radiopharmaceutical pharmacokinetics from the perspectives of a dose escalation study. Eur J Nucl Med Mol Imaging. 2025. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-025-07141-1\u003c/span\u003e\u003cspan address=\"10.1007/s00259-025-07141-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZboralski D, Osterkamp F, Christensen E, Bredenbeck A, Schumann A, Hoehne A, et al. Fibroblast activation protein targeted radiotherapy induces an immunogenic tumor microenvironment and enhances the efficacy of PD-1 immune checkpoint inhibition. Eur J Nucl Med Mol Imaging. 2023;50:2621\u0026ndash;35. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00259-023-06211-6\u003c/span\u003e\u003cspan address=\"10.1007/s00259-023-06211-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXie J, Zhang J, Wu H, Tang X, Liu J, Cheng G, Li P. The influences of age on T lymphocyte subsets in C57BL/6 mice. Saudi J Biol Sci. 2017;24:108\u0026ndash;13. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.sjbs.2016.09.002\u003c/span\u003e\u003cspan address=\"10.1016/j.sjbs.2016.09.002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWhite JR, Gong H, Colaizy TT, Moreland JG, Flaherty H, McElroy SJ. Evaluation of hematologic variables in newborn C57/BL6 mice up to day 35. Vet Clin Pathol. 2016;45:87\u0026ndash;95. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/vcp.12314\u003c/span\u003e\u003cspan address=\"10.1111/vcp.12314\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBaskar R, Dai J, Wenlong N, Yeo R, Yeoh KW. Biological response of cancer cells to radiation treatment. Front Mol Biosci. 2014;1:24. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3389/fmolb.2014.00024\u003c/span\u003e\u003cspan address=\"10.3389/fmolb.2014.00024\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMcConathy JMY, Rodon J, Goenka AH, Moy RH, Morse S, Demange A, Aimone P, Hope TA.. 671P LuMIERE: A phase I/II study evaluating safety, dosimetry, and preliminary activity of [177Lu]Lu-FAP-2286 in patients with advanced solid tumors. Ann Oncol. 2024;35:S526. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://10.1016/j.annonc.2024.08.737\u003c/span\u003e\u003cspan address=\"https://10.1016/j.annonc.2024.08.737\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"ejnmmi-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ejre","sideBox":"Learn more about [EJNMMI Research](http://ejnmmires.springeropen.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ejre/default.aspx","title":"EJNMMI Research","twitterHandle":"@officialEANM","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Fibroblast activation protein (FAP), cancer-associated fibroblast (CAF), targeted radionuclide therapy (TRT), pancreatic ductal adenocarcinoma (PDAC), Terbium-161","lastPublishedDoi":"10.21203/rs.3.rs-8237978/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8237978/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e \u003cp\u003eTerbium-161 (Tb-161) emits internal conversion and Auger electrons, in addition to beta-minus radiation, which might be of added benefit for targeted radionuclide therapy (TRT) compared to Lutetium-177 (Lu-177). We extensively compared Lu-177 and Tb-161 for fibroblast activation protein (FAP)-targeted TRT in a preclinical setting. To study this, FAP-2286 was labeled with Lu-177 and Tb-161 and characterized in vitro on FAP-expressing cells and ex vivo using patient tumor samples. Moreover, in vivo studies (i.e. biodistribution and efficacy) were performed using a clinically representative pancreatic ductal adenocarcinoma (PDAC) mouse model. Biodistribution was performed 1, 4, 24, and 48 h post injection of 5 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 or [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286. Subsequently, animals were treated with 4\u0026times;40 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 or [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 and with alternating doses of 2\u0026times;40 MBq/500 pmol of each radiopharmaceutical.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eNo difference in [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 uptake was observed in the cell models. In vivo studies did not show a survival benefit after 4\u0026times;40 MBq/500 pmol [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 or [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286, while Kaplan-Meier analyses demonstrated modestly prolonged survival after tandem therapy, in mice that first received [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 followed by [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286. Dosimetry calculations based on autoradiography on patient tumor samples showed that even with lower binding, a higher absorbed dose to the tumor can be accomplished with [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286.\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e \u003cp\u003eIn our vitro and in vivo studies, [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286 and [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286 demonstrated similar behavior. In the applied PDAC mouse model, FAP-TRT showed limited therapeutic efficacy, with a modest response observed in the tandem therapy group that first received [\u003csup\u003e177\u003c/sup\u003eLu]Lu-FAP-2286, followed by [\u003csup\u003e161\u003c/sup\u003eTb]Tb-FAP-2286.\u003c/p\u003e","manuscriptTitle":"Head-to-head comparison of [177Lu]Lu-FAP-2286 and [161Tb]Tb-FAP-2286 efficacy in a PDAC mouse model: Is there an added benefit of internal conversion and Auger electrons for FAP-TRT?","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-22 10:02:46","doi":"10.21203/rs.3.rs-8237978/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-12-17T08:26:45+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-17T07:57:23+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-17T07:02:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"EJNMMI Research","date":"2025-12-16T15:03:10+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ejnmmi-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ejre","sideBox":"Learn more about [EJNMMI Research](http://ejnmmires.springeropen.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ejre/default.aspx","title":"EJNMMI Research","twitterHandle":"@officialEANM","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"26ba5f68-ed1c-4ea4-92de-e7f3e1ec2147","owner":[],"postedDate":"December 22nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-01-12T16:10:51+00:00","versionOfRecord":{"articleIdentity":"rs-8237978","link":"https://doi.org/10.1186/s13550-026-01372-5","journal":{"identity":"ejnmmi-research","isVorOnly":false,"title":"EJNMMI Research"},"publishedOn":"2026-01-09 15:59:31","publishedOnDateReadable":"January 9th, 2026"},"versionCreatedAt":"2025-12-22 10:02:46","video":"","vorDoi":"10.1186/s13550-026-01372-5","vorDoiUrl":"https://doi.org/10.1186/s13550-026-01372-5","workflowStages":[]},"version":"v1","identity":"rs-8237978","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8237978","identity":"rs-8237978","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}