AAZTA5-squaramide ester: promising tool for 177Lu-labeling of monoclonal antibodies under mild conditions

Abstract Background Combining the advantages of both cyclic and acyclic chelator systems, AAZTA (1,4-bis(carboxymethyl)‐6‐[bis(carboxymethyl)]amino‐6‐methylperhydro‐1,4‐diazepine) is well suited for complexation of various diagnostic and therapeutic radiometals such as gallium-68, scandium-44 and lutetium-177 under mild conditions. Due to its specificity for primary amines and pH dependent binding properties, squaric acid (SA) represents an excellent tool for selective coupling of the appropriate chelator to different target vectors. Therefore, the aim of this study was to evaluate radiolabeling properties of the novel bifunctional AAZTA5-SA being coupled to a model antibody (bevacizumab) in comparison to DOTA-SA using the therapeutic nuclide lutetium-177. Results As proof-of-concept, bevacizumab was first functionalized with either AAZTA5-SA or DOTA-SA. After purification via fractionated size exclusion chromatography (SEC), the corresponding immunoconjugates were subsequently radiolabeled with lutetium-177 at pH 7 and room temperature (RT) as well as 37 °C. After 90 min, labeling of AAZTA5-SA-mAb resulted in almost quantitative radiochemical yields (RCY) of > 98% and > 99%, respectively. After purification via SEC, the radioconjugate [177Lu]Lu-AAZTA5-SA-mAb could be obtained with a purity of > 99% and an apparent specific activity of 4.5 GBq/µmol. In contrast, 177Lu-labeling of DOTA-SA-mAb showed negligible radiochemical yields of < 2% both at room temperature and 37 °C. In vitro complex stability measurements of [177Lu]Lu-AAZTA5-SA-mAb in human serum at 37 °C indicated > 99% protein bound activity within 15 days. In phosphate buffered saline (PBS), a slightly decreased stability of > 93% intact conjugate was observed over the same period. Conclusion Coupling of AAZTA5-SA to the monoclonal antibody bevacizumab allowed for 177Lu-labeling with almost quantitative radiochemical yields both at room temperature and 37 °C. Within 15 days, the resulting radioconjugate indicated very high in vitro complex stability both in human serum and PBS. Therefore, AAZTA5-SA is a promising tool for 177Lu-labeling of sensitive biomolecules such as antibodies for theranostic applications.