Individualised progesterone receptor modulator prevention strategies for triple-negative breast cancer in BRCA1 pathogenic variant carriers.

Our vision for personalised TNBC prevention includes predicting TNBC-risk in real time through the analysis of DNAme signatures in breast tissue (obtained through biopsy or more fine-needle aspiration which is more acceptable) or, eventually, in a more easily accessible epithelial surrogate sample. These markers would enable the ongoing assessment of primary prevention efficacy, for example, with mifepristone. They would also allow for personalised adjustments, such as changes in dosage, frequency or even medication choice which in addition to mifepristone includes drugs which have shown potential for breast cancer prevention (e.g. selective estrogen receptor modulators or thiazolidinediones 122 or which are specifically active in BRCA1 PV carriers (e.g. PARP inhibitors) 123 . Engagement with the patient community and integration of the aspects that matter most to them into the final solution, around risk communication and ease/invasiveness of sample collection and drug administration, would enhance the participatory nature of these endeavours (Fig. 3 ). Fig. 3 Approach to TNBC prevention in BRCA1 PV carriers using the PRM mifepristone and potential complementary approaches alongside real-time risk assessment. The blue bars in the centre of the cells reflect DNA methylation as a reflection of replicative age (the higher the number of blue bars, i.e. DNA methylation at PCGTs, the higher the replicative age of these cells). The risk state for individual cells increases from green to yellow to orange to red. A Scenario illustrating gradual development of mifepristone resistance where additional preventive treatments could temporarily offset resistance and sustain preventive efficacy; B Scenario showing sustained and high mifepristone sensitivity, where primary prevention remains effective without the need for complementary therapies. The blue bars in the centre of the cells reflect DNA methylation as a reflection of replicative age (the higher the number of blue bars, i.e. DNA methylation at PCGTs, the higher the replicative age of these cells). The risk state for individual cells increases from green to yellow to orange to red. A Scenario illustrating gradual development of mifepristone resistance where additional preventive treatments could temporarily offset resistance and sustain preventive efficacy; B Scenario showing sustained and high mifepristone sensitivity, where primary prevention remains effective without the need for complementary therapies.

Figure 4 shows the proposed design for a definitive study aimed at testing a personalised primary TNBC prevention approach in BRCA1 PV carriers. Fig. 4 Proposed trials design. Proposed design for a trial assessing a personalised primary prevention approach utilising the progesterone receptor modulator mifepristone. Proposed design for a trial assessing a personalised primary prevention approach utilising the progesterone receptor modulator mifepristone. Detailed discussions around the clinical trial design, including randomisation, control groups, statistics, inclusion/exclusion criteria, contraception, early BC detection, and risk communication during the trial, and more, are needed. In addition, a critical question is which intermediate surrogate markers reflective of the real time risk should be implemented and, most importantly, how these should be validated before inclusion in a clinical trial. The general criteria for such markers are as follows: (i) They should demonstrate a gradual increase with increasing risk across a range of age and disease statuses, as measured in normal breast tissue, normal breast tissue adjacent to cancer, and cancer tissue. We note that the lack of longitudinal, population-based collection of normal breast tissue limits the scope of specific assessment and validation possible. (ii) They must clearly indicate either a positive or negative response to an intervention. They should demonstrate a gradual increase with increasing risk across a range of age and disease statuses, as measured in normal breast tissue, normal breast tissue adjacent to cancer, and cancer tissue. We note that the lack of longitudinal, population-based collection of normal breast tissue limits the scope of specific assessment and validation possible. They must clearly indicate either a positive or negative response to an intervention. Consensus from the scientific and regulatory communities on the extent of evidence supporting a biomarker’s effectiveness would be needed before embarking on a prevention study. In light of the accrual challenges experienced by the BRCA-P trial and the current state of evidence for mifepristone in breast cancer prevention, a randomised phase IIB proof-of-concept trial with a short-term biomarker endpoint is a more immediately feasible first step than a large-scale randomised controlled trial (RCT) powered for TNBC incidence. Such a phase IIB trial would serve to assess drug acceptability, safety, and preliminary pharmacodynamic response, and could inform the design of a subsequent definitive prevention trial. The duration of drug intervention in such a trial would ideally be biomarker-guided - adapted in response to the individual’s risk trajectory as measured by intermediate surrogate markers - rather than fixed at a predetermined timepoint. An important consideration in selecting the biomarker endpoint for such a phase IIB trial is its biological relevance to the specific cancer subtype at risk. Background parenchymal enhancement (BPE) on breast MRI has been proposed as a potential short-term endpoint given that annual breast MRI is standard of care in BRCA1 PV carriers. However, a recently published large pooled analysis of the Breast Cancer Association Consortium (BCAC) demonstrated that mammographic density (percentage density) shows a markedly attenuated - effectively null - association with breast cancer risk specifically in BRCA1 PV carriers (OR per SD: 1.02, 95% CI 0.78–1.34), in contrast to BRCA2 carriers (OR per SD: 1.55, 95% CI 1.08–2.24), and attributes this finding to the predominance of the triple-negative subtype in BRCA1 -associated cancers 124 . Because BPE and mammographic density are inter-related imaging surrogates of fibroglandular tissue, and because women with TNBC characteristically have lower BPE and mammographic density compared to women with non-TNBC, BPE is likely to be insensitive - or even directionally misleading - as a pharmacodynamic endpoint for monitoring TNBC risk reduction in BRCA1 PV carriers. For this reason, DNA methylation (DNAme)-based biomarkers in accessible surrogate tissues (see below) are preferred as intermediate endpoints in this population. Due to fact that mifepristone has been shown to have an acceptable safety profile without liver toxicity, it is the most suitable PRM for consideration in the context a future breast cancer prevention trial. Despite evidence from mouse models and a very limited number of window trials, the evaluation of mifepristone 20 , 59 for breast cancer prevention has not been extensive. Data generated with mifepristone are supported with trials using other PRMs such as ulipristal acetate 20 , 60 . Large-scale evidence from adjuvant trials similar to that which exists for SERMs – agents that are central components of adjuvant breast cancer treatment – are unlikely to become available for PRMs. Data from ongoing contraceptive trials with mifepristone add to its long-term safety profiles 71 . Evaluation of mifepristone’s effectiveness could start within the context of a randomised clinical trial amongst very high-risk women. To walk towards this already challenging starting point, additional hurdles have to be overcome. Chiefly, these revolve around the negative political and cultural perceptions of mifepristone, which is primarily known as an abortion drug, and the limited access to it in countries where abortion laws are restrictive. Additionally, funding and regulatory challenges exist. The interest from the pharmaceutical industry is low, in part because of the association of mifepristone with abortion, but also because mifepristone is off-patent and prevention trials can be multi-national, long and expensive. Who will fund such a trial in the absence of pharma interest? Consensus from regulators on the design of clinical trials, and the biomarkers used within them, would also be needed to ensure results can be used to support implementation. Nevertheless, when compared with denosumab (the agent being tested in the BRCA-P trial), mifepristone has several important potential advantages for acceptability and feasibility. These are elaborated in the context of a possible stepwise trial approach described in the previous section. An additional topic requiring clarification is which drugs, when applied at low doses for short periods, could affect mifepristone-resistant cells (see Fig. 3 panel A). Several options exist and could potentially be applied in the same trial. These include intermittent fasting, a ketogenic diet/supplement (unpublished data), PARP inhibitors, which have particularly high antineoplastic activity in BRCA1 PV carriers compared to non-carriers 125 , and – still very speculatively – thiazolidinediones, which may help prevent breast cancer by activating PPARγ, which reduces cell proliferation, promotes cellular differentiation, and improves insulin and inflammatory signalling pathways 122 . The effectiveness of these approaches, particularly if applied in combination, will need to be considered. New, personalised approaches to primary breast cancer prevention are urgently needed and could be developed by building on the last 15 years of research into the pathogenesis of TNBC in BRCA1 PV carriers and methods to monitor risk. However, a community effort is needed to reach this goal of a new personalised cancer prevention approach. A consensus workshop, held in Innsbruck in September 2024, began this community effort. This group – the International Mifepristone Breast Cancer Prevention (IMBCP) Group – will now broaden its reach and continue to work towards ensuring that the necessary discussions take place, ultimately bringing about tangible change for women at the highest risk.Table 2 Table 2 The role of progesterone receptor modulators in breast cancer prevention Study Preventive effects Type of PRM Type of study Methodology Main Findings Ref Barrett et al., 2025 Yes Mifepristone Mouse model Medroxyprogesterone acetate and 7,12-dimethylbenzanthracene was used to induce mammary gland cancers in mice Mifepristone reduced mammary gland cancers by >50% 62 Lee et al., 2023 Likely Onapristone and ulipristal acetate Tamoxifen-insensitive mouse model Genetically modified C3(1)-TAg mice, which develop aggressive ER+ mammary tumors, were used to evaluate Z-endoxifen (5 mg/kg/day) as monotherapy and combined with selective progesterone receptor modulators ulipristal acetate (16.7 mg/kg/day) or onapristone (20 mg/kg/day). ( n   = 10–12 mice per group). Ulipristal acetate (81 mg/day) significantly delayed tumor onset ( p  = 0.03; latency 114 days, p  = 0.005), but did not reduce tumor growth or multiplicity; Onapristone (98 mg/day) showed no preventive effect. 146 Ranjan et al., 2021 Yes Mifepristone and telapristone Mouse Ovariectomized mice were randomized into four groups (9 mice/group): sham, estrogen (0.3 mg) and progesterone (30 mg), estrogen and progesterone with telapristone (30 mg), and estrogen and progesterone with mifepristone (30 mg). After 14 days of treatment, mammary glands were collected for RNA sequencing and splicing analysis. Estrogen and progesterone significantly increased MSC and luminal mature fractions and reduced luminal progenitor cells in breast. Moreover, they strongly activate Wnt signaling, most prominently in MSC. Progesterone receptor antagonists reversed progesterone driven tumorigenic effects in mammary cells. 147 Lee et al., 2021 Yes Telapristone acetate, ulipristal acetate, and mifepristone BRCA1 knockout mouse model Mice were treated with (30 mg/90 days) of mifepristone ( n  = 13), telapristone acetate (n = 10), or ulipristal acetate ( n  = 6) via subcutaneous pellets. The control group had 28 mice, Tumorigenesis was monitored, and histological analysis was performed at euthanasia. RNA was extracted from benign mammary glands for sequencing and gene expression analysis. Ulipristal acetate significantly reduced tumor development ( p  = 0.04), achieving a 37% reduction in adenocarcinoma compared with an 18% reduction by Telapristone and no effect by mifepristone. Ulipristal acetate delayed tumor onset by >2 months 148 Lee et al., 2016 Yes Telapristone acetate Rat/mouse Mice ( n  = 10–11) were treated with subcutaneous medroxyprogesterone acetate(MPA) (25 mg/pellet) alone or with telapristone acetate (25 mg/pellet), followed by weekly gavage of DMBA (1 mg) for 4 weeks. In the rat model ( n  = 10–11), a single injection of MNU (50 mg/kg) was followed by implantation of medroxyprogesterone acetate (MPA), progesterone, or telapristone acetate (identical doses to the mice study). Tumor formation and histology were assessed at study termination (65 days). In mouse, Telapristone acetate opposed the pro-hyperplastic effects of MPA ( p  = 0.002) In a rat mammary carcinogenesis model induced by MNU, MPA and progesterone increased tumor incidence and tumor burden compared with controls. Telapristone acetate significantly reduced tumor incidence when combined with MPA ( p  = 0.037) or progesterone ( p  = 0.032). The lowest tumor incidence (36%) was observed in rats treated with progesterone plus Telapristone acetate. Tumor burden was significantly reduced by Telapristone acetate in combination with progesterone compared with progesterone alone ( p  = 0.042). 149 Communal et al., 2016 Yes Ulipristal acetate Mouse xenograft model Breast tissue from 6 control and 5 BRCA1 PV carriers was xenografted into 4-week-old ovariectomized mice. Treatments included control, estradiol (1.5 mg/pellet), estradiol + progesterone (25 mg/pellet), or estradiol + progesterone + ulipristal acetate (30 mg/pellet) ( n  = 4 mice/group). Mice were sacrificed after 28 days for analysis. Ulipristal acetate inhibited progesterone driven proliferation in BRCA1 PV breast tissue. 150 Communal et al., 2012 Unclear Ulipristal acetate In vitro (Human) and xenograft athymic mouse model Normal human breast epithelial were xenografted in athymic mice ( n  = 4) treated with 20 mg UPA per pellet for 28 days. Ulipristal acetate had no effect on the mitotic index of xenografted human breast tissue exposed to gonadal hormones at levels similar to those in normal women. In vitro findings indicated that Ulipristal acetate reduced the proliferation of normal breast cells but promoted growth in breast cancer cells. 151 Wiehle et al., 2011 Yes Telapristone acetate Rat Rats at the age of 50 days were injected with N-methyl-N-nitrosourea to initiate mammary carcinogenesis, and 6 days later, Telapristone acetate (CDB-4124) pellets (30.0 and 3.0 mg/pellet) were implanted subcutaneously for a duration of 3 months. ( n  = 16 to 20 rats/group) Telapristone acetate (CDB-4124) demonstrated strong chemopreventive efficacy in rat mammary cancer models, with the following outcomes: - Increased tumor latency from (66 ± 24 to 87 ± 20 days) ( P  < 0.02) - Reduced tumor incidence from 85% → 35% ( P  < 0.001) - Decreased tumor multiplicity by from 3.0 → 1.1 tumors/animal ( P  < 0.001) - Lowered tumor burden from (2.6 → 0.26 g/animal) - Prevented spontaneous premalignant lesions at 200 mg/kg over 24 months 152 Gonzalez-Suarez et al., 2010 Yes Mifepristone Mouse Mice were treated with 10 mg/kg muRANK-muFc, PBS, or zoledronic acid. Tumor growth was monitored by palpation, and mice were euthanized when tumors exceeded a certain size. Proliferation assays were conducted using BrdU, and progesterone and mifepristone (10 mg/kg) were administered to examine hormonal effects. ( n  = 24-32). Mifepristone reduced 98% of progesterone-induced proliferation ( P  < 0.001 versus control) whereas RANK-Fc reduced proliferation by 82% ( P  < 0.001 versus control). 22 Kariagina et al., 2010 Yes Mifepristone Organoid T47D-Y, T47D-YA, and T47D-YB cells were cultured in serum-free media, treated with R5020 (20 nM) ± Iressa (500 nM) or mifepristone (100 nM) for 24 hours, followed by 25 nM Areg for 30 minutes. Estrogen and progesterone act synergistically to drive mammary epithelial proliferation, with combined estrogen and progesterone treatment inducing robust proliferation in ducts (24%) and lobules (39%), compared with minimal effects of estrogen alone (3-4%) and no effect of progesterone alone. In primary mammary organoids, Mifepristone significantly reduced the proliferative effect of Areg + progesterone, indicating that active progesterone receptor signalling is required for hormone-induced epithelial growth Mifepristone reduced the proportion of S-phase cells back to basal levels, confirming suppression of cell-cycle progression. 153 Lim et al., 2009 No Mifepristone and onapristone In vitro Mammary epithelial subsets from human tissue, including BRCA1 PV carriers, were analyzed using flow cytometry. Cells were cultured in Matrigel with supplemented medium, including mifepristone or onapristone. The clonogenic activity of luminal progenitor cells from BRCA1 PV carriers was not inhibited by mifepristone or onapristone despite higher progesterone receptor expression in these cells. 154 Poole et al., 2006 Yes Mifepristone Brca1/p53-deficient mouse model Mice were implanted with a pellet containing 35 mg/60-day constant-release mifepristone ( n  = 14) or a placebo pellet ( n  = 4). The study assessed the impact of mifepristone on tumor development and the role of BRCA1 in regulating progesterone receptors. Mifepristone prevented mammary tumorigenesis in Brca1/p53-deficient mice, with no palpable tumors detected up to 12 months of age in treated animals ( n  = 14), compared with 100% tumor penetrance and a median tumor latency of 6.6 months in control mice ( n  = 25). Mifepristone significantly reduced ductal branching and alveologenesis 61 Li et al., 1995 Yes PRM ZK114043 Mouse Whole organ cultures of mammary glands from estradiol/progesterone-primed virgin mice maintained in a serum-free medium with aldosterone, prolactin, insulin, and hydrocortisone. The cultures were treated with the PRM ZK114043 (5 ng/ml) for 4 days. ZK114043 promoted differentiation in mammary cells. ZK114043 showed potent antiproliferative and pro-differentiation effects in the normal mammary gland. ZK114043 strongly inhibited epithelial DNA synthesis ZK114043 induced a differentiated secretory phenotype, with increased lipid accumulation in alveolar cells. 155 Study Preventive effects Type of PRM Study Duration Type of study Sample Size ( N ) / Main Inclusion and Exclusion criteria Methodology Main Findings Ref Simões et al, 2025 Yes Ulipristal acetate: 5 mg daily 12 weeks Window-of-opportunity clinical trial N  = 24 Inclusion: Premenopausal women (25–45 years old) with regular cycles and BRCA1/2 mutation or women at moderate to high familial BC risk. Exclusion: history of breast, uterine, cervical or ovarian cancer; pregnancy/breastfeeding, prior breast enhancement, or use of hormonal therapies, steroids, anticoagulants, or CYP3A4 inhibitors; liver disease/cirrhosis Effect of ulipristal acetate on surrogate markers of breast cancer risk in 24 premenopausal women Significant reduction in Ki67 (8.2% → 2.9%, P < 0.0001) alongside a drop in progesterone levels (36 → <3 nmol/L, P < 0.0001), effectively suppressing the luteal phase. Decreased epithelial area in lobules and reduced acinar structure size following treatment. Luminal progenitor cells significantly reduced (from 43% to 30%, P  1.5, P < 0.05) after the treatment. 156 Utjés et al., 2024 Yes Mifepristone: 50 mg every other day 8 weeks Secondary Outcomes of a RCT N  = 16 Inclusion: Healthy premenopausal women (18–43 years old) with regular cycles, no treatment contraindications. Exclusion: history of cancer, pregnancy/breastfeeding Breast biopsies were collected from 16 women at baseline and after two months of treatment with 50 mg mifepristone every other day, yielding 32 paired samples. Breast cancer-related signatures and progesterone receptor signalling pathways were analyzed. 27 differentially expressed genes identified, mainly linked to extracellular matrix remodelling. Mifepristone-induced gene signature in vivo closely matched in vitro findings Gene signature associated with breast carcinogenesis and progesterone receptor status 157 Bartlett et al., 2022 Yes Mifepristone: 50 mg every other day; UPA: 5 mg daily 12 weeks RCT N  = 44 Inclusion: Healthy premenopausal women (18–45 years old) with regular cycles and confirmed BRCA1/2 mutation (or non-carrier controls). Exclusion: recent hormonal medication, pregnancy/breastfeeding, history of cancer, or prior bilateral salpingo-oophorectomy Two cohorts were studied: the first received mifepristone (50 mg every other day), and the second received ulipristal acetate (5 mg daily). Effects on breast cancer risk markers, including cell composition, proliferation, TP53 mutation, and methylation, were assessed in women with and without BRCA PV. Progesterone levels are higher in luteal phase in BRCA1 PV carriers, suggesting progesterone as a target for cancer risk reduction. Triple Negative Breast Cancers show elevated DNA methylation signatures reflecting higher mitotic age and higher proportion of luminal progenitor cells supporting the idea that luminal progenitor cells with high replicative age represent a predisposition to malignant transformation. Mifepristone reduces both mitotic age and luminal progenitor cell proportion in normal breast tissue in all controls and 64% of BRCA1/2 PV carriers. Moreover, Ulipristal acetate produces similar effects, validating the role of progesterone pathway targeting. Mifepristone decreases TP53 mutation frequency and the number of mutations in normal breast tissue of mitotic-age responders. 20 Westhoff et al., 2022 Yes Ulipristal acetate: 10 mg daily 12 weeks (84 days) RCT N  = 25 Inclusion: Healthy women (18–39 years old, BMI ≤ 35 kg/m²) with regular cycles, no baseline breast mass/ovarian cysts, and no contraindications to COCs. Exclusion: recent exogenous hormones, pregnancy, breastfeeding, or interacting medications. Women aged 18–39 received ulipristal acetate (10 mg daily) or combination oral contraceptives for 84 days, with breast cell proliferation and background parenchymal enhancement monitored. Ulipristal acetate reduced Ki67 by 84% (p = 0.040), while the COC group showed an 8% increase (p = 0.85) BPE scores decreased from 3 to 1 (p = 0.008) with ulipristal, with no change in the COC group 60 Engman et al., 2008 Yes Mifepristone: 50 mg every other day 12 weeks RCT N  = 28 Inclusion: Premenopausal women referred for leiomyoma surgery (due to menorrhagia/pressure symptoms). Exclusion: recent steroidal hormone therapy, history of cancer, abnormal baseline breast/pelvic imaging, urgent surgery needs, postmenopausal FSH levels, or significant systemic/hormonal dysfunction. 30 women, scheduled for surgical treatment of leiomyomas, randomized to either 50 mg mifepristone or placebo every other day, for 3 months. Mifepristone significantly reduced Ki-67 index after 3 months (P = 0.012) with lower inter-individual variability. Mifepristone did not affect cortisol but increased serum testosterone levels. Mifepristone reduced breast soreness and swelling but increased the incidence of hot flushes. 59 RCT Randomized Controlled trial; ER, Estrogen Receptor; EP Estrogen and Progesterone (EP); MSC Mammary stem cell; LP Luminal Progenitor; PV Pathogenic Variant; TAg SV40 Large T Antigen; DMBA Dimethylbenzanthracene; MPA Medroxyprogesterone Acetate; MNU N-Methyl-N-nitrosourea; muRANK murine RANK (Receptor Activator of NF-κB); muFc murine Fc region; PBS Phosphate-Buffered Saline; RANK-FC recombinant fusion protein consisting of the extracellular ligand-binding domain of RANK coupled to an immunoglobulin Fc region, which acts as a soluble decoy receptor to neutralize RANKL and thereby inhibit RANK signaling; PRM Progesterone Receptor Modulator; BPE Background Parenchymal Enhancement. The role of progesterone receptor modulators in breast cancer prevention Medroxyprogesterone acetate and 7,12-dimethylbenzanthracene was used to induce mammary gland cancers in mice Estrogen and progesterone significantly increased MSC and luminal mature fractions and reduced luminal progenitor cells in breast. Moreover, they strongly activate Wnt signaling, most prominently in MSC. Progesterone receptor antagonists reversed progesterone driven tumorigenic effects in mammary cells. Ulipristal acetate significantly reduced tumor development ( p  = 0.04), achieving a 37% reduction in adenocarcinoma compared with an 18% reduction by Telapristone and no effect by mifepristone. Ulipristal acetate delayed tumor onset by >2 months In mouse, Telapristone acetate opposed the pro-hyperplastic effects of MPA ( p  = 0.002) In a rat mammary carcinogenesis model induced by MNU, MPA and progesterone increased tumor incidence and tumor burden compared with controls. Telapristone acetate significantly reduced tumor incidence when combined with MPA ( p  = 0.037) or progesterone ( p  = 0.032). The lowest tumor incidence (36%) was observed in rats treated with progesterone plus Telapristone acetate. Tumor burden was significantly reduced by Telapristone acetate in combination with progesterone compared with progesterone alone ( p  = 0.042). Normal human breast epithelial were xenografted in athymic mice ( n  = 4) treated with 20 mg UPA per pellet for 28 days. Ulipristal acetate had no effect on the mitotic index of xenografted human breast tissue exposed to gonadal hormones at levels similar to those in normal women. In vitro findings indicated that Ulipristal acetate reduced the proliferation of normal breast cells but promoted growth in breast cancer cells. Telapristone acetate (CDB-4124) demonstrated strong chemopreventive efficacy in rat mammary cancer models, with the following outcomes: - Increased tumor latency from (66 ± 24 to 87 ± 20 days) ( P  < 0.02) - Reduced tumor incidence from 85% → 35% ( P  < 0.001) - Decreased tumor multiplicity by from 3.0 → 1.1 tumors/animal ( P  < 0.001) - Lowered tumor burden from (2.6 → 0.26 g/animal) - Prevented spontaneous premalignant lesions at 200 mg/kg over 24 months Estrogen and progesterone act synergistically to drive mammary epithelial proliferation, with combined estrogen and progesterone treatment inducing robust proliferation in ducts (24%) and lobules (39%), compared with minimal effects of estrogen alone (3-4%) and no effect of progesterone alone. In primary mammary organoids, Mifepristone significantly reduced the proliferative effect of Areg + progesterone, indicating that active progesterone receptor signalling is required for hormone-induced epithelial growth Mifepristone reduced the proportion of S-phase cells back to basal levels, confirming suppression of cell-cycle progression. Mifepristone prevented mammary tumorigenesis in Brca1/p53-deficient mice, with no palpable tumors detected up to 12 months of age in treated animals ( n  = 14), compared with 100% tumor penetrance and a median tumor latency of 6.6 months in control mice ( n  = 25). Mifepristone significantly reduced ductal branching and alveologenesis ZK114043 promoted differentiation in mammary cells. ZK114043 showed potent antiproliferative and pro-differentiation effects in the normal mammary gland. ZK114043 strongly inhibited epithelial DNA synthesis ZK114043 induced a differentiated secretory phenotype, with increased lipid accumulation in alveolar cells. N  = 24 Inclusion: Premenopausal women (25–45 years old) with regular cycles and BRCA1/2 mutation or women at moderate to high familial BC risk. Exclusion: history of breast, uterine, cervical or ovarian cancer; pregnancy/breastfeeding, prior breast enhancement, or use of hormonal therapies, steroids, anticoagulants, or CYP3A4 inhibitors; liver disease/cirrhosis Significant reduction in Ki67 (8.2% → 2.9%, P < 0.0001) alongside a drop in progesterone levels (36 → <3 nmol/L, P < 0.0001), effectively suppressing the luteal phase. Decreased epithelial area in lobules and reduced acinar structure size following treatment. Luminal progenitor cells significantly reduced (from 43% to 30%, P  1.5, P < 0.05) after the treatment. N  = 16 Inclusion: Healthy premenopausal women (18–43 years old) with regular cycles, no treatment contraindications. Exclusion: history of cancer, pregnancy/breastfeeding 27 differentially expressed genes identified, mainly linked to extracellular matrix remodelling. Mifepristone-induced gene signature in vivo closely matched in vitro findings Gene signature associated with breast carcinogenesis and progesterone receptor status N  = 44 Inclusion: Healthy premenopausal women (18–45 years old) with regular cycles and confirmed BRCA1/2 mutation (or non-carrier controls). Exclusion: recent hormonal medication, pregnancy/breastfeeding, history of cancer, or prior bilateral salpingo-oophorectomy Progesterone levels are higher in luteal phase in BRCA1 PV carriers, suggesting progesterone as a target for cancer risk reduction. Triple Negative Breast Cancers show elevated DNA methylation signatures reflecting higher mitotic age and higher proportion of luminal progenitor cells supporting the idea that luminal progenitor cells with high replicative age represent a predisposition to malignant transformation. Mifepristone reduces both mitotic age and luminal progenitor cell proportion in normal breast tissue in all controls and 64% of BRCA1/2 PV carriers. Moreover, Ulipristal acetate produces similar effects, validating the role of progesterone pathway targeting. Mifepristone decreases TP53 mutation frequency and the number of mutations in normal breast tissue of mitotic-age responders. N  = 25 Inclusion: Healthy women (18–39 years old, BMI ≤ 35 kg/m²) with regular cycles, no baseline breast mass/ovarian cysts, and no contraindications to COCs. Exclusion: recent exogenous hormones, pregnancy, breastfeeding, or interacting medications. Ulipristal acetate reduced Ki67 by 84% (p = 0.040), while the COC group showed an 8% increase (p = 0.85) BPE scores decreased from 3 to 1 (p = 0.008) with ulipristal, with no change in the COC group N  = 28 Inclusion: Premenopausal women referred for leiomyoma surgery (due to menorrhagia/pressure symptoms). Exclusion: recent steroidal hormone therapy, history of cancer, abnormal baseline breast/pelvic imaging, urgent surgery needs, postmenopausal FSH levels, or significant systemic/hormonal dysfunction. Mifepristone significantly reduced Ki-67 index after 3 months (P = 0.012) with lower inter-individual variability. Mifepristone did not affect cortisol but increased serum testosterone levels. Mifepristone reduced breast soreness and swelling but increased the incidence of hot flushes. RCT Randomized Controlled trial; ER, Estrogen Receptor; EP Estrogen and Progesterone (EP); MSC Mammary stem cell; LP Luminal Progenitor; PV Pathogenic Variant; TAg SV40 Large T Antigen; DMBA Dimethylbenzanthracene; MPA Medroxyprogesterone Acetate; MNU N-Methyl-N-nitrosourea; muRANK murine RANK (Receptor Activator of NF-κB); muFc murine Fc region; PBS Phosphate-Buffered Saline; RANK-FC recombinant fusion protein consisting of the extracellular ligand-binding domain of RANK coupled to an immunoglobulin Fc region, which acts as a soluble decoy receptor to neutralize RANKL and thereby inhibit RANK signaling; PRM Progesterone Receptor Modulator; BPE Background Parenchymal Enhancement.

Primary cancer prevention implies that in the organ at risk (i.e. breast) there are neither macroscopic nor microscopic signs of cancer or pre-cancer. Significant amounts of research focus on the detection of the earliest signs of cancer, utilising circulating cells 36 , 37 or circulating cell-products (e.g. cancer-specific altered DNA 9 , 38 – 40 , RNA 41 or proteins 42 , 43 ). However, none of these efforts have so far convincingly demonstrated a clinical benefit over existing imaging-based strategies, such as mammography or breast MRI. These imaging-based methods often lead to overdiagnosis 44 , 45 and do not effectively detect TNBC and other BCs with poor prognosis, which have higher mortality rates compared to screen detectable cancers 46 , 47 . PRS, which can be used to individualise BC risk prior to clinical manifestation, also have substantial limitations. A commonly used PRS, consisting of 313 common genetic variants preferentially predicts risk for estrogen-receptor positive BCs 48 . A new PRS, based on analysis of a more diverse cohort led by the US National Cancer Institute (Confluence), may help to overcome this issue 49 . PRS is being used in conjunction with other risk factors, such as age, to measure changing risk over time. Nevertheless, due to the nature of single nucleotide polymorphisms, a PRS per se cannot be used to monitor individual risk in response to an intervention. Two parameters that directly assess breast tissue and may reflect cancer risk have gained some attention: (i) mammographic density is associated with BC risk, however, the association is weaker and has not been evaluated extensively in women 35 years of age or younger 50 . Mammographic density also depends on menstrual cycle 51 . Most importantly, women with TNBC tend to have lower mammographic density, less MRI-assessed fibroglandular tissue, and lower background parenchymal enhancement compared to non-TNBC 14 . Therefore, although suitable for monitoring risk modulation in older women on SERMs 52 , 53 , imaging-based risk monitoring in young BRCA1 PV carriers at risk for TNBC is unlikely to be safe and feasible. (ii) Ki67, a marker for cell proliferation, has been used to assess SERM efficacy 54 , 55 . However, Ki67 expression in normal breast tissue is weakly associated with BC risk 56 . Additionally, it provides only a snapshot of current proliferation in breast tissue, is dependent on the menstrual cycle, and is difficult to assess in small breast biopsies with limited numbers of epithelial cells, leading to limited interlaboratory reproducibility 57 .

Breast cancer (BC) is the leading cause of cancer-related death among young women worldwide, and its prevalence is increasing 1 . Pathogenic variants (PVs) in the BRCA1 gene are responsible for approximately 1 to 3% of BCs globally, with 60 to 70% of these cancers being of basal cell origin and lacking expression of estrogen, progesterone, and HER2 receptors, making them triple negative breast cancers (TNBC) 2 . Among all BC subtypes, TNBC has the worst prognosis 3 and treatment options remain limited due to the absence of therapeutic targets. BRCA1 PV carriers have a 40% lifetime risk for TNBC, compared to a 1% lifetime risk in women without PVs; notably, this risk rises sharply from 25–30 years onwards 2 . Additionally, BRCA1 PV carriers have a 45% lifetime risk of ovarian cancer (OC), compared to 1.8% risk for women without PVs and the risk starts to rise from the age of 35 onwards 4 . Options for management of TNBC risk for BRCA1 PV carriers are rather scarce. Primary prevention strategies (aimed at preventing cancer development) are mostly limited to risk reducing bilateral mastectomy, which can lower BC risk by 90-95% 5 and remains the gold standard for breast cancer prevention. However, the procedure shows only a tendency towards improved survival 6 , has a higher risk of failure in women who smoke or have a high body mass index (BMI), carries a risk of post-surgical complications, and has negative consequences on self-perception and physical and mental well-being. Risk reduction through selective estrogen receptor modulators (SERMs; e.g. tamoxifen and raloxifene) and aromatase inhibitors 7 , 8 has shown efficacy in reducing estrogen receptor positive BC risk, but not mortality, and these drugs do not prevent TNBC. Secondary prevention strategies (aimed at early cancer detection) are currently limited to enhanced imaging-based surveillance (i.e. mammograms and breast MRI) and these remain a core aspect of the follow-up management of BRCA1 PV carriers. Blood-based early detection strategies (e.g. detection of circulating tumor DNA) have not shown great promise so far due to limited sensitivity 9 , 10 . Individual BC risk for BRCA1 PV carriers varies considerably, making risk stratification an important component of both primary and secondary prevention. These risks have been substantiated in recent years by comprehensive cohort studies. In particular, age-dependent prospective incidence rates have been empirically determined and allow age-dependent risk prediction 11 , 12 . This has significantly improved risk communication and informed decision-making on the uptake of preventive measures. CanRisk, an online tool based on personal, family and genetic risk factors, is now fully embedded in many cancer genetics services and implementation studies using CanRisk with other risk factors [i.e. polygenic risk score (PRS) and non-genetic risk factors] are underway in tertiary care (e.g. Precision HBOC, CanRisk-Clingen) and primary care (e.g. CanRisk-GP). Such comprehensive risk prediction has been shown to also modify individual breast cancer risk in BRCA1 PV carriers that might provide additional information for decision-making regarding prophylactic bilateral mastectomy 13 . Although mammographic density is a risk factor for BC, it is currently not well integrated into risk stratification, and – in particular for BRCA1 PV carriers – might be unsuitable due to lower mammographic density observed in TNBC 14 .

Progesterone Receptor Modulators (PRMs; also called antiprogestins), such as mifepristone, play a pivotal role in reproductive health through their effects on the cervix and uterus 58 . PRMs have also demonstrated effects on breast epithelial cell proliferation 20 , 59 – 61 and show BC preventive effects in mice 61 , 62 . PRMs are currently in use for emergency- 63 or on-demand contraception and are well-established for use in medical abortion 64 – 66 . Their efficacy for treating symptoms related to uterine fibroids 67 and other reproductive health conditions 68 was demonstrated many years ago. When administered at appropriate dosages and suitable delivery methods, PRMs like mifepristone offer a woman-centred contraceptive option with additional health benefits, supporting autonomy, while offering added non-contraceptive health benefits. Based on existing data, long-term treatment with mifepristone has found to be safe 69 , 70 and is undergoing further exploration 71 . One particular aspect of safety relates to the endometrium. PRMs have previously been observed to impact the endometrium but effects differ across PRM type, dose, timing of administration within the menstrual cycle, duration, and possibly also ethnicity 72 . Most of the evidence has been generated for mifepristone with less evidence for the other PRMs such as ulipristal acetate (UPA; now discontinued as a treatment for fibroids) and others that were under clinical development. Follicular phase administration has little effect on the endometrium, but depending on the dose used ovulation may be delayed or inhibited. Continuous administration of PRMs in low doses that do not affect ovulation can induce endometrial effects that are potentially contraceptive 73 . Prolonged use of doses that inhibit ovulation has led to concerns about endometrial hyperplasia due to unopposed estrogen. However, the endometrial changes that occur are a distinct histological entity known as PR modulator-associated endometrial changes (PAEC) 74 . Specifically, PAEC reveals an antiproliferative state and the endometrial glandular epithelium shows reduced mitotic activity compared to the proliferative phase although extensive cystic glandular dilatation is seen. Evidence indicates that PAEC regresses upon cessation of treatment 75 . Recent evidence demonstrates that three months of mifepristone exposure does not induce molecular changes associated with endometrial carcinogenesis 76 . A summary of current indications and potential side effects of mifepristone are displayed in Table 1 . Table 1 Side effects of mifepristone in studies for a range of indications Indication Treatment dose, duration Potential adverse effects Endometrium Liver function Refs Contraception 2–200 mg daily/weekly/ monthly, up to 12 months Nausea, breast distention, pelvic/abdominal pain, bloating and headache, have been reported (at doses of 10–50 mg weekly). PAEC (5 mg daily use for 6 months) 126 . Isolated reports of elevated transaminases 127 , 128 , returned to normal within two months. 126 – 135 Emergency contraception Single dose 10–50 mg Nausea, vomiting. N/A 58 , 136 Uterine fibroiids 2.5–50 mg daily or every other day, for 3–12 months Headache, nausea, vomiting, mood swings, diarrhea, decreased libido, weakness, fatigue, hot flashes, nervousness. Endometrial hyperplasia, PAEC (dilated cystic glandular formations). No atypia or malignancy. Isolated reports of elevated transaminases. 137 Endometriosis 5–100 mg daily for 3–6 months Nausea, vomiting, fatigue, hot flashes. PAEC. Isolated reports of elevated transaminases. 137 Obstetrics: Cervical ripening at term, induction of labor, delivery of still birth Single dose 50–200 mg (recommended dose) Hypoglycemia in newborn. N/A 138 , 139 Induced abortion, management of missed miscarriage, cervical pretreatment before surgical abortion Single dose 200–600 mg (recommended dose) Urticarial skin rash, self-resolving. N/A 140 – 143 Meningioma 200 mg daily for prolonged use (recommended dose) Fatigue, hot flashes, breast tenderness, nausea, mood swings, rash, partial alopecia, edema and biochemical hypothyroidism. Endometrial hyperplasia (no atypia). Not affected (evaluated in few studies). 144 Cushing syndrome 300–1200 mg daily for prolonged use (recommended dose) Hyper/hypokalemia, hyper/hypotension, hyponatremia, low cortisol levels, mineralocorticoid deficiency, maculopapular rash (in patients with hypercortisolism), edema, hyper/hypoglycemia, nausea/vomiting, headache, fatigue, secondary hypothyroidism. Endometrial hyperplasia (no atypia). Three reported cases of drug-induced liver injury (cholestatic liver injury) in patients with Cushing 145 . 137 , 144 PAEC PR modulator-associated endometrial changes; N/A not available Accumulating evidence suggests that PRMs can prevent breast cancers, including poor prognostic TNBCs. Side effects of mifepristone in studies for a range of indications Single dose 50–200 mg (recommended dose) 300–1200 mg daily for prolonged use (recommended dose) PAEC PR modulator-associated endometrial changes; N/A not available Accumulating evidence suggests that PRMs can prevent breast cancers, including poor prognostic TNBCs. Although UPA is also highly effective 60 , 77 , its association with liver injuries 78 precludes its use for BC risk reduction. A downstream effector of progesterone – RANKL – can be targeted with antibodies such as denosumab (Amgen), which is in clinical use to treat bone loss 79 . Unlike mifepristone 68 , denosumab is not known to be lipophilic and, hence, would not be enriched in the fat-containing breast tissue. Targeting RANKL has been shown to prevent mammary gland cancer in mice 22 , 80 but has no effect on contralateral breast cancer risk in postmenopausal BC patients 81 . The BRCA-P study is currently evaluating the BC risk reducing effect of denosumab in women carrying a germline BRCA1 pathogenic variant (EudraCT 2017-002505-35 and NCT04711109 ). The acceptance rate is, however, considerably impaired by potential participants’ wishes to become pregnant and the fear of side effects. Compared with denosumab, mifepristone has several features that may make it considerably more acceptable to young BRCA1 PV carriers in the context of a prevention trial. First, mifepristone is taken orally at home and does not require clinic-based injections, conferring a meaningful practical advantage over denosumab (given as a subcutaneous injection every six months) that is likely to improve both recruitment and long-term adherence. Second, because low-dose mifepristone has well-documented contraceptive efficacy, it could simultaneously serve as the required contraception during the trial period - a prerequisite in any prevention trial involving reproductive-age women - thereby reducing the perceived burden of participation compared to alternative prevention drugs, which necessitates separate contraception. Third, unlike denosumab, mifepristone is anticipated to reduce the risk of both breast cancer and ovarian cancer in BRCA1 PV carriers, given accumulating evidence that elevated or heightened progesterone responsiveness contributes to ovarian cancer risk in this population 33 – 35 . The prospect of a single oral agent that may simultaneously lower risk across both organs is a compelling patient-relevant argument that denosumab cannot offer. Fourth, short-term hormonal modulation with mifepristone has recently been shown not to induce oncogenic endometrial changes in BRCA1/2 PV carriers 76 , providing direct reassurance about safety in this specific population.