Abstract P1-10-01: Results from the breast cancer - anti progestin prevention study 1 (BC-APPS1) trial - a novel approach in breast cancer prevention

Abstract Background The luminal progenitor (LP) population in the normal breast is under the control of paracrine progesterone signalling and likely represents the cell of origin of estrogen receptor negative (ER-) BC. Exogenous progestins, as contraception or menopausal hormone replacement therapy (HRT), increase the risk of ER- and ER+ BC. We sought to examine the effect antagonism of progesterone signalling on the tissue composition and cellular hierarchy of normal human breast and thus the potential for progesterone receptor antagonism in breast cancer prevention. Methods BC-APPS1 is a single arm phase 2 pilot study in which premenopausal women at increased familial BC risk underwent vacuum assisted biopsy (VAB) in the luteal phase of the menstrual cycle prior to commencing a 12 week course of the selective progesterone receptor modulator (SPRM) ulipristal acetate (UA; 5mg daily). VAB was repeated in the final week of therapy. The primary endpoint of BC-APPS1 was change in epithelial Ki67 assessed by immunohistochemistry. Secondary endpoints included toxicity and additional tissue endpoints including immunofluorescence (IF) staining of LP markers, 2D mixed luminal/basal and 3D (mammosphere) colony formation assays, LP fraction by FACS (CD49f+/EPCAM+ cells), single cell and bulk RNAseq, epithelium/stromal laser capture microdissection-based proteomics and tissue stiffness analyses. Baseline samples were compared to a historic cohort of normal risk (n=25) and high risk (n=41) samples for IF analyses. Results Between 03/2016 and 03/2019 26 women were recruited to BC-APPS1 and 24 underwent paired biopsies. The trial met its primary endpoint with a significant reduction in median Ki67 between baseline (4.89%; IQR 4.36-10.42) and 12 weeks (2.41%; IQR 1.57-3.24; p<0.0001). Study procedures were well tolerated with minimal drug toxicity (all G1/2). 2 women discontinued UA due to anxiety: 1 drug induced and 1 induced following development of a small haematoma following VAB1. FACS analysis (n=17) demonstrated significant reduction in the LP proportion with UA treatment (median baseline 44.7% IQR 28.1-55.2 and 12 weeks 25.4% IQR 17.2-37.8; p<0.01). Breast tissue from high-risk women had increased expression of PR+ (10.8% vs 4.5%; p<0.01) and dual Sox9+Ki67+ cells (4.8% vs 0.8%, p<0.01). The Sox9+Ki67+ population reduced significantly with UA therapy in BC-APPS1 (4.4% vs 1.3%, p<0.05). In functional analyses both mammosphere and mixed luminal/basal colony formation reduced with UA treatment and single cell RNAseq (n=8 pairs) and epithelium/stromal laser capture microdissection-based proteomics (n=5 pairs) identified stromal components and remodelling as key pathways perturbed by UA treatment. Tissue stiffness, assessed by atomic force microscopy and previously shown to be positively associated with %mammographic density, was significantly reduced with UA treatment. Conclusions: High risk women have increased surrogate markers of BC risk including proliferating luminal progenitor cells which can be reduced by short term SPRM treatment with UA. Treatment is generally well tolerated and SPRM therapy is an attractive candidate for BC prevention. Longer term studies are warranted and stromal remodelling and breast tissue stiffness data suggest that mammographic density should be investigated as a potential surrogate biomarker of activity. Citation Format: Sacha Howell, Alice Greenhalgh, Robert Pedley, Suad Alghamdi, Amanda Caruso, Mujtaba Ansari, Tiago Moreira, Sue Astlet, Anthony Maxwell, Yit Lim, Hayley Brookes, Faiza Idries, Anthony Howell, D Gareth Evans, Michael Sherratt, Andrew Gilmore, Elaine Harkness, Walid Khaled, Alecia-Jane Twigger, Matt Roberts, Robert Clarke, Bruno Simoes. Results from the breast cancer - anti progestin prevention study 1 (BC-APPS1) trial - a novel approach in breast cancer prevention [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-10-01.