Bruno M. Simões, Robert Pedley, Curtis W. McCloskey, Matthew Roberts, Austin D. Reed, Alecia‐Jane Twigger, Pirashaanthy Tharmapalan, Amanda Caruso, S. Cabral, A. Wilby, Hannah Harrison, Yuxi Zhou, Alice Greenhalgh, Suad A. Alghamdi, Martina Forestiero, Jesica Lopez-Muñoz, Jasmin Roche, Ren Jie Tuieng, M. Aurangzeb Khan, Steven Squires, Susan Astley, Elaine F. Harkness, Angélica Santiago-Gómez, Katherine Spence, Jessica Ritchie, Susan Pritchard, Yit Yoong Lim, Michael J. Sherratt, Sebastiano Andò, Anthony Howell, D. Gareth Evans, Andrew Gilmore, Walid T. Khaled, Rama Khokha, Robert B. Clarke, Sacha J. Howell
Abstract Breast cancer is the leading cause of cancer-related death in women worldwide 1 . Here, in the Breast Cancer-Anti-Progestin Prevention Study 1 (BC-APPS1; NCT02408770 ), we assessed whether progesterone receptor antagonism with ulipristal acetate for 12 weeks reduces surrogate markers of breast cancer risk in 24 premenopausal women. We used multilayered OMICs and live-cell approaches as readouts for molecular features alongside clinical imaging and tissue micromechanics correlates. Ulipristal acetate reduced epithelial proliferation (Ki67) and the proportion, proliferation and colony formation capacity of luminal progenitor cells, the putative cell of origin of aggressive breast cancers 2 . MRI scans showed reduction in fibroglandular volume with treatment, whereas single-cell RNA sequencing, proteomics, histology and atomic force microscopy identified extracellular matrix remodelling with reduced collagen organization and tissue stiffness. Collagen VI was the most significantly downregulated protein after ulipristal acetate treatment, and we uncovered an unanticipated spatial association between collagen VI and SOX9 high luminal progenitor cell localization, establishing a link between collagen organization and luminal progenitor activity. Culture of primary human breast epithelial cells in a stiff environment increased luminal progenitor activity, which was antagonized by anti-progestin therapy, strengthening this mechanistic link. This study offers a template for biologically informed early-phase therapeutic cancer prevention trials and demonstrates the potential for premenopausal breast cancer prevention with progesterone receptor antagonists through stromal remodelling and luminal progenitor suppression.