Abstract OT1-01-07: PIONEER- Pre-operative wIndOw study of letrozole plus PR agonist megestrol acetate versus letrozole aloNE in post-menopausal patients with ER-positive breast cancer

Background: Published preclinical findings provided new insights into the functional 'cross-talk' between the oestrogen receptor (ER) and the progesterone receptor (PR) in breast cancer (BC) (Mohammed et al., Nature, 2015). Addition of a PR agonist to anti-oestrogens directly modifies ERa chromatin binding and the transcriptional response in breast cancer cells, and is anti-proliferative in vitro and in vivo. Megestrol Acetate (MA), an...