This Phase II trial, sponsored by St Vincent's Hospital, investigates the efficacy of pre-operative endocrine therapy with and without Prometrium (progesterone) in postmenopausal women with early-stage HR+ HER2- breast cancer. The study aims to explore potential additive effects of progesterone on standard endocrine therapies (letrozole and tamoxifen). Given the significant market for breast cancer therapies, particularly in hormone receptor-positive subtypes, successful outcomes could enhance treatment options and market share for involved therapies. The trial's findings may also influence future treatment guidelines and positioning of Prometrium in combination therapies, potentially impacting competitive dynamics in the oncology space.