Efficacy and Safety of Oral Progestogens (Megestrol Acetate and Medroxyprogesterone Acetate) in Heavily Pretreated Oestrogen Receptor-Positive Metastatic Breast Cancer: A 10-Year Multi-Site Study

Background: Oral progestogens, including megestrol acetate (MA) and medroxyprogesterone acetate (MPA), have largely been superseded by aromatase inhibitors, tamoxifen, and selective oestrogen receptor degraders (SERDs) in oestrogen receptor-positive (ER-positive) metastatic breast cancer. However, they remain an option as late-line therapy after failure of standard treatments. Contemporary data are limited, particularly in patients previously treated with CDK4/6 inhibitors. Methods: We conducted a multi-site retrospective analysis of patients with ER-positive metastatic breast cancer treated with MA or MPA between 2014 and 2024 at four hospital sites across London, United Kingdom. Patients were identified using pharmacy dispensing records. Progression-free survival (PFS) and overall survival (OS) were estimated using the Kaplan–Meier method and Cox regression. Subgroup analyses included prior CDK4/6 inhibitor exposure, histology and liver metastases. Results: A total of 116 patients were included. Median PFS was 2.4 months (95% CI 2.2–2.9), and median OS was 3.3 months (95% CI 2.7–4.9). Prior CDK4/6 inhibitor exposure was associated with shorter PFS (1.9 vs. 2.8 months; HR 1.59; 95% CI 1.08–2.35, p = 0.019) and a trend toward shorter OS (3.1 vs. 3.6 months; HR 1.18, 95% CI 0.80–1.75, p = 0.41). Similarly, liver metastases were associated with shorter PFS (2.3 vs. 2.8 months; HR 1.78, 95% CI 1.12–2.85, p = 0.015), with a trend toward worse OS (3.1 vs. 4.9 months; HR 1.45, 95% CI 0.93–2.25, p = 0.103). A subset of patients derived prolonged benefit, with a 6-month PFS rate of 16%. Toxicity was manageable; thromboembolic events and oedema occurred in 9% and 11% of patients respectively. Appetite improvement was reported in 10%. Conclusions: MA and MPA demonstrated modest but clinically relevant late-line activity in heavily pretreated, endocrine-refractory ER-positive metastatic breast cancer. While prior exposure to CDK4/6 inhibitors was associated with shorter PFS, patients without liver metastases appeared to derive the greatest benefit. These findings support a role for oral progestogens in selected patients who have exhausted standard therapeutic options.