Progression-free survival on endocrine therapy, before or after chemotherapy, in hormone receptor-positive HER2-negative metastatic breast cancer.

Purpose: A major question when treating HR+/HER2- metastatic breast cancer (MBC) is whether early introduction of chemotherapy (CT) increases endocrine resistance. We aimed to describe progression-free survival (PFS) under first endocrine therapy (ET) depending on whether given before or after CT in a large nationwide cohort, in the pre-CDK era.

Methods: The real-life retrospective ESME database includes all patients with MBC whose first-line treatment was initiated between 2008 and 2014 in one of the 18 French Comprehensive Cancer Centres. Our primary objective was to compare PFS from start of first ET in patients with HR+/HER2- MBC who received ET or CT first.

Results: We identified 6293 patients who received at least one ET line during their first two therapeutic lines for MBC. As first-line therapy, 3832 (60.9%) received ET alone (ET1 1st group), whilst 2461 (39.1%) received CT, including 2024 patients (32.2%) with maintenance ET after CT (ET1 after CT group). Median PFS under first ET was 12.4 months (95% CI 11.9-13.1) in ET 1st group vs. 12.6 months in ET1 after CT group (95% CI 12.1-13.4), HR 0.96 (95% CI 0.90-1.01, P = 0.1277).

Conclusions: PFS under first ET appears identical whether prescribed before or after chemotherapy. These data suggest chemotherapy does not promote endocrine resistance.