Real-world effectiveness of aromatase inhibitors and fulvestrant in HR+/HER2- advanced breast cancer: a snapshot of the last two years before conventional use of CDK 4/6 inhibitors in a Portuguese institution.

Background: Monotherapy with aromatase inhibitors and fulvestrant were the standard-of-care for hormone receptor-positive (HR+)/human epidermal growth factor receptor-type2 negative (HER2-) advanced breast cancer, before integration of cyclin-dependent kinase 4/6 inhibitors. Effectiveness data is essential for regulatory action, but little is known about real-world use of aromatase inhibitors and fulvestrant.

Methods: A retrospective cohort study was conducted resorting to data from a cancer registry to identify adult women with HR+/HER- advanced breast cancer exposed to aromatase inhibitors or fulvestrant (31 May 2017-31 March 2019) at the main oncology hospital in Portugal. Cases were updated with follow-up until death or cut-off (31 March 2021) and pseudoanonymized data extracted. Primary outcome was overall survival (OS) and secondary time to treatment failure (TTF), estimated using survival analysis and compared with published trials.

Results: 192 patients were distributed by subgroups according to the medicine. Letrozole: OS 30.8 (95% confidence interval (CI) 20.6-41.4); TTF 11.2 (95%CI 8.7-13.7). Exemestane: OS 22.1 (95%CI 9.7-34.6); TTF 6.0 (95%CI 4.1-7.8). Fulvestrant: OS 21.6 (95%CI 16.5-26.7); TTF 5.6 (95%CI 4.5-6.6).

Conclusions: Estimated effectiveness (OS) of letrozole and fulvestrant was, respectively, 3.2-3.5 months lower than reported. The clinical meaning seems uncertain and may be explained a higher proportion of worse prognostic characteristics in patients treated in the real-world.