Evaluation of First-Line Endocrine Therapy for HR+/HER2- Metastatic Breast Cancer: Efficacy, Tolerance and Prescribing Practices. A Prospective and Observational study

Introduction: Endocrine therapy is commonly employed for MBC (Metastatic Breast Cancer) patients due to its efficacy in modulating hormone receptors and its generally favorable side effect profile. This study aims to evaluate the effectiveness and tolerability of first-line endocrine therapy in HR+(Hormone receptor)/HER2-( Human epithelial growth factor receptor) metastatic breast cancer patients, to investigate differences among various endocrine therapies, and examine changes in prescribing practices influenced by evolving clinical guidelines. Methods: We conducted a multicenter, observational, prospective study involving 100 women aged over 18 years with de novo HR+, HER2- MBC. Data were collected between 2020 and 2023. The primary objective was to assess the ORR (Objective Response Rate) at 6 months under first-line endocrine therapy. Secondary objectives included evaluating the influence of types of endocrine therapy on the ORR, the evolution of the response at 2 years, clinical efficacy, and tolerability. Results: At 6 months, the ORR was 38%, with 3 complete responses (CR). No significant difference in ORR was observed between aromatase inhibitors and Faslodex (p=0.439). The clinical benefit rate was 80%. 84% of patients reporting mild to moderate adverse effects. Discussion: A first-line endocrine therapy for HR+/HER2- MBC offers substantial clinical benefit rate with an 80%, and manageable side effects. Despite a lower ORR, the therapy effectively controls disease progression and preserves quality of life. The results support the continued use of endocrine therapy while emphasizing the importance of managing adverse effects. Initial hesitations among practitioners were due to concerns about its efficacy. Conclusion: Endocrine therapy provides significant clinical benefits and maintains patient quality of life.