Introduction: Alpelisib is approved in combination with endocrine therapy (ET) to treat patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2−) progressive metastatic breast cancer (MBC). The SOLAR-1 trial demonstrated the efficacy of this oral agent and showed that, while alpelisib improves outcomes compared to placebo, it is also associated with clinically relevant adverse events (AEs). There is a pressing need for improved knowledge on the effectiveness and tolerability of this agent in real-world patient populations. Methods: We conducted a retrospective cohort study of patients with HR+, HER2− MBC treated with alpelisib and ET. We assessed the safety, tolerability, and effectiveness of alpelisib in a real-world population. Deidentified patient-, tumor-, and outcome-related data, including AEs, were collected and summarized. Kaplan-Meier methods were applied for survival analyses, and stratified analyses of interest were conducted. A p value <0.05 was considered statistically significant. Results: A total of 76 women treated with alpelisib + ET were included in our cohort. Most had been previously treated with cyclin-dependent kinase (CDK) 4/6 inhibitors and chemotherapy for MBC. The estimated median progression-free survival was 5.2 months (95% CI, 4.1–8.0). The median overall survival was longer among patients without prior everolimus therapy (hazard ratio, 4.28 [95% CI, 1.64–11.16]; p = 0.0012), and no significant outcome differences were observed between patients treated with different starting doses of alpelisib. Approximately 31.6% of patients permanently discontinued alpelisib due to AEs, and 32.9% had at least one dose reduction. The most common grade 3/4 AEs were hyperglycemia (21%), fatigue (13.2%), and diarrhea (10.5%). Conclusions: For progressive HR+, HER2− MBC, alpelisib + ET showed effectiveness in a real-world patient population that was comparable to published clinical trial data, regardless of starting dose. However, the effectiveness of alpelisib following previous everolimus exposure may be limited and, hence, should be a consideration to decide sequencing of therapy in these patients. Patients treated with alpelisib are at risk for clinically relevant AEs and require close monitoring.

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