Abstract
BACKGROUND: Combination therapy with cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors and hormonal therapy as the first-line and second-line treatments has already been shown to be effective in patients with hormone receptor-positive (HR+) human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC) in clinical trials. On the other hand, in clinical practice, CDK4/6 inhibitors are used not only as first-/second-line but also as later-line hormonal therapies, or for patients receiving prior chemotherapy in metastatic setting. However, the efficacy and safety of combination therapy in these patients remain unclear. In this study, we evaluate the clinical efficacy and safety of combination therapy with abemaciclib and hormonal therapy for chemotherapy-treated patients with HR+ HER2- MBC. METHODS: This multi-institutional prospective cohort study will involve a total of 300 chemotherapy-treated patients with HR+ HER2- MBC. The primary endpoint is progression-free survival (PFS). Secondary endpoints include overall survival, time to treatment failure, response rate, clinical benefit rate, and adverse events. The preplanned subpopulation analysis is the number of chemotherapy regimens for HR+ HER2- MBC (two or less vs. three or more), prior treatment history with CDK4/6 inhibitors other than abemaciclib (presence vs. absence) and menopausal status (pre vs. post). We also planned to determine PFS of the subpopulation treated with abemaciclib as maintenance therapy after chemotherapy. DISCUSSION: In this multi-institutional prospective cohort study, we evaluate the clinical efficacy and safety of combination therapy with abemaciclib and hormonal therapy for chemotherapy-treated patients with HR+ HER2- MBC. We also evaluate this combination therapy as maintenance therapy in patients who respond to early-line chemotherapy.