Abstract
Background: The combination of CDK4/6 inhibitors and endocrine therapy has greatly improved progression-free survival (PFS) in patients with hormone receptor-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer in many randomized controlled trials (RCTs). However, the key issue was the extent to which the benefit in PFS could translate into a prolongation of OS. Methods: We performed a systematical literature search of PubMed, Web of Science, Cochrane Central Register of Clinical Trials and Embase, as well as meeting online archives up to February 2020. The primary outcome was OS, and we performed indirect treatment comparisons depend on a meta-analysis. Results: Six RCTs were eligible including 3421 breast cancer patients. Compared to the endocrine therapy alone group, adding CDK4/6 inhibitors to endocrine therapy had significantly improved OS (HR=0.76, 95% CI=0.68-0.85, P<0.001). Moreover, the OS advantage was consistent in patients with different combined endocrine therapy, endocrine sensitivity status, sites of distant metastasis, menopausal status and age. Nevertheless, more adverse events were observed in patients treated with CDK4/6 inhibitors. The most common grade 3-4 adverse events were neutropenia (risk ratio [RR]=37.15, 95% CI=15.33-90.04), leucopenia (RR=25.58, 95% CI=13.23-49.46) and anaemia (RR=2.24, 95% CI=1.38-3.85). Conclusions: Our meta-analysis suggested that compared with endocrine therapy alone, the addition of CDK4/6 inhibitors significantly improved OS in patients with hormone receptor-positive, HER2-negative metastatic breast cancer. However, the addition of CDK4/6 inhibitors also increased the incidences of grade 3-4 adverse events.