Efficacy and Safety of Cyclin-Dependent Kinases 4 and 6 Inhibitors in HR+/HER2-Advanced Breast Cancer

Purpose: To assess the efficacy and safety of cyclin-dependent kinases 4 and 6 inhibitors (CDKi) combined with endocrine therapy (ET) in women with hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer (ABC) and compare the efficacy of different CDKi (palbociclib, ribociclib, or abemaciclib). Materials and Methods: This study based on randomized Phase 2 or 3 trials of CDKi plus ET compared with placebo plus ET for women with HR+/HER2-ABC and identify relevant randomized clinical trials (RCTs) published prior to February 2020. The primary endpoint was progression-free survival (PFS), the secondary endpoints included overall survival (OS), objective response rate (ORR), clinical benefit response (CBR) and safety. The PROSPERO registry number is 42018081105. Results: The results from eight trials including 4580 participants were pooled. Evidence indicated that the PFS of CDKi group was significantly prolonged (hazard ratio [HR] 0.55, 95% confidence interval [CI] 0.50-0.60, P < 0.01) compared with placebo group. The ORR and CBR were better (risk ratio [RR] 1.47, 95% CI 1.30-1.67, P < 0.01; 1.24, 95% CI 1.15-1.35, P < 0.01) in the CDKi group. The OS of CDKi group (HR 0.75, 95% CI 0.67-0.85, P < 0.01) was significantly longer than ET alone. Subgroup analyses confirmed that the benefit was consistent across most subgroups. Subgroup analyses showed no statistically significant difference of PFS among three CDKi: palbociclib vs ribociclib (HR 0.55, 95% CI 0.49-0.60, P = 0.34), palbociclib vs abemaciclib (HR 0.53, 95% CI, 0.47-0.59, P = 0.61), and ribociclib vs abemaciclib (HR 0.56, 95% CI, 0.51-0.62, P = 0.72). Treatment-related grade 3 or 4 hematologic adverse events (AEs) were more frequently in CDKi group. Conclusion: CDKi combined with ET can significantly prolong PFS and improve the ORR, CBR and OS in patients with HR+/HER2- ABC. However, the advantage of different CDKi has not been established.