Cyclin-dependent kinase 4 and 6 inhibitors in combination with neoadjuvant endocrine therapy in estrogen receptor-positive early breast cancer: a systematic review and meta-analysis

The combination of cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors and endocrine treatment has benefited patients with estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER + /HER2-) metastatic breast cancer; however, its effects in the neoadjuvant setting for ER + /HER2- early breast cancer (EBC) are unclear. Systematic searches were performed in PubMed, Embase, Cochrane Library, and major oncological meetings for trials of CDK4/6 inhibitors plus neoadjuvant endocrine treatment (NET) vs. NET/neoadjuvant chemotherapy (NACT) alone up to January 30, 2021. We assessed the efficacy of CDK4/6 inhibitors plus NET vs. NET/NACT alone in ER + /HER2- EBC. Six studies that included 803 patients treated with CDK4/6 inhibitors plus NET vs. NET/NACT alone were used. Compared with NET/NACT alone, CDK4/6 inhibitors plus NET increased the complete cell cycle arrest (CCCA) rate (OR, 9.00; 95% CI, 5.42-14.96; P < 0.001). Nonsignificant differences between CDK4/6 inhibitors and NET/NACT alone occurred in the preoperative endocrine prognostic index (PEPI)-0 rate (OR, 1.13; 95% CI, 0.59-2.18; P = 0.71), pathological complete response (pCR) rate (OR, 0.75; 95% CI, 0.13-4.29; P = 0.74), objective response rate (ORR) (OR, 0.70; 95% CI, 0.21-2.29; P = 0.55), and disease control rate (DCR) (OR, 1.16; 95% CI, 0.47-2.89; P = 0.74). CDK4/6 inhibitors plus NET indicated a high risk of neutropenia (OR, 56.43; 95% CI, 15.76-202.11; P < 0.001) as an adverse effect (AE) and elevated alanine aminotransferase (ALT) level (OR, 15.30; 95% CI, 2.02-115.98; P = 0.008) as grade 3/4 AEs. Compared with NET/NACT alone, CDK4/6 inhibitors plus NET increased CCCA rate in ER + /HER2- EBC patients. CDK4/6 inhibitors plus NET did not substantially improve the PEPI-0 rate, pCR rate, ORR, or DCR. The combination increased the risk of neutropenia and elevated ALT levels. In the neoadjuvant setting, addition of CDK4/6 inhibitors to NET may be an option for treating ER + /HER2- EBC.