LAUSR

Background: Cyclin-dependent kinase (CDK) 4/6 inhibitors have been advocated for adjuvant therapy of metastatic hormone receptor (HR)+/human epidermal growth factor receptor 2 (HER2)− breast cancer (BC). However, the efficiency of adding CDK 4/6 inhibitors to neoadjuvant therapy was not unequivocal. Objective: The aim of the study was to evaluate the efficiency and toxicity of neoadjuvant CDK 4/6 inhibitors + endocrine therapy (ET) versus neoadjuvant endocrine monotherapy or standard neoadjuvant chemotherapy in HR+/HER2− BC. Method: We searched PubMed, the Cochrane Library, Web of Science, and Embase online databases for randomized controlled trials and single-arm studies written in English until April 2021. Results: Five studies comparing CDK 4/6 inhibitors + ET as neoadjuvant treatments to ET alone and 2 studies comparing neoadjuvant CDK 4/6 inhibitors + ET to neoadjuvant chemotherapy were analysed. Neoadjuvant CDK 4/6 inhibitors + ET improved the rate of complete cell cycle arrest (CCCA: central Ki67 < 2.7%, odds ratio [OR] = 7.91, 95% confidence interval [CI] = 4.81–13.03, p < 0.001), increased the risk of adverse events (AEs; especially ≥3 AEs; AEs of all grades: OR = 9.10, 95% CI = 2.39–34.58, p = 0.001; AEs ≥3: OR = 12.24, 95% CI = 4.17–35.88, p < 0.001), led to no significant differences in pathological complete response (pCR) in patients with BC (OR = 0.34, 95% CI = 0.04–2.85, p = 0.318) compared to endocrine monotherapy. Moreover, subgroup analysis showed that the 3 types of CDK 4/6 inhibitors all improved the rate of CCCA (ribociclib: OR = 10.31, 95% CI = 3.59–29.61, p < 0.001; palbociclib: OR = 7.39, 95% CI = 1.26–43.40, p = 0.027, and abemaciclib: OR = 8.28, 95% CI = 3.41–20.11, p < 0.001). Compared to neoadjuvant chemotherapy, neoadjuvant CDK 4/6 inhibitors plus ET decreased the risk of AEs ≥3 (OR = 0.50, 95% CI = 0.29–0.87, p = 0.015) and showed similar ability to reach pCR (OR = 0.50, 95% CI = 0.12–2.07, p = 0.342) and reduce the residual cancer burden (RCB, RCB 0–1: OR = 0.47, 95% CI = 0.18–1.22, p = 0.121; RCB 2–3: OR = 2.30, 95% CI = 0.89–5.91, p = 0.084). Conclusions: The results suggested that combination therapy had increased efficacy and toxicity compared to endocrine monotherapy and showed similar efficacy to and better safety than neoadjuvant chemotherapy.