This nonrandomized, open-label, multi-cohort Phase 1b study (NCT02779751) investigated the safety and efficacy of abemaciclib plus pembrolizumab with/without anastrozole in patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC) without prior CDK4 and 6 inhibitor exposure. Patients were divided into two cohorts: treatment naïve (cohort 1) and pretreated (cohort 2). Patients received abemaciclib plus pembrolizumab with (cohort 1) or without (cohort 2) anastrozole over 21-day cycles. The primary objective was safety, and secondary objectives included efficacy and pharmacokinetics (PK). Cohort 1/2 enrolled 26/28 patients, respectively. Neutropenia (30.8/28.6%), AST increase (34.6/17.9%), ALT increase (42.3/10.7%), and diarrhea (3.8/10.7%) were the most frequent grade ≥3 adverse events in cohort 1/2, respectively. A total of two deaths occurred, which investigators attributed to treatment-related adverse events (AEs), both in cohort 1. Higher rates of all grade and grade ≥3 interstitial lung disease (ILD)/pneumonitis were observed compared to previously reported with abemaciclib and pembrolizumab monotherapy. The PK profiles were consistent between cohorts and with previous monotherapy studies. In cohorts 1/2, the overall response rate and disease control rate were 23.1/28.6% and 84.6/82.1%, respectively. Median progression-free survival and overall survivals were 8.9 (95% CI: 3.9-11.1) and 26.3 months (95% CI: 20.0-31.0) for cohort 2; cohort 1 data are immature. Abemaciclib plus pembrolizumab demonstrated antitumor activity, but high rates of ILD/pneumonitis and severe transaminase elevations occurred with/without anastrozole compared to the previous reporting. Benefit/risk analysis does not support further evaluation of this combination in the treatment of HR+, HER2- MBC.