Background: Abemaciclib is an orally administered, selective small molecule cyclin-dependent kinase (CDK)4 and 6 inhibitor, approved to treat HR+, HER2- MBC patients (pts) on a continuous twice daily dosing schedule as monotherapy or in combination with an aromatase inhibitor as initial endocrine based therapy or in combination with fulvestrant. Abemaciclib monotherapy increased tumor immunogenicity and synergized with anti-PD-1 to boost antitumor efficacy in murine models. Here we report safety and antitumor activity of abemaciclib plus pembrolizumab in HR+, HER2- MBC pts. Methods: This multicenter, nonrandomized, open-label, multi-cohort phase Ib study of abemaciclib plus pembrolizumab enrolled a cohort of endocrine resistant HR+, HER2- MBC pts who had received 1 or 2 prior chemotherapy regimens for MBC. No prior CDK4/6 inhibitor was allowed. Patients received 150mg abemaciclib orally every 12 hours plus pembrolizumab 200mg IV on day 1 every 21 days. Primary objective was to characterize safety of the abemaciclib plus pembrolizumab combination. Secondary objectives included objective response rate (ORR), progression-free survival (PFS), and overall survival (OS). Results: Of 28 pts enrolled, 15 (54%) received 1 line and 10 (36%) 2 lines of prior systemic chemotherapy in the locally advanced/metastatic setting. Safety of the combination was generally consistent with known side effects of abemaciclib and pembrolizumab and was generally manageable. Grade 3/4 adverse events in >2 pts included neutropenia (8 pts/29%), AST increase (5 pts/18%), diarrhea, and ALT increase (3 pts/11% each). Eight pts had confirmed partial response (29% ORR), and disease control rate (complete response [CR]+partial response [PR]+stable disease [SD]) was 82%. Clinical benefit rate (CR+PR+SD persisting for ≥6 months) was 46%. Median PFS and OS were 8.9 months (95% CI 3.9, 11.1) and 26.3 months (95% CI 20.0, 31.0), respectively. Conclusions: Combination of abemaciclib plus pembrolizumab demonstrated a generally tolerable safety profile with numerically higher rate of transaminase elevations than reported for the individual treatments. Compared to historical data for abemaciclib monotherapy in a similar pt population, a numerically higher but not obviously different ORR, PFS, and OS was observed. Clinical trial information: NCT02779751.