Background: Abemaciclib is an orally administered, selective small molecule cyclin-dependent kinase 4 and 6 inhibitor. In preclinical models, abemaciclib induced intratumor immune inflammation and synergized with PD-1 blockade to enhance antitumor efficacy in anti-PD-L1 refractory disease. Here, we report the safety and antitumor activity of abemaciclib plus the approved NSCLC treatment pembrolizumab in 2 cohorts for pts with nonsquamous and squamous NSCLC. Methods: Eligible pts for this nonrandomized, open-label, multicohort, phase 1b study were either chemotherapy-naive with ≥ 1% tumor cell (TC) PD-L1 staining, KRAS-mut nonsquamous NSCLC (Cohort A) or had a squamous subtype and received ≤ 1 prior platinum-containing chemotherapy regimen (Cohort B) for metastatic NSCLC. Primary endpoint was safety; secondary objectives included objective response rate (ORR), progression-free survival (PFS), and overall survival (OS). Results: Twenty-five pts with NSCLC were enrolled in each cohort. Most pts (68%) in Cohort B had received 1 prior line of chemotherapy. Safety profiles observed in both cohorts were largely consistent with previous reports for abemaciclib and pembrolizumab monotherapy. Grades 3/4 AEs in Cohorts A and B, respectively, included ALT increase (6 pts [24%]/ 0 pts), diarrhea (3 pts [12%]/ 0 pts), neutropenia (3 pts [12%]/ 0 pts), and pneumonitis (3 pts [12%]/ 1 pt [4%]). Six pts in Cohort A (24%) and 2 pts in Cohort B (8%) had a confirmed partial response for a disease control rate (CR+PR+SD) of 52% and 64%, respectively. In Cohort A, the ORR in pts with strong (≥50% TC) PD-L1 staining (n = 13) was 31% vs. 17% in pts with weak (1-49% TC) PD-L1 expression (n = 12). Median PFS and OS were 7.6 months (95% CI: 1.6, NR) and 22.0 months (95% CI: 9.9, NR) in Cohort A and 3.3 months (95% CI: 1.4, 5.2) and 6.0 months (95% CI: 3.7, 13.1) in Cohort B, respectively. Conclusions: Abemaciclib plus pembrolizumab resulted in a numerical higher rate of transaminase elevations and pneumonitis. Antitumor activity was remarkable in the KRAS-mut nonsquamous NSCLC but not noticeably higher as compared to historical data for pembrolizumab monotherapy. Clinical trial information: NCT02779751.