Background: For patients (pts) with ASCC, second-line or later treatment options have been limited. Pembrolizumab (pembro), an anti-PD-1 monoclonal antibody, has demonstrated antitumor activity in several tumor types (including ASCC) in the multicohort phase 1b KEYNOTE-028 study. KEYNOTE-158 (NCT02628067) is an open-label, phase 2, multicohort study that evaluates antitumor activity and safety of pembro in pts with previously treated advanced cancer. Results from the ASCC cohort are presented. Methods: Eligible pts were ≥18 y with histologically/cytologically documented metastatic and/or unresectable ASCC with prior treatment failure on or intolerance to standard first-line therapy, measurable disease per RECIST v1.1, ECOG PS of ≤1, and evaluable tissue sample for PD-L1 and biomarker analysis. PD-L1 expression was assessed by the PD-L1 IHC 22C3 pharmDx assay (Agilent Technologies). Pts received pembro 200 mg Q3W until disease progression, unacceptable AE, or completion of 35 cycles. The primary endpoint was ORR per RECIST v1.1 (assessed every 9 wk for 12 mo, then every 12 wk thereafter) by independent central review. Secondary endpoints were DOR, OS, PFS and safety. Results: 112 pts with ASCC were enrolled (81.3% women; median age, 61 y [range 32–79]; ≥2 prior therapies, 73.2%). At database cutoff (Dec 6, 2018) 10 pts (8.9%) had completed 35 cycles and 102 discontinued; median follow-up was 12.0 mo (range, 0.8–33.0) Five pts had CR and 8 had PR; ORR was 11.6% (95% CI, 6.3–19.0). Median DOR was not reached (range, 6.0+ to 29.1+ mo). Responses occurred in 11/75 pts (14.7%) with PD-L1 combined positive score (CPS) ≥1 and 2/30 pts (6.7%) with PD-L1 CPS < 1. Among all pts, median OS was 12.0 mo (95% CI, 9.1–15.4), and median PFS was 2.0 mo (95% CI, 2.0–2.1). 68 (60.7%) pts had treatment-related AEs, including 21 (18.8%) who had grade 3–5 events; there were no treatment-related deaths. 4 pts (3.6%) discontinued due to treatment-related AEs. 27 pts (24.1%) had immune-mediated AEs/infusion reactions. Conclusions: Pembro demonstrated antitumor activity and manageable toxicity in pts with heavily pretreated advanced ASCC, regardless of PD-L1 status. Clinical trial information: NCT02628067