Background: There is no established standard of care for unresectable locally advanced or metastatic CSCC. UV-induced DNA damage causes hypermutation in most CSCCs. Therefore, these tumors may be responsive to PD-1 checkpoint blockade. In the dose escalation portion of the phase 1 study of cemiplimab (REGN2810), a durable (19 + months) radiologic complete response was observed in a patient (pt) with metastatic CSCC (ASCO 2015, #3024). Methods: ECs enrolled pts with distantly metastatic (EC 7) and locally advanced (EC 8) CSCC. All patients received cemiplimab 3 mg/kg by intravenous infusion over 30 minutes every 2 weeks for up to 48 weeks. Research biopsies were performed at baseline and Day 29 (and at progression, if possible). Tumor measurements were performed every 8 weeks according to RECIST 1.1 to determine overall response rate (ORR). Data cutoff date was 31 Jan 2017. Results: 26 pts were enrolled (10 in EC 7 and 16 in EC 8): median age, 72.5 y (range, 56–88y); median PS 1 (range, 0–1); 21M:5F; median number of prior systemic therapy regimens, 1 (range, 0–2). Median exposure to cemiplimab was 7 doses (range, 1–22). The most common treatment-related adverse event of any grade was fatigue (19.2%). Each of the following ≥ Grade 3 related AEs occurred once: AST elevation, ALT elevation, arthralgia, and rash. ORR (PR + CR, including unconfirmed) and disease control rate (ORR+SD) were 52% (12/23; 4uPR, 5 PR, 2CR, 1 uCR) and 70% (16/23, including 4SD), respectively. Three patients are not yet evaluable. Median PFS and OS have not been reached, and only one patient has experienced PD during cemiplimab treatment after initial response. Correlative studies are in process, including PD-L1 status and whole exome tumor DNA sequencing. Conclusions: Cemiplimab is well tolerated and produces antitumor activity in patients with advanced CSCC. A pivotal trial of cemiplimab for patients with advanced CSCC is enrolling patients. Clinical trial information: NCT02383212