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9537 Background: Programmed death receptor-1 (PD-1)–blocking antibodies are approved as monotherapy treatment for patients (pts) with metastatic or locally advanced cutaneous squamous cell carcinoma (CSCC) who are not candidates for curative surgery or radiation. Cosibelimab is a high-affinity, fully human programmed death ligand-1 (PD-L1)–blocking antibody with a functional Fc domain capable of inducing antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) against tumor cells. Study CK-301-101 (NCT03212404) is a global, multicenter, multicohort, pivotal trial that enrolled pts with select advanced cancers for treatment with cosibelimab. Here we present the primary analysis of the registration-enabling expansion cohort in pts with metastatic CSCC. Methods: Adult pts with Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 who had metastatic (nodal and/or distant) CSCC not amenable to local therapy were eligible to participate. Cosibelimab was administered as a fixed dose of 800 mg every 2 weeks (Q2W) intravenously. The primary endpoint was confirmed objective response rate (ORR; complete response [CR] + partial response [PR]) assessed by independent central review according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 and the key secondary endpoint was duration of response. Results: Seventy-eight pts with metastatic CSCC were treated with cosibelimab and comprise the efficacy and safety populations (59M/19F; median age: 71 years). The confirmed ORR was 47.4% (95% CI: 36.0, 59.1; 6 CRs and 31 PRs) and the median duration of response was not reached at the time of data cutoff (median duration of follow-up: 15.2 months), with 76% of responses ongoing (range: 1.4-31.8+ months). The Kaplan–Meier estimated probability of maintaining a response at 6 and 24 months was 88.1% and 72.5%, respectively. Treatment-related adverse events (TRAEs) were reported in 54 pts (69.2%); 7 pts (9.0%) experienced at least 1 grade 3 TRAE (no grade 4 or grade 5 TRAEs were reported) with the most common being increased serum lipase in 2 pts. Conclusions: Treatment with cosibelimab monotherapy resulted in a robust ORR with durable responses and demonstrated a predictable and manageable safety profile in pts with metastatic CSCC, supporting its use in the treatment of this cancer. Clinical trial information: NCT03212404.