Background: CSCC is the second most common skin cancer. While the surgical cure rate for CSCC is > 95%, a proportion of pts are considered to have high risk for recurrence as assessed by immune status, primary disease stage, extent of nodal involvement, presence of extracapsular extension, and prior treatment. Post-operative RT is recommended for pts with high-risk features, but relapse with locoregional recurrence or distant metastases may still occur. This study evaluates the efficacy of cemiplimab, a human anti‒PD-1 monoclonal antibody, as an adjuvant therapy for pts with CSCC with high-risk features, after surgery and RT. Methods: This randomized, placebo-controlled, double-blind, multicenter, Phase 3 study will evaluate cemiplimab as an adjuvant treatment for pts with high-risk CSCC, based on surgical and clinicopathologic findings, who have completed surgery and post-operative RT (NCT03969004). Immunocompromised pts were excluded. The trial will enrol 412 pts from about 100 sites in North America, Europe, and Asia-Pacific regions. Pts with at least one of the following high-risk features are eligible: a) nodal disease with extracapsular extension b) in-transit metastases c) T4 lesion d) perineural invasion, and e) recurrent CSCC with at least one other risk factor. In Part 1 (blinded), pts will be randomized 1:1 to receive cemiplimab 350 mg or placebo intravenously every 3 weeks (Q3W) for up to 48 weeks. In optional Part 2 (unblinded), pts in the placebo arm who experience disease recurrence or pts in the cemiplimab arm who experience disease recurrence ≥3 months after completion of 48-week treatment in Part 1 will be eligible to receive open-label cemiplimab 350 mg Q3W for up to 96 weeks. Key objectives are to compare disease-free survival (primary) as well as overall survival, freedom from locoregional relapse, and distant relapse (secondary) of adjuvant cemiplimab vs placebo in pts with high-risk CSCC. This study is currently open for enrollment. Clinical trial information: NCT03969004