Background: CSCC is the second most common skin cancer with an estimated incidence of 1 million cases per year in the US. While the surgical cure rate for CSCC is > 95%, some pts have high risk of recurrence as assessed by immune status, primary disease stage, extent of nodal involvement, presence of extracapsular extension (ECE), and prior treatment. Postoperative RT is recommended for these pts but relapse with locoregional recurrence or distant metastases may still occur. C-POST is evaluating the efficacy of cemiplimab as adjuvant therapy for pts with high-risk CSCC. Here, we provide summary of the most recent study protocol amendment. Methods: C-POST is a randomized, placebo-controlled, double-blind, multicenter Phase 3 study to evaluate cemiplimab as adjuvant treatment for pts with high-risk CSCC, based on surgical and clinicopathologic findings, who completed surgery and postoperative RT (minimum total dose 50Gy, within 10 weeks before randomization) (NCT03969004). Pts with at least one of the following high-risk features are eligible: (1) nodal disease with (a) ECE and at least one node ≥20 mm or (b) at least three lymph nodes positive on surgical pathology report, regardless of ECE; (2) in-transit metastases; (3) T4 lesion; (4) perineural invasion; and (5) recurrent CSCC with at least one other risk factor. Pts with CSCC involvement in at least three lymph nodes (feature 1b) were added to the eligibility criteria, as this group was found to be at similar risk of CSCC recurrence as the initially planned study population. Protocol amendment now allows patients with chronic lymphocytic leukemia (CLL) who are not on active treatment to be enrolled. The study has two parts. In Part 1 (blinded), pts are randomly assigned 1:1 to receive cemiplimab 350 mg or placebo intravenously every 3 weeks for 12 weeks, followed by cemiplimab 700 mg or placebo every 6 weeks for 36 weeks. In optional Part 2 (unblinded), pts in the placebo arm who experience disease recurrence and pts in the cemiplimab arm who experience disease recurrence ≥3 months after completion of 48-week treatment in Part 1 are eligible to receive open-label cemiplimab for up to 96 weeks. The trial is expected to enrol 412 pts from about 100 sites in North and South America, Europe, and Asia-Pacific regions. Key primary objective is to compare disease-free survival; secondary objectives include evaluating overall survival, freedom from locoregional relapse, and distant relapse with adjuvant cemiplimab versus placebo in patients with high-risk CSCC. This study is once again open for enrolment following interruptions owing to the COVID-19 pandemic. Clinical trial information: NCT03969004.