Background: PSqCC is a rare tumor with poor prognosis and limited therapeutic options. The current standard of care for advanced disease has been palliative platinum-based chemotherapy, with only marginal survival benefit. Recent data has shown that the majority of PSqCC patients present high levels of programmed death-ligand 1 (PD-L1). The ORPHEUS trial is evaluating the efficacy and safety of INCMGA00012 –a programmed cell death 1 (PD-1) antagonist– in patients with unresectable, locally advanced or metastatic PSqCC. Methods: ORPHEUS is an international, multicenter, open-label, single-arm, phase 2 clinical trial. Eligible patients are male aged ≥18 years with locally advanced or metastatic PSqCC, ECOG performance status of 0-1, adequate organ function, a life expectancy of ≥12 weeks, and with no prior treatment with PD-1 or PD-L1/2 agents. A total of 18 patients will be enrolled to receive INCMGA00012 500 mg administered intravenously on day 1 of each 28-day cycle. Treatment will continue until progressive disease or unacceptable toxicity. Tumor assessments per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 and immune-related RECIST will be performed approximately every 8 weeks for the first 6 months and every 12 weeks thereafter until progressive disease. The primary endpoint is objective response rate. Secondary objectives include additional efficacy outcomes (clinical benefit rate, progression-free survival (PFS), 6-month PFS, duration of response, time to progression, overall survival, and maximum tumor shrinkage) and safety evaluated as per NCI-CTCAE 5.0. Exploratory objectives will evaluate efficacy based on immune-related RECIST; predictive and prognostic biomarkers; impact of INCMGA00012 on human immunodeficiency virus (HIV) control in patients known to be HIV-positive. The sample size calculation is based on an exact binomial test. At least 4 responders (22.2%) among 18 patients will be adequate to justify further investigation of this strategy. The analyses were designed to attain an 80% power, with a 10% dropout rate assumption, at a nominal one-sided α level of 5%. The response probabilities for null (H0) and alternative hypotheses (H1) were H0: ≤ 5% and H1: ≥ 25%, respectively. All 18 planned patients have been enrolled since the trial began in March 2020. Clinical trial information: NCT04231981.