Background: Checkpoint inhibitors (CPIs) are an effective treatment (tx) for many tumor types. Retifanlimab, an investigational humanized anti–PD-1 monoclonal antibody, has shown safety, pharmacology, and clinical activity consistent with the class. POD1UM-203 (NCT03679767) assessed efficacy and safety of retifanlimab in pts with selected solid tumors where CPI monotherapy is highly active. Methods: Eligible pts (≥18 y) had tx-naïve metastatic non-small cell lung cancer (NSCLC) with high PD-L1 expression (tumor proportion score ≥50%), cisplatin ineligible locally-advanced/metastatic urothelial cancer (UC) with PD-L1 expression (combined positive score ≥10%), unresectable/metastatic melanoma, or tx-naïve locally advanced/metastatic clear-cell renal cell carcinoma (RCC). Measurable disease (RECIST v1.1) was required. ECOG PS >1 and prior PD-1/PD-L1 directed tx were exclusions. Retifanlimab was administered as an IV infusion at 500 mg every 4 wks over 30 min. Primary endpoint was investigator-assessed objective response rate (ORR). Secondary endpoints were duration of response (DOR), disease control rate (DCR), progression-free survival, overall survival, safety, and pharmacokinetics. Results: A total of 121 pts (35 melanoma, 23 NSCLC, 29 UC, 34 RCC) received ≥1 dose of retifanlimab and were included in the analyses. Median duration of tx was 169 d (range, 1–442). The efficacy cut-off for the primary analysis occurred once all pts had been followed for at least 6 mo from the time of initial tx. Confirmed RECIST v1.1 responses were observed in all tumor types (Table) and were consistent with published ORR for other CPIs; median DOR was not reached for any tumor cohort and tx was ongoing at the time of data cutoff for 17, 11, 9, and 15 pts with melanoma, NSCLC, UC, and RCC, respectively. The most common tx-emergent AEs (TEAEs, >10% incidence) were asthenia (17.4%), arthralgia (14.9%), decreased appetite (14.0%), pruritus (12.4%), rash (10.7%), and urinary tract infection (10.7%); majority of TEAEs were low grade (≤ grade 2) and none led to tx discontinuation. Immune-related AEs occurred in 23 pts (19.0%), most common (>1% incidence) were hypothyroidism (7.4%), rash (4.1%), hyperthyroidism (2.5%), and pruritus (1.7%).Immune-related AEs led to dose delay in 5 pts (4.1%), but none led to tx discontinuation and/or dose interruption. Conclusions: Retifanlimab demonstrated antitumor activity and was generally well-tolerated in pts with melanoma, NSCLC, UC, or RCC comparable with approved CPIs for these tumor types. These results support ongoing further development of retifanlimab. Clinical trial information: NCT03679767