Carboplatin-paclitaxel With Retifanlimab or Placebo in Participants With Locally Advanced or Metastatic Squamous Cell Anal Carcinoma (POD1UM-303/InterAACT 2).

Inclusion Criteria: * Able to comprehend and willing to sign a written ICF for the study. * Are 18 years of age or older (or as applicable per local country requirements). * Histologically or cytologically verified, inoperable locally recurrent or metastatic SCAC. * No prior systemic therapy other than the following: a. Chemotherapy administered concomitantly with radiotherapy as a radiosensitizing agent is permitted. b. Prior neoadjuvant or adjuvant therapy if completed ≥ 6 months before study entry. * Has measurable disease per RECIST v1.1 as determined by local site investigator/radiology assessment. Tumor lesions situated in a previously irradiated area, or in an area subjected to other loco-regional therapy, are usually not considered measurable unless there has been demonstrated progression in the lesion. * Able and willing to provide adequate tissue sample and whole blood sample with central testing result prior to randomization. Biopsy for archival samples should have occurred within 9 months prior to randomization. * ECOG performance status 0 to 1. * If HIV-positive, then must be stable as defined by: a. CD4+ count ≥ 200/μL, b. Undetectable viral load per standard of care assay, c. Receiving antiretroviral therapy (ART/HAART) for at least 4 weeks prior to study enrollment, and have not experienced any HIV-related opportunistic infection for at least 4 weeks prior to study enrollment. * Willingness to avoid pregnancy or fathering children Exclu…