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.

1. Are 18 years of age or older (or as applicable per local country requirements).
2. 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.

1. 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.
2. 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.
3. ECOG performance status 0 to 1.
4. 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.
5. Willingness to avoid pregnancy or fathering children

Exclusion Criteria:

1. Has received prior PD-(L)1 directed therapy
2. Has received prior radiotherapy with or without radiosensitizing chemotherapy within 28 days of Cycle 1 Day 1 except for palliative radiation (30 Gy or less) which is restricted for 14 days of Cycle 1 Day 1 (note: all toxicities associated should have resolved to Grade ≤ 1).
3. Participants with laboratory outside of the protocol defined ranges.
4. History of second malignancy within 3 years (with exceptions).
5. Clinically significant pulmonary, cardiac, gastrointestinal or autoimmune disorders.
6. Active bacterial, fungal, or viral infections, including hepatitis A, B, and C and IV antibiotic use within 7 days of Cycle 1 Day 1.
7. Receipt of a live vaccine within 28 days of planned start of study therapy.
8. History of organ transplant, including allogeneic stem cell transplantation.
9. Known active CNS metastases and/or carcinomatous meningitis.
10. Known hypersensitivity to platinum, paclitaxel, another monoclonal antibody, or any of the excipients that cannot be controlled with standard measures (eg, antihistamines, corticosteroids).
11. Participant is pregnant or breastfeeding.
12. Current use of protocol defined prohibited medication.
13. Has pre-existing peripheral neuropathy that is ≥ Grade 2 by CTCAE v5.
14. Inability or unlikely, in the opinion of the investigator, to comply with the Protocol requirements