Background: Anal cancer is growing in annual incidence globally and human papillomavirus (HPV) remains the predominant risk factor underlying its development. Due to its relative rarity, clinical trials in anal cancer have historically been difficult to conduct and treatment options for metastatic disease remain limited. Carboplatin/paclitaxel (CP) was compared to cisplatin/5-fluorouracil (historical standard of care) in a recent randomized phase II clinical trial (InterAACT; EA2133) in treatment-naïve metastatic anal cancer, finding that response rates were equivocal, but that overall survival (OS) was significantly longer in the CP arm (20 months vs 12.3 months, p = 0.014). Additionally, reduced grade 3/4 toxicities were seen in the CP arm. NCI9673, a single-arm phase II study, established safety and efficacy of nivolumab in previously-treated metastatic anal cancer. Progression-free survival (PFS) was 4.1 months (95% CI 3.0-7.9) and OS was 11.5 months (95% CI 7.1-not estimable). Multiple randomized trials in lung cancer have demonstrated efficacy of platinum-based chemotherapy combined with checkpoint inhibitors. Together these studies form the rationale behind combining CP and nivolumab in treatment-naïve metastatic anal cancer. Methods: EA2176 (NCT04444921) is the first NCTN phase III randomized clinical trial in treatment-naïve metastatic anal cancer. Stratification factors include HIV status and history of chemoradiation for curative intent. Patients will be randomized to carboplatin (AUC = 5, Day 1) plus paclitaxel (80mg/m2, Days 1, 8, 15) +/- nivolumab 240mg IV (Cycle 1 = Days 1, 15; Cycle ≥2 = Day 1, 480mg) q 28-days until disease progression or treatment intolerance. CP will be given for up to 6 cycles, while nivolumab will be continued as maintenance for up to 2 years. The primary endpoint is PFS. Secondary objectives include OS, response rate, and toxicity. Goal enrollment is 205 patients and the study continues accrual. This sample size will provide 80% power at a two-sided α of 0.05 to detect a 4.8-month improvement in PFS assuming 8 months in the control arm. Novel correlative studies include sequential quantitative tumor-derived cell-free HPV ctDNA levels (serotypes 16 and 18; Sysmex-Inostics SafeSEQ NGS assay). Correlative funding provided in part by the Farrah Fawcett Foundation and Sysmex Inostics, Inc. Clinical trial information: NCT04444921