Background: ESCC is the predominant histological subtype of esophageal cancer, particularly in Asian countries. Platinum-based chemotherapy is the first-line standard therapy for patients with unresectable, locally advanced, recurrent or metastatic ESCC. The FP regimen is recommended as the preferred treatment by guidelines. However, the survival benefit conferred by this therapy leaves considerable space for improvement, with median OS being less than 1 year. Blockade of the immune checkpoint receptors has shown clinical benefits in multiple tumor types. Recent studies combining standard treatments with checkpoint inhibitors have shown encouraging efficacy and favorable safety profile in patients with unresectable, locally advanced, recurrent or metastatic ESCC. CS1001 (sugemalimab) is the first full-length, fully human immunoglobin G4 (IgG4, s228p) anti-programmed death-ligand 1 (PD-L1) monoclonal antibody developed by the OMT transgenic rat platform. In an ongoing Phase Ib trial, CS1001 in combination with FP regimen demonstrated an ORR of 67.6% (25/37) and an mPFS of 9.0 months with a manageable safety profile in unresectable, locally advanced or distantly metastatic ESCC (19 Feb 2020 data cutoff; Shen, L., et al, ESMO 2020). Methods: CS1001-304 is a randomized, double-blind Phase III study to compare the efficacy and safety of FP regimen with CS1001 or placebo as first-line treatment in ESCC. The study enrolls patients with histologically or cytologically confirmed unresectable locally advanced, recurrent or metastatic ESCC who have ECOG PS of 0-1, patients are not eligible for curative therapy (curative surgery or definitive chemoradiotherapy), and have not received any prior systemic anti-tumor therapy for locally advanced or metastatic disease. Approximately 540 patients will be randomized at 2:1 into CS1001 + FP and placebo + FP arms respectively, stratified by PD-L1 expression status (PD-L1 expression < 1% vs ≥ 1% and < 10% vs ≥10%), ECOG PS (0 vs 1) and distant metastasis (no vs yes). Patients randomized to either arm will receive FP regimen (fluorouracil: 800 mg/m²/day, continuous intravenous infusion [IV], D1-4 of each cycle; cisplatin: 80 mg/m², IV, D1 of each cycle), Q3W for up to 6 cycles in combination with CS1001 1200 mg or placebo (IV, D1 of each cycle), Q3W for up to 24 months. AEs will be monitored throughout the study and graded per NCI CTCAE v5.0. Tumor response will be assessed by RECIST v1.1 every 6 weeks in the first 12 months, and every 12 weeks thereafter. The primary endpoints are blinded independent central review (BICR)-assessed PFS and OS. Secondary endpoints include investigator-assessed PFS, BICR and investigator-assessed ORR and DoR, safety, PK profile, and immunogenicity. The study is actively enrolling patients in over 60 sites in China. Clinical trial information: NCT04187352