Study of Sugemalimab (or Placebo) Plus PGemOx Regimen in Participants With Extranodal NK/T-Cell Lymphoma

Inclusion Criteria: Has ENKTL histologically confirmed by the study center. Nasal and non-nasal ENKTL are both allowed. Has relapsed or refractory ENKTL after prior asparaginase-based chemotherapy or chemoradiotherapy. Has Eastern Cooperative Oncology Group (ECOG) performance status score of 0-2. Has at least one measurable lesion per Lugano 2014 classification. Is willing to provide stained tumor tissue sections and corresponding pathological reports or unstained tumor tissue sections (or tissue block) for central pathology review. Has adequate organ function. Has life expectancy of greater than 3 months. Exclusion Criteria: Has aggressive natural killer-cell leukemia, current central nervous system (CNS) involvement or is concomitant with hemophagocytic lymphohistiocytosis. Has known additional malignancy within 5 years prior to randomization. Has an active autoimmune disease or has had an autoimmune disease that may relapse. Has had a major surgical procedure within 28 days or radiotherapy within 90 days before the first dose of study treatment. Has active tuberculosis infection. Has a known history of human immunodeficiency virus (HIV) infection and/or acquired immune deficiency syndrome (AIDS). Has a known active Hepatitis B or C virus infection. Has received systemic anti-cancer therapy within 28 days before the first dose of study treatment, including chemotherapy, immunotherapy, biological therapy (e.g. cancer vaccine, cytokine therapy or growth factors to treat cancer). Has used traditional Chinese medicines or herbal preparations with anti-tumor indications within 7 days before the first dose of study treatment. Has received systemic corticosteroid or any other immunosuppressive therapy within 14 days before the first dose of study treatment. Has received any treatment of antibody or drug that targets at T-cell coregulatory pathways or immune checkpoint pathways. Has toxicity from prior anti-cancer treatment, except for alopecia and fatigue, that has not recovered to baseline or ≤ Grade 1 according to Common Terminology Criteria for Adverse Events (CTCAE) v5.0 before the first dose of study treatment. Has had allogenic hematopoietic stem cell transplantation (HSCT) within 5 years or autologous HSCT within 90 days before the first dose of study treatment. Has a known severe hypersensitivity to sugemalimab, its active substance and/or any of its excipients, or to other monoclonal antibodies. Female participants who are pregnant or breastfeeding. Is currently participating in or has participated in a trial of an investigational agent within 28 days before to the first dose of study treatment.