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…