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.