Safety and Efficacy of Pembrolizumab (MK-3475) Versus Placebo as Adjuvant Therapy in Participants With Hepatocellular Carcinoma (HCC) and Complete Radiological Response After Surgical Resection or Local Ablation (MK-3475-937 / KEYNOTE-937)

Inclusion Criteria:

• Has a diagnosis of HCC by radiological criteria and/or pathological confirmation.

• Has an eligibility scan (CT of the chest, triphasic CT scan or MRI of the abdomen,and CT or MRI of the pelvis) confirming complete radiological response ≥4 weeksafter complete surgical resection or local ablation. Randomization needs to occurwithin 12 weeks of the date of surgical resection or local ablation.

• Has no radiologic evidence of disease prior to enrollment.

• Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 7 days prior to Cycle 1, Day 1.

• Has a Child-Pugh class A liver score (5 to 6 points) within 7 days prior to Cycle 1,Day 1.

• Has alpha fetoprotein (AFP) concentration lower than 400 ng/mL within 28 days priorto Cycle 1, Day 1.

• Has controlled hepatitis B (Hep B).

• Has recovered adequately from toxicity and/or complications from the localintervention (surgical resection or local ablation) prior to starting studytreatment.

• If female, is not pregnant or breastfeeding, and at least one of the followingconditions applies: 1) Is not a woman of childbearing potential (WOCBP); or 2) Is aWOCBP and using a contraceptive method that is highly effective or be abstinent fromheterosexual intercourse as their preferred and usual lifestyle (a WOCBP must have anegative pregnancy test within 72 hours before the first dose of study treatment).

• If undergoing surgical resection, has submitted a tumor tissue sample duringScreening.

• Has adequate organ function.

Exclusion Criteria:

• Has a known additional malignancy that is progressing or has required activeantineoplastic treatment (including hormonal) or surgery within the past 3 years.

• Has had esophageal or gastric variceal bleeding within the last 6 months.

• Has clinically apparent ascites on physical examination.

• Has had clinically diagnosed hepatic encephalopathy in the last 6 months.

• Has received local therapy to liver ablation other than with radiofrequency ormicrowave ablation.

• Has a history of (noninfectious) pneumonitis/interstitial lung disease that requiredsteroids or has current pneumonitis/interstitial lung disease.

• Has an active infection requiring systemic therapy.

• Has dual active Hepatitis B Virus (HBV) and Hepatitis C Virus (HCV) infection atstudy entry.

• Has a known history of human immunodeficiency virus (HIV) infection.

• Has known active tuberculosis (TB; Bacillus tuberculosis).

• Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti PD-L2 agent orwith an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg,CTLA-4, OX-40, CD137).

• Has received prior systemic anti-cancer therapy for HCC including investigationalagents.

• Is receiving any of the following prohibited concomitant therapies:1) Antineoplasticsystemic chemotherapy or biological therapy; 2) Immunotherapy not specified in thisprotocol; 3) Investigational agents other than pembrolizumab; 4) Radiation therapy;

5. Oncological surgical therapy; or systemic glucocorticoids for any purpose otherthan to modulate symptoms from an AE that is suspected to have an immunologicetiology.

• Has received a live vaccine within 30 days prior to the first dose of studytreatment.

• Is currently participating in or has participated in a study of an investigationalagent or has used an investigational device within 4 weeks prior to Cycle 1, Day 1.

• Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapyor any other form of immunosuppressive therapy within 7 days prior to Cycle 1, Day

• Has severe hypersensitivity (≥Grade 3) to pembrolizumab and/or any of itsexcipients.

• Has an active autoimmune disease that has required systemic treatment in past 2years.

• Has a known psychiatric or substance abuse disorder that would interfere with theparticipant's ability to cooperate with the requirements of the study.

• Has had an allogenic tissue/solid organ transplant.