General Inclusion Criteria:
- Locally advanced or metastatic and/or unresectable HCC with diagnosis confirmed by histology/cytology or clinically by American Association for the Study of Liver Diseases (AASLD) criteria in cirrhotic patients
- Disease that is not amenable to curative surgical and/or locoregional therapies
- No prior systemic treatment (including systemic investigational agents) for locally advanced or metastatic and/or unresectable HCC
- Measurable disease (at least one untreated target lesion) according to RECIST v1.1
- ECOG Performance Status of 0-2 within 7 days prior to initiation of study treatment
- Child-Pugh B7 or B8 cirrhosis at screening and within 7 days prior to study treatment
- Adequate hematologic and end-organ function
- Life expectancy of at least 12 weeks
- Female participants of childbearing potential must be willing to avoid pregnancy and egg donation
General Exclusion Criteria:
- Pregnancy or breastfeeding
- Prior treatment with CD137 agonists or immune checkpoint blockade therapies
- Treatment with investigational therapy within 28 days prior to initiation of study treatment
- Treatment with locoregional therapy to liver within 28 days prior to initiation of study treatment, or non-recovery from side effects of any such procedure
- Treatment with systemic immunostimulatory agents
- Treatment with systemic immunosuppressive medication
- Treatment with a live, attenuated vaccine within 4 weeks prior to initiation of study treatment
- Inadequately controlled hypertension
- Active or history of autoimmune disease or immune deficiency
- History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan
- History of malignancy other than HCC within 3 years prior to screening, with the exception of malignancies with a negligible risk of metastasis or death
- Known fibrolamellar HCC, sarcomatoid HCC, other rare HCC variant, or mixed cholangiocarcinoma and HCC
- Symptomatic, untreated, or actively progressing central nervous system (CNS) metastases
- Prior allogeneic stem cell or solid organ transplantation
- Listed for liver transplantation
- Co-infection with hepatitis B virus (HBV) and hepatitis C virus (HCV)
- Untreated or incompletely treated esophageal and/or gastric varices with bleeding or that are at high risk for bleeding
- A prior bleeding event due to esophageal and/or gastric varices within 6 months prior to initiation of study treatment
- Grade ≥3 hemorrhage or bleeding event within 6 months prior to initiation of study treatment
- History of hepatic encephalopathy requiring hospitalization or treatment escalation within 6 months prior to study treatment, or any continued symptoms of encephalopathy despite medical management
- History, planned, or recommended placement of transjugular intrahepatic portosystemic shunt (TIPS)
- History of ascites requiring therapeutic paracentesis over the last 3 months
- History of spontaneous bacterial peritonitis within last 12 months