Background: The tyrosine kinase inhibitor sorafenib is the standard of care for first-line hepatocellular carcinoma (HCC). For patients with HCC after disease progression on sorafenib or for those with intolerance to sorafenib, no approved therapies are available. Because HCC is often driven by inflammation and is also associated with a suppressed immunoenvironment, there is a strong rationale to evaluate immunotherapy in patients with this type of cancer. The single-arm, multisite, phase 2 KEYNOTE-224 study (ClinicalTrials.gov, NCT02702414) was designed to evaluate the efficacy and safety of the anti–PD-1 antibody pembrolizumab in patients with previously treated advanced HCC. Methods: Approximately 100 patients will be enrolled. Inclusion criteria include age ≥18 years, histologically or cytologically confirmed diagnosis of HCC Barcelona Clinic Liver Cancer (BCLC) stage C disease or BCLC stage B disease not amenable to or refractory to locoregional therapy, and disease not amenable to a curative treatment approach (eg, transplantation, surgery, or ablation). Patients must also have measurable disease based on RECIST v1.1 as confirmed by central imaging vendor review, documented objective radiographic progression after stopping treatment with sorafenib or intolerance to sorafenib, Child-Pugh liver score A, ECOG performance status 0-1, and predicted life expectancy > 3 months. Patients will be allocated to receive pembrolizumab 200 mg IV every 3 weeks for up to 35 cycles (~2 years) or until disease progression, unacceptable toxicity, patient withdrawal of consent, or investigator decision. Response will be assessed every 9 weeks per RECIST v1.1 by central imaging vendor review. Adverse events (AEs) will be assessed throughout treatment and for 30 days thereafter (90 days for serious AEs) and graded per NCI CTCAE v4.0. The primary end point is objective response rate per RECIST v1.1 by central imaging vendor review. Secondary end points are overall survival; safety and tolerability; and duration of response, disease control rate, time to progression, and progression-free survival per RECIST v1.1 by central imaging vendor review. Enrollment in KEYNOTE-224 is ongoing. Clinical trial information: NCT02702414