TPS503 Background: There are no approved therapies for patients with hepatocellular carcinoma (HCC) after disease progression on sorafenib, or for patients with intolerance to sorafenib. HCC often arises in the background of chronic inflammation and is also associated with an immunosuppressed microenvironment, providing a strong rationale to evaluate immunotherapy in HCC. The randomized, double-blind, placebo-controlled phase 3 KEYNOTE-240 study (ClinicalTrials.gov, NCT02702401) was designed to compare the efficacy and safety of the anti–PD-1 antibody pembrolizumab + best supportive care (BSC) vs placebo + BSC in patients with previously treated advanced HCC. Methods: Eligibility criteria include age ≥18 years, confirmed diagnosis of Barcelona Clinic Liver Cancer (BCLC) stage C disease or BCLC stage B disease not amenable to or refractory to locoregional therapy, disease not amenable to a curative treatment approach (eg, transplantation, surgery, or ablation), documented progression after treatment with sorafenib or intolerance to sorafenib, Child-Pugh liver score A, and ECOG performance status 0-1. ~408 patients will be randomly assigned 2:1 to receive pembrolizumab 200 mg IV every 3 weeks (Q3W) + BSC or placebo Q3W + BSC for up to 35 cycles or until disease progression, unacceptable toxicity, patient withdrawal of consent, or investigator decision. Randomization will be stratified by geographic region, macrovascular invasion, and α-fetoprotein. BSC will be provided by the investigator per local treatment practices. Response will be assessed every 6 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. Primary objectives are comparison of progression-free survival per RECIST v1.1 by central imaging vendor review and overall survival between treatment arms. Secondary objectives are comparison of objective response rate, duration of response, disease control rate, and time to progression per RECIST v1.1 by central imaging vendor review, and evaluation of safety and tolerability. Enrollment in KEYNOTE-240 is ongoing. Clinical trial information: NCT02702401.