Pembrolizumab and stereotactic radiotherapy combined in subjects with advanced HCC: A phase II study (PEMRAD).

602 Background: Combination approaches of immune-checkpoint inhibitors and locoregional therapies have an increasing role in advanced HCC. Methods: The PEMRAD phase II trial investigated the efficacy of combination pembrolizumab and stereotactic body radiation (SBRT) following progression on sorafenib. Patients with advanced HCC, ECOG 0-1 and Childs-Pugh A were eligible. Patients received pembrolizumab on day 1 followed by SBRT starting Day 2 and delivered in five fractions. Pembrolizumab was continued every 21 days until progression, unacceptable toxicity or patient withdrawal. The primary endpoint was objective response rate (ORR) as measured by RECISTv 1.1. hypothesizing an increase to 40% with the combination. Tissue and liquid biopsies were collected for correlative analyses including multiplex IHC, longitudinal CyTOF and dynamic cytokine changes. Results: The trial was stopped early due to a shift in the treatment landscape. Between March 2018 and March 2023 18 patients of a planned 22 patients were enrolled and treated with combination therapy. Of these 10 (55%) had evidence of macrovascular invasion (MVI) and 15 (83%) had extrahepatic disease; viral hepatitis was the underlying etiology in 50%. The ORR was 41% meeting the proposed endpoint. Median number of cycles of pembrolizumab was 7.5 (2-35) .The median number of lesions treated by SBRT was 3 (1-5) and the median dominant gross tumour volume was 184 cc (IQR 69-285). The median PFS was 5.4 (2.8-9.9) months and median OS 12.6 (5.7-25.8) months. Of the 10 patients with macrovascular invasion 5 (50%) had evidence of a vascular thrombus response by RECIST criteria including 1 complete response. Treatment related adverse events ≥ grade 3 were reported in 5 patients (28%). One death due to myocarditis was attributed to treatment. Conclusions: In a poor prognostic group of patients in the second line setting, the combination of SBRT and pembrolizumab demonstrated high ORR and may have a specific role in patients with MVI. Correlative analyses will be presented at the meeting. Clinical trial information: NCT03316872 .