Final Results of a Phase I "RadVax" Trial of Hypofractionated Radiation Combined With Pembrolizumab in Patients With Metastatic Solid Tumors.

Purpose/Objective(s) Many patients treated with anti-PD-1 therapy do not show a clinical response. Preclinical studies suggest that adding hypofractionated radiotherapy (HFRT) to anti-PD1 can increase the efficacy of immunotherapy through several mechanisms including increased antigen presentation. We conducted a prospective trial testing the combination of pembrolizumab and HFRT in patients with metastatic solid tumors. Materials/Methods This prospective single-institution phase I trial tested pembrolizumab in combination with HFRT in patients with metastatic cancers (NSCLC, melanoma, pancreas, breast, others) and an ECOG performance status of 0-1. Melanoma and NSCLC patients were required to have progression of disease on anti-PD1, having received ≥ 2 doses of anti-PD1 and progression documented by RECIST v1.1. Patients were required to have an index lesion ≥1 cm that was amenable to HFRT and at least one other lesion that was not irradiated and could be followed for response using RECIST criteria. Pembrolizumab 200 mg IV every 3 weeks was administered beginning 1 week prior to the first fraction of radiation. The HFRT dose was 8 Gy x 3 fractions or 17 Gy x 1 fraction, determined by randomization during the Expansion phase. The primary objective was the safety of HFRT combined with pembrolizumab, with dose-limiting toxicity (DLT) defined as Grade ≥ 3 non-hematological toxicity related to the combination of Pembrolizumab and HFRT. The secondary objective was the radiographic response of metastatic lesions outside the radiation field as measured by RECIST. Results A total of 59 patients aged 27-90 years (median 60) were enrolled from March 2015 to December 2018 (24 in the Safety Phase and 35 in Expansion Phase). 40 patients (67.7%) had treatment-related AEs, of which 4 were grade 3 and none were grade 4. One patient experienced hepatitis, classified as DLT. While most patients did not have a radiologic response, in patients with metastatic melanoma, 7 of 16 (43.8%, exact 95% CI 19.8-70.1%) had an objective response of non-irradiated lesions to HFRT + pembrolizumab, including 3 complete and 4 partial responses. Responses are durable with 3/3 complete responders alive with no progression, and 3/4 partial responders alive with 2 having no evidence of progression. Among melanoma patients, only 2 of 7 (29%) responders received ipilimumab prior to enrollment, compared to 8 of 9 (89%) non-responders (P = 0.035). An increase in Ki67+ PD-1+ non-naive CD8 T-cells was observed in the blood 2 weeks after HFRT, but the magnitude did not correlate with likelihood of response. Responses were observed after either 17 Gy x 1 fraction or 8 Gy x 3 fractions, with no difference in response rate by fractionation. Conclusion This study suggests that HFRT administered with concurrent pembrolizumab is associated with acceptable toxicity and that in patients with metastatic melanoma progressing on anti-PD-1 therapy, this approach yields an ORR of 44%.