Re-Irradiation Plus Pembrolizumab: A Phase II Study for Patients with Recurrent Glioblastoma

Radiotherapy may enhance antitumor immune responses by several mechanisms, including induction of immunogenic cell death. We performed a phase 2 study of pembrolizumab with re-irradiation in patients with recurrent glioblastoma.

Sixty patients with recurrent glioblastoma received pembrolizumab with re-irradiation alone (cohort A, bevacizumab-naïve; n = 30) or with bevacizumab continuation (cohort B, n = 30). Dual primary endpoints, including the overall response rate and overall survival (OS) at either 12 (OS-12; cohort A) or 6 months (OS-6; cohort B), were assessed per cohort relative to historic benchmarks. Paired paraffin-embedded formalin-fixed tumor samples were assessed for immunologic biomarkers by IHC using digital quantification and co-detection-by-indexing (CODEX).

Study therapy was well tolerated, with most toxicities being grade ≤3. For cohort B, the primary endpoint of OS-6 was achieved (57%); however, survival was not improved for cohort A patients. The overall response rate was 3.3% and 6.7% for cohorts A and B, respectively. CODEX analysis of paired tumor samples from five patients revealed an increase of activated T cells in the tumor microenvironment after study therapy.

Compared with historic controls, re-irradiation plus pembrolizumab seemed to improve survival among bevacizumab-refractory patients but not among bevacizumab-naïve patients. CODEX revealed evidence of intratumoral infiltration of activated immune effector cells. A randomized, properly controlled trial of PD-1 blockade plus re-irradiation is warranted to further evaluate this regimen for bevacizumab-refractory patients, but alternative approaches are needed for bevacizumab-naïve patients.