Purpose of review: Checkpoint inhibitors (CPIs) represent the forefront of novel immunotherapeutic approaches for the treatment of solid cancers. However, the clinical development of CPIs in glioblastoma (GBM) has been challenging owing to an immunosuppressive tumor microenvironment and, possibly, low tumor mutation burden. Here, we review possible mechanisms responsible for the success of programmed cell death-1 (PD-1) blockade in patients with hypermutated GBM, recent clinical trials of anti-PD-1 monotherapy, trials incorporating neoadjuvant strategies, and trials of immunotherapy combination approaches in GBM. Mechanisms of resistance to immunotherapy and methods to overcome these challenges are also discussed.
Recent findings: Although two large phase III trials failed to demonstrate the superior efficacy of CPI in comparison with the standard of care in newly diagnosed and recurrent GBM, recent studies suggest that opportunities exist in some patients with GBM. A phase II study showed longer survival in patients with recurrent GBM who received neoadjuvant anti-PD-1 therapy than in those who received it as adjuvant therapy. In addition, cases of response to anti-PD-1 therapy in GBM patients with clonal hypermutator tumors have been reported.
Summary: Even though anti-PD-1 therapy does not seem to provide a benefit for molecularly unselected GBM patients, the success of PD-1 blockade in certain subsets of patients is encouraging.