Anti-PD-L1 Efficacy Can Be Enhanced by Inhibition of Myeloid-Derived Suppressor Cells with a Selective Inhibitor of PI3Kδ/γ

Checkpoint inhibitors are relatively inefficacious in head and neck cancers, despite an abundance of genetic alterations and a T-cell-inflamed phenotype. One significant barrier to efficacy may be the recruitment of myeloid-derived suppressor cells (MDSC) into the tumor microenvironment. Here we demonstrate functional inhibition of MDSC with IPI-145, an inhibitor of PI3K delta and PI3K gamma isoforms, which enhances responses to PD-L1 blockade. Combination therapy induced CD8(+) T lymphocyte-dependent primary tumor growth delay and prolonged survival only in T-cell-inflamed tumor models of head and neck cancers. However, higher doses of IPI-145 reversed the observed enhancement of anti-PD-L1 efficacy due to off-target suppression of the activity of tumor-infiltrating T lymphocytes. Together, our results offer a preclinical proof of concept for the low-dose use of isoform-specific PI3K delta/gamma inhibitors to suppress MDSC to enhance responses to immune checkpoint blockade. (C) 2017 AACR.