PI3Kγ in leukemia: class IB PI3 kinase reemerges as a cancer-intrinsic target

Phosphoinositide 3-kinase gamma (PI3Kγ), the only class IB PI3 kinase, is a cell-extrinsic immunotherapy target in solid tumors. PI3Kγ inhibition reprograms immunosuppressive myeloid cells to acquire immunostimulatory phenotypes, which promote antitumor cytotoxic T cell activity. Although PI3Kγ inhibition has no direct effect on solid tumor cells, several new studies have nominated PI3Kγ as a cell-intrinsic target in various leukemias, particularly acute myeloid leukemia (AML). Intrinsic dependency on PI3Kγ is present at baseline in leukemias with specific pathologic characteristics, is inducible by extrinsic inflammation in others, and may also be acquired with resistance to certain therapies. The discovery of leukemia PI3Kγ dependency has generated enthusiasm for immediate clinical trial evaluation of inhibitor monotherapy and combinations. Parallel laboratory evaluation is needed to develop an improved understanding of leukemia disease features associated with clinical inhibitor sensitivity that might suggest biomarker-directed patient enrichment strategies. In this review, we discuss recent progress credentialling PI3Kγ as a bona fide target in leukemia. We also highlight open questions, including a need to understand the mechanism of acquired resistance to PI3Kγ inhibition, how to optimally prioritize combination therapies to enhance PI3Kγ inhibitor utility, and how cell-extrinsic effects of PI3Kγ inhibition in the leukemia microenvironment might also contribute to clinical activity.