Abstract 4718: ZSTK474, a pan-PI3K inhibitor, enhances the anti-tumor activity of PD-1 blockade by decreasing Treg cells and increasing memory T cells
Authors: Isoyama Sho , Mori Shigeyuki , Sugiyama Daisuke , Nishikawa Hiroyoshi 
Published: August 2018

Abstract
The phosphoinositide 3-kinases (PI3Ks) transduce important signals involved in T-cell development and activation. Inhibition of the PI3Kδ isoform reportedly decreases regulatory T (Treg) cells and activates anti-tumor immunity in preclinical murine models. PI3K is therefore an attractive target for cancer immunotherapy. Here we explored whether ZSTK474, a selective PI3K inhibitor, which inhibits cancer proliferation by inhibition of activated PI3K in cancer cells, decreases Treg cells, resulting in activation of the anti-tumor immune responses leading to tumor growth inhibition in combination with anti-PD-1 antibody. In syngeneic murine models, ZSTK474 administration decreased Treg cell number in tumors and increased CD8+ T cells / Treg cells ratio, suggesting augmented anti-tumor immunity. ZSTK474 treatment with anti-PD-1 antibody showed far stronger tumor growth inhibition against NY-ESO-1 overexpressing CMS5a cells (CMS5a-NY-ESO-1) and increased tumor specific CD8+ T cells / Treg cells ratio in tumors compared with each monotherapy group. These anti-tumor effect by ZSTK474 was dependent on both CD8+ and CD4+ T cells because depletion of CD8+ or CD4+ T cells from tumor bearing mice abrogated the anti-tumor effect by ZSTK474. We then examined whether ZSTK474 inhibits human Treg cells. CD4+CD25+ Treg cells were isolated from human peripheral blood mononuclear cells (PBMCs) and stimulated with CD3/CD28 antibody and found that ZSTK474 selectively inhibited proliferation of human CD4+CD25+ Treg cells as compared to CD4+CD25- and CD8+ T cells. Furthermore, when PBMCs were stimulated with NY-ESO-1 peptide with/without ZSTK474, NY-ESO-1 specific CD8+ T cells were highly induced by adding ZSTK474. We next examined memory T cell responses in these mice. Re-challenge of CMS5a-NY-ESO-1 cells into tumor-free mice did not lead to tumor growth. Interestingly, when parental CMS5a cells were inoculated into the tumor-free mice, mice treated with anti-PD-1 antibody alone showed an aggressive tumor growth by CMS5a, whereas those treated with combination of anti-PD-1 antibody and ZSTK474 rejected the tumor. These results suggest that treatment with ZSTK474 may promote the development of memory responses against wide ranges of tumor antigens, probably due to changes of intracellular T-cell activation signal to memory formation. Actually, treatment with ZSTK474 increased frequency of memory precursor effector cell (MPEC) in tumor antigen-specific CD8+ T cells. These results suggest that ZSTK474 decreases Treg cells and activates anti-tumor immunity providing a rationale for the combination therapy of a PI3K inhibitor, ZSTK474 and PD-1 blockade.
Citation Format: Sho Isoyama, Shigeyuki Mori, Daisuke Sugiyama, Hiroyoshi Nishikawa. ZSTK474, a pan-PI3K inhibitor, enhances the anti-tumor activity of PD-1 blockade by decreasing Treg cells and increasing memory T cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4718.