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Myeloid derived suppressor cells (MDSCs) and polymorphonuclear cells (PMNs) are mediators of tumor immune evasion, however there are no approved clinical agents that directly antagonize the immunosuppressive activity of these cells. One of the mechanisms by which MDSCs and PMNs suppress anti-tumor immunity is expression of the enzyme arginase, which reduces L-arginine levels rendering T-cells and natural killer (NK)-cells unable to proliferate and mount an effective anti-tumor response. As a rationale to pursue arginase inhibition as a potential therapeutic strategy, we investigated the prevalence of arginase in tumors and peripheral blood from patients with various types of cancer. Our results confirm reports that multiple tumor types have arginase-expressing PMN infiltrates and that cancer patients have higher levels of plasma arginase and lower levels of plasma arginine compared to healthy volunteers. We also confirmed that in vitro-activated primary human T-cells and NK-cells require arginine to proliferate, secrete pro-inflammatory cytokines, and express the intracellular signaling molecule CD3ζ and cell surface activation marker PD-1. We developed CB-1158 to be a potent, selective, and orally-bioavailable small molecule inhibitor of arginase. CB-1158 potently inhibits recombinant human arginase 1 (IC50 = 98 nM), recombinant human arginase 2 (IC50 = 249 nM), and endogenous arginase from human neutrophils (IC50 = 160 nM). In a co-culture system, neutrophils strongly suppressed T-cell proliferation. The addition of CB-1158 blocked arginase activity, maintained arginine levels in the media, and allowed T-cells to proliferate in the presence of suppressive myeloid cells, suggesting that arginase is a major mechanism of myeloid cell suppression of lymphocyte proliferation and that CB-1158 can reverse the suppression. To extend these findings to MDSCs, we isolated peripheral monocytic MDSCs (M-MDSCs) or granulocytic MDSCs (G-MDSCs) from cancer patients and found that G-MDSCs expressed higher levels of arginase, depleted more arginine from cell-culture media, and were more suppressive to T-cell proliferation compared to M-MDSCs. Incubation of G-MDSCs with CB-1158 resulted in media arginine levels that could support T-cell proliferation. CB-1158 has high oral bioavailability in mice and rats. Twice-daily oral dosing of CB-1158 produced dose-dependent pharmacodynamic increases in plasma and tumor arginine levels and resulted in single-agent anti-tumor efficacy in the murine syngeneic tumor models Lewis Lung carcinoma (LLC1), Madison-109 lung carcinoma, and B16F10 melanoma. Antibody-mediated depletion of either CD8+ T-cells or NK-cells partially abrogated the anti-tumor effect of CB-1158 in the LLC1 and B16F10 models indicating that the mechanism of action of CB-1158 is immune cell-mediated through the actions of both T-cells and NK cells. Corroborating the immune-mediated anti-tumor efficacy in CB-1158-treated LLC1 tumors, following dosing with CB-1158 we observed increases in tumor infiltrating CD8+ T-cells; increased levels of tumor Th1 T-cell cytokines; increased expression of T-cell and NK-cell activation markers; and increased interferon-inducible genes. Based on the novel mechanism of action of CB-1158, there is potential for enhanced therapeutic benefit by combining CB-1158 with other immunotherapies. We observed improved anti-tumor activity by combining CB-1158 with either epacadostat or anti-PD-L1 in the B16F10 model, with low dose ionizing radiation in the Madison-109 model, and with gemcitabine in the LLC1 model. These results support the development of CB-1158, a first-in-class arginase inhibitor, as a novel immuno-oncology agent targeting the immunosuppressive effects of tumor-infiltrating myeloid cells. Citation Format: Susanne M. Steggerda, Mark Bennett, Jason Chen, Ethan Emberley, Matthew Gross, Tony Huang, Weiqun Li, Andy MacKinnon, Amani Makkouk, Gisele Marguier, Silinda Neou, Alison Pan, Tracy Wang, Melissa Works, Jing Zhang, Winter Zhang, Francesco Parlati. Arginase inhibitor CB-1158 elicits immune-mediated antitumor responses as a single agent and in combination with other immunotherapies [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B045.