Abstract A192: The potent and selective phosphoinositide-3-kinase-γ inhibitor, IPI-549, inhibits tumor growth in murine syngeneic solid tumor models through alterations in the immune suppressive microenvironment

Introduction The phosphoinositide-3-kinase (PI3K) lipid kinases transduce signals in response to various stimuli in different cell types. PI3K-γ is expressed in immune cells and has limited expression in epithelial cancer cells. Genetic inactivation of PI3K-γ highlights its role in the development and function of myeloid-derived cells that constitute a key component of the suppressive tumor microenvironment (Schmid 2011). Targeting PI3K-γ in tumor-associated myeloid cells could potentially relieve tumor immune tolerance, enabling the immune system to attack tumor cells more effectively. To date, potent and selective PI3K-γ inhibitors with drug-like properties have not been available to test this hypothesis. We now report the structure, biochemical, cellular, and in vivo properties of a potent and selective, small molecule inhibitor of PI3K-γ, IPI-549, and provide data to support the therapeutic potential of breaking tumor immune tolerance through PI3K-γ inhibition. Results Discovery efforts identified a highly selective inhibitor of PI3K-γ, IPI-549, with pharmaceutical properties suitable for further development. Binding studies with IPI-549 revealed a KD of 0.29 nM for PI3K-γ and enzymatic assays confirmed the selectivity of IPI-549 for PI3K-γ (>200-fold) over the other Class I PI3K isoforms. Comparison of IPI-549 to previously reported PI3K-γ inhibitors in this enzymatic confirmed its unique potency and selectivity for PI3K-γ. Cellular assays for individual Class I PI3K isoform activity demonstrated that IPI-549 is highly potent and specific for PI3K-γ (IC50 of 1.2 nM; >140-fold selectivity). In kinase screens, IPI-549 is selective for PI3K-γ over other kinases, receptors, ion channels, and transporters. In vitro assays demonstrated that IPI-549 blocked immune suppressive M2 murine macrophage polarization in response to IL-4 and MCSF1. Pharmacokinetic studies in mice demonstrated IPI-549 to be orally bioavailable with a long plasma half-life enabling selective inhibition of PI3K-γ relative to other Class I PI3K isoforms. To characterize IPI-5499s ability to inhibit PI3K-γ in vivo, mice with air pouches treated with IPI-549 showed dose responsive inhibition of PI3K-γ-dependent neutrophil migration. The effect of IPI-549 on tumor growth was tested in murine syngeneic solid tumor models. Mice treated with IPI-549 demonstrated significant tumor growth inhibition in multiple models. Studies to elucidate the mechanism of tumor growth inhibition indicated that IPI-549 affects suppressive myeloid cell numbers and/or function, leading to an increase in cytotoxic T-cell numbers and activity, as assessed by marker studies. Nude or CD8 T-cell depleted mice studies demonstrated a T-cell dependence of IPI-549-mediated tumor growth inhibition. Finally, in vivo studies with IPI-549 in combination with immune checkpoint inhibitors or following chemotherapy showed increased tumor growth inhibition compared to monotherapies. Conclusions IPI-549 is a potent and selective inhibitor of PI3K-γ with pharmaceutical properties that allow for the selective inhibition of PI3K-γ in vivo. Our findings provide evidence that targeted inhibition of PI3K-γ by IPI-549 can restore antitumor immune responses and inhibit solid tumor growth in preclinical models. Citation Format: Karen McGovern, Janid Ali, Erin Brophy, Alfredo Castro, Jonathan DiNitto, Catherine Evans, Kerrie Faia, Stanley Goldstein, Nicole Kosmider, Andre Lescarbeau, Tao Liu, Christian Martin, Somarajan Nair, Melissa Pink, Jennifer Proctor, Matthew Rausch, Sujata Sharma, John Soglia, Jeremy Tchaicha, Martin Tremblay, Vivian Villegas, Kerry White, David Winkler, Vito Palombella, Jeffery Kutok. The potent and selective phosphoinositide-3-kinase-γ inhibitor, IPI-549, inhibits tumor growth in murine syngeneic solid tumor models through alterations in the immune suppressive microenvironment. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A192.