Abstract 4022: PT2385, a novel HIF-2α antagonist, combines with checkpoint inhibitor antibodies to inhibit tumor growth in preclinical models by modulating myeloid cells and enhancing T cell infiltration

The majority of clear cell renal cell carcinoma (ccRCC) have deficiency in the gene encoding the von Hippel Landau (VHL) protein, as a result of DNA copy loss, non-sense mutations, and epigenetic silencing. Deficiency in VHL and its E3 ligase activity results in stabilization of the transcription factors hypoxia-inducible factor (HIF)-1α and HIF-2α. In ccRCC, HIF-2α has been proposed to function as an oncogenic driver, as its depletion in tumor cells results in inhibition of tumor growth.

We previously described the identification of a potent and selective small molecule antagonist of HIF-2α, PT2385 that disrupts the hetero-dimerization of HIF-2α with the aryl hydrocarbon receptor nuclear translocator. PT2385 inhibits growth of ccRCC tumor xenografts derived from cancer cell lines and from patients’ tumors. Inhibition of tumor growth is mediated by suppression of HIF-2α target genes, such as cyclin D1 and VEGFA, that promote tumor growth.

In addition to a direct role in the transcription regulation of growth-promoting genes in ccRCC, HIF-2α has also been implicated in the pro-tumorigenic property of the tumor microenvironment. HIF-2α is expressed in cells of the myeloid lineage, and HIF-2α expression in tumor-associated macrophages was reported in a number of tumor types. A role for HIF-2α in the polarization of macrophages to the M2 phenotype has also been described. The availability of PT2385 provides an opportunity to confirm the involvement of HIF-2α in immune suppression by the tumor microenvironment, and to assess the therapeutic utility of HIF-2α antagonism in tumors other than ccRCC. PT2385 was evaluated for its ability to inhibit tumor growth in syngeneic mouse tumor models. The mouse tumor cell lines used in these models do not express HIF-2α and PT2385 has no single agent efficacy. However, the combination of PT2385 with antibodies to immune checkpoint control molecules (PD-1, PD-L1, and CTLA4) yielded additive or synergistic efficacy in a model-dependent manner. Immune phenotyping of the treated tumors was performed by immunohistochemistry and flow cytometry. The analyses revealed that tumor growth inhibition by the combination regimens is accompanied by an increase in T cell infiltration and changes in macrophage and myeloid-derived suppressor cell populations in the tumors. Changes in cytokine expression were also observed.

Results of our studies show that combinations of PT2385 and immune checkpoint inhibitor antibodies are well tolerated and efficacious in preclinical models. The results are consistent with the hypothesis that HIF-2α plays an immunosuppressive role in the tumor microenvironment, and its inhibition provides an additional approach to reverse immune evasion by tumors. The combination of PT2385 and immune checkpoint inhibitors is being further evaluated in clinical trials.

Citation Format: Guangzhou Han, Christina Stevens, Zhaodan Cao, Shanhai Xie, Melissa Maddie, Barry Goggin, Eli Wallace, John Josey, Tai W. Wong. PT2385, a novel HIF-2α antagonist, combines with checkpoint inhibitor antibodies to inhibit tumor growth in preclinical models by modulating myeloid cells and enhancing T cell infiltration. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4022.

Abstract B267: AR-mTOR-1: A potent, selective mTORC 1/2 kinase inhibitor for the treatment of malignancy

Through alterations in the PTEN and PI3K genes, the PI3K / Akt pathway is constitutively activated in human cancers. mTOR kinase plays an unique role in this pathway as the key component of two independent signaling complexes (mTORC1 (raptor - rapamycin sensitive) and mTORC2 (rictor - rapamycin insensitive)) that are involved at multiple branch points in this signaling cascade. As such, inhibition of mTOR kinase inactivates both mTOR complexes and therefore serves as an attractive means to target this integral pathway for the treatment of human malignancy.

We report the biological and pharmaceutical evaluation of our selective mTOR 1/2 kinase inhibitor AR-mTOR-1. AR-mTOR-1 inhibits mTOR kinase with an IC50 of < 10 nM while maintaining selectivity against PI3K as well as a panel of additional lipid kinases, serine/threonine kinases and cytoplasmic and receptor tyrosine kinases. In mechanistic cellular assays, AR-mTOR-1 inhibits pAkt (Ser473), 4E-BP1 (Thr36/46) and pS6 (Ser235/6) with nanomolar potency, thus demonstrating inhibition of signaling from both mTORC1 and mTORC2 complexes. In line with its enzymatic selectivity over PI3K , AR-mTOR-1 does not significantly inhibit pAkt (Thr308) in cells. AR-mTOR-1 is broadly anti-proliferative in both epithelial and hematologic cancer cell lines, irrespective of mutational status, with IC50's ranging from 30 to 550 nM across 20 cell lines, suggesting the potential for broad clinical activity. Once daily dosing of AR-mTOR-1 in several mouse xenograft models, including PC3 prostate, U87 glioblastma, and H460 lung, results in robust anti-tumor activity. Finally, AR-mTOR-1 possesses desired in vitro and in vivo preclinical ADME properties including low clearance, high permeability and good absorption in three preclinical species. In total these data demonstrate that selectively targeting mTORC1 and mTORC2 with AR-mTOR-1 holds promise for broad spectrum clinical utility as a single agent across a wide array of cancer types.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B267.