Abstract P206: AB521 potently and selectively inhibits pro-tumorigenic gene transcription by Hypoxia-Inducible Factor (HIF)-2α in vitro and in vivo

Cells in the solid tumor microenvironment are frequently exposed to hypoxic conditions, necessitating molecular adaptations for survival. Of particular importance are transcriptional changes mediated by heterodimeric Hypoxia-Inducible Factor (HIF) proteins that consist of an oxygen-regulated α monomer (either HIF-1α, -2α, and -3α) coupled to a constitutively expressed β monomer (HIF-1β/ARNT). In normal oxygen conditions, HIF-2α is degraded following ubiquitination by the von Hippel-Lindau (pVHL) E3-ubiquitin ligase complex. Exposure to hypoxia, VHL mutation, or epigenetic silencing of pVHL leads to HIF-2α stabilization and transcription of pro-tumorigenic gene sets in both cancer and non-cancer cells. Inhibition of HIF-2α has been shown clinically to be an effective strategy to mitigate tumor growth, particularly in patients suffering from VHL disease or clear cell renal cell carcinoma (ccRCC), a cancer that has a particularly high prevalence of pVHL dysfunction. Applying a pharmacophore mapping and structure-based design approach, we identified a novel and potent small molecule HIF-2α inhibitor, AB521. AB521 avidly binds the HIF-2α PAS-B domain, preventing HIF-2α-mediated gene transcription. AB521 is characterized by a favorable preclinical pharmacokinetic profile and is projected to be suitable for once-daily dosing in humans. When delivered orally in mice, AB521 significantly regressed established 786-O xenograft tumors and decreased pharmacodynamic markers associated with HIF-2α in a dose-dependent manner. In vitro, AB521 potently inhibited HIF-2α-specific luciferase reporter transcription under high-serum conditions, VEGF protein secretion, colony formation in soft agar, and did not exhibit off-target cytotoxicity in 786-O cells. AB521 selectively inhibited HIF-2α-, but not HIF-1α-, mediated gene expression in hypoxic Hep3B hepatocellular carcinoma cells. AB521 also inhibited the transcriptional activity of endogenous HIF-2α in relevant human primary cell types, including endothelial cells and pro-tumorigenic M2-polarized macrophages. Importantly, inhibiting HIF-2α did not impact functionality of activated hypoxic human T cells, suggesting that AB521 would be favorable combination partner for I-O therapeutic agents. Indeed, expression of CD73, the primary enzyme responsible for synthesis of the immunosuppressive metabolite adenosine, was highly correlated with hypoxic signatures across several indications in publicly available bioinformatic datasets, suggesting combinations with adenosine pathway antagonists in ccRCC and beyond. In summary, AB521 is a novel and selective HIF-2α inhibitor with potent anti-tumor activity. Clinical evaluation of this molecule is expected to begin in the latter part of 2021.

Citation Format: Kelsey E. Sivick Gauthier, Dana Piovesan, Soonweng Cho, Kenneth V. Lawson, Patrick G. Schweickert, Alejandra Lopez, Suan Liu, Timothy Park, Artur Mailyan, Jeremy T. A. Fournier, Joel W. Beatty, Samuel L. Drew, Jarek Kalisiak, Balint Gal, Guillaume Mata, Zhang Wang, Brandon R. Rosen, Clayton Hardman, Matthaw P. Epplin, Kai Yu, Karl T. Haelsig, Lixia Jin, Elaine Ginn, Jennie Au, Cesar A. Meleza, Joel Tencer, Amber Pham, Hyock J. Kwon, Stephen W. Young, Manmohan Leleti, Jay P. Powers, Matthew J. Walters. AB521 potently and selectively inhibits pro-tumorigenic gene transcription by Hypoxia-Inducible Factor (HIF)-2α in vitro and in vivo [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P206.

Design, Synthesis, and Structure-Activity Relationship Optimization of Pyrazolopyrimidine Amide Inhibitors of Phosphoinositide 3-Kinase γ (PI3Kγ)

Phosphoinositide-3-kinase γ (PI3Kγ) is highly expressed in immune cells and promotes the production and migration of inflammatory mediators. The inhibition of PI3Kγ has been shown to repolarize the tumor immune microenvironment to a more inflammatory phenotype, thereby controlling immune suppression in cancer. Herein, we report the structure-based optimization of an early lead series of pyrazolopyrimidine isoindolinones, which culminated in the discovery of highly potent and isoform-selective PI3Kγ inhibitors with favorable drug-like properties. X-ray cocrystal structure analysis, molecular docking studies, and detailed structure-activity relationship investigations resulted in the identification of the optimal amide and isoindolinone substituents to achieve a desirable combination of potency, selectivity, and metabolic stability. Preliminary in vitro studies indicate that inhibition of PI3Kγ with compound 56 results in a significant immune response by increasing pro-inflammatory cytokine gene expression in M1 macrophages.

Abstract 1206: Discovery and characterization of AB521, a novel, potent, and selective hypoxia-inducible factor (HIF)-2α inhibitor

A hypoxic environment is a common characteristic of solid tumors. Cancer cells adapt to hypoxia and become more aggressive by up-regulation of genes associated with metabolism, growth, proliferation, angiogenesis, and erythropoiesis. Hypoxia-inducible factors (HIFs) are the central driving force for the cellular response to hypoxia and regulate a vast array of these genes. HIFs are heterodimers composed of an oxygen-sensitive HIF-α subunit (HIF-1α, HIF-2α, and HIF-3α) and a constitutively expressed HIF-1β subunit. HIF-2α is constitutively synthesized and regulated in an oxygen-dependent manner. Proline residues present in the oxygen-dependent degradation domain of the HIF-2α subunit are hydroxylated and subject to ubiquitination via the von Hippel-Lindau (pVHL) E3 ligase complex for subsequent proteasomal degradation. Under hypoxia or pseudohypoxia, caused by loss of VHL function, this process is inhibited, allowing HIF-2α translocation to the nucleus where, in complex with HIF-1β/ARNT, it promotes transcription of various pro-tumorigenic gene sets. Inhibition of HIF-2α has been demonstrated to be an effective strategy to mitigate tumor growth in the clinic, particularly in clear cell renal cell carcinoma (ccRCC).

Applying a pharmacophore mapping and structure-based design approach, we identified multiple novel series of small molecule HIF-2α inhibitors which avidly bind the HIF-2α PAS-B domain and disrupt dimerization with HIF-1β, preventing HIF-2α-mediated gene transcription. Interrogation of structure activity relationships and pharmacokinetic trends yielded highly optimized inhibitors, including AB521, which potently inhibits HIF-2α reporter transcription and VEGF secretion in a human ccRCC line. AB521 was confirmed to bind HIF-2α via multiple biochemical assays and inhibited HIF-2α-, but not HIF-1α-, mediated gene expression in a hepatocellular carcinoma cell line. Furthermore, AB521 is characterized by a favorable preclinical pharmacokinetic profile with high stability to human hepatocytes and no significant direct or time-dependent inhibition of the major CYP450 drug metabolizing enzymes. AB521 exhibits high bioavailability across preclinical species and is projected to possess a human pharmacokinetic profile suitable for once-daily dosing. In conclusion, we have discovered and extensively characterized a series of novel HIF-2α inhibitors, exemplified by AB521, which potently and selectively inhibits HIF-2α and possesses a favorable preclinical pharmacokinetic profile. The data described herein provides further support for the development of novel HIF-2α inhibitors for cancer therapy.

Citation Format: Kenneth V. Lawson, Kelsey E. Sivick Gauthier, Artur K. Mailyan, Jeremy T. Fournier, Joel W. Beatty, Samuel L. Drew, Jarek Kalisiak, Balint Gal, Guillaume Mata, Zhang Wang, Xuelei Yan, Lixia Jin, Elaine Ginn, Dana Piovesan, Jennifer Au, Cesar A. Meleza, Stephen W. Young, Matthew J. Walters, Manmohan Leleti, Jay P. Powers. Discovery and characterization of AB521, a novel, potent, and selective hypoxia-inducible factor (HIF)-2α inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1206.

Discovery of Potent and Selective 7-Azaindole Isoindolinone-Based PI3Kγ Inhibitors

The successful application of immunotherapy in the treatment of cancer relies on effective engagement of immune cells in the tumor microenvironment. Phosphoinositide 3-kinase γ (PI3Kγ) is highly expressed in tumor-associated macrophages, and its expression levels are associated with tumor immunosuppression and growth. Selective inhibition of PI3Kγ offers a promising strategy in immuno-oncology, which has led to the development of numerous potent PI3Kγ inhibitors with variable selectivity profiles. To facilitate further investigation of the therapeutic potential of PI3Kγ inhibition, we required a potent and PI3Kγ-selective tool compound with sufficient metabolic stability for use in future in vivo studies. Herein, we describe some of our efforts to realize this goal through the systematic study of SARs within a series of 7-azaindole-based PI3Kγ inhibitors. The large volume of data generated from this study helped guide our subsequent lead optimization efforts and will inform further development of PI3Kγ-selective inhibitors for use in immunomodulation.

Discovery of Potent and Selective PI3Kγ Inhibitors

The selective inhibition of the lipid signaling enzyme PI3Kγ constitutes an opportunity to mediate immunosuppression and inflammation within the tumor microenvironment but is difficult to achieve due to the high sequence homology across the class I PI3K isoforms. Here, we describe the design of a novel series of potent PI3Kγ inhibitors that attain high isoform selectivity through the divergent projection of substituents into both the "selectivity" and "alkyl-induced" pockets within the adenosine triphosphate (ATP) binding site of PI3Kγ. These efforts have culminated in the discovery of 5-[2-amino-3-(1-methyl-1H-pyrazol-4-yl)pyrazolo[1,5-a]pyrimidin-5-yl]-2-[(1S)-1-cyclopropylethyl]-7-(trifluoromethyl)-2,3-dihydro-1H-isoindol-1-one (4, IC₅₀ = 0.064 μM, THP-1 cells), which displays >600-fold selectivity for PI3Kγ over the other class I isoforms and is a promising step toward the identification of a clinical development candidate. The structure-activity relationships identified throughout this campaign demonstrate that greater γ-selectivity can be achieved by inhibitors that occupy an "alkyl-induced" pocket and possess bicyclic hinge-binding motifs capable of forming more than one hydrogen bond to the hinge region of PI3Kγ.

Abstract C050: A novel, potent, and selective hypoxia-inducible factor (HIF)-2α antagonist

The microenvironment of solid tumors is known to be hypoxic and requires induction of genes associated with metabolism, growth, proliferation, and angiogenesis for tumor cells to survive and metastasize. The master transcriptional regulator of hypoxia-induced genes is the Hypoxia-Inducible Factor (HIF), consisting of an oxygen-regulated alpha monomer, of which there are three isoforms (HIF-1α, HIF-2α, and HIF-3α), that can heterodimerize with a constitutively-expressed beta monomer (HIF-1β/ARNT) using Per-ARNT-SIM (PAS) protein-protein interaction domains. In normoxia, proline residues present in the oxygen-dependent degradation domain of the HIF-α subunits are hydroxylated, allowing for recognition by the von Hippel-Lindau (pVHL) E3-ubiquitin ligase complex and subsequent proteasomal degradation. Upon exposure to low oxygen conditions or in the case of VHL mutation or silencing, HIF-α subunits accumulate in the cell and mediate transcription of various pro-tumorigenic gene sets. In patients, overexpression of HIF is associated with poor prognosis, and both preclinical and clinical evidence is mounting that suggests inhibiting HIF-2α is a valid approach to destroy tumor cells, particularly in clear cell renal carcinoma, warranting development of next-generation inhibitors. Using a suite of in vitro and in vivo assays designed to evaluate HIF-2α-specific effects, herein we describe pharmacological properties associated with novel, potent, and selective small-molecule antagonists of HIF-2α. These compounds inhibited HIF-dependent reporter gene transcription as well as VEGF protein secretion in a human renal cell adenocarcinoma line. Compounds that were confirmed to bind the HIF-2α PAS-B domain by Microscale thermophoresis (MST) and Thermal shift assay (TSA) also significantly inhibited HIF-2α, but not HIF-1α, target gene expression in a hepatocellular carcinoma cell line (P<0.05). Further, NanoString analyses revealed pathway signatures upregulated in hypoxic conditions, including angiogenesis, metabolism, and metastasis, that were significantly decreased with antagonist treatment. Selective antagonists were evaluated in vivo to determine efficacy and PK-PD relationships. Additionally, syngeneic tumor models were used to characterize the effects of HIF-2α inhibition (alone or in combination with anti-PD-1 and anti-adenosine agents) on tumor growth, immune cell infiltration, and expression of genes associated with adenosine production/signaling. Collectively, these data support the rationale to target HIF-2α for the treatment of cancers, particularly indications with a HIF-2α-driven hypoxia signature, and describe the properties of novel and selective HIF-2α antagonists.

Citation Format: Kelsey E Sivick Gauthier, Kenneth V Lawson, Dana Piovesan, Matthew J Walters, Ada Chen, Xiaoning Zhao, Cesar Meleza, Nikki Kimura, Tim Park, Steve Young, Anh Tran, Samuel L Drew, Lixia Jin, Manmohan Leleti, Elaine Ginn, Jay P Powers. A novel, potent, and selective hypoxia-inducible factor (HIF)-2α antagonist [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C050. doi:10.1158/1535-7163.TARG-19-C050

Unified total synthesis of the natural products endiandric acid A, kingianic acid E, and kingianins A, D, and F

A measure of the strength of a synthetic strategy is its versatility: specifically, whether it allows structurally distinct targets to be prepared. Herein we disclose a unified approach for the total synthesis of natural products of three distinct structural types, all of which occur naturally as racemic mixtures. The point of divergence involves the terminal alkylation of a conjugated tetrayne, and culminates in a significantly shortened synthesis of endiandric acid A (8 steps), the first total synthesis of kingianic acid E (8 steps), and a second-generation synthesis of kingianins A, D, and F (11 steps). Evidence for redox catalysis in the biosynthesis of kingianic acid E is presented.