Renal cell carcinoma of clear cell type (ccRCC), the most common type, is characterized by inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene, which is observed in >85% of tumors. Because no other gene is mutated at a similar frequency, and VHL mutations are a known initiating event, VHL loss is regarded as the governing event in ccRCC development. Loss of VHL activates HIF-2, which is sufficient to reconstitute tumor development in ccRCC cell lines restored with VHL. Arguably, HIF-2 is the most attractive target for drug development in ccRCC. HIF-2 is implicated in cell survival, proliferation, pluripotency and angiogenesis. Among these pathways, only angiogenesis is targeted clinically, today. Targeting HIF-2 would disrupt multiple pro-oncogenic pathways simultaneously, but as a transcription factor, HIF-2 is considered undruggable. HIF- 2 is an obligatory heterodimer of a constitutive HIF-1? subunit?and a regulated HIF-2? subunit, which was discovered at UT Southwestern Medical Center (UTSW). Detailed structural studies of HIF-2??at UTSW identified a 2803 cavity within the PAS-B domain, a domain implicated in heterodimerization. Despite the fact that this cavity is buried, UTSW investigators determined that it was accessible and hypothesized that it may provide a foothold for a small molecule. Such a molecule may induce conformational changes transmitted to the surface of the domain disrupting the interaction with HI F-1? and antagonizing HIF-2 function. Proof-of-principle experiments confirmed this idea and an ingenious high-throughput screen involving 200,000 compounds led to the identification of small-molecule inhibitors of HIF-2 heterodimerization. The lead compound was optimized through medicinal chemistry efforts at UTSW and was subsequently licensed to Peloton Therapeutics, Inc., a company founded by UTSW investigators and located in the UTSW BioCenter. Peloton has developed this compound into a highly-potent and selective inhibitor of HIF-2 (HIF2-I) with appropriate pharmacokinetic properties and oral bioavailability. This drug induces regression of established ccRCC cell line xenografts, and preliminary experiments show remarkable activity in some patient-derived xenografts (PDX)/tumorgraft models. However, not every ccRCC tumorgraft responds.
The aims of this project are: (i) to identify a biomarker of HIF-2 dependency in ccRCC, (ii) to anticipate mechanisms of acquired resistance to HIF2-I, and (iii) to evaluate multiparametric MR imaging as a pharmacodynamic biomarker in a first-in-human phase I clinical trial of this first-in-class HIF2-I in metastatic ccRCC patients. The successful development of an inhibitor against HIF-2 serves as a paradigm for innovation. To our knowledge, this is the first isoform-specific inhibitor of a transcription factor of the non- nuclear hormone receptor family. This illustrates how ground-breaking basic science discoveries can be translated into innovative drugs through successful academic-industry partnerships.
Arguably, the most important driver of kidney cancer development is the HIF-2? protein, which heretofore has been considered ?undruggable.? HIF-2?, discovered and characterized at the atomic level by UT Southwestern scientists, was found to have a structural feature that could be exploited to develop a drug. Through ingenuity and perseverance a highly potent first-in-class drug was developed that will be evaluated for its activity against renal cancer in mice and humans.
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