Clear cell renal cell carcinoma (ccRCC) accounts for around 3% of adult malignancies. Its prognosis is closely related with disease stage, with 5-year survival rates ranging from almost 90% to less than 10% for stage I and IV disease, respectively. The last decade has witnessed major advances in the understanding of ccRCC biology and a number of effective treatments are available, such as antiangiogenic agents (multitargeted tyrokine kinase inhibitors or anti-VEGF strategies) and mTOR inhibitors (1). More recently, targeted therapy against the PD-1/PD-L1 immune checkpoint pathway has produced encouraging objective response rates in patients with metastatic ccRCC (2).
However, the development of acquired resistance to targeted therapies has heightened the need for investigation of novel approaches for ccRCC management. Hypoxia is a characteristic feature of solid tumors and the adaptation of cancer cells to hypoxia is instrumental in the development of aggressive phenotype and associated with a bad prognostic in patients (3). At the cellular level, the adaptation to hypoxia is mediated by the hypoxia-inducible factors (HIFs), consisting of an oxygen-sensitive α-subunit and a constitutively expressed β-subunit (also known as ARNT for aryl hydrocarbon receptor nuclear translocator), regulating the expression of target genes that promote angiogenesis, glycolysis, metastasis, increased tumor growth and resistance to treatments (3). HIF-1α and HIF-2α are the best characterized HIF-α subunits. Whereas HIF-1α is ubiquitously expressed, HIF-2α has a more restricted expression and is particularly present in highly vascularized organs or hypoxic tissues including kidney epithelial cells (4). Despite their substantial sequence similarity and co-expression in various cancer cell types, HIF-1α and HIF-2α play non-overlapping roles in tumor progression (5). The distinct roles of HIF-1α and HIF-2α in promoting tumor growth have been mainly defined in ccRCC because a majority of ccRCC patients (50–80%) exhibit a genetic inactivation of von Hippel-Lindau (VHL) gene resulting in the loss of pVHL, which normally mediates ubiquitination of HIF-α and its subsequent degradation, resulting in a constitutive expression of either HIF-1α and HIF-2α or HIF-2α alone (6). The role for HIF-2α as a key driver in the development and progression of the disease has been firmly established (7,8), whereas the activity of HIF-1α has been shown to be dispensable as its expression is often silenced (9).
These data suggest the specific function of HIF-2α in ccRCC highlight the therapeutic potential of HIF-2α antagonists in this disease. Although transcription factors are typically considered “undruggable”, the PAS-B domain of the HIF-2α subunit contains a large cavity within its hydrophobic core that provides a singular foothold for small-molecule to disrupt HIF-2α dimerization to ARNT (10,11). Recently published studies show that HIF-2α can be targeted by selective and orally active small-molecule inhibitors, paving the way to a novel strategy to treat ccRCC (12-14). A structure-based design approach developed by Peloton Therapeutics led to the discovery of closely related compounds including PT2399 and PT2385, which were evaluated for their biological activities. Two studies published in Nature clearly established that PT2399 specifically and functionally inhibited the dimerization of HIF-2α (but not HIF-1α) with its partner ARNT. As anticipated, specific HIF-2α target genes including VEGF-A, GLUT1, PAI-1 or CCND1 were significantly reduced demonstrating the on-target effects of PT2399. Using a large repertoire of ccRCC cell models (14), PT2399 was found to cause tumor regression in orthotopic xenografts, which correlated with reduced circulating tumor-derived VEGF. Similar findings were reported in the second study conducted with an extensive panel of patient-derived xenografts (13). However, differential responses to PT23099 were observed in both studies. Cell lines or patient-derived xenografts highly sensitive to PT2399 had stronger HIF-2α levels than poorly sensitive ones, suggesting that assessing HIF-2α activity/expression would be required as a predictive biomarker of efficacy. Importantly, prolonged treatment with PT2399 led to resistance, generated by mutations in both HIF-2α and its binding partner ARNT. Both mutations preserved HIF-2α dimerization despite the presence of PT2399 (13). These findings support the notion that second generation inhibitors and/or complementary approaches (15) would be ineluctable. In addition, acquisition of a p53 mutation might represent another mechanism of resistance to PT2399 as reported in the 786-O ccRCC cells despite their considerable amount of HIF-2α (14). These data suggest that mutations in some genes like p53 may impact response to HIF-2α antagonists similar to what has been observed in patients resistant to VEGF-targeted therapies (16). A third study published in Cancer Research describe the effects of a the related small-molecule inhibitor PT2385 (12). In line with the other studies, PT2385 showed a strong inhibitory on HIF-2α-controlled genes without any effect on HIF-1α-controlled genes, associated with a potent anti-tumor activity in xenograft models and decreased circulating VEGF-A level. Noteworthy, in animal studies, PT2385 did not exhibit the cardiovascular safety concerns observed with anti-VEGF therapies such as hypertension (17). PT2385 might thus represent a novel therapeutic option with more desirable safety profile than current anti-angiogenic approaches. As such, a Phase 1, dose-escalation trial of PT2385 is ongoing in patients with advanced ccRCC (ClinicalTrials.gov Identifier: NCT02293980).
In summary, the validation of HIF-2α as a bona fide target in ccRCC described in these studies in conjunction with the restricted expression of HIF-2α in normal adult physiology suggests that therapeutic approaches based on HIF-2α antagonists might be favorable for patients. However, the differential sensitivity to HIF-2α antagonists along with the potential mechanisms of resistance reported in these studies advocate for a need for companion predictive biomarkers as well as paving the way for second generation inhibitors or complementary inhibitory approaches.
Conflicts of Interest: The author has no conflicts of interest to declare.
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