Purpose: The heterodimeric transcription factor HIF-2 is arguably the most important driver of clear cell renal cell carcinoma (ccRCC). Although considered undruggable, structural analyses at the University of Texas Southwestern Medical Center (UTSW, Dallas, TX) identified a vulnerability in the a subunit, which heterodimerizes with HIF1b, ultimately leading to the development of PT2385, a first-in-class inhibitor. PT2385 was safe and active in a first-in-human phase I clinical trial of patients with extensively pretreated ccRCC at UTSW and elsewhere. There were no dose-limiting toxicities, and disease control !4 months was achieved in 42% of patients.Patients and Methods: We conducted a prospective companion substudy involving a subset of patients enrolled in the phase I clinical trial at UTSW (n ¼ 10), who were treated at the phase II dose or above, involving multiparametric MRI, blood draws, and serial biopsies for biochemical, whole exome, and RNAsequencing studies.Results: PT2385 inhibited HIF-2 in nontumor tissues, as determined by a reduction in erythropoietin levels (a pharmacodynamic marker), in all but one patient, who had the lowest drug concentrations. PT2385 dissociated HIF-2 complexes in ccRCC metastases, and inhibited HIF-2 target gene expression. In contrast, HIF-1 complexes were unaffected. Prolonged PT2385 treatment resulted in the acquisition of resistance, and we identified a gatekeeper mutation (G323E) in HIF2a, which interferes with drug binding and precluded HIF-2 complex dissociation. In addition, we identified an acquired TP53 mutation elsewhere, suggesting a possible alternate mechanism of resistance.Conclusions: These findings demonstrate a core dependency on HIF-2 in metastatic ccRCC and establish PT2385 as a highly specific HIF-2 inhibitor in humans. New approaches will be required to target mutant HIF-2 beyond PT2385 or the closely related PT2977 (MK-6482).

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Our results validate the IMDC model for first-line pazopanib in mRCC and confirm the effectiveness and safety of this treatment.

Highlights d Generation of a large PDX library from a diverse population d The PDX library is characterized by next-generation sequencing (exome and RNA-seq) d Interactive tool for selecting TG lines representative of RCC molecular subtypes d Precision diagnostics and therapeutic applications illustrated

5060 Background: Immunotherapy (IO)-based combinations have replaced tyrosine kinase inhibitors (TKI) as standard upfront treatment of metastatic RCC pts. Selection of 2L treatment after progression to novel combinations in 1st-line (1L) is mostly based on retrospective series or subgroups analysis of randomized trials. We aim to evaluate the activity and safety of sunitinib in advanced RCC after progression on an IO-based 1L approach. Methods: This is a prospective, non-randomized, open-label and multicenter phase II study. Pts with ECOG 0-2 and locally advanced or metastatic RCC after treatment in 1L with a prior PD-1 and/or PD-L1 and/or CTLA-4 inhibitor were included. Sunitinib was administered at 50 mg/day on a 4/2 schedule until disease progression. Primary endpoint was overall response rate (ORR) by RECIST v 1.1 criteria. Results: Twenty pts were enrolled in the study. Median age was 66 yo, 70% had intermediate prognosis by Heng's scale, and 88% ECOG 1. 45% of pts received atezolizumab, 30% pembrolizumab, 20% nivolumab and 15% ipilimumab-based regimens as 1L. ORR to 1L treatment was 45%. Median time from end of 1L to sunitinib onset date was 1.1 months (0.3-22.1). Two (10%) pts had partial response with sunitinib and 11 (55%) stable disease for a total disease control rate of 65%. With a median follow-up of 8.8 months, median PFS was 6.8 months (0.0-13.9) and median OS 13.6 months (10.5-16.6). Median duration of treatment was 4 months (0.9-16-2). Most common treatment-related adverse events, all grades, were asthenia (55%), dysgeusia (35%), diarrhea (30%), hypertension (30%), mucosal inflammation (25%), palmar-plantar erythrodysesthesia (25%), anemia (20%), neutropenia (20%) and nausea (15%); grade 3 were asthenia (15%), hypertension (10%) and neutropenia (10%). Conclusions: To our knowledge, the INMUNOSUN trial is the first study to evaluate prospectively the activity of a single agent TKI in a pure 2L setting of metastatic RCC pts treated with an IO-based approach upfront. ORR and PFS with sunitinib seem lower than expected when used as a 1L. Consistency in toxicity profile was observed. Clinical trial information: NCT03066427 .

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