Background: Hypoxia-inducible factor (HIF)-2α is a transcription factor that is a key oncogenic driver in RCC. MK-6482 is a first-in-class small molecule HIF-2α inhibitor that blocks the heterodimerization of HIF-2α with HIF-1β and induces regression in mouse xenograft RCC models. Methods: Pts with advanced ccRCC who had received at least 1 prior therapy were enrolled in an expansion cohort from the first-in-human phase 1/2 study of MK-6482 in advanced solid tumors (NCT02974738). Pts were administered 120 mg of MK-6482 orally once daily. Primary end point: safety. Key secondary end points: ORR, duration of response (DOR), and PFS. Results: Fifty-five pts were enrolled in the dose expansion cohort. Median (range) number of prior therapies was 3 (1-9); 67% received anti–PD-1 and anti-VEGF agents. Five pts (9%) were favorable risk, 40 (73%) were intermediate risk, and 10 (18%) were poor risk by IMDC criteria. With a median follow-up of 13 mo the most common all-grade, all-cause AEs > 30% were anemia (75%), fatigue (67%), dyspnea (47%), nausea (33%), and cough (31%). Anemia (26%) and hypoxia (15%) were the most common grade 3 AEs. No grade 4/5 drug-related AEs were observed. ORR was 24% with 13 confirmed PRs. Thirty-one pts (56%) had SD, with a disease control rate (CR+PR+SD) of 80%. Thirty-five of 52 (67%) pts with baseline and postbaseline assessments had tumor shrinkage. Median DOR was not reached; 81% of pts had response ≥6 mo per Kaplan-Meier estimate. Sixteen pts (29%) continued treatment beyond 12 mo. By IMDC risk, 2/5 pts with favorable risk had PR (ORR = 40%), 10/40 with intermediate risk had PR (ORR = 25%), and 1/10 with poor risk had PR (ORR = 10%); disease control rate was 100%, 80%, and 70%, respectively. Median PFS for the total population was 11.0 mo; the 12 mo PFS rate was 49%. Median PFS for favorable, intermediate, and poor IMDC risk was 16.5, 11.0, and 6.9 mo, respectively. As of May 15, 2019, 30 pts (55%) discontinued due to PD, 2 (4%) due to AEs. Sixteen pts (29%) had treatment ongoing. Conclusions: MK-6482 is well tolerated with a favorable safety profile and demonstrated promising single-agent activity in heavily pretreated pts with ccRCC across IMDC risk groups. A phase 3 trial in a similar population is planned. Clinical trial information: NCT02974738