Background: CB-839 is a first-in-class inhibitor of GLS that targets tumor glutamine utilization, a pathway upregulated in kidney cancer. CB-839 combines with E or Cabo in pre-clinical RCC models to inhibit metabolic pathways resulting in enhanced anti-tumor activity. The CX-839-001 study includes cohorts evaluating the safety, efficacy and recommended Phase 2 dose (RP2D) of CB-839 in combination with E (CBE) or Cabo (CB-Cabo) in pts with mRCC. Here we report results from the CBE and CB-Cabo cohorts. Methods: Eligibility criteria included mRCC with cc or pap histology, ECOG 0-1, RECIST measurable disease and, for cc pts, treatment with at least 1 prior anti-VEGF therapy. Pts received escalating doses of CB-839 (PO BID) in a 3+3 design, either 400-800 mg with E (10 mg PO QD) or 600-800 mg with Cabo (60 mg PO QD). Tumor response (RECIST 1.1) was assessed every 8 wks. Results: 27 pts (22 cc, 3 pap, 2 other) were enrolled in CB-E escalation (n = 17) and expansion (n = 10) cohorts (7 at 400 mg, 13 at 600 mg, 7 at 800 mg) and 11 pts (9 cc, 2 pap) in CB-Cabo escalation cohorts (6 at 600 mg, 5 at 800 mg). Median prior lines of therapy was 2 for CBE (including 64% of cc pts with ≥2 prior anti-VEGF therapies) and 3 for CB-Cabo (range 0-6). A maximum tolerated dose was not reached; 800 mg was selected as the RP2D for CB-839. The most common treatment-related Gr≥3 AEs were fatigue (11%), anemia, hyperglycemia, and neutropenia (7% each) for CBE (N = 27) and diarrhea (14%) and platelet count decreased (14% including 1 DLT at 600 mg) for CB-Cabo (N = 7). Of 24 evaluable CBE pts there was 1 PR and 21 SD for a disease control rate (DCR = CR + PR + SD) of 92% with a median PFS of 7.1 mo (95% CI 4 - 11). For 9 evaluable CB-Cabo pts the ORR and DCR were 44% (4 PR) and 100%, respectively. Conclusions: The tumor metabolism inhibitor CB-839 combines with full dose of RCC targeted therapies, E and Cabo, with good tolerability and encouraging clinical activity in heavily-pretreated mRCC pts. CB-Cabo ORR and DCR of 44% and 100% are promising, and CBE DCR of 92% and PFS of 7.1 mo compare favorably to historical E monotherapy. Randomized Phase 2 studies have been initiated (CBE) or are planned (CB-Cabo). Clinical trial information: NCT02071862