BACKGROUND On the basis of potential additive or synergistic immunostimulatory antitumor effects, in this phase 1 study, the authors evaluated the combination of sunitinib and tremelimumab (CP-675206; an antibody against cytotoxic T-lymphocyte-associated antigen 4 [CTLA4]) in patients with metastatic renal cell carcinoma (mRCC) was evaluated. METHODS Adult patients with mRCC who had received ≤ 1 previous systemic treatment received tremelimumab (6 mg/kg, 10 mg/kg, or 15 mg/kg) intravenously once every 12 weeks and oral sunitinib (50 mg daily for 4 weeks then 2 weeks off or 37.5 mg daily as a continuous dose). The primary objective was to determine the maximum tolerated dose (MTD). Secondary objectives were to assess antitumor activity, safety, and pharmacokinetics. RESULTS Twenty-eight patients were enrolled. Two of 5 patients who received 50 mg sunitinib plus tremelimumab 6 mg/kg experienced dose-limiting toxicities (DLTs), and no further enrollment to the combination with sunitinib 50 mg dosing was pursued. Among patients who received continuous sunitinib 37.5 mg daily, 1 of 7 patients who received tremelimumab 10 mg/kg plus sunitinib suffered a sudden death, and 3 of 6 patients who received tremelimumab 15 mg/kg plus sunitinib experienced DLTs. An expansion cohort (n = 7) was enrolled at tremelimumab 10 mg/kg plus sunitinib 37.5 mg daily; 3 of those patients experienced DLTs. Overall, rapid-onset renal failure was the most common DLT. Nine of 21 patients who were evaluable for response achieved partial responses (43%; 95% confidence interval, 22%-66%), and 4 of those responses were ongoing at the time of the current report. CONCLUSIONS In this study of tremelimumab plus sunitinib, rapid-onset acute renal failure was observed unexpectedly, and further investigation of tremelimumab doses > 6 mg/kg plus sunitinib 37.5 mg daily is not recommended. Preclinical investigation may be warranted to understand the mechanism of renal toxicity.