Background: Activin receptor-like kinase 1 (ALK1) is a novel target in angiogenesis involved in blood vessel maturation and stabilization. Concurrent targeting of ALK1 and vascular endothelial growth factor (VEGF) signaling results in dual angiogenic blockade and augmented inhibition of tumor growth in RCC xenograft models. Dalantercept (Dal) is an ALK1 receptor-fusion protein that acts as a ligand trap and has demonstrated monotherapy activity with an acceptable safety profile in a completed phase I study. Methods: Part 1 of this study evaluated the safety, tolerability, and preliminary activity of escalating dose levels of Dal plus fixed dose axitinib in pts with advanced RCC. 3-6 pts each received Dal (0.6, 0.9, or 1.2 mg/kg) SC Q3W and axitinib 5 mg PO BID. Key eligibility: predominantly clear cell RCC, 1 prior VEGFR TKI, ≤3 prior tx. Results: As of September 12, 2014, 29 pts were enrolled in three cohorts (n=6, 9, 14) at dose levels of 0.6, 0.9 and 1.2 mg/kg, respectively. At the 1.2 mg/kg dose level, 1 DLT of grade 3 abdominal and back pain occurred (n=1) in addition to more Dal associated edema events including peripheral edema (n=6), fluid overload (n=1), ascites (n=1), and pleural effusion (n=1). The 0.9 mg/kg dose level was well tolerated (3 pts with low grade edema; no pleural effusions or ascites) and selected for Part 2. Frequent AEs regardless of attribution: fatigue, diarrhea, dysphonia, nausea, peripheral edema, creatinine rise, epistaxis, hypertension, arthralgia, hand-foot rash, and cough. 28 pts were evaluable for response by RECIST 1.1. The ORR was 25% (n=7) and 31.3% (n=5) among pts who received ≥ 2 prior txs (n=16), including VEGF targeted tx, mTOR inhibitors, and immune tx. Of those evaluable for disease control at 6 mos. (n=25, 3 active pts have not reached 6 mos.), 52% (n=13) remained progression free. PFS data are maturing and will be presented. Conclusions: The combination of dalantercept and axitinib is well tolerated and associated with encouraging activity in pts with prior VEGF, mTOR, and immune therapies. Part 2 of this study randomizes pts to dalantercept + axitinib vs. placebo + axitinib and is actively accruing patients. Clinical trial information: NCT01727336