Background: Resistance to VEGF-targeted therapy is a major challenge in contemporary treatment of metastatic renal cell carcinoma (mRCC), and endoglin (CD105) activation may be an important mechanism leading to resistance. Endoglin is an essential angiogenic receptor expressed on proliferating tumor vessels and mRCC cancer stem cells that is upregulated following VEGF inhibition. TRC105 is an anti-endoglin monoclonal antibody that potentiates bevacizumab (Bev) and VEGF receptor tyrosine kinase inhibitors (VEGFR TKI) in preclinical models. Methods: Heavily-pretreated mRCC pts with ECOG PS 0-1, and acceptable organ function were treated with TRC105 weekly (8 mg/kg and then 10 mg/kg) in combination with axitinib (initially at 5 mg PO BID and then escalated per patient tolerance to a maximum of 10 mg PO BID). Results: Eighteen mRCC pts (median age=61.5; M:F 16:2; median number of prior therapies=3, including >1 VEGFR TKI, clear cell=13, prior axitinib allowed) were treated. TRC105 dose escalation proceeded from 8 mg/kg (n=3) to 10 mg/kg (n=15) without dose limiting toxicity. Low grade AEs characteristic of each drug were not increased in frequency or severity at the recommended phase 2 doses of the two drugs. Three pts (18%) were PR by RECIST and 8 of 17 pts (47%) exhibited >10% tumor reduction. Median PFS is not mature and is at least 5.8 months in the overall population and at least 5.9 months in ccRCC pts. The single patient who progressed on axitinib immediately prior to study entry remains progression free at month 5 with minor tumor regression. Conclusions: TRC105 at 8 and 10 mg/kg was well tolerated with axitinib in mRCC pts with signs of activity. A multicenter randomized phase II trial of axitinib +/- TRC105 is accruing at this time. Clinical trial information: NCT01806064