Background: CD105 (endoglin) is an endothelial cell membrane receptor highly expressed on angiogenic tumor vessels that is essential for angiogenesis and upregulated by hypoxia and VEGF inhibition. TRC105 is an anti-CD105 monoclonal antibody being studied in phase II trials that potentiates chemotherapy in preclinical models. Methods: Pts with advanced solid tumors (for purposes of dose escalation) or pts with metastatic HER2-negative breast cancer (following completion of dose escalation), ECOG PS 0-1, and normal organ function were treated with intravenously administered TRC105 wkly plus capecitabine at 1,000 mg/m² BID for 14 of every 21 days, and assessed for safety, PK, and response. Results: Fourteen patients (median age = 52; M:F 4:10; median of 3 prior regimens; 10 with breast and 4 with colorectal cancer) were enrolled. Dose escalation proceeded from 7.5 mg/kg TRC105 to the recommended single agent phase II dose of 10 mg/kg of TRC105 in combination with capecitabine, without development of dose limiting toxicity. Fourteen pts were treated at 7.5 mg/kg (n=4) or 10 mg/kg (n=10) TRC105 wkly + 1,000 mg/m² BID/14d capecitabine of repeating 21 day cycles. Patients experienced expected TRC105 related adverse events of grade 1 or grade 2 infusion reaction, epistaxis, gingival bleeding, telangiectasia, headache, rash, and fatigue. Grade 3 headache and grade 3 vomiting were each seen in one patient. Adverse events characteristic of each individual drug were not increased in frequency or severity when the two drugs were administered together. Mucocutaneous telangiectasia, a marker of TRC105 target modulation, was observed at both dose levels. A heavily pretreated male breast cancer patient remained on study for 9 months with a RECIST-defined partial response. Stable disease beyond 9 weeks was observed in three patients. Conclusions: The recommended single agent phase II dose of 10 mg/kg TRC105 wkly was well tolerated in combination with capecitabine. The combination treatment could be advanced in HER2-negative breast or colorectal cancer. Clinical trial information: NCT01326481.