Background: Endoglin is a receptor expressed on tumor vessels that is overexpressed in certain tumors, including choriocarcinoma. Anecdotally Bev does not appear to be an active single agent for treatment of choriocarcinoma. Endoglin is upregulated following vascular endothelial growth factor (VEGF) inhibition and may mediate resistance to Bev. TRC105 is an endoglin antibody that may be active in patients with gestational trophoblastic neoplasia (GTN), including choriocarcinoma Methods: We performed a compassionate use study of TRC105 in combination with Bev in two pts with refractory and metastatic choriocarinoma who had exhausted known surgical and chemotherapeutic options. Patients were treated with TRC105 at 10 mg/kg iv weekly with Bev 10 mg/kg iv every two weeks. Results: Pt 1 is a 38 year old woman whose metastatic choriocarinoma progressed following prior therapy with six multi-agent chemotherapy regimens including autologous stem cell transplant. She received 8 monthly cycles of TRC105 and Bev. Beta hCG normalized during the 4th cycle and has remained in within the normal range 4 months after her last treatment, indicating an ongoing complete response. Pt 2 is a 33 year old woman whose metastatic choriocarcinoma progressed following eight prior multi-agent chemotherapy regimens. She was noted to have progression with a rising beta hCG at the end of the initial month of dosing. Treatments were well tolerated by both patients. Toxicities have included grade 1 epistaxis, rash, thrombocytopenia, and hoarseness, grade 2 gingivitis, and grade 3 hypertension, headache, and fatigue. A TRC105 dose reduction to 8 mg/m2 was required with the 4th cycle secondary to gingivitis in Patient 1. Correlative studies are ongoing, including assessment of tumor endoglin expression. Conclusions: TRC105 combined with bev was well tolerated and exhibited activity, including durable complete response in choriocarcinoma, a tumor known to densely express endoglin. Further investigation of TRC105 as a single agent or in combination with Bev is warranted. Assessment of tumor endoglin expression may identify GTN patients who are most responsive to TRC105 therapy.