Phase I clinical trial of bavituximab (Bavi) and paclitaxel (P) in patients (pts) with HER2-negative metastatic breast cancer (MBC).

567 Background: Bavituximab is a novel tumor vascular targeting agent. It is an unconjugated, chimeric immunoglobulin G1 monoclonal antibody directed against phosphatidylserine (PS). PS is externally expressed on endothelial cells when exposed to hypoxia and/or other physiological stressors frequently observed in tumor-associated vasculature. On attaching to PS, Bavi triggers antitumor effects by inducing antibody-dependent cellular cytotoxicity and promoting antitumor immunity. Methods: We conducted a phase I clinical trial of Bavi in combination with P in pts with HER-2 negative MBC. Pts were treated with weekly P (80mg/m2 for 3/ 4 weeks) and weekly Bavi (3mg/kg for 4/ 4 wks). Microparticle generation, activation and circulating endothelial cell apoptotic markers were measured by flow cytometry. Results: 14 pts with MBC were enrolled. Median age at MBC diagnosis was 50yrs. Seven pts had triple negative MBC; 4 pts presented with de-novo metastatic disease. Prior treatments included chemotherapy in the adjuvant/neoadjuvant setting (8); metastatic setting (2); adjuvant hormonal therapy (3). Best responses to date include complete response (1), partial response (6), stable disease (1), progressive disease (2) and too early to evaluate (4). Bavi related toxicities include grade2/3 infusion related reactions in 2 pts (1 discontinued Bavi). Bone pain, fatigue, headache and neutropenia were the most common adverse effects. 1 pt had a catheter associated upper extremity thrombosis requiring anticoagulation. Laboratory correlative studies revealed no evidence of platelet activation of PAC-1 or P-Selectin. Median platelet and endothelial microparticles decreased from baseline in response to therapy. Conclusions: Bavi is a novel vascular targeting agent that is well tolerated in combination with P. Early results show promise in terms of clinical responses with 8 of 10 evaluable patients having clinical benefit. Early biomarker results suggest no effect of therapy on platelet activation but decreases in circulating microparticles is observed. Clinical trial information: NCT01288261.