Anionic phospholipids, principally phosphatidylserine (PS), become exposed on the external surface of vascular endothelial cells in tumors, providing an excellent marker for tumor vascular targeting. The murine antibody 3G4 has been previously shown to bind to anionic phospholipids in a β2-glycoprotein I (β2GP1)-dependent manner. This antibody inhibits tumor growth in a variety of rodent tumor models that may involve stimulating antibody-dependent cellular cytotoxicity (ADCC) toward tumor vessels. We have generated chimeric mouse/human (bavituximab) and humanized versions of 3G4 for the treatment of human cancer. In the present study, we evaluate the ability of the chimeric and humanized antibodies to exert anti-tumor effects through various mechanisms, including ADCC, on cellular targets expressing PS. We also compare binding kinetics to β2GP1 and demonstrate in vivo targeting to tumor blood vessels. In the presence of bavituximab or humanized 3G4, human peripheral blood mononuclear cells mediate specific killing of human tumor cells treated with paclitaxel or etoposide. Selective targeting of blood vessels within human MDA-MB-435 tumors in nude rats is shown following a single intravenous dose of bavituximab. No toxicities of bavituximab or humanized 3G4 are observed following intravenous administration in rats. The binding kinetics of both bavituximab and humanized 3G4 antibodies to β2GP1 are equivalent using surface plasmon resonance. These data demonstrate that humanized 3G4 has comparable target recognition and effector function to bavituximab and, therefore, could be used to treat human cancer patients.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA