VB4-845 Oportuzumab monatox is an antineoplastic. Chemically, oportuzumab is a single chain variable fragment of a monoclonal antibody which binds to epithelial cell adhesion molecule (EpCAM, the tumor-associated calcium signal transducer 1). Oportuzumab is fused with Pseudomonas aeruginosa exotoxin A (which is reflected by the monatox in the drug's name).