naptumomab estafenatox naptumomab estafenatox naptumomab estafenatox (abr-217620) is a drug being developed for the treatment of various types of cancer like non-small cell lung carcinoma and renal cell carcinoma. naptumomab estafenatoxschematic image of naptumomab estafenatox. vh, vl: variable (antigen binding) domains of antibody fragment ch1, cl: constant domains of antibody fragment sag: superantigen sea/e-120monoclonal antibodytypefab fragmentsourcemousetarget5t4clinical dataatc codenoneidentifierscas number676258-98-3 nchemspidernoneunii93t929w6lcchemical and physical dataformulac3255h5025n855o1050s18molar mass73513.02 g·mol−1 ny (what is this?) (verify) mechanism of action chemically, it is a fusion protein consisting of the antigen-binding fragment (fab) of a monoclonal antibody with the superantigen staphylococcal enterotoxin a (sea/e-120, "estafenatox"). the fab binds to 5t4, an antigen expressed by various tumor cells, and the superantigen induces an immune response by activating t lymphocytes.