An antibody-drug conjugate composed of an antibody directed against 5T4 and conjugated, via the stable linker maleimidocaproyl (mc), to the microtubule inhibitor monomethyl auristatin phenylalanine (MMAF), with potential antineoplastic activity. Upon administration, the antibody moiety of PF-06263507 selectively binds to cells expressing the 5T4 oncofetal antigen. After internalization and enzymatic cleavage of the immunoconjugate within the tumor cell cytosol, free MMAF binds to tubulin and inhibits its polymerization, which may result in G2/M phase arrest and tumor cell apoptosis. 5T4, a transmembrane glycoprotein, is overexpressed by a variety of cancer cell types; its expression is correlated with increased invasiveness.