An antibody-drug conjugate (ADC) containing a humanized immunoglobulin G1 kappa (IgG1k) monoclonal antibody directed against prostate-specific membrane antigen (PSMA) and site-specifically conjugated to two of the microtubule-disrupting toxin amberstatin (AS269), with potential antineoplastic activity. Upon administration of anti-PSMA ADC ARX517, the monoclonal antibody moiety selectively binds to PSMA, a protein which is abundantly expressed on the surface of metastatic and hormone-refractory prostate cancer cells. Upon internalization, amberstatin binds to tubulin and inhibits its polymerization, resulting in G2/M phase arrest and tumor cell apoptosis. The site-specific conjugation of the cytotoxic agent to the antibody improves the biophysical properties of ARX517, increases payload stability and optimizes its efficacy.