Phase 1 study designThe phase 1, open-label, dose-escalation clinical trial was conducted in men with hormone-refractory prostate cancer that had progressed despite prior treatment with taxane-based chemotherapy regimens. In addition to assessing PSMA ADC's safety and tolerability, the study included evaluations of pharmacodynamics, changes in tumor burden, and changes in PSA and CTC values compared to baseline. PSA is a secreted protein that is distinct from PSMA. The initial 12-week clinical trial period evaluated up to four intravenous doses of PSMA ADC administered at three-week intervals. Following completion of the four doses, patients were offered, at their physicians' discretion, the option to continue treatment with PSMA ADC for up to an additional 39 weeks. About PSMA ADC PSMA, a protein that is a validated biomarker of prostate cancer, is expressed on the surface of prostate cancer cells as well as on blood vessels supplying other solid tumors. PSMA ADC comprises a fully human monoclonal antibody selectively targeting PSMA linked to a chemotherapeutic drug. Using technology licensed from Seattle Genetics, Inc., the PSMA antibody is linked to monomethyl auristatin E, a compound that inhibits cell proliferation by disrupting the cellular "backbone" (i.e., microtubules) required for replication. The resultant antibody-drug conjugate attaches to the PSMA protein on the surface of prostate cancer cells and is designed to internalize into the cancer cell, release active anti-cancer drug, and destroy the malignant cell. Unlike traditional chemotherapy, PSMA ADC is designed to deliver the drug selectively to prostate cancer cells by targeting PSMA. In pre-clinical studies, PSMA ADC exhibited a high level of tumor-specific activity. About Prostate Cancer Prostate cancer is the most common form of cancer affecting men in the United States and is the second leading cause of cancer deaths among men each year. The American Cancer Society estimates that in 2013, 238,590 new cases of prostate cancer will be diagnosed and approximately 29,720 American men will die from the disease.