J591 in Patients With Advanced Prostate Cancer and Unfavorable Circulating Tumor Cell Counts

J591 is a de-immunized monoclonal antibody against the extracellular domain of prostate-specific membrane antigen (PSMA). Background data demonstrate that mAb Hu-J591 doses as low as 20 mg may lead to a decrease in CTCs and that doses up to 300 mg are safe. With a goal of establishing less frequent, potentially more convenient and cost-effective future therapy, the investigators propose a single-center, open-label, dose de-escalation study to determine the effect of mAb Hu-J591 on circulating tumor cells in subjects with metastatic PC. Four dose levels are planned with a minimum of 6 and maximum of 24 subject enrollment starting with a single infusion of 300 mg of mAb Hu-J591. In the initial phase of the study a cohort of 6 subjects will receive 300 mg of mAb Hu-J591 and blood samples will be collected pre and post-infusion for the detection of CTCs, PSA and to assess hematological toxicity at screening, baseline, week 1, week 4, week 8, and week 12. In addition, patients will receive a dose of 89Zr-DFO-huJ591 (5 mCi) 2-4 weeks prior to treatment and will undergo 89Zr-DFO-huJ591 PET imaging 1-3 weeks pre-treatment (optional) as well as repeat 89Zr-DFO-huJ591 infusion (5 mCi) at 11 weeks and PET/CT imaging at 12 weeks (optional). If less than four subjects respond by dropping their CTC counts from more than or equal to 5 CTCs/7.5 mL whole blood to less than 5 CTCs/7.5 mL whole blood in the initial cohort, the trial will be ended at this dose. If four or more subjects respond, an additional 6 subjects will be accrued to the second dose level (200 mg). The same decision rule will be applied to evaluate two additional lower dose levels (100 mg, 50 mg) with 6 subjects enrolled in each cohort. Imaging studies will be performed prior to treatment at screening and at 3 months or as clinically indicated to assess disease response. Subjects who tolerate the initial infusion well (\< grade 2 toxicities) may be re-treated every 3 months (12 weeks) at the same dose-level until progression.