The purpose of the initial (phase I) portion of this study is to find a dose level and administration schedule of the study drug, 225Ac-J591 that can be given without severe side effects. Following selection of the proper dose for each treatment regimen, the purpose of the phase II portion of the study is to test if the regimen of 225Ac-J591 leads to treatment response.
   
This research study is being done because the standard treatments for prostate cancer that has spread beyond the prostate gland are intended to minimize the adverse effects of the disease and make men live longer. These treatments, however, are not curative so additional treatments are needed.
   
Prostate-specific membrane antigen (PSMA) is a protein that is on the surface of most prostate cancer cells. It is absent from most other normal places in the body, but is present to a lower degree in the kidney, small intestine, salivary glands, and brain. J591 is a monoclonal antibody (an engineered protein) which recognizes PSMA. Actinium-225 (225Ac) is a small radioactive particle that emits alpha-particles (damaging/ionizing radiation). 225Ac-J591 is the combination compound that has the radioactive particle linked to J591. It is designed so that J591 will recognize PSMA and drags the radioactive particle 225Ac with it wherever it goes. This drug is used not FDA approved for any indication and is considered experimental.

The treatment phase is comprised of 8 weeks for the fractionated dose cohort and 8 weeks past last dose of 225Ac-J591 for the multiple dose regimen (for subjects receiving 4 cycles, 26 weeks is expected). .The study medication will be administered as a single fractionated cycle day 1 and day 15 in the fractionated dose regimen and as a single dose per cycle repeated every 6 weeks in the multiple dose regimen.