Targeting metastatic prostate cancer with radiolabeled monoclonal antibody J591 to the extracellular domain of prostate specific membrane antigen

Purpose: We performed an interim analysis of imaging data collected in 2 phase I radioimmunotherapy trials to determine the ability of monoclonal antibody (mAb) J591 directed to the extracellular domain of prostate specific membrane antigen (PSMA) to target sites of known metastatic prostate cancer accurately. Materials and Methods: Patients with progressing hormone independent prostate cancer were entered in 2 phase I dose finding trials with radiolabeled mAb J591. J591 is the first mAb targeting the extracellular domain of PSMA as well as the first de-immunized (humanized) mAb to PSMA to be tested in humans. These trials were primarily designed to assess dose limiting toxicity, maximum tolerated dose, pharmacokinetics and organ dosimetry. Planar gamma camera imaging studies obtained on the first 53 patients were reviewed and compared to sites of metastatic prostate cancer visualized on conventional imaging studies including bone scan, computerized tomography and/or magnetic resonance imaging. In 1 trial 29 patients received (111)indium-J591 for imaging followed by (90)yttrium-J591 for therapy. In the parallel trial 24 patients were treated with (177)lutetium-J591, an isotope that can be imaged directly. Results: Of 53 patients reviewed 46 (87%) had evidence of metastatic disease on conventional scans. Overall, of the 43 evaluable patients J591 accurately targeted bone and/or soft tissue lesions in 42 (98%). J591 accurately targeted bone lesions in 32 of 34 (94%) and soft tissue lesions in 13 of 18 (72%) evaluable patients. Conclusions: Radiolabeled J591 accurately targets bone and soft tissue metastatic prostate cancer sites, and may be useful for targeting therapeutic and/or diagnostic imaging agents.