Targeted Alpha Radionuclide Therapy of Prostate Cancer Using a Small Molecule Inhibitor of Prostate-specific Membrane Antigen (PSMA)

Objectives: PSMA is a well-recognized target for imaging and therapy of prostate cancer (PCa), as it is expressed in nearly all PCa lesions. Radiolabeled PSMA inhibitors localize rapidly to tumor lesions, including soft-tissue and bone metastases, but also show high uptake in kidneys and salivary glands. The aim of the present study was to evaluate the potential of 213Bi-PSMA I&T a DOTA-chelated urea-based PSMA inhibitor, as therapeutic agent in PSMA-expressing tumor cells in vitro and in vivo. Materials and Methods: PSMA I&T was labeled with 111In for biodistribution studies and with 213Bi for therapeutic studies. The tumor targeting of 111In-PSMA I&T was evaluated in mice with subcutaneous LNCaP tumors and PSMA-transfected LS174T tumors. Biodistribution was determined 2 h after injection of 0.1, 0.3, 1, or 10 nmol of 111In-PSMA I&T. Blocking of PSMA-specific binding to kidneys was investigated by co-injecting 2-(Phosphonomethyl)pentane-1,5-dioic acid (PMPA). Next, the therapeutic efficacy of 213Bi-PSMA I&T was tested in vitro in LNCaP cells and in vivo in mice with LNCaP xenografts. Cells were incubated for 20 min with 213Bi-PSMA I&T, 213Bi-DTPA, or medium, and fixed at 1, 2, 4, 24, and 48 h after treatment. Mice were injected with 213Bi-PSMA I&T (0.2 nmol, 5 MBq) and biodistribution was determined at 1 h after injection. In addition, tumor and kidneys were dissected 1 h and 24 h after injection. Fixed cells and tissues were stained with an anti-53BP1 antibody, an anti-GEMININ antibody and a TUNEL detection kit. Results: 111In-PSMA I&T uptake in LNCaP and LS174T-PSMA tumors, kidneys, spleen, lung and salivary glands was dependent on amount of injected peptide. Optimal doses for tumor targeting were between 0.1 and 0.3 nmol. Co-injection of 10 nmol PMPA resulted in a 2.5-fold decrease in renal uptake, and a 1.2-fold decrease in tumor uptake. Treatment of LNCaP cells with 213Bi-PSMA I&T caused a decrease in cellular proliferation, accumulation of DNA double-strand breaks, and induction of apoptosis. The biodistribution of 213Bi-PSMA I&T and 111In-PSMA I&T were similar. 213Bi-PSMA I&T induced DNA-double strand breaks in LNCaP tumors, from which a significant amount was not repaired at 24 h after injection. Conclusion: PSMA I&T is a promising targeting agent for radionuclide therapy of PCa. In this study we showed that renal accumulation of this tracer can be efficiently reduced by co-injection of PMPA, while tumor uptake was preserved. Our preliminary results reveal a good targeting and therapeutic effect of 213Bi-PSMA I&T in LNCaP cells in vitro and in vivo.JRC.E.5-Nuclear chemistr