213Bi-labeled PSMA-targeting agents for alpha radionuclide therapy of prostate cancer

137 Objectives Prostate specific membrane antigen (PSMA) is a valuable target for specific delivery of diagnostic and therapeutic radionuclides to prostate tumors. Up to now, PSMA-targeted radionuclide therapy strategies have mainly focused on beta-emitting radionuclides. Potentially, the therapeutic efficacy could be significantly enhanced by using alpha-emitting radionuclides. Here we report on two novel 213Bi-labeled PSMA-targeting agents for prostate cancer treatment. Methods Small-molecule inhibitor PSMA I&T and PSMA nanobody JVZ-008 were labeled with 213Bi. In vitro, LNCaP cells were incubated for 20 min with 213Bi-PSMA I&T, 213Bi-JVZ-008, 213Bi-DTPA, or medium, and fixed at 1, 2, 4, 24, and 48 h after treatment. Mice engrafted with LNCaP tumors were injected with 5 MBq 213Bi-PSMA I&T (0.2 nmol + 10 nmol 2-PMPA for renal protection) or 213Bi-JVZ-008 (0.7 nmol + 4 mg gelofusin for renal protection) and their biodistribution was determined at 1 h after injection. In addition, tumors of treated and untreated mice were collected 1 h and 24 h post treatment. Fixed cells and tumors were stained for 53BP1 and H2AX to determine the presence of DNA double strand breaks (DSBs). Results Treatment of LNCaP cells with 213Bi-PSMA I&T and 213Bi-JVZ-008 induced DSBs in vitro, as indicated by the presence of increased numbers of 53BP1 and H2AX nuclear foci. In vivo, tumor uptake of 213Bi-PSMA I&T and 213Bi-JVZ-008 at 1 h post injection was 5.75±2.70 %ID/g and 2.68±0.56 %ID/g, respectively. The two tracers showed similar tumor-to-kidney ratios. 213Bi-PSMA I&T and 213Bi-JVZ-008 both induced DSBs in LNCaP tumors at 1 h after injection, which were still observed 24 h after injection. At early time points, the DNA damage was visible as single foci and linear tracks, i.e. alpha-tracks. Alpha-tracks were no longer detectable 24 h post treatment. Conclusions Preclinical evaluation of 213Bi-PSMA I&T and 213Bi-JVZ-008 revealed good tumor targeting and induction of DSBs in PSMA-expressing LNCaP xenografts. These promising preliminary results warrant further evaluation of the therapeutic efficacy and toxicity of 213Bi-labeled tracers for treatment of metastasized PCa.