An Improved 211At-Labeled Agent for PSMA-Targeted α-Therapy

Abstract

α-Particle emitters targeting the prostate-specific membrane antigen (PSMA) proved effective in treating patients with prostate cancer who were unresponsive to the corresponding β-particle therapy. ²¹¹At is an α-emitter that may engender less toxicity than other α-emitting agents. We synthesized a new ²¹¹At-labeled radiotracer targeting PSMA that resulted from the search for a pharmacokinetically optimized agent. Methods: A small series of ¹²⁵I-labeled compounds was synthesized from tin precursors to evaluate the effect of the location of the radiohalogen within the molecule and the presence of lutetium in the chelate on biodistribution. On that basis, ²¹¹At-3-Lu was selected and evaluated in cell uptake and internalization studies, and biodistribution and PSMA-expressing (PSMA+) PC3 PIP tumor growth control were evaluated in experimental flank and metastatic (PC3-ML-Luc) models. A long-term (13-mo) toxicity study was performed for ²¹¹At-3-Lu, including tissue chemistries and histopathology. Results: The radiochemical yield of ²¹¹At-3-Lu was 17.8% ± 8.2%. Lead compound ²¹¹At-3-Lu demonstrated total uptake within PSMA+ PC3 PIP cells of 13.4 ± 0.5% of the input dose after 4 h of incubation, with little uptake in control cells. In SCID mice, ²¹¹At-3-Lu provided uptake that was 30.6 ± 4.8 percentage injected dose per gram (%ID/g) in PSMA+ PC3 PIP tumor at 1 h after injection, and this uptake decreased to 9.46 ± 0.96 %ID/g by 24 h. Tumor-to-salivary gland and tumor-to-kidney ratios were 129 ± 99 at 4 h and 130 ± 113 at 24 h, respectively. Deastatination was not significant (stomach, 0.34 ± 0.20 %ID/g at 4 h). Dose-dependent survival was demonstrated at higher doses (>1.48 MBq) in both flank and metastatic models. There was little off-target toxicity, as demonstrated by hematopoietic stability, unchanged tissue chemistries, weight gain rather than loss throughout treatment, and favorable histopathologic findings. Conclusion: Compound ²¹¹At-3-Lu or close analogs may provide limited and acceptable toxicity while retaining efficacy in management of prostate cancer.

Keywords: 211At; PSMA; murine models; prostate cancer; radiopharmaceutical therapy; α-emitter.