Preclinical Evaluation of 213Bi- and 225Ac-Labeled Low-Molecular-Weight Compounds for Radiopharmaceutical Therapy of Prostate Cancer.

Prostate-specific membrane antigen (PSMA) targeted radiopharmaceutical therapy is a new treatment option for patients with advanced prostate cancer refractory to other treatments. Previously we synthesized a β-particle-emitting low-molecular-weight compound, 177Lu-L1, which demonstrated reduced off-target effects in a xenograft model of prostate cancer. Here we leveraged that scaffold to synthesize α-particle-emitting analogs of L1, 213Bi-L1, and 225Ac-L1 to evaluate their safety and cell kill effect in PSMA+ xenograft models. Methods: Radiochemical synthesis, cell uptake, cell kill effect, and biodistribution of 213Bi-L1 and 225Ac-L1 were evaluated. The efficacy of 225Ac-L1 was determined in human PSMA+ subcutaneous and micrometastatic models. Subacute toxicity at 8 weeks and chronic toxicity at one year after administration were evaluated for 225Ac-L1. Radiation absorbed dose of 225Ac-L1 was determined using the biodistribution data and α-camera imaging. Results:213Bi-/225Ac-L1 demonstrated specific cell uptake and cell kill in PSMA+ cells. Biodistribution of 213Bi-L1 and 225Ac-L1 revealed specific uptake of radioactivity within PSMA+ lesions. Treatment studies of 225Ac-L1 demonstrated activity-dependent, specific inhibition of tumor growth in the PSMA+ flank tumor model. 225Ac-L1 also showed an increased survival benefit in the micrometastatic model compared to 177Lu-L1. Activity-escalated acute and chronic toxicity studies of 225Ac-L1 revealed off-target radiotoxicity, mainly in kidneys and liver. The estimated maximum tolerated activity was ~1 MBq/kg. α-camera imaging of 225Ac-L1 revealed high renal cortical accumulation at 2 h followed by fast clearance at 24 h. Conclusion:225Ac-L1 demonstrated activity-dependent efficacy with minimal treatment-related organ radiotoxicity issues. 225Ac-L1 is a promising therapeutic for further clinical evaluation.