Preclinical evaluation of PSMA-TO1- a PSMA-targeting ligand with increased circulating half-life

1504 Introduction: Prostate-specific membrane antigen (PSMA) is an important biological target in molecular radiotherapy, for which there exists a variety of PSMA-targeting agents. In this work, we present a preclinical evaluation of PSMA-TO1, a ligand developed aiming at a prolonged circulating half-life through increased lipophilicity. The aim of this study was to directly compare PSMA-TO1 as an imaging and therapeutic agent with PSMA-11 and PSMA-617, the two best known PSMA ligands. First, we compared imaging properties, biodistribution and tumor uptake of 68Ga-/ 177Lu-labeled PSMA-TO1 versus PSMA-617 and PSMA-11 in a mouse model of prostate cancer. Second, we assessed treatment efficacy of 225Ac-labeled PSMA-TO1 and PSMA-617 against a mouse model of systemic disease. Methods: NSG mice bearing subcutaneous C4-2 tumors underwent PET/CT imaging with each of the following compounds: 68Ga-PSMA-11, 68Ga-PSMA-617, and 68Ga-PSMA-TO1 (n=3 mice) and tumor uptake was analyzed. We then performed a biodistribution study in mice with subcutaneous C4-2 tumors with either 30 MBq 177Lu-PSMA-617 or 177Lu-PSMA-TO1 (n=25 mice per ligand). Following treatment, mice were sacrificed at 5 time points up to 7 days after administration for tumor and organ collection and activity quantification. Finally, mice were inoculated via intracardiac injection of C4-2 cells to establish widespread microscopic visceral and bone lesions, as previously described (1). These mice were treated 5 weeks after inoculation with either 40 kBq 225Ac-PSMA-617 or 225Ac-PSMA-TO1 (n=10 mice per ligand, and n=5 for untreated controls) and efficacy was assessed by overall survival. Results: PET images of 68Ga-labeled PSMA-TO1, PSMA-11, and PSMA-617 all showed high accumulation in the tumor with the greatest uptake observed using PSMA-617, followed by PSMA-TO1 then PSMA-11. Serial ex-vivo counts of 177Lu-PSMA-617 and 177Lu-PSMA-TO1-treated tumors revealed faster tumor clearance for PSMA-617. While the tumor retention of PSMA-TO1 is superior to PSMA-617, kidney uptake is also increased in this mouse model. In the 225Ac survival study, both PSMA-617 and PSMA-TO1 significantly prolonged median survival from 7.7 weeks in untreated mice to 14.5 weeks (p<0.0001) and 17.8 weeks (p<0.0001), respectively. The survival difference observed in favor of the mice treated with 225Ac-PSMA-TO1 compared to those treated with 225Ac-PSMA-617 was significant (p=0.0002). There was no observed difference in toxicity between treatment groups. Conclusions: In a mouse model of prostate cancer, PSMA-TO1 exhibits increased tumor retention compared to PSMA-617, albeit at the expense of higher kidney uptake. The significant survival benefit incurred by mice treated with 225Ac-PSMA-TO1 versus PSMA-617 may support further investigations into the potential utility of this compound.