Evaluation of ²¹²Pb-PSMA radioligand therapy in an immunocompetent prostate cancer model

Introduction: Prostate cancer (PC) is the second leading cause of cancer-related death in men, and metastatic castration-resistant prostate cancer (mCRPC) remains incurable. Prostate-specific membrane antigen (PSMA) is a validated therapeutic target. Targeted alpha therapies (TAT) employing radionuclides such as lead-212 (²¹²Pb) and actinium-225 (²²⁵Ac) are in development, delivering high-linear-energy-transfer radiation with potent cytotoxic and potentially immunomodulatory effects. However, currently available PSMA-positive preclinical models rely on human xenografts, which do not allow for the study of radioligand therapy-induced immune modulation. Objective: The immunomodulatory effects of PSMA-targeted alpha therapy are believed to play a key role in the anti-tumor efficacy towards PC. However, the lack of immunocompetent in vivo models of PSMA-positive PC has been a major hurdle in mechanistic and translational studies. To overcome this, we established a syngeneic prostate cancer model expressing murine PSMA (mPSMA) and developed a high-affinity PSMA-directed TAT tool compound specific to mPSMA. Methodology: Murine bone-metastatic prostate cancer cell line, RM1-BM, which recapitulates late-stage TP53-mutant disease, was engineered to express murine FOLH1 (mPSMA) using the PiggyBac transposon system. Expression of mPSMA was confirmed both in vitro and in vivo by western blotting and immunohistochemistry. The affinity and specificity of the TAT agent ²¹²Pb-PS0001 (AdvanCell) for murine and human PSMA were assessed using inhibition and cytotoxicity assays. Biodistribution and therapeutic efficacy were assessed in immunocompetent C57BL/6 mice bearing mPSMA-RM1-BM tumors. Blood and tissue samples collected from treated animals underwent multiomics analyses to characterize immune response modulation induced by ²¹²Pb-PS0001. Results: RM1-BM-mFOLH1 cells expressed mPSMA at levels comparable to human PSMA-positive C4-2 cells. ²¹²Pb-PS0001 exhibited high-affinity, selective binding and robust tumor uptake in RM1-BM-mFOLH1 models, with minimal off-target accumulation. Therapeutic evaluation demonstrated that a single administration of ²¹²Pb-PS0001 significantly inhibited RM1-BM-mFOLH1 tumor growth in immunocompetent mice, whereas untreated controls showed progressive disease. Notably, several treated animals achieved durable tumor control, suggesting potential induction of an anti-tumor immune response. Conclusion: By establishing a syngeneic, mPSMA-positive metastatic prostate cancer model, we enabled the evaluation of PSMA-TAT in an immunocompetent setting. Preliminary studies with ²¹²Pb-PS0001 demonstrated selective PSMA targeting, favorable pharmacokinetics, and potent anti-tumor efficacy, supporting its utility as a PSMA-TAT tool compound for investigating immune-mediated mechanisms of action in prostate cancer.