JHU-2545 Selectively Shields Salivary Glands and Kidneys during PSMA-Targeted Radiotherapy

ABSTRACT PURPOSE Prostate-specific membrane antigen (PSMA) radiotherapy is a promising treatment for metastatic castration-resistant prostate cancer (mCRPC) with several beta or alpha particle-emitting radionuclide-conjugated small molecules showing efficacy in late stage patients. However, PSMA is also expressed in kidneys and salivary glands where specific uptake causes dose-limiting xerostomia and potential for nephrotoxicity. The PSMA inhibitor 2- (phosphonomethyl)pentanedioic acid (2-PMPA) can prevent kidney uptake in mice, but also blocks tumor uptake, precluding its clinical utility. Selective delivery of 2-PMPA to non-malignant tissues could improve the therapeutic window of PSMA radiotherapy. EXPERIMENTAL DESIGN A tri-alkoxycarbonyloxy alkyl (TrisPOC) prodrug of 2-PMPA, JHU-2545, was synthesized to enhance 2-PMPA delivery to non-malignant tissues. Preclinical pharmacokinetic and imaging experiments were conducted prior to assessment in 3 mCRPC patients receiving PSMA PET and radiotherapy. RESULTS JHU-2545 resulted in 3- and 53-fold greater exposure of 2-PMPA in rodent salivary glands (18.0 ± 0.97 h*nmol/g) and kidneys (359 ± 4.16 h*nmol/g) versus prostate tumor xenograft (6.79 ± 0.19 h*nmol/g). JHU-2545 also blocked rodent kidneys and salivary glands uptake of the PSMA PET tracers 68 Ga-PSMA-11 and 18 F-DCFPyL by up to 85% without effect on tumor. In a mCRPC patient, JHU-2545 treatment prior to 68 Ga-PSMA-617 administration reduced kidney SUV max by 76% without effect on metastatic lesions. When administered prior to injection of the beta emitter 177 Lu-PSMA-617, JHU-2545 shielded both the salivary glands (72% Gy reduction) and kidneys (45% Gy reduction) without effect on metastases’ dose. CONCLUSIONS JHU-2545 pre-treatment raises the cumulative dose limit and improves the safety and efficacy profile of PSMA radiotherapy. STATEMENT OF TRANSLATIONAL RELEVANCE Prostate Specific Membrane Antigen (PSMA) molecular radiotherapy has emerged as a promising treatment for metastatic castration-resistant prostate cancer (mCRPC), but endogenous expression of PSMA in kidneys and salivary glands causes uptake into these organs resulting in dose-limiting toxicities. We describe the discovery of JHU-2545, a PSMA inhibitor prodrug that selectively blocks kidney and salivary gland uptake of PSMA theranostics without altering tumor uptake in both preclinical models and in mCRPC patients. Pretreatment of JHU-2545 thereby improves the safety and efficacy profile of the multiple PSMA radiotherapies in development.