Charged-linker modification of PSMA-617-based inhibitors affects the targeting and pharmacokinetic profile

Introduction: The application of [¹⁷⁷Lu]Lu-Glu-urea-Lys-2NaI-ChX-DOTA ([_¹⁷⁷Lu]Lu-PSMA-617) for treatment of metastatic castration-resistant prostate cancer (mCRPC) has demonstrated major clinical benefits. FDA approval is expected in the near future. However, treatment with the beta-emitting [¹⁷⁷Lu]Lu-PSMA-617 and, in particular, with the potentially more effective alpha-emitting [²²⁵Ac]Ac-PSMA-617 is limited by side effects like xerostomia. Charged-linker modifications of the diagnostic PSMA inhibitors PSMA-11 and PSMA-1007 affected their pharmacokinetic profile, yielding improved imaging quality and tumor-to-background contrasts. In this work, this strategy was applied to the therapeutic variant PSMA-617 and resulted in new histidine/glutamic acid-modified structures with different pharmacokinetic properties.

Methods: A small set of structurally related linker-modified PSMA-targeting inhibitors were synthesized by varying histidine and/or glutamic acid moieties introduced at three different positions within the linker of PSMA-617. The substances were radiolabeled with gallium-68 and lutetium-177 to investigate the chemical and stability properties. Lipophilicity was determined (n-octanol/PBS-system) and biological characteristics were studied in competitive cell and internalization assays with PSMA-expressing LNCaP cells. For assessment of pharmacokinetic profiles, small-animal PET imaging studies with ⁶⁸Ga-labelled compounds were performed in BALB/c nu/nu mice bearing LNCaP xenografts.

Results: The series of linker-modified PSMA inhibitors demonstrated high radiolytic stability. Complexation reactions of the precursors with both radionuclides resulted in high yields (>95%). Lipophilicity determined as logD at pH 7.4 in n-octanol/PBS of the new substances was found to be lower compared to the PSMA-617 reference compound. The investigated compounds showed a high affinity to PSMA (Ki ranging from 9.97 ± 2.82 nM to 75.4 ± 42.3 nM) and high specific in vitro internalization. Modifications with glutamic acid led to a two-fold increased affinity, while the introduction of glutamic acid and histidine into the linker region significantly improved surface binding and internalization rates (p < 0.05) compared to PSMA-617. Dynamic small-animal PET scans showed fast clearance from kidneys and bloodstream with a favorable pharmacokinetic profile and high uptake in the LNCaP tumors.

Conclusions: Modifications of the linker region of PSMA-617 significantly influences the targeting and pharmacokinetic profile of the studied inhibitors. However, considering the overall properties, none of the ligands exhibited a clear advantage as compared to the parent compound. This study, however, adds important insights into PSMA-617-related linker modifications for further optimizations of ligand properties.