PSMA-targeted radiotherapy of prostate cancer (PCa) has emerged recently as a promising approach to the treatment of disseminated disease. A small number of ligands have been evaluated in patients, and while early tumor response is encouraging, relapse rate is high, and these compounds localize to the parotid, salivary and lacrimal glands as well as the kidney, leading to dose-limiting toxicities and adverse events affecting quality of life. We envision that dual targeting ligands displaying high affinity for PSMA and appropriate affinity for human serum albumin (HSA) may demonstrate a higher therapeutic index and be suitable for treatment of PCa by targeted alpha therapy (TAT). Methods: Six novel urea-based ligands with varying affinities for PSMA and HSA were synthesized, labeled with (131)I and evaluated by in vitro binding and uptake assays in LNCaP cells. Four compounds were advanced for further evaluation in a preclinical model of PCa. The compounds were compared to MIP-1095, a PSMA ligand currently in clinical evaluation. Results: The compounds demonstrated affinity for PSMA on the order of 4-40 nM, and affinity for HSA in the range 1-53 µM. Compounds with relatively high affinity for HSA (≤ 2 µM) showed high and sustained blood pool activity and reduced uptake in the kidneys. (131)I-RPS-027, with IC50 (PSMA) = 15 nM and Kd (HSA) = 11.2 µM, clears from the blood over the course of 48h, shows good tumor uptake (10 %ID/g) and retention, and greater than a 5-fold decrease in kidney uptake relative to MIP-1095. The tumor-to-kidney ratio of (131)I-RPS-027 is greater than 3:1 at 24h post injection increasing to 7:1 by 72h. Conclusion: RPS-027 shows dual targeting to PSMA and albumin, resulting in high tumor uptake, highly favorable tissue distribution and a promising therapeutic profile in a preclinical model of prostate cancer. In comparison to existing ligands proposed for targeted therapy of prostate cancer, RPS-027 has tumor-to-tissue ratios that predict a significant reduction in side effects during therapy. Using iodine/radioiodine as a surrogate for the radiohalogen (211)At, we therefore propose dual targeting ligands such as RPS-027 as next generation radiopharmaceuticals for TAT using (211)At.
Journal of nuclear medicine : official publication, Society of Nuclear Medicine. 2017 Apr 27 [Epub ahead of print]
James M Kelly, Alejandro Amor-Coarasa, Anastasia Nikolopoulou, Till Wüstemann, Peter Barelli, Dohyun Kim, Clarence Williams, Xiwei Zheng, Cong Bi, Bao Hu, J David Warren, David S Hage, Stephen G DiMagno, John W Babich
Weill Cornell Medical College, United States., University of Nebraska-Lincoln, United States., University of Illinois at Chicago, United States.