Synthesis, (123)I-Radiolabeling Optimization, and Initial Preclinical Evaluation of Novel Urea-Based PSMA Inhibitors with a Tributylstannyl Prosthetic Group in Their Structures

Prostate-specific membrane antigen (PSMA) has been identified as a target for the development of theranostic agents. In our current work, we describe the design and synthesis of novel N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-(S)-L-lysine (DCL) urea-based PSMA inhibitors with a chlorine-substituted aromatic fragment at the lysine ε-nitrogen atom, a dipeptide including two phenylalanine residues in the L-configuration as the peptide fragment of the linker, and 3- or 4-(tributylstannyl)benzoic acid as a prosthetic group in their structures for radiolabeling. The standard compounds [(127)I]PSMA-m-IB and [(127)I]PSMA-p-IB for comparative and characterization studies were first synthesized using two alternative synthetic approaches. An important advantage of the alternative synthetic approach, in which the prosthetic group (NHS-activated esters of compounds) is first conjugated with the polypeptide sequence followed by replacement of the Sn(Bu)(3) group with radioiodine, is that the radionuclide is introduced in the final step of synthesis, thereby minimizing operating time with iodine-123 during the radiolabeling process. The obtained DCL urea-based PSMA inhibitors were radiolabeled with iodine-123. The radiolabeling optimization results showed that the radiochemical yield of [(123)I]PSMA-p-IB was higher than that of [(123)I]PSMA-m-IB, which were 74.9 ± 1.0% and 49.4 ± 1.2%, respectively. The radiochemical purity of [(123)I]PSMA-p-IB after purification was greater than 99.50%. The initial preclinical evaluation of [(123)I]PSMA-p-IB demonstrated a considerable affinity and specific binding to PC-3 PIP (PSMA-expressing cells) in vitro. The in vivo biodistribution of this new radioligand [(123)I]PSMA-p-IB showed less accumulation than [(177)Lu]Lu-PSMA-617 in several normal organs (liver, kidney, and bone). These results warrant further preclinical development, including toxicology evaluation and experiments in tumor-bearing mice.