Triphosgene-Free Solid-Phase Synthesis of Urea-Containing PSMA-Targeting Peptides for Radiotheranostics: A Comparative Study

Abstract

Prostate-specific membrane antigen (PSMA) is a validated target for prostate cancer imaging and radiotherapy, with most high-affinity ligands incorporating the lysine–urea–glutamate (KuE) pharmacophore. Despite emerging triphosgene-free approaches, current synthetic strategies still largely rely on hypertoxic triphosgene for asymmetric urea formation. Herein, we report a comparative evaluation of triphosgene-free acyl-transfer reagents for on-resin KuE formation under solid-phase peptide synthesis (SPPS) conditions. Among the reagents examined, 1,1′-carbonyldiimidazole (CDI) emerged as the most effective, enabling near-quantitative urea formation under mild, SPPS-compatible conditions. Application of the optimised CDI-mediated protocol on TentaGel® resin bearing an HMPB linker enabled efficient fully solid-phase assembly of PSMA-617 in >80% crude purity and 31% isolated yield. Purified PSMA-617 was radiolabelled with ⁶⁸Ga in >99% radiochemical purity, confirming chemical integrity. Additionally, the PSMA-I&T backbone and four PSMA-617 analogues were synthesised in high crude purities, demonstrating rapid on-resin ligand construction for structure–activity relationship (SAR) studies.