Combination of the amide‐to‐triazole substitution strategy with alternative structural modifications for the metabolic stabilization of tumor‐targeting, radiolabeled peptides

Radiolabeled peptides play a key role in nuclear medicine to selectively deliver radionuclides to malignancies for diagnosis (imaging) and therapy. Yet, their efficiency is often compromised by low metabolic stability. The use of 1,4‐disubstituted 1,2,3‐triazoles (1,4‐Tzs) as stable amide bond bioisosteres can increase the half‐life of peptides in vivo while maintaining their biological properties. Previously, the amide‐to‐triazole substitution strategy was used for the stabilization of the pansomatostatin radioligand [111In]In‐AT2S, resulting in the mono‐triazolo‐peptidomimetic [111In]In‐XG1, a radiotracer with moderately enhanced stability in vivo and retained ability to bind multiple somatostatin receptor (SSTR) subtypes. However, inclusion of additional 1,4‐Tz led to a loss of affinity towards SST2R, the receptor overexpressed by most SSTR‐positive cancers. To enhance further the stability of [111In]In‐XG1, alternative modifications at the enzymatically labile position Thr10‐Phe11 were employed. Three novel 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA)‐peptide conjugates were synthesized with a 1,4‐Tz (Asn5‐Ψ[Tz]‐Phe6) and either a β‐amino acid (β‐Phe11), reduced amide bond (Thr10‐Ψ[NH]‐Phe11), or N‐methylated amino acid (N‐Me‐Phe11). Two of the new peptidomimetics were more stable in blood plasma in vitro than [111In]In‐XG1. Yet none of them retained high affinity towards SST2R. We demonstrate for the first time the combination of the amide‐to‐triazole substitution strategy with alternative stabilization methods to improve the metabolic stability of tumor‐targeting peptides.