Macrocyclization of native peptides through (thio)urea crosslinking of two amines

Cyclopeptides have emerged as promising candidates for drug development. However, owing to the scarcity of efficient cyclization strategies for natural peptides, the development of diverse cyclopeptides remains a formidable challenge. In this study, a variety of macrocyclic peptides featuring (thio)urea bridges are developed through the site-selective lysine‑lysine crosslinking of native peptides, employing N, N'-carbonyldiimidazole or N, N'-thiocarbonyldiimidazole as  bridging reagents. In particular, dual thiourea-bridged macrocyclic peptides are further synthesized by using a one-pot two-step multicomponent macrocyclization. These strategies enable the chemoselective macrocyclization of peptides bearing diverse nucleophilic residues to afford (thio)urea-bridged macrocyclic analogs, as exemplified by the successful transformation of the targeted peptide RGD (Arg-Gly-Asp) and the antimicrobial peptide Anoplin analogue. Biological evaluation demonstrates that the cyclopeptides, particularly the thiourea- and bis-thiourea-bridged analogs, exhibit enhanced target binding affinity, potent antitumor activity, as well as improved membrane permeability and stability. Molecular docking elucidates the structural origin of the enhanced bioactivity, suggesting that the reinforced molecular interactions might be a potential underlying cause.