Synthesis of a Novel Esterase-Sensitive Cyclic Prodrug System for Peptides That Utilizes a “Trimethyl Lock”-Facilitated Lactonization Reaction

Abstract

This paper describes a unique strategy for preparing cyclic prodrugs of peptides that have increased metabolic stability and increased cell membrane permeability when compared to the linear peptides. By taking advantage of a unique “trimethyl lock”-facilitated lactonization system, an esterase-sensitive cyclic prodrug of a model hexapeptide H-Trp-Ala-Gly-Gly-Asp-Ala-OH was synthesized by linking the N-terminal amino group to the C-terminal carboxyl group. The key intermediate for both approaches was compound 9 with Boc-Ala attached to the phenol hydroxyl group of the “trimethyl lock” linker through an ester bond, which can then be incorporated into the peptide using a normal coupling reagent for peptide synthesis. The synthesis of the linear peptides was accomplished using both solution-phase and solid-phase approaches with the solution-phase approach having the advantage of using the key intermediate 9 most efficiently. Cyclization using standard high-dilution techniques provided cyclic prodrug 13. In 90% human plasma, prodrug 13 released the original peptide, as designed, through an apparent esterase-catalyzed hydrolysis of the phenol ester bond.