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.

Cyclization-activated Prodrugs

“…This has been exploited by medicinal chemists, especially by Borchardt and co-workers, to develop two-step activation prodrugs [6]. Thus, these authors carried out covalent attachment of model drugs to the carboxyl group of the hydrocinnamic acid moiety while masking the o-hydroxyl substituent as a precursor structure sensitive to either reductases [76][77][78], esterases [79][80][81] or phosphatases [82]. Consequently, the o-hydroxyl group could be released in a first enzymatically-promoted transformation, after which fast lactonization would lead to drug release (Scheme 14).…”

“…Scheme 14. Application of the "trimethyl lock" concept by Borchardt and co-workers for the design of two-step prodrugs (X=RCOO or PO 2 O; R 1 =H, OH) [6,[76][77][78][79][80][81][82]. In what concerns similar esterase-sensitive pro-prodrugs of anisidine as model amine, half-lives determined at 37 ºC in different media were as follows: 4030 min in phosphate buffer (pH 7.4), 11.9 min in the same buffer containing porcine liver esterase, 53.7 min in plasma and 475 min in plasma containing diisopropylfluorophosphate.…”

“…Hence, these pro-prodrugs are enzymatically, rather than chemically, activated in biological media [79]. These observations prompted the same authors to design an esterase-sensitive cyclic two-step prodrug of a model hexapeptide [80,81]. This was also a "trimethyl lock"-based system that, in 90% human plasma, released the parent peptide (t 1/2 =504 min) by an apparent esterase-catalyzed hydrolysis of the phenol ester bond followed by fast lactonization (Scheme 15A).…”

“…This was also a "trimethyl lock"-based system that, in 90% human plasma, released the parent peptide (t 1/2 =504 min) by an apparent esterase-catalyzed hydrolysis of the phenol ester bond followed by fast lactonization (Scheme 15A). Moreover, this cyclic two-step prodrug presented a 70-fold increase relatively to the linear peptide in terms of permeation across Caco-2 cell monolayers [80,81]. Scheme 15.…”

“…Scheme 15. Cyclic two-step prodrugs of peptides based on: (A) the "trimethyl lock" [80,81] and, (B) coumarinic acid derivatives [85][86][87][88][89][90][91]. Another approach, quite similar to the "trimethyl lock" concept, for the design of two-step prodrugs has been based on coumarinic acid derivatives, as these equally undergo facile lactonization [83].…”