Synthesis of a novel esterase-sensitive cyclic prodrug of a hexapeptide using an (acyloxy)alkoxy promoiety

Synthetic methodology for preparing novel esterase-sensitive cyclic prodrugs of peptides with increased protease stability and cell membrane permeability compared to linear peptides is described. Cyclic prodrug 1 of the hexapeptide H-Trp-Ala-Gly-Gly-Asp-Ala-OH linked by the N-terminal amino group to the C-terminal carboxyl group via an (acyloxy)alkoxy promoiety was synthesized. A convergent synthetic approach involving Boc[[(alaninyloxy)methyl]carbonyl]-N-tryptophan (2) and H-Ala-Gly-Gly-Asp(OBzl)-OTce (3) was used. The key fragment 2 has the promoiety inserted between the Ala and the Trp residues. Fragment 3 was synthesized by a solution-phase approach using standard Boc-amino acid chemistry. These fragments were coupled to produce the protected linear hexapeptide, which after deprotection was cyclized using standard high-dilution techniques to yield cyclic prodrug 1. In pH 7.4 buffer (HBSS) at 37 degrees C, cyclic prodrug 1 was shown to degrade quantitatively to the hexapeptide (t(1/2) = 206 +/- 11 min). The rate of hydrolysis of cyclic prodrug 1 was significantly faster in human blood (t(1/2) = 132 +/- 4 min) than in HBSS. Paraoxon, a known inhibitor of esterases, slowed this hydrolysis of cyclic prodrug 1 to a value (t(1/2) = 198 +/- 9 min) comparable to the chemical stability. In human blood, cyclic prodrug 1 was shown to be 25-fold more stable than the linear hexapeptide.