ESTERASE-SENSITIVE CYCLIC PRODRUGS OF PEPTIDES - EVALUATION OF AN ACYLOXYALKOXY PROMOIETY IN A MODEL HEXAPEPTIDE

Purpose. To evaluate a cyclic acyloxyalkoxycarbamate prodrug of a mode l hexapeptide (H-Trp-Ala-Gly-Gly-Asp-Ala-OH) as a novel approach to en hance the membrane permeation of the peptide and stabilize it to metab olism. Methods. Conversion to the linear hexapeptide was studied at 37 degrees C in aqueous buffered solutions and in various biological mil ieus having measurable esterase activities. Transport and metabolism c haracteristics were assessed using the Caco-2 cell culture model. Resu lts. In buffered solutions the cyclic prodrug degraded chemically to t he linear hexapeptide in stoichiometric amounts. Maximum stability was observed between pH 3-4. In 90% human plasma (t(1/2) = 100 +/- 4 min) and in homogenates of the rat intestinal mucosa(t(1/2)= 136 +/- 4 min ) and rat liver (t(1/2) = 65 +/- 3 min), the cyclic prodrug disappeare d faster than in buffered solution, pH 7.4 (t(1/2) = 206 +/- 25 min). Pretreatment of these media with paraoxon significantly decreased the degradation rate of the prodrug. When applied to the apical side of Ca co-2 cell monolayers, the cyclic prodrug (t(1/2) = 282 +/- 25 min) was significantly more stable than the hexapeptide (t(1/2) = 14 min) and at least 76-fold more able to permeate (P-app = 1.30 +/- 0.15 x 10(-7) cm/s) than the parent peptide (P-app less than or equal to 0.17 x 10( -8) cm/s). Conclusions. Preparation of a cyclic peptide using an acylo xyalkoxy promoiety reduced the lability of the peptide to peptidase me tabolism and substantially increased its permeation through biological membranes. In various biological media the parent peptide was release d from the prodrug by an apparent esterase-catalyzed reaction, sensiti ve to paraoxon inhibition.