Utilization of an In Situ Rat Absorption Model to Assess the Gut Permeability of Three Peptidic HIV-1 Protease Inhibitors
Rop, Timothy J.
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Peptides represent a class of therapeutic pharmaceuticals with great potential, as several peptidic HIV protease inhibitors with molecular weights of approximately 800 to 1000 look especially promising in the treatment of AIDS. Obstacles such as enzymatic degradation, drug metabolism, and "first pass" liver clearance are all important factors that reduce the oral bioavailability of such peptides, but poor absorption across the intestinal epithelium is the major barrier to effective oral administration. Recent in vitro investigation disclosed a strong direct correlation between the heptane-ethylene glycol partition coefficients and the effective permeabilities across Caco-2 cells for a series of six peptidic HIV-1 protease inhibitors. The findings suggest that the major determinant of desolvation energy, and thus peptide permeability to the gut wall, is not necessarily the number of hydrogen bonding sites inherent in the structure of the peptide, but more precisely the hydrogen bonding potential of these sites. An in situ rat mesenteric vein absorption model was employed to verify this relationship for three of the six peptidic inhibitors. Briefly, a constant infusion was utilized to perfuse peptide solutions through an isolated loop of gut while both the mesenteric blood exiting the section and the intestinal perfusate were continuously collected. Amounts of drug in both the perfusate and the blood were quantified by HPLC to allow the calculation of both appearance and disappearance permeabilities. Neither appearance nor disappearance permeability coefficients support the relationship found in vitro between the heptane-ethylene glycol partition coefficient and peptide permeability. Further investigation is required to confirm this finding and to interpret its significance.