In Vitro Release and Rectal Absorption of Barbital and Aminopyrine from Aqueous Polyacrylic Acid Gel

Abstract

This study was designed t o evaluate aqueous polyacrylic acid gel (Carbopol gel), relative to its suitability for use as a vehicle for drug delivery. Barbital-Na and aminopyrine, used as model acidic and basic drugs, were completely dissolved into the aqueous gel base at 5 rng/ml and 50 mg/ml, respectively. In the release experiment using micropore membrane, higher concentration of polyacrylic acid in the gel resulted in higher viscosity, and consequently lower release rates of both drugs. Higher pH of barbital gel preparation resulted in higher fraction of ionized molecules of barbital in the gel preparation causing a higher barbital release rate. While the release rate of aminopyrine from gel preparation was the lowest at the region of pH 6.8, the ionized molecules of aminopyrine in gel base was 98.5%. The rectal absorption of barbital from the gel preparation at the pH 5.8-8.3 range had relation with the results of permeability through artificial intestinal lipid barrier and accorded with pH partition hypothesis. The permeability rate of aminopyrine through the artificial intestinal lipid barrier and the rectal absorption of aminopyrine from gel preparations did not have a marked difference at the pH 5.8-8.3 range. The release of both drugs from gel base was not a rate-limiting factor on the rectal absorption of both drugs from the gel bases.