Possible mechanisms for the enhancement of rectal absorption of hydrophilic drugs and polypeptides by aqueous polyacrylic acid gel.

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Abstract
The mechanisms of absorption enhancing effect by polyacrylic acid gel were investigated in rats. Polyacrylic acid gel significantly enhanced rectal absorption of phenol red, a hydrophilic compound. The enhanced absorption was concentration dependent in the range of 0.01-0.05% w/v polyacrylic acid in gel. The gel also significantly increased the influx and the net influx on water movement in rat rectum. Further, the histological change in rectal epithelium after administration of the polyacrylic acid gel was examined when transmission electron microscopy. The structural changes, which were removal of the mucus and fenestrations of the intercellular spaces 5 and 10 min after administration of polyacrylic acid gel, were observed. These changes were reversible and returned relatively soon to normal levels. Thus, the absorption promoting effect by polyacrylic acid gel may occur mainly with increased water influx as a driving force.