Absorption and Excretion of Drugs. XXV. On the Mechanism of Rectal Absorption of Sulfonamides

Abstract

The rates of absorption of several sulfonamides were measured in the rectum of the anesthetized rat. Their absorption was considerably affected by the pH of solution. The unionized form was in general readily absorbed, while the ionized form was more slowly absorbed. Sulfonamides which are highly lipid-soluble were more readily absorbed than those which are poorly lipid-soluble. And there is a slightly acidic zone at the rectum-blood barrier, like the intestinal-blood barrier. The results suggest that sulfonamides are absorbed from the rectum by a passive transport, and that the patterns of absorption in the rectum are similar as a gastro-intestinal absorption of drugs. When the permeability constants were plotted against the absorption velocity constants, the good agreement was obtained among the spots. The diffusion constant as well as lipid-solubility is an important factor for rectal absorption.