ABSORPTION OF DRUGS FROM THE RAT COLON

¹ Laboratory of Chemical Pharmacology, National Heart Institute, National Institutes of Health, Bethesda, Maryland

The relative rates of absorption of a large number of acidic and basic drugs were measured in the colon of the anesthetized rat.

The degree of ionization and the lipid- solubility of organic electrolytes in solution are the most important factors determining their rates of absorption. Weak acids and bases were in general readily absorbed while stronger, highly ionized acids and bases were more slowly absorbed. The absorption of acidic drugs was increased and that of basic drugs decreased when the colonic contents were made more acidic. Weak organic electrolytes that are highly lipid-soluble were more readily absorbed than those which are poorly lipid-soluble.

The results suggest that many drugs are absorbed from the colon by simple diffusion of the unionized moiety across a lipoidal harrier.

The similarities of the patterns of absorption in the colon, small intestine and stomach indicate a common mechanism for the gastrointestinal absorption of drugs.