Black lipid membranes as a model for intestinal absorption of drugs.

Black lipid membranes were generated in isotonic buffer (pH 4-5 and pH 6-5) from egg phosphatidylcholine and intestinal lipid, and the permeability to salicylamide, salicylic acid, p-aminobenzoic acid and tryptophan of these membranes was studied. Electrical resistance of intestinal lipid membranes was higher than that of phosphatidylcholine membranes. The presence of cholesterol produced an increase in the electrical resistance of black lipid membranes and a small decrease in the permeability of membranes to drugs. The permeability coefficient of salicylamide, an uncharged drug, was much larger than the coefficients of the charged drugs examined. The values for salicylic acid and p-aminobenzoic acid were much larger than comparable values predicted from their partition coefficients. Intestinal lipid membranes were more permeable to acidic drugs than phosphatidylcholine membranes. It is suggested that phospholipids and other lipid components of the small intestine may play an important role in the membrane permeability to acidic drugs. This method may be of interest in studying the complex processes of drug absorption from intestine.