Transport characteristics of diphenhydramine in human intestinal epithelial Caco-2 cells: contribution of pH-dependent transport system.

Transport characteristics of diphenhydramine, an antihistamine, were studied in cultured human intestinal Caco-2 cell monolayers to elucidate the mechanisms of its intestinal absorption. Diphenhydramine accumulation in the monolayers increased rapidly and was influenced by extracellular pH (pH 7.4 > 6.5 > 5.5). Diphenhydramine uptake was temperature dependent, saturable, and not potential sensitive. Kinetic analysis revealed that the apparent Km values were constant (0.8-1.0 mM) in all pH conditions tested, whereas Vmax values decreased at the lower pH. The initial uptake of diphenhydramine was competitively inhibited by another antihistamine, chlorpheniramine, with a Ki value of 1.3 mM. On the other hand, cimetidine and tetraethylammonium, typical substrates for the renal organic cation transport system, had no effect. Moreover, biological amines and neurotransmitters, such as histamine, dopamine, serotonin, and choline, also had no effect on the diphenhydramine accumulation. Finally, diphenhydramine uptake was stimulated by preloading monolayers with chlorpheniramine (trans-stimulation effect). These findings indicate that diphenhydramine transport in Caco-2 cells is mediated by a specific transport system. This pH-dependent transport system may contribute to the intestinal absorption of diphenhydramine.