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

Transport characteristics of diphenhydramine, an antihistamine, were studie d in cultured human intestinal Caco-2 cell monolayers to elucidate the mech anisms of its intestinal absorption. Diphenhydramine accumulation in the mo nolayers increased rapidly and was influenced by extracellular pH (pH 7.4 > 6.5 > 5.5). Diphenhydramine uptake was temperature dependent, saturable, a nd not potential sensitive. Kinetic analysis revealed that the apparent K-m , values were constant (0.8-1.0 mM) in all pH conditions tested, whereas V- max,, values decreased at the lower pH. The initial uptake of diphenhydrami ne was competitively inhibited by another antihistamine, chlorpheniramine, with a K-i, value of 1.3 mM. On the other hand, cimetidine and tetraethylam monium, typical substrates for the renal organic cation transport system, h ad no effect. Moreover, biological amines and neurotransmitters, such as hi stamine, dopamine, serotonin, and choline, also had no effect on the diphen hydramine accumulation. Finally, diphenhydramine uptake was stimulated by p reloading monolayers with chlorpheniramine (trans-stimulation effect). Thes e findings indicate that diphenhydramine transport in Caco-2 cells is media ted by a specific transport system. This pH-dependent transport system may contribute to the intestinal absorption of diphenhydramine.