Purpose: The transepithelial transport characteristics of the antihistamine, diphenhydramine, were studied in human intestinal Caco-2 cell monolayers to elucidate the mechanisms of its intestinal absorption.
Methods: The transepithelial transport and the cellular accumulation of diphenhydramine were measured using Caco-2 cell monolayers grown in Transwell chambers.
Results: The transepithelial transport of diphenhydramine from the apical to basolateral side was saturable, and the flux and cellular accumulation of diphenhydramine were dependent on the apical extracellular pH (pH 7.4 > 6.5 > 5.5). Transport and accumulation of diphenhydramine from the apical side were inhibited by another antihistamine, chlorpheniramine, while typical substrates for the renal organic cation transport system such as tetraethylammonium, cimetidine and guanidine had no effect. The transepithelial transport and cellular accumulation of diphenhydramine from the basolateral side were also pH-dependent and inhibited by chlorpheniramine. In addition, intracellular diphenhydramine preloaded was preferentially effluxed to the apical side, suggesting the involvement of the secretory pathway in diphenhydramine transport. Furthermore, diphenhydramine uptake from both the apical and basolateral sides was stimulated by preloading monolayers with chlorpheniramine (trans-stimulation effect).
Conclusions: Transepithelial transport of diphenhydramine across Caco-2 cells is mediated by pH-dependent, specific transport systems that exist in both the apical and basolateral membranes.