Caco-2 cells were preloaded for 30 min at 37°C with 1 mM chlorpheniramine or diphenhydramine (pH 7.4). After removing this medium, cell monolayers were washed once with ice-cold incubation medium and then incubated for 1 min at 37°C with 5 mM diphenhydramine or chlorpheniramine (pH 7.4). Thereafter, intracellular accumulation of drugs was measured. Each value represents the mean ± S.E. of three monolayers.

• ↵2-150 P < .01, significant difference from nonpreloaded value.