Inhibitory Effect of Flavonoids on the Efflux ofN‐Acetyl 5‐Aminosalicylic Acid Intracellularly Formed in Caco‐2 Cells

N‐acetyl 5‐aminosalicylic acid (5‐AcASA) that was intracellularly formed from 5‐aminosalicylic acid (5‐ASA) at 200 μM was discharged 5.3, 7.1, and 8.1‐fold higher into the apical site than into the basolateral site during 1, 2, and 4‐hour incubations, respectively, in Caco‐2 cells grown in Transwells. The addition of flavonols (100 μM) such as fisetin and quercetin with 5‐ASA remarkably decreased the apically directed efflux of 5‐AcASA. When 5‐ASA (200 μM) was added to Caco‐2 cells grown in tissue culture dishes, the formation of 5‐AcASA decreased, and, in addition, the formed 5‐AcASA was found to be accumulated within the cells in the presence of such flavonols. Thus, the decrease in 5‐AcASA efflux by such flavonols was attributed not only to the inhibition ofN‐acetyl‐conjugation of 5‐ASA but to the predominant cellular accumulation of 5‐AcASA. Various flavonoids also had both of the effects with potencies that depend on their specific structures. The essential structure of flavonoids was an absence of a hydroxyl substitution at the C5 position on the A‐ring of flavone structure for the inhibitory effect on theN‐acetyl‐conjugation of 5‐ASA, and a presence of hydroxyl substitutions at the C3′or C4′position on the B‐ring of flavone structure for the promoting effect on the cellular accumulation of 5‐AcASA. Both the decrease in 5‐AcASA apical efflux and the increase in 5‐AcASA cellular accumulation were also caused by MK571 and indomethacin, inhibitors of MRPs, but not by quinidine, cyclosporin A, P‐glycoprotein inhibitors, and mitoxantrone, a BCRP substrate. These results suggest that certain flavonoids suppress the apical efflux of 5‐AcASA possibly by inhibiting MRPs pumps located on apical membranes in Caco‐2 cells.