PREDICTION OF DRUG INTERACTION IN VITRO -FOCUSING ON CYTOCHROME P-450 3A AND P-GLYCOPROTEIN-

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Using cultured Caco-2 cell monolayers, we tried to investigate the involvement of cytochrome P-450 (CYP) 3A4 and P-glycoprotein (P-gp) and to predict drug interaction of orally administered agents in vitro. Probucol, a lipid-lowering drug, caused P-gp-independent suppression of cyclosporine A (a substrate of CYP3A4 and P-gp) transport across Caco-2 cell monolayers. Pretreatment with various inducers failed to augment the P-gp-induced drug extrusion into the apical side of Caco-2 cell monolayers. Microsome fraction from Caco-2 cells even treated with CYP inducers did not demonstrate oxidization of nifedipine which is mediated by CYP3A4.We thus performed ex vivo experiments using intact rats. Microsome fractions from the intestinal epithelium and liver showed oxidation of nifedipine (averaged production rate ; 35 and 160 pmol/min/mg protein, respectively). We constructed ex vivo intestinal perfusion system and examined the absorption and secretion of P-gp substrate drug. P-gp inhibitor (verapamil, ketoconazole) increased the transport of digoxin, a P-gp substrate, to the adventitial side, and decreased the secretion into the luminal perfusate, indicating that this system can be used to assess the involvement of P-gp in the drug absorption at the intestinal tract. Treatment with phenobarbital, a CYP inducer, increased CYP3A4 activity in the liver (+ 130 %), but not intestinal epithelium. Another inducer, rifampicin, elevated CYP3A4 activity in the intestinal epithelium (+ 32 %) without affecting the activity in the liver. Rifampicin also enhanced the P-gp activity in the intestine. These results from ex vivo study suggest that CYP inducers may cause tissue-specific induction of CYP or P-gp activity. The mechanism of tissue-specific induction of drug metabolism or drug extrusion needs to be clarified. It may be helpful to understand the mechanism of drug interaction.