Prediction of in vivo drug-drug interactions based on mechanism-based inhibition from in vitro data: inhibition of 5-fluorouracil metabolism by (E)-5-(2-Bromovinyl)uracil.

The fatal drug-drug interaction between sorivudine, an antiviral drug, and 5-fluorouracil (5-FU) has been shown to be caused by a mechanism-based inhibition. In this interaction, sorivudine is converted by gut flora to (E)-5-(2-bromovinyl)uracil (BVU), which is metabolically activated by dihydropyrimidine dehydrogenase (DPD), and the activated BVU irreversibly binds to DPD itself, thereby inactivating it. In an attempt to predict this interaction in vivo from in vitro data, inhibition of 5-FU metabolism by BVU was investigated by using rat and human hepatic cytosol and human recombinant DPD. Whichever enzyme was used, increased inhibition was observed that depended on the preincubation time of BVU and enzyme in the presence of NADPH and BVU concentration. The kinetic parameters obtained for inactivation represented by k(inact) and K'(app) were 2.05 +/- 1.52 min(-1), 69.2 +/- 60.8 microM (rat hepatic cytosol), 2.39 +/- 0.13 min(-1), 48.6 +/- 11.8 microM (human hepatic cytosol), and 0.574 +/- 0.121 min(-1), 2.20 +/- 0.57 microM (human recombinant DPD). The drug-drug interaction in vivo was predicted quantitatively based on a physiologically based pharmacokinetic model, using pharmacokinetic parameters obtained from the literature and kinetic parameters for the enzyme inactivation obtained in the in vitro studies. In rats, DPD was predicted to be completely inactivated by administration of BVU and the area under the curve of 5-FU was predicted to increase 11-fold, which agreed well with the reported data. In humans, a 5-fold increase in the area under the curve of 5-FU was predicted after administration of sorivudine, 150 mg/day for 5 days. Mechanism-based inhibition of drug metabolism is supposed to be very dangerous. We propose that such in vitro studies should be carried out during the drug-developing phase so that in vivo drug-drug interactions can be predicted.

Prediction of in vivo drug-drug interactions between tolbutamide and various sulfonamides in humans based on in vitro experiments.

Drug-drug interactions between tolbutamide and sulfonamides have extensively been reported. We attempted to predict the in vivo interaction between tolbutamide and sulfonamides from the in vitro metabolic inhibition studies. The inhibition constant (K(i)) was derived from the inhibitory effects of eight sulfonamides (sulfaphenazole, sulfadiazine, sulfamethizole, sulfisoxazole, sulfamethoxazole, sulfapyridine, sulfadimethoxine, and sulfamonomethoxine) on tolbutamide metabolism. We found that the inhibitory effect of sulfaphenazole was greatest among the eight sulfonamides examined. Furthermore, the contribution of each P450 enzyme to tolbutamide metabolism was investigated by using recombinant P450 enzymes. Although cytochrome P450 (CYP) 2C8, 2C9, and 2C19 metabolized tolbutamide, the main enzyme involved was CYP2C9. The K(i) values of several sulfonamides were comparable between human liver microsomes and recombinant CYP2C9. The maximum unbound plasma concentration of sulfonamides in the portal vein was calculated from literature data on the pharmacokinetics of sulfonamides. Using the K(i) values obtained from in vitro inhibition studies, the degree of increase in tolbutamide area under the plasma concentration-time curve (AUC) was predicted. About 4.8- and 1.6-fold increases in tolbutamide AUC were predicted by coadministration of sulfaphenazole and sulfamethizole, respectively, which agreed well with the reported increases in humans. Furthermore, the increase in tolbutamide AUC by coadministration of sulfadiazine, sulfisoxazole, and sulfamethizole was predicted to be 1.5- to 2. 6-fold, although the corresponding in vivo effects have not been reported. It is concluded that some of these sulfonamides have to be carefully coadministered with CYP2C9 substrates such as tolbutamide although coadministration of sulfaphenazole needs the greatest care.