When the metabolism of a drug is competitively or noncompetitively inhibited by another drug, the degree of in vivo interaction can be evaluated from the [I] u /K i ratio, where [I] u is the unbound concentration around the enzyme and K i is the inhibition constant of the inhibitor. In the present study, we evaluated the metabolic inhibition potential of drugs known to be inhibitors or substrates of cytochrome P450 by estimating their [I] u /K i ratio using literature data.The maximum concentration of the inhibitor in the circulating blood ([I] max ), its maximum unbound concentration in the circulating blood ([I] max,u ), and its maximum unbound concentration at the inlet to the liver ([I] in,max,u ) were used as [I] u , and the results were compared with each other. In order to calculate the [I] u /K i ratios, the pharmacokinetic parameters of each drug were obtained from the literature, together with their reported K i values determined in in vitro studies using human liver microsomes.For most of the drugs with a calculated [I] in,max,u /K i ratio less than 0.25, which applied to about half of the drugs investigated, no in vivo interactions had been reported or "no interaction" was reported in clinical studies. In contrast, the [I] max,u /K i and [I] max /K i ratio was calculated to be less than 0.25 for about 90% and 65% of the drugs, respectively, and more than a 1.25-fold increase was reported in the area under the concentration-time curve of the co-administered drug for about 30% of such drugs. These findings indicate that the possibility of underestimation of in vivo interactions (possibility of false-negative prediction) is greater when [I]

Irinotecan hydrochloride (CPT-11) is an anticancer agent with unpredictable bouts of diarrhea as a dose-limiting toxic side-effect. Since the biliary excretion of its active metabolite (SN-38) and SN-38 glucuronide (SN38-Glu), which are mediated by the multidrug resistance associated protein-2 (MRP2/ABCC2), has been proposed to be related to this gastrointestinal toxicity, we have attempted here to examine the potential of various therapeutic agents to interact with the biliary excretion in order to identify MRP2 inhibitors to prevent this toxicity. The inhibition constants (K(i)) of 26 compounds were examined for the transport of a typical MRP2 substrate in isolated canalicular membrane vesicles. Of these, 13 compounds inhibited the transport with K(i) values from 0.0461 to 281 microM. Three inhibitors (probenecid, sulfobromophthalein and glycyrrhizin) were also found to inhibit the biliary excretion of SN-38 and SN38-Glu in rats in vivo, and the degrees of inhibition were compatible with the estimated values based on the ratios of K(i) and unbound concentrations in circulating plasma. A similar estimation of the potential inhibitory effect in human was also examined by considering both the K(i) of each therapeutic agent and its unbound concentration both in circulating plasma and the inlet to the liver. The predicted degrees of inhibition by most compounds were minimal whereas approximately 75% inhibition was predicted for probenecid. Thus, probenecid may be a candidate which can be used clinically to inhibit the biliary excretion of CPT-11 metabolites, whereas an interaction between most of the other compounds and MRP2 is more unlikely.

The active transport of solutes mediated by the bile salt export pump (BSEP/ABCB11) and multidrug resistance associated protein-2 (MRP2/ABCC2) are thought to involve bile acid-dependent and -independent bile formation, respectively. To evaluate the potential of therapeutic agents as inhibitors of such transporters on bile canalicular membranes, we examined the inhibition of the primary active transport of typical substrates by 15 drugs, clinically known to cause cholestasis in canalicular membrane vesicles. The inhibition by most of the compounds in rat canalicular membrane vesicles (CMVs) was minimal or observed at much higher concentrations than obtained in clinical situations. However, cloxacillin, cyclosporin A and midecamycin inhibited BSEP, and cyclosporin A and midecamycin inhibited MRP2 with an inhibition constant close to the clinical concentration. By comparing the inhibition potential between rat and human CMVs, the inhibition of BSEP- and MRP2-mediated transport by midecamycin and cyclosporin A was relatively similar whereas the inhibitory effect on BSEP-mediated transport by cloxacillin and glibenclamide was more marked in humans than in rats. These results suggest that the majority of cholestasis-inducing drugs have a minimal inhibitory effect on rat BSEP and MRP2 although species differences in inhibitory potential should be considered, especially in the case of BSEP.

There have been no reports of the quantitative prediction of induction for drug-metabolizing enzymes in humans. We have tried to predict such enzyme induction in humans from in vitro data obtained using human hepatocytes. The in vitro and in vivo data on enzyme induction by inducers, such as rifampicin, phenobarbital and omeprazole, were collected from the published literature. The degree of enzyme induction in humans was compared with that predicted from in vitro data on human hepatocytes. Using the in vivo data, we calculated the hepatic intrinsic clearance of typical CYP substrates, such as midazolam and caffeine, before and after inducer treatment and estimated the induction ratios of hepatic intrinsic clearance following treatment. In the in vitro studies, the amount of mRNA or enzyme and enzyme activity in human hepatocytes, with or without an inducer, were compared and the induction ratios were estimated. The unbound mean concentration was taken as an index of drug exposure and the induction ratios in the in vivo and in vitro studies were compared. The unbound mean concentrations of inducers used in the in vitro studies were higher than those in the in vivo studies. The maximum induction ratios by inducers in the in vitro studies were higher than those in the in vivo studies. The induction ratio for rifampicin, omeprazole, troglitazone, dexamethasone and phenobarbital increased as the unbound mean concentration increased to reach a constant value. The induction of CYP3A and 1A was analyzed by the Emax model. The maximum induction ratio (Emax) and the concentration at half maximum induction (EC50) for rifampicin, omeprazole, troglitazone, dexamethasone and phenobarbital were 12.3, 0.847 micromol/L, 2.36, 0.225 micromol/L, 6.86, 0.002 micromol/L, 8.30, 9.32 micromol/L, and 7.62, 58.4 micromol/L, respectively. The Emax and EC50 of omeprazole for CYP1A were 12.02 and 0.075 micromol/L, respectively. The predicted induction ratio of all those inducers, except for omeprazole, based on the Emax and EC50 values obtained from the in vitro data were similar to the observed values. On the whole, a good correlation between the observed and predicted induction ratio of omeprazole was observed (r=0.768, p<0.05), although the predicted induction ratio was higher than the observed value. In conclusion, the present study suggests that it is possible to predict quantitatively the CYP3A enzyme induction from hepatocyte data.