It is of great importance to predict in vivo pharmacokinetics in humans based on in vitro data. We summarize recent findings of the quantitative prediction of the hepatic metabolic clearance from in vitro studies using human liver microsomes, hepatocytes, or P450 isozyme recombinant systems. Furthermore, we propose a method to predict pharmacokinetic alterations caused by drug-drug interactions that is based on in vitro metabolic inhibition studies using human liver microsomes or human enzyme expression systems. Although we attempt to avoid the false negative prediction, the inhibitory effect was underestimated in some cases, indicating the possible contribution of the active transport into hepatocytes and/or interactions at the processes other than the hepatic metabolism, such as the metabolism and transport processes during gastrointestinal absorption.
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ABSTRACT:Clinical studies have revealed that plasma concentrations of midazolam after oral administration are greatly increased by coadministration of erythromycin and clarithromycin, whereas azithromycin has little effect on midazolam concentrations.
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]
Although I(inlet,u,max) overestimated the unbound concentration in the liver, the tolbutamide/sulfaphenazole interaction could be successfully predicted by using a fixed value of I(inlet,u,max) indicating that the unbound concentration of sulfaphenazole in the liver after its clinical dose is by far larger than the concentration to inhibit CYP2C9-mediated metabolism and that care should be taken when it is co-administered with drugs that are substrates of CYP2C9.
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