Inhibitory drug metabolites may contribute to drug-drug interactions (DDIs). The aim of this study was to determine the importance of inhibitory metabolites of itraconazole (ITZ) in in vivo cytochrome p450 (CYP) 3A4 inhibition. The pharmacokinetics of ITZ and midazolam (MDZ) were determined in six healthy volunteers in four sessions after administration of MDZ with and without oral ITZ. After doses of 50, 200, and 400 mg of ITZ, the clearance of orally administered MDZ decreased by 27, 74, and 83%, respectively. The in vivo half maximal inhibitory concentration (IC 50 ) for ITZ ranged from 5 to 132 nmol/l in the six subjects. The metabolites of ITZ were estimated to account for ~50% of the total CYP3A4 inhibition, with the relative contribution increasing with time after ITZ dosing. Of the total of 18 interactions observed, 15 (84%) could be predicted within a twofold error margin, with improved accuracy observed when ITZ metabolites were included in the predictions. This study shows that the metabolites of ITZ contribute to CYP3A4 inhibition and need to be accounted for in quantitative rationalization of ITZ-mediated DDIs.Many clinically important cytochrome P450 (CYP) inhibitors have been shown to possess circulating metabolites, 1 and many of these metabolites are predicted to contribute to in vivo drug-drug interactions (DDIs). 2 Classic examples of inhibitors with circulating inhibitory metabolites include three potent CYP inhibitors: itraconazole (ITZ),3 fluoxetine,4 and bupropion.5 Norfluoxetine, the primary circulating metabolite of fluoxetine, has been shown to be responsible for the persistent CYP2D6 inhibition after administration of fluoxetine,6 although no quantitative prediction of CYP2D6 inhibition by norfluoxetine as compared with fluoxetine was shown. Using atomoxetine as an example, quantitative predictions of the contribution of metabolites to in vivo DDIs were performed, and, despite the overall weak in vivo inhibition of CYP3A4 and CYP2D6 by atomoxetine, the metabolites were predicted to be mainly responsible for the interaction. 7 The metabolites of ITZ have been predicted to account for ~50% of the overall CYP3A4 inhibition in vivo. This prediction is based on in vivo disposition data of ITZ and its three NIH Public Access RESULTSSix healthy volunteers (one woman and five men), in the age range of 22-42 (mean 34) years and within 20% of optimal body mass index, completed the study. An additional female subject was enrolled, but she withdrew after the midazolam (MDZ) control session because of nausea and vomiting. No other side effects or possible adverse effects were observed in any of the subjects. All subjects had normal liver and kidney function and were in good health. Each subject was homozygous for CYP3A5*3 and hence was expected to lack expression of functional CYP3A5 protein. Individuals with potential CYP3A5 expression based on genotype were excluded from the study because CYP3A5 has been shown not to metabolize ITZ. 3 MDZ pharmacokinetics were measured after escalati...

An endogenous probe for CYP3A activity would be useful for early identification of in vivo CYP3A4 inhibitors. The aim of this study was to determine whether formation clearance (CLf) of the sum of 6β-hydroxycortisol and 6β-hydroxycortisone is a useful probe of CYP3A4 inhibition in vivo. In human liver microsomes, formation of 6β-hydroxycortisol and 6β-hydroxycortisone was catalyzed by CYP3A4, and itraconazole inhibited these reactions with IC50,u of 3.1nM and 3.4nM, respectively. The in vivo IC50,u of itraconazole towards the combined CLf of 6β-hydroxycortisone and 6β-hydroxycortisol was 1.6nM. The greater inhibitory potency in vivo is likely due to circulating inhibitory itraconazole metabolites. The maximum in vivo inhibition was 59% suggesting that fm,CYP3A4 for cortisol and cortisone 6β-hydroxylation is approximately 60%. Based on significantly decreased CLf of 6β-hydroxycortisone and 6β-hydroxycortisol after 200mg and 400mg single doses of itraconazole, this endogenous probe can be used to detect moderate and potent CYP3A4 inhibition in vivo.

The effect of food on pharmacokinetic properties of drugs is a commonly observed occurrence affecting about 40% of orally administered drugs. Within the pharmaceutical industry, significant resources are invested to predict and characterize a clinically relevant food effect. Here, the predictive performance of physiologically based pharmacokinetic (PBPK) food effect models was assessed via de novo mechanistic absorption models for 30 compounds using controlled, pre-defined in vitro, and modeling methodology. Compounds for which absorption was known to be limited by intestinal transporters were excluded in this analysis. A decision tree for model verification and optimization was followed, leading to high, moderate, or low food effect prediction confidence. High (within 0.8- to 1.25-fold) to moderate confidence (within 0.5- to 2-fold) was achieved for most of the compounds (15 and 8, respectively). While for 7 compounds, prediction confidence was found to be low (> 2-fold). There was no clear difference in prediction success for positive or negative food effects and no clear relationship to the BCS category of tested drug molecules. However, an association could be demonstrated when the food effect was mainly related to changes in the gastrointestinal luminal fluids or physiology, including fluid volume, motility, pH, micellar entrapment, and bile salts. Considering these findings, it is recommended that appropriately verified mechanistic PBPK modeling can be leveraged with high to moderate confidence as a key approach to predicting potential food effect, especially related to mechanisms highlighted here.

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PHRA represents a useful experimental system for the evaluation of the metabolic fates of low clearance compounds in drug development.

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Pantoprazole (a proton‐pump inhibitor) is used to treat gastroesophageal reflux disease (GERD) and Zollinger‐Ellison syndrome. In vitro and in vivo metabolism of pantoprazole had been studied and its metabolites were identified by LC‐MS and NMR. In vitro, major metabolites of pantoprazole were pantoprazole sulfone, pantoprazole sulfide, 6‐hydroxy‐pantoprazole sulfide, 4′‐O‐demethyl‐pantoprazole sulfide, pantoprazole thioether‐glucoside. In vivo, most of the metbaolites were in the forms of sulfate conjugates and some minor metabolites were as glucuronide conjugates.

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