Itraconazole (ITZ) is metabolized in vitro to three inhibitory metabolites: hydroxy-ITZ (OH-ITZ), keto-ITZ, and N-desalkyl-ITZ (ND-ITZ). The goal of this study was to determine the contribution of these metabolites to drug-drug interactions caused by ITZ. Six healthy volunteers received 100 mg ITZ orally for seven days and pharmacokinetic analysis was conducted at day 1 and day 7 of the study. The extent of CYP3A4 inhibition by ITZ and its metabolites was predicted using this data. ITZ, OH-ITZ, keto-ITZ and ND-ITZ, were detected in plasma samples of all volunteers. A 3.9-fold decrease in the hepatic intrinsic clearance of a CYP3A4 substrate was predicted using the average unbound steady-state concentrations (C ss,ave,u ) and liver microsomal inhibition constants for ITZ, OH-ITZ, keto-ITZ, and ND-ITZ. Accounting for circulating metabolites of ITZ significantly improved the in vitro to in vivo extrapolation of CYP3A4 inhibition compared to a consideration of ITZ exposure alone.

Under the guidance of the International Consortium for Innovation and Quality in Pharmaceutical Development (IQ), scientists from 20 pharmaceutical companies formed a Victim Drug-Drug Interactions Working Group. This working group has conducted a review of the literature and the practices of each company on the approaches to clearance pathway identification (f CL ), estimation of fractional contribution of metabolizing enzyme toward metabolism (f m ), along with modeling and simulation-aided strategy in predicting the victim drug-drug interaction (DDI) liability due to modulation of drug metabolizing enzymes. Presented in this perspective are the recommendations from this working group on: 1) strategic and experimental approaches to identify f CL and f m , 2) whether those assessments may be quantitative for certain enzymes (e.g., cytochrome P450, P450, and limited uridine diphosphoglucuronosyltransferase, UGT enzymes) or qualitative (for most of other drug metabolism enzymes), and the impact due to the lack of quantitative information on the latter. Multiple decision trees are presented with stepwise approaches to identify specific enzymes that are involved in the metabolism of a given drug and to aid the prediction and risk assessment of drug as a victim in DDI. Modeling and simulation approaches are also discussed to better predict DDI risk in humans. Variability and parameter sensitivity analysis were emphasized when applying modeling and simulation to capture the differences within the population used and to characterize the parameters that have the most influence on the prediction outcome.

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.

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...

Interest in determining safe and efficacious doses for drug administration in pediatric patients has increased dramatically in recent years. However, published pediatric clinical studies have failed to increase proportionally with adult clinical study publications. In order to assess the current state of pediatric dose determination and the supporting role of physiologically based pharmacokinetic modeling and simulation in determining pediatric dose, the pediatric clinical literature (2006-2016) and case examples of pediatric PBPK modeling efforts were reviewed. The objective of this assessment was to investigate the contribution of PBPK to our understanding of the differences between children and adults, which lead to differences in drug dose. Pediatric and adult dose data were available for 31 small molecule drugs. In general, pediatric dose was well-correlated with adult data, with an apparent tendency for higher body weight- or body surface area-normalized pediatric dose. Overall performance of pediatric PBPK modeling approaches was considered to adequately predict observed data. However, model performance was dependent upon age group simulated, with approximately half of neonatal predictions falling outside of 1.5-fold of observed. In conclusion, there is a clear need for further refinement of starting dose in pediatric phase 1 studies, and utilization of PBPK could lead to reduced numbers of patients required to establish safe and efficacious doses in the pediatric population.

This subteam under the Drug Metabolism Leadership Group (Innovation and Quality Consortium) investigated the quantitative role of circulating inhibitory metabolites in drug–drug interactions using physiologically based pharmacokinetic (PBPK) modeling. Three drugs with major circulating inhibitory metabolites (amiodarone, gemfibrozil, and sertraline) were systematically evaluated in addition to the literature review of recent examples. The application of PBPK modeling in drug interactions by inhibitory parent–metabolite pairs is described and guidance on strategic application is provided.

Oligonucleotide microarrays were used to study the variability of pharmacokinetics and drug metabolism (PKDM)-related gene expression in 75 normal human livers. The objective was to define and use absorption, distribution, metabolism and excretion (ADME) gene expression variability to discern co-regulated genes and potential surrogate biomarkers of inducible gene expression. RNA was prepared from donor tissue and hybridized on Agilent microarrays against an RNA mass balanced pool from all donors. Clustering of PKDM gene sets revealed donors with distinct patterns of gene expression that grouped genes known to be regulated by the nuclear receptor, pregnane X-receptor (PXR). Fold range metrics and frequency distributions from the heterogeneous human population were used to define the variability of individual PKDM genes in the 75 human livers and were placed in context by comparing expression data with basal ADME gene expression variability in an inbred and diet/environment controlled population of 27 Rhesus livers. The most variable genes in the hepatic transcriptome were mainly related to drug metabolism, intermediary metabolism, inflammation and cell cycle control. Unique patterns of expression across 75 individuals of inducible ADME gene expression allowed their expression to be correlated with the expression of many other genes. Correlated genes for AhR, CAR and PXR responsive genes (CYP1A2, CYP2B6 and CYP3A4) were identified that may be co-regulated and, therefore, provide clues to the identity of surrogate gene or protein markers for CYP induction. In conclusion, microarrays were used to define the variable expression of hepatic ADME genes in a diverse human population, the expression variability of ADME genes was compared with the expression variability in an inbred population of Rhesus monkeys, and genes were defined that may be co-regulated with important inducible CYP genes.

Assuming that valerenic acid serum concentrations correlate with the pharmacological activity of valerian, the timing of the valerenic acid peak concentration is consistent with the standard dosage recommendation to take valerian 30 min to 2 h before bedtime. Ongoing studies are evaluating the relationship between valerenic acid serum concentrations and objective measures of sleep in patients.