Enantiospecific separation and quantitation of mephenytoin and its metabolites nirvanol and 4'-hydroxymephenytoin in human plasma and urine by liquid chromatography/tandem mass spectrometry

Prediction of in vivo clearance and associated variability of CYP2C19 substrates by genotypes in populations utilizing a pharmacogenetics-based mechanistic model

It is important to examine the cytochrome P450 2C19 (CYP2C19) genetic contribution to drug disposition and responses of CYP2C19 substrates during drug development. Design of such clinical trials requires projection of genotype-dependent in vivo clearance and associated variabilities of the investigational drug, which is not generally available during early stages of drug development, but is essential for CYP2C19 substrates with multiple clearance pathways. This study evaluated the utility of pharmacogenetics-based mechanistic modeling in predicting such parameters. Hepatic CYP2C19 activity and variability within genotypes were derived from in vitro S-mephenytoin metabolic activity in genotyped human liver microsomes (N = 128). These data were then used in mechanistic models to predict genotype-dependent disposition of CYP2C19 substrates (i.e., S-mephenytoin, citalopram, pantoprazole, and voriconazole) by incorporating in vivo clearance or pharmacokinetics of wild-type subjects and parameters of other clearance pathways. Relative to the wild-type, the CYP2C19 abundance (coefficient of variation percentage) in CYP2C19*17/*17, *1/*17, *1/*1, *17/null, *1/null, and null/null microsomes was estimated as 1.85 (117%), 1.79 (155%), 1.00 (138%), 0.83 (80%), 0.38 (130%), and 0 (0%), respectively. The subsequent modeling and simulations predicted, within 2-fold of the observed, the means and variabilities of urinary S/R-mephenytoin ratio (36 of 37 genetic groups), the oral clearance of citalopram (9 of 9 genetic groups) and pantoprazole (6 of 6 genetic groups), and voriconazole oral clearance (4 of 4 genetic groups). Thus, relative CYP2C19 genotype-dependent hepatic activity and variability were quantified in vitro and used in a mechanistic model to predict pharmacokinetic variability, thus allowing the design of pharmacogenetics and drug-drug interaction trials for CYP2C19 substrates.

Breaking the sensitivity limitations of cytochrome P450 oxidation product: dansyl chloride derivatisation of 4-OH mephenytoin, a CYP2C19 metabolite and its application to in vitro CYP inhibition assay

A rapid selective and sensitive liquid chromatography/tandem mass spectrometry (LC-MS/MS) method was developed for the quantitative determination of derivatised cytochrome P450-2C19 oxidation product (dansyl-4-OH mephenytoin) and its underivatised form (4-OH mephenytoin). Samples were anaysed on C18 column (Waters Xbridge, 50 mm×4.6 mm, 3.5 μm particle size) with the mobile phase consisting of 0.1% formic acid in water and 0.1% formic acid in acetonitrile. A gradient method with a short run time of 2.5 min and 3.5 min was developed for the analysis of dansyl-4-OH mephenytoin and 4-OH mephenytoin, respectively. The standard curve was linear (r(2)=0.9972 for 4-OH mephenytoin; r(2)=0.9946 for dansyl-4-OH mephenytoin) over the concentration range of 0.16 to 40 ng/mL for both derivatised and underivatised forms. The CV (%) and relative error (RE) for inter and intraassay at three QC levels for dansyl-4-OH mephenytoin was 0.97-5.85% and -9.80 to 2.51%, respectively. Whereas, for 4-OH mephenytoin the CV (%) and RE (%) at three QC levels was 0.82-3.47% and -6.69 to -0.01%, respectively. The developed method was validated for various parameters such as linearity, precision & accuracy, extraction recovery, matrix effect, autosampler stability and was proved to be consistent across three QC levels with overall CV (%) less than 15. Dansylation helped in increasing the sensitivity of hydroxy mephenytoin by 100-200 fold. Given the simplicity involved in derivatisation process, we believe that this novel methodology will change the current approaches used for the enhancing the detection sensitivity of 4-OH mephenytoin.

Chiral chromatographic resolution of antiepileptic drugs and their metabolites: a challenge from the optimization to the application

A large number of the antiepileptic drugs (AEDs) presently available for clinical practice are chiral compounds while others, although achiral, may originate pharmacologically active chiral metabolites in vivo. The well-known implications of chirality in pharmacokinetics and pharmacodynamics demand the investigation of pharmacological properties for a racemic mixture and each enantiomer. To achieve these objectives, appropriate chiral analytical methods must be available. This article provides the first review of the current state of the art in chiral chromatographic methods available for quantifying enantiomers of AEDs in distinct matrices. Particular attention is paid to the methodological aspects and optimization strategies that successfully allow enantiomeric chromatographic separation of chiral AEDs and/or metabolites. Furthermore, the relevance of these methods in supporting the discovery and development of chiral AEDs is emphasized. In parallel and whenever available, the principal validation parameters are herein considered and related to the stage of drug discovery and development. In an attempt to optimize anticonvulsant activity and simultaneously diminish toxic effects, many pharmaceutical companies have started to manufacture single enantiomers. Therefore, chiral chromatographic techniques will be essential and the information herein compiled can be used as a framework for developing them.

High-performance liquid chromatography-tandem mass spectrometry in the identification and determination of phase I and phase II drug metabolites

Applications of tandem mass spectrometry (MS/MS) techniques coupled with high-performance liquid chromatography (HPLC) in the identification and determination of phase I and phase II drug metabolites are reviewed with an emphasis on recent papers published predominantly within the last 6 years (2002–2007) reporting the employment of atmospheric pressure ionization techniques as the most promising approach for a sensitive detection, positive identification and quantitation of metabolites in complex biological matrices. This review is devoted to in vitro and in vivo drug biotransformation in humans and animals. The first step preceding an HPLC-MS bioanalysis consists in the choice of suitable sample preparation procedures (biomatrix sampling, homogenization, internal standard addition, deproteination, centrifugation, extraction). The subsequent step is the right optimization of chromatographic conditions providing the required separation selectivity, analysis time and also good compatibility with the MS detection. This is usually not accessible without the employment of the parent drug and synthesized or isolated chemical standards of expected phase I and sometimes also phase II metabolites. The incorporation of additional detectors (photodiode-array UV, fluorescence, polarimetric and others) between the HPLC and MS instruments can result in valuable analytical information supplementing MS results. The relation among the structural changes caused by metabolic reactions and corresponding shifts in the retention behavior in reversed-phase systems is discussed as supporting information for identification of the metabolite. The first and basic step in the interpretation of mass spectra is always the molecular weight (MW) determination based on the presence of protonated molecules [M+H]⁺ and sometimes adducts with ammonium or alkali-metal ions, observed in the positive-ion full-scan mass spectra. The MW determination can be confirmed by the [M-H]^- ion for metabolites providing a signal in negative-ion mass spectra. MS/MS is a worthy tool for further structural characterization because of the occurrence of characteristic fragment ions, either MSⁿ analysis for studying the fragmentation patterns using trap-based analyzers or high mass accuracy measurements for elemental composition determination using time of flight based or Fourier transform mass analyzers. The correlation between typical functional groups found in phase I and phase II drug metabolites and corresponding neutral losses is generalized and illustrated for selected examples. The choice of a suitable ionization technique and polarity mode in relation to the metabolite structure is discussed as well.

Artemisinin antimalarials moderately affect cytochrome P450 enzyme activity in healthy subjects

The aim of this study was to investigate which principal human cytochrome P450 (CYP450) enzymes are affected by artemisinin and to what degree the artemisinin derivatives differ with respect to their respective induction and inhibition capacity. Seventy-five healthy adults were randomized to receive therapeutic oral doses of artemisinin, dihydroartemisinin, arteether, artemether or artesunate for 5 days (days 1-5). A six-drug cocktail consisting of caffeine, coumarin, mephenytoin, metoprolol, chlorzoxazone and midazolam was administered orally on days -6, 1, 5 and 10 to assess the activities of CYP1A2, CYP2A6, CYP2C19, CYP2D6, CYP2E1 and CYP3A, respectively. Four-hour plasma concentrations of parent drugs and corresponding metabolites and 7-hydroxycoumarin urine concentrations were quantified by liquid chromatography-tandem mass spectrometry. The 1-hydroxymidazolam/midazolam 4-h plasma concentration ratio (CYP3A) was increased on day 5 by artemisinin [2.66-fold (98.75% CI: 2.10-3.36)], artemether [1.54 (1.14-2.09)] and dihydroartemisinin [1.25 (1.06-1.47)] compared with day -6. The S-4'-hydroxymephenytoin/S-mephenytoin ratio (CYP2C19) was increased on day 5 by artemisinin [1.69 (1.47-1.94)] and arteether [1.33 (1.15-1.55)] compared with day -6. The paraxanthine/caffeine ratio (CYP1A2) was decreased on day 1 after administration of artemisinin [0.27 (0.18-0.39)], arteether [0.70 (0.55-0.89)] and dihydroartemisinin [0.73 (0.59-0.90)] compared with day -6. The alpha-hydroxymetoprolol/metoprolol ratio (CYP2D6) was lower on day 1 compared with day -6 in the artemisinin [0.82 (0.70-0.96)] and dihydroartemisinin [0.83 (0.71-0.96)] groups, respectively. In the artemisinin-treated subjects this decrease was followed by a 1.34-fold (1.14-1.58) increase from day 1 to day 5. These results show that intake of artemisinin antimalarials affect the activities of several principal human drug metabolizing CYP450 enzymes. Even though not significant in all treatment groups, changes in the individual metrics were of the same direction for all the artemisinin drugs, suggesting a class effect that needs to be considered in the development of new artemisinin derivatives and combination treatments of malaria.