Quantitative determination of dextromethorphan and three metabolites in urine by reverse-phase high-performance liquid chromatography

Effect of a Triphasic Oral Contraceptive on Drug-Metabolizing Enzyme Activity as Measured by the Validated Cooperstown 5+1 Cocktail

The effects of a common oral contraceptive preparation on the activity of 7 drug-metabolizing enzymes were investigated using the validated Cooperstown 5+1 Cocktail. In a randomized crossover fashion, 10 premenopausal women received caffeine, dextromethorphan, omeprazole, intravenous midazolam, and warfarin + vitamin K with and without a triphasic oral contraceptive (ethinyl estradiol 35 microg) and varying doses of daily norgestimate (0.18, 0.215, and 0.25 mg). Bioequivalence testing showed nonequivalence in drug versus no-drug treatment on the activity of drug-metabolizing enzymes (as reflected by metabolite ratios following probe drug administration); the activity of CYP1A2, CYP2C19, and NAT-2 decreased following the oral contraceptive, whereas the activity of CYP2C9 and CYP2D6 increased. No effects on xanthine oxidase or hepatic CYP3A were seen. Application of a non-parametric statistical testing approach revealed a significant difference only for CYP1A2 and CYP2C19. This triphasic oral contraceptive may have a clinically significant effect on the activity of some drug-metabolizing enzymes.

Resveratrol modulates drug- and carcinogen-metabolizing enzymes in a healthy volunteer study

Resveratrol has been shown to exhibit cancer-preventive activities in preclinical studies. We conducted a clinical study to determine the effect of pharmacologic doses of resveratrol on drug- and carcinogen-metabolizing enzymes. Forty-two healthy volunteers underwent baseline assessment of cytochrome P450 (CYP) and phase II detoxification enzymes. CYP1A2, CYP2D6, CYP2C9, and CYP3A4 enzyme activities were measured by the metabolism of caffeine, dextromethorphan, losartan, and buspirone, respectively. Blood lymphocyte glutathione S-transferase (GST) activity and GST-pi level and serum total and direct bilirubin, a surrogate for UDP-glucuronosyl transferase (UGT) 1A1 activity, were measured to assess phase II enzymes. After the baseline evaluation, study participants took 1 g of resveratrol once daily for 4 weeks. Enzyme assessment was repeated upon intervention completion. Resveratrol intervention was found to inhibit the phenotypic indices of CYP3A4, CYP2D6, and CYP2C9 and to induce the phenotypic index of 1A2. Overall, GST and UGT1A1 activities were minimally affected by the intervention, although an induction of GST-pi level and UGT1A1 activity was observed in individuals with low baseline enzyme level/activity. We conclude that resveratrol can modulate enzyme systems involved in carcinogen activation and detoxification, which may be one mechanism by which resveratrol inhibits carcinogenesis. However, pharmacologic doses of resveratrol could potentially lead to increased adverse drug reactions or altered drug efficacy due to inhibition or induction of certain CYPs. Further clinical development of resveratrol for cancer prevention should consider evaluation of lower doses of resveratrol to minimize adverse metabolic drug interactions.

Predictive value of animal models for human cytochrome P450 (CYP)-mediated metabolism: a comparative study in vitro

One major challenge in drug development is defining of the optimal animal species to serve as a model of metabolism in man. The study compared the hepatic drug metabolism characteristics of humans and six widely used experimental animal species. Classical in vitro model enzyme assays with known human cytochrome P450 (CYP) enzyme selectivity were employed and optimized to target human hepatic CYP forms. The profile of CYP activities best resembling the human was seen in mouse followed by monkey, minipig, and dog liver microsomes, with rats displaying the most divergent. The widest interindividual variability was found in CYP3A-mediated midazolam -hydroxylase, and omeprazole sulphoxidase activities in human and monkey liver microsomes. These data demonstrate that if hepatic xenobiotic-metabolizing characteristics were to be the sole reason for the selection of animal species for toxicity studies, then the rat might not be the most appropriate model to mimic human CYP activity patterns.

Effects of repeated green tea catechin administration on human cytochrome P450 activity

PURPOSE

Preclinical studies suggested that green tea or green tea catechins can modulate the activities of drug-metabolizing enzymes. We conducted this clinical study to determine the effect of repeated green tea catechin administration on human cytochrome P450 (CYP) enzyme activities.

METHODS

Forty-two healthy volunteers underwent a 4-week washout period by refraining from tea or tea-related products. At the end of the washout period, study participants received a cocktail of CYP metabolic probe drugs, including caffeine, dextromethorphan, losartan, and buspirone for assessing the activity of CYP1A2, CYP2D6, CYP2C9, and CYP3A4, respectively. Blood and urine samples before and 8 h after probe drug administration were collected to determine parent drug and metabolite concentrations for measurements of baseline CYP enzyme activities. Following the baseline evaluation, study participants underwent 4 weeks of green tea catechin intervention at a dose that contains 800 mg epigallocatechin gallate (EGCG) daily. The green tea catechin product was taken on an empty stomach to optimize the p.o. bioavailability of EGCG. The EGCG dose given in this study exceeded the amounts provided by average green tea consumption. Upon completion of the green tea catechin intervention, the postintervention CYP enzyme activities were evaluated as described above.

RESULTS

There are large between-subject variations in CYP enzyme activities in healthy individuals. Four weeks of green tea catechin intervention did not alter the phenotypic indices of CYP1A2, CYP12D6, and CYP12C9, but resulted in a 20% increase (P = 0.01) in the area under the plasma buspirone concentration-time profile, suggesting a small reduction in CYP3A4 activity.

CONCLUSIONS

We conclude that repeated green tea catechin administration is not likely to result in clinically significant effects on the disposition of drugs metabolized by CYP enzymes.

Simultaneous determination of dextromethorphan, dextrorphan, and guaifenesin in human plasma using semi-automated liquid/liquid extraction and gradient liquid chromatography tandem mass spectrometry

A method for the simultaneous determination of dextromethorphan (DEX), dextrorphan (DET), and guaifenesin (GG) in human plasma was developed, validated, and applied to determine plasma concentrations of these compounds in samples from six clinical pharmacokinetic (PK) studies. Semi-automated liquid handling systems were used to perform the majority of the sample manipulation including liquid/liquid extraction (LLE) of the analytes from human plasma. Stable-isotope-labeled analogues were utilized as internal standards (ISTDs) for each analyte to facilitate accurate and precise quantification. Extracts were analyzed using gradient liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Use of semi-automated LLE with LC-MS/MS proved to be a very rugged and reliable approach for analysis of more than 6200 clinical study samples. The lower limit of quantification was validated at 0.010, 0.010, and 1.0 ng/mL of plasma for DEX, DET, and GG, respectively. Accuracy and precision of quality control (QC) samples for all three analytes met FDA Guidance criteria of +/-15% for average QC accuracy with coefficients of variation less than 15%. Data from the thorough evaluation of the method during development, validation, and application are presented to characterize selectivity, linearity, over-range sample analysis, accuracy, precision, autosampler carry-over, ruggedness, extraction efficiency, ionization suppression, and stability. Pharmacokinetic data are also provided to illustrate improvements in systemic drug and metabolite concentration-time profiles that were achieved by formulation optimization.

In vitro metabolism and interactions of the fungicide metalaxyl in human liver preparations

In order to provide additional information for risk assessment of the fungicide metalaxyl, the main objectives were (1) to elucidate the interactions of metalaxyl with different human liver cytochrome P450 enzymes, (2) to tentitatively identify and (semi)quantify metabolites in vitro and (3) to identify human CYP enzymes responsible for metabolism. The mean inhibitory concentrations (IC(50)) for 7-pentoxyresorufin-O-dealkylation (CYP2B) and bupropion hydroxylation (2B6) were 48.9 and 41.7μM, respectively. The biotransformation reactions were hydroxylation, (di)demethylation and lactone formation. In human liver microsomes predominant metabolites were two hydroxymetalaxyl derivatives or atropisomers of one of the derivatives. On the basis of previous rat studies these could be N-(2-hydroxymethyl-6-methylphenyl)-N-(methoxyacetyl)alanine methyl ester and/or N-(2,6-dimethyl-5-hydroxyphenyl)-N-(methoxyacetyl)alanine methyl ester. The amounts of didemethylmetalaxyl N-(2,6-dimethylphenyl)-N-(hydroxyacetyl)alanine and lactone 4-(2,6-dimethylphenyl)-3-methylmorpholine-2,5-dione were higher in homogenates than microsomes. The carcinogenic 2,6-dimethylaniline was not detected. Among the nine major human CYPs, CYP3A4 was the only one responsible for metalaxyl hydroxylation, while CYP2B6 was the major isoform responsible for (di)demethylation and lactone formation.

Comparison of Human Tracheal/Bronchial Epithelial Cell Culture and Bovine Nasal Respiratory Explants for Nasal Drug Transport Studies

Ten drug compounds with varying physicochemical properties and transporter substrate specificities were investigated to compare their in vitro permeabilities across bovine nasal respiratory explants and the EpiAirway system, both established models for the assessment of nasal drug absorption. Permeability across the bovine explants and EpiAirway correlated well with the partitioning behavior of compounds whose clogDC values were greater than 0. The permeabilities of all ten compounds were well-correlated between the two tissue models, with the permeability values through the EpiAirway tissues being approximately 10-fold higher than through the bovine explants due to the thickness differences between the models. For more lipophilic compounds, the in vitro permeabilities measured with both tissue systems were also predictive of the reported in vivo nasal bioavailabilities. Deviations from these correlations were observed for compounds reported to be substrates of p-glycoprotein or OCT transporters, and differences were also seen between the permeabilities measured in the tissue models for these compounds. Both models can be used to estimate the systemic bioavailability of moderately lipophilic compounds administered intranasally, while each may have particular advantages or disadvantages in estimating the bioavailability of drug compounds that are subject to local mucosal metabolism or to carrier-mediated uptake or efflux.

Determination of dextromethorphan and its metabolite dextrorphan in human urine using high performance liquid chromatography with atmospheric pressure chemical ionization tandem mass spectrometry: a study of selectivity of a tandem mass spectrometric assay

Analytical method for the simultaneous determination of dextromethorphan (1) and dextrorphan (2) in urine, based on solid-phase extraction of drug from acidified hydrolyzed biological matrix, were developed. The analytes (1 and 2) and the internal standard (levallorphan, 3, IS) were detected by high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS) in positive ionization mode using a heated nebulizer (HN) probe and monitoring their precursor-->product ion combinations of m/z 272-->215, 258-->201, and 284-->201 for 1, 2, and 3, respectively, in multiple reaction monitoring mode. The analytes and IS were chromatographed on a Keystone Prism reverse phase (50 mm x 2.0 mm) 5 microm column using a mobile phases consisting of a 35/65 or 27/73 mixtures of methanol/water containing 0.1% TFA adjusted to pH 3 with ammonium hydroxide pumped at 0.4 ml/min for 1 and 2, respectively. The limits of reliable quantification of 1 and 2 were 2 and 250 ng/ml, respectively, when 1 ml of urine was processed. The absence of matrix effect was demonstrated by analysis of neat standards and standards spiked into urine extracts originating from five different sources. The linear ranges of the assay were 2-200 and 250-20,000 ng/ml for 1 and 2, respectively. Assay selectivity was evaluated by monitoring the "cross-talk" effects from other metabolites into the MS/MS channels used for monitoring 1, 2, and 3. In addition, an interfering peak originating from an unknown metabolite of 1 into the quantification of dextromethorphan was detected, requiring an effective chromatographic separation of analytes from other metabolites of 1. The need for careful assessment of selectivity of the HPLC-MS/MS assay in the presence of metabolites, and the assessment of matrix effect, are emphasized.

Simple chromatography method for simultaneous determination of dextromethorphan and its main metabolites in human plasma with fluorimetric detection

Dextromethorphan, the innocuous non-narcotic antitussive agent, is the most widely used probe drug to assess CYP2D6 function both in vivo and in vitro. For this reason a simple and selective high performance liquid chromatography method with fluorimetric detection for simultaneous quantitation of dextromethorphan, and its main metabolites in human plasma was developed and validated. The method involved a simple and rapid protein precipitation protocol, using a mixture of ZnSO(4) and methanol. The analysis was performed on a 3 microm, C(18) Tracer Excel 15 cm x 0.4 cm i.d. column by gradient elution in which Mobile phase A consisted of potassium dihydrogen phosphate buffer (pH = 3, 0.01 M):methanol:tetrahydrofuran (68.5:31:0.5), and mobile phase B consisted of methanol:tetrahydrofuran (93.25:6.75). Linear calibration curves were obtained in the range of 10-500 ng/ml for dextromethorphan, dextrorphan and hydroxymorphinan. The limit of quantitation (LOQ) was 10 ng/ml for each compound. The maximum within and between days precisions were 7.4 and 7.8%, respectively. The accuracies at four different concentration levels ranged from 88.2 to 111.5%. The recoveries were between 88.0 and 108.6%. The assay method was successfully applied to determine dextromethorphan metabolic ratio after an oral dose of 30 mg of dextromethorphan hydrobromide.

Comparative studies on the cytochrome p450-associated metabolism and interaction potential of selegiline between human liver-derived in vitro systems

Selegiline was used as a model compound in a project aimed at comparing, evaluating, and integrating different in vitro approaches for the prediction of cytochrome p450 (p450)-catalyzed hepatic drug metabolism in humans (EUROCYP). Metabolic predictions were generated using homology modeling, cDNA-expressed p450 enzymes, human liver microsomes, primary cultured human hepatocytes, and precision-cut human liver slices. All of the in vitro systems correctly indicated the formation of two dealkylated metabolites, desmethylselegiline and methamphetamine. The metabolic instability of selegiline was demonstrated by all of the in vitro systems studied. Estimates of clearance varied from 16 l/h to 223 l/h. With the exception of one approach, all systems underpredicted the in vivo clearance in humans (236 l/h). Despite this, all approaches successfully classified selegiline as a high clearance compound. Homology modeling suggested the participation of CYP2B6 in the demethylation of selegiline and of CYP2D6 in the depropargylation of the drug. Studies with recombinant expressed enzymes and with human hepatic microsomal fraction supported the involvement of CYP2B6 but not of CYP2D6. These techniques also suggested the involvement of CYP1A2, CYP2C8, and CYP2C19 in the biotransformation of selegiline. In vitro, CYP2B6 was the most active form of p450 involved in selegiline metabolism. Metabolism by several enzymes operating in parallel implies a low interaction potential for the drug. None of the techniques alone was able to predict all aspects of the metabolic and kinetic behavior of selegiline in vivo. However, when used as an integrated package, all significant characteristics were predictable.

A sensitive LC-MS/MS assay for the determination of dextromethorphan and metabolites in human urine--application for drug interaction studies assessing potential CYP3A and CYP2D6 inhibition

The commonly used antitussive dextromethorphan can be used to simultaneously assess potential cytochrome P450 3A (CYP3A) and CYP2D6 inhibition during drug development. The metabolism of dextromethorphan to dextrorphan and subsequently to 3-hydroxymorphinan are via the 2D6 pathway, while the metabolism of dextromethorphan to 3-methoxymorphinan is via the 3A pathway. A sensitive and specific LC-MS/MS assay has been developed to determine the human urine concentrations of dextromethorphan and three metabolites (dextrorphan, 3-methoxymorphinan and 3-hydroxymorphinan) in support of drug interaction studies. Urine samples (0.5 ml), after enzymatic hydrolysis of the conjugates and containing 3-ethylmorphine as an internal standard, were extracted with chloroform under basic conditions. Following concentration and reconstitution, the samples were analyzed by LC-MS/MS. The assay was linear over the range of 5.00-500 ng/ml for dextromethorphan and 3-methoxymorphinan; and 200-3000 ng/ml for dextrorphan and 3-hydroxymorphinan using a Perkin-Elmer Sciex triple quadrupole mass spectrometer (API 300). The intra- and inter-day relative standard deviation (RSD) across three validation runs over the entire concentration range for all analytes was less than 15%. Accuracy determined at three or four concentrations (9.00, 200, and 400 ng/ml for dextromethorphan and 3-methoxymorphinan; 250, 400, 1300 and 2500 ng/ml for dextrorphan and 3-hydroxymorphinan) ranged between 96.3 and 113.8%. The stability of analytes in urine was demonstrated for 9 months at -20 degrees C, 24 h under ambient conditions and for up to three freeze/thaw cycles. The method described herein is suitable for the rapid and efficient measurement of dextromethorphan and different metabolites to estimate potential CYP3A inhibition by drug candidates and for screening of extensive and poor metabolizers of CYP2D6 in the heterogeneous population. The method has subsequently been validated on a Sciex API 3000 with lower limit of quantitation; 1.00 ng/ml for dextromethorphan and 3-methoxymorphinan; 60.0 ng/ml for dextrorphan and 100 ng/ml for 3-hydroxymorphinan.

Development of a rapid and sensitive high-performance liquid chromatographic method to determine CYP2D6 phenotype in human liver microsomes

Dextromethorphan is a probe substrate to determine CYP2D6 phenotype. The conversion of dextromethorphan to dextrorphan by CYP2D6 accounts for approximately 60% of total metabolism. Most analytical methods utilize complicated labor- and time-intensive sample processing methods with several liquid-liquid extraction (LLE) steps. Our goal was to develop a non-LLE based rapid and sensitive HPLC method, to measure dextromethorphan metabolism in human liver microsomes. A solid-phase filtration based reverse-phase HPLC method with fluorescence detection was developed and validated. Human liver (n = 6) microsomal incubations were carried out with dextromethorphan, under optimum conditions. The analytes were separated by one-step centrifugal filtration with Nanosep separation units. The filtrate was injected ( 50 microL) into a Waters Alliance 2690 HPLC system. Metabolic incubations were also conducted to determine levels using LLE for comparisons. The Nanosep separation step reduced the extraction time from 3h to 40 min. The limit of quantitation was 23.8 nM (9.7 ng/mL), recovery was approximately 98%, the mean precision values were <10% RSD for the controls (80, 320 and 640 nM) and mean percentage error was <5%. Michaelis-Menten parameters were determined to distinguish CYP2D6 phenotypes. A rapid and sensitive HPLC method is reported, which may be suitable for automation and allows phenotyping of human liver microsomes.

Carbamazepine: a 'blind' assessment of CVP-associated metabolism and interactions in human liver-derived in vitro systems

1. The ability of various in vitro systems for CYP enzymes (computer modelling, human liver microsomes, precision-cut liver slices, hepatocytes in culture, recombinant enzymes) to predict various aspects of in vivo metabolism and kinetics of carbamazepine (CBZ) was investigated. 2. The study was part of the EUROCYP project that aimed to evaluate relevant human in vitro systems to study drug metabolism. 3. CBZ was given to the participating laboratories without disclosing its chemical nature. 4. The most important enzyme (CYP3A4) and metabolic route (10,11-epoxidation) were predicted by all the systems studied. 5. Minor enzymes and routes were predicted to a different extent by various systems. 6. Prediction of a clearance class, i.e. slow clearance, was correctly predicted by microsomes, slices, hepatocytes and recombinant enzymes (CYP3A4). 7. The 10,11-epoxidation of CBZ by the recombinant CYP3A4 was enhanced by the addition of exogenous cytochrome-b5, leading to a considerable over-prediction. 8. Induction potency of CBZ was predicted in cultured hepatocytes in which 7-ethoxycoumarin O-deethylase was used as an index activity. 9. It seems that for a principally CYP-metabolized substance such as CBZ, all liver-derived systems provide useful information for prediction of metabolic routes, rates and interactions.

High-performance liquid chromatography assay for simultaneous determination of dextromethorphan and its main metabolites in urine and in microsomal preparations

An HPLC method has been developed and validated for the determination of dextromethorphan, dextrorphan, 3-methoxymorphinan and 3-hydroxymorphinan in urine samples. Deconjugated compounds were extracted on silica cartridges using dichloromethane/hexane (95:05, v/v) as an eluent. Chromatographic separation was accomplished on a Phenyl analytical column serially connected with a Nitrile analytical column. The mobile phase consisted of a mixture of an aqueous solution, containing 1.5% acetic acid and 0.1% triethylamine, and acetonitrile (75:25, v/v). Compounds were monitored using a fluorescence detector. Calibration curves were linear over the range investigated (0.2-8.0 microM) with correlation coefficients >0.999. The method was reproducible and precise. Coefficients of variation and deviations from nominal values were both below 10%. For all the analytes, recoveries exceeded 77% and the limits of detection were 0.01 microM. The validated assay proved to be suitable for the determination of DEM metabolic indexes reported to reflect the enzymatic activity of the cytochrome P450s, CYP2D6 and CYP3A, both in vivo, when applied to urine samples from patients, and in vitro, when applied to samples from the incubation of liver microsomes with dextromethorphan.

Characterization of xenobiotic-metabolizing cytochrome P450 (CYP) forms in ringed and grey seals from the Baltic Sea and reference sites

Earlier studies have shown that members of the cytochrome P4501 (CYP1) enzyme family are constitutively expressed, and are elevated in the livers of ringed seals (Phoca hispida) and grey seals (Halichoerus grypus) living in the heavily polluted Baltic Sea. In this study, we compared the expression profiles of several additional CYP enzymes in the liver and extrahepatic tissues of Baltic ringed and grey seals with the corresponding CYP expression in seals from relatively unpolluted waters. We used marker enzyme activity levels, diagnostic inhibitors and immunoblot analysis to assess members of the CYP2A, CYP2B, CYP2C, CYP2D, CYP2E and CYP3A sub-families. Coumarin 7-hydroxylation (COH), a marker of CYP2A activity, was high in the liver and the lungs of all the studied seal populations. The presence of a putative CYP2A form in these seals was further supported by the strong inhibition of COH activity by a chemical inhibitor and by an anti-CYP2A5 antibody. However, antibodies to human and rodent CYP2B, CYP2C and CYP2E forms did not recognize any proteins in these seal species. Dextromethorphan O-demethylation (marker for CYP2D activity) and chlorzoxazone 6-hydroxylation (marker for CYP2E activity) were measurable in the livers of all the seals we studied. Both activities were elevated in the Baltic seal populations, showed a strong positive correlation with CYP1A activity and were at least partly inhibited by a typical CYP1A inhibitor, alpha-naphthoflavone. Further studies are needed to determine the presence and characteristics of CYP2D and CYP2E enzymes in ringed and grey seals. Testosterone 6beta-hydroxylation, a CYP3A marker, showed a relatively high level of activity in the livers of both seal species and was potently inhibited by ketoconazole, a CYP3A-selective inhibitor. The putative CYP3A activity showed an opposing geographical trend to that of CYP2D and CYP2E, since it was elevated in the control area. CYP3A protein levels, revealed by immunoblotting, showed a positive correlation with testosterone 6beta-hydroxylation. We conclude tentatively that CYP2A- and CYP3A-like enzymes are expressed in ringed and grey seals, but that CYP2B- and CYP2C-like ones are not. Further information on the individual contaminant profile is needed before any conclusions can be drawn on a possible connection between the varying CYP expressions and the contaminant load.

Rapid bioanalytical determination of dextromethorphan in canine plasma by dilute-and-shoot preparation combined with one minute per sample LC-MS/MS analysis to optimize formulations for drug delivery

The determination of dextromethorphan in canine plasma is used to demonstrate the high throughput bioanalytical approach of automated dilute-and-shoot (DAS) sample preparation followed by a 1 min isocratic liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. Dilute-and-shoot preparation is commonly used for the determination of drugs in several biological matrices such as urine and saliva, but is not typically used with plasma samples because the amount of protein present in plasma can lead to a variety of problems including column failure. As a result, plasma sample preparation usually removes protein by precipitation, extraction or filtration; however, the dilute-and-shoot approach solubilizes proteins throughout the chromatographic portion of the assay. The attributes of this approach are compared with a previously validated liquid/liquid extraction procedure for determination of dextromethorphan in plasma. Accuracy and precision of both methods are similar. The lower limit of quantitation (LLOQ) of the dilute-and-shoot approach is much higher at 2 ng/ml versus 5 pg/ml with the liquid/liquid extraction; however, the sample throughput of the preparation portion of the dilute-and-shoot approach is more than 50-fold greater. The ruggedness of the dilute-and-shoot method was thoroughly investigated because of the problems traditionally associated with the direct injection of diluted plasma onto an LC-MS/MS instrument. With the optimal conditions, greater than 1,000 injections of diluted plasma have been successfully performed on a single column in less than 19 h making this technique an excellent approach for the rapid preparation and high throughput of plasma samples containing drug levels in the ng/ml range or higher. Application of this methodology to measure the levels of dextromethorphan in canine plasma to evaluate drug delivery from various formulations is also presented.

On the assessment of drug metabolism by assays of codeine and its main metabolites

Codeine and its main metabolites appear to have advantages for assessing drug metabolic phenotypes. The authors have further developed a high-performance liquid chromatography (HPLC) method for the quantification of codeine and six of its metabolites in urine. Quantification was performed by electrochemical detection for morphine, normorphine, morphine-6-glucuronide, and the internal standard 4-O-methyldopamine; and by ultraviolet detection for codeine, norcodeine, and morphine-3-glucuronide. The method had a detection limit of 2 nmol/L(-1) for morphine and normorphine, 4 nmol/L(-1) for morphine-6-glucuronide, 3 nmol/L for the internal standard, 20 nmol/L(-1) for morphine-3-glucuronide, and 60 nmol/L(-1) for codeine and norcodeine. The coefficients of variations were <9% for intraday and <10% for interday analyses. The recovery of codeine and its metabolites ranged from 55% (for morphine-3-glucuronide) to 90% (for codeine, norcodeine, morphine, and morphine-6-glucuronide). Eleven healthy volunteers were phenotyped for CYP2D6 using codeine as well as debrisoquine and dextromethorphan. Ten subjects were extensive metabolizers (EM) and one a poor metabolizer (PM) of codeine, debrisoquine, and dextromethorphan. Significant correlations between the metabolic ratios (MRs) of the different probe drugs were obtained (r2 > 0.95, p < 0.001). This HPLC method is simple, sensitive, accurate, and reproducible for assessing the CYP2D6 phenotype.

Effects of psychopharmacotherapy on phenotypic expression of cytochrome P450 2D6 in patients genotyped for CYP2D6 mutations

No Abstract

Intra-individual variability and influence of urine collection period on dextromethorphan metabolic ratios in healthy subjects

The aim of the study was to evaluate intra-individual variability in metabolic ratios (MRs) of dextromethorphan (DM) in healthy volunteers and to compare the MRs in urine collected 0-4, 0-8 and 0-24 h post-dose. Urinary molar ratios of DM to dextrorphan (MR1) and of DM to methoxymorphinan (MR2) were obtained after a single oral 27.5 mg dose of DM hydrobromide to ten healthy male and four female Caucasians (ten extensive metabolizers (EM) and four poor metabolizers (PM) of DM) to probe activities of CYP2D6 and CYP3A, respectively. Seven EM and one PM received DM on three additional occasions within 2 months. For the seven EM, the intra-individual variability (CVw) in the MRs obtained in the three urine collections ranged from 11 to 93% (MR1) and from 8 to 77% (MR2). The mean CVw estimated separately for the 4, 8 and 24 h urines by two-way analysis of variance reached 58, 57 and 44% for the MR1 and 50, 42 and 31% for the MR2, respectively. For all 14 subjects, the log-transformed ratios (MR1) obtained in the 24 h urines were highly correlated with those in either the 8 h (rs = 0.967, P < 0.0001) or 4 h urines (rs = 0.946, P < 0.0001). Correlation between the log-transformed MR2s were weaker (24 h vs. 8 h: rs = 0.829, P < 0.0001, 24 h vs. 4 h: rs = 0.831, P < 0.0001). The MR1s in 4 h and 8 h urines were only 2 and 9% less than those in 24 h urines (median differences) and varied from 48 and 47% below to 85 and 55% above (95% -CI for the differences). However, the MR2s in the 4 h and 8 h urines were shifted towards higher values by 49 and 23% and the corresponding 95% -CI limits were: 16-164% (4 h vs. 24 h) and 30-119% (8 h vs. 24 h). In conclusion, MR1 values in the 4 h urine collection agree well with those in longer collections and their use in epidemiological studies can be recommended. The intra-individual variability of approximately 50% in the MR1 has to be taken into account in clinical studies with within-subject design. Accurate determination of the MR2 requires at least a 24 h period of urine collection.

Analysis of dextrorphan, a metabolite of dextromethorphan, using gas chromatography with electron-capture detection

Dextromethorphan, a constituent of many over-the-counter cough syrups, is used as a probe drug for phenotyping subjects for their cytochrome P450 2D6 (CYP2D6) enzyme activity and for measuring CYP2D6 activity of preparations such as microsomes. In such studies, formation of the metabolite dextrorphan is used as indicator of the activity of this CYP enzyme. The present report describes an electron-capture gas chromatographic procedure developed for detection and quantification of dextrorphan in human liver microsomal preparations in vitro. After basification of the incubation mixture, dextrorphan was derivatized with pentafluorobenzoyl chloride under aqueous conditions prior to analysis on a gas chromatograph equipped with a capillary column, an electron capture detector, and a printer-integrator. Para-hydroxymephenytoin was carried through the procedure as internal standard. The procedure, which involves the derivatization of dextrorphan under aqueous conditions, is rapid and involves the use of the relatively economical procedure of electron-capture gas chromatography. The derivative is stable and possesses excellent chromatographic properties.

Influence of pretreatment with immunosuppressants on O-demethylation of dextromethorphan in isolated perfused rat liver

Changes in the immune system induced by exogenous or endogenous factors may be accompanied with modifications of the activity of the drug metabolising enzymes in the liver. Some immunostimulatory agents are known to suppress the oxidative metabolism mediated by cytochromes P450. Possible effects of substances which suppress the immune responses of the organism have not been fully understood yet. The present study was undertaken to investigate the influence of immunosuppressants cyclophosphamide and dexamethasone on the CYP2D 1-dependent metabolism of dextromethorphan (DEM) in the isolated perfused liver from male rat donors (Wistar albino, 250-310 g). Recirculatory perfusion system was used with Williams' medium E (Sigma Chemicals Co.) as a perfusion medium (120 mL). DEM was administered as a 1 mg bolus into the perfusion solution at the start of each experiment after 20 min preperfusion. Samples of perfusate for HPLC determination of DEM and its O-demethylated metabolite dextrorphan (DOR) were taken at 15 min intervals for 120 min. The results have shown a rapid conversion of DEM to DOR in the isolated rat liver preparations. Pharmacokinetic parameters in the livers from intact rats were as follows: t1/2 DEM = 19.1+/-4.10 min, k(m) = 0.035+/-0.008 min(-1), Cl(m) = 4.21+/-0.97 mL x min(-1), AUC(DOR) = 2160+/-201 microg x min x L(-1). Pretreatment of rats with dexamethasone (4 mg/kg/day, iv, x 3 days) led to a significant increase in the concentration of dextrorphan in the recirculating solution, but it did not substantially change the kinetic constants of DOR formation (km = 0.036+/-0.004 min(-1), Cl(m) = 4.27+/-0.43 mL x min(-1)). Cyclophosphamide (100 mg/kg, ip, 1 dose on day 5 before perfusion) induced nearly twofold increase in the DOR concentrations in perfusate and thus highly significant (p < 0.01) changes of the kinetic parameters characterizing the increased rate of conversion of DEM to DOR (t1/2 DEM = 12.1+/-0.90 min, km = 0.055+/-0.004 min(-1), Cl(m) = 7.09+/-1.37 mL x min(-1), AUC(DOR) = 3602+/-154 microg x min x L(-1)). Considering that both cyclophosphamide and dexamethasone belong to the most widely used immunosuppressive drugs, their potential to promote the CYP2D-mediated metabolism might have a clinical impact in combined therapy of autoimmune or other diseases.

Quantitation of levorphanol in human plasma at subnanogram per milliliter levels using capillary gas chromatography with electron-capture detection

A gas chromatographic method for the determination of levorphanol in human plasma is described. The method utilizes extractive alkylation with tetrabutylammonium cation as the phase-transfer catalyst and pentafluorobenzyl bromide as the alkylating agent, and employs a structural analog, d-3-hydroxy-N-ethylmorphinan, as the internal standard. The pentafluorobenzyl ethers formed are separated by capillary gas chromatography and detected by electron capture. The method has good precision and accuracy for concentrations ranging from 0.25 ng/ml to 100 ng/ml and has been used to measure plasma concentrations as part of a study to evaluate the management of chronic neuropathic pain with levorphanol.

CYP2D6 polymorphism is not crucial for the disposition of selegiline

OBJECTIVE

To determine the possible impact of CYP2D6 polymorphism on the pharmacokinetics and pharmacodynamics of selegiline.

METHODS

Five poor metabolizers and 8 extensive metabolizers of debrisoquin (INN, debrisoquine) were given 10 mg selegiline hydrochloride. The concentrations of selegiline and its main metabolites in serum were determined for 4 days. The pharmacodynamics were quantitated by measuring platelet monoamine oxidase type B activity for 3 weeks. In addition, the effect of selegiline and its main metabolites on the CYP2D6-catalyzed dextromethorphan O-demethylase activity and the effect of quinidine on the metabolism of selegiline were studied in human liver microsomes.

RESULTS

Peak serum concentrations of selegiline were reached rapidly and ranged from 1 to 32 nmol/L. The metabolite concentrations were considerably higher and remained so for a longer period. There were no significant differences in the pharmacokinetic parameters of selegiline, desmethylselegiline, and l-amphetamine between poor metabolizers and extensive metabolizers. However, the area under the serum concentration-time curve (AUC) values of l-methamphetamine were, on average, 46% higher (P = .01) in poor metabolizers than in extensive metabolizers. No significant correlations were found between debrisoquin metabolic ratio and AUC values of selegiline or its metabolites, except for l-methamphetamine (rs = 0.90; P < .001). The maximum monoamine oxidase type B inhibition was 97% in both groups. The inhibitory potency of selegiline, desmethylselegiline, and l-methamphetamine toward dextromethorphan O-demethylase was very low (50% inhibitory concentration values from 160 to 580 mumol/L). Quinidine (< or = 100 mumol/L) did not inhibit the formation of desmethylselegiline or l-methamphetamine from selegiline.

CONCLUSIONS

CYP2D6 is not important in the primary elimination of selegiline, and the biological effect of selegiline seems to be similar in poor metabolizers and extensive metabolizers of debrisoquin. The inhibitory effect of selegiline and its main metabolites on CYP2D6 activity seems to be negligible.

Effect of fluvoxamine therapy on the activities of CYP1A2, CYP2D6, and CYP3A as determined by phenotyping

OBJECTIVE

To determine the effect of 150 mg/day fluvoxamine on the activities of CYP1A2, CYP2D6, CYP3A, N-acetyltransferase-2 (NAT2), and xanthine oxidase (XO) by phenotyping with caffeine, dextromethorphan, and midazolam.

METHODS

Oral caffeine (2 mg/kg), oral dextromethorphan (30 mg), and intravenous midazolam (0.025 mg/kg) were administered to 10 white male volunteers every 14 days for 4 months and to 10 white premenopausal female volunteers during the midfollicular and midluteal phases of the menstrual cycle for 4 complete cycles (8 total phenotyping measures). The first 6 phenotyping measures were used to establish baseline activity. Subjects were given 150 mg/day fluvoxamine for the fourth month or cycle of the study. Enzyme activity for CYP1A2, CYP2D6, NAT2, and XO was expressed as urinary metabolite ratios. Midazolam plasma clearance was used to express CYP3A activity.

RESULTS

No difference between baseline and weeks 2 and 4 of fluvoxamine therapy was observed for NAT2 or XO metabolite ratios. For CYP1A2, CYP2D6, and CYP3A phenotypes, significant differences existed between baseline and fluvoxamine therapy. For CYP1A2, the mean urinary metabolite ratio (+/-SD) was 7.53 +/- 7.44 at baseline and 4.30 +/- 2.82 with fluvoxamine ( P = .012). Mean CYP2D6 molar urinary dextromethorphan ratios before and after fluvoxamine therapy were 0.00780 +/- 0.00694 and 0.0153 +/- 0.0127, respectively (P = .011). Midazolam clearance decreased from 0.0081 +/ 0.0024 L/min/kg at baseline to 0.0054 +/- 0.0021 L/min/kg with therapy (P = .0091). For CYP1A2, CYP2D6, and CYP3A, fluvoxamine therapy changed the phenotyping measures by a median of -44.4%, 123.5%, and -34.4%, respectively.

CONCLUSIONS

We concluded that fluvoxamine may cause significant inhibition of CYP1A2, CYP2D6, and CYP3A activity. This metabolic inhibition may have serious implications for a variety medications.

Cytochrome P450 specificity of metabolism and interactions of oxybutynin in human liver microsomes

Oxybutynin has an extensive first pass metabolism after oral administration, the main active metabolite being N-desethyloxybutynin. The purpose of this study was to investigate the CYP isoform specificity of oxybutynin N-deethylation and possible interactions. Oxybutynin N-deethylation in human liver microsomes in vitro was potently inhibited by ketoconazole (IC50 4.5 microM), less and variably by itraconazole and not by quinidine or several other reference inhibitors, suggesting that CYP3A enzymes are predominant catalysts of the reaction. Recombinant CYP3A5 enzyme had higher activity in oxybutynin N-deethylation than recombinant CYP3A4. Ketoconazole inhibited oxybutynin N-deethylation by the recombinant CYP3A4 and CYP3A5 almost completely, whereas itraconazole inhibited the activity of CYP3A4 more potently than that of CYP3A5. Oxybutynin inhibited CYP3A4- and CYP2D6- associated activities (testosterone 6 beta-hydroxylase and dextromethorphan O- demethylase, respectively) in human liver microsomes. CYP1A1/2-, CYP2A6-, CYP2C9- and CYP2E1-associated activities were inhibited less potently or not at all by oxybutynin when compared with reference inhibitors. Although the reasons for the weak and variable inhibition by itraconazole remain to be studied, it seems that oxybutynin is predominantly metabolized by CYP3A4 and CYP3A5 but not by CYP2D6. However, it seems to have some affinity also to the latter enzyme.

A sensitive assay of metoprolol and its major metabolite alpha-hydroxy metoprolol in human plasma and determination of dextromethorphan and its metabolite dextrorphan in urine with high performance liquid chromatography and fluorometric detection

A reverse-phase High Performance Liquid Chromatographic (HPLC) method was developed for the analysis of metoprolol in the large number of human plasma samples obtained in in vitro-in vivo correlations (IVIVC) and bioavailability studies of extended release formulations of metoprolol tartrate. The metabolite, alpha-hydroxy metoprolol (OH-met), could also be quantified. The analytes were extracted from the plasma using solid phase columns, separated on a C-4 analytical column followed by fluorimetric detection. The linearity, precision, accuracy, stability, selectivity and ruggedness were validated for the concentration ranges of 1-400 ng ml-1 for metoprolol and 0.5-200 ng ml-1 for OH-met. The same chromatographic conditions were slightly modified to quantify dextromethorphan and its metabolite dextrorphan in urine in the concentration range 0.052-0.05 microgram ml-1 as a method for screening for fast metabolizers.

Omission of the deconjugation step in urine analysis and the unaltered outcome of CYP2D6 phenotyping with dextromethorphan

The present study was aimed at determining whether the deconjugation step in chemical analysis could be omitted without altering the outcome of phenotyping CYP2D6 with dextromethorphan. This drug and its metabolite, dextrorphan, were assayed by high-performance liquid chromatography (HPLC) in urine. Urinary levels of dextromethorphan and dextrorphan with and without enzymatic (beta-glucuronidase) treatment of urine and the metabolic ratios for dextromethorphan were determined in 45 subjects. Although the enzymatic treatment did not alter the urinary concentration of dextromethorphan in both phenotypes, it increased the urinary concentration of dextrorphan in both poor and extensive metabolizers by 3.7- and 12.8-fold, respectively. A urinary unconjugated dextromethorphan/unconjugated dextrorphan metabolic ratio of 2.00 and a total dextromethorphan/total dextrorphan metabolic ratio of 0.30, respectively, identified three poor metabolizers. Enzymatic treatment decreased the urinary antimode value. Moreover, the urinary metabolic ratio based on unconjugated dextrorphan and dextromethorphan correlated well with that based on assay of total dextrorphan and dextromethorphan (rs = 0.9458, P < 0.001). The results show that urinary analysis of dextrorphan and dextromethorphan omitting the enzymatic deconjugation step is a fast, reliable and sensitive method and could be used for studying CYP2D6 type genetic polymorphism in man.

Highly sensitive high-performance liquid chromatographic-tandem mass spectrometric method for the analysis of dextromethorphan in human plasma

A stable-isotope-dilution HPLC-tandem mass spectrometry-based method was developed for the determination of dextromethorphan in human plasma. Plasma samples were prepared for analysis by solid-phase extraction on octadecylsilane extraction cartridges. Dextromethorphan and the deuterium-labeled dextromethorphan internal standard were chromatographed on a short reversed-phase column and detected by a selected-reaction-monitoring scheme. Linear standard curves were obtained over three orders of magnitude and the limit of quantitation for dextromethorphan was 50 pg/ml, using a 1-ml plasma sample. The combination of HPLC and electrospray tandem mass spectrometry resulted in a rapid, selective and sensitive method for the analysis of dextromethorphan in plasma. The method was applied for the evaluation of the pharmacokinetic profile of dextromethorphan in human volunteers following peroral administration.

Automated determination of dextromethorphan and its main metabolites in human plasma by high-performance liquid chromatography and column switching

An automated column-switching technique coupled to isocratic high-performance liquid chromatography (HPLC) with fluorescence detection was developed for simultaneous determination of dextromethorphan and its three major metabolites, dextrorphan, hydroxymorphinan, and methoxymorphinan. After cleavage of conjugates by incubation with glucuronidasearylsulfatase at 37 degrees C for 15 h, plasma samples were injected directly into the HPLC system. Dextromethorphan and metabolites were retained on a cleanup column (10 x 4.6 mm internal diameter [ID]) filled with cyanopropyl (CN) material (Hypersil CPS, 10-microns article size) while interfering proteins and lipids were washed to waste. After column switching, the drugs were eluted from the cleanup column and separated on Spherisorb CN material (5-microns particle size, column size 250 x 4.6 mm ID). Fluorescence detection was carried out with an excitation wavelength of 220 nm and an emission wavelength of 305 nm. Sample cleanup and HPLC separation were completed within 20 min. Regression analyses found linearity (r > 0.99) between drug concentration and detector response over a wide range-5-220 ng/ml for dextromethorphan, 5-550 ng/ml for dextrorphan, 5-500 ng/ml for hydroxymorphinan, and 5-200 ng/ml for methoxymorphinan. The limit of quantification was approximately 5 ng/ml, and the recovery was > 90% for all compounds. At concentrations of 20-500 ng/ml, the intra- and interassay coefficients of variation ranged from 3.5 to 14.6% and from 7.0 to 14.0%, respectively. The method is suitable for in vivo phenotyping of CYP2D6 activity, which catalyzes the O-demethylation of dextromethorphan to dextrorphan, and is also applicable to pharmacokinetic studies in man.

High performance liquid chromatography determination of dextromethorphan and its metabolites in urine using solid-phase extraction

A high performance liquid chromatography assay coupled with fluorescence detection was developed for the determination of dextromethorphan and its metabolites in urine. The products and the internal standard, pholcodine, were separated on an Alltima C18, 5 microns column (250 x 4.6 mm), using a mobile phase containing sodium dodecyl sulphate (1 mM) in a mixture of acetonitrile-sodium dihydrogen phosphate (0.01 M) 40.5:59.5, v/v) (pH* = 2.5). A novel solid-phase extraction procedure with strong cation exchange, non end-capped, Isolute SCX cartridges allows good recovery of the products (mean 85% or more). For all analytes, the assay is sensitive (LOQ 25 ng ml-1, using 200 microliters urine), reproducible (RSD < 15%) and accurate (< 15% deviation of the nominal value) over the range evaluated. This method can be used to measure dextromethorphan and its metabolites to phenotype individuals as poor or extensive metabolizers of drugs metabolized by CYP2D6.

Quantification of dextromethorphan and metabolites: a dual phenotypic marker for cytochrome P450 3A4/5 and 2D6 activity

A sensitive and selective liquid chromatographic procedure using fluorimetric detection was developed to quantify dextromethorphan (DTM), 3-methoxymorphinan (3MM), dextrorphan (DT), 3-hydroxymorphinan (3OH) and two internal standards, codeine (COD) and ethylmorphine (ETM), in urine. Precision and accuracy of the assay were determined over a concentration range of 5-3200 ng/ml urine for DTM, 5-400 ng/ml urine for 3MM, 400-40 000 ng/ml urine for DT and 200-16 000 ng/ml urine for 3OH, by assaying freshly prepared calibration standards and replicates of six quality control (QC) samples on separate days. All of the inter-day and intra-day coefficients of variation (C.V.s) were less than 20% except for a low QC for 3MM. The inter-day and intra-day accuracies were less than 20% for the low QCs, less than 15% for the medium QCs and less than 12% for the high QCs, for all compounds. The limit of quantification (LOQ) was 2 ng/ml urine for DTM and 3MM, 250 ng/ml urine for DT, and 100 ng/ml urine for 3OH. Absolute recovery was 76% for DTM, 74% for 3MM, 77% for DT, 46% for 3OH, 73% for ETM, and 57% for COD. The frequency distribution of the CYP2D6 metabolic ratio (DTM/DT) illustrated a bimodal distribution whereas, the CYP3A metabolic ratio (DTM/3MM) exhibited a unimodal distribution in overnight urine samples of volunteers who ingested 30 mg dextromethorphan hydrobromide. The CYP2D6 metabolic ratio significantly correlated with 3MM/3OH (r=0.82) and DTM/3OH (r=0.95) but did not correlate with the CYP3A metabolic ratio (r=0.27).

Dextromethorphan as an in vivo probe for the simultaneous determination of CYP2D6 and CYP3A activity

Dextromethorphan (DM) is O-demethylated into dextrorphan (DEX) in humans by the cytochrome P450 designated as CYP2D6 and N-demethylated into 3-methoxymorphinan (3MM) via CYP3As. Clinically, DM has been successfully used as an index of CYP2D6 and this paper describes analytical and clinical data that will help evaluate the use of DM hydrobromide as a probe of CYP3A activity. DM and its three demethylated metabolites were measured in a 4-h spot urine sample using a HPLC method employing solid-phase extraction (C(18)), analysis on a phenyl column [mobile phase, methanol-acetonitrile-phosphate buffer (10 mM, pH 3.5, 20:25:55, v/v)] and fluorescence detection (excitation at lambda=228 nm, no emission cut-off filter). The urinary molar ratio DM-DEX was used to assess CYP2D6 activity while DM-3MM was used for CYP3As. The DM-3MM ratios were sensitive to the co-administration of selective CYP3A inhibitors grapefruit juice and erythromycin. In addition, in healthy volunteers and cancer patients, the N-demethylation of DM correlated with the CYP3A-mediated metabolism of verapamil and tamoxifen. DM appears to be a promising way to simultaneously phenotype patients for CYP2D6 and CYP3As.

Coadministration of nefazodone and benzodiazepines: IV. A pharmacokinetic interaction study with lorazepam

This study was conducted to determine the potential for an interaction between nefazodone (NEF), a new antidepressant, and lorazepam (LOR) after single- and multiple-dose administration in a randomized, double-blind, parallel-group, placebo-controlled study in healthy male volunteers. A total of 12 subjects per group received either placebo (PLA) twice daily, 2 mg of LOR twice daily, 200 mg of NEF twice daily, or the combination of 2 mg of LOR and 200 mg of NEF (LOR+NEF) twice daily for 7 days. Plasma samples were collected after dosing on day 1 and day 7 and before the morning dose on days 4, 5, and 6 for the determination of LOR, NEF, and NEF metabolites hydroxy (HO)-NEF, m-chlorophenylpiperazine (mCPP), and dione by validated high-performance liquid chromatography methods. Steady-state levels in plasma were reached by day 4 for LOR, NEF, HO-NEF, mCPP, and dione. Noncompartmental pharmacokinetic analysis showed that there was no effect of LOR on the single dose or steady-state pharmacokinetics of NEF, HO-NEF, or dione after coadministration. The steady-state mCPP Cmax values decreased 36% for the LOR+NEF group in comparison to that when NEF was given alone. There was no effect of NEF on the pharmacokinetics of LOR after coadministration. The absence of an interaction appears to be attributable to LOR's metabolic clearance being dependant on conjugation rather than hydroxylation. Overall, no change in LOR or NEF dosage is necessary when the two drugs are coadministered.

Coadministration of nefazodone and benzodiazepines: III. A pharmacokinetic interaction study with alprazolam

This study was conducted to determine the potential for an interaction between nefazodone, a new antidepressant, and alprazolam after single- and multiple-dose administration in a randomized, double-blind, parallel-group, placebo-controlled study in 48 healthy male volunteers. A group of 12 subjects received either placebo twice daily, 1 mg of alprazolam twice daily, 200 mg of nefazodone twice daily, or the combination of 1 mg of alprazolam and 200 mg of nefazodone twice daily for 7 days. Serial blood samples were collected after dosing on day 1 and day 7 and before the morning dose on days 4, 5, and 6 for the determination of alprazolam and its metabolites alpha-hydroxyalprazolam (AOH) and 4-hydroxyalprazolam (4OH) and nefazodone and its metabolites hydroxynefazodone (HO-nefazodone), m-chlorophenylpiperazine (mCPP), and a triazole dione metabolite (dione) by validated high-performance liquid chromatography methods. Steady-state levels in plasma were reached by day 4 for alprazolam, 4OH, nefazodone, HO-nefazodone, mCPP, and dione. Noncompartmental pharmacokinetic analysis showed that at steady state, alprazolam Cmax and AUCtau values significantly increased approximately twofold and 4OH Cmax and AUCtau values significantly decreased by 40 and 26%, respectively, when nefazodone was coadministered with alprazolam. There was no effect of alprazolam on the single-dose or steady-state pharmacokinetics of nefazodone, HO-nefazodone, or dione after the coadministration of alprazolam and nefazodone. However, the mean steady-state mCPP Cmax and AUCtau significantly increased by approximately threefold and t1/2 values significantly increased by approximately twofold after the coadministration of alprazolam and nefazodone in comparison to those when nefazodone was given alone. Competitive inhibition between alprazolam and nefazodone metabolism at cytochrome P450 3A4 may be responsible for the pharmacokinetic interaction when alprazolam and nefazodone were coadministered. No adjustment of nefazodone dosage is required when nefazodone and alprazolam are coadministered. Because alprazolam concentrations in plasma are increased in the presence of nefazodone, a reduction in alprazolam dosage is recommended when the two agents are coadministered.

Pharmacokinetic and pharmacodynamic evaluation during coadministration of nefazodone and propranolol in healthy men

Potential interactions between nefazodone (200 mg every 12 hours) and propranolol (40 mg every 12 hours) were assessed in 18 healthy male volunteers in an open-label, randomized, three-way crossover study. The nature, frequency, and severity of adverse events during coadministration of nefazodone and propranolol were similar to those observed with either treatment alone. There were no clinically significant effects on vital signs, electrocardiographic results, or laboratory parameters. With coadministration, the maximum peak concentration (Cmax) and area under the concentration-time curve over the dosing interval (AUC tau) of propranolol decreased 29% and 14%, respectively; Cmax and AUC tau of 4-hydroxy-propranolol decreased 15% and 21%, respectively. Despite decreased plasma concentrations of the beta-antagonists, the reduction in exercise-induced tachycardia and post-exercise double product was slightly greater with coadministration than with propranolol alone. Administration of nefazodone alone did not significantly affect either pharmacologic parameter. The pharmacokinetics of nefazodone and its metabolites were largely unaffected during coadministration. Coadministration of propranolol and nefazodone results in modest pharmacokinetic inequivalencies, but no clinically significant alterations of the pharmacodynamics of propranolol.

Simultaneous determination of dextrorphan and guaifenesin in human plasma by liquid chromatography with fluorescence detection

A sensitive liquid chromatographic (LC) method was developed and validated for the simultaneous determination of dextrorphan and guaifenesin in human plasma using fluorescence detection. Dextrorphan and guaifenesin were extracted from plasma by a liquid-liquid extraction procedure using chloroform containing laudanosine as the internal standard. A cyano column (15 cm x 46 mm i.d., Spherisorb 5-CN) and a mobile phase containing acetonitrile-triethylamine-distilled water (10:1:89, v/v/v) (pH 6) were used. The concentration-response relationship for dextrorphan was found to be linear over a concentration range of 23-515 ng ml-1 with a lower limit of detection of 20 ng ml-1; the accuracy of the method would fall (95% confidence limit) within 9.53% and 11.07% of the true value for the inter-and intra-day, respectively; the inter- and intra-day precision, as measured by RSD, ranged from 1.88% to 30.07% (mean 2.28%) and from 4.69% to 7.51% (mean 5.67%) over the dynamic concentration range of the method (33-326 ng ml-1). The concentration-response relationship for guaifenesin was found to be linear over a concentration range of 181-8136 ng ml-1 with a lower detection limit of 30 ng ml-1; the accuracy of the method would fall (95% confidence limit) within 9.78% and 8.04% of the true value for the inter- and intra-day, respectively; the inter- and intra-day precision, as measured by the RSD, ranged from 2.55 to 6.07% (mean 3.90%) and from 3.12 to 3.90% (mean 3.52%) over the dynamic concentration range of the method (435-6430 ng ml-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Deficient C-oxidation of nicotine

In most people, nicotine is extensively (70% to 80%) metabolized to cotinine by C-oxidation. In a clinical trial, a 57-year-old woman was found to have the expected plasma levels of nicotine but unusually low plasma levels of cotinine both when smoking cigarettes and while receiving transdermal nicotine. To characterize her metabolism, simultaneous infusions of deuterium-labeled nicotine (d2) and cotinine (d4) were administered, with comparison to 20 other control smokers. The clearance of nicotine was unusually low (6.5 ml/min/kg versus 17.2 ml/min/kg), and the half-life of nicotine significantly longer (348 minutes versus 138 minutes) in the index case subject compared with the control subjects. The clearance of cotinine was normal. The index case subject converted only 9% of nicotine to cotinine, compared with 72% for the control subjects. As far as we know, this is the first person with deficient C-oxidation of nicotine to be characterized. Deficient C-oxidation of nicotine is associated with a long half-life of nicotine and deficient generation of cotinine, both of which could influence the risks and addictiveness of tobacco use in affected individuals.

High-dose dextromethorphan in amyotrophic lateral sclerosis: phase I safety and pharmacokinetic studies

Much interest has focused on the role of glutamate-mediated excitotoxicity in the etiopathogenesis of amyotrophic lateral sclerosis (ALS). We therefore conducted a phase I study of high-dose dextromethorphan (DM) in ALS. DM is a selective, noncompetitive antagonist of the N-methyl-D-aspartate subtype of the glutamate receptor. Thirteen patients were given DM in an escalating dose fashion, to a target of 10 mg/kg/day or the maximum tolerable dose, and then maintained on this dose for up to 6 months. Total daily doses ranged from 4.8 to 10 mg/kg (median, 7 mg/kg). Side effects were dose limiting in most patients. The most common side effects were light-headedness, slurred speech, and fatigue. Detailed pharmacokinetic and neuropsychology studies were performed. This study demonstrates the feasibility of long-term administration of high-dose DM in ALS, as well as in other conditions associated with glutamate excitotoxicity.

Influence of hepatic impairment on the pharmacokinetics of nefazodone and two of its metabolites after single and multiple oral doses

The pharmacokinetics of nefazodone, a new antidepressant, and two of its active metabolites, hydroxy-nefazodone and m-chlorophenylpiperazine, were determined after single and repeated oral escalating doses of 50, 100 and 200 mg, in healthy volunteers (n = 13) and patients with mild (n = 13) or severe (n = 6) hepatic impairment. All subjects were classified according to their dextromethorphan oxidation capacity. In healthy volunteers, nefazodone was rapidly absorbed after which the plasma concentrations declined with an apparent elimination half-life ranging from 2.7 +/- 1.7 h to 10.2 +/- 4.4 h according to the dosage. Hydroxy-nefazodone appeared rapidly in plasma and its time-course (half-life ranging 1.4 +/- 0.9 h to 6.5 +/- 1.6 h) paralleled that of nefazodone, while mCPP showed low and variable concentrations. The disproportionately longer half-life and more markedly increased Cmax and AUC0-48 which was observed with dosage and treatment duration, and moreover AUC0-12 at steady state significantly higher (P < 0.05) than AUC0-infinity after single dose demonstrated the non-linearity of the pharmacokinetics of nefazodone and hydroxy-nefazodone. The constant molar AUC0-48 hydroxy-nefazodone/nefazodone ratio (0.32 +/- 0.04) and the close correlation (r2 = 0.95) between kinetic parameters of nefazodone and hydroxy-nefazodone suggest that nefazodone hydroxylation is not a saturable process. The kinetics of nefazodone and metabolites were significantly affected by severe but not by mild liver insufficiency. As a consequence, on a pharmacokinetic basis nefazodone should be used with caution in severely hepatic impaired patients.

Genetic basis for a lower prevalence of deficient CYP2D6 oxidative drug metabolism phenotypes in black Americans

Debrisoquin hydroxylase (CYP2D6) is a cytochrome P450 enzyme that catalyzes the metabolism of > 30 commonly prescribed medications. Deficiency in CYP2D6 activity, inherited as an autosomal recessive trait, was found to be significantly less common in American blacks (1.9%) than whites (7.7%). To determine the genetic basis for this difference, inactivating CYP2D6 mutations were assessed by allele-specific PCR amplification and RFLP analyses of genomic DNA from 126 unrelated whites and 127 unrelated blacks. Blacks had a twofold lower frequency (8.5 versus 23%, P = 0.001) of the CYP2D6(B) mutation (point mutation at intron 3/exon 4 splice site), while complete deletion of the CYP2D6 gene (5.5% blacks, 2.4% whites), and the CYP2D6(A) mutation (single nucleotide deletion in exon 5; 0.24% blacks, 1.4% whites) were not different between the two groups. The prevalence of heterozygous genotypes was significantly lower in blacks (25 versus 42% of extensive metabolizers, P = 0.009), consistent with the observed prevalence of the deficient trait in blacks and whites. We conclude that the same CYP2D6 mutations lead to a loss of functional expression in blacks and whites, but American blacks have a lower prevalence of the deficient trait due to a lower frequency of the CYP2D6(B) mutation. This could explain racial differences in drug effects and disease risk.

Oxidative polymorphism of dextromethorphan in a Burundi population

The wide availability, metabolism by the same cytochrome P450 as debrisoquine and, above all, the inocuity of dextromethorphan (DMP) favour the frequent choice of this drug as the test substance in determining oxidation phenotypes. 100 healthy Burundian volunteers (94 m and 6 f) in this study ingested 50 mg DMP bromhydrate, i.e. 38.5 mg of DMP base. Urine was collected for 8 h following the dose and TLC was used to analyse it. The method was particularly useful in view of its low cost, speed and the ease of applying it to a large study group. 5% of the Burundian subjects were poor metabolizers.

Effect of quinidine on the dextromethorphan O-demethylase activity of microsomal fractions from human liver

1. The kinetics of dextromethorphan O-demethylation were measured in microsomes prepared from five human livers, both in the absence and in the presence of quinidine. 2. For each liver and over the concentration range of dextromethorphan examined (4.2-3400 microM), this reaction involved an enzymatic component of high affinity, with an apparent Michaelis-Menten constant (Km) of 4.6 +/- 1.8 microM (mean +/- s.d.) and a maximum velocity (Vmax) of 4.2 +/- 3.5 nmol mg-1 h-1 (mean +/- s.d.). 3. Quinidine was a potent and competitive inhibitor of the activity of this component (mean Ki +/- s.d. of 0.025 +/- 0.008 microM) as it is for other oxidation reactions which have already been found to co-segregate with the debrisoquine-type polymorphism. 4. With microsomes from four of the five livers studied, there was evidence of a second enzymatic component of activity characterized by a similar Vmax and about 20-fold higher Km compared with the high affinity component. The activity of this low affinity component was unaffected by quinidine in the concentrations studied.