Improved high-performance liquid chromatographic determination of debrisoquine and 4-hydroxydebrisoquine in human urine following direct injection

Effect of Age and Postoperative Time on Cytochrome P450 Enzyme Activity Following Liver Transplantation

This study evaluates the changes in cytochrome P450 (CYP) enzyme activity in orthotopic liver transplant (OLTx) patients in relation to recipient age and postoperative time. Thirty-eight stable OLTx patients, separated into younger and older age groups, and 21 healthy subjects were given a 5-drug cocktail including chlorzoxazone (CYP2E1), caffeine (CYP1A2), dapsone (CYP3A4), mephenytoin (CYP2C19), and debrisoquin (CYP2D6). The phenotypic indexes were determined for each associated enzyme. Compared to young healthy subjects, the CYP2E1 capacity was significantly increased in younger and older OLTx patients (P < .001), while the CYP2C19 capacity was decreased significantly in younger and older OLTx patients within 30 days postoperatively (P < .01). The CYP2D6 capacity was significantly lower after 30 days postoperatively in older OLTx patients (P < .05). The authors conclude that within 30 days postoperatively, CYP2E1 capacity was markedly elevated in OLTx patients, while 2C19 function was significantly reduced. CYP2D6 capacity was impaired after 30 days postoperatively. Younger and older OLTx patients experienced similar changes in major CYP450 enzyme capacity following liver transplantation.

Clinical assessment of CYP2D6-mediated herb-drug interactions in humans: effects of milk thistle, black cohosh, goldenseal, kava kava, St. John's wort, and Echinacea

Cytochrome P450 2D6 (CYP2D6), an important CYP isoform with regard to drug-drug interactions, accounts for the metabolism of approximately 30% of all medications. To date, few studies have assessed the effects of botanical supplementation on human CYP2D6 activity in vivo. Six botanical extracts were evaluated in three separate studies (two extracts per study), each incorporating 16 healthy volunteers (eight females). Subjects were randomized to receive a standardized botanical extract for 14 days on separate occasions. A 30-day washout period was interposed between each supplementation phase. In study 1, subjects received milk thistle (Silybum marianum) and black cohosh (Cimicifuga racemosa). In study 2, kava kava (Piper methysticum) and goldenseal (Hydrastis canadensis) extracts were administered, and in study 3 subjects received St. John's wort (Hypericum perforatum) and Echinacea (Echinacea purpurea). The CYP2D6 substrate, debrisoquine (5 mg), was administered before and at the end of supplementation. Pre- and post-supplementation phenotypic trait measurements were determined for CYP2D6 using 8-h debrisoquine urinary recovery ratios (DURR). Comparisons of pre- and post-supplementation DURR revealed significant inhibition (approximately 50%) of CYP2D6 activity for goldenseal, but not for the other extracts. Accordingly, adverse herb-drug interactions may result with concomitant ingestion of goldenseal supplements and drugs that are CYP2D6 substrates.

Clinical assessment of effects of botanical supplementation on cytochrome P450 phenotypes in the elderly: St John's wort, garlic oil, Panax ginseng and Ginkgo biloba

OBJECTIVES

Elderly patients are more likely to ingest prescription medications concurrently with botanical supplements, and may therefore be vulnerable to herb-drug interactions. Phytochemical-mediated modulation of cytochrome P450 (CYP) activity may underlie many herb-drug interactions. Some evidence suggests that CYP activity may decrease in the elderly. If so, herb-mediated changes in CYP activity may take on greater clinical relevance in this population. In this study, single timepoint, phenotypic metabolic ratios were used to determine whether long-term supplementation of St John's wort, garlic oil, Panax ginseng, and Ginkgo biloba affected CYP1A2, CYP2D6, CYP2E1 or CYP3A4 activity in elderly subjects.

METHODS

Twelve healthy volunteers between the ages of 60 and 76 years (mean age 67 years) were randomly assigned to receive each botanical supplement for 28 days followed by a 30-day washout period. Probe drug cocktails of midazolam, caffeine, chlorzoxazone and debrisoquine were administered before and at the end of supplementation. Pre- and post-supplementation phenotypic ratios were determined for CYP3A4, CYP1A2, CYP2E1 and CYP2D6 using 1-hydroxymidazolam/midazolam serum ratios (1-hour), paraxanthine/caffeine serum ratios (6-hour), 6-hydroxychlorzoxazone/chlorzoxazone serum ratios (2-hour) and debrisoquine urinary recovery ratios (8-hour), respectively. The content of purported 'active' phytochemicals was determined for each supplement.

RESULTS

Comparisons of pre- and post-St John's wort phenotypic ratios revealed significant induction of CYP3A4 (approximately 140%) and CYP2E1 activity (approximately 28%). Garlic oil inhibited CYP2E1 activity by approximately 22%. P. ginseng inhibition of CYP2D6 was statistically significant, but the magnitude of the effect (approximately 7%) did not appear to be clinically relevant. None of the supplements tested in this study appeared to affect CYP1A2 activity.

CONCLUSIONS

Elderly subjects, like their younger counterparts, are susceptible to herb-mediated changes in CYP activity, especially those involving St John's wort. Pharmacokinetic herb-drug interactions stemming from alterations in CYP activity may adversely affect drug efficacy and/or toxicity. When compared with earlier studies that employed young subjects, the data suggest that some age-related changes in CYP responsivity to botanical supplementation may exist. Concomitant ingestion of botanical supplements with prescription medications, therefore, should be strongly discouraged in the elderly.

Drug disposition of chiral and achiral drug substrates metabolized by cytochrome P450 2D6 isozyme: case studies, analytical perspectives and developmental implications

The concepts of drug development have evolved over the last few decades. Although number of novel chemical entitities belonging to varied classes have made it to the market, the process of drug development is challenging, intertwined as it is with complexities and uncertainities. The intention of this article is to provide a comprehensive review of novel chemical entities (NCEs) that are substrates to cytochrome P450 (CYP) 2D6 isozyme. Topics covered in this review aim: (1) to provide a framework of the importance of CYP2D6 isozyme in the biotransformation of NCEs as stand-alones and/or in conjunction with other CYP isozymes; (2) to provide several case studies of drug disposition of important drug substrates, (3) to cover key analytical perspectives and key assay considerations to assess the role and involvement of CYP2D6, and (4) to elaborate some important considerations from the development point of view. Additionally, wherever applicable, special emphasis is provided on chiral drug substrates in the various subsections of the review.

Cytochrome P450 mRNA expression in peripheral blood lymphocytes as a predictor of enzyme induction

OBJECTIVE

Previous reports have supported the concept that messenger ribonucleic acid (mRNA) concentrations for cytochrome P450 (CYP) enzymes in peripheral blood mononuclear cells may be predictive of systemic enzyme activity. We investigated whether changes in mRNA expression for CYP1A2,CYP2C19, CYP2D6 and CYP3A4 in peripheral blood lymphocytes (PBLs) may serve as surrogate markers for changes in CYP enzyme activity following the administration of rifampin.

METHODS

On day 1 and day 9 of the study, 12 healthy volunteers were administered caffeine 100 mg, debrisoquine 10 mg and omeprazole 40 mg orally, along with midazolam 0.025 mg/kg intravenously. Blood samples and urine were collected for 8 h after drug administration. The subjects took rifampin 300 mg (n = 6) or 600 mg (n = 6) daily on days 2-8. Total RNA was isolated from PBLs on day 1 and day 9, and mRNA expression for the CYP enzymes and hGAPDH were determined by means of quantitative, real-time, reverse-transcriptase polymerase chain reaction. CYP1A2 activity was estimated by calculating the plasma paraxanthine to caffeine AUC ratio (caffeine metabolic ratio; CMR), CYP2C19 activity by the 2-h omeprazole hydroxylation index (HI), CYP2D6 activity by the urinary debrisoquine recovery ratio (DBRR) and CYP3A4 activity by midazolam clearance.

RESULTS

Median midazolam clearance (0.362 to 0.740 l/kg/h), omeprazole HI (0.752 to 0.214), CMR (0.365 to 0.450) and DBRR (0.406 to 0.479) all changed significantly following rifampin, consistent with the expected enzyme induction. CYP1A2,CYP2D6 and CYP3A4 mRNA content were measurable in all samples. CYP2C19 mRNA was inconsistently detectable. There were no significant correlations between changes in enzyme activity and mRNA expression by Spearman's rank order correlation.

CONCLUSION

The results do not support the use of mRNA expression assays for CYP1A2, CYP2C19, CYP2D6 and CYP3A4 enzymes in PBLs as surrogates for quantifying changes in systemic enzyme activity in the setting of enzyme induction.

In vivo effects of goldenseal, kava kava, black cohosh, and valerian on human cytochrome P450 1A2, 2D6, 2E1, and 3A4/5 phenotypes

OBJECTIVES

Phytochemical-mediated modulation of cytochrome P450 (CYP) activity may underlie many herb-drug interactions. Single-time point phenotypic metabolic ratios were used to determine whether long-term supplementation of goldenseal ( Hydrastis canadensis ), black cohosh ( Cimicifuga racemosa ), kava kava ( Piper methysticum ), or valerian ( Valeriana officinalis ) extracts affected CYP1A2, CYP2D6, CYP2E1, or CYP3A4/5 activity.

METHODS

Twelve healthy volunteers (6 women) were randomly assigned to receive goldenseal, black cohosh, kava kava, or valerian for 28 days. For each subject, a 30-day washout period was interposed between each supplementation phase. Probe drug cocktails of midazolam and caffeine, followed 24 hours later by chlorzoxazone and debrisoquin (INN, debrisoquine), were administered before (baseline) and at the end of supplementation. Presupplementation and postsupplementation phenotypic trait measurements were determined for CYP3A4/5, CYP1A2, CYP2E1, and CYP2D6 by use of 1-hydroxymidazolam/midazolam serum ratios (1-hour sample), paraxanthine/caffeine serum ratios (6-hour sample), 6-hydroxychlorzoxazone/chlorzoxazone serum ratios (2-hour sample), and debrisoquin urinary recovery ratios (8-hour collection), respectively. The content of purported "active" phytochemicals was determined for each supplement.

RESULTS

Comparisons of presupplementation and postsupplementation phenotypic ratio means revealed significant inhibition (approximately 40%) of CYP2D6 (difference, -0.228; 95% confidence interval [CI], -0.268 to -0.188) and CYP3A4/5 (difference, -1.501; 95% CI, -1.840 to -1.163) activity for goldenseal. Kava produced significant reductions (approximately 40%) in CYP2E1 only (difference, -0.192; 95% CI, -0.325 to -0.060). Black cohosh also exhibited statistically significant inhibition of CYP2D6 (difference, -0.046; 95% CI, -0.085 to -0.007), but the magnitude of the effect (approximately 7%) did not appear to be clinically relevant. No significant changes in phenotypic ratios were observed for valerian.

CONCLUSIONS

Botanical supplements containing goldenseal strongly inhibited CYP2D6 and CYP3A4/5 activity in vivo, whereas kava inhibited CYP2E1 and black cohosh weakly inhibited CYP2D6. Accordingly, serious adverse interactions may result from the concomitant ingestion of goldenseal supplements and drugs that are CYP2D6 and CYP3A4/5 substrates. Kava kava and black cohosh may interact with CYP2E1 and CYP2D6 substrates, respectively. Valerian appears to be less likely to produce CYP-mediated herb-drug interactions.

Effect of chronic disulfiram administration on the activities of CYP1A2, CYP2C19, CYP2D6, CYP2E1, and N-acetyltransferase in healthy human subjects

AIMS

Short-term disulfiram administration has been shown to selectively inhibit CYP2E1 activity but the effects of chronic disulfiram administration on the activities of drug metabolizing enzymes is unclear. The purpose of this study was to evaluate the effects of disulfiram given for 11 days on selected drug metabolizing enzyme activities.

METHODS

Seven healthy volunteers were given disulfiram 250 mg daily for 11 days. Activities of the drug metabolizing enzymes CYP1A2, CYP2C19, CYP2D6, CYP2E1 and N-acetyltransferase were determined using the probe drugs caffeine, mephenytoin, debrisoquine, chlorzoxazone, and dapsone, respectively. Chlorzoxazone was administered before disulfiram administration and after the second and eleventh doses of disulfiram, while the other probe drugs were given before disulfiram administration and after the eleventh disulfiram dose.

RESULTS

Disulfiram administration markedly inhibited chlorzoxazone 6-hydroxylation by more than 95%, but did not affect metabolism of debrisoquine or mephenytoin. Caffeine N3-demethylation was decreased by 34% (P < 0.05). Monoacetyldapsone concentrations were markedly elevated by disulfiram administration resulting in a nearly 16-fold increase in the dapsone acetylation index, calculated as the plasma concentration ratio of monoacetyldapsone to dapsone. CYP-mediated dapsone N-hydroxylation was not significantly altered.

CONCLUSIONS

These data suggest that disulfiram-mediated inhibition is predominantly selective for CYP2E1. The magnitude of CYP2E1 inhibition was similar after both acute and chronic disulfiram administration. The effects on caffeine N3-demethylation (CYP1A2) and dapsone metabolism suggest that chronic disulfiram administration may affect multiple drug metabolizing enzymes, which could potentially complicate the use of chronically administered disulfiram as a diagnostic inhibitor of CYP2E1.

Enantioselectivity of debrisoquine 4-hydroxylation in Brazilian Caucasian hypertensive patients phenotyped as extensive metabolizers

Debrisoquine (D), an antihypertensive drug metabolized to 4-hydroxydebrisoquine (4-OHD) by CYP2D6, is commonly used as an in vivo probe of CYP2D6 activity and can be used to phenotype individuals as either extensive (EMs) or poor metabolizers (PMs) of such drugs as beta-adrenergic blockers, tricyclic antidepressants, and class 1C antiarrhythmics. This report describes reversed-phase HPLC systems by which D and 4-OHD or S-(+) and R-(-)-4-OHD in urine are more selectively quantified without the need for derivatization techniques. We also studied the urinary excretion of R-(-)- and S-(+)-4-hydroxydebrisoquine in EM hypertensive patients in order to determine weather 4-OHD formation exhibits enantioselectivity. Twelve patients with mild to severe essential hypertension were admitted to the study. They received a single tablet of Declinax containing 10 mg debrisoquine sulfate. All the urine excreted during the following 8 h was collected. The debrisoquine metabolic ratio (DMR) was calculated as % of dose excreted as D/% of dose excreted as 4-OHD and the debrisoquine recovery ratio (DRR) was calculated as % of dose excreted as 4-OHD/% of dose excreted as D+4-OHD. Debrisoquine and its metabolite were determined in urine by HPLC using a reversed-phase Select B LiChrospher column, a mobile phase of 0.25 N acetate buffer, pH 5-acetonitrile (9:1, v/v) and a fluorescence detector. The limit of quantitation was determined to be 25.0 ng/ml for D and 18.75 ng/ml for 4-OHD. Intra- and inter-day relative standard deviations (RSDs) were less than 10%. All hypertensive patients studied showed a DMR of less than 12.6 or a DRR higher than 0.12 and were classified as EMs. Direct enantioselective separation on chiral stationary phase involved resolution of S-(+)-4-OHD and R-(-)-4-OHD on a Chiralcel OD-R column with a mobile phase of 0.125 N sodium perchlorate, pH 5-acetonitrile-methanol (85:12:3, v/v/v). The quantitation limit of each enantiomer was 3.75 ng/ml of urine. Intra- and inter-day RSDs were less than 10% for each enantiomer. A high degree of enantioselectivity in the 4-hydroxylation of D favouring the S-(+) enantiomer was observed, resulting in R-(-)-4-OHD not detected in the urine of the EM hypertensive patients studied.

A micromethod for quantitation of debrisoquine and 4-hydroxydebrisoquine in urine by liquid chromatography

We describe a new simple, selective and sensitive micromethod based on HPLC and fluorescence detection to measure debrisoquine (D) and 4-hydroxydebrisoquine (4-OHD) in urine for the investigation of xenobiotic metabolism by debrisoquine hydroxylase (CYP2D6). Four hundred microl of urine was required for the analysis of D and 4-OHD. Peaks were eluted at 8.3 min (4-OHD), 14.0 min (D) and 16.6 min for the internal standard, metoprolol (20 microg/ml). The 5-microm CN-reverse-phase column (Shimpack, 250 x 4.6 mm) was eluted with a mobile phase consisting of 0.25 M acetate buffer, pH 5.0, and acetonitrile (9:1, v/v) at 0.7 ml/min with detection at lambdaexcitation = 210 nm and lambdaemission = 290 nm. The method, validated on the basis of measurements of spiked urine, presented 3 ng/ml (D) and 6 ng/ml (4-OHD) sensitivity, 390-6240 ng/ml (D) and 750-12000 ng/ml (4-OHD) linearity, and 5.7/8.2% (D) and 5.3/8.2% (4-OHD) intra/interassay precision. The method was validated using urine of a healthy Caucasian volunteer who received one 10-mg tablet of Declinax(R), po, in the morning after an overnight fast. Urine samples (diuresis of 4 or 6 h) were collected from zero to 24 h. The urinary excretion of D and 4-OHD, Fel (0-24 h), i.e., fraction of dose administered and excreted into urine, was 6.4% and 31.9%, respectively. The hydroxylation capacity index reported as metabolic ratio was 0.18 (D/4-OHD) for the person investigated and can be compared to reference limits of >12.5 for poor metabolizers (PM) and <12.5 for extensive metabolizers (EM). In parallel, the recovery ratio (RR), another hydroxylation capacity index, was 0.85 (4-OHD: SigmaD + 4-OHD) versus reference limits of RR <0.12 for PM and RR >0. 12 for EM. The healthy volunteer was considered to be an extensive metabolizer on the basis of the debrisoquine test.

Determination of a 'GW cocktail' of cytochrome P450 probe substrates and their metabolites in plasma and urine using automated solid phase extraction and fast gradient liquid chromatography tandem mass spectrometry

A mass spectrometry based method for the simultaneous determination of an in vivo Greenford-Ware or 'GW cocktail' of CYP450 probe substrates and their metabolites in both human plasma and urine is described. The probe substrates, caffeine, diclofenac, mephenytoin, debrisoquine, chlorzoxazone and midazolam, together with their respective metabolites and stable isotope labelled internal standards, are simultaneously extracted from the biological matrix using solid phase extraction in 96-well microtitre plate format, automated by means of a custom built Zymark robotic system. The extracts are analysed by fast gradient high performance liquid chromatography (HPLC) with detection by tandem mass spectrometry (MS/MS) using thermally and pneumatically assisted electrospray ionisation in both positive and negative ion modes and selected reaction monitoring. The methods are specific, accurate and precise with intra- and inter-assay precision (%CV) of less than 15% for all analytes.

Debilitating reaction following the initial dose of tramadol

OBJECTIVE

To describe a debilitating reaction following a single oral dose of tramadol.

CASE SUMMARY

A 32-year-old white man with no prior medical problems, allergies, or previous medication reactions experienced ataxia, dilation of the pupils, numbness in his arms and legs, tremulousness, and dysphoria lasting approximately 4 hours following an initial tramadol dose (100 mg). The patient recovered with no sequelae.

DISCUSSION

Central nervous system (CNS) stimulation during therapy with tramadol was reported in 7% of patients in clinical trials. These reactions are usually mild and transient. This report describes a debilitating CNS-mediated reaction to an initial dose of tramadol in an otherwise healthy adult. The patient was phenotyped for CYP2D6 activity, the major metabolic pathway for tramadol elimination, and was determined to be an extensive metabolizer with very high CYP2D6 activity.

CONCLUSIONS

The exact mechanism of the adverse response is not known; however, based on phenotyping results, we suspect that it may be related to high concentrations of the active O-desmethyl metabolite of tramadol.

Validation of the five-drug "Pittsburgh cocktail" approach for assessment of selective regulation of drug-metabolizing enzymes

OBJECTIVES

To determine whether the probe drugs caffeine, chlorzoxazone, dapsone, debrisoquin (INN, debrisoquine), and mephenytoin can be simultaneously administered as a metabolic cocktail to estimate in vivo cytochrome P450 (CYP) and N-acetyltransferase enzyme activities.

METHODS

Fourteen healthy nonsmoking male volunteers (mean age +/- SD, 21.6 +/- 2.2 years) received 100 mg caffeine, 250 mg chlorzoxazone, 100 mg dapsone, 10 mg debrisoquin, and 100 mg mephenytoin individually and in four and five-drug combinations in a randomized manner using a 7 x 7 Latin square. Each drug or drug combination was given orally after an overnight fast, with a minimum 1-week washout between administrations. In each session, urine was collected from 0 to 8 hours and plasma was obtained at 4 and 8 hours after drug administration. Plasma and metabolite concentrations were used to estimate phenotypic trait measures for the efficiency of each drug's metabolism.

RESULTS

The phenotypic indexes determined for caffeine, chlorzoxazone, dapsone, debrisoquin, and mephenytoin were not significantly different when given alone than when given in combination. The median percentage change of the trait measures observed during administration of all five compounds compared with individual administration ranged from -10.7% for the 6-hydroxychlorzoxazone to chlorzoxazone plasma ratio to +2.2% for the debrisoquin recovery ratio.

CONCLUSIONS

The results of this study show that caffeine, chlorzoxazone, dapsone, debrisoquin, and mephenytoin in low doses can be simultaneously administered without metabolic interaction. This cocktail approach can thus simultaneously provide independent in vivo phenotypic measures for multiple CYP enzymes and N-acetyltransferase.