The influence of enzyme induction on polymorphic sparteine oxidation

Exploring the Use of Serum-Derived Small Extracellular Vesicles as Liquid Biopsy to Study the Induction of Hepatic Cytochromes P450 and Organic Anion Transporting Polypeptides

Liver-derived small extracellular vesicles (sEVs), prepared from small sets of banked serum samples using a novel two-step protocol, were deployed as liquid biopsy to study the induction of cytochromes P450 (CYP3A4, CYP3A5, and CYP2D6) and organic anion transporting polypeptides (OATP1B1 and OATP1B3) during pregnancy (nonpregnant (T0), first, second, and third (T3) trimester women; N = 3 each) and after administration of rifampicin (RIF) to healthy male subjects. Proteomic analysis revealed induction (mean fold-increase, 90% confidence interval) of sEV CYP3A4 after RIF 300 mg × 7 days (3.5, 95% CI = 2.5-4.5, N = 4, P = 0.029) and 600 mg × 14 days (3.7, 95% CI = 2.1-6.0, N = 5, P = 0.018) consistent with the mean oral midazolam area under the plasma concentration time curve (AUC) ratio in the same subjects (0.28, 95% CI = 0.22-0.34, P < 0.0001; and 0.17, 95% CI = 0.13-0.22, P < 0.0001). Compared with CYP3A4, liver sEV CYP3A5 protein (subjects genotyped CYP3A5*1/*3) was weakly induced (≤ 1.5-fold). It was also possible to measure liver sEV-catalyzed dextromethorphan (DEX) O-demethylation to dextrorphan (DXO), correlated with sEV CYP2D6 expression (r = 0.917, P = 0.0001; N = 10) and 3-hour plasma DXO-to-DEX concentration ratio (r = 0.843, P = 0.002, N = 10), and show that CYP2D6 was not induced by RIF. Nonparametric analysis of liver sEV revealed significantly higher CYP3A4 (3.2-fold, P = 0.003) and CYP2D6 (3.7-fold, P = 0.03) protein expression in T3 vs. T0 women. In contrast, expression of both OATPs in liver sEV was unaltered by RIF administration and pregnancy.

Distinct Effects of Inflammation on Cytochrome P450 Regulation and Drug Metabolism: Lessons from Experimental Models and a Potential Role for Pharmacogenetics

Personalized medicine strives to optimize drug treatment for the individual patient by taking into account both genetic and non-genetic factors for drug response. Inflammation is one of the non-genetic factors that has been shown to greatly affect the metabolism of drugs—primarily through inhibition of cytochrome P450 (CYP450) drug-metabolizing enzymes—and hence contribute to the mismatch between the genotype predicted drug response and the actual phenotype, a phenomenon called phenoconversion. This review focuses on inflammation-induced drug metabolism alterations. In particular, we discuss the evidence assembled through human in-vitro models on the effect of inflammatory mediators on clinically relevant CYP450 isoform levels and their metabolizing capacity. We also present an overview of the current understanding of the mechanistic pathways via which inflammation in hepatocytes may modulate hepatic functions that are critical for drug metabolism. Furthermore, since large inter-individual variability in response to inflammation is observed in human in-vitro models and clinical studies, we evaluate the potential role of pharmacogenetic variability in the inflammatory signaling cascade and how this can modulate the outcome of inflammation on drug metabolism and response.

From Endogenous Compounds as Biomarkers to Plasma-Derived Nanovesicles as Liquid Biopsy; Has the Golden Age of Translational Pharmacokinetics-Absorption, Distribution, Metabolism, Excretion-Drug-Drug Interaction Science Finally Arrived?

It is now established that a drug's pharmacokinetics (PK) absorption, distribution, metabolism, excretion (ADME) and drug-drug interaction (DDI) profile can be modulated by age, disease, and genotype. In order to facilitate subject phenotyping and clinical DDI assessment, therefore, various endogenous compounds (in plasma and urine) have been pursued as drug-metabolizing enzyme and transporter biomarkers. Compared with biomarkers, however, the topic of circulating extracellular vesicles as "liquid biopsy" has received little attention within the ADME community; most organs secrete nanovesicles (e.g., exosomes) into the blood that contain luminal "cargo" derived from the originating organ (proteins, messenger RNA, and microRNA). As such, ADME profiling of plasma exosomes could be leveraged to better define genotype-phenotype relationships and the study of ontogeny, disease, and complex DDIs. If methods to support the isolation of tissue-derived plasma exosomes are successfully developed and validated, it is envisioned that they will be used jointly with genotyping, biomarkers, and modeling tools to greatly progress translational PK-ADME-DDI science.

Cytochrome P450 Enzymes Involved in Metoprolol Metabolism and Use of Metoprolol as a CYP2D6 Phenotyping Probe Drug

Metoprolol is used for phenotyping of cytochrome P450 (CYP) 2D6, a CYP isoform considered not to be inducible by inducers of the CYP2C, CYP2B, and CYP3A families such as rifampicin. While assessing CYP2D6 activity under basal conditions and after pre-treatment with rifampicin in vivo, we surprisingly observed a drop in the metoprolol/α-OH-metoprolol clearance ratio, suggesting CYP2D6 induction. To study this problem, we performed in vitro investigations using HepaRG cells and primary human hepatocytes (before and after treatment with 20 μM rifampicin), human liver microsomes, and CYP3A4-overexpressing supersomes. While mRNA expression levels of CYP3A4 showed a 15- to 30-fold increase in both cell models, mRNA of CYP2D6 was not affected by rifampicin. 1'-OH-midazolam formation (reflecting CYP3A4 activity) increased by a factor of 5-8 in both cell models, while the formation of α-OH-metoprolol increased by a factor of 6 in HepaRG cells and of 1.4 in primary human hepatocytes. Inhibition studies using human liver microsomes showed that CYP3A4, 2B6, and 2C9 together contributed 19.0 ± 2.6% (mean ± 95%CI) to O-demethylation, 4.0 ± 0.7% to α-hydroxylation, and 7.6 ± 1.7% to N-dealkylation of metoprolol. In supersomes overexpressing CYP3A4, metoprolol was α-hydroxylated in a reaction inhibited by the CYP3A4-specific inhibitor ketoconazole, but not by the CYP2D6-specific inhibitor quinidine. We conclude that metoprolol is not exclusively metabolized by CYP2D6. CYP3A4, 2B6, and 2C9, which are inducible by rifampicin, contribute to α-hydroxylation, O-demethylation, and N-dealkylation of metoprolol. This contribution is larger after CYP induction by rifampicin but is too small to compromise the usability of metoprolol α-hydroxylation for CYP2D6 phenotyping.

Effect of fermented red ginseng on cytochrome P450 and P-glycoprotein activity in healthy subjects, as evaluated using the cocktail approach

Aims

We assessed the drug interaction profile of fermented red ginseng with respect to the activity of major cytochrome (CYP) P450 enzymes and of a drug transporter protein, P‐glycoprotein (P‐gp), in healthy volunteers.

Methods

This study was an open‐label crossover study. The CYP probe cocktail drugs caffeine, losartan, dextromethorphan, omeprazole, midazolam and fexofenadine were administered before and after 2 weeks of fermented red ginseng administration. Plasma samples were collected, and tolerability was assessed. Pharmacokinetic parameters were calculated, and the 90% confidence intervals (CIs) of the geometric mean ratios of the parameters were determined from logarithmically transformed data. Values were compared between before and after fermented red ginseng administration using analysis of variance (anova).

Results

Fifteen healthy male subjects were evaluated, none of whom were genetically defined as a poor CYP2C9, CYP2C19 or CYP2D6 metabolizer based on genotyping. Before and after fermented red ginseng administration, the geometric least‐square mean metabolic ratio (90% CI) was 0.901 (0.830–0.979) for caffeine (CYP1A2) to paraxanthine, 0.774 (0.720–0.831) for losartan (CYP2C9) to EXP3174, 1.052 (0.925–1.197) for omeprazole (CYP2C19) to 5‐hydroxyomeprazole, 1.150 (0.860–1.538) for dextromethorphan (CYP2D6) to dextrorphan, and 0.816 (0.673–0.990) for midazolam (CYP3A4) to 1‐hydroxymidazolam. The geometric mean ratio of the area under the curve of the last sampling time (AUC_last) for fexofenadine (P‐gp) was 1.322 (1.112–1.571).

Conclusion

No significantly different drug interactions were observed between fermented red ginseng and the CYP probe substrates following the two‐week administration of concentrated fermented red ginseng. However, the inhibition of P‐gp was significantly different between fermented red ginseng and the CYP probe substrates. The use of fermented red ginseng requires close attention due to the potential for increased systemic exposure when it is used in combination with P‐gp substrate drugs.

CYP2D6 Is Inducible by Endogenous and Exogenous Corticosteroids

Although cytochrome P450 (CYP) 2D6 has been widely considered to be noninducible on the basis of human hepatocyte studies, in vivo data suggests that it is inducible by endo- and xenobiotics. Therefore, we investigated if the experimental conditions routinely used in human hepatocyte studies may be a confounding factor in the lack of in vitro induction of CYP2D6. Sandwich cultured human hepatocytes (SCHH) were preincubated with or without dexamethasone (100 nM) for 72 hours before incubation with 1μM endogenous (cortisol or corticosterone) or exogenous (dexamethasone or prednisolone) corticosteroids. At 72 hours, CYP2D6 mRNA, protein, and activity were quantified by real-time quantitative polymerase chain reaction, quantitative proteomics, and formation of dextrorphan from dextromethorphan, respectively. In the absence of supplemental dexamethasone, CYP2D6 activity, mRNA, and protein were significantly and robustly (>10-fold) induced by all four corticosteroids. However, this CYP2D6 induction was abolished in cells preincubated with supplemental dexamethasone. These data show, for the first time, that CYP2D6 is inducible in vitro but the routine presence of 100 nM dexamethasone in the culture medium masks this induction. Our cortisol data are in agreement with the clinical observation that CYP2D6 is inducible during the third trimester of pregnancy when the plasma concentrations of cortisol increase to ∼1μM. These findings, if confirmed in vivo, have implications for predicting CYP2D6-mediated drug-drug interactions and call for re-evaluation of regulatory guidelines on screening for CYP2D6 induction by xenobiotics. Our findings also suggest that cortisol may be a causative factor in the in vivo induction of CYP2D6 during pregnancy.

Impact of physiological, pathological and environmental factors on the expression and activity of human cytochrome P450 2D6 and implications in precision medicine

With only 1.3-4.3% in total hepatic CYP content, human CYP2D6 can metabolize more than 160 drugs. It is a highly polymorphic enzyme and subject to marked inhibition by a number of drugs, causing a large interindividual variability in drug clearance and drug response and drug-drug interactions. The expression and activity of CYP2D6 are regulated by a number of physiological, pathological and environmental factors at transcriptional, post-transcriptional, translational and epigenetic levels. DNA hypermethylation and histone modifications can repress the expression of CYP2D6. Hepatocyte nuclear factor-4α binds to a directly repeated element in the promoter of CYP2D6 and thus regulates the expression of CYP2D6. Small heterodimer partner represses hepatocyte nuclear factor-4α-mediated transactivation of CYP2D6. GW4064, a farnesoid X receptor agonist, decreases hepatic CYP2D6 expression and activity while increasing small heterodimer partner expression and its recruitment to the CYP2D6 promoter. The genotypes are key determinants of interindividual variability in CYP2D6 expression and activity. Recent genome-wide association studies have identified a large number of genes that can regulate CYP2D6. Pregnancy induces CYP2D6 via unknown mechanisms. Renal or liver diseases, smoking and alcohol use have minor to moderate effects only on CYP2D6 activity. Unlike CYP1 and 3 and other CYP2 members, CYP2D6 is resistant to typical inducers such as rifampin, phenobarbital and dexamethasone. Post-translational modifications such as phosphorylation of CYP2D6 Ser135 have been observed, but the functional impact is unknown. Further functional and validation studies are needed to clarify the role of nuclear receptors, epigenetic factors and other factors in the regulation of CYP2D6.

Effect of Ginkgo biloba special extract EGb 761® on human cytochrome P450 activity: a cocktail interaction study in healthy volunteers

PURPOSE

We assessed the human in vivo metabolic drug interaction profile of Ginkgo biloba extract EGb 761® with respect to the activities of major cytochrome P450 (CYP) enzymes.

METHODS

A single-center, open-label, randomized, three-fold crossover, cocktail phenotyping design was applied. In random order, the following treatments were administered to 18 healthy men and women for 8 days each: placebo twice daily, EGb 761® 120 mg twice daily, and EGb 761® 240 mg in the morning and placebo in the evening. In the morning of day 8, administration was performed together with the orally administered phenotyping cocktail (enzyme, metric): 150 mg caffeine (CYP1A2, paraxanthine/caffeine plasma ratio 6-h postdose), 125 mg tolbutamide (CYP2C9, plasma concentration 24-h postdose), 20 mg omeprazole (CYP2C19, omeprazole/5-hydroxy omeprazole plasma ratio 3-h postdose), 30 mg dextromethorphan (CYP2D6, dextromethorphan/dextrorphan plasma ratio 3-h postdose), and 2 mg of midazolam (CYP3A, plasma concentration 6-h postdose). Formally, absence of a relevant interaction was assumed if the 90% confidence intervals (CIs) for EGb 761®/placebo ratios of the metrics were within the 0.70-1.43 range.

RESULTS

EGb 761®/placebo ratios for phenotyping metrics were close to unity for all CYPs. Furthermore, respective CIs were within the specified margins for all ratios except CYP2C19 for EGb 761® 120 mg twice daily (90% CI 0.681-1.122) and for CYP2D6 for EGb 761® 240 mg once daily (90% CI 0.667-1.281). These findings were attributed to the intraindividual variability of the metrics used. All treatments were well tolerated.

CONCLUSION

EGb 761® has no relevant effect on the in vivo activity of the major CYP enzymes in humans and therefore has no relevant potential to cause respective metabolic drug-drug interactions.

The risk of recurrence in breast cancer patients treated with tamoxifen: polymorphisms of CYP2D6 and ABCB1

CYP2D6 plays a major role in the metabolism of tamoxifen, and polymorphism of P-glycoprotein has been associated with resistance of many drug therapies. This study investigates the clinical impact of genetic variants of CYP2D6 and ABCB1 in breast cancer patients treated with tamoxifen. Blood samples from 95 breast cancer patients treated with tamoxifen were collected and genotyped for CYP2D6 and ABCB1 variants using allele-specific PCR method. Recurrence risks were calculated using Kaplan-Meier analysis and compared using the log-rank test. Patients carrying CYP2D6*10/*10 and heterozygous null allele (IM) showed higher risks of developing recurrence and metastasis (OR 13.14; 95% CI 1.57-109.94; P = 0.004) than patients with CYP2D6*1/*1 and *1/*10 genotypes. Patients with homozygous CC genotypes of ABCB1 C3435T showed a shorter time to recurrence. Patients who were CYP2D6 IM and homozygous CC genotype of C3435T have statistically significant higher risks of recurrence (P = 0.002). Similarly, median time to recurrence in these patients was only 12 months (95% CI = 0.79-23.2) compared to those without this combination which was 48 months (95% CI = 14.7-81.2). Patients with CYP2D6 IM and homozygous CC genotype of ABCB1 C3435T have shorter times to recurrence. The results confirmed the findings of previous studies and support FDA recommendation to perform pre-genotyping in patients before the choice of therapy is determined in breast cancer patients.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Genetic polymorphism of hepatocyte nuclear factor-4alpha influences human cytochrome P450 2D6 activity

Hepatocyte nuclear factor-4 alpha (HNF4A) is an essential transcriptional regulator for many genes that are expressed preferentially in the liver. Among the important functions of the liver is drug metabolism in response to xenobiotic exposure. Recent studies have suggested that HNF4A regulates the expression of cytochrome P450 (CYP), including CYP2D6 and CYP3A4, which show large individual variations in their activities. To understand the genetic factors that influence individual CYP activities, a genetic variant of HNF4A and the effects of genetic variants of HNF4A on CYP activity were investigated. Here, we report the identification of a novel coding variant of HNF4A that influences CYP2D6 activity in humans. After direct sequencing, a polymorphism search revealed the HNF4A G60D variant in Koreans. This variant was unable to bind to the recognition site in the CYP2D6 promoter and therefore lacked the regulatory function for this gene. Human liver specimens with the heterozygous HNF4A G60D genotype showed a tendency toward lower levels of CYP2D6 activity than the wild-type genotype in the same genetic background of CYP2D6. Furthermore, human subjects with the HNF4A G60D genotype tended to have lower CYP2D6 activity than those with the wild-type HNF4A. The HNF4A G60D variant was detected at low frequency in Asian populations, including Koreans, Chinese, and Vietnamese, and was not found in Africans or Caucasians.

CONCLUSION

This is the first report to show that the genetic polymorphism of liver-enriched nuclear receptor HNF4A influences downstream CYP2D6 function in human subjects.

Interaction study of the combined use of paroxetine and fosamprenavir-ritonavir in healthy subjects

Human immunodeficiency virus-infected patients have an increased risk for depression. Despite the high potential for drug-drug interactions, limited data on the combined use of antidepressants and antiretrovirals are available. Theoretically, ritonavir-boosted protease inhibitors may inhibit CYP2D6-mediated metabolism of paroxetine. We wanted to determine the effect of fosamprenavir-ritonavir on paroxetine pharmacokinetics and vice versa and to evaluate the safety of the combination. Group A started with 20 mg paroxetine every day for 10 days; after a wash-out period of 16 days, subjects received paroxetine (20 mg every day) plus fosamprenavir-ritonavir (700/100 mg twice a day) from days 28 to 37. Group B received the regimens in reverse order. On days 10 and 37, pharmacokinetic curves were recorded. Twenty-six healthy subjects (18 females, 8 males) were included. Median (range) age and weight were 44.4 (18.2 to 64.3) years and 68.8 (51.0 to 89.4) kg. Three subjects were excluded (two because of adverse events; one for nonadherence). Addition of fosamprenavir-ritonavir to paroxetine resulted in a significant decrease in paroxetine exposure: the geometric mean ratios (90% confidence intervals) of paroxetine plus fosamprenavir-ritonavir to paroxetine alone were 0.45 (0.41 to 0.49) for the area under the concentration-time curve from 0 to 24 h (AUC(0-24)), 0.49 (0.45 to 0.53) for the maximum concentration of the drug in plasma (C(max)), and 0.75 (0.71 to 0.80) for the apparent elimination half-life (t(1/2)). The free fraction of paroxetine showed a median (interquartile range) increase of 30% (18 to 42%) after the addition of fosamprenavir-ritonavir. The AUC(0-12), C(max), C(min), and t(1/2) of amprenavir and ritonavir were similar to those of historical controls. No serious adverse events occurred. Fosamprenavir-ritonavir reduced total paroxetine exposure by 55%. This is partly explained by protein displacement of paroxetine. We think that this interaction is clinically relevant and that titration to a higher dose of paroxetine may be necessary to accomplish the needed antidepressant effect.

Selective Induction of Human Hepatic Cytochromes P450 2B6 and 3A4 by Metamizole

The pyrazolone drug metamizole is a widely used analgesic. Analysis of liver microsomes from patients treated with metamizole revealed selectively higher expression of cytochromes P450, CYP2B6 and CYP3A4 (3.8- and 2.8-fold, respectively), and 2.9-fold higher bupropion hydroxylase activity compared with untreated subjects. Further investigation of metamizole and various derivatives on different potential target genes in human primary hepatocytes demonstrated time- and concentration-dependent induction by metamizole of CYP2B6 (7.8- and 3.1-fold for mRNA and protein, respectively, at 100 muM) and CYP3A4 (2.4- and 2.9-fold, respectively), whereas other genes (CYP2C9, CYP2C19, CYP2D6, NADPH:cytochrome P450 reductase, ABCB1, constitutive androstane receptor (CAR), pregnane X receptor (PXR)) were not substantially altered. Using reporter gene assays, we show that metamizole is not acting as a direct ligand to either PXR or CAR, suggesting a phenobarbital-like mechanism of induction. These data warrant further studies to elucidate the drug-interaction potential of metamizole, especially in patients with long-term treatment.

Evaluation of probe drugs and pharmacokinetic metrics for CYP2D6 phenotyping

INTRODUCTION

Cytochrome P450 2D6 (CYP2D6) is one of the most important enzymes catalyzing biotransformation of xenobiotics in the human liver. This enzyme's activity shows a high degree of interindividual variability caused in part by its genetic polymorphism, the so-called debrisoquine/sparteine polymorphism. The genetic component influencing CYP2D6 activity can be determined by genotyping. However, genotyping alone is not sufficient to accurately predict an individual's actual CYP2D6 activity, as this is also influenced by other factors. For the determination of the exact actual enzymatic activity ("phenotyping"), adequate probe drugs have to be administered prior to measurements of these compounds and/or their metabolites in body fluids. PROBE DRUGS: Debrisoquine, sparteine, metoprolol or dextromethorphan represent well-established probe drugs while tramadol has been recently investigated for this purpose. The enzymatic activity is reflected by various pharmacokinetic metrics such as the partial clearance of a parent compound to the respective CYP2D6-mediated metabolite or metabolic ratios. Appropriate metrics need to fulfill pre-defined validation criteria.

METHODS

In this review, we have compiled a list of such criteria useful to select the best metrics to reflect CYP2D6 activity. A comprehensive Medline search for reports on CYP2D6 phenotyping trials with the above mentioned probe drugs was carried out.

CONCLUSION

Application of the validation criteria suggests that dextromethorphan and debrisoquine are the best CYP2D6 phenotyping drugs, with debrisoquine having the problem of very limited availability as a therapeutic drug. However, the assessment of the best dextromethorphan CYP2D6 phenotyping metric/procedure is still ongoing.

Pharmacogenetics in drug regulation: promise, potential and pitfalls

Pharmacogenetic factors operate at pharmacokinetic as well as pharmacodynamic levels-the two components of the dose-response curve of a drug. Polymorphisms in drug metabolizing enzymes, transporters and/or pharmacological targets of drugs may profoundly influence the dose-response relationship between individuals. For some drugs, although retrospective data from case studies suggests that these polymorphisms are frequently associated with adverse drug reactions or failure of efficacy, the clinical utility of such data remains unproven. There is, therefore, an urgent need for prospective data to determine whether pre-treatment genotyping can improve therapy. Various regulatory guidelines already recommend exploration of the role of genetic factors when investigating a drug for its pharmacokinetics, pharmacodynamics, dose-response relationship and drug interaction potential. Arising from the global heterogeneity in the frequency of variant alleles, regulatory guidelines also require the sponsors to provide additional information, usually pharmacogenetic bridging data, to determine whether data from one ethnic population can be extrapolated to another. At present, sponsors explore pharmacogenetic influences in early clinical pharmacokinetic studies but rarely do they carry the findings forward when designing dose-response studies or pivotal studies. When appropriate, regulatory authorities include genotype-specific recommendations in the prescribing information. Sometimes, this may include the need to adjust a dose in some genotypes under specific circumstances. Detailed references to pharmacogenetics in prescribing information and pharmacogenetically based prescribing in routine therapeutics will require robust prospective data from well-designed studies. With greater integration of pharmacogenetics in drug development, regulatory authorities expect to receive more detailed genetic data. This is likely to complicate the drug evaluation process as well as result in complex prescribing information. Genotype-specific dosing regimens will have to be more precise and marketing strategies more prudent. However, not all variations in drug responses are related to pharmacogenetic polymorphisms. Drug response can be modulated by a number of non-genetic factors, especially co-medications and presence of concurrent diseases. Inappropriate prescribing frequently compounds the complexity introduced by these two important non-genetic factors. Unless prescribers adhere to the prescribing information, much of the benefits of pharmacogenetics will be squandered. Discovering highly predictive genotype-phenotype associations during drug development and demonstrating their clinical validity and utility in well-designed prospective clinical trials will no doubt better define the role of pharmacogenetics in future clinical practice. In the meantime, prescribing should comply with the information provided while pharmacogenetic research is deservedly supported by all concerned but without unrealistic expectations.

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.

Influence of rifampicin on the expression and function of human intestinal cytochrome P450 enzymes

AIMS

To investigate the potential induction by rifampicin of intestinal CYP2C8, CYP2C9, CYP2D6 and CYP3A4 using preparations of human enterocytes.

METHODS

Using a multilumen perfusion catheter shed human enterocytes were collected from 6 healthy subjects before and after 10 days of 600 mg day(-1) oral rifampicin administration. The protein expression of CYP2C8, CYP2C9, CYP2D6 and CYP3A4 as well as that of CYP3A4 mRNA was determined using Western blotting and RT-PCR, respectively.

RESULTS

CYP3A4 mRNA expression in shed enterocytes increased from 74.6 +/- 44.2 to 143.2 +/- 68.4 a.u. (P < 0.05, 95% CI: 21.8-115.3). Expression of CYP2C8 and CYP2C9 increased from 5.1 +/- 0.9 to 10.4 +/- 2.3 pmol mg(-1) protein (P < 0.01, 95% CI: 2.8-7.7) and from 4.2 +/- 1.4 to 5.7 +/- 1.1 pmol mg(-1) protein (P < 0.01, 95% CI: 0.6-2.4), respectively. No significant difference in CYP2D6 expression before and during rifampicin intake was observed. Rifampicin administration also resulted in a significant induction of CYP3A4 protein (34.1 +/- 10.7 vs. 113.9 +/- 31.1 pmol mg(-1) protein (P < 0.001, 95% CI: 51.8-107.6)). Ex vivo incubation of enterocyte homogenates with verapamil resulted in a significantly increased production of the metabolites formed via CYP3A4 (D-617: 125.9 +/- 118.8 vs. 277.2 +/- 145.5 pmol min(-1) mg(-1) protein (P < 0.05, 95% CI: 30.1-272.5); norverapamil: 113.0 +/- 57.9 vs. 398.4 +/- 148.2 pmol min(-1) mg(-1) protein (P < 0.05, 95% CI: 47.2-523.6)).

CONCLUSION

Our findings indicate that shed enterocytes are a useful tool to study the expression, regulation and function of drug metabolizing enzymes. Induction of intestinal CYP2C8 and CYP2C9 might contribute in part to rifampicin - mediated drug interactions, in addition to their hepatic counterparts and intestinal and hepatic CYP3A4.

Inhibitory Effect of Antituberculosis Drugs on Human Cytochrome P450-Mediated Activities

The potential for drug-drug interactions mediated by the inhibition of cytochrome P-450 (CYP) were concerned during antituberculosis therapy. However, the information regarding human CYP inhibition by antituberculosis drugs is limited to isoniazid. In the current study, we examined the inhibitory effects of pyrazinamide and ethionamide, both of which are chemically related to isoniazid, on the CYP-mediated activities in human liver microsomes and compared them to that of isoniazid. No remarkable effects on any CYP activities were observed by pyrazinamide and ethionamide. In contrast, in addition to the reported inhibitory effect of isoniazid on CYP1A2, CYP2A6, CYP2C19, and CYP3A activities, our results newly showed its effect on CYP2C9 and CYP2E1 activities. Isoniazid showed potent direct inhibitory effect on S-warfarin 7-hydroxylation, while a preincubation step in the presence of NADPH was needed to inhibit chlorzoxazone 6-hydroxylation. Furthermore, irreversible inhibition of CYP2C19 activity by isoniazid was also observed in the dilution study. These results suggested that pyrazinamide and ethionamide did not seem to cause drug interactions mediated by the inhibition of CYP. In contrast, isoniazid might contribute to the severe drug interactions by a different inhibitory mechanism depending on each of the CYP isozymes, in addition to the reported observations.

CYP2D6 genotypes and depressive symptoms during late pregnancy and postpartum

The aim of this exploratory was to investigate the theory of a relation between cytochrome P450 2D6 (CYP2D6) genotype and depressive symptoms in late pregnancy and/or postpartum. We studied 145 women with depressive symptoms. CYP2D6 genotype was analysed in leukocyte DNA by polymerase chain reaction (PCR). There were no significant differences in CYP2D6 genotypes between the groups of women being depressed during and/or after pregnancy. The frequencies of CYP2D6 genotypes did not differ from other European studies. This study cannot confirm that depressive symptoms in late pregnancy and postpartum are connected with CYP2D6 genotype. It is, however, noteworthy that the frequency of ultrarapid metabolizers was higher than in a general Caucasian population. This warrants further exploration in a greater study sample, but should also be investigated in a general population with major depression.

Pharmacokinetic interactions with rifampicin : clinical relevance

The antituberculosis drug rifampicin (rifampin) induces a number of drug-metabolising enzymes, having the greatest effects on the expression of cytochrome P450 (CYP) 3A4 in the liver and in the small intestine. In addition, rifampicin induces some drug transporter proteins, such as intestinal and hepatic P-glycoprotein. Full induction of drug-metabolising enzymes is reached in about 1 week after starting rifampicin treatment and the induction dissipates in roughly 2 weeks after discontinuing rifampicin. Rifampicin has its greatest effects on the pharmacokinetics of orally administered drugs that are metabolised by CYP3A4 and/or are transported by P-glycoprotein. Thus, for example, oral midazolam, triazolam, simvastatin, verapamil and most dihydropyridine calcium channel antagonists are ineffective during rifampicin treatment. The plasma concentrations of several anti-infectives, such as the antimycotics itraconazole and ketoconazole and the HIV protease inhibitors indinavir, nelfinavir and saquinavir, are also greatly reduced by rifampicin. The use of rifampicin with these HIV protease inhibitors is contraindicated to avoid treatment failures. Rifampicin can cause acute transplant rejection in patients treated with immunosuppressive drugs, such as cyclosporin. In addition, rifampicin reduces the plasma concentrations of methadone, leading to symptoms of opioid withdrawal in most patients. Rifampicin also induces CYP2C-mediated metabolism and thus reduces the plasma concentrations of, for example, the CYP2C9 substrate (S)-warfarin and the sulfonylurea antidiabetic drugs. In addition, rifampicin can reduce the plasma concentrations of drugs that are not metabolised (e.g. digoxin) by inducing drug transporters such as P-glycoprotein. Thus, the effects of rifampicin on drug metabolism and transport are broad and of established clinical significance. Potential drug interactions should be considered whenever beginning or discontinuing rifampicin treatment. It is particularly important to remember that the concentrations of many of the other drugs used by the patient will increase when rifampicin is discontinued as the induction starts to wear off.

Cytochrome P450 2D6: overview and update on pharmacology, genetics, biochemistry

Of about one dozen human P450 s that catalyze biotransformations of xenobiotics, CYP2D6 is one of the more important ones based on the number of its drug substrates. It shows a very high degree of interindividual variability, which is primarily due to the extensive genetic polymorphism that influences expression and function. This so-called debrisoquine/sparteine oxidation polymorphism has been extensively studied in many different populations and over 80 alleles and allele variants have been described. CYP2D6 protein and enzymatic activity is completely absent in less than 1% of Asian people and in up to 10% of Caucasians with two null alleles, which do not encode a functional P450 protein product. The resulting "poor metabolizer" (PM) phenotype is characterized by the inability to use CYP2D6-dependent metabolic pathways for drug elimination, which affect up to 20% of all clinically used drugs. The consequences are increased risk of adverse drug reactions or lack of therapeutic response. Today, genetic testing predicts the PM phenotype with over 99% certainty. At the other extreme, the "Ultrarapid Metabolizer" (UM) phenotype can be caused by alleles carrying multiple gene copies. "Intermediate Metabolizers" (IM) are severely deficient in their metabolism capacity compared to normal "Extensive Metabolizers" (EM), but in contrast to PMs they express a low amount of residual activity due to the presence of at least one partially deficient allele. Whereas the intricate genetics of the CYP2D6 polymorphism is becoming apparent at ever greater detail, applications in clinical practice are still rare. More clinical studies are needed to show where patients benefit from drug dose adjustment based on their genotype. Computational approaches are used to predict and rationalize substrate specificity and enzymatic properties of CYP2D6. Pharmacophore modeling of ligands and protein homology modeling are two complementary approaches that have been applied with some success. CYP2D6 is not only expressed in liver but also in the gut and in brain neurons, where endogenous substrates with high-turnover have been found. Whether and how brain functions may be influenced by polymorphic expression are interesting questions for the future.

Gene expression of cytochrome P450 1B1 and 2D6 in leukocytes in human pregnancy

We investigated the influence of human pregnancy on gene expression of two cytochrome P450 enzymes in white blood cells. Cytochrome P450 1B1 (CYP1B1) catalyses oestradiol 4-hydroxylation, and may participate in the endocrine regulation of oestrogens. Cytochrome P450 2D6 (CYP2D6) metabolises many commonly used drugs, and previous studies have suggested that it is induced during pregnancy. CYP1B1 and CYP2D6 were therefore considered to be of interest in human pregnancy. As it is not ethically possible to take liver biopsies from healthy mothers during pregnancy, easily accessible cells that express the genes were used as a surrogate tissue. White blood cells were collected from eighteen pregnant women, and were used to measure CYP1B1 and CYP2D6 ribonucleic acid (RNA). The analysis was repeated after pregnancy, the women, thus, serving as their own controls. Real-time reverse transcriptase - polymerase chain reaction methods were used with 18S ribosomal RNA as an internal control. A slight, but not significant, increase in gene activity of CYP1B1 was detected during pregnancy. Expression of CYP2D6 in blood was extremely low, and induction of CYP2D6 during pregnancy could not be confirmed. In conclusion, gene expression of CYP1B1 and CYP2D6 in leukocytes was not significantly up-regulated in the third trimester of pregnancy, but a trend indicating an altered metabolism during pregnancy was detected.

Patient-tailored antiemetic treatment with 5-hydroxytryptamine type 3 receptor antagonists according to cytochrome P-450 2D6 genotypes

PURPOSE

The use of serotonin 5-hydroxytryptamine type 3 receptor antagonists has substantially reduced, but not eliminated, nausea and vomiting in cancer chemotherapy. This study sought to investigate whether efficacy of antiemetic treatment with ondansetron and tropisetron depends on cytochrome P-450 2D6 (CYP2D6) genotype, hypothesizing that the rapid and particularly the ultrarapid metabolizers of these drugs are at risk of being undertreated.

PATIENTS AND METHODS

Included in the study were 270 cancer patients receiving their first day of chemotherapy. Nausea and vomiting were documented using standardized interviews. The intensity of nausea was measured with visual analog scales before and twice during the chemotherapy. The relationship between the CYP2D6 genotypes and the tropisetron serum concentrations 3 and 6 hours after drug administration was analyzed in a subgroup of 42 patients. CYP2D6 genotyping was carried out by polymerase chain reaction-restriction fragment length polymorphism analysis.

RESULTS

Genetically defined poor metabolizers had higher serum concentrations of tropisetron than all other patients (P <.03). Approximately 30% of all patients receiving chemotherapy experienced nausea and vomiting. Genetically defined ultrarapid meta-bolizers of CYP2D6 substrates had higher frequency of vomiting within the first 4 hours (P <.001) and within the period 5 to 24 hours (P <.03) after treatment than all the other patients; the tendency for nausea was similar. This difference was more pronounced in patients treated with tropisetron than in those treated with ondansetron.

CONCLUSION

Antiemetic treatment with tropisetron or ondansetron could be improved by adjustment for the CYP2D6 genotype; approximately 50 subjects would have to be genotyped to protect one patient from severe emesis.

Mimicking gene defects to treat drug dependence

The genetic basis for drug dependence has focused on genes that encode receptors involved in the reinforcing properties of drugs of abuse or that determine drug-taking behavior (e.g. impulsivity, etc.). Pharmacogenetic variations in the patterns of metabolism among individuals can also importantly modulate the risk of drug dependence. Cytochrome P450 drug metabolizing enzymes (CYPs), can activate (e.g. codeine to morphine) or deactivate (e.g. nicotine to cotinine) drugs of abuse. Some CYPs are polymorphic, that is, there are gene mutations which result in individuals with no (null mutations) or decreased enzyme activity (e.g. CYP2D6*10). Individuals with two null mutations appear in the population as phenotypic poor metabolizers. Using in vitro studies, we have identified drugs of abuse that are substrates of the polymorphic enzymes CYP2D6 (codeine, amphetamines, dextromethorphan), CYP2A6 (nicotine) and CYP2C19 (flunitrazepam). In human experimental studies, we have shown that CYP phenotype and genotype affect abuse liability of CYP2D6 metabolized drugs of abuse. In addition, we inhibited CYP2D6 and decreased individuals' risk of dependence experimentally (codeine, dextromethorphan) and treated codeine dependence. In epidemiologic studies CYP2D6 and CYP2A6 null mutations protect individuals from becoming codeine and tobacco dependent, respectively. With respect to CYP2A6, individuals with mutations, smoke fewer cigarettes and can quit more easily. Inhibiting CYP2A6 (e.g. tranylcypromine, methoxsalen) decreases smoking and the activation of procarcinogens. By mimicking these gene defects the risk of dependence can be decreased in individuals and new treatments developed.

Plasma concentrations of risperidone and 9-hydroxyrisperidone: effect of comedication with carbamazepine or valproate

To evaluate the pharmacokinetic interaction between risperidone and the mood-stabilizing agents carbamazepine and valproic acid, steady state plasma concentrations of risperidone and 9-hydroxyrisperidone (9-OH-risperidone) were compared in patients treated with risperidone alone (controls, n = 23) and in patients comedicated with carbamazepine (n = 11) or sodium valproate (n = 10). The three groups were matched for sex, age, body weight, and antipsychotic dosage. Plasma concentrations of risperidone and 9-OH-risperidone did not differ between valproate-comedicated patients and controls. By contrast, the concentrations of both compounds were lower in patients taking carbamazepine, although the difference reached statistical significance only for the metabolite (p < 0.001). The sum of the concentrations of risperidone and 9-OH-risperidone in patients receiving carbamazepine (median 44 nmol/L) was also significantly lower than in patients receiving valproate (168 nmol/L) and in controls (150 nmol/L). In five patients assessed with and without carbamazepine comedication, dose-normalized plasma risperidone and 9-OH-risperidone concentrations were significantly lower when the patients received combination therapy than when they received risperidone alone. In three patients assessed with and without valproate, no major changes in the levels of risperidone and its metabolite were observed. These findings demonstrate that carbamazepine markedly decreases the plasma concentrations of risperidone and its active 9-OH-metabolite, probably by inducing CYP3A4-mediated metabolism. This interaction is likely to be clinically significant. Conversely, valproic acid does not cause any major change in plasma antipsychotic levels.

Rifampicin treatment greatly increases the apparent oral clearance of quinidine

We investigated the effect of cytochrome P450 induction by rifampicin on the in vivo oxidative metabolism of quinidine. The pharmacokinetics of a 200 mg oral single dose quinidine were studied before and after one week of daily treatment with 600 mg rifampicin in six healthy young male volunteers. Biomarker reactions of cytochrome P450 isozyme activities in the form of caffeine, sparteine, mephenytoin, tolbutamide and cortisol metabolism were applied. The median total apparent oral clearance and partial clearance by 3-hydroxylation of quinidine increased 9 times. The partial clearance by N-oxidation increased 6 times. The Cmax and the elimination half life were reduced 3 times. No statistically significant changes were found for quinidine tmax and renal clearance. The cortisol metabolic ratio increased 5 times, while no statistically significant effects were seen for other CYP marker reactions. The results indicate that the inductive effect of rifampicin is likely to be of clinical relevance particularly when used concomitantly with drugs metabolized by CYP3A4.

Nortriptyline E-10-hydroxylation in vitro is mediated by human CYP2D6 (high affinity) and CYP3A4 (low affinity): implications for interactions with enzyme-inducing drugs

The human cytochrome P450 (CYP) isoforms mediating nortriptyline 10-hydroxylation have been identified using kinetic studies on heterologously expressed human CYPs and chemical inhibition studies on human liver microsomes. Nortriptyline was metabolized to E-10-hydroxynortriptyline by human lymphoblast-expressed CYPs 2D6 (Km 2.1 microM) and 3A4 (Km 37.4 microM) with high and low affinity, respectively, whereas CYPs 1A2, 2A6, 2B6, 2C9, 2C19, and 2E1 had no detectable activity. Human liver microsomal nortriptyline E-10-hydroxylation displayed biphasic kinetics. The high-affinity component (Km 1.3 +/- 0.4 microM, n = 11 livers) was selectively inhibited by the CYP 2D6 inhibitor quinidine, whereas the CYP3A4 inhibitor ketoconazole selectively inhibited the low-affinity component (K(m) 24.4 +/- 7 microM, n = 11 livers). Inhibition by ketoconazole increased with increasing substrate concentration, whereas the reverse was true for quinidine. The Vmax of the low-affinity component in human liver microsomes was significantly correlated (r2 = 0.84) with the relative activity factor for CYP3A4, a measure of the amount of catalytically active enzyme. A simulation of the relative contribution of CYPs 2D6 and 3A4 to net nortriptyline hydroxylation rate suggested that the relative contribution of CYP3A4 is only 20% even at the higher end of the therapeutic range. Induction of CYP3A4 will increase its importance and increase the net metabolic rate, whereas inhibition of CYP3A4 will be of little importance due to its minimal relative contribution under uninduced conditions. The identification of CYP3A4 as a low-affinity nortriptyline E-10-hydroxylase explains the ability of poor metabolizers of debrisoquin to hydroxylate nortriptyline, as well as the increased in vivo clearance via this pathway caused by CYP3A4-inducing drugs such as pentobarbital, carbamazepine, and rifampin.

Time course of enzyme induction in humans: effect of pentobarbital on nortriptyline metabolism

To study the effect of induction we gave six male volunteers 10 mg nortriptyline three times a day for 4 weeks and 0.2 gm pentobarbital on days 8 to 21. Plasma and urinary levels of nortriptyline and metabolites were measured. The rate and extent of induction of the enzyme(s) were estimated by a model with use of nortriptyline concentrations. There was a marked decrease of nortriptyline levels after 2 days of pentobarbital treatment. Total clearance of nortriptyline increased more than twofold (range, 1.6-fold to 4.1-fold). Apparent metabolic clearance by 10-hydroxylation increased markedly. The decrease in nortriptyline levels was more rapid than the increase after pentobarbital cessation, fitting with the theory of the model. The induction of nortriptyline metabolism is probably mainly the result of an increase in a non-CYP 2D6 P450 isozyme, possibly CYP 3A4 or a CYP 2C form. More knowledge of induction characteristics of drugs should lead to better predictions of decreased effects and appearance of adverse effects. The kinetic model used for analysis of our data could then be useful.

Genetic polymorphism of drug metabolizing enzymes: new mutations in CYP2D6 and CYP2A6 genes in Japanese

Genetic polymorphism of drug metabolizing enzymes, particularly cytochrome P450(CYP), is an important cause of adverse drug reactions. Multiple gene mutations in CYP have been shown to be phenotype. The occurrence of genetic polymorphism has been seen in genes for CYP1A1, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A5. This review discusses the molecular mechanism of two genetic polymorphisms, debrisoquine/sparteine (CYP2D6) coumarin (CYP2A6) polymorphisms. In addition, elucidation of gene mutations of CYP2D6 and CYP2A6 in Japanese will be discussed.

Induction of CYP2D6 in pregnancy

Expression of the drug-metabolizing enzyme cytochrome P4502D6 (CYP2D6) is predominantly under genetic control, and enzyme-inducing drugs have little influence on its activity. We studied the activity of CYP2D6 during pregnancy. One hundred forty pregnant women were genotyped for CYP2D6. Seventeen of them (four poor metabolizers, seven heterozygous extensive metabolizers, and six extensive metabolizers) were phenotyped with dextromethorphan in late pregnancy and 7 to 11 weeks after parturition. During pregnancy the dextromethorphan/dextrorphan metabolic ratio was significantly reduced (p = 0.0015) among homozygous and heterozygous extensive metabolizers, indicating increased CYP2D6 activity. In contrast, poor metabolizers showed an increased metabolic ratio during pregnancy. These results are consistent with previous findings of a marked increase in metabolism of the CYP2D6 substrate metoprolol during pregnancy. Both studies indicate an increase in CYP2D6 activity during pregnancy, which may be caused by an induction of the CYP2D6 enzyme.

Cytochrome P450 enzymes: interpretation of their interactions with selective serotonin reuptake inhibitors. Part II

The SSRIs have been used as an example to show how one might interpret the available evidence to draw conclusions about the relationships between drugs and P450s. Under what circumstances might one apply the knowledge of such relationships? First, the clinical implications must be considered when drugs with a narrow therapeutic index are coprescribed with other drugs that may affect P450s. For example, good clinical practice demands that before a TCA is coprescribed with another drug, the physician be aware of the potential for the second drug to interact with CYP2D6. Second, it may be helpful to consider P450 enzymes when adverse events occur during polypharmacy. It may happen that a known side effect of one drug occurs. Rather than attributing this to patient sensitivity, the physician should consider the possibility that a pharmacokinetic drug interaction increased plasma drug concentration, which in turn enhanced the probability of such an occurrence. Even when a pharmacokinetic drug interaction is considered as a possible cause, an appreciation of the role of P450s may lead to the realization that an interaction was not only possible but that it was likely. Finally, copharmacy can be used intentionally to produce controlled interactions. Indeed, planned pharmacokinetic drug interactions at the level of P450s have been proposed to reduce cyclosporine dosage requirements, to reduce variability of TCA levels, and to manipulate the contribution of alternative metabolic pathways to minimize toxic effects. As long as pharmaceuticals are metabolized by the P450 system, interactions with the various isozymes will be inescapable. It is fortunate that understanding them is becoming more tractable.

Assessment of individual CYP2D6 activity in extensive metabolizers with renal failure: comparison of sparteine and dextromethorphan

OBJECTIVES

To examine whether the variability of CYP2D6 activity in patients with chronic renal failure can be assessed, particularly among subjects with the extensive metabolizer phenotype, by use of standard in vivo indexes of CYP2D6 activity derived from oral administration of dextromethorphan and sparteine.

METHODS

A single 100 mg oral dose of sparteine and a single 40 mg oral dose of dextromethorphan were administered on two occasions to 12 patients with chronic renal failure (creatinine clearance ranging from 20 to 70 ml/min) and 12 age- and sex-matched healthy subjects. Sparteine clearances, sparteine metabolic ratio, and urinary recovery of dextrorphan were calculated. Patients and healthy control subjects were not selected on the basis of their CYP2D6 phenotypes.

RESULTS

Chronic renal failure was associated with a decrease in sparteine partial metabolic clearance to dehydrosparteine (median of 322 ml/min and range of 62 to 670 ml/min in patients with renal failure versus median of 635 ml/min and range of 77 to 1276 ml/min in normal subjects; p < 0.02). Sparteine apparent oral clearance (p < 0.03) and renal clearance (p < 0.001) decreased in patients with renal failure. However, sparteine metabolic ratio was not significantly altered in patients with renal failure and showed that all patients were extensive metabolizers of sparteine. Although fractional urinary excretion of dextrorphan decreased in patients with renal failure (median, 24.4%; range, 9.7% to 55.9%) compared with control (median, 47.5%; range, 24.1% to 72.1%) (p = 0.02), it also showed that all subjects were extensive metabolizers of dextromethorphan. The amount of dextromethorphan excreted in urine correlated with creatinine clearance independently from CYP2D6 activity measured as sparteine partial metabolic clearance. However, it did not correlate with sparteine metabolic ratio or with fractional urinary excretion of dehydrosparteine.

CONCLUSION

Assessment of CYP2D6 activity by use of dextromethorphan and sparteine is possible in extensive metabolizer patients with chronic renal failure. However, in these subjects, dextromethorphan and sparteine do not reflect CYP2D6 activity in the same way.

Interethnic differences in drug metabolism: influence of genetic and environmental factors on debrisoquine hydroxylation phenotype

Genetic and environmental factors are determinants of the interindividual and interethnic variability in drug metabolism. The metabolism of several important drugs (e.g. haloperidol) cosegregates with that of debrisoquine. Thus, interethnic differences in debrisoquine hydroxylation polymorphism (CYP2D6) might be partly responsible for the variation in haloperidol disposition between races. The influence of tobacco, ethanol, caffeine, gender, and oral contraceptive use on the debrisoquine metabolic ratio (MR) has been analyzed in 633 Spanish healthy volunteers. MR was also determined in panels of healthy volunteers. 18 smokers were studied during a tobacco abstinence period, and 31 women three times during the same menstrual cycle. Among EMs, debrisoquine MR was significantly (P < 0.05) lower during smoking cessation (mean antilog +/- SD, 0.48 +/- 0.29) compared to a smoking period (0.61 +/- 0.23). During the lutheal phase of the menstrual cycle, debrisoquine MR was also significantly (P < 0.01) lower (0.33 +/- 0.41) compared to the ovulatory-phase (0.41 +/- 0.34) and the phase before ovulation (0.44 +/- 0.36). Among EMs, it is suggested that debrisoquine MR may be modified by tobacco smoking and sexual hormones. The clinical relevance of these findings remains unclear.

Prediction of CYP2D6-mediated polymorphic drug metabolism (sparteine type) based on in vitro investigations

Discovery of genetic polymorphism in drug metabolism has contributed a great deal to understanding the variability in dose-concentration relationships introduced by genetic factors, thereby elucidating the mechanisms responsible for unexpected drug reactions. This knowledge should find its way into clinical practice in order to make therapy more efficient and safe. Moreover, genetic factors in drug metabolism should be taken into account during drug development. Therefore, in vitro methods for identifying the metabolic pattern of new compounds during early stages of drug development should be improved. This review summarizes in vitro methods available to identify genetic polymorphism in drug oxidation, in particular the CYP2D6-related polymorphism.

Phenobarbital induces the 2-hydroxylation of desipramine

The kinetics of a single oral dose of desipramine (DMI; 100 mg) were studied in eight epileptic patients chronically treated with phenobarbital (PB) and in eight drug-free healthy controls. All subjects were extensive metabolizers with respect to the genetically determined CYP2D6-related metabolic polymorphism. Compared with controls, epileptic patients exhibited lower peak plasma DMI concentrations (74 +/- 24 vs. 107 +/- 32 nmol/L; means +/- SD, p < 0.05), smaller DMI area-under-the-curve values (1,943 +/- 461 vs. 3,234 +/- 1,145 nmol L-1 h; p < 0.01), and shorter DMI elimination half-lives (15.1 +/- 2.1 vs. 20.6 +/- 3.4 h; p < 0.01). The proportion of the dose excreted as 2-hydroxydesipramine (2-OH-DMI) was significantly higher in the patients (54 +/- 8 vs. 40 +/- 9%; p < 0.05). In one single poor metabolizer volunteer, a 3-week treatment with PB was associated with no major changes in DMI kinetics, but the urinary excretion of 2-OH-DMI tended to increase. These results suggest that PB is an inducer of the 2-hydroxylation of DMI, a reaction primarily catalyzed by CYP2D6, but do not provide further information on the specific P450 isoenzyme(s) being induced.

The effect of carbamazepine on the 2-hydroxylation of desipramine

The effect of carbamazepine (CBZ, 200 mg twice daily for 28 days) on the kinetics of a single oral dose of desipramine (DMI, 100 mg) was investigated in six healthy volunteers. Compared with a control session, treatment with CBZ caused a marked increase in DMI apparent oral clearance (from 1.05 +/- 0.40 to 1.38 +/- 0.52 1 h per kg, means +/- SD, P < 0.01) and a significant shortening in DMI half-life (from 22.1 +/- 3.5 to 17.8 +/- 3.5 h, P < 0.01). The amount of 2-hydroxydesipramine (2-OH-DMI) excreted in urine over a 24-h period was significantly increased during CBZ intake (from 75 +/- 15 to 92 +/- 16 mumol, P < 0.01). These findings suggest that CBZ induces the 2-hydroxylation of DMI, a reaction primarily catalyzed by the polymorphic CYP2D6 isozyme. This interaction may have considerable practical significance.

Induction and autoinduction properties of rifamycin derivatives: a review of animal and human studies

Animal studies have demonstrated that the mouse and rabbit are far more responsive to the inductive properties of rifamycin derivatives than the rat and guinea pig. The rat hepatic cytochrome P450 system seems to be resistant to the action of rifampicin unless very high doses are used. Mouse hepatic microsomal mixed-function oxidase activity is markedly increased by repeated dosing with rifampicin, whereas administration of rifabutin may be ineffective. In humans, both rifampicin and rifabutin are extensively metabolized and induce their own metabolism. The induced metabolic pathways remain essentially unknown. Under autoinduction conditions, the elimination half-life of rifampicin decreases, whereas that of rifabutin is not altered. Although the effects of repeated administration of rifampicin and rifabutin on the various forms of cytochrome P450 in humans have not been extensively examined, there is convincing evidence that the P4503A subfamily is induced by either drug, whereas the P4501A subfamily and P4502D6 do not appear to be affected by rifampicin. Limited reliable information is available concerning the induction of human glucuronyltransferase activities by rifampicin and rifabutin which, however, do not seem to influence zidovudine glucuronide formation in healthy subjects.

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

sigma recognition sites in brain and peripheral tissues. Characterization and effects of cytochrome P450 inhibitors

Binding to sigma sites in subcellular fractions of brain and in crude homogenates from peripheral tissues of the guinea pig was characterized with the [3H]ligands (+)pentazocine and di(2-tolyl)guanidine (DTG). The inhibitory effects of representative sigma compounds and cytochrome P450 inhibitors were evaluated in guinea pig tissues, and the effects of cytochrome P450 induction on sigma binding in the rat were investigated. For both ligands, the majority of sites were localized to the microsomal fractions. The KD values for [3H](+)pentazocine- or [3H]DTG-labeled sigma sites in guinea pig liver and testes were 2-fold lower than those in brain and heart. The number of sites labeled by [3H](+)pentazocine varied, with an order of liver > testes > brain > heart. In contrast, the Bmax values for [3H]DTG-defined sigma sites were greatest in testes, followed by liver, brain and heart. The rank order of potency for representative sigma and P450 compounds was similar in brain, liver and testes for both [3H]ligands, and the potency of selective compounds to displace sigma binding in guinea pig liver failed to correlate with their abilities to inhibit cytochrome P450IID1 activity in human liver. Following induction of cytochrome P450IIB1 with phenobarbital or cytochrome P450IA1 with beta-naphthoflavone, neither the affinity nor the number of sigma sites was altered in rat brain or liver. These results suggest that sigma sites in the periphery are similar to those in the brain, and that the sigma binding site is not identical with cytochrome P450IIB1, P450IA1 or P450IID1.

Clinical pharmacokinetics: current requirements and future perspectives from a regulatory point of view

1. There is an increasing appreciation of the relevance of pharmacokinetics of drugs during evaluation of their safety for human clinical use. Regulatory requirements for clinical pharmacokinetic data have progressively evolved to emphasize and address these safety implications. 2. Historically the dose schedules usually recommended have been too high, often with serious consequences. Therefore, the need to establish reliable dose response (both therapeutic and toxic) relationships must be an important objective. 3. Concurrent developments in our understanding of the pharmacological effects (therapeutic or toxic) of metabolites, the interethnic and interindividual differences in drug responses and the toxicological aspects of drug chirality now provide compelling reasons for the roles of bioactivation, pharmacogenetics and stereochemical factors to be addressed in pharmacokinetic studies during the clinical development of drugs. 4. Apart from the traditional pharmacokinetic studies following single and multiple doses in healthy volunteers, patients and special subgroups, reliable dose-response curves for therapeutic and toxic effects must be established in well-designed controlled studies using a wide range of doses. Often, doses lower than those recommended have a much improved risk/benefit ratio. 5. Secondary pharmacology of the drug and its active metabolites must be defined for assessment of safety (adverse reactions and pharmacokinetic and pharmacodynamic drug-drug interactions) in high dose/concentration situations. 6. The enzyme systems responsible for the metabolism of a drug must be identified followed by rational investigations of drug-drug and drug-disease interactions both from the efficacy and safety viewpoints. Factors responsible for alterations in the functional expression of this enzyme system must be identified and the safety and efficacy implications of these findings at interethnic, inter- and intraindividual levels must be fully explored during all phases of the clinical development of the drug. This should lead to carefully designed patient subgroup-specific dose schedules which maximize the risk/benefit ratio for all patients. 7. Drugs operate in a chiral environment and, not surprisingly, enantiomers of a drug differ significantly in their pharmacokinetics and pharmacodynamics. The possibility of interactions between enantiomers of a drug and of enantioselective interactions should be examined. These should be thoroughly investigated and the decision to market a racemic mixture or one of its enantiomers must be justified. 8. Analysis of population pharmacokinetics offers an approach by which to examine the roles of various factors which are likely to be clinically relevant for the safe and effective use of drugs.(ABSTRACT TRUNCATED AT 400 WORDS)

Reproducibility over time of mephenytoin and debrisoquine hydroxylation phenotypes

Mephenytoin and debrisoquine hydroxylation phenotypes were determined twice in 15 Spanish healthy volunteers with an interval of about one year. The phenotype assignment did not change in any subject for either debrisoquine or mephenytoin. Among extensive metabolisers of mephenytoin, there was a slight increase (P = 0.04) of the mephenytoin-S/R enantiomeric ratio over the study period. The family members of a poor metaboliser of mephenytoin were phenotyped, and the heterozygous extensive metabolisers were found to have higher mephenytoin-S/R ratios than other extensive metabolisers suggesting a correlation between the genotype and the S/R ratio.

Oxidative polymorphism of debrisoquine is not related to the risk of Alzheimer's disease

Oxidative polymorphism of debrisoquine has been studied in patients suffering from many spontaneous disorders which show genetic and/or environmental factors in their pathogenesis. To elucidate whether any relationship exists between this genetic polymorphism and the risk of developing Alzheimer disease (AD) we determined the oxidative phenotype and metabolic ratio (MR) of debrisoquine (DBQ) in 47 patients with AD or senile dementia of Alzheimer type (SDAT) and 837 healthy controls. The patients were free of drugs during at least the previous 30 days; all the controls were free of drugs. Three patients (6.38%) and 42 controls (5.02%) were classified as poor metabolizers (PM) of DBQ (non-significant difference). The distribution of MR values in the AD/SDAT patients showed non-significant differences when compared with controls. There was no relation between oxidative polymorphism of DBQ and age at onset of the disease. These results suggest that DBQ oxidative genetic polymorphism cannot be considered as a risk factor for developing AD-SDAT.

Stereoselective and isozyme-selective drug interactions

A surprisingly large number of marketed drugs are racemic mixtures. The pharmacokinetic literature on racemic drugs contains a vast amount of information on drug-drug interactions derived from the measurement of total drug concentrations in plasma and urine. The appreciation of the role of stereochemistry in drug interactions with racemic warfarin resulted in a long-overdue scientific rigor being applied to the study of drug interactions. It also compelled us to recognize that much of the literature was uninterpretable. A better understanding of oxidative metabolism, particularly the complexity of the cytochrome P-450 family of enzymes, has also strengthened the scientific basis of drug interactions. We now recognize that investigators and clinicians must consider both stereoselectivity and isozyme selectivity in the study of drug interactions to understand the nature of the interaction so as to more effectively use new and potent drugs.

Debrisoquine metabolism in Chinese patients with Alzheimer's and Parkinson's diseases

We determined the oxidative phenotype and metabolic ratio of debrisoquine in 96 Chinese patients with Alzheimer's disease (n = 12), Parkinson's disease (n = 55), and using patients with stroke and cervical spondylosis as controls (n = 29). We did not find any difference in debrisoquine metabolic phenotype among Parkinson's disease, Alzheimer's disease, and control patients as judged by chi-square analysis. In addition, the metabolic ratio of all our patients was less than 12.6. The result suggested that Chinese patients with Parkinson's disease and Alzheimer's disease metabolize debrisoquine at a velocity not different from that of their Western counterparts even though the frequency distribution of debrisoquine metabolism phenotyping in these two populations is quite different.

Oxidative polymorphism of debrisoquine is not related to human colo-rectal cancer

The oxidative polymorphism of debrisoquine (DBQ) has been determined in 89 patients with colo-rectal cancer and in 556 normal control subjects. Four patients and 34 controls, with a metabolic ratio greater than 12.6, were classified as poor metabolisers of DBQ (n.s.). No difference was found in the distribution of the frequencies of the MR of DBQ between patients and controls. It is concluded that polymorphic oxidation of DBQ is not related to the risk of developing colo-rectal cancer in human beings.

Lack of a relationship between the polymorphism of debrisoquine oxidation and lung cancer

1. Determination of debrisoquine oxidation phenotype was carried out in 119 healthy subjects, 135 patients with chronic bronchitis and 153 patients with lung cancer, all of Caucasian origin. 2. A non-Gaussian distribution of the log D/HD ratio was observed in the three groups. 3. Assuming an antimode of 1.12, the proportion of PMs was found to be 6.7% in healthy subjects, 8.9% in chronic bronchitics and 6.5% in patients with lung cancer. These differences were not significant. 4. The presence of a lung tumour itself had no influence on phenotype in a group of 14 patients who were phenotyped before and after surgery. 5. We conclude that a link between debrisoquine phenotype and lung cancer is unlikely.