Geographical/interracial differences in polymorphic drug oxidation. Current state of knowledge of cytochromes P450 (CYP) 2D6 and 2C19

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

Lack of Interaction of Milnacipran with the Cytochrome P450 Isoenzymes Frequently Involved in the Metabolism of Antidepressants

OBJECTIVE

To compare the pharmacokinetics of milnacipran in extensive metabolisers (EMs) and poor metabolisers (PMs) of sparteine and mephenytoin, and to assess the influence of multiple administrations of milnacipran on the activity of cytochrome P450 (CYP) isoenzymes through its own metabolism and through various probes, namely CYP2D6 (sparteine/dextromethorphan), CYP2C19 (mephenytoin), CYP1A2 (caffeine) and CYP3A4 (endogenous 6-beta-hydroxy-cortisol excretion).

METHODS

Twenty-five healthy subjects, 12 EMs for both sparteine/dextromethorphan and mephenytoin, nine EMs for mephenytoin and PMs for sparteine/dextromethorphan (PM(2D6)) and four PMs for mephenytoin and EMs for sparteine/dextromethorphan (PM(2C19)) were administered milnacipran as a single 50 mg capsule on day 1 followed by a 50 mg capsule twice daily for 7 days. The pharmacokinetics of milnacipran and its oxidative metabolites were assessed after the first dose (day 1) and after multiple administration (day 8), and were compared for differences between CYP2D6 and CYP2C19 PMs and EMs. Metabolic tests were performed before (day -2), during (days 1 and 8) and after (day 20) milnacipran administration.

RESULTS

Milnacipran steady state was rapidly achieved. Metabolism was limited: approximately 50% unchanged drug, 30% as glucuronide and 20% as oxidative metabolite (mainly F2800 the N-dealkyl metabolite). Milnacipran administration to PM2D6 and PM2C19 subjects did not increase parent drug exposure or decrease metabolite exposure. Milnacipran oxidative metabolism is not mediated through CYP2D6 or CYP2C19 polymorphic pathways nor does it significantly interact with CYP1A2, CYP2C19, CYP2D6 or CYP3A4 activities.

CONCLUSION

Limited reciprocal pharmacokinetic interaction between milnacipran and CYP isoenzymes would confer flexibility in the therapeutic use of the drug when combined with antidepressants. Drug-drug interaction risk would be low, even if the combined treatments were likely to inhibit CYP2D6 and CYP2C19 isoenzyme activities.

Fenproporex N-dealkylation to amphetamine—enantioselective in vitro studies in human liver microsomes as well as enantioselective in vivo studies in Wistar and Dark Agouti rats

Fenproporex (FP) is known to be N-dealkylated to R(-)-amphetamine (AM) and S(+)-amphetamine. Involvement of the polymorphic cytochrome P450 (CYP) isoform CYP2D6 in metabolism of such amphetamine precursors is discussed controversially in literature. In this study, the human hepatic CYPs involved in FP dealkylation were identified using recombinant CYPs and human liver microsomes (HLM). These studies revealed that not only CYP2D6 but also CYP1A2, CYP2B6 and CYP3A4 catalyzed this metabolic reaction for both enantiomers with slight preference for the S(+)-enantiomer. Formation of amphetamine was not significantly changed by quinidine and was not different in poor metabolizer HLM compared to pooled HLM. As in vivo experiments, blood levels of R(-)-amphetamine and S(+)-amphetamine formed after administration of FP were determined in female Dark Agouti rats (fDA), a model of the human CYP2D6 poor metabolizer phenotype (PM), male Dark Agouti rats (mDA), an intermediate model, and in male Wistar rats (WI), a model of the human CYP2D6 extensive metabolizer phenotype. Analysis of the plasma samples showed that fDA exhibited significantly higher plasma levels of both amphetamine enantiomers compared to those of WI. Corresponding plasma levels in mDA were between those in fDA and WI. Furthermore, pretreatment of WI with the CYP2D inhibitor quinine resulted in significantly higher amphetamine plasma levels, which did not significantly differ from those in fDA. The in vivo studies suggested that CYP2D6 is not crucial to the N-dealkylation but to another metabolic step, most probably to the ring hydroxylation. Further studies are necessary for elucidating the role of CYP2D6 in FP hydroxylation.

Neuromuscular pharmacodynamics of rocuronium in patients with major burns

Rocuronium, which has a short onset time and is free of hyperkalemic effects, could be considered for rapid-sequence induction of anesthesia in patients with burns. In this study, we assessed the neuromuscular pharmacodynamics of rocuronium in patients with major burns. Adults aged 18-59 yr who had a major burn injury (n = 56) and a control group of 44 nonburned patients were included. Rocuronium was used at 3 times (0.9 mg/kg) or 4 times (1.2 mg/kg) the 95% effective dose. Anesthesia consisted of propofol and fentanyl with nitrous oxide and oxygen. Neuromuscular block was monitored with an acceleromyograph by using train-of-four stimulation. The onset time to 95% neuromuscular block was prolonged in burned compared with nonburned patients (115 +/- 58 s versus 68 +/- 16 s for 0.9 mg/kg; 86 +/- 20 s versus 57 +/- 11 s for 1.2 mg/kg). Dose escalation shortened the onset time, prolonged the duration of action, and improved intubating conditions in burned patients. All recovery profiles were significantly shorter in burned versus nonburned groups with both doses. Resistance to the neuromuscular effects of rocuronium was partially overcome by increasing the dose. A dose up to 1.2 mg/kg provides good tracheal intubating conditions after major burns.

Identification of cytochrome P450 enzymes involved in the metabolism of 4'-methoxy-alpha-pyrrolidinopropiophenone (MOPPP), a designer drug, in human liver microsomes

1. The metabolism of 4'-methoxy-alpha-pyrrolidinopropiophenone (MOPPP), a novel designer drug, to its demethylated major metabolite 4'-hydroxy-pyrrolidinopropio-phenone (HO-PPP) was studied in pooled human liver microsomes (HLM) and in cDNA-expressed human hepatic cytochrome P450 (CYP) enzymes. 2. CYP2C19 catalysed the demethylation with apparent Km and Vmax values of 373.4 +/- 45.1 microM and 6.0 +/- 0.3 pmol min(-1) pmol(-1) CYP, respectively (mean +/- SD). Both CYP2D6 and HLM exhibited clear biphasic profiles with apparent K(m,1) values of 1.3 +/- 0.4 and 22.0 +/- 6.5 microM, respectively, and V(max,1) values of 1.1 +/- 0.1 pmol min(-1) pmol(-1) CYP and 169.1 +/- 20.5 pmol min(-1) mg(-1) protein, respectively. 3. Percentages of intrinsic clearances of MOPPP by particular CYPs were calculated using the relative activity factor (RAF) approach with (S)-mephenytoin-4'-hydroxylation or bufuralol-1'-hydroxylation as index reactions for CYP2C19 or CYP2D6, respectively. 4. MOPPP, HO-PPP and the standard 3',4'-methylenedioxy-pyrrolidinopropio-phenone (MDPPP) were separated and analysed by liquid chromatography-mass spectrometry in the selected-ion monitoring (SIM) mode. 5. The CYP2D6 specific chemical inhibitor quinidine (3 microM) significantly (p<0.0001) inhibited HO-PPP formation by 91.8 +/- 0.5% (mean +/- SEM) in incubation mixtures with HLM and 2 microM MOPPP. 6. It can be concluded from the data obtained from kinetic and inhibition studies that polymorphically expressed CYP2D6 is the enzyme mainly responsible for MOPPP demethylation.

CYP2C19Polymorphism and Proton Pump Inhibitors

Proton pump inhibitors such as omeprazole (esomeprazole), lansoprazole, pantoprazole and rabeprazole are eliminated by the hepatic route and the polymorphic CYP2C19 is mainly involved in their metabolism. In different populations three phenotypes have been identified: extensive metabolizers, poor metabolizers and individuals carrying one wild type and one mutant allele (het extensive metabolizers). Systemic exposure to the proton pump inhibitors as expressed by the AUC (area under the plasma level time profiles) is 5-12-times higher in poor metabolizers than in extensive metabolizers. As the pharmacodynamic response (elevation of intragastric pH) to the proton pump inhibitors is related directly to their AUC, a much higher pH can be monitored over 24 hr in poor metabolizers than in extensive metabolizers. Furthermore, clinical efficacy of all proton pump inhibitors depend on maintaining intragastric pH above certain threshold levels and significantly higher eradication rates of Helicobacter pylori have been observed in patients of the poor metabolizers and het extensive metabolizers phenotype if compared to extensive metabolizers. Likewise, limited data suggest that proton pump inhibitors-induced healing rates in gastro-oesophageal reflux disease are apparently higher in poor metabolizers/het extensive metabolizers than in extensive metabolizers of CYP2C19. Therefore initial genotyping for this enzyme and higher dosage in extensive metabolizers is likely to improve the clinical efficacy of proton pump inhibitors.

Allele-specific change of concentration and functional gene dose for the prediction of steady-state serum concentrations of amitriptyline and nortriptyline in CYP2C19 and CYP2D6 extensive and intermediate metabolizers

BACKGROUND

Recently, new polymorphisms were described in connection with intermediate and ultrarapid CYP2D6 metabolism. These may allow a much desired prediction of metabolic activity within the extensive metabolizer group. The functional consequences are still being discussed with few data available for clinical patients.

METHODS

We conducted a prospective, blinded two-center study seeking correlations between CYP2C19 (*2,*3, and *4; conventional PCR) and CYP2D6 genotypes (*1 to *10, *35, and *41; real-time and multiplex PCR) and drug concentrations (Emit and HPLC) in 50 Caucasians receiving amitriptyline (AT; 75 mg twice a day).

RESULTS

Eighteen CYP2C19 heterozygotes (*1/*2) had higher AT (P = 0.033) and lower nortriptyline (NT; P = 0.059) concentrations than 30 homozygotes (*1/*1). For CYP2D6, we calculated two new indices, i.e., the allele-specific change of concentration on identical background (ASCOC) and a quantitative functional gene dose. The ASCOC describes the change in NT concentration attributable to a mutant allele compared with the wild type. We found significantly higher concentrations for alleles *4 (95.6%; P <0.0001), *10 (63.3%; P <0.001), and *41 (39.8%; P <0.0001) but not for *2 and *35. Assigning of semiquantitative gene doses of 0, 0.5, or 1 to each allele instead of applying the current classification system (predicted phenotypes: 3 intermediate metabolizers, 46 extensive metabolizers, and 1 ultrarapid metabolizer) produced significant NT concentration differences: gene doses of 0.5 (n =3), 1 (n = 14), 1.5 (n = 11), 2 (n = 21) and 3 (n = 1; P <0.00001).

CONCLUSIONS

AT and NT concentrations can be predicted within the group of CYP2D6 extensive metabolizers. The ASCOC provides substantial advantages compared with current methods of analysis. CYP2D6 but not CYP2C19 correlates with the sum of both concentrations used to guide AT therapy.

Association between cytochrome P450 2D6 genotype and harm avoidance

Genetic polymorphisms within the serotonergic and dopaminergic neurotransmitter systems may be associated with heightened harm avoidance and novelty seeking. It is also conceivable that polymorphisms in enzymes that mediate the metabolism of endogenous amines within the brain may influence these components of temperament. The CYP2D6 enzyme is expressed at low levels in the brain, and has been shown to mediate the formation of the neurotransmitters serotonin and dopamine from trace amines. Some 5-10% of Caucasians are CYP2D6 deficient due to inactivating mutations in the CYP2D6 gene, and are termed poor metabolizers (PMs). In this study, we investigated whether temperament varied significantly between PMs and CYP2D6 extensive metabolizers (EMs) using the Temperament and Character Inventory (TCI). CYP2D6 genotypes were determined for 121 depressed patients. Of these patients, 113 were inferred from genotype as being EMs and eight as PMs. A significant difference in temperament was observed between inferred CYP2D6 EM and PM individuals. CYP2D6 PMs had significantly lower harm avoidance scores (P = 0.003) than EMs. Furthermore, analysis of the harm avoidance sub-scales revealed that the CYP2D6 PMs scored significantly lower on "fear of uncertainty" (P < 0.001), fatigability (P = 0.009), and shyness (P = 0.038) than EMs, but did not differ significantly from EMs on the worry/pessimism sub-scale. No significant difference in character scores was detected between inferred CYP2D6 EMs and PMs. Our findings suggest that the CYP2D6 polymorphism may impact on personality, and one potential mechanism for this would be by influencing the generation of endogenous neurotransmitters in the brain.

The design, preparation and SAR of novel small molecule sodium (Na + ) channel blockers

A parallel strategy incorporating predictive modeling of both sodium site 2 blocking activity and cytochrome p450 CYP2D6 enzyme activity as well as experimental data from ADME profiling (eADME) has been applied to the design of new small molecule sodium channel blockers. New structural motifs were identified, which combined sodium channel activity with decreased ADME liabilities. Compounds 10h (site 2, IC(50) =531 nM) and 7j (site 2, IC(50) =149 nM) were identified from two structural classes as sodium channel blockers with favorable in vitro eADME profiles.

Interpretations of 'appropriate' minority inclusion in clinical research

The range of possible interpretations of the phrase "appropriate representation" has left investigators struggling with the practical application of the National Institutes of Health guidelines on the inclusion of minorities in research. At least three goals might be reached by including minorities in clinical research: to test specific hypotheses about differences by race and ethnicity; to generate hypotheses about possible differences by race and ethnicity; and to ensure the just distribution of the benefits and burdens of participation in research, regardless of whether there are expected differences in outcome by race or ethnicity. In this paper, we describe possible interpretations of "appropriate representation," as well as provide a general approach that investigators might use to address this issue. To expand scientific knowledge about the health of minority populations, investigators should be expected to state which goal they have selected and why that goal is appropriate as compared with other possible goals.

Evaluation of Fast Disintegrating Lansoprazole Tablet in Human Subjects

Fast disintegrating lansoprazole tablet (LFDT) has been developed as a multiple unit formulation and evaluated using human subjects as compared to the conventional lansoprazole (LPZ) capsule containing enteric coated granules. Twelve healthy male volunteers, who were confirmed as extensive metabolizers (EMs) based on the plasma levels of LPZ sulphone metabolite, were enrolled into the study and genotype of CYP2C19 was confirmed. They kept 30 mg LFDT in their mouths for 2 min and the saliva was recovered without swallow. Eight subjects did not show LPZ in their serum after intake. Although LPZ was detected in 4 subjects' serum, their concentrations were less than 5 ng/mL. LPZ was thought to be not absorbed from the oral cavity. LFDT was orally administered to 12 healthy male EMs at two doses, 15 mg and 30 mg, and serum LPZ concentrations were measured. The mean C(max) and AUC(0-24) were 474.1+/-254.0 ng/mL and 1105.3+/-1101.4 ng.h/mL (15 mg) and 992.8+/-384.3 ng/mL and 2216.5+/-1270.1 ng.h/mL (30 mg). By comparing to that obtained after oral administration of the same doses of LPZ capsule, serum LPZ concentration vs. time curve was almost the same level, i.e., C(max) and AUC(0-24) did not have significant differences. From these results, LFDT has been shown to be equivalent to LPZ capsule and will show the same acid suppressing effects in the clinical situation.

Characterization of the CYP2C8 Active Site by Homology Modeling

To compare the features of the active sites of CYP2C8, CYP2C9, and CYP2C19, homology modeling was performed based on the crystallographic coordinates of mammalian CYP2C5. It was found that CYP2C8 has a much larger pocket than the other forms due to the existence of an additional pocket. The approach to the additional pocket is comprised of Ile102, Ser114, Leu208, Val366, and Ile476, and the side chains of Ser114, Val366, and Ile476, which are smaller than the corresponding residues in the other CYPs, enable access to the pocket. The general features of the active site in the CYP2C8 model are similar to those of the previously constructed CYP3A4 model, which may account for the 2 CYPs sharing some of their substrates. The CYP2C8 model was validated by examining the bound orientation of paclitaxel and showing that it is consistent with the formation of the 6-beta hydroxylated derivative during metabolism. Docked paclitaxel was found to form a hydrogen bond with the side chain of Asn 99, which is a characteristic residue of CYP2C8 and is located in the additional pocket. Descriptors for CYP2C8 and CYP2C9 substrates were also examined with the molecular operating environment (MOE). The descriptor by which CYP2C8 and CYP2C9 substrates were classified most distinctly was found to be molar refractivity, which might be related to the longer shape and more polar nature of the active site of CYP2C8 in the CYP2C subfamily.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Cytochrome P450 dependent metabolism of the new designer drug 1-(3-trifluoromethylphenyl)piperazine (TFMPP)

1-(3-Trifluoromethylphenyl)piperazine (TFMPP) is a designer drug with serotonergic properties. Previous studies with male Wistar rats (WI) had shown, that TFMPP was metabolized mainly by aromatic hydroxylation. In the current study, it was examined whether this reaction may be catalyzed by cytochrome P450 (CYP)2D6 by comparing TFMPP vs. hydroxy TFMPP ratios in urine from female Dark Agouti rats, a model of the human CYP2D6 poor metabolizer phenotype (PM), male Dark Agouti rats, an intermediate model, and WI, a model of the human CYP2D6 extensive metabolizer phenotype. Furthermore, the human hepatic CYPs involved in TFMPP hydroxylation were identified using cDNA-expressed CYPs and human liver microsomes. Finally, TFMPP plasma levels in the above mentioned rats were compared. The urine studies suggested that TFMPP hydroxylation might be catalyzed by CYP2D6 in humans. Studies using human CYPs showed that CYP1A2, CYP2D6 and CYP3A4 catalyzed TFMPP hydroxylation, with CYP2D6 being the most important enzyme accounting for about 81% of the net intrinsic clearance, calculated using the relative activity factor approach. The hydroxylation was significantly inhibited by quinidine (77%) and metabolite formation in poor metabolizer genotype human liver microsomes was significantly lower (63%) compared to pooled human liver microsomes. Analysis of the plasma samples showed that female Dark Agouti rats exhibited significantly higher TFMPP plasma levels compared to those of male Dark Agouti rats and WI. Furthermore, pretreatment of WI with the CYP2D inhibitor quinine resulted in significantly higher TFMPP plasma levels. In conclusion, the presented data give hints for possible differences in pharmacokinetics in human PM and human CYP2D6 extensive metabolizer phenotype subjects relevant for risk assessment.

Culture, demographics, and critical care issues: an overview

The population dynamic and the immigration trends in the United States continue to challenge health care professionals who each day must serve an increasingly diverse population. Today's physicians must not only have a solid background in medical sciences but they must also have knowledge of how culture, race, and ethnicity impact how patients view and accept traditional Western practices. Whether doctors and patients are close in the "context spectrum" will often determine their ability to communicate beyond the spoken language. According to a report of the American Medical Association, by the year 2000, out of a total 812,770 physicians, only 2.5% were Black, 3.5% Hispanic, and 8.9% Asian. Only a fraction of a percent was American Native/Alaskan Native. Therefore, the majority of the physicians are Caucasian, and it could be assumed that they would likely be accustomed to high-context communication styles. The gross of the demographic changes and population increases in the United States during the past 10 years can be attributed to immigration from regions of the world where low-context communication styles are prevalent. Such differences between physicians and patients can create difficult, tense situations in an already charged atmosphere as can be that of a critical care unit.

Hepatic drug metabolism and transport in patients with kidney disease

The disposition of many drugs is altered in patients with acute (AKD) and chronic kidney disease (CKD). A decline in renal clearance of several drugs has been correlated significantly with residual renal function (ie, creatinine clearance) of subjects. Reductions in nonrenal clearance of some compounds also have been reported and associated with clearance of markers of oxidative and/or conjugative metabolism or P-glycoprotein-mediated transport. Although initial accounts of reduced hepatic microsomal cytochrome P-450 (CYP) content and activity in animal models of AKD and CKD were published almost 25 years ago, it is only in the last decade that technical advances in molecular biology and clinical pharmacology have enabled researchers to begin to characterize the phenotypic expression of individual enzymes and, importantly, distinguish the molecular and/or genetic basis for these changes. The selective modulation of hepatic CYP enzyme activity observed in kidney disease is caused, at least in part, by differentially altered expression of several CYP isoforms. This review summarizes data available through June 2003 regarding the effect of AKD and CKD on drug metabolism. Knowledge of the impact and nature of these alterations associated with kidney disease may facilitate the individualization of medication management in this patient population.

Analysis of Sepik populations of Papua New Guinea suggests an increase of CYP2C19 null allele frequencies during the colonization of Melanesia

The cytochrome P450 (CYP) isozyme CYP2C19 metabolizes clinically important drugs, including the anti-malarial proguanil currently used for multi-drug resistant Plasmodium falciparum malaria. CYP2C19 activity varies among geographical regions due to high frequencies of two null alleles (CYP2C19*2/*3) in Asian and especially Pacific populations. Previously, we reported an unprecedentedly high frequency of CYP2C19 poor metabolizers (PM) within populations of Vanuatu, which suggested even higher PM frequencies in Papua New Guinea. We examined CYP2C19 allele frequencies of three malarious populations from inland East Sepik Province, Papua New Guinea to evaluate this prediction and the use of proguanil in malaria treatment programs. These Papua New Guinean populations have PM frequencies intermediate between island South-east Asia and Vanuatu, most likely resulting from genetic drift during the settlement of the Pacific. This study highlights the medical consequences of population origins and the need for a better understanding of the genetic diversity of our global species.

Pharmacogenetics of cytochrome p4502D6: genetic background and clinical implication

Interindividual differences in the pharmacokinetics of a number of drugs are often due to hereditary polymorphisms of drug-metabolizing enzymes. Most important is cytochrome p4502D6 (CYP2D6), also known as debrisoquine/sparteine hydroxylase. It catalyzes hydroxylation or demethylation of more than 20% of drugs metabolized in the human liver, such as neuroleptics, antidepressants, some beta-blockers and many others like codeine. About 7%-10% of Caucasians lack any CYP2D6 activity due to deletions and frame-shift or splice-site mutations of the gene. About 1%-3% of Middle-Europeans, but up to 29% of Ethiopians display gene duplications, leading to elevated so-called ultrarapid metabolization rates. Meanwhile there is now a much better understanding of the genetic background of poor, intermediate, extensive and ultrarapid metabolizers, enabling a more precise DNA genotyping-based prediction of plasma levels. Since there is evidence that deteriorated drug elimination partly accounts for drug side-effects, CYP2D6 genotyping could contribute to an individualized and therefore optimized drug therapy.

Genotype???phenotype associations for common CYP3A4 and CYP3A5 variants in the basal and induced metabolism of midazolam in European- and African-American men and women

CYP3A activity in adults varies between individuals and it has been suggested that this has a genetic basis, possibly related to variant alleles in CYP3A4 and CYP3A5 genes. Accordingly, genotype-phenotype associations were investigated under constitutive and induced conditions. Midazolam's systemic and oral clearances, and the erythromycin breath test (ERBT) were determined in 57 healthy subjects: 23 (11 men, 12 women) European- and 34 (14 men, 20 women) African-Americans. Studies were undertaken in the basal state and after 14-15 days pretreatment with rifampin. DNA was characterized for the common polymorphisms CYP3A4*1B, CYP3A5*3, CYP3A5*6 and CYP3A5*7 by direct sequencing, and for exon 21 and exon 26 variants of MDR1 by allele-specific, real-time polymerase chain reaction. In 95% of subjects, the basal systemic clearance of midazolam was unimodally distributed and variability was less than four-fold whereas, in 98% of the study population, oral clearance varied five-fold. No population or sex-related differences were apparent. Similar findings were observed with the ERBT. Rifampin pretreatment markedly increased the systemic (two-fold) and oral clearance (16-fold) of midazolam, and the ERBT (two-fold) but the variabilities were unchanged. No associations were noted between these phenotypic measures and any of the studied genotypes, except for oral clearance and its fold-increase after rifampin. These were related to the presence of CYP3A4*1B and the inversely linked CYP3A5*3 polymorphism, with the extent of induction being approximately 50% greater in CYP3A5*3 homozygotes compared to wild-type subjects. In most healthy subjects, variability in intestinal and hepatic CYP3A activity, using midazolam as an in-vivo probe, is modest and common polymorphisms in CYP3A4 and CYP3A5 do not appear to have important functional significance.

Potential inhibition of cytochrome P450 3A4 by propofol in human primary hepatocytes

AIM: Hepatic cytochrome P450 isoenzymes constitute a superfamily of hemoproteins that play a major role in the metabolism of endogenous compounds and in the detoxification of xenobiotic molecules. P450 3A4 is one of the most important forms in human being, and mediates the metabolism of around 70% of therapeutic drugs and endogenous compounds. Propofol, a widely used intravenous anesthetic drug, is known to inhibit cytochrome P450 activities in isolated rat hepatocytes. The goal of this study was to evaluate the potential efficacy of propofol on P450 3A4 in a dose-dependent manner to understand its drug-drug interaction.

METHODS: Hepatocytes were isolated from liver specimens from hepatic angioma patients undergone hepatic surgery. Primary incubated hepatocytes were treated with 0, 0.01, 0.05, 0.1, 0.5, and 1.0 mM propofol for 24 hours. P450 3A4 activity was measured with Nash’s colorimetry. The protein expression was assessed by Western blot analysis.

RESULTS: A dose-dependent inhibitory effect of propofol was observed in cytochrome P450 3A4 activity. A minimal dosage of propofol (0.01 mM) induced a significant inhibition of P450 3A4 activity, although its regular dosages (0.01-0.1 mM) showed no inhibitory effect on the cellular protein expression of P450 3A4.

CONCLUSION: Propofol may be a potential CYP3A4 inhibitor as this anesthetic can inhibit isoenzyme activity significantly and reduce the metabolic rate of CYP3A4 substrates. This inhibition occurs at post-expression level, and concentration of propofol used clinically does not affect CYP3A4 protein expression. propofol may thus induce drug interaction of cytochrome P450 3A4 activity at the dosage used clinically.

Identification of cytochrome p450 enzymes involved in the metabolism of 4'-methyl-alpha-pyrrolidinopropiophenone, a novel scheduled designer drug, in human liver microsomes

4'-Methyl-alpha-pyrrolidinopropiophenone (MPPP) is a new drug of abuse. It is believed to have an abuse potential similar to that of amphetamines. Previous studies with Wistar rats had shown that MPPP was metabolized mainly by hydroxylation in position 4' followed by dehydrogenation to the corresponding carboxylic acid. The aim of the study presented here was to identify the human hepatic cytochrome p450 (p450) enzymes involved in the biotransformation of MPPP to 4'-hydroxymethyl-pyrrolidinopropiophenone. Baculovirus-infected insect cell microsomes and human liver microsomes were used for this purpose. Only CYP2C19 and CYP2D6 catalyzed this hydroxylation. The apparent Km and Vmax values for the latter were 9.8 +/- 2.5 microM and 13.6 +/- 0.7 pmol/min/pmol p450, respectively. CYP2C19 was not saturable over the tested substrate range (2-1000 microM) and interestingly showed a biphasic kinetic profile with apparent Km,1 and Vmax,1 values of 47.2 +/- 12.5 microM and 8.1 +/- 1.4 pmol/min/pmol p450, respectively. Experiments with pooled human liver microsomes also revealed biphasic nonsaturable kinetics with apparent Km,1 and Vmax,1 values of 57.0 +/- 20.9 microM and 199.7 +/- 59.7 pmol/min/mg of protein for the high affinity enzyme, respectively. Incubation of 2 microM MPPP with 3 microM of the CYP2D6-specific inhibitor quinidine resulted in significant (p < 0.01) turnover inhibition (11.8 +/- 1.6% of control). Based on kinetic data corrected for the relative activity factors, CYP2D6 is the enzyme mainly responsible for MPPP hydroxylation, confirmed by CYP2D6 inhibition studies.

A discordance between cytochrome P450 2D6 genotype and phenotype in patients undergoing methadone maintenance treatment

AIMS

To assess CYP2D6 activity and genotype in a group of patients undergoing methadone maintenance treatment (MMT).

METHODS

Blood samples from 34 MMT patients were genotyped by a polymerase chain reaction-based method, and results were compared with CYP2D6 phenotype (n = 28), as measured by the molar metabolic ratio (MR) of dextromethorphan (DEX)/dextrorphan (DOR) in plasma.

RESULTS

Whereas 9% of patients (3/34) were poor metabolizers (PM) by genotype, 57% (16/28) were PM by phenotype (P < 0.005). Eight patients, who were genotypically extensive metabolizers (EM), were assigned as PM by their phenotype. The number of CYP2D6*4 alleles and sex were significant determinants of CYP2D6 activity in MMT patients, whereas other covariates (methadone dose, age, weight) did not contribute to variation in CYP2D6 activity.

CONCLUSIONS

There was a discordance between genotype and in vivo CYP2D6 activity in MMT patients. This finding is consistent with inhibition of CYP2D6 activity by methadone and may have implications for the safety and efficacy of other CYP2D6 substrates taken by MMT patients.

Genetic polymorphism of CYP2D6 in Chinese subjects in Malaysia

BACKGROUND

Although Malaysian Chinese share an origin with the mainland Chinese, their evolution has been influenced by intermarriages. With a gene such as CYP2D6, which is highly polymorphic, it is expected that the Malaysian Chinese would exhibit a polymorphism profile different from those of the Chinese populations in other geographical locations.

OBJECTIVE

To study the genotype distribution of CYP2D6 among the Chinese people in Malaysia.

METHOD

We obtained DNA from 236 Chinese individuals in Malaysia and used PCR-based methods to identify any common CYP2D6 alleles.

RESULTS

A total of 236 subjects were enrolled and were successfully genotyped. Malaysian Chinese were relatively heterogeneous in terms of their CYP2D6 genotypes with nine genotypes recorded. CYP2D6*4, *5, *9, *10 and *17 were detected with the most common genotype being *1/*10. No subject had genotypes that predicted poor metabolic activity. However, 40% showed genotypes (e.g. CYP2D6*10/*10, *17, *4 and *9 and *9/*9) that predicted an intermediate metabolizer phenotype. Another subject carried the defective CYP2D6*17 allele and six carried the defective CYP2D6*9 allele. Both these alleles have not been reported in other earlier Chinese studies.

CONCLUSION

This study revealed that, in terms of CYP2D6 polymorphism, Malaysian Chinese were a heterogeneous group of people. Although sharing some similarities with other Orientals, they also seemed to have some notable differences. The alleles CYP2D6*4, *5, *9, *10 and *17 were all detected. CYP2D6*3 was however absent.

Rapid detection of the known SNPs of CYP2C9 using oligonucleotide microarray

AIM: Cytochrome P450 2C9 (CYP2C9) is a polymorphic enzyme responsible for the metabolism of a large number of clinically important drugs. Individuals with mutant enzymes may risk serious side effects under routine therapy with certain drugs metabolized by CYP2C9. In order to facilitate the detection of the known SNPs of CYP2C9, an allele-specific oligonucleotide (ASO) based microarray was made.

METHODS: An oligonucleotide microarray was made to facilitate the SNP (single nucleotide polymorphism) screening and was applied for the detection of CYP2C9 polymorphism in 62 high blood pressure (HBP) patients who received Irbesartan for treatment. Part of the genotyping results was confirmed by direct sequencing. And the relation between CYP2C9 polymorphism and therapeutic outcome of Irbesartan was statistically analyzed.

RESULTS: Heterozygous alleles of CYP2C9*1/*3 were found in 7 out of 62 subjects. No mutant alleles of CYP2C9*2, *4 and *5 and no homozygous mutant alleles were detected. The 7 heterozygous CYP2C9*1/*3 and 13 random wild type DNA samples were subjected to direct sequencing with purified PCR products and same genotyping results were obtained with the 20 DNA samples. There was no significant difference in the odds of effectiveness of Irbesartan between the wild type (normal) group and CYP2C9*1/*3 (mutant) group (P > 0.05).

CONCLUSION: The oligonucleotide microarray made in this study is a reliable assay for detecting the CYP2C9 known alleles and the heterozygous CYP2C9*1/*3 has no significant effects on the therapeutic outcome of Irbesartan.

Adverse drug reaction monitoring in Jena. Relevance of polymorphic drug metabolizing enzymes for inducing adverse drug reactions

Inter-subject variability in therapeutic drug response and drug toxicity is a major problem in clinical practice. In this field genetic polymorphisms of drug metabolizing enzymes play an important role. In a multicenter study supported by the German Federal Institute for Drugs and Medical Devices (BfArM, Z 12.01-68502-201) adverse drug reactions (ADRs) leading to hospital admission to departments of internal medicine have been registered and evaluated. The aim of the presented part of the study was to look for evident differences in genotypes for polymorphic drug metabolizing enzymes between adverse drug reaction cases and controls. All cases found in the local area--Jena and Weimar--were genotyped for N-acetyl-transferase 2 (NAT2), cytochrom P450 (CYP) 2D6 and 2C19 in comparison to a control population of the same region. The investigation on genotype was carried out for about 2 years (2000-2002). 254 blood samples from patients of the ADR study were analyzed. The genotype of drug metabolizing enzymes was determined by means of polymerase chain reaction using allel specific primers or restriction enzyme analysis. Within all ADRs cases genotyped, no exceptional frequencies for slow acetylators or poor metabolizers (PM) of CYP2D6 or CYP2C19 were found. About 65% of the individuals with ADR genotypically displayed a slow acetylator state. 6.3% PM for CYP2D6, including CYP2D6*3, *4 and *6 alleles, and 2.0% PM frequency for CYP2C19 (*2) have been found in ADR cases. A direct connection between PM genotype and the ADR observed may be assumed only in few of them. Further investigations on genotype and ADR-associated drugs require a much larger sample of patients to obtain more data allowing to focus an association on specific drugs, ADR and polymorphisms genotype of drug metabolizing enzymes might be useful.

Bioactivation of cyclophosphamide: the role of polymorphic CYP2C enzymes

Several-fold differences have been observed among patients in the biotransformation of cyclophosphamide. The aim of this study was to investigate the contribution of CYP2C9 and CYP2C19 and their polymorphisms to the variability of cyclophosphamide activation. The formation of 4-hydroxycyclophosphamide was studied in microsomes from a total of 32 different genotyped human livers, as well as in yeast microsomes expressing different genetic variants of CYP2C9 and CYP2C19. The kinetic data obtained in the yeast system revealed that the intrinsic clearance (V(max)/K(m)) of cyclophosphamide by CYP2C9.2 and CYP2C9.3 samples was approximately threefold lower than that by CYP2C9.1. However, in liver microsomes, there were no statistically significant differences in the intrinsic clearance of 4-hydroxycyclophosphamide formation between the group of seven CYP2C9*1/*1 livers and the remaining nine with one or two variant CYP2C9 alleles ( P>0.7). We found a statistically significant correlation ( r(s)=0.65, P=0.003) between 4-hydroxylation of cyclophosphamide and 5'-hydroxylation of R-omeprazole, a measure of CYP2C19 activity in human liver microsomes ( n=19). No correlation was found between 4-hydroxylation of cyclophosphamide and the formation rate of hydroxycelecoxib, mainly catalysed by CYP2C9 ( r(s)=0.17, P=0.55, n=32). In conclusion, based on the correlation with the formation of R-5'-hydroxyomeprazole, CYP2C19 may partly contribute to the bioactivation of cyclophosphamide in human liver microsomes, while the role of CYP2C9 appears minor.

Strategies and molecular probes to investigate the role of cytochrome P450 in drug metabolism: focus on in vitro studies

Drug metabolism is the major determinant of drug clearance and, because of polymorphic or inducible expression of drug-metabolising cytochrome P450s (CYPs), is the factor most frequently responsible for interindividual differences in pharmacokinetics. A number of well characterised CYP substrates and inhibitors have been identified that allow precise measurements of individual CYP isoforms. Their use, alone or in combination, facilitates the phenotype characterisation of hepatocytes in vitro and in vivo. Two procedures are used for in vitro investigation of the metabolic profile of a drug: incubation with microsomes and incubation with metabolically competent cells. The major limitation of microsomes is that they express phase I activities, but only part of phase II activities, and can only be used for short incubation times. When intact cells are used, gene expression, metabolic pathways, cofactors/enzymes and plasma membrane are largely preserved, but fully differentiated cells such as primary cultured hepatocytes need to be used, since hepatoma cell lines have only very low and partial CYP expression. CYP-engineered cells or their microsomes ('supersomes') have made the identification of the CYPs involved in the metabolism of a drug candidate straightforward and easier. Inhibition of CYP is an undesirable feature for a drug candidate, and needs to be addressed by examining whether the drug candidate inhibits the metabolism of other compounds or whether other compounds inhibit the metabolism of the drug candidate. Such experiments can be conducted both with microsomes and in cells. The major limitation of microsomes is that inhibition parameters may not accurately reflect the situation in vivo, since the contribution of drug transport is not considered. The best picture of a potential drug-drug interaction can be obtained in metabolically competent hepatocytes. Screening of CYP inducers cannot be done in microsomes. It requires the use of a cellular system fully capable of transcribing and translating CYP genes, and can be monitored in vitro as an increase in enzyme mRNA or activity. Human hepatocytes in primary culture respond well to enzyme inducers during the first few days; this ability is lost thereafter. Rat hepatocytes are much less stable and soon become unresponsive to inducers. Hepatoma cell lines respond poorly to inducers, although the induction of a few isoenzymes has been reported. Primary cultured hepatocytes are still the unique in vitro model that allows global examination of the inductive potential of a drug.

The use of proton pump inhibitors in children: a comprehensive review

Proton pump inhibitors (PPIs) belong to a group of chemically related compounds whose primary function is the inhibition of acid production in the final common metabolic pathway of gastric parietal cells. PPIs are highly selective and effective in their action and have few short- or long-term adverse effects. These pharmacologic features have made the development of PPIs the most significant advancement in the management of acid peptic related disorders in the last two decades. There are numerous published adult studies that describe the pharmacology, efficacy and safety of these anti-secretory agents; however, in the pediatric population, there are very few comparable studies, particularly multicenter studies with significant patient enrollment. In preparing this article, our aim was to perform a comprehensive review of the literature on the clinical pharmacology and use of PPIs in the pediatric population, and to briefly review some recent articles. Relevant literature was identified by performing MEDLINE/Pubmed searches from January 1990 to December 2001. Combinations of the following search terms were use to analyze these databases: proton pump inhibitor, children, pediatrics, gastroesophageal reflux disease (GERD), esophagitis, intestinal metaplasia, Helicobacter pylori, omeprazole, lansoprazole, pantoprazole, rabeprazole, esomeprazole, and safety. Abstracts from the 14th annual conference of the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition (NASPGHAN) 2001, and the Disease and Digestive Week 2001, were also included in the review. All pediatric studies reviewed were limited to either omeprazole or lansoprazole. The dosage range used for the management of GERD and related disorders with lansoprazole was 0.73-1.66 mg/kg/day (maximum 30 mg/day). The dosage range for GERD management using omeprazole was 0.3-3.5 mg/kg (maximum 80 mg/day). The dosage range for omeprazole used for H. pylori was 0.5-1.5 mg/kg/day, with a maximum dosage of 40 mg/day, and lansoprazole-containing regimens for H. pylori eradication used dosages ranging from 0.6-1.2 mg/kg/day, with a maximum dosage of 30 mg/day. Few severe adverse events were reported with the use of either drug. Eradication rates for H. pylori were 56-87% for lansoprazole-based triple therapy, and 75-94% for omeprazole-based eradication regimens. To date, there are no published controlled trials of sufficient power comparing the efficacy of the five commercially available PPIs in children, for a variety of acid peptic diseases. Studies suggest that PPIs are highly effective for the management of GERD and related disorders, and are a critically needed component of triple therapy to eradicate H. pylori. PPIs have a very good tolerability profile in adults and children, but long-term tolerability studies are needed, particularly in the pediatric population. Multicenter studies are critically needed to evaluate the second-generation PPIs, to compare PPI efficacy to each other, and to assess the importance of developmental and genetic pharmacology of these drugs in children with acid-peptic disease.

Pharmacogenetics of the proton pump inhibitors: a systematic review

Cytochrome P450 (CYP) 2C19 mediates the major metabolic transformations of the proton pump inhibitors (PPIs) omeprazole, pantoprazole, lansoprazole, esomeprazole, and rabeprazole. Genetic polymorphism of CYP2C19 can lead to significant phenotypic variation in the activity of this isoenzyme and thus in the metabolism of PPIs. We systematically reviewed the pharmacogenetic studies of PPIs with respect to the effects of CYP2C19 polymorphism on the clinical outcomes of PPI therapy. We searched MEDLINE (January 1966-August 2002) and EMBASE (January 1988-August 2002) for English-language articles on the pharmacogenetics of PPIs; the search was supplemented by a bibliographic review of all relevant articles. Seventeen pertinent citations were identified, and the quality (level) of evidence for each was categorized according to the rating scale of the United States Preventive Services Task Force. We found that the relationship between CYP2C19 genetic polymorphism and clinical outcomes after PPI therapy has not yet been clearly delineated. Virtually all pharmacogenetic studies of PPIs have been performed in Japanese men; thus, the clinical relevance of CYP2C19 genetic polymorphism in non-Asian patients and women is unknown. Differences among dual- and triple-therapy drug regimens make it difficult to compare H. pylori eradication studies and assess their applicability to current practice patterns. Drug adherence, a pivotal factor in the success of eradication therapy, was addressed in only four trials. Future directions for research include performing more studies with larger sample sizes, particularly in non-Asian populations and women; measuring plasma PPI concentrations to directly correlate H. pylori infection and ulcer cure rates with plasma drug availability; expanding the study population to patients with gastroesophageal reflux disease; and exploring the influence of CYP3A4 in the success or failure of PPI therapy. Although CYP2C19 genotyping is currently only a research instrument, it may be a valuable clinical tool in select patients to ensure optimal PPI therapy.

Different alterations of cytochrome P450 3A4 isoform and its gene expression in livers of patients with chronic liver diseases

AIM: To determine whether parenchymal cells or hepatic cytochrome P450 protein was changed in chronic liver diseases, and to compare the difference of CYP3A4 enzyme and its gene expression between patients with hepatic cirrhosis and obstructive jaundice, and to investigate the pharmacologic significance behind this difference.

METHODS: Liver samples were obtained from patients undergoing hepatic surgery with hepatic cirrhosis (n = 6) and obstructive jaundice (n = 6) and hepatic angeioma (controls, n = 6). CYP3A4 activity and protein were determined by Nash and western bloting using specific polychonal antibody, respectively. Total hepatic RNA was extracted and CYP3A4cDNA probe was prepared according the method of random primer marking, and difference of cyp3a4 expression was compared among those patients by Northern blotting.

RESULTS: Compared to control group, the CYP3A4 activity and protein in liver tissue among patients with cirrhosis were evidently reduced. (P < 0.01) Northern blot showed the same change in its mRNA levels. In contrast, the isoenzyme and its gene expression were not changed among patients with obstructive jaundice.

CONCLUSION: Hepatic levels of P450s and its CYP3A4 isoform activity were selectively changed in different chronic liver diseases. CYP3A4 isoenzyme and its activity declined among patients with hepatic cirrhosis as expression of cyp3a4 gene was significantly reduced. Liver's ability to eliminate many clinical therateutic drug substrates would decline consequently, These findings may have practical implications for the use of drugs in patients with cirrhosis and emphasize the need to understand the metabolic fate of therapeutic compounds. Elucidation of the reasons for these different changes in hepatic CYP3A4 may provide insight into more fundamental aspects and mechanisms of imparied liver function.

Stereoselective determination of the CYP2C19 probe drug mephenytoin in human urine by gas chromatography-mass spectrometry

A sensitive, specific and reproducible gas chromatographic assay utilizing mass-selective detection has been developed for the stereoselective determination of mephenytoin (MP) in human urine. Following extraction of urine samples using methyl tert.-butyl ether, separation of R- and S-MP was achieved with a chiral capillary column; detection and quantitation were accomplished by mass spectrometry in the single ion monitoring mode (m/z 104 and 189). Excellent linearity was observed for both enantiomers over the concentration range of 5-1000 ng/ml with corresponding correlation coefficients (r)>0.99. The intra- and inter-day precision and accuracy were within +/-5%. This method employs a simplified processing procedure, demonstrates improved extraction recovery, and provides at least 5-fold greater sensitivity than previously reported assays. This method is well suited for the phenotypic evaluation of CYP2C19 activity using mephenytoin.

Pharmacokinetics of fluvoxamine in relation to CYP2C19 phenotype and genotype

OBJECTIVE

To evaluate the pharmacokinetics of fluvoxamine (FLV) in poor metabolizers (PMs) versus extensive metabolizers (EMs) of cytochrome P450 (CYP)2C19.

METHODS

This was a prospective, open-label study conducted at the Clinical Research Unit School of Pharmacy. Fifty-seven healthy, nonsmoking volunteers aged 21-40 years participated. Subjects abstained from caffeinated products 12 hours prior to and during each testing period. To assess CYP2C19 activity, blood samples were collected from each subject prior to and two hours after a single dose of omeprazole 20 mg. Once PMs were identified, a sample population of EMs were selected for comparison between the two groups regarding FLV disposition. A single 100 mg FLV dose was given to EMs and PMs; blood samples for FLV analysis were obtained prior to drug administration and 0.5, 1, 2, 3, 4 6, 8, 12 and 24 hours later. A blood sample one day prior to FLV administration was also obtained for CYP2C 19 and CYP2D6.

RESULTS

Four PMs were identified with the omeprazole phenotype probe and had a mean +/- SD hydroxylation index of 1.335 +/- 0.271. Nine EMs were selected based upon a hydroxylation index between 0.100 and 0.400 (mean 0.193 +/- 0.079). FLV pharmacokinetic parameters (AUC, elimination half-life, Cmax and Tmax) did not significantly differ between the two groups. Genotype analysis for CYP2C19 revealed a mutant allele for the *2 which confirmed phenotype detection of PM status. Genotype analysis for CYP2D6*3 and *4 alleles showed that all PMs of CYP2C19 were EMs of

CONCLUSIONS

FLV disposition and dosing is unlikely to be affected by CYP2C19 polymorphism.

Clinical significance of the cytochrome P450 2C19 genetic polymorphism

Cytochrome P450 2C19 (CYP2C19) is the main (or partial) cause for large differences in the pharmacokinetics of a number of clinically important drugs. On the basis of their ability to metabolise (S)-mephenytoin or other CYP2C19 substrates, individuals can be classified as extensive metabolisers (EMs) or poor metabolisers (PMs). Eight variant alleles (CYP2C19*2 to CYP2C19*8) that predict PMs have been identified. The distribution of EM and PM genotypes and phenotypes shows wide interethnic differences. Nongenetic factors such as enzyme inhibition and induction, old age and liver cirrhosis can also modulate CYP2C19 activity. In EMs, approximately 80% of doses of the proton pump inhibitors (PPIs) omeprazole, lansoprazole and pantoprazole seem to be cleared by CYP2C19, whereas CYP3A is more important in PMs. Five-fold higher exposure to these drugs is observed in PMs than in EMs of CYP2C19, and further increases occur during inhibition of CYP3A-catalysed alternative metabolic pathways in PMs. As a result, PMs of CYP2C19 experience more effective acid suppression and better healing of duodenal and gastric ulcers during treatment with omeprazole and lansoprazole compared with EMs. The pharmacoeconomic value of CYP2C19 genotyping remains unclear. Our calculations suggest that genotyping for CYP2C19 could save approximately 5000 US dollars for every 100 Asians tested, but none for Caucasian patients. Nevertheless, genotyping for the common alleles of CYP2C19 before initiating PPIs for the treatment of reflux disease and H. pylori infection is a cost effective tool to determine appropriate duration of treatment and dosage regimens. Altered CYP2C19 activity does not seem to increase the risk for adverse drug reactions/interactions of PPIs. Phenytoin plasma concentrations and toxicity have been shown to increase in patients taking inhibitors of CYP2C19 or who have variant alleles and, because of its narrow therapeutic range, genotyping of CYP2C19 in addition to CYP2C9 may be needed to optimise the dosage of phenytoin. Increased risk of toxicity of tricyclic antidepressants is likely in patients whose CYP2C19 and/or CYP2D6 activities are diminished. CYP2C19 is a major enzyme in proguanil activation to cycloguanil, but there are no clinical data that suggest that PMs of CYP2C19 are at a greater risk for failure of malaria prophylaxis or treatment. Diazepam clearance is clearly diminished in PMs or when inhibitors of CYP2C19 are coprescribed, but the clinical consequences are generally minimal. Finally, many studies have attempted to identify relationships between CYP2C19 genotype and phenotype and susceptibility to xenobiotic-induced disease, but none of these are compelling.

Genotyping of drug targets: a method to predict adverse drug reactions?

In the last decades, advances in molecular biology have led to modern pharmacogenetics, which started as a science that focused on investigating drug metabolising enzymes and genetic determinants of pharmacokinetic variability. As more evidence has become available on the structure of drug targets and the genes coding for them, increasing attention has been directed towards pharmacodynamic explanations of variability in therapeutic response as well as in the risk for adverse drug reactions. Traditionally, genetic drug safety research has focused on variations in single genes whose functions are known to be related to given adverse drug reactions. A few such examples, malignant hyperthermia, the long QT syndrome, venous thromboembolic disease, tardive dyskinesia, and drug addiction, are presented in this article. In the future, results from the Human Genome Project together with tools such as DNA microarray technology, high-output screening systems and advanced bioinformatics, will permit a more thorough elucidation than is currently possible of the genetic components of adverse drug reactions. By screening for a large number of single nucleotide polymorphisms (SNPs), SNP patterns associated with adverse drug reactions can be discovered even though the functions of the SNPs as such are completely unknown. On the basis of these findings, it can be expected that pharmacogenetic research will identify situations where a drug should be avoided in certain individuals in order to reduce the risk for adverse drug reactions. If so, it will be feasible to use molecular diagnostics to select drugs that are safe for the individual patient.

Correlation of tramadol pharmacokinetics and CYP2D6*10 genotype in Malaysian subjects

The aim of the present study is to investigate the influence of the CYP2D6*10 allele on the disposition of tramadol hydrochloride in Malaysian subjects. A single dose of 100 mg tramadol was given intravenously to 30 healthy orthopaedic patients undergoing various elective surgeries. After having obtained written informed consents, patients were genotyped for CYP2D6*10: the most common CYP2D6 allele among Asians by means of allele-specific polymerase chain reaction. The presence of other mutations (CYP2D6*1, *3, *4, *5, *9 and *17) was also investigated. Tramadol was extracted from 1 ml serum with an n-hexane: ethylacetate combination (4:1) after alkalinisation with ammonia (pH 10.6). Serum concentrations were measured by means of high-performance liquid chromatography. The pharmacokinetics of tramadol was studied during the 24 h after the dose. As among other Asians, the allele frequency for CYP2D6*10 among Malaysians was high (0.43). Subjects who were homozygous for CYP2D6*10 had significantly (P=0.046) longer mean serum half-life of tramadol than subjects of the normal or the heterozygous group (Kruskal-Wallis test). When patients were screened for the presence of other alleles, the pharmacokinetic parameter values were better explained. CYP2D6 activity may play a main role in determining tramadol pharmacokinetics. The CYP2D6*10 allele particularly was associated with higher serum levels of tramadol compared with the CYP2D6*1 allele. However, genotyping for CYP2D6*10 alone is not sufficient to explain tramadol disposition.

Biologic and molecular mechanisms for sex differences in pharmacokinetics, pharmacodynamics, and pharmacogenetics: Part I

There are pharmacological differences between women and men that have important clinical consequences. For several drugs, there is a higher incidence in women of drug-induced QT prolongation and a potentially fatal arrhythmia, torsades de pointes. This may be a reflection of the longer baseline QT interval in women. A difference in cardiovascular disease between women and men is that women have a higher mortality rate after myocardial infarction (MI). Women also have a higher rate of hemorrhagic stroke after receiving thrombolytic therapy for an MI. Differences in effectiveness of analgesics have been demonstrated, with kappa opioids providing pain relief for women but not men. Drugs may have different pharmacokinetics in women and men because of differences in phase I and phase II enzymes that metabolize drugs. Conflicting results about biological sex differences have been reported for the major drug metabolizing enzyme, cytochrome P450 3A4 (3A4) and may be related to a role for P-glycoprotein, a cell membrane transporter, reported as two times higher in male livers than those of females. It has been reported that boys need a higher dose of 6-mercaptopurine, which is metabolized by thiopurine methyltransferase (TPMT). TPMT is reported to be 14% higher in male human liver biopsies than those from females. Verapamil, a drug for angina and hypertension, has different clearance and side effects in men and women. Ethnic/racial variations have also been demonstrated with the drug metabolizing enzymes, CYP2C9, 2C19, and 2D6.

Explaining interindividual variability of docetaxel pharmacokinetics and pharmacodynamics in Asians through phenotyping and genotyping strategies

PURPOSE

To explain the variability of docetaxel pharmacokinetics through study of CYP3A phenotype and genotype, and MDR1 genotype.

PATIENTS AND METHODS

We studied the pharmacokinetics and pharmacodynamics of docetaxel in patients in whom it was indicated and who had not received known CYP3A4 substrates. Midazolam was administered intravenously to these patients at least 2 days before docetaxel treatment, and systemic clearances of both drugs were correlated. Patients were characterized for polymorphisms in the CYP3A4 promoter region, CYP3A5, and the C3435T polymorphism of MDR1.

RESULTS

Thirty-two patients were enrolled, of whom 31 had full pharmacokinetic data sets. Docetaxel clearance correlated with midazolam clearance, body-surface area, serum albumin, and performance status. Docetaxel and midazolam clearances were normally distributed. In multiple linear regression analyses, midazolam clearance and performance status were the only significant covariates of docetaxel clearance, and the area under the curve of docetaxel, serum levels of alpha-1-acid glycoprotein, and ALT were significant predictors of nadir neutrophil count. No polymorphisms were detected in the 5' regulatory region of CYP3A4. Nine patients of 25 studied were homozygous for the CYP3A5*3 genotype, and had lower mean clearance of midazolam but not docetaxel. The T/T genotype at the C3435T of MDR1, which is associated with reduced P-glycoprotein function, was found in eight of 27 patients.

CONCLUSION

Midazolam may be used as a probe drug for CYP3A activity to predict docetaxel clearances, hence reducing interindividual variability. Homozygotes for CYP3A5*3 and C3435T of MDR1 are common in our population, and their effects on pharmacokinetics of relevant substrates should be studied further.

Rapid and Reliable Method for Cytochrome P450 2D6 Genotyping

Background: Single-nucleotide polymorphisms and single-base deletions within the cytochrome P450 2D6 (CYP2D6) gene have been associated with a poor metabolizer (PM) phenotype and display a frequency of 7–10% in the Caucasian population.

Methods: We developed a reliable and rapid procedure to identify five major PM-associated mutations (CYP2D6*4, *7, and *8) and deletions (CYP2D6*3 and *6) by real-time PCR with subsequent fluorometric melting point analysis of the PCR product. These polymorphisms within the CYP2D6 gene were detected by use of two primer pairs and five different pairs of hybridization probes. DNA extracted from whole blood of 323 individuals was analyzed, and results were compared with genotypes obtained by allele-specific multiplex PCR. In case of uncertain results, additional sequence analysis was performed.

Results: Genotyping results by real-time PCR were 100% reliable, whereas conventional allele-specific multiplex PCR produced uncertain results for 12.1% of samples, as confirmed by sequence analysis. Costs for reagents and consumables were considerably higher for the real-time PCR technology, but labor time was reduced by 2 h compared with allele-specific PCR. The allele frequencies in the population investigated were 0.186 for allele *4, 0.026 for allele *5, 0.009 for allele *3, 0.031 for allele *6, and 0.002 for allele *8. The defective CYP2D6*7 allele was not found. In addition, three additional mutations were detected, one of them displaying a PM genotype.

Conclusion: Genotyping of CYP2D6 by real-time PCR with fluorometric melting point analysis is a rapid and reliable method.

Comparison of oral omeprazole and endoscopic ethanol injection therapy for prevention of recurrent bleeding from peptic ulcers with nonbleeding visible vessels or fresh adherent clots

OBJECTIVES

Omeprazole is a potent inhibitor of gastric acid secretion. Recently it was reported that p.o. omeprazole therapy reduced the rebleeding rate in patients with nonbleeding visible vessels or adherent clots. The aim of this study was to ascertain whether p.o. administration of omeprazole can be an effective alternative to endoscopic injection therapy in peptic ulcers with stigmata of recent hemorrhage.

METHODS

A total of 101 patients who had peptic ulcer bleeding based on endoscopic findings of nonbleeding visible vessels or fresh adherent clots were randomly assigned to receive omeprazole (40 mg p.o. every 12 h) or endoscopic ethanol injection therapy.

RESULTS

Rebleeding rates were 22.9% (11 of 48) in the omeprazole group and 20.8% (11 of 53) in the endoscopic injection therapy group. The rebleeding rates of clinical significance were 14.6% and 13.2%, respectively. There was no significance difference in the rebleeding rate, requirement for surgery, total units of blood transfused, or mortality between the two groups.

CONCLUSIONS

Oral omeprazole administration is comparable to endoscopic ethanol injection therapy for prevention of rebleeding in patients with nonbleeding visible vessels or adherent clots.

Evidence for environmental influence on CYP2D6-catalysed debrisoquine hydroxylation as demonstrated by phenotyping and genotyping of Ethiopians living in Ethiopia or in Sweden

Black Africans show lower rates of CYP2D6- and CYP2C19-dependent drug metabolism compared to Caucasians of the same apparent genotype. To determine if environmental factors are responsible for this difference, the genotypes and phenotypes of CYP2D6 and CYP2C19 among Ethiopians living in Sweden (n = 70) were assessed and compared to our previously published data from Ethiopians living in Ethiopia (n = 114) and Swedish Caucasians (n = 134). There was no significant difference in CYP2C19 genotype or phenotype as assessed by mephenytoin between Ethiopians in Sweden or in Ethiopia. However, Swedes were significantly more rapid for CYP2C19 activity than both Ethiopian groups (P < 0.01). A comparison of the debrisoquine MR among individuals of the same CYP2D6 genotype revealed that Swedes exhibited the highest rate of debrisoquine metabolism, followed by Ethiopians in Sweden and Ethiopians in Ethiopia. The difference between the Ethiopian groups was significant (P < 0.02 using a univariate test ANOVA) and amounted to approximately 50% of the magnitude of the MR difference between Swedes and Ethiopians in Ethiopia. It is tempting to speculate that inhibitory dietary factors may explain the differences seen between the two Ethiopian groups and that these components in the past might have contributed to dietary stress-mediated selection of duplicated and multiduplicated active CYP2D6 genes, as frequently seen in Ethiopians. In conclusion, the results indicate a significant influence of environmental factors as an explanation for the difference in capacity for CYP2D6, but not CYP2C19 metabolism between Caucasians and Black Africans. Additional factors remain to be elucidated to fully explain the interethnic differences in CYP2D6 activity.

Treatment of codeine dependence with inhibitors of cytochrome P450 2D6

Codeine is O-demethylated by cytochrome P450 2D6 (CYP2D6) to form the more potent drug morphine, accounting for much of codeine's analgesic and dependence-producing properties. Because morphine production can be decreased by inhibition of CYP2D6, the authors hypothesized that CYP2D6 inhibition could be used to treat codeine dependence. A randomized, double-blind, placebo-controlled trial was conducted. All patients received brief behavioral therapy. Two weeks of baseline monitoring were followed by 8 weeks of daily treatment with fluoxetine or quinidine (two potent CYP2D6 inhibitors) or placebo. Thirty patients were assessed (all white, age 40 + 12 years using 127 + 79 mg/day of codeine [mean + SD]), and 17 entered treatment. Eight patients remained in the study by treatment week 8. Quinidine > fluoxetine > placebo inhibited CYP2D6 as reflected in the change of the O-demethylation of dextromethorphan, a specific CYP2D6 probe. At treatment week 8, placebo, quinidine, and fluoxetine reduced mean daily codeine intake by 57%, 56%, and 51% of baseline intake respectively; there was no difference among treatment groups. In this small sample, CYP2D6 inhibitors did not appear to have a useful role in the treatment of codeine dependence.

Pharmacogenetics of bipolar disorder

To review the pharmacogenetics of bipolar disorders, the authors searched databases for genetic association and linkage studies involving response to long-term prophylactic lithium treatment, as well as treatment with antidepressants or clozapine. Significant ethnic variations in the metabolism and efficacy of antidepressants, as well as clozapine, have been reported by several groups. Systematic studies suggest that that genetic factors affect the response to prophylactic lithium treatment. Numerous associations between the three traits of interest and candidate gene polymorphisms have been proposed. Among these, an association between the serotonin transporter gene and response to serotonin reuptake inhibitors appears robust. Considerable interest has also focused on serotonergic gene polymorphisms and response to clozapine. Response to pharmacotherapy in bipolar disorders may be mediated by genetic factors, but the role played by heritability is unknown.

The gastroprokinetic and antiemetic drug metoclopramide is a substrate and inhibitor of cytochrome P450 2D6

Metoclopramide is increasingly prescribed for conditions previously treated with cisapride, but its metabolic enzymology and drug interactions are poorly understood. Using human liver microsomes (HLMs) and recombinant human cytochromes P450 (P450), we identified the major route of metoclopramide oxidation and the P450 isoforms involved. We also documented the ability of metoclopramide to inhibit the P450 system, using isoform-specific substrate reaction probes of CYP1A2, 2C19, 2C9, 2D6, 2E1, and 3A4. Metoclopramide was predominantly N-dealkylated to monodeethylmetoclopramide, a metabolite that has not so far been described in humans. Formation rate of this metabolite followed Michaelis-Menten kinetics (K(m), 68 +/- 16 microM; V(max), 183 +/- 57 pmol/min/mg of protein; n = 3 HLMs). Of the isoform-specific inhibitors tested, 1 microM quinidine was a potent inhibitor of metoclopramide (25 microM) monodeethylation [by an average of 58.2%; range, approximately 38% (HL09-14-99) to 78.7% (HL161)] with K(i) values highly variable among the HLMs tested (K(i), mean +/- S.D., 2.7 +/- 2.8 microM; range, 0.15 microM in HL66, 2.4 microM in HL09-14-99, and 5.7 microM in HLD). Except troleandomycin, which inhibited metoclopramide metabolism in only one HLM (by approximately 23% in HL09-14-99), the effect of other inhibitors was minimal. Among the recombinant human P450 isoforms examined, monodeethylmetoclopramide was formed at the highest rate by CYP2D6 (V = 4.5 +/- 0.3 pmol/min/pmol of P450) and to a lesser extent by CYP1A2 (0.97 +/- 0.15 pmol/min/pmol of P450). The K(m) value derived (approximately 53 microM) was close to that from HLMs (68 microM). Metoclopramide is a potent inhibitor of CYP2D6 at therapeutically relevant concentrations (K(i) = 4.7 +/- 1.3 microM), with negligible effect on other isoforms tested. Further inhibition of CYP2D6 was observed when metoclopramide was preincubated with HLMs and NADPH-generating system before the substrate probe was added (maximum rate of inactivation, K(inact) = 0.02 min(-1), and the concentration required to achieve the half-maximal rate of inactivation, K'(i) = 0.96 microM), suggesting mechanism-based inhibition. Metoclopramide elimination is likely to be slowed in poor metabolizers of CYP2D6 or in patients taking inhibitors of this isoform, whereas metoclopramide itself could reduce the clearance of CYP2D6 substrate drugs.

CYP2D6 allele frequency in European Caucasians, Asians, Africans and their descendants

Over 40 cytochrome P450 (CYP) 2D6 allelic variants have been discovered thus far. The alleles may be classified on the basis of the level of activity for which they encode CYP2D6 enzymes, into functional, non-functional and reduced function groups. CYP2D6 allele frequency is known to vary amongst racial/ethnic groups. Generally, for European Caucasians and their descendants, the functional group of alleles are predominant, with a frequency of 71%. Non-functional alleles represent 26% of the variability, mainly CYP2D6*4. In Asians and their close descendants, functional alleles represent only ~ 50% of the frequency of CYP2D6 alleles. Asians and Pacific Islanders have a high frequency (median = 41%) of a reduced function allele, CYP2D6*10, contributing to the population shift to the right of metabolic rates indicating slower metabolism. Information concerning Amerindians from North (Canada), Central and South America indicate comparatively low frequencies of CYP2D6*10, perhaps a "founders" effect. The frequency of functional alleles in Africans and African Americans is also about 50%. Both Africans and African Americans have reduced function alleles representing 35% of allele variation, mainly CYP2D6*17. African Americans, however, have more than twice the median frequency of nonfunctional alleles compared with Africans (14.5% vs 6.3%). Non-functional and reduced function alleles represent about 50% of allele frequency in Black populations but a much greater variety than carried in Asians. Since alleles which encode for no or reduced functioning clearly affect metabolic activity of drugs mediated by CYP2D6, studies are needed in populations in which these alleles play a major role in order to assure optimal dosing recommendations are based on empirical pharmacogenetics.

Genetic polymorphisms and metabolism of endocrine disruptors in cancer susceptibility

Epidemiological studies have estimated that approximately 80% of all cancers are related to environmental factors. Individual cancer susceptibility can be the result of several host factors, including differences in metabolism, DNA repair, altered expression of tumor suppressor genes and proto-oncogenes, and nutritional status. Xenobiotic metabolism is the principal mechanism for maintaining homeostasis during the body's exposure to xenobiotics. The balance of xenobiotic absorption and elimination rates in metabolism can be important in the prevention of DNA damage by chemical carcinogens. Thus the ability to metabolize and eliminate xenobiotics can be considered one of the body's first protective mechanisms. Variability in individual metabolism has been related to the enzymatic polymorphisms involved in activation and detoxification of chemical carcinogens. This paper is a contemporary literature review on genetic polymorphisms involved in the metabolism of endocrine disruptors potentially related to cancer development.

Enantioselective analysis of citalopram and metabolites in adolescents

Studies of the antidepressant effect and pharmacokinetics of citalopram have been performed in adults, but the effects on children and adolescents have only been studied to a minor extent despite its increasing use in these age groups. The aim of this study was to investigate a group of adolescents treated for depression, with respect to the steady-state plasma concentrations of the enantiomers of citalopram and its demethylated metabolites desmethylcitalopram and didesmethylcitalopram. Moreover, the authors studied the genotypes for the polymorphic cytochrome P450 enzymes CYP2D6 and CYP2C19 in relation to the different enantiomers. The S/R ratios of citalopram and desmethylcitalopram found in this study of 19 adolescents were similar to studies involving older patients. The concentrations of the R-(-)- and S-(+)-enantiomers of citalopram and desmethylcitalopram were also in agreement with values from earlier studies, the R-(-)-enantiomer (distomer) being the major enantiomer. The results indicate that the use of oral contraceptives may have some influence on the metabolism of citalopram. This might be because of an interaction of the contraceptive hormones with the CYP2C19 enzyme.