Abstract:The cytochrome P450 enzyme CYP1A2 mediates the rate-limiting step in the metabolism of many drugs including theophylline, clozapine, and tacrine as well as in the bioactivation of procarcinogens. CYP1A2 activity shows both pronounced intra-and interindividual variability, which is, among other factors, related to smoking causing enzyme induction, to drug intake and to dietary factors which may result in induction or inhibition. In contrast to these exogenous factors, genetic influences on enzyme activity seem to be less pronounced. Therefore, phenotyping of CYP1A2, i.e. the determination of the actual activity of the enzyme in vivo, represents a useful approach both for scientific and clinical applications. CYP1A2 is almost exclusively expressed in the liver. Since liver tissue cannot be obtained for direct phenotyping, a probe drug which is metabolized by CYP1A2 has to be given. Proposed probe drugs include caffeine, theophylline, and melatonin. Caffeine is most often used because of the predominating role of CYP1A2 in its overall metabolism and the excellent tolerability. Various urinary, plasma, saliva, and breath based CYP1A2 caffeine metrics have been applied. While caffeine clearance is considered as the gold standard, the salivary or plasma ratio of paraxanthine to caffeine in a sample taken approximately 6 hr after a defined dose of caffeine is a more convenient, less expensive but also fully validated CYP1A2 phenotyping metric. CYP1A2 phenotyping is applied frequently in epidemiologic and drugdrug interaction studies, but its clinical use and usefulness remains to be established.The human cytochrome P450 enzyme CYP1A2 plays an important role in the metabolism of several clinically used drugs. It is one of the major P450 enzymes and accounts for approximately 13% of the total content of this enzyme group in the human liver (Shimada et al. 1994). CYP1A2 mRNA content shows an up to 40-fold variability between individuals (Schweikl et al. 1993) and corresponding variability of enzyme activity and drug metabolism (Potkin et al. 1994).Genotyping and phenotyping are the two methods that are used today to assess the in vivo activity of drug-metabolizing enzymes. Genotyping works well to predict enzyme activity if a major fraction of variability is attributable to known polymorphisms. Although it allows cost-and timeeffective characterization of many significant genes at once, it will fail to provide reasonably exact estimates of enzyme activity when other major factors of influence (e.g. liver disease, enzyme induction/inhibition) are prevalent. To a great extent, this applies to CYP1A2. Consequently, the optimal method of describing actual enzyme activity would be phenotyping with a carefully selected probe compound or endogenous substance which provides the most clinically

Phenotyping for drug metabolizing enzymes and transporters is used to assess quantitatively the effect of an intervention (e.g., drug therapy, diet) or a condition (e.g., genetic polymorphism, disease) on their activity. Appropriate selection of test drug and metric is essential to obtain results applicable for other substrates of the respective enzyme/transporter. The following phenotyping metrics are recommended based on the level of validation and on practicability: CYP1A2, paraxanthine/caffeine in plasma 6 h after 150 mg caffeine; CYP2C9, tolbutamide plasma concentration 24 h after 125 mg tolbutamide; CYP2C19, urinary excretion of 4'-OH-mephenytoin 0-12 h after 50 mg mephenytoin; CYP2D6, urinary molar ratio debrisoquine/4-OH-debrisoquine 0-8 h after 10 mg debrisoquine; and CYP3A4, plasma clearance of midazolam after 2 mg midazolam (all drugs given orally).

The present meta-analysis provides quantitative estimates on clinical potency of individual PPIs that may be helpful when switching between PPIs and for assessing the cost-effectiveness of specific PPIs. However, our estimates must be viewed with caution because only a limited dose range has been tested and not exactly the same study conditions were applied for the different substances.

Isoniazid is metabolized by the genetically polymorphic arylamine N-acetyltransferase type 2 (NAT2). A greater number of high-activity alleles are related to increased acetylation capacity and in some reports to low efficacy and toxicity of isoniazid. The objective of this study was to assess individual isoniazid exposure based on NAT2 genotype to predict a personalized therapeutic dose. Isoniazid was administered to 18 healthy Caucasians (age 30 ؎ 6 years, body weight 74 ؎ 10 kg, five women) in random order as a 200-mg infusion, a 100-mg oral, and a 300-mg oral single dose. For the assessment of NAT2 genotype, common single nucleotide polymorphisms identifying 99.9% of variant alleles were characterized. Noncompartmental pharmacokinetics and compartmental population pharmacokinetics were estimated from isoniazid plasma concentrations until 24 h postdose by high-pressure liquid chromatography. The influence of NAT2 genotype, drug formulation, body weight, and sex on dose-normalized isoniazid pharmacokinetics was assessed by analysis of variance from noncompartmental data and confirmed by population pharmacokinetics. Eight high-activity NAT2*4 alleles were identified. Sex had no effect; the other factors explained 93% of the variability in apparent isoniazid clearance (analysis of variance). NAT2 genotype alone accounted for 88% of variability. Individual isoniazid clearance could be predicted as clearance (liters/hour) ‫؍‬ 10 ؉ 9 ؋ (number of NAT2*4 alleles). To achieve similar isoniazid exposure, current standard doses presumably appropriate for patients with one high-activity NAT2 allele may be decreased or increased by approximately 50% for patients with no or two such alleles, respectively. Prospective clinical trials are required to assess the merits of this approach.

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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.

The increase of concentrations observed for many drugs when administered concomitantly with grapefruit juice was attributed to inhibition of cytochrome P450 enzymes by naringenin, the aglycone of the grapefruit flavonoid naringin. However, this explanation is equivocal, and formation of naringenin after ingestion of grapefruit juice has not been proved. We investigated renal excretion of naringin, naringenin, and its glucuronides after administration of 20 ml grapefruit juice (621 mumol/L naringin) per kilogram of body weight to six healthy adults. Urine was collected for 24 hours, and flavonoids were measured by HPLC in aliquots with and without glucuronidase pretreatment. Naringin or naringin glucuronides were not found. Naringenin and its glucuronides appeared in urine after a median lag-time of 2 hours and reached 0.012% to 0.37% and 5.0% to 57%, respectively, of the molar naringin dose. In additional investigations, low concentrations (< 4 mumol/L) of naringenin glucuronides, but neither naringin nor naringenin were found in plasma samples from previous grapefruit juice interaction studies, and metabolization of naringin to naringenin occurred during 24 hours of incubation (37 degrees C) in three of five feces samples tested. The data suggest that cleavage of the sugar moiety, presumably by intestinal bacteria, is the first step of naringin metabolism. Naringenin formation is thought to be the crucial step in determination of bioavailability of the compound, which undergoes rapid glucuronidation. The pronounced interindividual variability of naringin kinetics provides a possible explanation for some of the apparently contradictory results of drug interaction studies with grapefruit and naringin.

Doses of CYP1A2 substrates with a narrow therapeutic range should be decreased immediately on cessation of heavy smoking. As a rule of thumb, a stepwise daily dose reduction of approximately 10% until the fourth day after smoking cessation is proposed, which should be accompanied by therapeutic drug monitoring.