Omeprazole does not enhance the metabolism of phenacetin, a marker of CYP1A2 activity, in healthy volunteers

Hepatic Drug-Metabolizing Enzyme Induction and Implications for Preclinical and Clinical Risk Assessment

Hepatic drug metabolizing enzyme (DME) induction complicates the development of new drugs owing to altered efficacy of concomitant treatments, reduction in exposure resulting from autoinduction, and potential generation of toxic metabolites. Risk assessment of DME induction during clinical evaluation is confounded by several uncertainties pertaining to hazard identification and dose response analysis. Hepatic DME induction rarely leads to clinical evidence of altered metabolism and toxicity in the patient, which typically occur only if the DME induction is relatively severe. High drug doses are associated with a greater likelihood of hepatic DME induction and downstream effects; therefore, drugs of low potency requiring higher dosing tend to lead to a greater risk of drug–drug interactions. Vigilance in clinical trials for increased or diminished drug effect and, specifically, pharmacokinetic studies in the presence of other drugs and concomitant diseases are necessary for a drug risk assessment profile.

Efforts to remove hepatic DME-inducing drugs from development can be facilitated with current in vitro and in vivo assessments and will improve with the development of newer technologies. A carefully tailored case-by-case approach will lead to the development of efficacious drugs with an acceptable risk/benefit profile available to patients.

Proton pump inhibitors: actions and reactions

Proton pump inhibitors are the second most commonly prescribed drug class in the United States. The increased utilization of PPIs parallels the rising incidence of reflux disease. Owing to their clinical efficacy and relative lack of tachyphylaxis, PPIs have largely displaced H-2 receptor antagonists in the treatment of acid peptic disorders. The elevation of intragastric pH and subsequent alterations of gastric physiology induced by PPIs may yield undesired effects within the upper GI tract. The ubiquity of the various types of H(+), K(+)-ATPase could also contribute to non-gastric effects. PPIs may influence physiology in other ways, such as inducing transepithelial leak.

CYP1A Induction and Human Risk Assessment: An Evolving Tale of in Vitro and in Vivo Studies: TABLE 1

CYP1A1 and 1A2 play critical roles in the metabolic activation of carcinogenic polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic amines/amides (HAAs), respectively, to electrophilic reactive intermediates, leading to toxicity and cancer. CYP1As are highly inducible by PAHs and halogenated aromatic hydrocarbons via aryl hydrocarbon receptor-mediated gene transcription. The impact of CYP1A induction on the carcinogenic and toxic potentials of environmental, occupational, dietary, and therapeutic chemicals has been a central focus of human risk evaluation and has broadly influenced the fields of cancer research, toxicology, pharmacology, and risk assessment over the past half-century. From the early discovery of CYP1A induction and its role in protection against chemical carcinogenesis in intact animals, to the establishment of CYP1A enzymes as the principal cytochromes P450 for bioactivation of PAHs and HAAs in in vitro assays, to the recent realization of an essential protective role of CYP1A in benzo[a]pyrene-induced lethality and carcinogenesis with CYP1A knockout mice, the understanding of the interrelation between CYP1A induction and chemical safety has followed a full circle. This unique path of CYP1A research underscores the importance of whole animal and human studies in chemical safety evaluation.

Drug Interactions with Paracetamol

Paracetamol (acetaminophen) is one of the most commonly used analgesic antipyretic drugs worldwide, and it is widely available by prescription and over the counter (OTC). Fortunately, few clinically significant drug interactions have been documented. There is probable potentiation of hepatotoxicity following an overdose from the paracetamol metabolite NAPQI by enzyme-inducing drugs. There is considerable controversy regarding the possible interaction with warfarin in its potential to increase its anticoagulant effects because of discrepancies between observational studies and those in healthy volunteers. Otherwise, no serious adverse drug interactions with therapeutic doses of paracetamol have been confirmed in humans. Because the absorption of paracetamol is so dependent on gastric emptying, other drugs that alter gastric emptying can change its pharmacokinetics; but this would not cause serious adverse effects. Although animal experiments have demonstrated that many compounds can modify paracetamol hepatotoxicity, these are unlikely to be important at therapeutic doses.

Clinically significant pharmacokinetic interactions between dietary caffeine and medications

Caffeine from dietary sources (mainly coffee, tea and soft drinks) is the most frequently and widely consumed CNS stimulant in the world today. Because of its enormous popularity, the consumption of caffeine is generally thought to be safe and long term caffeine intake may be disregarded as a medical problem. However, it is clear that this compound has many of the features usually associated with a drug of abuse. Furthermore, physicians should be aware of the possible contribution of dietary caffeine to the presenting signs and symptoms of patients. The toxic effects of caffeine are extensions of their pharmacological effects. The most serious caffeine-related CNS effects include seizures and delirium. Other symptoms affecting the cardiovascular system range from moderate increases in heart rate to more severe cardiac arrhythmia. Although tolerance develops to many of the pharmacological effects of caffeine, tolerance may be overwhelmed by the nonlinear accumulation of caffeine when its metabolism becomes saturated. This might occur with high levels of consumption or as the result of a pharmacokinetic interaction between caffeine and over-the-counter or prescription medications. The polycyclic aromatic hydrocarbon-inducible cytochrome P450 (CYP) 1A2 participates in the metabolism of caffeine as well as of a number of clinically important drugs. A number of drugs, including certain selective serotonin reuptake inhibitors (particularly fluvoxamine), antiarrhythmics (mexiletine), antipsychotics (clozapine), psoralens, idrocilamide and phenylpropanolamine, bronchodilators (furafylline and theophylline) and quinolones (enoxacin), have been reported to be potent inhibitors of this isoenzyme. This has important clinical implications, since drugs that are metabolised by, or bind to, the same CYP enzyme have a high potential for pharmacokinetic interactions due to inhibition of drug metabolism. Thus, pharmacokinetic interactions at the CYP1A2 enzyme level may cause toxic effects during concomitant administration of caffeine and certain drugs used for cardiovascular, CNS (an excessive dietary intake of caffeine has also been observed in psychiatric patients), gastrointestinal, infectious, respiratory and skin disorders. Unless a lack of interaction has already been demonstrated for the potentially interacting drug, dietary caffeine intake should be considered when planning, or assessing response to, drug therapy. Some of the reported interactions of caffeine, irrespective of clinical relevance, might inadvertently cause athletes to exceed the urinary caffeine concentration limit set by sports authorities at 12 mg/L. Finally, caffeine is a useful and reliable probe drug for the assessment of CYP1A2 activity, which is of considerable interest for metabolic studies in human populations.

Possible enhancement of the first-pass metabolism of phenacetin by ingestion of grape juice in Chinese subjects

AIMS

This serendipitous study revealed an unexpected effect of Jufeng grape juice on the CYP1A2-mediated metabolism of phenacetin. Investigation of the inhibition of CYP1A2 by grapefruit juice was involved but a translation error led to the grape juice substitution.

METHODS

Twelve healthy subjects took a single oral dose of phenacetin (900 mg) on two randomized occasions together with 200 ml water or grape juice. Plasma phenacetin and paracetamol concentrations were assessed by h.p.l.c.

RESULTS

Ingestion of grape juice was associated with reduced plasma phenacetin concentrations, while paracetamol levels were unaffected. Paracetamol to phenacetin AUC ratios increased from 13.9+/-3.1 to 24.3+/-3.8 after ingestion of grape juice.

CONCLUSIONS

These findings suggest enhanced first-pass metabolism of phenacetin, due to CYP1A2 activation by grape juice or to desaturation of CYP1A2 isoenzymes secondary to a slower rate of phenacetin absorption.

Pharmacokinetic interaction between acetaminophen and lansoprazole

Because of its minimal gastric toxicity, acetaminophen is the analgesic of choice for patients with gastric acid-related disorders. Because proton pump inhibitors are widely used, concomitant prescription of acetaminophen and lansoprazole would be prevalent. This crossover study was conducted to investigate an acetaminophen-lansoprazole interaction. On one occasion, each of six healthy, fasted, male volunteers ingested 1.0 g acetaminophen dissolved in 200 mL water. On another occasion, at least 1 week apart, 30 mg lansoprazole was administered orally, simultaneously with acetaminophen, after pretreatment with the same dose of lansoprazole once daily for 2 days. Plasma acetaminophen concentrations were measured at 0, 0.25, 0.5, 0.75, 1, 2, 3, 5, and 8 hours after dosing. The peak plasma concentration of acetaminophen and the time to its occurrence were significantly higher and shorter, respectively, during the lansoprazole session than during the control session. Neither the elimination half-life nor the area under the curve was significantly different between the two sessions. Lansoprazole hastens the absorption of acetaminophen solution, but little modifies its elimination rate and bioavailability.

Effect of active and passive cigarette smoking on CYP1A2-mediated phenacetin disposition in Chinese subjects

The effect of active and passive cigarette smoking on CYP1A2-mediated phenacetin disposition was evaluated in a controlled study of 36 healthy Chinese subjects. Each subject was administered a single oral dose of phenacetin (900 mg), and frequent blood samples were taken for up to 12 hours for simultaneous high-pressure liquid chromatography determinations of plasma concentrations of phenacetin and metabolically derived paracetamol. Compared with values observed in controls not exposed to cigarette smoking, subjects who smoked 7 to 40 (median, 20) cigarettes per day exhibited a 2.5-fold higher phenacetin apparent oral clearance (7.2, 4.3-12.0 L x h(-1) x kg(-1) vs 2.9, 1.8-4.6 L x h(-1) x kg(-1) [geometric means, 95% confidence intervals]; n = 12, p < 0.05). In subjects exposed to passive smoking, phenacetin's apparent oral clearance (3.6, 2.0-46.6 L x h(-1) x kg(-1), n = 12) was intermediate between the values observed in the two other groups. Plasma paracetamol levels were moderately lower in active smokers than in passive smokers and controls. These results demonstrated that, in contrast to results found in previous studies, Chinese subjects were fully susceptible to the inducing effect of cigarette smoke on CYP1A2 activity.

Pharmacokinetics and effect on caffeine metabolism of the proton pump inhibitors, omeprazole, lansoprazole, and pantoprazole

AIMS

To study the pharmacokinetics of three proton pump inhibitors, omeprazole, lansoprazole, and pantoprazole, as well as any potential influence on CYP1A2 activity (measured by means of rate of caffeine metabolism) of these compounds at single dose and repeated dose administration.

METHODS

Fourteen healthy males, classified as 12 extensive metabolizers (EMs) and two poor metabolizers (PMs) according to the urinary S/R mephenytoin ratio, completed this open, randomized, three-way cross-over study. In each of the three 7-day treatment periods either omeprazole (20 mg), lansoprazole (30 mg) or pantoprazole (40 mg) in therapeutically recommended doses was administered once daily, and the pharmacokinetics of the proton pump inhibitors as well as the rate of caffeine metabolism was measured on days 1 and 7.

RESULTS

In the EMs there was an increase in AUC from day 1 to day 7 for omeprazole. In the PMs the AUC of both omeprazole and lansoprazole was unchanged during repeated dosing, while for pantoprazole there was a tendency to a slight decrease. The AUC at steady state was for all three proton pump inhibitors 5 fold higher in PMs compared with EMs, indicating that the same proportion of the dose, irrespective of compound, is metabolized by CYP2C19. No induction of CYP1A2 was evident for any of the compounds in either EMs or PMs.

CONCLUSIONS

The approximately 5 fold difference in AUC between EMs and PMs indicates that approximately 80% of the dose for all three proton pump inhibitors is metabolized by the polymorphically expressed CYP2C19. None of the three proton pump inhibitors, administered in therapeutically recommended doses, is an inducer of CYP1A2--neither in PMs nor in EMs.

The effect of omeprazole pretreatment on acetaminophen metabolism in rapid and slow metabolizers of S-mephenytoin

Omeprazole, a widely used and potent gastric proton pump inhibitor, induces cytochrome P450 (CYP) 1A2 in humans. Induction is most pronounced in slow metabolizers of S-mephenytoin because CYP2C19 (S-mephenytoin hydroxylase) is responsible for the elimination of omeprazole. Acetaminophen (INN, paracetamol), a widely used and effective analgesic and antipyretic agent, causes serious hepatic and renal toxicity at high doses by conversion of acetaminophen to the toxic intermediate N-acetyl-p-benzoquinone imine (NAPQI) through CYP1A2, CYP2E1, and CYP3A4. This study evaluated whether omeprazole pretreatment in five rapid and five slow metabolizers of S-mephenytoin could increase thioether (an estimate of NAPQI production) metabolite formation from acetaminophen. The results of this study show that, despite induction of CYP1A2 activity in slow metabolizers (a 75% increase in plasma clearance of caffeine), the formation of NAPQI from acetaminophen was not increased after 7 days of omeprazole administration (40 mg/day). This suggests that induction of CYP1A2 activity by omeprazole is unlikely to increase the risk of acetaminophen hepatotoxicity.

Drug interactions with proton pump inhibitors

Omeprazole, lansoprazole and pantoprazole are all mainly metabolised by the polymorphically expressed cytochrome P450 (CYP) isoform CYP2C19 (S-mephenytoin hydroxylase). All 3 proton pump inhibitors have a very limited potential for drug interactions at the CYP level. Small effects on CYP reported for these compounds are usually of no clinical relevance. No dose related adverse effects have been identified, suggesting that the small proportion of slow metabolisers is at no additional risk for clinically important drug interactions. The absorption of some compounds, e.g. benzylpenicillin (penicillin G), are altered during treatment with proton pump inhibitors as a result of the increased intragastric pH. A synergy has been confirmed between omeprazole and amoxicillin or clarithromycin in the antibacterial effect against Helicobacter pylori.

Beta-naphthoflavone induced CYP1A1 and 1A3 proteins in the liver of rainbow trout (Oncorhynchus mykiss)

1. Two CYP1A proteins, designated HAP 1 and HAP 2, were isolated from the liver of the beta-naphthoflavone (BNF)-treated rainbow trout. The proteins were initially resolved by chromatography on a DEAE sepharose column and were further purified by hydroxyl-apatite chromatography. 2. Both HAP 1 and HAP 2 proteins exhibited high 7-ethoxyresorufin, methoxy-resorufin and phenacetin O-dealkylase activities and were good catalysts for the oxidation of 7,12-dimethylbenz[a]anthracene (DMBA). No qualitative difference was observed between the two proteins in their ability to catalyse the formation of the individual metabolites of DMBA. 3. The two purified proteins showed identical amino acid sequence for the first 13 amino acids. However, the 14th amino acid was valine for HAP 1 protein and alanine for HAP 2 protein. 4. Alignment of the amino acid sequences showed that HAP 1 protein was identical to the deduced protein of the previously reported trout CYP1A2 (renamed CYP1A1) gene for the first 24 amino acids at the N-terminal region. HAP 2 protein corresponded to the deduced protein sequence of CYP1A3 gene for the first 14 amino acids. However, unlike the deduced sequences of CYP1A1 and 1A3 the N-terminal methionine was absent in the purified proteins. 5. We conclude that HAP 1 and HAP 2 are the products of the CYP1A1 and CYP1A3 genes respectively, and are found in the liver of the BNF-treated rainbow trout.

The influence of ethnic factors and gender on CYP1A2-mediated drug disposition: a comparative study in Caucasian and Chinese subjects using phenacetin as a marker substrate

To assess potential ethnic and gender-related differences in the expression of cytochrome CYP1A2-mediated activity, the pharmacokinetics of phenacetin (a CYP1A2 substrate) and its metabolite paracetamol were compared in 20 Caucasian and 20 Chinese subjects after administration of a single oral 900 mg phenacetin dose. Peak plasma concentrations and apparent oral clearance values for phenacetin did not differ between the two groups (geometric means: 3.4 micrograms/ml and 1.56 ml h-1 kg-1, respectively, for Caucasians vs. 4.7 micrograms/ml and 1.25 ml h-1 kg-1, respectively, for Chinese, after excluding one Caucasian with aberrantly low plasma phenacetin values). Pharmacokinetic parameters for metabolically derived paracetamol were also similar in the two groups. When subjects were divided into subgroups according to gender, phenacetin apparent oral clearance values were found to be lower in Chinese women compared with both Chinese men and Caucasian subjects of either sex. It is concluded that there are no major interethnic differences in the expression of CYP1A2-related activity between Caucasians and Chinese, although Chinese women as a subgroup may exhibit comparatively lower enzyme activity.

Pharmacokinetics, metabolism and interactions of acid pump inhibitors. Focus on omeprazole, lansoprazole and pantoprazole

This review updates and evaluates the currently available information regarding the pharmacokinetics, metabolism and interactions of the acid pump inhibitors omeprazole, lansoprazole and pantoprazole. Differences and similarities between the compounds are discussed. Omeprazole, lansoprazole and pantoprazole are all mainly metabolished by the polymorphically expressed cytochrome P450 (CYP) isoform S-mephenytoin hydroxylase (CYP2C19), which means that within a population a few individuals (3% of Caucasians) metabolise the compounds slowly compared with the majority of the population. For all 3 compounds, the area under the plasma concentration-versus-time curve (AUC) for a slow metaboliser is, in general, approximately 5 times higher than that in an average patient. Since all 3 compounds are considered safe and well tolerated, and no dosage-related adverse drug reactions have been identified, this finding seems to be of no clinical relevance. The acid pump inhibitors seem to be similarly handled in the elderly, where a somewhat slower elimination can be demonstrated compared with young individuals. In patients with renal insufficiency, omeprazole is eliminated as in healthy individuals, whereas the data on lansoprazole and pantoprazole are unresolved. In patients with hepatic insufficiency, as expected, the elimination rates of all 3 compounds are substantially decreased. No clinically relevant effects on specific endogenous glandular functions, such as the adrenal (cortisol), the gonads or the thyroid, were demonstrated for omeprazole and pantoprazole, whereas a few minor concerns have been raised regarding lansoprazole. The absorption of some compounds, e.g. digoxin, might be altered as a result of the increased gastric pH obtained during treatment with acid pump inhibitors, and, accordingly, similar effects are expected irrespective of which acid pump inhibitor is given. The effect of the acid pump inhibitors on enzymes in the liver has been intensely debated, and some authors have claimed that lansoprazole and pantoprazole have less potential than omeprazole to interact with other drugs metabolised by CYP. However, after assessment of available data in this area, the conclusion is that all 3 acid pump inhibitors have a very limited potential for drug interactions at the CYP level. In addition, the small effects on CYP reported for these compounds are rarely of any clinical relevance, considering the normal intra- (and inter-)individual variations in metabolism observed for most drugs. In conclusion, omeprazole, lansoprazole and pantoprazole are structurally very similar, and an evaluation of available data indicates that also with respect to pharmacokinetics, metabolism and interactions in general they demonstrate very similar properties, even though omeprazole has been more thoroughly studied with regard to different effects.