Effects of omeprazole on intragastric pH and plasma gastrin are dependent on the CYP2C19 polymorphism

Refractory Gastroesophageal Reflux Disease: Diagnosis and Management

Gastroesophageal reflux disease (GERD) is common, with increasing worldwide disease prevalence and high economic burden. A significant number of patients will remain symptomatic following an empiric proton pump inhibitor (PPI) trial. Persistent symptoms despite PPI therapy are often mislabeled as refractory GERD. For patients with no prior GERD evidence (unproven GERD), testing is performed off antisecretory therapy to identify objective evidence of pathologic reflux using criteria outlined by the Lyon consensus. In proven GERD, differentiation between refractory symptoms (persisting symptoms despite optimized antisecretory therapy) and refractory GERD (abnormal reflux metrics on ambulatory pH impedance monitoring and/or persistent erosive esophagitis on endoscopy while on optimized PPI therapy) can direct subsequent management. While refractory symptoms may arise from esophageal hypersensitivity or functional heartburn, proven refractory GERD requires personalization of the management approach, tapping from an array of non-pharmacologic, pharmacologic, endoscopic, and surgical interventions. Proper diagnosis and management of refractory GERD is critical to mitigate undesirable long-term complications such as strictures, Barrett's esophagus, and esophageal adenocarcinoma. This review outlines the diagnostic workup of patients presenting with refractory GERD symptoms, describes the distinction between unproven and proven GERD, and provides a comprehensive review of the current treatment strategies available for the management of refractory GERD.

Prediction of Omeprazole Pharmacokinetics and its Inhibition on Gastric Acid Secretion in Humans Using Physiologically Based Pharmacokinetic-Pharmacodynamic Model Characterizing CYP2C19 Polymorphisms

PURPOSE

To develop a whole physiologically based pharmacokinetic-pharmacodynamic (PBPK-PD) model to describe the pharmacokinetics and anti-gastric acid secretion of omeprazole in CYP2C19 extensive metabolizers (EMs), intermediate metabolizers (IMs), poor metabolizers (PMs) and ultrarapid metabolizers (UMs) following oral or intravenous administration.

METHODS

A PBPK/PD model was built using Phoenix WinNolin software. Omeprazole was mainly metabolized by CYP2C19 and CYP3A4 and the CYP2C19 polymorphism was incorporated using in vitro data. We described the PD by using a turn-over model with parameter estimates from dogs and the effect of a meal on the acid secretion was also implemented. The model predictions were compared to 53 sets of clinical data.

RESULTS

Predictions of omeprazole plasma concentration (72.2%) and 24 h stomach pH after administration (85%) were within 0.5-2.0-fold of the observed values, indicating that the PBPK-PD model was successfully developed. Sensitivity analysis demonstrated that the contributions of the tested factors to the plasma concentration of omeprazole were Vmax,2C19 ≈ Papp > Vmax,3A4 > Kti, and contributions to its pharmacodynamic were Vmax,2C19 > kome > kms > Papp > Vmax,3A4. The simulations showed that while the initial omeprazole dose in UMs, EMs, and IMs increased 7.5-, 3- and 1.25-fold compared to those of PMs, the therapeutic effect was similar.

CONCLUSIONS

The successful establishment of this PBPK-PD model highlights that pharmacokinetic and pharmacodynamic profiles of drugs can be predicted using preclinical data. The PBPK-PD model also provided a feasible alternative to empirical guidance for the recommended doses of omeprazole.

Refractory Gastroesophageal Reflux Disease: A Management Update

Gastroesophageal reflux disease (GERD) is one of the most frequent gastrointestinal disorders. Proton pump inhibitors (PPIs) are effective in healing lesions and improving symptoms in most cases, although up to 40% of GERD patients do not respond adequately to PPI therapy. Refractory GERD (rGERD) is one of the most challenging problems, given its impact on the quality of life and consumption of health care resources. The definition of rGERD is a controversial topic as it has not been unequivocally established. Indeed, some patients unresponsive to PPIs who experience symptoms potentially related to GERD may not have GERD; in this case the definition could be replaced with “reflux-like PPI-refractory symptoms.” Patients with persistent reflux-like symptoms should undergo a diagnostic workup aimed at finding objective evidence of GERD through endoscopic and pH-impedance investigations. The management strategies regarding rGERD, apart from a careful check of patient's compliance with PPIs, a possible change in the timing of their administration and the choice of a PPI with a different metabolic pathway, include other pharmacologic treatments. These include histamine-2 receptor antagonists (H2RAs), alginates, antacids and mucosal protective agents, potassium competitive acid blockers (PCABs), prokinetics, gamma aminobutyric acid-B (GABA-B) receptor agonists and metabotropic glutamate receptor-5 (mGluR5) antagonists, and pain modulators. If there is no benefit from medical therapy, but there is objective evidence of GERD, invasive antireflux options should be evaluated after having carefully explained the risks and benefits to the patient. The most widely performed invasive antireflux option remains laparoscopic antireflux surgery (LARS), even if other, less invasive, interventions have been suggested in the last few decades, including endoscopic transoral incisionless fundoplication (TIF), magnetic sphincter augmentation (LINX) or radiofrequency therapy (Stretta). Due to the different mechanisms underlying rGERD, the most effective strategy can vary, and it should be tailored to each patient. The aim of this paper is to review the different management options available to successfully deal with rGERD.

Influence of CYP2C19 Polymorphisms on the Pharmacokinetics of Omeprazole in Elderly Subjects

Omeprazole blocks the gastric H⁺ /K⁺  adenosine triphosphatase, thus inhibiting gastric acid secretion, and is metabolized by cytochrome P450 (CYP) 2C19. Due to the physiological changes in the elderly, there are different pharmacokinetic consequences compared to young people. The aim of this study was to evaluate the pharmacokinetic profiles of omeprazole in 15 elderly participants according to the CYP2C19 genotype. The concentration-time profiles of omeprazole and its metabolites, 5-hydroxy (5-OH) omeprazole and omeprazole sulfone, were similar between the CYP2C19 extensive metabolizer (EM) and intermediate metabolizer groups. In contrast, when comparing the EM group and CYP2C19 poor metabolizer (PM) group, the EM/PM geometric mean ratio (95% confidence interval) of area under the plasma concentration-time curve from time of dosing to the last measurable concentration was 0.52 (0.27-1.01) and that of the IM group was 0.71 (0.32-1.59), indicating that the exposure of omeprazole in the PM group was increased. The exposure of 5-OH omeprazole was significantly decreased in the PM group when compared to the EM group, with an EM/PM geometric mean ratio (95% confidence interval) of 2.20 (1.50-3.22). In conclusion, the tendency of drug exposure according to the CYP2C19 genotype in the elderly and young adults was similar in that the exposure level was highest in the PM group. However, when compared to young adults, the difference between the genotype groups was smaller in the elderly.

Pharmacogenomic considerations for medications in the perioperative setting

Several high-profile examples of adverse outcomes from medications used in the perioperative setting are well known (e.g., malignant hyperthermia, prolonged apnea, respiratory depression, inadequate analgesia), leading to an increased understanding of genetic susceptibilities underlying these risks. Pharmacogenomic information is increasingly being utilized in certain areas of medicine. Despite this, routine preoperative genetic screening to inform medication risk is not yet standard practice. In this review, we assess the current readiness of pharmacogenomic information for clinical consideration for several common perioperative medications, including description of key pharmacogenes, pharmacokinetic implications and potential clinical outcomes. The goal is to highlight medications for which emerging or considerable pharmacogenomic information exists and identify areas for future potential research.

Proton Pump Inhibitor-Refractory Gastroesophageal Reflux Disease

Proton pump inhibitor (PPI)-refractory gastroesophageal reflux disease (GERD) is defined by the presence of troublesome GERD symptoms despite PPI optimization for at least 8 weeks in the setting of ongoing documented pathologic gastroesophageal reflux. It arises from a dysfunction in protective systems to prevent reflux and as propagation of physiologic reflux events. Treatment possibilities include pharmacologic options, invasive management strategies, and endoluminal therapies. Management strategy should be personalized to the patient's needs and mechanistic dysfunction. This article reviews the definition, mechanisms, and management options for PPI-refractory GERD.

Effects of CYP2C19 Genetic Polymorphisms on PK/PD Responses of Omeprazole in Korean Healthy Volunteers

The aim of this study was to examine the effects of CYP2C19*2 and *3 genetic polymorphisms on omeprazole pharmacokinetic (PK) and pharmacodynamic (PD) responses. Twenty-four healthy Korean volunteers were enrolled and given 20 mg omeprazole orally once daily for 8 days. The genotypes of CYP2C19 single nucleotide polymorphisms (SNPs) (*2, *3, and *17) were screened. The plasma concentrations of omeprazole, omeprazole sulfone, and 5-hydroxy (5-OH) omeprazole were determined by liquid chromatography with tandem mass spectrometry (LC-MS/MS). The noncompartmental method was used for the determination of PK parameters. Change of mean pH and proportion (%) of time of gastric pH above 4.0 were estimated. The poor metabolizer (PM) group had the lowest metabolic ratio and exhibited the highest area under the curve (AUC) for omeprazole among the CYP2C19 phenotype groups. The PM group showed the greatest change of mean pH and the highest % time of gastric pH above 4.0. The relationship between AUC of omeprazole and % time of gastric pH above 4.0 was confirmed. The study demonstrates that CYP2C19*2 and *3 influence the PKs and PDs of omeprazole in Korean healthy volunteers.

Partnership in optimizing management of reflux symptoms: a treatment algorithm for over-the-counter proton-pump inhibitors

BACKGROUND

Uncomplicated heartburn and acid regurgitation are increasingly treated and managed using over-the-counter medications. However, with over-the-counter availability of antacids, alginates, histamine 2 receptor antagonists (H2RAs), and proton-pump inhibitors (PPIs), consumers need guidance as to appropriate options and how to use them.

METHODS

Relevant guidelines, studies, and reviews were identified via literature searches of PubMed/Medline and Google Scholar, as well as cross-referencing from the identified papers.

RESULTS

Antacids, alginates, and H2RAs are best suited to management of occasional heartburn, taken either before provocative meals or other triggers or on demand when symptoms arise. Over-the-counter PPIs are appropriate options across the range of symptom severity/frequency typically encountered in the pharmacy, but may be particularly appropriate for treatment of those with frequent and/or very bothersome heartburn. A 2-4 week course of daily PPIs can lead to complete resolution of frequent heartburn. Counseling is important to ensure that patients understand that failure of symptoms to resolve or a rapid return of symptoms while taking a PPI is an indication to consult a doctor, whereas a return of symptoms after a period of months may be an indication for just another course of treatment. The need for effective communication and for ensuring use of the correct medication in the over-the-counter setting puts much of the responsibility for management of heartburn and acid regurgitation on the pharmacist. A proposed algorithm that details when and how to use available over-the-counter medications is presented. This algorithm also highlights alarm features and atypical symptoms indicative of other underlying conditions that should be referred directly to a physician.

CONCLUSION

Implementation of a simple algorithm will empower pharmacists and consumers alike and ensure consistent and appropriate care.

Update on the pharmacogenomics of proton pump inhibitors

Proton pump inhibitors (PPIs) are widely used for the treatment of gastroesophageal reflux disease as well as other acid-related disorders. PPIs are metabolized primarily via the CYP2C19 and CYP3A4 isoenzymes; their activity is influenced both by exogenous and endogenous (pharmacogenetic) factors. The CYP2C19 polymorphism affects the metabolism of PPIs, causing large individual pharmacokinetic variations. Differences in the CYP2C19-mediated metabolism can produce marked interpatient variability in acid suppression, in drug-interaction potential and in clinical efficacy. Understanding the pharmacokinetic properties of PPIs and examining the pharmacogenetic alterations may help clinicians optimize PPI therapy and administer individual treatment, especially to nonresponder patients with gastroesophageal reflux disease or ulcer or after failed eradication therapy.

Role of omeprazole dosage and cytochrome P450 2C19 genotype in patients receiving omeprazole-amoxicillin dual therapy for Helicobacter pylori eradication

STUDY OBJECTIVE

To determine the factors that may influence Helicobacter pylori eradication in patients receiving omeprazole-amoxicillin dual therapy.

DESIGN

Prospective, randomized study.

SETTING

University-affiliated hospital in Taiwan.

PATIENTS

A total of 128 adults (age range 20-75 yrs) with H. pylori-positive duodenal ulcer were enrolled; 121 completed the final evaluation.

INTERVENTION

Patients were randomly assigned to one of four omeprazole-amoxicillin treatment groups, with each treatment administered for 2 weeks: O2A2 group (33 patients)--omeprazole 20 mg twice/day plus amoxicillin 500 mg 4 times/day; O2A1 group (32 patients)--omeprazole 20 mg twice/day plus amoxicillin 250 mg 4 times/day; O1A2 group (32 patients)--omeprazole 20 mg once/day plus amoxicillin 500 mg 4 times/day; and O1A1 group (31 patients)--omeprazole 20 mg once/day plus amoxicillin 250 mg 4 times/day.

MEASUREMENTS AND MAIN RESULTS

Data were collected on H. pylori status, histologic parameters, antibiotic resistance, intragastric pH, cytochrome P450 (CYP) 2C19 genotype, and adverse reactions. The intent-to-treat cure rates (95% confidence interval [CI]) in groups O2A2, O2A1, O1A2, and O1A1 were 76% (95% CI 59-87%), 72% (95% CI 54-84%), 50% (95% CI 34-66%) and 52% (95% CI 35-68%), respectively. Eradication of H. pylori infection was statistically significantly dependent on omeprazole dosage, CYP2C19 genotype, age, gastritis status, and H. pylori density. All CYP2C19 poor metabolizers were cured, whereas the H. pylori cure rate in CYP2C19 extensive metabolizers varied from 44-76% in the different treatment groups. Eradication of H. pylori was favored in the omeprazole higher dose groups versus the lower dose groups (79% vs 53%, p=0.004). No secondary antibiotic resistance was found. Thirty-seven (95%) of 39 patients who failed with the initial treatment were cured by subsequent antibiotic susceptibility-driven proton pump inhibitor-based triple therapy.

CONCLUSION

Provided a maintenance dose of amoxicillin is given every 6 hours, eradication of H. pylori infection was significantly dependent on omeprazole dosage, CYP2C19 genotype, age, gastritis status, and H. pylori density.

Intragastric acidity during administration of generic omeprazole or esomeprazole - a randomised, two-way crossover study including CYP2C19 genotyping

BACKGROUND

Generic omeprazole has been approved in many countries for the treatment of acid-related gastrointestinal disorders. However, clinical studies comparing generic to original proton pump inhibitors are limited.

AIMS

To compare the effect of generic omeprazole 20 mg/day with esomeprazole 20 mg/day on intragastric acidity and to investigate the influence of the CYP2C19 metabolizer status.

METHODS

In this randomised, single-blinded, two-way crossover study, 24 healthy Helicobacter pylori-negative subjects, received generic omeprazole (Omep; Hexal AG, Holzkirchen, Germany) 20 mg once daily or esomeprazole 20 mg once daily for five consecutive days. Twenty-four-hour intragastric pH was recorded on day 5 of each treatment. CYP2C19 status was determined by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

Over all, there were no statistically significant differences between generic omeprazole and esomeprazole with respect to median intragastric pH (3.5 and 3.9, P = 0.07), the total hours with intragastric pH >4 (10.4 and 11.3, P = 0.29), and during upright (9.6 and 9.1, P = 0.77) or supine (2.2 and 2.2, P = 0.94) position. However, in CYP2C19 rapid metabolizers, esomeprazole was superior to omeprazole, with the percentage of time with intragastric pH >3.0 and pH >3.5 being higher with esomeprazole than with generic omeprazole [Δ = 9% (P = 0.026) and Δ = 8% (P = 0.046), respectively].

CONCLUSIONS

Overall, generic omeprazole 20 mg appears to provide a similar intragastric acid control when compared with esomeprazole 20 mg. However, esomeprazole might be advantageous in subjects with a rapid CYP2C19 metabolizer status.

Proton pump inhibitors and rebound acid hypersecretion

Rebound acid hypersecretion is defined as an increase in gastric acid secretion above pre-treatment levels after cessation of antisecretory therapy. Proton pump inhibitors (PPIs) have been proved to be more effective than histamine-2 receptor antagonists in suppressing intragastric acidity and have therefore become the main choice for management of acid-related complaints. Recent studies have shown that PPI therapy-elicited rebound acid hypersecretion could in turn induce acid-related symptoms. This makes it difficult for patients to cease PPI therapy. Hypergastrinemia is an important mechanism that leads to rebound acid hypersecretion. To reduce the occurrence of rebound acid hypersecretion, efforts should be made to restrict indications for using PPIs.

Relationship between the Acid-Inhibitory Effects of Two Proton Pump Inhibitors and CYP2C19 Genotype in Japanese Subjects: A Randomized Two-Way Crossover Study

This two-way crossover study investigated possible differences between the proton pump inhibitors, omeprazole and rabeprazole, in their effect on gastric acid secretion in Japanese subjects with differing cytochrome P450, family 2, subfamily C, polypeptide 19 (CYP2C19) genotypes. A total of 23 Helicobacter pylori-negative healthy volunteers received omeprazole 20 mg/day and rabeprazole 10 mg/day. Each drug treatment was given for a continuous 7-day period allocated in random order, with an interval of at least 1 week between drug treatment periods to allow for wash-out. Intragastric pH was measured on days 1 and 7. Overall median intragastric pH levels at 7 and 8 h after the first administration were significantly higher with omeprazole. There was no significant difference in intragastric pH in homozygous extensive metabolizers, whereas intragastric pH was significantly higher with omeprazole in combined data from heterozygous extensive metabolizers and poor metabolizers at 6, 7 and 8 h after the first drug administration. There were no significant differences in intragastric pH between omeprazole and rabeprazole irrespective of genotype on day 7 of administration. In conclusion, on day 1 the time to onset of the antisecretory action of 20 mg/day omeprazole was more rapid than that of 10 mg/day rabeprazole in Japanese individuals who have a higher incidence of the CYP2C19 poor metabolizer genotype, however by day 7 no difference in antisecretory effect was found, regardless of genotype.

Influence of CYP2C19 on the relationship between pharmacokinetics and intragastric pH of omeprazole administered by successive intravenous infusions in Chinese healthy volunteers

OBJECTIVES

To explore the effect of cytochrome P450 2C19 (CYP2C19) polymorphisms on the relationship between the pharmacokinetics and pharmacodynamics of omeprazole administered by intravenous successive infusions in Chinese healthy volunteers.

METHODS

A total of 21 subjects [7 homozygous extensive metabolizers (homEMs), 9 heterozygous extensive metabolizers (hetEMs), 5 poor metabolizers (PMs)] received a 5-day course of omeprazole (40 mg) administered as a single dose daily during a 30-min period. Plasma concentrations were monitored by sampling at very short intervals for the first 8.5 h post-omeprazole administration and at 24 h post-administration, and intragastric pH was recorded on days 1 and 5.

RESULTS

After a single dose, both the area under the plasma concentration-time curve (AUC) and peak concentration (C(max)) were higher in PMs than in EMs. Both the mean half-life (t((1/2))) and total clearance in PMs were significantly higher and lower than those in homEMs and EMs, respectively. Mean AUC and C(max) ratios in homEMs, hetEMs, and PMs were 1.0:1.1:1.4 and 1.0:1.0:1.1, respectively. Relative to the values determined after a single dose in EMs, after repeated doses, the intragastric pH, AUC, C(max), and t((1/2)) had increased significantly, while the total clearance had decreased significantly. Mean AUC and C(max) ratios in homEMs, hetEMs, and PMs were 1.4:1.4:1.5 and 1.2:1.2:1.3, respectively, compared to those of a single dose. The mean intragastric pH was significantly higher in PMs than in EMs after the fifth dose.

CONCLUSIONS

There is a relationship between the pharmacokinetics and pharmacodynamics of omeprazole, with the latter depending in part on the duration of administration as evidenced by a higher AUC or C(max) and intragastric pH resulting from repeated dosing.

CYP2C19 genotypes in the pharmacokinetics/pharmacodynamics of proton pump inhibitor-based therapy of Helicobacter pylori infection

IMPORTANCE OF THE FIELD

Proton pump inhibitors (PPIs) are potent gastric acid inhibitors. Therapies with a PPI and antibiotics are used to cure Helicobacter pylori (H. pylori) infection, which is closely related to many gastrointestinal diseases. Most PPIs are mainly metabolized by cytochrome 2C19 (CYP2C19). The genetic polymorphisms of CYP2C19 may lead to the differences in pharmacokinetics (PK), pharmacodynamics (PD) and clinical efficacy of PPIs.

AREAS COVERED IN THIS REVIEW

The roles of PPIs on the eradication of H. pylori are summarized. The impact f CYP2C19 polymorphism on the PK and PD of PPIs is addressed and related to the present status of therapy for H. pylori infection. The opinions on the strategy of PPIs-based therapies of H. pylori infection are provided.

WHAT THE READER WILL GAIN

Update the factors that may influence the PPIs-based therapies of H. pylori infection.

TAKE HOME MESSAGE

The eradication rates of H. pylori infection are significantly different between patients who are CYP2C19 extensive metabolizers and poor metabolizers, partly because of the differences in the PK and PD of PPIs. Nonetheless, the differences can be improved by adjusting the regimens of PPIs and using antibiotics that have less H. pylori-resistance.

A clinical guide to using intravenous proton-pump inhibitors in reflux and peptic ulcers

Intravenous (IV) proton-pump inhibitors (PPIs) are potent gastric acid suppressing agents, and their use is popular in clinical practice. Both IV and oral PPIs have similarly short half-lives, and their effects on acid secretion are similar, thus their dosing and dosage intervals appear to be interchangeable. The possible exception is when sustained high pHs are required to promote clot stabilization in bleeding peptic ulcers. Continuous infusion appears to be the only form of administration that reliably achieves these high target pHs. IV PPI is indicated in the treatment of high-risk peptic ulcers, complicated gastroesophageal reflux, stress-induced ulcer prophylaxis, Zollinger-Ellison syndrome, and whenever it is impossible or impractical to give oral therapy. The widespread use of PPIs has been controversial. IV PPIs have been linked to the development of nosocomial pneumonia in the intensive care setting and to spontaneous bacterial peritonitis in cirrhotic patients. This review discusses the use of IV PPI in different clinical scenarios, its controversies, and issues of appropriate use.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

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

Pharmacokinetic- pharmacodynamic analysis of the role of CYP2C19 genotypes in short-term rabeprazole-based triple therapy against Helicobacter pylori

AIMS

The aim was to explore the role of CYP2C19 polymorphism in short-term rabeprazole-based triple therapy against Helicobacter pylori infection.

METHODS

Patients with H. pylori infection were tested for CYP2C19 genotype as poor metabolizers (PMs) or extensive metabolizers (EMs, homozygous EM or heterozygous EM) and given rabeprazole for 7 days. Antibiotics (clarithromycin and amoxicillin) were given on days 1-4, days 4-7, or days 1-7. A direct link model with an effect compartment was used in the population pharmacokinetic-pharmacodynamic analysis. The status of H. pylori infection was evaluated.

RESULTS

Rabeprazole clearance was lower in CYP2C19 PMs than in EMs (with average values of 10.7 vs. 16.8 l h(-1) in PMs and EMs, respectively), resulting in higher plasma levels in the former group. The values of EC(50) and k(eo) of gastrin response increased with multiple doses of rabeprazole. The k(eo) values were lower in CYP2C19 PMs than in EMs on day 1 (0.012 vs. 0.017 x 10(-4) l min(-1)), and higher than in EMs on day 4 (0.804 vs. 0.169 x 10(-4) l min(-1)) of rabeprazole treatment. The predicted gastrin-time profile showed a higher response in CYP2C19 PMs than in EMs on days 4 and 7. Helicobacter pylori was eradicated in all CYP2C19 PMs except in one patient infected by a resistant strain. In contrast, in CYP2C19 EMs the eradication rates ranged from 58 to 85%.

CONCLUSIONS

CYP2C19 genotypes play a role in H. pylori eradication therapy. Rabeprazole-based short-term triple therapy may be applicable in CYP2C19 PMs for H. pylori eradication.

Gastroesophageal reflux disease: an Asian perspective

The prevalence of gastroesophageal reflux disease (GERD) ranges from 2.5% to 7.1% in most population-based studies in Asia. There is evidence that GERD and its complications are rising, coinciding with a decline in Helicobacter pylori (H. pylori) infection. Asian GERD patients share similar risk factors and pathophysiological mechanisms with their Western counterparts. Possible causes for the lower prevalence of GERD include less obesity and hiatus hernia, a lesser degree of esophageal dysmotility, a high prevalence of virulent strains of H. pylori, and low awareness. Owing to the lack of precise translation for 'heartburn' in most Asian languages, reflux symptoms are often overlooked or misinterpreted as dyspepsia or chest pain. Furthermore, a symptom-based diagnosis with a therapeutic trial of the proton pump inhibitor (PPI) may be hampered by the high prevalence of H. pylori-related disease. The risk stratification for prompt endoscopy, use of a locally-validated, diagnostic symptom questionnaire, and response to H. pylori'test and treat' help improve the accuracy of the PPI test for diagnoses. PPI remain the gold standard treatment, and 'on-demand' PPI have been shown to be a cost-effective, long-term treatment. The clinical course of GERD is benign in most patients in Asia. The risk of progression from non-erosive reflux disease to erosive esophagitis is low, and treatment response to a conventional dose of PPI is generally higher. Although H. pylori eradication may lead to more resilient GERD in a subset of patients, the benefits of H. pylori eradication outweigh the risks, especially in Asian populations with a high incidence of gastric cancer.

Relative potency of proton-pump inhibitors-comparison of effects on intragastric pH

AIM

Comparative potency of proton-pump inhibitors (PPIs) is an important clinical issue. Most available trials have compared the different PPIs at one or a few selected specific dosages, making it difficult to derive quantitative equivalence dosages. Here we derived PPI dose equivalents based on a comprehensive assessment of dose-dependent effects on intragastric pH.

METHODS

All available clinical studies reporting the effects of PPIs on mean 24-h intragastric pH were sought from electronic databases including Medline. Studies included were restricted to those targeting the Caucasian population, and healthy volunteers or gastroesophageal reflux disease (GERD) patients. The dose-effect relationships for mean 24-h intragastric pH and for percentage of time with pH > 4 in 24 h were analyzed for each PPI using pharmacodynamic modeling with NONMEM and a model integrating all available data.

RESULTS

Fifty-seven studies fulfilled the inclusion criteria. Based on the mean 24-h gastric pH, the relative potencies of the five PPIs compared to omeprazole were 0.23, 0.90, 1.00, 1.60, and 1.82 for pantoprazole, lansoprazole, omeprazole, esomeprazole, and rabeprazole, respectively. Compared with healthy volunteers, patients with GERD needed a 1.9-fold higher dose and Helicobacter pylori-positive individuals needed only about 20% of the dose to achieve a given increase in mean 24-h intragastric pH.

CONCLUSION

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.

Effect of some natural products either alone or in combination on gastritis induced in experimental rats

Gastritis, an inflammatory state in gastric mucosa, can be induced experimentally in various ways. The present study considered the iodoacetamide model (Iodo). Omega-3 fatty acids (fish oil), black seed oil, and curcuminoids (natural products) in addition to omeprazole (synthetic proton-pump inhibitor) were tested. Supplementation of 0.1% iodoacetamide to drinking water of experimental rats for two consecutive weeks resulted in: (i) increased serum nitric oxide (NO) and gastrin, and decreased pepsinogen, (ii) depletion of gastric mucosal glutathione (GSH), and (iii) increased gastric mucosal lipid peroxidation (MDA), but failed to affect gastric mucosal myeloperoxidase (MPO) activity. Histological examination showed marked neutrophilic infiltration after 1 week of iodoacetamide administration and shedding of apical cell layer with pale edematous vacuolated gastric gland cells and thickening of muscularis mucosa after 2 weeks of iodoacetamide intake. Individual administration of omega-3 fatty acids 12 mg/kg, black seed oil 50 mg/kg, and curcuminoids 50 mg/kg body weight orally daily for 3 weeks decreased MDA, gastrin, and NO, and normalized mucosal GSH but failed to affect serum pepsinogen level. Combined administration of these natural products for 3 weeks normalized MPO activity, and other effects were nearly the same as with individual use. Omeprazole administration 30 mg/kg body weight orally daily for 3 weeks induced a similar response except for an observed increase in serum gastrin and pepsinogen levels.

Refractory gastrooesophageal reflux disease

The introduction of proton pump inhibitors (PPIs) has facilitated the treatment of gastrooesophageal reflux disease (GORD) enormously; however, treatment of GORD still fails in a small proportion of patients. This small proportion of therapy-resistant patients encompasses a substantial part of the working load of physicians and has become a common clinical problem. A strong variability in acid-suppressive effect of PPI treatment exists depending on compliance, Helicobacter pylori status and genotype. Nocturnal acid breakthrough does not seem to be a major determinant of refractory GORD. Recent data, however, show that PPI-refractory GORD can result from nonacid reflux episodes. It is wise to reconsider the diagnosis of GORD in patients who are PPI-refractory. Most patients in whom a PPI is not effective do not have GORD, instead they suffer from other disorders such as functional dyspepsia. If after a thorough history is taken the suspicion of GORD is still high, the next step would be to perform upper endoscopy and reflux monitoring. In case patients truly have PPI-refractory GORD, therapy can be aimed at oesophageal hypersensitivity or a surgical solution can be sought.

Effects of CYP2C19 genetic polymorphism on the pharmacokinetics and pharmacodynamics of omeprazole in Chinese people

BACKGROUND

To investigate whether the pharmacodynamics and pharmacokinetics of omeprazole (OPZ) are dependent of the CYP2C19 genotype status in Chinese people.

METHODS

Eighteen healthy subjects were voluntary to participate in the study, whose CYP2C19 genotype status were determined by polymerase chain reaction-restriction fragment length polymorphism method. There were six homozygous extensive metabolizers, six heterozygous extensive metabolizers and six poor metabolizers (PMs). All subjects were Helicobacter pylori-negative, determined by serology method and (13)C-urea breath test. After d1 and d8 orally received OPZ 20 mg once daily in the morning, intragastric pH values were monitored for 24 h by Digitrapper pH. Meanwhile, blood samples were collected at various time-points until 24 h after administration. The serum concentrations of OPZ were measured by liquid chromatography.

RESULTS

After single or repeated doses, the PMs showed a significantly higher mean area under the serum concentration-time curves (AUC) values than that observed in the homozygous extensive metabolizers or the heterozygous extensive metabolizers, with a relative ratio of 1.0 : 1.1 : 4.2 and 1.0 : 1.3 : 3.3 (homozygous extensive metabolizers:heterozygous extensive metabolizers:poor metabolizers), respectively. After a single dose of OPZ, significant differences in intragastric pH median, pH > 3 holding time and pH > 4 holding time were observed among the three groups. After repeated doses, the PMs showed a significantly higher intragastric pH values than that observed in the homozygous extensive metabolizers or the heterozygous extensive metabolizers.

CONCLUSION

The pharmacodynamic effects of OPZ and its pharmacokinetics depend on the CYP2C19 genotype status in Chinese people.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Pathogenesis of Helicobacter pylori Infection

SUMMARY

Helicobacter pylori is the first formally recognized bacterial carcinogen and is one of the most successful human pathogens, as over half of the world's population is colonized with this gram-negative bacterium. Unless treated, colonization usually persists lifelong. H. pylori infection represents a key factor in the etiology of various gastrointestinal diseases, ranging from chronic active gastritis without clinical symptoms to peptic ulceration, gastric adenocarcinoma, and gastric mucosa-associated lymphoid tissue lymphoma. Disease outcome is the result of the complex interplay between the host and the bacterium. Host immune gene polymorphisms and gastric acid secretion largely determine the bacterium's ability to colonize a specific gastric niche. Bacterial virulence factors such as the cytotoxin-associated gene pathogenicity island-encoded protein CagA and the vacuolating cytotoxin VacA aid in this colonization of the gastric mucosa and subsequently seem to modulate the host's immune system. This review focuses on the microbiological, clinical, immunological, and biochemical aspects of the pathogenesis of H. pylori .

Pathogenesis of Helicobacter pylori Infection

SUMMARY

Helicobacter pylori is the first formally recognized bacterial carcinogen and is one of the most successful human pathogens, as over half of the world's population is colonized with this gram-negative bacterium. Unless treated, colonization usually persists lifelong. H. pylori infection represents a key factor in the etiology of various gastrointestinal diseases, ranging from chronic active gastritis without clinical symptoms to peptic ulceration, gastric adenocarcinoma, and gastric mucosa-associated lymphoid tissue lymphoma. Disease outcome is the result of the complex interplay between the host and the bacterium. Host immune gene polymorphisms and gastric acid secretion largely determine the bacterium's ability to colonize a specific gastric niche. Bacterial virulence factors such as the cytotoxin-associated gene pathogenicity island-encoded protein CagA and the vacuolating cytotoxin VacA aid in this colonization of the gastric mucosa and subsequently seem to modulate the host's immune system. This review focuses on the microbiological, clinical, immunological, and biochemical aspects of the pathogenesis of H. pylori .

Pathogenesis of Helicobacter pylori Infection

SUMMARY

Helicobacter pylori is the first formally recognized bacterial carcinogen and is one of the most successful human pathogens, as over half of the world's population is colonized with this gram-negative bacterium. Unless treated, colonization usually persists lifelong. H. pylori infection represents a key factor in the etiology of various gastrointestinal diseases, ranging from chronic active gastritis without clinical symptoms to peptic ulceration, gastric adenocarcinoma, and gastric mucosa-associated lymphoid tissue lymphoma. Disease outcome is the result of the complex interplay between the host and the bacterium. Host immune gene polymorphisms and gastric acid secretion largely determine the bacterium's ability to colonize a specific gastric niche. Bacterial virulence factors such as the cytotoxin-associated gene pathogenicity island-encoded protein CagA and the vacuolating cytotoxin VacA aid in this colonization of the gastric mucosa and subsequently seem to modulate the host's immune system. This review focuses on the microbiological, clinical, immunological, and biochemical aspects of the pathogenesis of H. pylori .

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori is the first formally recognized bacterial carcinogen and is one of the most successful human pathogens, as over half of the world's population is colonized with this gram-negative bacterium. Unless treated, colonization usually persists lifelong. H. pylori infection represents a key factor in the etiology of various gastrointestinal diseases, ranging from chronic active gastritis without clinical symptoms to peptic ulceration, gastric adenocarcinoma, and gastric mucosa-associated lymphoid tissue lymphoma. Disease outcome is the result of the complex interplay between the host and the bacterium. Host immune gene polymorphisms and gastric acid secretion largely determine the bacterium's ability to colonize a specific gastric niche. Bacterial virulence factors such as the cytotoxin-associated gene pathogenicity island-encoded protein CagA and the vacuolating cytotoxin VacA aid in this colonization of the gastric mucosa and subsequently seem to modulate the host's immune system. This review focuses on the microbiological, clinical, immunological, and biochemical aspects of the pathogenesis of H. pylori.

Should the presence of polymorphisms of CYP2C19 enzymes influence the choice of the proton pump inhibitor for treatment of Helicobacter pylori infection?

There are variations in the CYP2C19 genotypes, that are important for the metabolism of PPIs. Patients who are heterozygotes for the mutation, but especially homozygotes, have a much slower metabolism, which will result in more profound acid suppression. Studies have been published, that suggest that the success rate of anti-Helicobacter therapy is in part related to the CYP2C19 genotype of the patient. However, it is important to keep in mind that most studies that have evaluated this have been carried out in Asia, in particular in Japan, where the prevalence of poor metabolizers (PM) is much higher than, for example, in Caucasians. The systematic review published in this issue suggests that particularly for omeprazole in combination with amoxicillin or amoxicillin and clarithromycin the success rate is much lower when compared to other proton pump inhibitors (PPIs). However, there was marked heterogeneity when the results were pooled in formal meta-analysis. Study quality was suboptimal and other factors such as resistance to antibiotics may explain the observed differences in success rates. More clinical trial data are needed before we can accept the conclusions of this meta-analysis.

Papel del polimorfismo genético CYP2C19 en los efectos adversos a fármacos y en el riesgo para diversas enfermedades

There are a great number of polymorphic genes in the human genome. Many of them codify enzymes that metabolizes drugs and xenobiotic agents, including carcinogens. Among the better known of them, there are a number of isozymes of the microsomal oxidative system (CYP3A4, CYP2C9, CYP2C19 y CYP2D6). This article reviews the following issues: a) frequency of presentation of the "poor metabolizer" genotype and/or phenotype for substrates of CYP2C19; b) role of CYP2C19 polymorphism on the metabolism of some drugs (mephenytoine and other antiepileptic drugs, proton pump inhibitors, several antidepressants and anxyolitics, the antimalaria aggent proguanyl, and propranolol, among others, use this metabolic pathway), and c) possible role of CYP2C19 polymorphism in the risk for development of neoplasia and other diseases (systemic lupus erythematosus, psoriasis, hip osteonecrosis, Alzheimer's disease, amyotrophic lateral sclerosis, essential tremor).

Smoking increases the treatment failure for Helicobacter pylori eradication

PURPOSE

Treatment failure for Helicobacter pylori (H. pylori) eradication is encountered in approximately 10-20% of patients, and many studies have pointed to a link with smoking. To investigate the effects of smoking on eradication outcome, we performed a meta-analysis.

METHODS

A PubMed search was performed to retrieve articles published up to August 2005. Pooled odds ratio (OR) and differences rate for H. pylori eradication failure in smokers compared with nonsmokers were used as summary statistics. Meta-regression was used for examining the source of heterogeneity.

RESULTS

Twenty-two published studies (5538 patients), which provided information on eradication failure according to smoking status, were included in the analysis. The summary OR for eradication failure among smokers relative to nonsmokers was 1.95 (95% confidence interval [CI]: 1.55-2.45; P <.01). It corresponds with the differences in eradication rates between smokers and nonsmokers (8.4% [95% CI: 3.3-13.5%, P <.01]). Meta-regression analysis demonstrated that a high proportion of nonulcer dyspepsia patients in studies revealed a higher failure rate among smokers, compared with a low proportion of nonulcer dyspepsia.

CONCLUSIONS

Our meta-analysis demonstrated that smoking increases the treatment failure rate for H. pylori eradication.

CYP2C19 genotype and the PPIs--focus on rabeprazole

Amongst all the proton pump inhibitors (PPI), the hepatic metabolism of rabeprazole is least dependent on the CYP4502C19 system. Rabeprazole is therefore the PPI least affected by CYP4502C19 genetic polymorphism. This unique feature of rabeprazole complements rabeprazole's fast onset of action, and may lead to profound and consistent inhibition of gastric acid secretion in the treatment of acid-related disorders.

Effect of orally administered omeprazole on abomasal luminal pH in dairy calves fed milk replacer

The objective of this study was to determine whether oral administration of omeprazole, a proton-pump inhibitor, increased abomasal luminal pH in calves fed milk replacer. Four male dairy calves with cannulae in the abomasal body suckled milk replacer (60 ml/kg body weight every 12 h) and were administered a non-enteric-coated omeprazole (4 mg/kg body weight every 24 h) in a paste formulation for five successive days. Abomasal luminal pH was continuously measured using miniature glass pH electrodes. On the first day of omeprazole administration, there was a significant (P<0.05) increase in mean 24-h pH from 2.89 to 4.17. The mean 24-h pH on days 2, 3, 4 and 5 of omeprazole administration were 3.85, 4.02, 3.97 and 3.39 respectively. We conclude that oral administration of non-enteric-coated omeprazole increased abomasal luminal pH in calves fed milk replacer, but that the effect may decrease over time.

Acid-suppressive efficacy of a reduced dosage of rabeprazole: comparison of 10 mg twice daily rabeprazole with 20 mg twice daily rabeprazole, 30 mg twice daily lansoprazole, and 20 mg twice daily omeprazole by 24-hr intragastric pH-metry

Rabeprazole achieves more potent acid suppression than other proton pump inhibitors. Therefore it is administered at reduced as well as high dosages in eradication therapy for Helicobacter pylori; however, there is incomplete assessment of the efficacy of a reduced dosage of rabeprazole as might be employed in therapy. In this study, we evaluated acid-suppressive efficacy of a reduced dosage of rabeprazole on day 7 by 24-hr pH-metry in 10 healthy male cytochrome P-450 2C19 extensive metabolizers without Helicobacterpylori infection and compared the results with those of high dosages of rabeprazole, lansoprazole, and omeprazole. Median intragastric pH value, pH >3 holding time ratio (pH>3HT), pH>4HT, pH>5HT, pH>6HT, and pH>7HT for 24 hr with rabeprazole, 10 mg twice daily, were not significantly different from those of rabeprazole, 20 mg twice daily, lansoprazole, 30 mg twice daily, and omeprazole, 20 mg twice daily. In conclusion, for acid-suppressive efficacy, a reduced dosage of rabeprazole is comparable to high dosages of rabeprazole, lansoprazole, and omeprazole.

Effect of CYP2C19 polymorphism on the safety and efficacy of omeprazole in Japanese patients with recurrent reflux oesophagitis

BACKGROUND

The polymorphic enzyme cytochrome P450 2C19 affects omeprazole metabolism. This influence on metabolism might affect serum gastrin levels, and safety, during long-term treatment of reflux oesophagitis.

AIM

To examine the relationship between cytochrome P450 2C19 genotype and the safety profile of long-term omeprazole treatment.

METHODS

A total of 119 Japanese patients with recurrent reflux oesophagitis underwent cytochrome P450 2C19 genotyping prior to receiving daily omeprazole 10 mg or 20 mg for 6-12 months, during which adverse event frequency, serum gastrin levels and endoscopic findings were monitored.

RESULTS

The incidences of adverse events, serious adverse events and adverse events leading to withdrawal did not differ between homozygous extensive metabolizer (n = 46), heterozygous extensive metabolizer (n = 53) or poor metabolizer (n = 20) groups. In all genotype groups, serum gastrin increased during the first 3 months of dosing but stabilized thereafter. No significant differences were seen either in the rate of reflux oesophagitis healing or symptom improvement among genotype groups.

CONCLUSIONS

Long-term treatment with omeprazole was well-tolerated in Japanese patients, irrespective of their cytochrome P450 2C19 metabolic genotype, indicating that dose adjustment depending on metabolic genotype is not required during treatment with omeprazole.

Omeprazole treatment of Korean patients: effects on gastric pH and gastrin release in relation to CYP2C19 geno- and phenotypes

This study aimed to investigate the effect of omeprazole on intragastric pH and gastrin release as well as the plasma concentration of omeprazole in relation to CYP2C19 genotypes after repeated doses in Korean patients. Twenty-six Korean patients with acid related disease were genotyped for CYP2C19 by allele specific PCR (wt/wt, CYP2C19*1/*1; wt/mut, CYP2C19*1/*2 or *1/*3; mut/mut, CYP2C19*2/*2, *2/*3 or *3/*3). Intragastric pH was monitored during 24 hr, and the plasma concentrations of omeprazole, hydroxyomeprazole, omeprazole sulfone and meal-stimulated gastrin were measured during 4 hr before and after 8 consecutive daily doses of 20 mg omeprazole. Unexpectedly the AUCs of omeprazole in the three genotypes were similarly high on Day 8. The mean 24 hr pH increased significantly in all three genotypes (paired t-test; P<0.0001), and the AUCs (4 hr) of gastrin in all patients increased markedly from 129+/-73 to 298+/-142 pMhr (P<0.0001). However, there was no statistically significant difference between the three genotypes in the mean pH and gastrin AUCs on Day 8. After 8 consecutive doses of 20 mg omeprazole, the gastric pH and the plasma gastrin were increased significantly in all three CYP2C19 genotypes, which were confirmed by high plasma concentrations of omeprazole in all three genotype groups. We suggest that the reason why the wt/wt had high concentrations of omeprazole similar to those in the other two genotype groups is that some of them were old with low CYP2C19 activity. In these patients omeprazole accumulated from the first to the eighth dose similar to that in the heterozygotes.

Toward individualized pharmaceutical care of East Asians: the value of genetic testing for polymorphisms in drug-metabolizing genes

Research into the relationship between genetics and drug response has focused on polymorphisms in genes that encode drug-metabolizing enzymes, particularly the genes of cytochrome P450 superfamily 2, which affect the clearance of the anticoagulant warfarin, proton pump inhibitors, tricyclic antidepressants, and many other clinically relevant drugs. Much of this work has targeted East Asians, a genetically distinguishable and populous group. Researchers have identified polymorphisms that inactivate gene function, compared polymorphism frequencies in East-Asian and Caucasian populations, and determined the effects on the pharmacokinetic parameters of drugs. Detection in an individual of polymorphisms known to inactivate a drug-metabolizing enzyme is predictive of poor metabolism of drugs processed by that pathway, which itself may be predictive of an atypical drug response. Genetic tests can be used to screen for individuals with poor metabolizer phenotypes, with the ultimate goal of predicting the clinical effects of drugs.

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.

Managing gastroesophageal reflux disease: from pharmacology to the clinical arena

GERD is a condition affecting patients throughout the 24-hour period, although the nighttime interval may require special consideration because of the pharmacologic profile of the agents used to treat GERD, and the normal physiologic processes rendering nighttime GERD particularly damaging. GERD patients should be managed with appropriate therapy proportional to the frequency and severity of their symptoms. PPIs are the most potent inhibitors of acid secretion, and with a thorough knowledge of their pharmacologic properties, clinicians can be helped in identifying strategies that can maximize the benefits of their potency (see Table 2). PPIs offer significant benefit to persons requiring longer-term therapy because they are potent agents and offer ease of dosing and favorable drug interaction and adverse effect profiles. However, it is necessary that clinicians understand the physiology and pharmacology of acid secretion to use them appropriately. Inevitably, proper therapeutic treatment demands that variables such as pharmacokinetics, ethnicity (metabolic profile), and the normal physiology of acid secretion be considered when choosing an appropriate PPI.

Omeprazole disposition in children following single-dose administration

Omeprazole is frequently used to treat gastroesophageal reflux in infants and children despite the lack of age-specific pharmacokinetic and dosing information in the approved product labeling. To address this challenge, the authors examined the potential influence of development and cytochrome P450 2C19 (CYP2C19) genotype on omeprazole disposition by conducting two pharmacokinetic (PK) studies in children and adolescents (ages 2-16 years) after a single oral 10- or 20-mg dose of the drug. Plasma omeprazole concentrations were determined by HPLC-MS from seven plasma samples obtained over a 6-hour postdose period. Pharmacokinetic parameters were determined by noncompartmental methods. Subjects were genotyped for CYP2C19 by PCR-RFLP. Data were available from 37 patients (19 female), 10 of whom were < or = 5 years of age. No drug-associated adverse events were observed. The numbers of functional CYP2C19 alleles per subject in the cohort were 2 (n = 25), 1 (n = 11), and 0 (n = 1). Pharmacokinetic parameters (mean +/- SD, range) were as follows: tmax (2.1 +/- 1.2, 1-6 h), Cmax (331.1 +/- 333.6, 20.8-885.8 ng/mL), AUC0-->infinity (809.5 +/- 893.8, 236.9-1330.9 ng/mL.h), t1/2 (0.98 +/- 0.22, 0.7-1.4 h), and CL/F (1.8 +/- 1.4, 0.3-5.8 L/h/kg). Comparison of mean AUC0-->infinity values normalized for dose (i.e., per 1 mg/kg) between subjects with one versus two functional CYP2C19 alleles revealed no statistically significant difference. In addition, the CL/F and apparent elimination rate constant (lambda z) for omeprazole were not significantly different for subjects with one versus two functional CYP2C19 alleles. No association between age and CL/F, t1/2, or lambda z was observed. The range of t1/2 values for omeprazole was similar to those reported in adults (1-1.5 h).

CONCLUSIONS

(1) in children ages 2 to 16 years receiving 10 or 20 mg of omeprazole as a single oral dose, the PK are quite comparable to values reported for adults, and (2) in pediatric patients who are CYP2C19 extensive metabolizers, there was no association between genotype and the pharmacokinetics of omeprazole.

Treatment of acid-related diseases in the elderly with emphasis on the use of proton pump inhibitors

Proton pump inhibitors (PPIs) have revolutionised the treatment of acid-related disorders, and they have also made it possible to define the spectrum of acid inhibition required for optimal treatment in each disorder. Five PPIs are now available: the older drugs, omeprazole, lansoprazole and pantoprazole, and the two newest, rabeprazole and esomeprazole. These agents have predominantly been developed in the younger adult population, and data for the elderly population are limited. Subtle differences have emerged between the old and the new PPIs in their pharmacokinetic, pharmacodynamic and efficacy profiles. The degree of clinical relevance of these differences in the adult population is in question. However, according to this review, based on the available data for the elderly and by inference from the adult population, the differences are highly relevant in the elderly population. Studies of the pharmacokinetics of older PPIs demonstrated considerable variation in drug clearance that was reflected in a wide range of efficacy related to acid suppression with standard dosages. The newer PPIs offer several advantages over older agents, particularly in terms of rapid, profound and consistent acid inhibition. Consistent acid inhibition is particularly important in the elderly since clinical response is often difficult to judge in this patient group. An individual's cytochrome P450 (CYP) 2C19 genotype predicts the degree of acid suppression and consequently the clinical efficacy of the PPIs. The older PPIs are predominantly metabolised by CYP2C19, with this being of more importance for omeprazole and lansoprazole than pantoprazole. The hepatic metabolism of rabeprazole is predominantly by nonenzymatic reactions and minimally by CYP-mediated reactions, which therefore confers an advantage over older PPIs in that genetic polymorphisms for CYP2C19 do not significantly influence rabeprazole clearance, clinical efficacy or potential for drug interactions. The metabolism of esomeprazole involves CYP2C19 but to a lesser extent than its predecessor omeprazole. Furthermore, esomeprazole has a more rapid onset of action and less variation in clearance rates than omeprazole. Drug clearance decreases with age independently of CYP2C19 status, exaggerating some of the differences between the PPIs and increasing the risk of drug interactions.

Time-dependent amplified pharmacokinetic and pharmacodynamic responses of rabeprazole in cytochrome P450 2C19 poor metabolizers

STUDY OBJECTIVES

To determine the pharmacokinetic and pharmacodynamic rationale for the optimum regimen of rabeprazole in the treatment of Helicobacter pylori infection in patients who are cytochrome P450 (CYP) 2C19 poor metabolizers or extensive metabolizers.

DESIGN

Prospective, multiple-dose pharmacokinetic and pharmacodynamic study.

SETTING

University-affiliated medical center in Taiwan.

SUBJECTS

Twelve healthy volunteers (aged 20-30 yrs) who were identified as CYP2C19 poor metabolizers (six subjects) or extensive metabolizers (six).

INTERVENTION

Each subject received rabeprazole 20 mg twice/day for 3 consecutive days and once/day on the fourth day.

MEASUREMENTS AND MAIN RESULTS

Pharmacokinetic and pharmacodynamic parameters were compared between CYP2C19 poor and extensive metabolizers on day 1 and day 4 of dosing. The mean +/- SD values of area under the concentration-time curve of rabeprazole and rabeprazole thioether were significantly higher in poor metabolizers than in extensive metabolizers on day 1 (5357 +/- 883 vs 1131 +/- 512 ng x hr/ml and 1703 +/- 432 vs 561 +/- 358 ng x hr/ml, respectively; p<0.001) and on day 4 (5601 +/- 669 vs 1619 +/- 778 ng x hr/ml and 1914 +/- 378 vs 511 +/- 360 ng x hr/ml, respectively; p<0.001). However, no significant difference was noted between day 1 and day 4 of dosing within the same genotype groups. Only CYP2C19 poor metabolizers had significantly higher plasma gastrin levels on day 4 compared with those levels on day 1 (p<0.05). The pharmacokinetic-pharmacodynamic relationship of rabeprazole appears to be time dependent.

CONCLUSION

The pharmacokinetic and pharmacodynamic data suggest that CYP2C19 poor metabolizers might be subject to advantageous conditions, especially after day 4, for treating H. pylori infection with rabeprazole.

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.

Polymorphism of CYP2C19 and gastric emptying in patients with proton pump inhibitor-resistant gastric ulcers

The aim of the present study was to investigate whether CYP2C19 polymorphism status and gastric emptying are related to healing in patients with gastric ulcers. We studied the CYP2C19 status in seven patients with proton pump inhibitor (PPI)-resistant ulcers, 21 with PPI-sensitive ulcers and 46 healthy volunteers using polymerase chain reaction restriction fragment length polymorphism to detect CYP2C19m1 mutation in exon 5 and CYP2C19m2 mutation in exon 4. Gastric emptying was evaluated using the 13C-acetate breath test. The frequency of phenotypes, indicated by genotypes, did not differ significantly between the three patient groups. The peak time of 13C excretion in patients with PPI-resistant ulcers was significantly longer than that of patients with PPI-sensitive ulcers and healthy volunteers. Our results suggest that rate of gastric emptying, but not CYP2C19 polymorphism, is likely to be an important factor in the delayed healing of patients with PPI-resistant gastric ulcer.