A study of the interaction of omeprazole and warfarin in anticoagulated patients

Elevated INR in a COVID-19 patient after concomitant administration of azvudine and anticoagulants

Background: Azvudine (FNC) is a promising treatment candidate for managing coronavirus disease 2019 (COVID-19). However, drug interactions with azvudine have been poorly studied, especially with no reported cases of azvudine with anticoagulants such as warfarin and rivaroxaban. Case summary: The patient was diagnosed with lower limb venous thrombosis and took warfarin regularly. The international normalized ratio (INR) was stable (2.0-3.0). However, the INR increased to 7.52 after administering azvudine. The patient had no other factors justifying this change. This increase in INR occurred again with the administration of azvudine in combination with rivaroxaban, and the INR increased to 18.91. After azvudine administration was stopped, the INR did not increase when rivaroxaban was used alone. Conclusion: Azvudine, warfarin, and rivaroxaban might have previously unidentified drug interactions that increased the INR. Therefore, the INR must be closely monitored when they are concomitantly administered in COVID-19 patients.

Warfarin dose adjustment required after cannabidiol initiation and titration

PURPOSE

A case of a possible interaction between cannabidiol and warfarin is presented along with a brief overview of cytochrome enzymes involved in these drugs' metabolism.

SUMMARY

A 46-year-old male taking warfarin for treatment of a deep venous thrombosis was initiated on a Food and Drug Administration (FDA)-approved cannabidiol product (Epidiolex, Greenwich Biosciences) for intractable epilepsy. The patient's International Normalized Ratio (INR) was monitored closely during cannabidiol initiation and dose titration. The patient required a nearly 20% warfarin dose reduction to maintain an INR within the goal range after starting therapy with cannabidiol. There is 1 other case report describing a clinically significant interaction between cannabidiol (specifically Epidiolex) and warfarin in a patient receiving warfarin who was enrolled in a study involving the initiation and titration of cannabidiol; that patient developed a supratherapeutic INR of 6.86 and required a 30% reduction in the weekly warfarin dose to reachieve the goal INR.

CONCLUSION

A previously published report suggesting an interaction between cannabidiol and warfarin is supported by this case report. INR should be monitored frequently in patients taking warfarin who begin to take FDA-approved cannabidiol. Additional studies should be performed to clarify the interaction potential of cannabidiol and warfarin.

Association of marked prolongation of prothrombin time-international normalized ratio with warfarin and endoscopic nasobiliary drainage for biliary fistula after left hemihepatectomy

WHAT IS KNOWN AND OBJECTIVE

Vitamin K deficiency is known to cause impaired coagulation. We report a case of marked prolongation of the prothrombin time-international normalized ratio (PT-INR) associated with warfarin and vitamin K deficiency caused by endoscopic nasobiliary drainage (ENBD).

CASE PRESENTATION

Oral administration of warfarin was initiated in a 67-year-old man after left hemihepatectomy. He developed a biliary fistula after surgery that was treated by ENBD, which resulted in significant prolongation of the PT-INR.

WHAT IS NEW AND CONCLUSION

The effect of warfarin was enhanced in this patient due to reduced absorption of vitamin K as a result of external biliary drainage.

Proton pump inhibitors co-prescribed with warfarin reduce warfarin control as measured by time in therapeutic range

INTRODUCTION

Warfarin is an oral anticoagulant that requires ongoing monitoring with time in therapeutic range (TTR), a common measure of the quality of warfarin control and likelihood of adverse events including bleeds. Numerous factors can influence these warfarin outcomes including drug interactions. Proton pump inhibitors (PPIs) have been reported to interact with warfarin but there remain conflicting reports with regard to the impact on bleeds and limited data on TTR. Therefore, the aim of this study was to determine the effect of PPIs on warfarin control using TTR and bleeds as endpoints.

METHODS

Retrospective data were collected for patients managed by a warfarin management clinic in Queensland. Data collected included current medications, reported bleeds and INR results to calculate TTR. Patients were grouped as taking PPIs or not taking PPIs. Analysis of TTR and bleeds occurred for these groups both before and after exclusions of other medication reported to interact with warfarin.

RESULTS

Of the 4494 included patients, almost half (44.5%) were taking PPIs with esomeprazole most commonly (34.8%) prescribed. Patients taking PPIs had significantly reduced TTR compared with patients not taking PPIs both before (78.5 ± 9.7% vs 81.7 ± 10.2%, P < 0.0001) and after (84.4 ± 5.1% vs 89.4 ± 8.0%, P < 0.0001) excluding all other interacting medications with warfarin. Patients taking PPIs also had significantly higher incidence of minor bleeds compared with patients not taking PPIs (32.4% vs 26.1%, P = 0.0487).

DISCUSSION

Patients taking PPIs with warfarin had significantly lower TTRs and higher incidence of minor bleeds. This combination warrants additional caution and monitoring with warfarin control as measured by TTR potentially affected.

Drug Interactions with Antiulcer Agents: Considerations in the Treatment of Acid-Peptic Disease

All of the antiulcer agents have been implicated in drug interactions. These agents generally influence the absorption, metabolism, or elimination of other medications. However, these interactions can lead to alterations in pharmacodynamic response. The mechanisms by which antiulcer agents produce drug interactions differ among the agents. It is beyond the scope of this article to review all of the drug interactions that have been reported with antiulcer agents. However, it is the intent to provide the reader with a detailed understanding of the mechanisms by which antiulcer agents may interact with other medications and to provide insight into factors that may influence the potential magnitude or clinical consequences of these interactions. An understanding of antiulcer drug interactions will aid pharmacists in assisting clinicians with drug selection and/or monitoring of drug interactions. Specifically, pharmacists can assist with the identification of potential antiulcer drug interactions and develop strategies designed to minimize adverse consequences of these interactions.

Pharmacokinetic drug interaction profiles of proton pump inhibitors: an update

Proton pump inhibitors (PPIs) are used extensively for the treatment of gastric acid-related disorders, often over the long term, which raises the potential for clinically significant drug interactions in patients receiving concomitant medications. These drug–drug interactions have been previously reviewed. However, the current knowledge is likely to have advanced, so a thorough review of the literature published since 2006 was conducted. This identified new studies of drug interactions that are modulated by gastric pH. These studies showed the effect of a PPI-induced increase in intragastric pH on mycophenolate mofetil pharmacokinetics, which were characterised by a decrease in the maximum exposure and availability of mycophenolic acid, at least at early time points. Post-2006 data were also available outlining the altered pharmacokinetics of protease inhibitors with concomitant PPI exposure. New data for the more recently marketed dexlansoprazole suggest it has no impact on the pharmacokinetics of diazepam, phenytoin, theophylline and warfarin. The CYP2C19-mediated interaction that seems to exist between clopidogrel and omeprazole or esomeprazole has been shown to be clinically important in research published since the 2006 review; this effect is not seen as a class effect of PPIs. Finally, data suggest that coadministration of PPIs with methotrexate may affect methotrexate pharmacokinetics, although the mechanism of interaction is not well understood. As was shown in the previous review, individual PPIs differ in their propensities to interact with other drugs and the extent to which their interaction profiles have been defined. The interaction profiles of omeprazole and pantoprazole sodium (pantoprazole-Na) have been studied most extensively. Several studies have shown that omeprazole carries a considerable potential for drug interactions because of its high affinity for CYP2C19 and moderate affinity for CYP3A4. In contrast, pantoprazole-Na appears to have lower potential for interactions with other medications. Lansoprazole and rabeprazole also seem to have a weaker potential for interactions than omeprazole, although their interaction profiles, along with those of esomeprazole and dexlansoprazole, have been less extensively investigated. Only a few drug interactions involving PPIs are of clinical significance. Nonetheless, the potential for drug interactions should be considered when choosing a PPI to manage gastric acid-related disorders. This is particularly relevant for elderly patients taking multiple medications, or for those receiving a concomitant medication with a narrow therapeutic index.

Monitoring the distribution of warfarin in blood plasma

Warfarin is an anticoagulant drug extensively used in the treatment and prevention of thrombotic disorders. Previous studies have shown that warfarin binds extensively to blood plasma proteins and that only a small fraction of the drug is unbound and thus available for therapeutic function. Both warfarin's narrow therapeutic window and the susceptibility of anticoagulant function to patient-dependent factors necessitate regular monitoring. In this study, we have shown that the lifetimes for each of the various bound and free forms of the drug in blood plasma can be quantified in situ by time-correlated single-photon counting fluorescence spectroscopy over the clinically significant concentration range. A relationship between the blood coagulation and the distribution of fluorescence lifetimes was observed. The in situ detection of clinically relevant concentrations of warfarin in its respective bound and unbound forms could provide a prognostic tool for use in patient treatment.

Hemarthrosis in a patient on warfarin receiving ceftaroline: a case report and brief review of cephalosporin interactions with warfarin

OBJECTIVE

To report a possible interaction between warfarin and ceftaroline, resulting in hemarthrosis, and provide readers with an understanding of mechanisms of interaction between cephalosporins and warfarin.

CASE SUMMARY

Ceftaroline was prescribed for an 85-year-old female with a therapeutic international normalized ratio (INR) hospitalized for the treatment of cellulitis. She was subsequently readmitted with shoulder pain and a supratherapeutic INR. The patient was diagnosed with hemarthrosis, presumably related to elevated INR. Evaluation using the drug interaction probability scale for warfarin and ceftaroline yielded a score consistent with a possible or probable interaction.

DISCUSSION

Cephalosporins may interact with warfarin through a variety of mechanisms, including potentiation of hypoprothrombinemia related to certain side chain groups, inhibition of P-glycoprotein, or alteration of gastrointestinal flora. All mechanisms reported in the medical literature as of April 2012 are briefly examined, but the latter is the most reasonable mechanism for a ceftaroline interaction with warfarin.

CONCLUSIONS

Health care providers should consider closely monitoring patients receiving antibiotics with activity against Enterobacteriaceae and warfarin, even if no direct mechanism of interaction has been reported. Further research regarding a ceftaroline-warfarin interaction is warranted.

Adverse drug reactions and drug interactions as causes of hospital admission in oncology

CONTEXT

Although several studies have evaluated the frequency of adverse drug reactions (ADRs) and drug-drug interactions (DDIs) in general medicine, few studies have looked at the epidemiology of adverse drug events (ADEs) in oncology.

OBJECTIVES

We sought to investigate how many hospital admissions in oncology are related to a DDI or an ADR.

METHODS

All cancer patients admitted to an oncology ward during an eight-month period had their charts retrospectively evaluated for reasons of hospitalization, using a 4-point scale (definitely, probably, possibly, or unlikely associated) to classify admissions by their probability of being associated with either a DDI or an ADR.

RESULTS

From September 2007 to May 2008, there were 550 hospital admissions and 458 were eligible. Among unplanned admissions (n=298), 39 (13.0%, 95% confidence interval [CI] 9.4%-17.4%) were considered to be associated with an ADE, 33 (11.0%, 95% CI 7.7%-15.2%) with an ADR, and six (2.0%, 95% CI 0.7%-4.3%) with a DDI. The most common DDIs involved warfarin, captopril, and anti-inflammatory agents, and the most frequent ADR was neutropenic fever post-chemotherapy. Most patients were discharged completely recovered, but two patients died.

CONCLUSION

Approximately one in 10 unplanned hospitalizations of cancer patients is associated with an ADE. Prospective and population-based studies are warranted to evaluate their magnitude in oncology.

Drug interactions in oncology: how common are they?

BACKGROUND

Drug-drug interactions (DDIs) comprise an important problem in medical oncology practice. We systematically reviewed the frequency of DDIs in oncology.

METHODS

We searched PubMed for eligible articles and on-line databases for abstracts of major oncology meetings.

RESULTS

Eight studies reported on the frequency of DDIs: six evaluated the frequency of potential DDIs, while two studies reported on real DDIs, i.e. interactions that had clinical consequences. Studies of potential DDIs found that approximately one-third of patients are exposed to dangerous drug doublets, with the most common ones involving warfarin and anticonvulsants. One study of real DDIs found that 2% of hospitalized cancer patients had a DDI as the cause of admission.

CONCLUSIONS

Drug interactions comprise an important issue in oncology, with approximately one-third of ambulatory cancer patients being at risk of DDIs. Data are limited on the clinical consequences of drug interactions among cancer patients.

Probable interaction between warfarin and marijuana smoking

OBJECTIVE

To report a probable interaction between warfarin and marijuana smoking, resulting in increased international normalized ratio (INR) values and bleeding complications.

CASE SUMMARY

A 56-year-old white male had been receiving chronic warfarin therapy for 11 years after mechanical heart valve replacement. He was admitted to the hospital with a diagnosis of upper gastrointestinal bleeding. Upon admission, his INR value was supratherapeutic at 10.41, and his hemoglobin level was 6.6 g/dL. He received 4 units of fresh frozen plasma and one 10-mg dose of oral vitamin K; his INR was 1.8 the next day. He was discharged 7 days after admission. Fifteen days after hospital discharge, he was readmitted with a constant nosebleed and increased bruising. His INR value was 11.55. After treatment, he was discharged with an INR value of 1.14. The patient smoked marijuana more frequently throughout the period of these 2 hospitalizations due to his depression. He was counseled by the pharmacist on the potential interaction of warfarin and marijuana. The patient decided to stop smoking marijuana after the third counseling session. During the 9 months that he did not smoke marijuana, his INR values ranged from 1.08 to 4.40 with no significant bleeding complications.

DISCUSSION

Marijuana may increase warfarin anticoagulant effect by inhibiting its metabolism, and to a lesser extent, displacing warfarin from protein-binding sites. Other causes (eg, nonadherence) of the patient's increased INR were ruled out. Using the Horn Drug Interaction Probability Scale, our patient's warfarinmarijuana interaction appeared to be probable.

CONCLUSIONS

To our knowledge, there have been no other reported cases of warfarin-marijuana interaction. While more clinical reports would be useful to confirm this interaction, clinicians should be aware of its probability so as to manage patients appropriately.

Drug interaction studies with dexlansoprazole modified release (TAK-390MR), a proton pump inhibitor with a dual delayed-release formulation: results of four randomized, double-blind, crossover, placebo-controlled, single-centre studies

BACKGROUND AND OBJECTIVE

Most proton pump inhibitors are extensively metabolized by cytochrome P450 (CYP) isoenzymes, as are many other drugs, giving rise to potential drug-drug interactions. Dexlansoprazole modified release (MR) [TAK-390MR] is a modified-release formulation of dexlansoprazole (TAK-390), an enantiomer of lansoprazole, which employs an innovative Dual Delayed Release technology designed to prolong the plasma dexlansoprazole concentration-time profile following once-daily oral administration. As with lansoprazole, dexlansoprazole is metabolized mainly by CYP3A and CYP2C19. Based on in vitro studies, dexlansoprazole has the potential to inhibit activity of these isoenzymes and also may induce human hepatic CYP1A and CYP2C9 activity. To determine whether dexlansoprazole has an effect on these isoenzymes in vivo, drug interaction studies with dexlansoprazole MR were conducted.

METHODS

Four separate randomized, double-blind, two-way crossover, placebo-controlled, single-centre studies were conducted in healthy volunteers to evaluate the effect of dexlansoprazole on the pharmacokinetics of four test substrates (diazepam, phenytoin, theophylline [administered as intravenous aminophylline] and warfarin), which were selected based on in vitro and/or in vivo data that suggest a potential drug interaction with CYP isoenzymes or potentially coadministered narrow therapeutic index drugs. In each study, dexlansoprazole MR 90 mg or placebo was administered once daily for 9 or 11 days in each period. Subjects received a single dose of test substrate in each study period. Pharmacokinetic parameters of the test substrates were estimated using noncompartmental methods. A conclusion of no effect of dexlansoprazole MR on the test substrate was made if the 90% confidence intervals (CIs) for the ratios of the central values for the observed maximum plasma drug concentration (C(max)) and the area under the plasma concentration-time curve (AUC) of test substrate administered with dexlansoprazole MR versus placebo were within 0.80-1.25 based on an analysis of variance model. The potential for a pharmacodynamic interaction was also assessed for warfarin using prothrombin time, measured as the international normalized ratio. Routine safety assessments were conducted in these studies.

RESULTS

Mean C(max) and AUC values were generally similar for each test substrate when administered with multiple once-daily doses of dexlansoprazole MR or placebo. The 90% CIs for the bioavailability of these test substrates administered with dexlansoprazole MR relative to that obtained when the substrates were administered with placebo were within the bioequivalency range of 0.80-1.25, indicating that multiple doses of dexlansoprazole MR had no effect on the pharmacokinetics of these drugs. Additionally, dexlansoprazole MR had no effect on the pharmacodynamics of warfarin. Administration of these drugs with dexlansoprazole MR 90 mg or placebo was well tolerated; the only serious adverse event, which led to a subject's discontinuation from the study, was considered unrelated to study drugs.

CONCLUSIONS

Coadministration of dexlansoprazole MR with diazepam, phenytoin or theophylline did not affect the pharmacokinetics of these drugs, and therefore is unlikely to alter the pharmacokinetic profile of other drugs metabolized by CYP2C19, CYP2C9, CYP1A2 and perhaps CYP3A. Additionally, dexlansoprazole MR coadministered with warfarin did not affect the pharmacokinetics of the warfarin enantiomers and had no effect on the anticoagulant activity of warfarin. Dexlansoprazole MR was well tolerated in these trials of healthy subjects.

Stereoselective inhibition of cytochrome P450 forms by lansoprazole and omeprazolein vitro

The stereoselectivity of the inhibitory interaction potential of lansoprazole and omeprazole isomers on six human cytochrome P450 forms was evaluated using human liver microsomes. Lansoprazole enantiomers showed stereoselective inhibition of CYP2C9-catalysed tolbutamide 4-methylhydroxylation, CYP2C19-catalysed S-mephenytoin 4'-hydroxylation, CYP2D6-catalysed dextromethorphan O-demethylation, CYP2E1-catalysed chlorzoxazone 6-hydroxylation and CYP3A4-catalysed midazolam 1-hydroxylation, whereas omeprazole only inhibited CYP2C19 stereoselectively. Of the P450 forms tested, CYP2C19-catalysed S-mephenytoin 4'-hydroxylation was extensively inhibited by both the lansoprazole and omeprazole enantiomers in a competitive and stereoselective manner; the S-enantiomers of both drugs inhibited the hydroxylation more than the R-enantiomers. The estimated K(i) values determined for CYP2C19-catalysed S-mephenytoin 4'-hydroxylation were 0.6, 6.1, 3.4 and 5.7 microM for S-lansoprazole, R-lansoprazole, S-omeprazole and R-omeprazole, respectively. The results indicate that although both lansoprazole and omeprazole are strong inhibitors of CYP2C19, the inhibition of CYP2C19 by lansoprazole is highly stereoselective, whereas the inhibition by omeprazole is less stereoselective. In addition, S-lansoprazole, the most potent CYP2C19 inhibitor, is not a good CYP2C19-selective inhibitor owing to its inhibition of other P450 forms.

The role of cytochrome P2C19 in R-warfarin pharmacokinetics and its interaction with omeprazole

Previous studies reported omeprazole to be an inhibitor of cytochrome P450 (CYP) 2C19 and suggested the pharmacokinetic interaction of omeprazole with R-warfarin. The aim of this study was to compare possible effects of omeprazole on the stereoselective pharmacokinetics and pharmacodynamics of warfarin between CYP2C19 genotypes. Seventeen subjects, of whom 10 were homozygous extensive metabolizers (hmEMs) and seven were poor metabolizers (PMs) for CYP2C19, were enrolled in this randomized crossover study, and they ingested 20 mg omeprazole or placebo once daily for 11 days. On day 7, they administered a single dose of 10 mg racemic warfarin. The plasma concentrations of warfarin enantiomers and prothrombin time expressed as international normalized ratio were monitored up to 120 hours. During the placebo phase, area under the plasma concentration-time curve (AUC) and elimination half-life (t1/2) of R-warfarin in PMs was significantly greater than those in hmEMs (AUC[0-infinity], 42,938/34,613 ng h/mL [PM/hmEM], P = 0.004; t1/2, 48.8/40.8 hours [PM/hmEM], P = 0.013). Omeprazole treatment significantly increased the AUC(0-infinity) (41,387 ng h/mL, P = 0.004) and t1/2 (46.4 hours, P = 0.017) of R-warfarin in hmEMs to levels comparable to those in the PMs. There were no differences in S-warfarin pharmacokinetics between the CYP2C19 genotypes (AUC[0-infinity], 15,851/16,968 ng*h/mL [PM/hmEM]; t1/2, 22.7/25.4 h [PM/hmEM]), or between the two treatment phases (AUC[0-infinity], 14,756/18,166 ng h/mL [PM/hmEM]; t1/2, 27.0/25.4 hours [PM/hmEM] in the omeprazole phase) as well as anticoagulant effects. These results indicate that CYP2C19 activity was one of determinants on the R-warfarin disposition because the pharmacokinetics of warfarin enantiomers were different between the CYP2C19 genotypes and the omeprazole affected the R-warfarin pharmacokinetics of CYP2C19 in only hmEMs. However, the phamacodynamic effect of the interaction of warfarin with omeprazole would be of minor clinical significance.

Control of gastric acid secretion in health and disease

Recent milestones in the understanding of gastric acid secretion and treatment of acid-peptic disorders include the (1) discovery of histamine H(2)-receptors and development of histamine H(2)-receptor antagonists, (2) identification of H(+)K(+)-ATPase as the parietal cell proton pump and development of proton pump inhibitors, and (3) identification of Helicobacter pylori as the major cause of duodenal ulcer and development of effective eradication regimens. This review emphasizes the importance and relevance of gastric acid secretion and its regulation in health and disease. We review the physiology and pathophysiology of acid secretion as well as evidence regarding its inhibition in the management of acid-related clinical conditions.

The effect of etoricoxib on the pharmacodynamics and pharmacokinetics of warfarin

The effects of etoricoxib on pharmacodynamic and pharmacokinetic parameters of warfarin were determined in healthy men and women. Subjects titrated with warfarin to an international normalized ratio for prothrombin time of 1.4 to 1.7 during a 28-day prestudy period were randomly assigned in crossover fashion to be coadministered etoricoxib (120 mg) or matching placebo over two 21-day continuous periods. On day 21, a 24-hour pharmacokinetic profile of both S(-) and R(+) warfarin, as well as international normalized ratio values, were determined. Etoricoxib increased the international normalized ratio by 13% (90% confidence interval: 8%, 19%; P </= .001). Etoricoxib had no effect on the pharmacokinetics of S(-) warfarin but led to a modest increase in the AUC(24 h) ( approximately 10%) of R(+) warfarin. This increase in the international normalized ratio is not likely to be clinically important in most patients; however, the international normalized ratio of patients coadministered oral anticoagulants and etoricoxib should be closely monitored, particularly during initiation of therapy.

Safety issues relating to long-term treatment with histamine H2-receptor antagonists

H2-receptor antagonist therapy is associated with a low incidence of adverse reactions. Adverse events reported in clinical trials of ranitidine in daily doses of up to 1200 mg include headache, tiredness and mild gastrointestinal disturbances, but the incidence is similar to or less than that for placebo. High doses of cimetidine (> 5 g/day) can cause reversible impotence or gynaecomastia. While ranitidine exhibits no clinically significant drug-drug interactions, cimetidine interacts with many drugs metabolized by cytochrome P450. In contrast to ranitidine and cimetidine, where safety data are available for up to 10 years of continuous therapy, experience with famotidine and nizatidine is limited. The safety of long-term H2-receptor antagonist therapy needs to be considered in relation to the potential consequences of prolonged acid suppression, including the risk of proliferation of gastric flora and the risk of developing enterochromaffin-like cell hyperplasia, which could in turn, theoretically, lead to gastric malignancy. Such problems have not been observed in patients during long-term therapy at low or full doses of H2-receptor antagonists. Standard doses of currently available H2-receptor antagonists permit acid secretion in response to food and other stimuli, and this daily acid tide prevents persistent bacterial colonization.

Effects of Daily Ingestion of Cranberry Juice on the Pharmacokinetics of Warfarin, Tizanidine, and Midazolam—Probes of CYP2C9, CYP1A2, and CYP3A4

Case reports suggest that cranberry juice can increase the anticoagulant effect of warfarin. We investigated the effects of cranberry juice on R-S-warfarin, tizanidine, and midazolam; probes of CYP2C9, CYP1A2, and CYP3A4. Ten healthy volunteers took 200 ml cranberry juice or water t.i.d. for 10 days. On day 5, they ingested 10 mg racemic R-S-warfarin, 1 mg tizanidine, and 0.5 mg midazolam, with juice or water, followed by monitoring of drug concentrations and thromboplastin time. Cranberry juice did not increase the peak plasma concentration or area under concentration-time curve (AUC) of the probe drugs or their metabolites, but slightly decreased (7%; P=0.051) the AUC of S-warfarin. Cranberry juice did not change the anticoagulant effect of warfarin. Daily ingestion of cranberry juice does not inhibit the activities of CYP2C9, CYP1A2, or CYP3A4. A pharmacokinetic mechanism for the cranberry juice-warfarin interaction seems unlikely.

Pharmacokinetic drug interaction profiles of proton pump inhibitors

Proton pump inhibitors are used extensively for the treatment of gastric acid-related disorders because they produce a greater degree and longer duration of gastric acid suppression and, thus, better healing rates, than histamine H(2) receptor antagonists. The need for long-term treatment of these disorders raises the potential for clinically significant drug interactions in patients receiving proton pump inhibitors and other medications. Therefore, it is important to understand the mechanisms for drug interactions in this setting. Proton pump inhibitors can modify the intragastric release of other drugs from their dosage forms by elevating pH (e.g. reducing the antifungal activity of ketoconazole). Proton pump inhibitors also influence drug absorption and metabolism by interacting with adenosine triphosphate-dependent P-glycoprotein (e.g. inhibiting digoxin efflux) or with the cytochrome P450 (CYP) enzyme system (e.g. decreasing simvastatin metabolism), thereby affecting both intestinal first-pass metabolism and hepatic clearance. Although interactions based on the change of gastric pH are a group-specific effect and thus may occur with all proton pump inhibitors, individual proton pump inhibitors differ in their propensities to interact with other drugs and the extent to which their interaction profiles have been defined. The interaction profiles of omeprazole and pantoprazole have been studied most extensively. A number of studies have shown that omeprazole carries a considerable potential for drug interactions, since it has a high affinity for CYP2C19 and a somewhat lower affinity for CYP3A4. In contrast, pantoprazole appears to have lower potential for interactions with other medications. Although the interaction profiles of esomeprazole, lansoprazole and rabeprazole have been less extensively investigated, evidence suggests that lansoprazole and rabeprazole seem to have a weaker potential for interactions than omeprazole. Although only a few drug interactions involving proton pump inhibitors have been shown to be of clinical significance, the potential for drug interactions should be taken into account when choosing a therapy for gastric acid-related disorders, especially for elderly patients in whom polypharmacy is common, or in those receiving a concomitant medication with a narrow therapeutic index.

Tibolone (Livial) enhances warfarin-induced anticoagulation in postmenopausal women

OBJECTIVE

To investigate the potential drug interaction between tibolone and warfarin in healthy postmenopausal women.

METHODS AND RESULTS

The study was designed as a double-blind, randomized, placebo-controlled, two-way crossover study in postmenopausal women. After stabilization of the International Normalized Ratio (INR; a standardized prothrombin time, PT) between 1.4 and 2.0 with warfarin, subjects were randomized to receive either tibolone (2.5mg/day) or placebo for 21 days. After a 7-day wash-out period (during which warfarin treatment was continued) the treatments were crossed over. Primary efficacy parameters were INR and coagulation Factors II, VII, VIIa and X (means of measurements at Days 18 and 20 and Days 46 and 48). Treatment with tibolone induced a statistically significant increase in INR (estimate of mean difference=0.40; P=0.002), and a statistically significant decrease in coagulation factors. Treatments were generally well tolerated and no clinically significant adverse events were observed.

CONCLUSIONS

Tibolone enhances warfarin-induced anticoagulation in postmenopausal women, as reflected by increases in INR and decreases in coagulation Factors II, VII, VIIa and X, compared to placebo. It is advisable to monitor for changes in coagulation status during (and after discontinuation of) simultaneous use of tibolone and warfarin.

Safety of potent gastric acid inhibition

The proton pump inhibitors are a very effective drug group for the control of gastric acid secretion, which makes them of great use in the medical practice setting, while at the same time they represent one of the treatment groups widely used in Western European countries. These factors lead to this drug group being prescribed in all age populations, quite often in polymedicated patients and with pluripathology, and on many occasions during prolonged periods of time. All these determinant factors sometimes make the safety profile of proton pump inhibitors disputable. In this respect all of them have been shown to have little adverse events and are safe in long-term treatment. The risk of drug interactions when prescribed in association with other drugs is low and their repercussion in the medical practice setting is quite exceptional as they require few dosage adjustments in patients with severe concomitant diseases and in elderly patients. Finally, their safety is high in pregnant women and in children, although further studies in this population are required to corroborate this evidence.

Effect of gemfibrozil on the pharmacokinetics and pharmacodynamics of racemic warfarin in healthy subjects

AIMS

Case reports suggest that gemfobrozil can increase the anticoagulant effect of warfarin. Because gemfibrozil inhibits CYP2C9 in vitro, we studied its effects on the pharmacokinetics and pharmacodynamics of racemic warfarin.

METHODS

In a randomized cross-over study, 10 healthy subjects ingested 600 mg gemfibrozil or placebo twice daily for 8 days. On day 3, they were administered a single dose of 10 mg racemic R-S-warfarin orally. The concentrations of R- and S-warfarin in plasma and thromboplastin time were monitored up to 168 h.

RESULTS

Gemfibrozil decreased the mean (+/-SD) area under the plasma concentration-time curve [AUC((0-infinity))] of S-warfarin by 11%, from 19.9 +/- 5.2 mg l(-1) h to 17.6 +/- 4.7 mg l(-1) h (95% CI on the difference -3.7, -0.78; P < 0.01) and that of R-warfarin by 6% from 31.3 +/- 7.5 mg l(-1) h during the gemfibrozil phase to 29.5 +/- 6.9 mg l(-1) h during the placebo phase (95% CI -3.3, -0.33; P < 0.05). There were no significant differences in the elimination half-lives of S- or R-warfarin between the phases. Gemfibrozil did not alter the anticoagulant effect of warfarin.

CONCLUSION

Unexpectedly, gemfibrozil slightly decreased the plasma concentrations of R- and S-warfarin. Displacement of warfarin from plasma albumin by gemfibrozil or its interference with the absorption of warfarin could explain the present findings. Usual therapeutic doses of gemfibrozil seem to have limited effects on the pharmacokinetics and pharmacodynamics of single dose warfarin in healthy subjects.

Effect of omeprazole on the anticoagulant activity and the pharmacokinetics of warfarin enantiomers in rats

The effect of the proton pump inhibitor omeprazole on the anticoagulation and the pharmacokinetics of warfarin enantiomers was studied in rats. Omeprazole given intraperitoneally in a daily dose of 0.67 mg/kg over 8 days had no significant effect on the absorption, distribution and the total serum clearance values of the S- and R-enantiomers of warfarin. Omeprazole did not affect the pre-treatment baseline blood coagulation and the in vitro rat serum protein binding of warfarin enantiomers. In vitro study with rat liver microsomes showed that omeprazole had an inhibitory effect on the hydroxylation of warfarin enantiomers. Results obtained from in vivo urinary excretion study revealed that omeprazole inhibited the formation clearance of both S- and R-form oxidative metabolites, but increased that of the overall reductive metabolites, and the renal clearance of S- and R-enantiomers, of warfarin. As a consequence, the total serum clearance values for warfarin enantiomers remained unchanged and the hypoprothrombinaemic response produced by warfarin was not affected.

Impact of cotherapy with some proton pump inhibitors on medical claims among HMO patients already using other common drugs also cleared by cytochrome P450

Adverse drug event (ADE) rates resulting from coadministration of proton pump inhibitors (PPIs) and other drugs with potential for interaction with PPIs (DPIs) are unknown. This retrospective study assesses the occurrence of such ADEs and their potential impact on medical care costs by reviewing integrated medical and pharmacy claims. Managed care patients receiving one or more DPIs were identified. Within this sample, those who were also prescribed omeprazole or lansoprazole (DPI + PPI) were included. A second cohort (DPI alone) was created, matching for age, gender, and DPI use. Rates of ADEs were followed for 6 months after entry. Among PPI users, 58% used one or more DPIs, whereas 7% of DPI subjects used a PPI. Among claims arising from ADEs occurring in more than 1% of patients, 14 occurred in the DPI + PPI group and 2 occurred in the DPI-alone cohort, respectively, a highly significant difference. Crude odds ratios for the risks of specific ADEs were significantly increased for cotherapy with a PPI and warfarin, clarithromycin, corticosteroids, carbamazepine, nifedipine, or diclofenac. After adjustment, the first three associations remained significant. Coprescription of a PPI with potentially interacting drugs was common in practice and associated with significantly increased claims for medical care.

General medicine and surgery for dental practitioners. Part 3: gastrointestinal system

Diseases of the gastrointestinal (GI) system can be relevant to the dental surgeon for several reasons. The mouth may display signs of the disease itself, for example the cobblestone mucosa, facial or labial swelling of Crohn's disease, or the osteomata of Gardner's syndrome. These are well covered elsewhere and not discussed further here. The sequelae of GI disease, for example gastric reflux producing dental erosion, iron deficiency anaemia and treatment such as corticosteroid therapy may all have a bearing on management and choice of anaesthesia.

A summary of Food and Drug Administration-reported adverse events and drug interactions occurring during therapy with omeprazole, lansoprazole and pantoprazole

BACKGROUND

Pantoprazole is claimed to have a lower potential for drug interaction than other proton pump inhibitors.

AIM

To estimate the frequency of adverse events and drug interactions reported to the Food and Drug Administration in patients receiving omeprazole, lansoprazole or pantoprazole.

METHODS

The study involved a search of the Food and Drug Administration's database for adverse events and drug interactions with omeprazole, lansoprazole or pantoprazole as primary or secondary suspect drug. An estimate of the amount of drug dispensed during the adverse event collection period (from US drug launch) was obtained from the International Medical Statistics health database.

RESULTS

Of the suspected drug interactions recorded, vitamin K antagonist interactions, although rare, were the most common. The frequency of vitamin K antagonist interactions was 0.09 per million packages for omeprazole and 0.11 per million packages for lansoprazole and pantoprazole. Interactions with benzodiazepines or phenytoin were even rarer, being reported in less than 10 patients on each proton pump inhibitor.

CONCLUSION

The frequency of reported drug interactions was low for omeprazole, lansoprazole and pantoprazole and vitamin K antagonist interactions were by far the most common. These potentially important drug interactions, although rare, were no less frequent on pantoprazole than on omeprazole or lansoprazole, suggesting a class effect.

Gastroesophageal reflux in children: pathogenesis, prevalence, diagnosis, and role of proton pump inhibitors in treatment

A substantial percentage of infants, children and adolescents experience gastroesophageal reflux disease (GERD) and its accompanying symptoms, as well as disease complications. The diagnosis of GERD in children is made based upon the child's history, and data derived primarily from pH monitoring tests and endoscopy. In those children with confirmed reflux disease, the options for management parallel those recommended in adult patients, with the first step consisting of lifestyle changes. Surgical procedures may also be performed; however, these are rarely recommended prior to an adequate course of pharmacologic therapy, and appropriate case selection is important. Among the current pharmacotherapeutic options available in the US, the prokinetic agents and the acid-inhibitory agents (histamine-2 receptor antagonists, proton pump inhibitors) are the most widely used. The clinical utility of the prokinetic agents has been limited by the recent withdrawal of cisapride from the US marketplace and the potential for irreversible central nervous system complications with metoclopramide. Numerous clinical studies performed in adults, and several studies involving children, have demonstrated that the proton pump inhibitors are more effective than the histamine-2 receptor antagonists in the relief of GERD symptoms and healing of erosive esophagitis. In children, omeprazole and lansoprazole may be administered as the intact oral capsule, or in those who are unable or unwilling to swallow, the granule contents of the capsule may be mixed with soft foods (e.g. apple sauce) or fruit drinks/liquid dietary supplements prior to oral administration with no detrimental effects on pharmacokinetics, bioavailability, or pharmacodynamics. Studies performed with omeprazole and lansoprazole in children have shown pharmacokinetic parameters that closely resemble those observed in adults. In over a decade of use in adults, the proton pump inhibitor class of agents has been found to have a good safety profile. Studies involving children have also shown these agents to be well tolerated. In numerous drug-drug interaction studies performed with these two proton pump inhibitors, relatively few clinically significant interactions have been observed.

The nature and frequency of potential warfarin drug interactions that increase the risk of bleeding in patients with atrial fibrillation

PURPOSE

To determine the frequency with which atrial fibrillation (AF) patients receiving warfarin are prescribed interacting drugs that could increase bleeding risks.

METHODS

We retrospectively examined medical records for 704 Medicare beneficiaries > or = 65 years of age discharged from Kansas hospitals with AF. We identified all patients receiving warfarin and examined discharge prescriptions for drugs that could increase bleeding risk either by increasing the international normalized ratio (INR) or directly inhibiting hemostasis.

RESULTS

Of 256 patients discharged on warfarin, 138 (54%) were prescribed another medication that could increase bleeding risk. Among these patients, 106 (41%) were discharged with a total of 150 prescriptions for drugs that could interact with warfarin to increase the INR. Antibiotics accounted for 67% of these prescriptions. Fifty-three patients (21%) received 56 prescriptions for drugs which could inhibit hemostasis. These were primarily antiplatelet drugs with 61% of the prescriptions for aspirin. Patients with coronary artery disease were more likely than others to be prescribed warfarin plus antiplatelet agents (OR = 2.80; p = 0.04). More than one interacting drug was prescribed for 20% of the patients.

CONCLUSIONS

AF patients discharged on warfarin were frequently prescribed concomitant medications that increase bleeding risks. These patients should be closely monitored and counseled to watch for signs of bleeding.

Effect of omeprazole on oral and intravenous RS-methadone pharmacokinetics and pharmacodynamics in the rat

The effect of omeprazole on oral and intravenous (iv) RS-methadone pharmacokinetics and pharmacodynamics was studied in awake, freely moving rats, which were divided in four groups: oral RS-methadone (3 mg/kg) was given (a) to a control group (CO(oral)) (n = 65) and (b) to an omeprazole pretreated group (OP(oral)) (n = 77), and iv RS-methadone (0.35 mg/kg) was administered (c) to a control group (CO(iv)) (n = 86) and (d) to an omeprazole pretreated group (OP(iv)) (n = 86). Omeprazole (2 mg/kg) was given iv 2 h before RS-methadone. Plasma concentrations of RS-methadone (Cp) were determined by high-performance liquid chromatography and analgesic response by tail flick for 0-180 min (oral) and 0-120 min (iv). RS-Methadone rate of absorption (mean +/- SE) was faster in OP(oral) (k(01) = 0.31 +/- 0.04 min(-1)) than in CO(oral) (k(01) = 0.05 +/- 0.007 min(-1)), consequently plasma peak concentrations (C(max)) were greater (197.41 +/- 33.70 ng/mL versus 83.54 +/- 7.97 ng/mL) and the time to reach C(max) (t(max)) was shorter (11.23 +/- 1.32 min versus 39.18 +/- 1.74 min). Mean area under the Cp-time curve (AUC(0-infinity)) and hence bioavailability of oral RS-methadone were increased by omeprazole without significant changes in the elimination. Omeprazole did not affect the pharmacokinetics of iv RS-methadone. The changes of the analgesic effect of RS-methadone as a function of time were similar in all four groups. In the CO(oral) group, Cp and analgesic effect were defined by the E(max) model. The relationship between Cp and drug effect in the OP(oral) group showed a counterclockwise hysteresis (k(e0) of 0.018 +/- 0.006 min(-1)). For the iv groups (CO(iv) and OP(iv)), the Cp-analgesic effect relationship was described by an E(max) sigmoid model and omeprazole did not affect the pharmacodynamic parameters. It is concluded that omeprazole causes an increase in the bioavailability of oral RS-methadone without modifying the analgesic response but affecting the Cp-effect relationship.

Pharmacogenetics of warfarin elimination and its clinical implications

Warfarin is one of the most widely prescribed oral anticoagulants. However, optimal use of the drug has been hampered by its >10-fold interpatient variability in the doses required to attain therapeutic responses. Pharmacogenetic polymorphism of cytochrome P450 (CYP) may be associated with impaired elimination of warfarin and exaggerated anticoagulatory responses to the drug in certain patients. Clinically available warfarin is a racemic mixture of (R)- and (S)-warfarin, and the (S)-enantiomer has 3 to 5 times greater anticoagulation potency than its optical congener. Both enantiomers are eliminated extensively via hepatic metabolism with low clearance relative to hepatic blood flow. CYP2C9 is almost exclusively responsible for the metabolism of the pharmacologically more active (S)-enantiomer. Several human allelic variants of CYP2C9 have been cloned, designated as CYP2C9*1 (reference sequence or wild-type allele), CYP2C9*2, CYP2C9*3 and CYP2C9*4, respectively. The allelic frequencies for these variants differ considerably among different ethnic populations. Caucasians appear to carry the CYP 2C9*2 (8 to 20%) and CYP2C9*3 (6 to 10%) variants more frequently than do Asians (0% and 2 to 5%, respectively). The metabolic activities of the CYP2C9 variants have been investigated in vitro. The catalytic activity of CYP2C9*3 expressed from cDNA was significantly less than that of CYP2C9*1. Human liver microsomes obtained from individuals heterozygous for CYP2C9*3 showed significantly reduced (S)-warfarin 7-hydroxylation as compared with those obtained from individuals genotyped as CYP2C9*1. The influence of the CYP2C9*3 allele on the in vivo pharmacokinetics of (S)-warfarin has been studied in Japanese patients. Patients with the homozygous CYP2C9*3 genotype, as well as those with the heterozygous CYP2C9*1/*3 genotype, had significantly reduced clearance of (S)-warfarin (by 90 and 60%, respectively) compared with those with homozygous CYP2C9*1. The maintenance dosages of warfarin required in Japanese patients with heterozygous and homozygous CYP2C9*3 mutations were significantly lower than those in patients with CYP2C9*1/*1. In addition, 86% of British patients exhibiting adequate therapeutic responses with lower maintenance dosages of warfarin (<1.5 mg/day) had either the CYP2C9*2 or CYP2C9*3 mutation singly or in combination, whereas only 38% of randomly selected patients receiving warfarin carried the corresponding mutations. Furthermore, the former group showed more frequent episodes of major bleeding associated with warfarin therapy. These data indicate that the CYP2C9*3 allele may be associated with retarded elimination of (S)-warfarin and the resulting clinical effects. However, relationships between CYP2C9 genotype, enzyme activity, metabolism of probe substrates, dosage requirements and bleeding complications should be interpreted with caution, and further studies are required.

Drug interaction studies with esomeprazole, the (S)-isomer of omeprazole

Esomeprazole, the (S)-isomer of omeprazole, is the first proton pump inhibitor (PPI) developed as a single isomer for the treatment of patients with acid related diseases. Because of the extensive use of PPIs, the documentation of the potential for drug interactions with esomeprazole is of great importance. Altered absorption or metabolism are 2 of the major mechanisms for drug-drug interactions. Since intragastric pH will increase with esomeprazole treatment, it can be hypothesised that the absorption of drugs with pH-sensitive absorption (e.g. digoxin and ketoconazole) may be affected. Esomeprazole does not seem to have any potential to interact with drugs that are metabolised by cytochrome P450 (CYP) 1 A2, 2A6, 2C9, 2D6 or 2E1. In drug interaction studies with diazepam, phenytoin and (R)-warfarin, it was shown that esomeprazole has the potential to interact with CYP2C19. The slightly altered metabolism of cisapride was also suggested to be the result of inhibition of a minor metabolic pathway for cisapride mediated by CYP2C19. Esomeprazole did not interact with the CYP3A4 substrates clarithromycin (2 studies) or quinidine. Since the slightly increased area under the concentration-time curve (AUC) of cisapride could be explained as an inhibition of CYP2C19, the data on these 3 CYP3A4 substrates indicate that esomeprazole does not have the potential to inhibit this enzyme. The minor effects reported for diazepam, phenytoin, (R)-warfarin, and cisapride are unlikely to be of clinical relevance. Clarithromycin interacts with the metabolism of esomeprazole resulting in a doubling of the AUC of esomeprazole. The increased plasma concentrations of esomeprazole are unlikely to have any safety implications. It can be concluded that the potential for drug-drug interactions with esomeprazole is low, and similar to that reported for omeprazole.

Drug interactions update: drugs, herbs, and oral anticoagulation

Management of warfarin drug interactions is often complicated by lack of information regarding interactions with new drugs and with herbal medicinals. The pharmaceutical industry has increased both the number and quality of drug interaction studies prior to marketing new agents. Interactions may still occur in patients, however, despite negative pre-marketing studies in healthy volunteers. The clinical significance and intensity of warfarin interactions with prescription drugs (e.g., celecoxib, proton pump inhibitors, and selective serotonin reuptake inhibitors) can often be predicted on the basis of known metabolic characteristics of the drugs and warfarin enantiomers. Drug interactions with herbal medicinals are much more difficult to characterize and predict because of the lack of federal regulations regarding safety, efficacy, and manufacturing standards. Published case reports of interactions between warfarin and even the most widely used herbal medicinals are limited. Practitioners are encouraged to report such interactions through the FDA MedWatch program.

Proton pump inhibitors and their drug interactions: an evidence-based approach

The proton pump inhibitors (PPIs) are the most effective antisecretory agents used to treat acid-related disorders. As such, they are frequently prescribed for patients who are concurrently using other medications. PPIs may interact with other drugs through numerous mechanisms. The most important include competitive inhibition of hepatic cytochrome P (CYP) 450 enzymes involved in drug metabolism, and alteration of the absorption of other drugs via changes in gastric pH levels. Poor metabolizers, who lack CYP2C19, may be particularly predisposed to drug interactions. Although the potential for drug interactions is high, few clinically significant interactions have been reported for the PPIs. Nevertheless, caution is indicated when certain drugs are co-prescribed with these agents. The incidence of clinically significant drug interactions increases proportionately with the number of drugs taken and with the age of the patient. The drug interaction with the greatest clinical importance is the reduction in benzodiazepine clearance by omeprazole.

Oral anticoagulants. Pharmacologic issues for use in the elderly

There has been a marked expansion of the indications for oral anticoagulant therapy, particularly among the elderly. Despite the documented benefits, the use of warfarin remains strikingly low among patients 80 years of age and older. Elderly patients often exhibit an enhanced dose response to warfarin. On average, steady-state warfarin doses decrease by 11% per decade of age. Pharmacokinetic changes in the elderly are negligible. Pharmacodynamic differences have not been well characterized. Initiating warfarin dosing in the elderly should be done cautiously, with doses of 5 mg or less. Doses should be adjusted downward in the presence of congestive heart failure, advanced obstructive lung disease, liver disease, malignancy, protracted diarrhea, enteral feedings, or concurrent potentiating medications. Numerous medications interfere with the anticoagulant response of warfarin. The most powerful potentiating drugs are those that interfere with the metabolism of (S)-warfarin. Examples include amiodarone, trimethoprim-sulfamethoxazole, and metronidazole. These drugs should be prescribed with caution in the elderly and mandate frequent INR monitoring during the induction period. An extensive assessment of patient-specific factors that might increase the hazards related to warfarin therapy needs to be conducted and documented before initiating oral anticoagulant therapy. Patients and their caregivers need to understand the risks and benefits, and to recognize signs of abnormal bleeding and the need for frequent monitoring. Patients should be encouraged to maintain consistency in their vitamin K intake and should strive to meet the recommended dietary allowance for vitamin K. To improve anticoagulation control, physicians and other health care providers need to be aware of the many warfarin drug interactions and be cognizant of the increased dose response of warfarin seen in the elderly. Concurrent prescription of multiple drugs known to affect warfarin's anticoagulant response should be minimized and use of nonselective nonsteroidal anti-inflammatory drugs should be limited given their deleterious effects on the gastric mucosa. Transitions from inpatient care to subacute care and back to outpatient care are particularly vulnerable periods for patients' anticoagulation control. Enhanced provider communication and more seamless transitions help to ensure optimal INR follow-up and timely warfarin dose adjustment if indicated.

Selection of drugs to treat gastro-oesophageal reflux disease: the role of drug interactions

Gastro-oesophageal reflux disease is probably the most common acid-peptic disease in Western countries, and the successful treatment of mild to moderate disease with pharmacotherapy has become commonplace. A large number of effective drugs are now available, and so the decision-making process for physicians increasingly relies on considerations other than pure efficacy. Cost, adverse effects and drug interactions have therefore become important, particularly in the most vulnerable patients - children, the elderly and patients who are ill and are taking medications that may influence the efficacy of antireflux therapy. Important drug interactions with antacids include the prevention of the absorption of antibacterials such as tetracycline, azithromycin and quinolones. H2 antagonists, proton pump inhibitors and prokinetic agents undergo metabolism by the cytochrome P450 (CYP) system present in the liver and gastrointestinal tract. Cimetidine is an inhibitor of CYP3A and it may cause significant interactions with drugs of narrow therapeutic range and low bioavailability that are metabolised by these enzymes. The gastroparietal proton pump inhibitors lansoprazole, omeprazole and pantoprazole are all primarily metabolised by a genetically polymorphic enzyme, CYP2C19, that is absent from approximately 3% of Caucasians and 20% of Asians. These drugs may also interact with CYP3A, but to a lesser extent. Interactions with prokinetic agents carry the greatest potential for harm. Metoclopramide is a dopamine antagonist that may cause extrapyramidal effects when administered alone at high concentrations, or when coadministered with antipsychotic agents such as haloperidol or phenothiazines. Cisapride is clearly able to prolong the electrocardiographic QT interval and cause lethal ventricular arrhythmias when its metabolism is slowed by interaction with inhibitors of CYP3A, such as erythromycin, ketoconazole or itraconazole.

Pharmacokinetic considerations in the eradication of Helicobacter pylori

As Helicobacter pylori plays an important role in the aetiopathogenesis of peptic ulcer, therapeutic strategies aimed at maintaining long term remission have shifted from the control of intragastric pH to targeting H. pylori. According to recent international guidelines the clinical goals--rapid ulcer healing and prevention of relapse--can be best accomplished by combination therapy consisting of an antisecretory drug (proton pump inhibitor or ranitidine) and 2 antimicrobial agents (preferable amoxicillin, clarithromycin or metronidazole). When applying such multidrug regimens, possible synergy between the agents suggests that pharmacokinetic considerations might help to improve H. pylori eradication rates, which should be above 85 to 90% on an intention-to-treat basis. The present review summarises the pharmacokinetic properties and interaction potential of all drugs presently used in the various H. pylori eradication regimens, with emphasis on particular patient populations such as the elderly and those with renal impairment. The drugs considered are omeprazole, lansoprazole, pantoprazole, rabeprazole, ranitidine and ranitidine bismutrex, bismuth salts, amoxicillin, clarithromycin, azithromycin, roxithromycin, metronidazole, tinidazole and tetracycline. When addressing the clinically important questions of the efficacy, safety and costs of the recommended regimens, the impact of drug disposition on H. pylori eradication should not be neglected.

Co-prescription of H2 receptor blockers and proton pump inhibitors with warfarin in general practice

Acid-suppressing drugs and anticoagulants are used increasingly in general practice. Warfarin is potentiated by some acid-suppressing drugs, notably cimetidine and omeprazole, through interference of the cytochrome P450 system. This study aimed to ascertain the extent of co-prescribing of warfarin and acid-suppressing drugs in general practice. We conducted a retrospective survey of the records of all patients prescribed acid-suppressing drugs over a 2-year period to ascertain those who had also taken warfarin; we also made a cross-sectional survey of all patients on warfarin to ascertain those who had taken acid-suppressing drugs. From a general practice population of 45 574 patients in northern England, 3423 (7.5%) had been prescribed acid-suppressing drugs during the previous 24 months. Of 274 patients who had been on warfarin, 44 (16.1%) had also taken acid-suppressing drugs (26 H2 receptor blockers and 18 proton-pump inhibitors). The commonest reasons for anticoagulation were thrombo-embolic disease (40.9%), atrial fibrillation (36.4%), valvular heart disease (18.2%), and surgical prophylaxis (4.5%). The indications for concurrent acid-suppressing drugs were: 'dyspepsia' 38.6%, reflux 22.7%, oesophagitis 13.6%, duodenal ulcer 13.6%, gastric ulcer 4.5%, unknown 6.8%. There have been no studies from primary care to evaluate the possible clinical effects of the concomitant use of acid-suppressing drugs and warfarin; some fluctuations in coagulation control, particularly in patients taking the combinations intermittently, may be due to this.

Proton pump inhibitors. Pharmacology and rationale for use in gastrointestinal disorders

Proton pump inhibitors (PPIs) are drugs which irreversibly inhibit proton pump (H+/K+ ATPase) function and are the most potent gastric acid-suppressing agents in clinical use. There is now a substantial body of evidence showing improved efficacy of PPIs over the histamine H2 receptor antagonists and other drugs in acid-related disorders. Omeprazole 20 mg/day, lansoprazole 30 mg/day, pantoprazole 40 mg/day or rabeprazole 20 mg/day for 2 to 4 weeks are more effective than standard doses of H2-receptor antagonists in healing duodenal and gastric ulcers. Patients with gastric ulcers should receive standard doses of PPIs as for duodenal ulcers but for a longer time period (4 to 8 weeks). There is no conclusive evidence to support the use of a particular PPI over another for either duodenal or gastric ulcer healing. For Helicobacter pylori-positive duodenal ulceration, a combination of a PPI and 2 antibacterials will eradicate H. pylori in over 90% of cases and significantly reduce ulcer recurrence. Patients with H. pylori-positive gastric ulcers should be managed similarly. PPIs also have efficacy advantages over ranitidine and misoprostol and are better tolerated than misoprostol in patients taking nonsteroidal anti-inflammatory drugs (NSAIDs). In endoscopically proven gastro-oesophageal reflux disease, standard daily doses of the PPIs are more effective than H2-receptor antagonists for healing, and patients should receive a 4 to 8 week course of treatment. For severe reflux, with ulceration and/or stricture formation, a higher dose regimen (omeprazole 40 mg, lansoprazole 60 mg, pantoprazole 80 mg or rabeprazole 40 mg daily) appears to yield better healing rates. There is little evidence that PPIs lead to resolution of Barrett's oesophagus or a reduction of subsequent adenocarcinoma development, but PPIs are indicated in healing of any associated ulceration. In Zollinger-Ellison syndrome, PPIs have become the treatment of choice for the management of gastric acid hypersecretion.

Pharmacokinetic drug interactions with anti-ulcer drugs

The safety profile of any pharmacological agent is defined on the basis of its toxicity, tolerability and potential for pharmacokinetic and/or pharmacodynamic interactions with other compounds, which may belong to the same or to a different pharmacological class. Drug-drug interactions are important in clinical practice because short and long term therapeutic regimens frequently require coadministration of different drugs. The pharmacological treatment of gastric and duodenal ulcers (and of related syndromes) includes older and newer compounds, which have different mechanisms of action and exert different therapeutic effects. These compounds are widely prescribed in combination with other drugs being given for the treatment of concomitant diseases. This article reviews pharmacokinetic interactions with anti-ulcer drugs, paying particular attention to those which have clinically relevant adverse effects. Drugs mentioned in the literature as causing any pharmacokinetic interaction with anti-ulcer compounds are considered in this article.

Role of cytochrome P450 enzymes in drug-drug interactions

Many adverse drug-drug interactions are attributable to pharmacokinetic problems and can be understood in terms of alterations of P450-catalyzed reactions. Much is now known about the human P450 enzymes and what they do, and it has been possible to apply this information to issues related to practical problems. A relatively small subset of the total number of human P450s appears to be responsible for a large fraction of the oxidation of drugs. The three major reasons for drug-drug interactions involving the P450s are induction, inhibition, and possibly stimulation, with inhibition appearing to be the most important in terms of known clinical problems. With the available knowledge of human P450s and reagents, it is possible to do in vitro experiments with drugs and make useful predictions. The results can be tested in vivo, again using assays based on our knowledge of human P450s. This approach has the capability of not only improving predictions about which drugs might show serious interaction problems, but also decreasing the number of in vivo interaction studies that must be performed. These approaches should improve with further refinement and technical advances.

Lack of effect of omeprazole in oral acenocoumarol anticoagulant therapy

BACKGROUND

Omeprazole is eliminated almost completely by hepatic metabolism within the cytochrome P-450 system and might inhibit the oxidative metabolism of other drugs. This is particularly relevant for compounds with a narrow therapeutic range, such as acenocoumarol. In this study we evaluated the effect of omeprazole use in patients receiving continuous acenocoumarol therapy.

METHODS

One thousand and fifty-seven patients receiving long-term oral acenocoumarol combined with omeprazole were selected retrospectively. In 118 of these patients omeprazole was considered the only factor of possible influence on anticoagulant therapy. The control group consisted of 299 age- and sex-matched patients taking acenocoumarol without interfering medication. Dose adjustment of acenocoumarol on starting omeprazole therapy was indicated by clinically relevant changes in coagulation time.

RESULTS

No adaptation of the anticoagulant dose was necessary in 74 of 118 omeprazole patients (62.7%), compared with 169 of 299 controls (56.5%). A higher dose was necessary in 30 of 118 omeprazole patients (25.4%), compared with 84 of 299 controls (28.0%). In 14 of 118 omeprazole patients (11.9%) a lowering of the anticoagulant dose was required, compared with 46 of 299 controls (15.4%).

CONCLUSIONS

We found no evidence of any interaction between omeprazole and acenocoumarol. It seems likely that omeprazole can be administered safely to patients treated with acenocoumarol.

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.

Human P450 metabolism of warfarin

The anticoagulant drug warfarin occurs as a pair of enantiomers that are differentially metabolized by human cytochromes P450 (CYP). R-warfarin is metabolized primarily by CYP1A2 to 6- and 8-hydroxywarfarin, by CYP3A4 to 10-hydroxywarfarin, and by carbonyl reductases to diastereoisomeric alcohols. S-warfarin is metabolized primarily by CYP2C9 to 7-hydroxywarfarin. Potential warfarin-drug interactions could occur with any of a very wide range of drugs that are metabolized by these P450s, and a number of such interactions have been reported. The efficacy of warfarin is affected primarily when metabolism of S-warfarin is altered.

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.

Clinically important drug interactions with anticoagulants. An update

Coumarin derivatives combine 3 unfavorable properties which make them prone to potentially life threatening drug-drug interactions: (i) high protein binding; (ii) cytochrome P450 dependent metabolism; and (iii) a narrow therapeutic range. An entire list of drugs which are supposed to interact with coumarins (mostly with warfarin) comprises about 250 different compounds. Noteworthy are the interactions with cardiovascular or antilipidaemic drugs which are often coadministered with coumarins: amiodarone, propafenone and fibrates. Cardiovascular drugs which are obviously devoid or proven to be devoid of an interaction are angiotensin converting enzyme (ACE) inhibitors, calcium antagonists, beta-blockers and cardiac glycosides. There are several other drugs which enhance the hypoprothrombinaemic response to coumarins by various mechanisms: inhibitors of the elimination of the eutomer S-(-)-warfarin (e.g. miconazole, phenylbutazone), combined with protein binding displacement (e.g., sulfinpyrazone, phenylbutazone), synergistic hypoprothrombinaemia (e.g. cefazoline). Furthermore, bleeding complications may occur with drugs affecting platelet function [aspirin (acetylsalicylic acid) and several nonsteroidal anti-inflammatories (NSAIDs)]. Strong inducers of coumarin metabolism are rifampicin (rifampin) and carbamazepine. Biphasic interactions may occur where a drug first enhances the hypoprothrombinaemic response to a coumarin but has a sustained inducing effect on coumarin metabolism (e.g. phenytoin or sulfinpyrazone). The complex response of coumarins to concomitant drug therapy makes it difficult to predict the occurrence and degree of a deterioration of anticoagulant control in individual patients. For clinical practice, it seems advisable that one should monitor for changes in prothrombin time when adding or deleting any newly approved drug or any drug suspected (e.g. on the basis of this review) to cause an interaction to patients on coumarin therapy. The onset of the adverse prothrombin time response might be from between 1 to 2 days up to 3 weeks (in case of phenprocoumon) after starting a concomitant drug regimen. With amiodarone, an adverse prothrombin time response might occur up to 2 months after initiating therapy. For heparins, only a drug interaction with aspirin or nitroglycerin seems clinically relevant due to the possibility of coadministration during acute cardiac events. Both drugs are shown to enhance the activated partial thromboplastin time response to heparin.