Differences between white subjects and Chinese subjects in the in vivo inhibition of cytochrome P450s 2C19, 2D6, and 3A by omeprazole

Acid suppressant use in association with incidence and severe outcomes of COVID-19: a systematic review and meta-analysis

PURPOSE

Several observational studies have presented conflicting results on the association between the use of proton pump inhibitors (PPIs) or histamine H2 receptor antagonist (H2RA) and the risk of coronavirus disease 2019 (COVID-19). This systematic review and meta-analysis aimed to examine this association.

METHODS

In July 2021, PubMed, Embase, Cochrane Central Register of Controlled Trials, and Web of Science were searched for articles investigating the relationship between the two main acid suppressants and COVID-19. Studies showing the effect estimates as hazard ratio (HR) for severe outcomes or incidence of COVID-19 were evaluated using a random-effects model.

RESULTS

A total of 15 retrospective cohort studies with 18,109 COVID-19 cases were included in the current meta-analysis. PPI use was significantly associated with severe outcomes of COVID-19 (hazard ratio [HR] = 1.53; 95% confidence interval [CI]: 1.20-1.95) but not with the incidence of COVID-19, whereas H2RA use was significantly associated with decreased incidence (HR = 0.86, 95% CI: 0.76-0.97). For subgroup analyses of PPIs, increased severe outcomes of COVID-19 were observed in < 60 years, active use, in-hospital use, and Asians. For subgroup analyses of H2RAs, decreased severe outcomes of COVID-19 were observed in > 60 years, while in-hospital use and use in Asia were associated with higher disease severity.

CONCLUSIONS

Close observation can be considered for COVID-19 patients who use PPIs to prevent severe outcomes. However, caution should be taken because of substantial heterogeneity and plausible protopathic bias.

Antihypertensive Effect Of Amlodipine In Co-Administration With Omeprazole In Patients With Hypertension And Acid-Related Disorders: Cytochrome P450-Associated Aspects

Background

CYP2C19 and CYP3A are the main enzymes involved in omeprazole metabolism, while CYP3A is the principal enzyme family for amlodipine biotransformation. Concomitant use of these drugs in patients with hypertension and acid-related disorders (ARD) might lead to drug-drug interaction.

Purpose

The aim of the study was to find if adding omeprazole for treating ARD to amlodipine long-term therapy of hypertension influenced blood pressure of CYP2C19 polymorphism carriers.

Patients and methods

Fifty-one patients diagnosed with hypertension and ARD were enrolled in the study. Evaluation of antihypertensive therapy was performed by office (OBPM) and ambulatory (ABPM) blood pressure monitoring. Peripheral venous blood was collected for DNA extraction and real-time polymerase chain reaction was performed for CYP2C19*2^G681A (rs4244285), CYP2C19*3^G636A (rs4986893) and CYP2C19*17^C-806T (rs12248560) polymorphisms analysis.

Results

Of 51 patients there were 21 extensive metabolizers (EMs), 18 ultrarapid metabolizers (UMs) and 12 intermediate metabolizers (IMs). The results of OBPM showed that antihypertensive effect was significantly more pronounced in IMs compared to EMs or UMs and the average group value in the following parameters: average office systolic blood pressure (BP), dynamics of the average office systolic BP. According to dynamics of diastolic BP, the antihypertensive effect was also significantly higher in IMs than in UMs and the average group value. The results of ABPM revealed that there was a significantly more pronounced antihypertensive effect in IMs compared to all other analyzed groups according to the dynamics of both daytime systolic and 24 hr diastolic BP. The average daytime diastolic BP and its dynamics, the average 24 hr systolic BP and its dynamics were higher in IMs compared to EMs and UMs.

Conclusion

Adding omeprazole to long-term amlodipine therapy in patients with hypertension and ARD may lead to a significantly more pronounced antihypertensive effect in patients genotyped CYP2C19 IMs.

Omeprazole increases the efficacy of a soluble epoxide hydrolase inhibitor in a PGE₂ induced pain model

Epoxyeicosatrienoic acids (EETs) are potent endogenous analgesic metabolites produced from arachidonic acid by cytochrome P450s (P450s). Metabolism of EETs by soluble epoxide hydrolase (sEH) reduces their activity, while their stabilization by sEH inhibition decreases both inflammatory and neuropathic pain. Here, we tested the complementary hypothesis that increasing the level of EETs through induction of P450s by omeprazole (OME), can influence pain related signaling by itself, and potentiate the anti-hyperalgesic effect of sEH inhibitor. Rats were treated with OME (100mg/kg/day, p.o., 7 days), sEH inhibitor TPPU (3mg/kg/day, p.o.) and OME (100mg/kg/day, p.o., 7 days)+TPPU (3mg/kg/day, p.o., last 3 days of OME dose) dissolved in vehicle PEG400, and their effect on hyperalgesia (increased sensitivity to pain) induced by PGE2 was monitored. While OME treatment by itself exhibited variable effects on PGE2 induced hyperalgesia, it strongly potentiated the effect of TPPU in the same assay. The significant decrease in pain with OME+TPPU treatment correlated with the increased levels of EETs in plasma and increased activities of P450 1A1 and P450 1A2 in liver microsomes. The results show that reducing catabolism of EETs with a sEH inhibitor yielded a stronger analgesic effect than increasing generation of EETs by OME, and combination of both yielded the strongest pain reducing effect under the condition of this study.

Omeprazole preferentially inhibits the metabolism of (+)-(S)-citalopram in healthy volunteers

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Citalopram (CITA) pharmacokinetics are enantioselective in healthy volunteers and the metabolism of (+)-(S)-CITA to (+)-(S)-DCITA is dependent on CYP2C19. Omeprazole is a potent CYP2C19 inhibitor.

WHAT THIS STUDY ADDS

This study indicates that omeprazole induces a loss of enantioselectivity in the CITA pharmacokinetics because of the selective inhibition of (+)-(S)-CITA metabolism.

AIM

The study assessed the influence of omeprazole on the kinetic disposition of the (+)-(S)-citalopram (CITA) and (-)-(R)-CITA enantiomers in healthy volunteers.

METHODS

In a cross-over study, healthy volunteers (n = 9) phenotyped as extensive metabolizers of CYP2C19 and CYP2D6 and with an oral midazolam clearance ranging from 10.9 to 149.3 ml min(-1) kg(-1) received a single dose of racemic CITA (20 mg orally) in combination or not with omeprazole (20 mg day(-1) for 18 days). Serial blood samples were collected up to 240 h after CITA administration. CITA and demethylcitalopram (DCITA) enantiomers were analyzed by LC-MS/MS using a Chiralcel OD-R column.

RESULTS

The kinetic disposition of CITA was enantioselective in the absence of treatment with omeprazole, with the observation of a greater proportion of plasma (-)-(R)-CITA [AUC S:R ratio of 0.53 (95% CI 0.41, 0.66) for CITA and 1.08 (95% CI 0.80, 1.76) for DCITA] than (+)-(S)-CITA. Racemic CITA administration to healthy volunteers in combination with omeprazole showed a loss of enantioselectivity in CITA pharmacokinetics with an increase of approximately 120% in plasma (+)-(S)-CITA concentrations [AUC S:R ratio of 0.95 (95% CI 0.72, 1.10) for CITA and 0.95 (95% CI 0.44, 1.72) for DCITA].

CONCLUSIONS

The administration of multiple doses of omeprazole preferentially inhibited (+)-(S)-CITA metabolism in healthy volunteers. Although omeprazole increased plasma concentrations of (+)-(S)-CITA by approximately 120%, it is difficult to evaluate the clinical outcome because the range of plasma CITA concentrations related to maximum efficacy and minimum risk of adverse effects has not been established.

WITHDRAWN: Proton pump inhibitor treatment for acute peptic ulcer bleeding

BACKGROUND

Randomised controlled trials (RCTs) evaluating the clinical effect of proton pump inhibitors (PPIs) in peptic ulcer (PU) bleeding yield conflicting results.

OBJECTIVES

To evaluate the efficacy of PPIs in acute bleeding from PU using evidence from RCTs.

SEARCH STRATEGY

We searched CENTRAL, The Cochrane Library (Issue 4, 2004), MEDLINE (1966 to November 2004), EMBASE (1980 to November 2004), proceedings of major meetings to November 2004, and reference lists of articles. We contacted pharmaceutical companies and experts in the field.

SELECTION CRITERIA

RCTs of PPI treatment (oral or intravenous) compared with placebo or H(2)-receptor antagonist (H(2)RA) in acute bleeding from PU.

DATA COLLECTION AND ANALYSIS

Two reviewers extracted data independently, assessed study validity, summarised studies and undertook meta-analysis. The influence of study characteristics on the outcomes was examined by subgroup analyses and meta-regression.

MAIN RESULTS

Twenty-four RCTs comprising 4373 participants in total were included. Statistical heterogeneity was found among trials for rebleeding (P = 0.04), but not for all-cause mortality (P = 0.24) or surgery (P = 0.45). There was no significant difference in all-cause mortality rates between PPI and control treatment; pooled rates were 3.9% on PPI versus 3.8% on control (odds ratio (OR) 1.01; 95% CI 0.74 to 1.40). PPIs significantly reduced rebleeding compared to control; pooled rates were 10.6% with PPI versus 17.3% with control treatment (OR 0.49; 95% CI 0.37 to 0.65). PPI treatment significantly reduced surgery compared with control; pooled rates were 6.1% on PPI versus 9.3% on control (OR 0.61; 95% CI 0.48 to 0.78). There was no evidence to suggest that results on mortality and rebleeding were dependent on study quality, route of PPI administration, type of control treatment or application of initial endoscopic haemostatic treatment. PPIs significantly reduced surgery compared with placebo but not when compared with H(2)RA. There was no evidence to suggest that study quality, route of PPI administration or application of initial endoscopic haemostatic treatment influenced results on surgery. PPI treatment appeared more efficacious in studies conducted in Asia compared to studies conducted elsewhere. All-cause mortality was reduced only in Asian studies; reductions in rebleeding and surgery were quantitatively greater in Asian studies. Among patients with active bleeding or non-bleeding visible vessel, PPI treatment reduced mortality (OR 0.53; 95% CI 0.31 to 0.91), rebleeding and surgery.

AUTHORS' CONCLUSIONS

PPI treatment in PU bleeding reduces rebleeding and surgery compared with placebo or H(2)RA, but there is no evidence of an overall effect on all-cause mortality.

Intravenous pantoprazole as an adjuvant therapy following successful endoscopic treatment for peptic ulcer bleeding

BACKGROUND

Several studies have suggested that proton pump inhibitors are efficacious in preventing rebleeding when administered immediately after endoscopic treatments. However, there are limited clinical outcome data on the use of intravenous pantoprazole.

OBJECTIVE

To evaluate the efficacy of intravenous pantoprazole after successful endoscopic treatment for peptic ulcer bleeding using evidence from randomized controlled trials (RCTs).

METHODS

The Cochrane Library, MEDLINE, EMBASE and several Chinese databases up to July 2008 were searched. RCTs that compared the relative effectiveness of intravenous pantoprazole with placebo, H2 receptor antagonist or other agents for patients with peptic ulcer bleeding who were pretreated with successful endoscopic therapies were retrieved.

RESULTS

Five RCTs comprising a total of 821 participants were included in the final meta-analysis. Overall, there were significant differences in ulcer rebleeding (RR 0.31; 95% CI 0.18 to 0.53; pooled rates were 4.7% for pantoprazole and 15.0% for control), surgical intervention (RR 0.28, 95% CI 0.09 to 0.83; pooled rates were 1.4% in pantoprazole group versus 6.5% in control) and total length of hospital stay (weighted mean difference -1.53; 95% CI -1.91 to -1.16), but not on mortality (RR 0.72, 95% CI 0.29 to 1.81; pooled mortality rates were 1.9% for pantoprazole versus 2.8% for control) and blood transfusion requirements (weighted mean difference -0.53; 95% CI for random effects -1.04 to -0.02) when compared with control treatments. A series of subgroup analyses supported the results from the main analysis.

CONCLUSIONS

Intravenous administration of pantoprazole after endoscopic therapy for peptic ulcer bleeding reduces rates of ulcer rebleeding, surgical intervention and overall duration of hospital stay, but not mortality and blood transfusion requirements compared with placebo, H2 receptor antagonist or somatostatin.

Proton pump inhibitors: actions and reactions

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

A pharmacokinetic study of etravirine (TMC125) co-administered with ranitidine and omeprazole in HIV-negative volunteers

AIMS

Etravirine is a next-generation non-nucleoside reverse transcriptase inhibitor (NNRTI) with activity against wild-type and NNRTI-resistant HIV. Proton pump inhibitors and H(2)-antagonists are frequently used in the HIV-negative-infected population, and drug-drug interactions have been described with other antiretrovirals. This study evaluated the effect of steady-state omeprazole and ranitidine on the pharmacokinetics of a single dose of etravirine.

METHODS

In an open-label, randomized, one-way, three-period crossover trial, HIV-negative volunteers randomly received a single dose of 100 mg etravirine alone (treatment A); 11 days of 150 mg ranitidine b.i.d. (treatment B); and 11 days of 40 mg omeprazole q.d. (treatment C). A single dose of 100 mg etravirine was co-administered on day 8 of sessions 2 and 3. Each session was separated by a 14-day wash-out.

RESULTS

Nineteen volunteers (seven female) participated. When a single dose of etravirine was administered in the presence of steady-state ranitidine, etravirine least squares means ratios (90% confidence interval) for AUC(last) and C(max) were 0.86 (0.76, 0.97) and 0.94 (0.75, 1.17), respectively, compared with administration of etravirine alone. When administered with steady-state omeprazole, these values were 1.41 (1.22, 1.62) and 1.17 (0.96, 1.43), respectively. Co-administration of a single dose of etravirine and ranitidine or omeprazole was generally safe and well tolerated.

CONCLUSIONS

Ranitidine slightly decreased etravirine exposure, whereas omeprazole increased it by approximately 41%. The increased exposure of etravirine when co-administered with omeprazole is attributed to CYP2C19 inhibition. Considering the favourable safety profile of etravirine, these changes are not clinically relevant. Etravirine can be co-administered with proton pump inhibitors and H(2) antagonists without dose adjustments.

Intravenous esomeprazole for prevention of peptic ulcer re-bleeding: rationale/design of Peptic Ulcer Bleed study

BACKGROUND

A limited number of trials have investigated the efficacy of proton pump inhibitors for peptic ulcer bleeding, and some study design issues have been identified.

AIM

To present the design of a large trial evaluating the effects of intravenous esomeprazole on clinical outcomes in high-risk patients who have undergone endoscopic haemostasis for peptic ulcer bleeding.

METHODS

The Peptic Ulcer Bleed study is an international, randomized, double-blind, placebo-controlled trial comparing either esomeprazole 80 mg intravenous bolus infusion for 30 min followed by esomeprazole 8 mg/h intravenously for 71.5 h, or placebo infusion for 72 h, after successful endoscopic haemostasis in patients with peptic ulcer bleeding and associated high-risk stigmata. All patients will receive once daily oral esomeprazole 40 mg for 27 days after intravenous therapy. The primary end point is the rate of clinically significant re-bleeding during the first 72 h after endoscopy. Secondary end points include: rate of re-bleeding during the first 7 and 30 days after treatment; length of hospitalization; mortality; blood transfusion; endoscopic re-treatment and surgery.

RESULTS

Expected 2008.

CONCLUSIONS

The carefully designed protocol and quality control measures represent a pragmatic approach to contemporary challenges in peptic ulcer bleeding management and, it is hoped, qualify the Peptic Ulcer Bleed study as a new standard for future interventional studies.

RANDOMIZED TRIAL COMPARING PANTOPRAZOLE INFUSION, BOLUS AND NO TREATMENT ON GASTRIC pH AND RECURRENT BLEEDING IN PEPTIC ULCERS

BACKGROUND

To study the effects of pantoprazole on gastric pH and recurrent bleeding after endoscopic treatment for bleeding peptic ulcers.

METHODS

After endoscopic haemostasis, patients were randomly assigned to infusion group (pantoprazole 80 mg i.v. bolus followed by continuous infusion of 8 mg/h for 3 days), bolus group (pantoprazole 80 mg i.v. bolus followed by 40 mg i.v. bolus every 12 h for 3 days) and no-treatment group (no acid suppression in the first 3 days). Gastric pH was monitored. Rebleeding rate within 30 days, the need for surgery, transfusion requirement, total hospital stay, mortality rate and gastric pH were compared.

RESULTS

One hundred and sixty-eight patients were included, with 15 patients excluded from the analysis. There were 54 patients in the infusion group, 49 in the bolus group and 50 in the no-treatment group. There was fewer rebleeding (3.7 vs 16.0%, P = 0.034), less operative intervention (0 vs 8.0%, P = 0.034) and shorter hospital stay (6.4 vs 8.2 days, P = 0.040) in the infusion group compared with that in no-treatment group. When the bolus group was compared with no-treatment group, there were fewer rebleed (4.1 vs 16.0%, P = 0.049) and less blood transfusion (1.5 vs 2.9 units, P = 0.007). There was no difference in mortality among the three groups. Patients who received either pantoprazole infusion or bolus had significantly higher mean pH and longer duration of pH above 6 compared with the no-treatment group. There was no difference in the rebleeding rate, transfusion requirement, need for operation and hospital stay between the infusion and bolus groups. The mean pH and the duration of pH above 6 were also similar.

CONCLUSION

Pantoprazole either as infusion or bolus decreased rebleeding after endoscopic treatment for bleeding peptic ulcer.

Proton pump inhibitor treatment for acute peptic ulcer bleeding

BACKGROUND

Randomised controlled trials (RCTs) evaluating the clinical effect of proton pump inhibitors (PPIs) in peptic ulcer (PU) bleeding yield conflicting results.

OBJECTIVES

To evaluate the efficacy of PPIs in acute bleeding from PU using evidence from RCTs.

SEARCH STRATEGY

We searched CENTRAL, The Cochrane Library (Issue 4, 2004), MEDLINE (1966 to November 2004), EMBASE (1980 to November 2004), proceedings of major meetings to November 2004, and reference lists of articles. We contacted pharmaceutical companies and experts in the field.

SELECTION CRITERIA

RCTs of PPI treatment (oral or intravenous) compared with placebo or H(2)-receptor antagonist (H(2)RA) in acute bleeding from PU.

DATA COLLECTION AND ANALYSIS

Two reviewers extracted data independently, assessed study validity, summarised studies and undertook meta-analysis. The influence of study characteristics on the outcomes was examined by subgroup analyses and meta-regression.

MAIN RESULTS

Twenty-four RCTs comprising 4373 participants in total were included. Statistical heterogeneity was found among trials for rebleeding (P = 0.04), but not for all-cause mortality (P = 0.24) or surgery (P = 0.45). There was no significant difference in all-cause mortality rates between PPI and control treatment; pooled rates were 3.9% on PPI versus 3.8% on control (odds ratio (OR) 1.01; 95% CI 0.74 to 1.40). PPIs significantly reduced rebleeding compared to control; pooled rates were 10.6% with PPI versus 17.3% with control treatment (OR 0.49; 95% CI 0.37 to 0.65). PPI treatment significantly reduced surgery compared with control; pooled rates were 6.1% on PPI versus 9.3% on control (OR 0.61; 95% CI 0.48 to 0.78). There was no evidence to suggest that results on mortality and rebleeding were dependent on study quality, route of PPI administration, type of control treatment or application of initial endoscopic haemostatic treatment. PPIs significantly reduced surgery compared with placebo but not when compared with H(2)RA. There was no evidence to suggest that study quality, route of PPI administration or application of initial endoscopic haemostatic treatment influenced results on surgery. PPI treatment appeared more efficacious in studies conducted in Asia compared to studies conducted elsewhere. All-cause mortality was reduced only in Asian studies; reductions in rebleeding and surgery were quantitatively greater in Asian studies. Among patients with active bleeding or non-bleeding visible vessel, PPI treatment reduced mortality (OR 0.53; 95% CI 0.31 to 0.91), rebleeding and surgery.

AUTHORS' CONCLUSIONS

PPI treatment in PU bleeding reduces rebleeding and surgery compared with placebo or H(2)RA, but there is no evidence of an overall effect on all-cause mortality.

Pharmacokinetics of proton pump inhibitors in children

The use of proton pump inhibitors (PPIs) has become widespread in children and infants for the management of paediatric acid-related disease. Pharmacokinetic profiles of only omeprazole and lansoprazole have been well characterised in children over 2 years of age with acid-related diseases. Few data have been recently published regarding the pharmacokinetics of pantoprazole in children, and none are available for rabeprazole or esomeprazole. The metabolism of PPI enantiomers has never been studied in the paediatric population. A one-compartment model best describes the pharmacokinetic behaviour of omeprazole, lansoprazole and pantoprazole in children, with important interindividual variability for each pharmacokinetic parameter. Like adults, PPIs are rapidly absorbed in children following oral administration; the mean time to reach maximum plasma concentration varies from 1 to 3 hours. Since these agents are acid labile, their oral formulations consist of capsules containing enteric-coated granules. No liquid formulation is available for any of the PPIs. Thus, for those patients unable to swallow capsules, extemporaneous liquid preparations for omeprazole and lansoprazole have been reported; however, neither the absolute nor the relative bioavailabilities of these oral formulations have been studied in children. Intravenous formulations are available for omeprazole (in Europe), lansoprazole and pantoprazole. PPIs are rapidly metabolised in children, with short elimination half-lives of around 1 hour, similar to that reported for adults. All PPIs are extensively metabolised by the liver, primarily by cytochrome P450 (CYP) isoforms CYP2C19 and CYP3A4, to inactive metabolites, with little unchanged drug excreted in the urine. Similar to that seen in adults, the absolute bioavailability of omeprazole increases with repeated dosing in children; this phenomenon is thought to be due to a combination of decreased first-pass elimination and reduced systemic clearance. The apparent clearance (CL/F) of omeprazole, lansoprazole and pantoprazole appears to be faster for children than for adults. A higher metabolic capacity in children as well as differences in the extent of PPI bioavailability are most likely responsible for this finding. This may partly account for the need in children for variable and sometimes considerably greater doses of PPIs, on a per kilogram basis, than for adults to achieve similar plasma concentrations. Furthermore, no studies have been able to demonstrate a statistically significant correlation between age and pharmacokinetic parameters among children. Despite the small number of very young infants studied, there is some evidence for reduced PPI metabolism in newborns. The limited paediatric data regarding the impact of CYP2C19 genetic polymorphism on PPI metabolism are similar to those reported for adults, with poor metabolisers having 6- to 10-fold higher area under the concentration-time curve values compared with extensive metabolisers. Finally, because a pharmacokinetic/pharmacodynamic relationship exists for PPIs, the significant interindividual variability in their disposition may partly explain the wide range of therapeutic doses used in children. Further studies are needed to better define the pharmacokinetics of PPIs in children <2 years of age.

Systematic review and meta-analysis: proton-pump inhibitor treatment for ulcer bleeding reduces transfusion requirements and hospital stay--results from the Cochrane Collaboration

BACKGROUND

Proton-pump inhibitors reduce re-bleeding and surgical intervention, but not mortality, after ulcer bleeding.

AIM

To examine the effects of proton-pump inhibitor treatment on transfusion requirements and length of hospital stay in patients with ulcer bleeding.

METHODS

For the Cochrane Collaboration meta-analysis of randomized-controlled trials of proton-pump inhibitor therapy for ulcer bleeding, outcomes of transfusion requirements and hospital stay were summarized, respectively, as mean (+/-s.d.) units transfused and hospital days. We calculated weighted mean difference with 95% confidence interval. We also performed subgroup analyses according to geographical origin of the randomized-controlled trials.

RESULTS

There was significant heterogeneity among randomized-controlled trials for either outcome. Overall, proton-pump inhibitor treatment marginally reduced transfusion requirements (WMD = -0.6 units; 95% CI: -1.1 to 0; P = 0.05) and length of hospitalization (WMD = -1.1 days; 95% CI: -1.5 to -0.7; P < 0.0001). Most of the randomized-controlled trials did not state precise criteria for administering blood transfusion and discharging patients, thereby limiting the strength of conclusions on the pooled effects.

CONCLUSIONS

Proton-pump inhibitor treatment for ulcer bleeding produces small, but potentially important, reductions in transfusion requirements and length of hospitalization.

Systematic review: proton-pump inhibitor failure in gastro-oesophageal reflux disease--where next?

Proton-pump inhibitor failure has become a common clinical dilemma in gastrointestinal clinics and has been increasingly encountered at the primary care level as well. Underlying mechanisms are diverse and may overlap. Most patients who have proton-pump inhibitor failure are likely to originate from the non-erosive reflux disease phenotype. Currently, available diagnostic modalities provide limited clues to the exact underlying cause. Treatment relies primarily on escalating dosing of proton-pump inhibitors. However, new insights into the pathophysiology of proton-pump inhibitor failure are likely to provide alternative therapeutic options.

Systematic review and meta-analysis: enhanced efficacy of proton-pump inhibitor therapy for peptic ulcer bleeding in Asia--a post hoc analysis from the Cochrane Collaboration

BACKGROUND

Proton-pump inhibitors reduce re-bleeding rates after ulcer bleeding. However, there is significant heterogeneity among different randomized-controlled trials.

AIM

To see whether proton-pump inhibitors for ulcer bleeding produced different clinical outcomes in different geographical locations.

METHODS

This was a post hoc analysis of our Cochrane Collaboration systematic review and meta-analysis of proton-pump inhibitor therapy for ulcer bleeding. Sixteen randomized-controlled trials conducted in Europe and North America were pooled and re-analysed separately from seven conducted in Asia. We calculated pooled rates for 30-day all-cause mortality, re-bleeding and surgical intervention and derived odds ratios and numbers needed to treat with 95% confidence intervals.

RESULTS

There was no significant heterogeneity for any outcome. Reduced all-cause mortality was seen in the Asian randomized-controlled trials (odds ratios = 0.35; 95% confidence interval: 0.16-0.74; number needed to treat = 33), but not in the others (odds ratios = 1.36; 95% confidence interval: 0.94-1.96; number needed to treat--incalculable). There were significant reductions in re-bleeding and surgery in both sets of randomized-controlled trials, but the effects were quantitatively greater in Asia.

CONCLUSIONS

Proton-pump inhibitor therapy for ulcer bleeding has been more efficacious in Asia than elsewhere. This may be because of an enhanced pharmacodynamic effect of proton-pump inhibitors in Asian patients.

Ethnic Differences in Genetic Polymorphisms of CYP2D6, CYP2C19, CYP3As and MDR1/ABCB1

Metabolic capacities for debrisoquin, sparteine, mephenytoin, nifedipine, and midazolam, which are substrates of polymorphic CYP2D6, CYP2C19, and CYP3A, have been reported to exhibit, in many cases, remarkable interindividual and ethnic differences. These ethnic differences are partly associated with genetic differences. In the case of the drug transporter ABCB1/MDR1, interindividual differences in its transporter activities toward various clinical drugs are also attributed to several ABCB1/MDR1 genetic polymorphisms. In this review, the existence and frequency of various low-activity alleles of drug metabolizing enzymes as well as populational drug metabolic capacities are compared among several different races or ethnicities. Distribution of nonsynonymous ABCB1/MDR1 SNPs and haplotype frequency in various races are summarized, with the association of nonsynonymous SNPs with large functional alterations as a rare event.

Proton pump inhibitor treatment for acute peptic ulcer bleeding

BACKGROUND

Peptic ulcer (PU) bleeding is associated with substantial morbidity, mortality and healthcare cost. Randomised controlled trials (RCTs) evaluating the clinical effect of proton pump inhibitors (PPIs) in peptic ulcer bleeding have yielded conflicting results.

OBJECTIVES

To evaluate the efficacy of PPIs in the management of acute bleeding from PU using evidence from RCTs.

SEARCH STRATEGY

We performed a search of CENTRAL, The Cochrane Library (Issue 3, 2003), MEDLINE (1966 to February 2003) and EMBASE (1980 to February 2003) and proceedings of recent major meetings through to February 2003. We searched the reference lists of articles and contacted pharmaceutical companies and experts in the field for additional published or unpublished data.

SELECTION CRITERIA

RCTs of PPI treatment (oral or intravenous) compared with either placebo or H(2)-receptor antagonist (H(2)RA) in patients with acute bleeding from PU were included if they met pre-defined criteria.

DATA COLLECTION AND ANALYSIS

Two reviewers extracted data independently on a purpose-designed data extraction form. Validity of included studies was assessed by adequacy of randomisation method and other pre-defined criteria. Studies were summarised and meta-analysis was undertaken. The influence of factors on the outcomes was assessed.

MAIN RESULTS

Twenty-one RCTs with a total of 2915 participants were included. Statistical heterogeneity was found among trials for rebleeding (P = 0.05), but not for mortality (P = 0.26) or surgery (P = 0.42). There was no significant difference in mortality rates between PPI and control treatment; pooled rates were 5.2% on PPI versus 4.6% on control (odds ratio (OR) 1.11; 95% CI 0.79 to 1.57). PPI treatment significantly reduced rates of surgical intervention compared with control; pooled rates were 8.4% on PPI versus 13.0% on control (OR 0.59; 95% CI 0.46 to 0.76). PPIs significantly reduced rebleeding compared to control; pooled rates were 10.6% with PPI (range: 0% to 24.4%) versus 18.7% with control treatment (range: 2.3% to 39.1%), the OR was 0.46 (95% CI 0.33 to 0.64). Results on mortality and rebleeding rates were independent of route of PPI administration, type of control treatment or application of initial endoscopic haemostatic treatment. Surgical intervention rates varied with type of control (PPI significantly reduced surgical intervention rates compared with placebo and not when compared with H(2)RA) but not with route of PPI administration or application of initial endoscopic haemostatic treatment.

REVIEWERS' CONCLUSIONS

PPI treatment in PU bleeding reduces rebleeding and surgical intervention rates in studies comparing treatment with placebo or H(2)RA, but there is no evidence of an effect on mortality.

CYP2C19 pharmacogenetics in the clinical use of proton-pump inhibitors for gastro-oesophageal reflux disease: variant alleles predict gastric acid suppression, but not oesophageal acid exposure or reflux symptoms

BACKGROUND

The rate of metabolic inactivation of proton-pump inhibitors is determined by polymorphisms of CYP2C19. It is not known if CYP2C19 variant alleles affect responses to proton-pump inhibitor therapy in gastro-oesophageal reflux disease (GERD).

AIM

To determine if the CYP2C19 genotype is associated with clinical effectiveness of proton-pump inhibitors during GERD therapy.

METHODS

GERD patients undergoing ambulatory gastric and oesophageal pH monitoring were genotyped for CYP2C19 polymorphisms.

RESULTS

Sixty subjects were enrolled. Forty-four subjects had two wild-type alleles, 15 had one variant, and one had two variant CYP2C19 alleles. The presence of a variant allele was significantly associated with a lower odds of gastric acid breakthrough during proton-pump inhibitor therapy [odds ratio 5.14, 95% confidence interval (CI) 1.17-22.61]. The presence of a variant allele was not associated with a lower odds of significant oesophageal acid exposure (odds ratio 2.50, 95% CI 0.60-10.52), or the occurrence of symptoms (incidence rate ratio 1.06, 95% CI 0.54-2.06).

CONCLUSIONS

These results indicate that factors other than gastric acid secretion are important determinants of reflux in GERD patients. This suggests that CYP2C19 genotype testing will not be useful in proton-pump inhibitor therapy of GERD, except perhaps in identifying patients at risk for hypochlorhydria and consequent hypergastrinemia.

Symptomatic response to lansoprazole predicts abnormal acid reflux in endoscopy-negative patients with non-cardiac chest pain

AIM

To determine whether symptomatic response to lansoprazole predicts abnormal acid reflux in endoscopy-negative patients with non-cardiac chest pain.

METHODS

Patients who complained of chest pain, but had normal coronary angiography, were asked to undergo upper endoscopy. Those without gastric and oesophageal lesions were recruited for 24-h ambulatory oesophageal pH monitoring, and were randomly given lansoprazole 30 mg or placebo, both daily for 4 weeks. Chest pain symptoms were recorded before and 1 month after treatment on a locally validated questionnaire. The symptom score was calculated by multiplying the severity and frequency of the symptom, and symptom improvement was defined as > 50% reduction in symptom score.

RESULTS

Overall, 68 patients, 36 on lansoprazole and 32 on placebo, completed the trial. The symptom score was reduced significantly in both groups (P < 0.001). In the lansoprazole group, more patients with than without abnormal reflux showed symptom improvement (92% vs. 33%; odds ratio = 22; 95% confidence interval, 2.3-201.8; chi2 = 10.9; P = 0.001), giving a sensitivity, specificity, positive predictive value, negative predictive value and accuracy of 92%, 67%, 58%, 94% and 75%, respectively. In the placebo group, the rates of symptom improvement were similar between those with and without abnormal reflux (33% vs. 35%, P = N.S.).

CONCLUSIONS

Treatment with lansoprazole is a useful test in diagnosing endoscopy-negative gastro-oesophageal reflux disease in Chinese patients with non-cardiac chest pain.

Clinical significance of the cytochrome P450 2C19 genetic polymorphism

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

Mechanisms of clinically relevant drug interactions associated with tacrolimus

The clinical management of tacrolimus, a macrolide used as immunosuppressant after transplantation, is complicated by its narrow therapeutic index in combination with inter- and intraindividually variable pharmacokinetics. As a substrate of cytochrome P450 (CYP) 3A enzymes and P-glycoprotein, tacrolimus interacts with several other drugs used in transplantation medicine, which also are known CYP3A and/or P-glycoprotein inhibitors and/or inducers. In clinical studies, CYP3A/P-glycoprotein inhibitors and inducers primarily affect oral bioavailability of tacrolimus rather than its clearance, indicating a key role of intestinal P-glycoprotein and CYP3A. There is an almost complete overlap between the reported clinical drug interactions of tacrolimus and those of cyclosporin. However, in comparison with cyclosporin, only few controlled drug interaction studies have been carried out, but tacrolimus drug interactions have been extensively studied in vitro. These results are inconsistent and are of poor predictive value for clinical drug interactions because of false negative results. P-glycoprotein regulates distribution of tacrolimus through the blood-brain barrier into the brain as well as distribution into lymphocytes. Interaction of other drugs with P-glycoprotein may change tacrolimus tissue distribution and modify its toxicity and immunosuppressive activity. There is evidence that ethnic and gender differences exist for tacrolimus drug interactions. Therapeutic drug monitoring to guide dosage adjustments of tacrolimus is an efficient tool to manage drug interactions. In the near future, progress can be expected from studies evaluating potential pharmacokinetic interactions caused by herbal preparations and food components, the exact biochemical mechanism underlying tacrolimus toxicity, and the potential of inhibition of CYP3A and P-glycoprotein to improve oral bioavailability and to decrease intraindividual variability of tacrolimus pharmacokinetics.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Dapsone and sulfones in dermatology: overview and update

In their 60-year history, dapsone and the sulfones have been used as both antibacterial and anti-inflammatory agents. Dapsone has been used successfully to treat a range of dermatologic disorders, most successfully those characterized by abnormal neutrophil and eosinophil accumulation. This article reviews and updates the chemistry, pharmacokinetics, clinical application, mechanism of action, adverse effects, and drug interactions of dapsone and the sulfones in dermatology.

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.

Interrelationship between substrates and inhibitors of human CYP3A and P-glycoprotein

PURPOSE

CYP3A and P-gp both function to reduce the intracellular concentration of drug substrates, one by metabolism and the other by transmembrane efflux. Moreover, it has been serendipitously noted that the two proteins have many common substrates and inhibitors. In order to test this notion more fully, systematic studies were undertaken to determine the P-gp-mediated transport and inhibitory characteristics of prototypical CYP substrates.

METHODS

L-MDR1, LLC-PK1, and Caco-2 cells were used to evaluate established CYP substrates as potential P-gp substrates and inhibitors in vitro, and mdr1a deficient mice were used to assess the in vivo relevance of P-gp-mediated transport.

RESULTS

Some (terfenadine, erythromycin and lovastatin) but not all (nifedipine and midazolam) CYP3A substrates were found to be P-gp substrates. Except for debrisoquine, none of the prototypical substrates of other common human CYP isoforms were transported by P-gp. Studies in mdr1a disrupted mice confirmed that erythromycin was a P-gp substrate but the CYP3A-inhibitor ketoconazole was not. In addition, CYP3A substrates and inhibitors varied widely in their ability to inhibit the P-gp-mediated transport of digoxin.

CONCLUSIONS

These results indicate that the overlap in substrate specificities of CYP3A and P-gp appears to be fortuitous rather than indicative of a more fundamental relationship.

Inhibition and induction of cytochrome P450 and the clinical implications

The cytochrome P450s (CYPs) constitute a superfamily of isoforms that play an important role in the oxidative metabolism of drugs. Each CYP isoform possesses a characteristic broad spectrum of catalytic activities of substrates. Whenever 2 or more drugs are administered concurrently, the possibility of drug interactions exists. The ability of a single CYP to metabolise multiple substrates is responsible for a large number of documented drug interactions associated with CYP inhibition. In addition, drug interactions can also occur as a result of the induction of several human CYPs following long term drug treatment. The mechanisms of CYP inhibition can be divided into 3 categories: (a) reversible inhibition; (b) quasi-irreversible inhibition; and (c) irreversible inhibition. In mechanistic terms, reversible interactions arise as a result of competition at the CYP active site and probably involve only the first step of the CYP catalytic cycle. On the other hand, drugs that act during and subsequent to the oxygen transfer step are generally irreversible or quasi-irreversible inhibitors. Irreversible and quasi-irreversible inhibition require at least one cycle of the CYP catalytic process. Because human liver samples and recombinant human CYPs are now readily available, in vitro systems have been used as screening tools to predict the potential for in vivo drug interaction. Although it is easy to determine in vitro metabolic drug interactions, the proper interpretation and extrapolation of in vitro interaction data to in vivo situations require a good understanding of pharmacokinetic principles. From the viewpoint of drug therapy, to avoid potential drug-drug interactions, it is desirable to develop a new drug candidate that is not a potent CYP inhibitor or inducer and the metabolism of which is not readily inhibited by other drugs. In reality, drug interaction by mutual inhibition between drugs is almost inevitable, because CYP-mediated metabolism represents a major route of elimination of many drugs, which can compete for the same CYP enzyme. The clinical significance of a metabolic drug interaction depends on the magnitude of the change in the concentration of active species (parent drug and/or active metabolites) at the site of pharmacological action and the therapeutic index of the drug. The smaller the difference between toxic and effective concentration, the greater the likelihood that a drug interaction will have serious clinical consequences. Thus, careful evaluation of potential drug interactions of a new drug candidate during the early stage of drug development is essential.

Canadian Native Indians exhibit unique CYP2A6 and CYP2C19 mutant allele frequencies

Many human cytochromes P450 (CYP) enzymes are polymorphically expressed, resulting in interindividual and interethnic differences in the metabolism of substrate drugs. Little is known about the genetic variation of CYP enzymes in Canadian Native Indians. We therefore determined the CYP2C19 and CYP2A6 mutant allele frequencies in 159 Canadian Native Indians and compared them with white and Asian subjects. Canadian Native Indians differed significantly from both white and Asian populations in allelic patterns of both CYP2C19 (19.1% CYP2C19*2 and 0% CYP2C19*3) and CYP2A6 (0.9% CYP2A6*2 and 13.9% CYP2A6*3). In addition, analysis of the Canadian Native Indian data suggested that there may be an association between the presence of the CYP2C19 and CYP2A6 mutant alleles such that the co-occurrence of these 2 alleles is higher than would be predicted on the basis of their individual frequencies in this population.

Genetic determinants of drug responsiveness and drug interactions

Six cytochrome P450 enzymes mediate the oxidative metabolism of most drugs in common use: CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. These enzymes have selective substrate specificity, and their activity is characterized by marked interindividual variation. Some of these systems (CYP2C19, CYP2D6) are polymorphically distributed; thus, a subset of the population may be genetically deficient in enzyme activity. Phenotyping procedures designed to identify subjects with impaired metabolism who may be at increased risk for drug toxicity have been developed and validated. This has been supplemented in recent years by the availability of genetic analysis and the identification of specific alleles that are associated with altered (i.e., reduced, deficient, or increased) enzyme activity. The potential of genotyping to predict pharmacodynamics holds great promise for the future because it does not involve the administration of exogenous compound and is not confounded by drug therapy. Drug interactions caused by the inhibition or induction of oxidative drug metabolism may be of great clinical importance because they may result in drug toxicity or therapeutic failure. Further understanding of cytochrome P450 complexity may allow, through a combined in vitro-in vivo approach, the reliable prediction and possible prevention of deleterious drug interactions.

Syncopal episodes associated with cisapride and concurrent drugs

OBJECTIVE

To report a case of QT prolongation and syncopal episodes resulting from concomitant use of cisapride and agents known to inhibit its metabolism.

CASE SUMMARY

A 53-year-old white woman was involved in two motor vehicle accidents on the same day after experiencing syncopal episodes. Cardiac and neurologic evaluations were negative; the syncopal episodes were attributed to QT prolongation associated with the concomitant use of cisapride and agents known to inhibit its metabolism.

DISCUSSION

This is the first case published in the English-language literature describing QT prolongation resulting from the concomitant use of cisapride and agents known to inhibit its metabolism. Clarithromycin inhibits CYP3A4, the isoenzyme responsible for the metabolism of cisapride. Concomitant administration of cisapride with agents known to inhibit CYP3A4 (i.e., azole antifungals, erythromycin, clarithromycin) may result in elevated cisapride concentrations. Elevated cisapride concentrations have been associated with QT prolongation, syncopal episodes, and cardiac dysrhythmias.

CONCLUSIONS

Acquired QT prolongation is a well-recognized adverse effect of several drugs. Recognition of newer drugs and drug combinations that place patients at risk for this potentially fatal adverse event is imperative for appropriate monitoring and prevention.

Determinants of interindividual variability and extent of CYP2D6 and CYP1A2 inhibition by paroxetine and fluvoxamine in vivo

Major depression may require antidepressant treatment for several years. This necessitates consideration of the long-term effects of antidepressants on multiple clinical endpoints. The antidepressants paroxetine and fluvoxamine are potent in vitro inhibitors of CYP2D6 and CYP1A2 isozymes, respectively. CYP2D6 and CYP1A2 are important for the clearance of 30 or more frequently used medications. Moreover, CYP1A2 also contributes to metabolism of 17beta-estradiol and metabolic activation of environmental procarcinogens (e.g., arylamines in cigarette smoke). The aim of this study was to assess the determinants of interindividual variability and extent of CYP2D6 and CYP1A2 inhibition during paroxetine and fluvoxamine treatment. Healthy volunteers and patients received caffeine (100 mg) and dextromethorphan (30 mg) at baseline and at steady state of paroxetine (10-20 mg/day, 5-74 days, N = 13) or fluvoxamine (50-100 mg/day, 5-43 days, N = 8). The caffeine metabolic ratio (CMR) and the log O-demethylation ratio (ODMR) of dextromethorphan in overnight urine were used as in vivo indices of the CYP1A2 and CYP2D6 isozyme activities, respectively. All subjects had an extensive metabolizer phenotype for CYP2D6. After fluvoxamine treatment, baseline CMR 5.1 +/- 1.4 (mean +/- SD) decreased to 2.7 +/- 1.1 (p < 0.01). Paroxetine did not have a significant effect on CMR (p > 0.05). In seven of eight subjects in the fluvoxamine group, posttreatment CMR was comparable with the minimum CMR value (2.0) attainable in nonsmoking healthy volunteers. After paroxetine treatment, log ODMR changed from a baseline value of -2.28 +/- 0.37 to -1.13 +/- 0.44, indicating significant inhibition of CYP2D6 (p < 0.001). Subjects' CYP2D6 phenotype did not change after paroxetine treatment. Fluvoxamine had no significant effect on log ODMR (p > 0.05). The extent of inhibition of CYP2D6 and CYP1A2 by paroxetine and fluvoxamine, respectively, displayed a positive correlation with baseline enzyme activity (p < 0.05). In addition, a negative association was found between the plasma paroxetine concentration and the CYP2D6 activity after paroxetine treatment (r = -0.47, p < 0.05). These data indicate that paroxetine and fluvoxamine treatment with minimum clinically effective doses significantly inhibit CYP2D6 and CYP1A2, respectively. The extent of inhibition of CYP2D6 by paroxetine and of CYP1A2 by fluvoxamine is dependent in part on the baseline enzyme activity. The interindividual variability in CYP2D6 inhibition by paroxetine can also be explained by variability in plasma paroxetine concentration. Most patients treated with fluvoxamine (50-100 mg/day) will reach population minimums for CYP1A2 activity. These results have potential implications for interindividual variability in the risk for drug-drug interactions mediated by CYP2D6 and CYP1A2 as well as for the disposition of 17beta-estradiol and environmental procarcinogens.