Inhibition of drug metabolism in human liver microsomes by nizatidine, cimetidine and omeprazole

ACVIM consensus statement: Support for rational administration of gastrointestinal protectants to dogs and cats

The gastrointestinal (GI) mucosal barrier is continuously exposed to noxious toxins, reactive oxygen species, microbes, and drugs, leading to the development of inflammatory, erosive, and ultimately ulcerative lesions. This report offers a consensus opinion on the rational administration of GI protectants to dogs and cats, with an emphasis on proton pump inhibitors (PPIs), histamine type-2 receptor antagonists (H2 RAs), misoprostol, and sucralfate. These medications decrease gastric acidity or promote mucosal protective mechanisms, transforming the management of dyspepsia, peptic ulceration, and gastroesophageal reflux disease. In contrast to guidelines that have been established in people for the optimal treatment of gastroduodenal ulcers and gastroesophageal reflux disease, effective clinical dosages of antisecretory drugs have not been well established in the dog and cat to date. Similar to the situation in human medicine, practice of inappropriate prescription of acid suppressants is also commonplace in veterinary medicine. This report challenges the dogma and clinical practice of administering GI protectants for the routine management of gastritis, pancreatitis, hepatic disease, and renal disease in dogs and cats lacking additional risk factors for ulceration or concerns for GI bleeding. Judicious use of acid suppressants is warranted considering recent studies that have documented adverse effects of long-term supplementation of PPIs in people and animals.

Substituted tetrazoles as multipurpose screening compounds

Tetrazoles are small functional heterocycles that are suited to serve simultaneously as aromatic platform for diversity and as functional interaction motif. Furthermore, the tetrazole ring and its deprotonated tetrazolate counterpart are metal ion complexing ligands that possess a rich variety of binding and bridging modes. We recently demonstrated that fragments containing the tetrazole moiety and a metal chelating hydrazide group are well suited to discover selective screening hits with high ligand efficiency for a given protein target. Here, we report the synthesis and characterization of new polydentate tetrazole-containing screening compounds and their synthetic precursors as well as their deposition in a multipurpose screening library in the frame of the EU-OPENSCREEN network. The pure and well-characterized screening compounds could be useful to aid drug discovery programs for multiple or hitherto undruggable targets by enclosure of under-represented tetrazole derivatives.

Measuring Drug Metabolism Kinetics and Drug-Drug Interactions Using Self-Assembled Monolayers for Matrix-Assisted Laser Desorption-Ionization Mass Spectrometry

The competition of two drugs for the same metabolizing enzyme is a common mechanism for drug-drug interactions that can lead to altered kinetics in drug metabolism and altered elimination rates in vivo. With the prevalence of multidrug therapy, there is great potential for serious drug-drug interactions and adverse drug reactions. In an effort to prevent adverse drug reactions, the FDA mandates the evaluation of the potential for metabolic inhibition by every new chemical entity. Conventional methods for assaying drug metabolism (e.g., those based on HPLC) have been established for measuring drug-drug interactions; however, they are low-throughput. Here we describe an approach to measure the catalytic activity of CYP2C9 using the high-throughput technique self-assembled monolayers for matrix-assisted laser desorption-ionization (SAMDI) mass spectrometry. We measured the kinetics of CYP450 metabolism of the substrate, screened a set of drugs for inhibition of CYP2C9 and determined the Ki values for inhibitors. The throughput of this platform may enable drug metabolism and drug-drug interactions to be interrogated at a scale that cannot be achieved with current methods.

Interindividual variability of CYP2C19-catalyzed drug metabolism due to differences in gene diplotypes and cytochrome P450 oxidoreductase content

Large interindividual variability has been observed in the metabolism of CYP2C19 substrates in vivo. The study aimed to evaluate sources of this variability in CYP2C19 activity, focusing on CYP2C19 diplotypes and the cytochrome P450 oxidoreductase (POR). CYP2C19 gene analysis was carried out on 347 human liver samples. CYP2C19 activity assayed using human liver microsomes confirmed a significant a priori predicted rank order for (S)-mephenytoin hydroxylase activity of CYP2C19*17/*17 > *1B/*17 > *1B/*1B > *2A/*17 > *1B/*2A > *2A/*2A diplotypes. In a multivariate analysis, the CYP2C19*2A allele and POR protein content were associated with CYP2C19 activity. Further analysis indicated a strong effect of the CYP2C19*2A, but not the *17, allele on both metabolic steps in the conversion of clopidogrel to its active metabolite. The present study demonstrates that interindividual variability in CYP2C19 activity is due to differences in both CYP2C19 protein content associated with gene diplotypes and the POR concentration.

Physiologically based pharmacokinetic modeling of disposition and drug-drug interactions for atorvastatin and its metabolites

Atorvastatin is the most commonly used of all statins to lower cholesterol. Atorvastatin is extensively metabolized in both gut and liver to produce several active metabolites. The purpose of the present study is to develop a physiologically based pharmacokinetic (PBPK) model for atorvastatin and its two primary metabolites, 2-hydroxy-atorvastatin acid and atorvastatin lactone, using in vitro and in vivo data. The model was used to predict the pharmacokinetic profiles and drug-drug interaction (DDI) effect for atorvastatin and its metabolites in different DDI scenarios. The predictive performance of the model was assessed by comparing predicted results to observed data after coadministration of atorvastatin with different medications such as itraconazole, clarithromycin, cimetidine, rifampin and phenytoin. This population based PBPK model was able to describe the concentration-time profiles of atorvastatin and its two metabolites reasonably well in the absence or presence of those drugs at different dose regimens. The predicted maximum concentration (Cmax), area under the concentration-time curve (AUC) values and between-phase ratios were in good agreement with clinically observed data. The model has also revealed the importance of different metabolic pathways on the disposition of atorvastatin metabolites. This PBPK model can be utilized to assess the safety and efficacy of atorvastatin in the clinic. This study demonstrated the feasibility of applying PBPK approach to predict the DDI potential of drugs undergoing complex metabolism.

Adverse effects reported in the use of gastroesophageal reflux disease treatments in children: a 10 years literature review

Gastroesophageal reflux (GER) is commonly observed in children, particularly during the first year of life. Pharmacological therapy is mostly reserved for symptomatic infants diagnosed with GER disease (GERD), usually as defined in a recent consensus statement. The purpose of the present article was to review the reported adverse effects of pharmacological agents used in the treatment of paediatric GERD. We conducted this review using the electronic journal database Pubmed and Cochrane database systematic reviews using the latest 10-year period (1 January 2003 to 31 December 2012). Our search strategy included the following keywords: omeprazole, esomeprazole, lansoprazole, pantoprazole, rabeprazole, rantidine, cimetidine, famotidine, nizatidine, domperidone, metoclopramide, betanechol, erythromycin, baclofen, alginate. We used Pubmed's own filter of: 'child: birth-18 years'. All full articles were reviewed and we only included randomized controlled trials retrieved from our search. We addressed a summary of our search on a drug-by-drug basis with regard to its mechanism of action and clinical applications, and reviewed all of the adverse effects reported and the safety profile of each drug. Adverse effects have been reported in at least 23% of patients treated with histamine H2 receptor antagonists (H2 RAs) and 34% of those treated with proton pump inhibitors (PPIs), and mostly include headaches, diarrhoea, nausea (H2 RAs and PPIs) and constipation (PPIs). Acid suppression may place immune-deficient infants and children, or those with indwelling catheters, at risk for the development of lower respiratory tract infections and nosocomial sepsis. Prokinetic agents have many adverse effects, without major benefits to support their routine use.

Modeling chemical interaction profiles: II. Molecular docking, spectral data-activity relationship, and structure-activity relationship models for potent and weak inhibitors of cytochrome P450 CYP3A4 isozyme

Polypharmacy increasingly has become a topic of public health concern, particularly as the U.S. population ages. Drug labels often contain insufficient information to enable the clinician to safely use multiple drugs. Because many of the drugs are bio-transformed by cytochrome P450 (CYP) enzymes, inhibition of CYP activity has long been associated with potentially adverse health effects. In an attempt to reduce the uncertainty pertaining to CYP-mediated drug-drug/chemical interactions, an interagency collaborative group developed a consensus approach to prioritizing information concerning CYP inhibition. The consensus involved computational molecular docking, spectral data-activity relationship (SDAR), and structure-activity relationship (SAR) models that addressed the clinical potency of CYP inhibition. The models were built upon chemicals that were categorized as either potent or weak inhibitors of the CYP3A4 isozyme. The categorization was carried out using information from clinical trials because currently available in vitro high-throughput screening data were not fully representative of the in vivo potency of inhibition. During categorization it was found that compounds, which break the Lipinski rule of five by molecular weight, were about twice more likely to be inhibitors of CYP3A4 compared to those, which obey the rule. Similarly, among inhibitors that break the rule, potent inhibitors were 2–3 times more frequent. The molecular docking classification relied on logistic regression, by which the docking scores from different docking algorithms, CYP3A4 three-dimensional structures, and binding sites on them were combined in a unified probabilistic model. The SDAR models employed a multiple linear regression approach applied to binned 1D ¹³C-NMR and 1D ¹⁵N-NMR spectral descriptors. Structure-based and physical-chemical descriptors were used as the basis for developing SAR models by the decision forest method. Thirty-three potent inhibitors and 88 weak inhibitors of CYP3A4 were used to train the models. Using these models, a synthetic majority rules consensus classifier was implemented, while the confidence of estimation was assigned following the percent agreement strategy. The classifier was applied to a testing set of 120 inhibitors not included in the development of the models. Five compounds of the test set, including known strong inhibitors dalfopristin and tioconazole, were classified as probable potent inhibitors of CYP3A4. Other known strong inhibitors, such as lopinavir, oltipraz, quercetin, raloxifene, and troglitazone, were among 18 compounds classified as plausible potent inhibitors of CYP3A4. The consensus estimation of inhibition potency is expected to aid in the nomination of pharmaceuticals, dietary supplements, environmental pollutants, and occupational and other chemicals for in-depth evaluation of the CYP3A4 inhibitory activity. It may serve also as an estimate of chemical interactions via CYP3A4 metabolic pharmacokinetic pathways occurring through polypharmacy and nutritional and environmental exposures to chemical mixtures.

In vitro-to-in vivo predictions of drug-drug interactions involving multiple reversible inhibitors

INTRODUCTION

Predictions of drug-drug interactions (DDIs) are commonly performed for single inhibitors, but interactions involving multiple inhibitors also frequently occur. Predictions of such interactions involving stereoisomer pairs, parent/metabolite combinations and simultaneously administered multiple inhibitors are increasing in importance. This review provides the framework for predicting inhibitory DDIs of multiple inhibitors with any combination of reversible inhibition mechanism.

AREAS COVERED

The review provides an overview of the reliability of the in vitro determined reversible inhibition mechanism. Furthermore, the article provides a method to predict DDIs for multiple reversible inhibitors that allows substituting the inhibition constant (K(i)) with an inhibitor affinity (IC(50)) value determined at S << K(M).

EXPERT OPINION

A better understanding and the prediction methods of DDIs, resulting from multiple inhibitors, are important. The inhibition mechanism of a reversible inhibitor is often equivocal across studies and unreliable. Determination of the K(i) requires the assignment of reversible inhibition mechanism but in vitro-to-in vivo prediction of DDI risk can be achieved for multiple inhibitors from estimates of the inhibitor affinity (IC(50)) only, regardless of the inhibition mechanism.

[Drug-induced hyperglycemia: a study in the French pharmacovigilance database]

OBJECTIVE

To analyse drugs inducing hyperglycemia by using data reported to the French spontaneous reporting system and recorded in the French PharmacoVigilance Database (FPVD).

METHODS

All cases with a report of hyperglycemia and/or diabetes in the French database between 1985 and 2008 were included in the study. We estimated the risk of hyperglycemia linked to drugs by the case/non-case method. Cases were reports including hyperglycemia and non cases all other reports. This risk was estimated through calculation of reporting odds ratios (ROR).

RESULTS

During this period, 1219 reports including the words "hyperglycemia and/or diabetes" were registered (0.34% of the database). This adverse drug reaction occurred 1 fold over 4 in diabetics or as a part of HIV infection. Effect was "serious" in approximatively 50% of cases. We found an increase of risk during exposition with methylprednisolone [ROR=43.5; 95% CI (37.3-50.8)], tacrolimus [ROR=25; 95% CI (17.9-34.8)], olanzapine [ROR=19.9; 95% CI (14.9-26.5)], prednisone [ROR=18.9; 95% CI (15.7-22.8)] or pentamidine [ROR=15.4; 95% CI (8.2-28.3)].

CONCLUSION

Drug classes most frequently found in FPVD linked to hyperglycemia are antiretroviral, steroidal anti-inflammatory, second generation neuroleptic, immunosuppressive and diuretic drugs.

Histamine2-receptor antagonists are an alternative to proton pump inhibitor in patients receiving clopidogrel

BACKGROUND & AIMS

Previous observational studies reported that concomitant use of clopidogrel and proton pump inhibitors (PPIs) in patients with prior acute coronary syndrome (ACS) was associated with adverse cardiovascular outcomes. We investigated whether H(2)-receptor antagonist (H(2)RA) is an alternative to PPI in patients with ACS.

METHODS

We conducted a population-based retrospective cohort study of 6552 patients in Taiwan discharged for ACS between 2002 and 2005. Patients were divided into 5 cohorts: clopidogrel plus H(2)RA (n = 252), clopidogrel plus PPI (n = 311), clopidogrel alone (n = 5551), H(2)RA alone (n = 235), and PPI alone (n = 203). The primary outcome was rehospitalization for ACS or all-cause mortality within 3 month of rehospitalization.

RESULTS

The 1-year cumulative incidence of the primary outcome was 26.8% (95% CI: 21.5%-33.0%) in the clopidogrel plus H(2)RA cohort and 33.2% (95% CI: 27.8%-39.4%) in the clopidogrel plus PPI cohort, compared with 11.6% (95% CI: 10.8%-12.5%) in the clopidogrel alone cohort (P < .0001). No significant difference was observed between the PPI alone cohort (11.0%; 95% CI: 7.1%-16.8%), the H(2)RA alone cohort (11.8%; 95% CI: 8.2%-16.8%), and the clopidogrel alone cohort in terms of the primary outcome. The number needed to harm was 7 with concomitant H(2)RA and 5 with concomitant PPI. On multivariate analysis, concomitant H(2)RA and PPI were independent risk factors predicting adverse outcomes (adjusted hazard ratios, 2.48 and 3.20, respectively; P < .0001).

CONCLUSIONS

Concomitant use of clopidogrel and H(2)RA or PPI after hospital discharge for ACS is associated with increased risk of adverse outcomes.

No dose adjustment on coadministration of the PDE4 inhibitor roflumilast with a weak CYP3A, CYP1A2, and CYP2C19 inhibitor: an investigation using cimetidine

This nonrandomized, fixed-sequence, 2-period crossover study investigated potential pharmacokinetic interactions between the phosphodiesterase 4 inhibitor roflumilast, currently in clinical development for the treatment of chronic obstructive pulmonary disease, and the histamine 2 agonist cimetidine. Participants received roflumilast, 500 µg once daily, on days 1 and 13. Cimetidine, 400 mg twice daily, was administered from days 6 to 16. Pharmacokinetic analysis of roflumilast and its active metabolite roflumilast N-oxide was performed, and the ratio of geometric means for roflumilast alone and concomitantly with steady-state cimetidine was calculated. The effect of cimetidine on the total PDE4 inhibitory activity (tPDE4i; total exposure to roflumilast and roflumilast N-oxide) was also calculated. Coadministration of steady-state cimetidine increased mean tPDE4i of roflumilast and roflumilast N-oxide by about 47%. The maximum plasma concentration (C(max)) of roflumilast increased by about 46%, with no effect on C(max) of roflumilast N-oxide. The increase in tPDE4i of roflumilast and roflumilast N-oxide following coadministration with cimetidine was mainly due to the inhibitory effect of cimetidine on cytochrome P450 (CYP) isoenzymes CYP1A2, CYP3A, and CYP2C19. These moderate changes indicate that dose adjustment of roflumilast is not required when coadministered with a weak inhibitor of CYP1A2, CYP3A, and CYP2C19, such as cimetidine.

New insights into the structural characteristics and functional relevance of the human cytochrome P450 2D6 enzyme

To date, the crystal structures of at least 12 human CYPs (1A2, 2A6, 2A13, 2C8, 2C9, 2D6, 2E1, 2R1, 3A4, 7A1, 8A1, and 46A1) have been determined. CYP2D6 accounts for only a small percentage of all hepatic CYPs (< 2%), but it metabolizes approximately 25% of clinically used drugs with significant polymorphisms. CYP2D6 also metabolizes procarcinogens and neurotoxins, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, and indolealkylamines. Moreover, the enzyme utilizes hydroxytryptamines and neurosteroids as endogenous substrates. Typical CYP2D6 substrates are usually lipophilic bases with an aromatic ring and a nitrogen atom, which can be protonated at physiological pH. Substrate binding is generally followed by oxidation (5-7 A) from the proposed nitrogen-Asp301 interaction. A number of homology models have been constructed to explore the structural features of CYP2D6, while antibody studies also provide useful structural information. Site-directed mutagenesis studies have demonstrated that Glu216, Asp301, Phe120, Phe481, and Phe483 play important roles in determining the binding of ligands to CYP2D6. The structure of human CYP2D6 has been recently determined and shows the characteristic CYP fold observed for other members of the CYP superfamily. The lengths and orientations of the individual secondary structural elements in the CYP2D6 structure are similar to those seen in other human CYP2 members, such as CYP2C9 and 2C8. The 2D6 structure has a well-defined active-site cavity located above the heme group with a volume of approximately 540 A(3), which is larger than equivalent cavities in CYP2A6 (260 A(3)), 1A2 (375 A(3)), and 2E1 (190 A(3)), but smaller than those in CYP3A4 (1385 A(3)) and 2C8 (1438 A(3)). Further studies are required to delineate the molecular mechanisms involved in CYP2D6 ligand interactions and their implications for drug development and clinical practice.

Relevance of the aryl hydrocarbon receptor (AhR) for clinical toxicology

INTRODUCTION

The aryl hydrocarbon receptor (AhR) is a cellular signaling molecule infamous for mediating the toxicity of dioxins and related compounds.

AIM

The aim of this review is to provide a background of AhR and to examine critically its role in chemical toxicity, in physiological systems, and its interaction with drugs and other compounds.

TOXICITY

The AhR is essential for the toxicity of dioxins and related chemicals. The AhR mediates the exquisite sensitivity of animals to dioxins, where as little as 2 ng/kg/day can yield striking adverse effects. PHYSIOLOGICAL ROLE OF AHR: The wide variety of adverse effects of dioxin argues for an important role of the AhR in a variety of physiological systems. Recent investigations have highlighted the role of AhR in the development of the brain and vasculature. DRUGS AND OTHER CHEMICAL ACTIVATORS OF AHR: The development of AhR agonists during drug development programs is sometimes inadvertent, but sometimes the target of development, and is yet further confirmation of the likely importance of AhR signaling in constitutive physiology. The presence of AhR agonists in the diet such as indolo-(3,2-b)-carbazole and 3,3'-diindolylmethane (metabolized from indole 3-carbinol), flavonoids, and sulforaphane and of endogenous activators of this signaling system such as eicosanoids, indirubin, bilirubin, cAMP, and tryptophan are suggestive that AhR activation is a normal physiological process and that it is the persistent and high-level stimulation of AhR by dioxins that is responsible for toxicity.

CONCLUSIONS

AhR-mediated toxicity and physiology are highly relevant to clinical toxicology and drug development.

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.

Pharmacological management of atypical antipsychotic-induced weight gain

Excessive bodyweight gain was reported during the 1950s as an adverse effect of typical antipsychotic drug treatment, but the magnitude of bodyweight gain was found to be higher with the atypical antipsychotic drugs that were introduced after 1990. Clozapine and olanzapine produce the greatest bodyweight gain, ziprasidone and aripiprazole have a neutral influence, and quetiapine and risperidone cause an intermediate effect. In the CATIE study, the percentage of patients with bodyweight gain of >7% compared with baseline differed significantly between the antipsychotic drugs, i.e. 30%, 16%, 14%, 12% and 7% for olanzapine, quetiapine, risperidone, perphenazine (a typical antipsychotic) and ziprasidone, respectively (p<0.001). Appetite stimulation is probably a key cause of bodyweight gain, but genetic polymorphisms modify the bodyweight response during treatment with atypical antipsychotics. In addition to nutritional advice, programmed physical activity, cognitive-behavioural training and atypical antipsychotic switching, pharmacological adjunctive treatments have been assessed to counteract excessive bodyweight gain. In some clinical trials, nizatidine, amantadine, reboxetine, topiramate, sibutramine and metformin proved effective in preventing or reversing atypical antipsychotic-induced bodyweight gain; however, the results are inconclusive since few randomized, placebo-controlled clinical trials have been conducted. Indeed, most studies were short-term trials without adequate statistical power and, in the case of metformin, nizatidine and sibutramine, the results are contradictory. The tolerability profile of these agents is adequate. More studies are needed before formal recommendations on the use of these drugs can be made. Meanwhile, clinicians are advised to use any of these adjunctive treatments according to their individual pharmacological and tolerability profiles, and the patient's personal and family history of bodyweight gain and metabolic dysfunction.

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.

Review article: The prevalence and clinical relevance of cytochrome P450 polymorphisms

BACKGROUND

Most drugs currently used in clinical practice are effective in only 25% to 60% of patients, while adverse drug reactions (ADRs) as a consequence of treatment are estimated to cost billions of US dollars and tens of thousands of deaths.

AIM

To review the prevalence and clinical significance of cytochrome P450 polymorphisms.

RESULTS

The cytochrome P450 enzyme families 1-3 are responsible for 70 to 80% of all phase I dependent drug metabolisms. In 90% metabolic activity dependents on six enzymes: CYP1A2, CYP3A, CYP2C9, CYP2C19, CYP2D6 and CYP2E1. Polymorphisms in the CYP450 gene can influence metabolic activity of the subsequent enzymes. A poor metabolizer (PM) has no or very poor enzyme activity. A consequence of PM is drug toxicity if no other metabolic route is available, or when multiple drugs are metabolized by the same cytochrome. In that case dose reduction is an option to prevent toxic effects.

CONCLUSIONS

In the future genotyping should be considered to identify patients who might be at risk of severe toxic responses, in order to guide appropriate individual dosage. Medical therapy should be a close cooperation between clinicians, pharmacologists and laboratory specialists, leading to reduced therapeutic errors, ADRs and health care costs.

Pharmacokinetics and safety of saquinavir/ritonavir and omeprazole in HIV-infected subjects

We investigated the pharmacokinetics and safety of saquinavir/ritonavir when administered with omeprazole simultaneously and 2 h apart to human immunodeficiency virus (HIV) subjects. Saquinavir/ritonavir 12-h pharmacokinetics was assessed with and without omeprazole 40 mg. Subjects were randomized to group A (saquinavir/ritonavir and omeprazole simultaneously/2 h apart) or group B (saquinavir/ritonavir and omeprazole 2 h apart/simultaneously). Saquinavir/ritonavir pharmacokinetics was assessed on days 1, 8, and 22. Within-subject changes were evaluated by geometric mean ratios and 90% confidence interval (CI). Twelve subjects completed the study. GM (90% CI) for saquinavir area under the curve (AUC)(0-12) (ng h/ml), trough concentration (C(trough)) (ng/ml), and maximum concentration (C(max)) (ng/ml) were 14,698 (13,242-20,636), 433 (368-758), 2,513 (2,243-3,329) without omeprazole; 22,646 (18,536-131,861), 750 (619-1,280), 3,890 (3,223-5,133) with omeprazole simultaneously; and 24,549 (20,884-38,894), 851 (720-1,782), 4,141 (3,554-5,992) with omeprazole 2 h earlier. Simultaneous administration of omeprazole significantly increased saquinavir AUC(0-12), C(trough), and C(max) by 54, 73, and 55%, whereas staggered administration by 67, 97, and 65%. No grade 3/4 toxicity or lab abnormalities were observed. In the presence of omeprazole, saquinavir plasma exposure is significantly increased in HIV-infected subjects whether administered simultaneously or 2 h apart.

Gastrointestinal medications

Medications to address gastrointestinal disorders are among the most commonly dispensed somatic medications. The authors examine proton pump inhibitors, H(2) blockers, 5-HT(3) receptor-antagonists, and a few other drugs that are used to address this domain of medical concerns. The metabolic pathways, interactions with the P-glycoprotein transporter, and capabilities of inhibiting or inducing metabolic enzymes are elucidated for each drug. Specific drug-drug interactions with each agent are also detailed, including both psychotropic and non-psychotropic agents. Also, the article explores how different genotypic variants for specific cytochrome P450 enzymes have an impact on the effectiveness and likelihood of drug-drug interactions relating to specific gastro-intestinal medications.

Cytochrome P450 and Its Role in Veterinary Drug Interactions

Cytochrome P450 (CYP) enzymes are common sites of drug interactions in human beings. Drugs may act as inhibitors or inducers of CYPs, leading to altered clearance of a second drug. Clinically relevant drug interactions involving various CYP isoforms in people, including CYP1A2, CYP2C9, CYP2D6, and CYP3A4, have been well documented. Analogous interactions are beginning to be characterized in dogs, for which canine CYPs share many of the same substrate ranges as in human beings.

Effect of omeprazole on the pharmacokinetics of saquinavir-500 mg formulation with ritonavir in healthy male and female volunteers

INTRODUCTION

Recent studies have described reduced absorption of certain protease inhibitors when administered with agents known to increase gastric pH. No clinically significant interactions between saquinavir absorption and gastric pH have previously been shown. We evaluated the effect of omeprazole, a proton-pump-inhibitor, on the pharmacokinetics of the recently developed saquinavir-500 mg formulation co-administered with ritonavir.

METHODS

Eighteen healthy subjects (n = 6 women and 12 men) received 1000/100 mg saquinavir/ritonavir twice daily in an open-label study for 15 days. On days 11-15, subjects were administered omeprazole 40 mg daily with the morning dose. Serial plasma samples were collected for 12-h pharmacokinetic profiles of saquinavir and ritonavir on days 10 and 15 and safety analysis on days 1, 4, 10, 15 and 29.

RESULTS

The geometric mean and 95% confidence interval (CI), for the area under time-concentration curve (AUC; ng h/ml), trough plasma concentration (C trough; ng/ml) and maximum observed plasma concentration (Cmax; ng/ml) of saquinavir were 20599 (14396-29360) and 37511 (28733-48970); 737 (482-1127) and 1521 (1039-2227); 3227 (2370-4393) and 5611 (4507-7710) on days 10 and 15, respectively, with geometric mean ratios of 1.82, 2.06 and 1.75. No significant changes were observed in saquinavir elimination half life, ritonavir pharmacokinetic parameters or in safety laboratory tests. No unexpected adverse events attributed to study medication were noted.

CONCLUSIONS

In the presence of omeprazole, total saquinavir plasma exposure is significantly increased (82% increase in AUC). The mechanism of this interaction requires elucidation. Despite the significant increase in saquinavir exposure, no short term toxicities were observed.

The effect of cimetidine on dextromethorphan O-demethylase activity of human liver microsomes and recombinant CYP2D6

Clinically, cimetidine therapy impairs the clearance of various drugs metabolized by CYP2D6, such as desipramine and sparteine. Cimetidine is known to reversibly inhibit CYP2D6 in vitro; however, Ki values are greater than plasma concentrations observed in vivo. There is evidence suggesting that this drug may act as an inactivator of cytochrome P450 (P450) enzymes after metabolic activation. Therefore, the purpose of this study was to determine whether cimetidine acts as a mechanism-based inactivator of CYP2D6. Dextromethorphan O-demethylation was used as a probe of CYP2D6 activity. The Vmax and Km of this reaction were 0.82 +/- 0.06 nmol/min/nmol of P450 and 4.1 +/- 0.1 microM, respectively, in pooled human liver microsomes; and 15.9 +/- 0.8 nmol/min/nmol P450 and 1.4 +/- 0.6 microM, respectively, with recombinant CYP2D6. With human liver microsomes, cimetidine competitively inhibited CYP2D6 (Ki = 38 +/- 5 microM) and was a mixed inhibitor of recombinant CYP2D6 (Ki = 103 +/- 17 microM). Preincubation of human liver microsomes with cimetidine and NADPH did not increase the inhibitory potency of cimetidine; however, preincubation with recombinant CYP2D6 resulted in enzyme inactivation that could be attenuated by the CYP2D6 inhibitor quinidine. The KI and kinact were estimated to be 77 microM and 0.03 min-1, respectively, and the half-life of inactivation was 25 min. Therefore, cimetidine may represent a class of compounds capable of inactivating specific cytochromes P450 in vivo, but for which conditions may not be achievable in vitro using human liver microsomes.

In vivo metabolic activity of CYP2C19 and CYP3A in relation to CYP2C19 genetic polymorphism in chronic liver disease

To study whether chronic liver disease (CLD) and genetic polymorphism affect the hepatic activity of cytochrome P450 (CYP) isoforms, we compared in vivo CYP2C19 and CYP3A activities using 3-hour omeprazole hydroxylation index (plasma concentration ratio of omeprazole to its 5-hydroxylated metabolite; a higher index indicates lower CYP2C19 activity) and partial formation clearance of cortisol to 6beta-hydroxycortisol (CL(cortisol-->6beta-HC)) in 31 CLD patients (9 with chronic hepatitis; 22 with cirrhosis comprising 20 Child-Pugh type A, 1 type B, and 1 type C) and 30 healthy subjects with different CYP2C19 genotypes. The mean (+/-SEM) omeprazole hydroxylation index in CLD patients with homozygous extensive metabolizer (EM) genotype (*1/*1, n = 8), heterozyous EM (*1/*2, n = 11; *1/*3, n = 6) genotypes and poor metabolizer (PM) genotypes (*2/*2, n = 3; *3/*3, n = 3) were 17.15 +/- 2.12, 20.02 +/- 2.63, and 26.04 +/- 3.15, respectively, which were significantly higher compared with control subjects with the corresponding CYP2C19 genotypes (0.81 +/- 0.09, 1.55 +/- 0.20, and 15.5 +/- 1.52). CLD patients with PM genotype had significantly (P < .05) higher omeprazole hydroxylation indexes than did those with homozygous EM genotype, and those with heterozygous EM genotypes had intermediate values. The mean CL(cortisol-->6beta-HC) decreased significantly (P < .001) in CLD patients compared with control subjects (1.19 +/- 0.12 versus 2.26 +/- 0.24 mL/min). Multiple regression analysis showed that CLD, serum albumin level, and CYP2C19 genotype correlated significantly (P < .05) with the omeprazole hydroxylation index, whereas no significant correlation was observed between CL(cortisol-->6beta-HC) and other variables, except CLD. Because CLD and genetic polymorphism of CYP2C19 act additively to reduce CYP2C19 activity, genotyping these patients may be of value in averting adverse reactions of drugs that depend on CYP2C19 for elimination.

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.

Impact of gastric acid suppressants on cytochrome P450 3A4 and P-glycoprotein: consequences for FK506 assimilation

BACKGROUND

Cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (PGP) are important determinants of the oral bioavailability and clearance of tacrolimus. Cimetidine and omeprazole are known modulators of several CYPs in vitro. In the present study, the impact of cimetidine and omeprazole on tacrolimus exposure and on CYP3A4/PGP activity in vivo was examined.

METHODS

In a cohort of 48 renal transplant recipients who switched standard ulcer prophylaxis with 400 mg of cimetidine daily to 20 mg of omeprazole, dose/weight normalized trough levels of tacrolimus during a 5-day interval before and after switch were compared and further studied using multivariate analysis. In a cohort of 6 healthy volunteers, the effect of a 5-day course of ranitidine, cimetidine, and omeprazole on overall CYP, CYP3A4, and PGP activity in vivo was assessed with the (13)C-aminopyrin breath test and the combined per oral and intravenous (14)C-erythromycin breath and urine test.

RESULTS

Dose/weight normalized trough levels of tacrolimus decreased significantly (-15%) after switch from cimetidine to omeprazole. In healthy volunteers, a significant increase of intestinal CYP3A4 activity was observed after omeprazole, whereas no change was noted after cimetidine/ranitidine. Overall CYP activity was significantly decreased after cimetidine and remained unchanged after omeprazole/ranitidine. No effects on PGP or hepatic CYP3A4 were seen.

CONCLUSION

Switching treatment with cimetidine to omeprazole in renal transplant recipients is associated with a decrease of dose/weight normalized trough levels of tacrolimus. Studies in healthy volunteers suggest that this may be explained by an increase of intestinal CYP3A4 activity.

CYP isoforms involved in the metabolism of clarithromycin in vitro: comparison between the identification from disappearance rate and that from formation rate of metabolites

To clarify whether CYP2C19 is involved in the overall metabolism of clarithromycin (CAM) or not, in vitro studies using human liver microsomes and recombinant CYPs were performed by an approach based on the disappearance rate of parent compound from the incubation mixture. In addition, the results of disappearance rate were compared with those obtained from the formation rates of the major metabolites of CAM, 14-(R)-hydroxy-CAM and N-demethyl-CAM. The intrinsic clearance (CL(int)) values determined from the disappearance of CAM in nine different human liver microsomes were highly correlated with the testosterone 6beta-hydroxylation activity (r=0.957, p<0.001). The CL(int) of CAM was markedly reduced by selective inhibitors of CYP3A4 (ketoconazole and troleandomycin) and by polyclonal antibodies raised against CYP3A4/5 in human liver microsomes. Among the 11 isoforms of recombinant human CYP, only CYP3A4 revealed the metabolic activity for the disappearance of CAM. These results were fairly consistent with those obtained from the conventional approach based on the formation of major metabolites of CAM. Comparison of the kinetic parameters estimated from the disappearance rate of CAM and the formation rates of 14-(R)-hydroxy-CAM and N-demethyl-CAM indicates that N-demethylation and 14-(R)-hydroxylation account for 65% of CL(int) derived from the disappearance of CAM in human liver microsomes. The findings suggest that CYP3A4 plays a predominant role in the overall metabolic clearance of CAM as well as in the formation of 14-(R)-hydroxy-CAM and N-demethyl-CAM. CYP2C19 does not appear to be involved in the overall metabolism of CAM at least in human liver microsomes. A combination of the disappearance rate of a parent compound and the formation rate of metabolites appears to be a useful approach for estimating the percentage contribution of the formation of metabolites to the overall metabolic clearance of a parent compound in vitro.

Comparison of inhibitory effects of the proton pump-inhibiting drugs omeprazole, esomeprazole, lansoprazole, pantoprazole, and rabeprazole on human cytochrome P450 activities

The human clearance of proton pump inhibitors (PPIs) of the substituted benzimidazole class is conducted primarily by the hepatic cytochrome P450 (P450) system. To compare the potency and specificity of the currently used PPIs (i.e., omeprazole, esomeprazole, lansoprazole, pantoprazole, and rabeprazole) as inhibitors of four cytochrome P450 enzymes (CYP2C9, 2C19, 2D6, and 3A4), we performed in vitro studies using human liver microsomal preparations and recombinant CYP2C19. Sample analysis was done using selected reaction monitoring liquid chromatography/tandem mass spectometry. With several systems for CYP2C19 activity (two marker reactions, S-mephenytoin 4'-hydroxylation and R-omeprazole 5-hydroxylation, tested in either human liver microsomes or recombinant CYP2C19), the five PPIs showed competitive inhibition of CYP2C19 activity with K(i) of 0.4 to 1.5 microM for lansoprazole, 2 to 6 microM for omeprazole, approximately 8 microM for esomeprazole, 14 to 69 microM for pantoprazole, and 17 to 21 microM for rabeprazole. Pantoprazole was a competitive inhibitor of both CYP2C9-catalyzed diclofenac 4'-hydroxylation and CYP3A4-catalyzed midazolam 1'-hydroxylation (K(i) of 6 and 22 microM, respectively), which were at least 2 times more potent than the other PPIs. All PPIs were poor inhibitors of CYP2D6-mediated bufuralol 1'-hydroxylation with IC(50) > 200 microM. The inhibitory potency of a nonenzymatically formed product of rabeprazole, rabeprazole thioether, was also investigated and showed potent, competitive inhibition with K(i) values of 6 microM for CYP2C9, 2 to 8 microM for CYP2C19, 12 microM for CYP2D6, and 15 microM for CYP3A4. The inhibitory potency of R-omeprazole on the four studied P450 enzymes was also studied and showed higher inhibitory potency than its S-isomer on CYP2C9 and 2C19 activities. Our data suggest that, although the inhibitory profiles of the five studied PPIs were similar, lansoprazole and pantoprazole are the most potent in vitro inhibitors of CYP2C19 and CYP2C9, respectively. Esomeprazole showed less inhibitory potency compared with omeprazole and its R-enantiomer. The inhibitory potency of rabeprazole was relatively lower than the other PPIs, but its thioether analog showed potent inhibition on the P450 enzymes investigated, which may be clinically significant.

Lack of effect of omeprazole or of an aluminium hydroxide/magnesium hydroxide antacid on the pharmacokinetics of lumiracoxib

OBJECTIVE

To evaluate the effects of multiple doses of omeprazole and of a single dose of an aluminium hydroxide/magnesium hydroxide (Al/Mg) antacid on the single-dose plasma pharmacokinetics of lumiracoxib.

STUDY DESIGN

Open-label, randomised, three-period, crossover study.

POPULATION STUDIED

Healthy subjects aged 18-65 years.

METHODS

Fourteen subjects who met eligibility criteria were each administered three treatments in random order: (A) lumiracoxib 400 mg as a single oral dose; (B) oral omeprazole 20 mg once daily for 4 consecutive days, then lumiracoxib 400 mg as a single oral dose just prior to oral omeprazole 20 mg on day 5; and (C) lumiracoxib 400 mg as a single oral dose immediately prior to a 20 mL dose of Al/Mg antacid (magnesium hydroxide 800 mg and aluminium hydroxide 900 mg). The interval between each lumiracoxib dose was 7 days. Analysis of variance was performed to determine whether lumiracoxib alone differed from lumiracoxib plus omeprazole or from lumiracoxib plus Al/Mg antacid for overall exposure (area under the concentration-time curve from zero to infinity [AUC( infinity )]) and peak concentration (C(max)), with treatment sequence, subject, period and treatment as factors. Ratios of geometric means between lumiracoxib plus omeprazole and lumiracoxib plus Al/Mg antacid to lumiracoxib alone (reference) were calculated for AUC( infinity ) and C(max). If the mean ratios, with 90% CIs, fell within the interval 0.80-1.25, the treatments were considered equivalent.

RESULTS

Arithmetic mean plasma lumiracoxib concentration-time profiles were similar for all treatments, with a rapid rise in concentration after administration, reaching C(max) values (mean +/- SD) of 9.24 +/- 1.96, 8.81 +/- 2.30, and 10.43 +/- 3.24 mg/L within 2-3 hours for treatments A, B and C, respectively. AUC( infinity ) was similar for the three treatments (36.75 +/- 7.73, 34.88 +/- 8.40 and 35.50 +/- 5.72 mg. h/L). All ratios of geometric means with 90% CIs fell within the interval used for establishing bioequivalence, except for the C(max) comparison between lumiracoxib plus Al/Mg antacid and lumiracoxib alone, which was 1.11 (0.95, 1.31).

CONCLUSIONS

Coadministration of lumiracoxib with omeprazole or with an Al/Mg antacid had no clinically significant effect on lumiracoxib single-dose plasma pharmacokinetics. Lumiracoxib can, therefore, be administered concurrently with either of these agents without need for lumiracoxib dosage alteration.

Drug–drug interactions of Z-338, a novel gastroprokinetic agent, with terfenadine, comparison with cisapride, and involvement of UGT1A9 and 1A8 in the human metabolism of Z-338

In the present study, the inhibitory properties of N-[2-(diisopropylamino)ethyl]-2-[(2-hydroxy-4,5-dimethoxybenzoyl)amino]-1,3-thiazole-4-carboxamide monohydrochloride trihydrate (Z-338), a novel gastroprokinetic agent, were investigated and compared with those of cisapride to establish its potential for drug-drug interactions. There was no notable inhibition of terfenadine metabolism or of any of the isoforms of cytochrome P450 (CYP1A1/2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1 and 3A4) by Z-338 in in vitro studies using human liver microsomes. Z-338 was mainly metabolized to its glucuronide by UGT1A9 (UDP glucoronosyltransferase 1 family, polypeptide A9) and UGT1A8, and did not show marked inhibition of P-glycoprotein activity. On the other hand, cisapride strongly inhibited CYP3A4 and markedly inhibited CYP2C9. Furthermore, we used the whole-cell patch-clamp technique to investigate the effects of Z-338 and cisapride on potassium currents in human embryonic kidney (HEK) 293 cells transfected with the human ether-a-go-go-related gene (hERG). Z-338 had no significant effect on hERG-related current at the relatively high concentration of 10 microM. In contrast, the inhibition by Z-338 was very small compared with that of cisapride at 10 nM, which was a thousand-fold lower concentration. In the prediction method for the drug interaction between terfenadine and cisapride based on the K(i) and PK parameters, we suggest the possibility that terfenadine mainly affect the QT interval, since its plasma concentration would be markedly increased, but cisapride may not be changed. Thus, in contrast with cisapride, Z-338 did not inhibit CYP and the hERG channel, and is predominantly metabolized by glucuronide conjugation, Z-338 is considered unlikely to cause significant drug-drug interactions when coadministered with CYP substrates at clinically effective doses.

Treatment of the metabolic disturbances caused by antipsychotic drugs: focus on potential drug interactions

The risk of excessive bodyweight gain, glucose dysregulation and hyperlipidaemia is differentially increased by conventional and atypical antipsychotic drugs. Switching or combining agents may be sufficient in some cases, but in many instances additional drug treatment will be required. This includes oral antidiabetics, insulin and agents to treat hyperlipidaemia, hypertension and platelet dysfunction, among others. Numerous pharmacokinetic and pharmacodynamic interactions with the antipsychotics are possible, although few have been tested in formal studies. After reviewing the literature, the authors provide preliminary guidelines to assist clinicians in drug selection for this complex and fragile clinical population.

Extrapyramidal Symptoms Related to Adjunctive Nizatidine Therapy in an Adolescent Receiving Quetiapine and Paroxetine

Weight gain is a serious problem with recently introduced atypical antipsychotic agents. Nizatidine, a histamine2 (H2)-receptor antagonist, may help reduce this weight gain. To our knowledge, no adverse effects have been reported when nizatidine is given at recommended doses with atypical antipyschotic agents. We describe, however, an adolescent who was receiving quetiapine and paroxetine for schizophrenia and depression, and developed extrapyramidal symptoms (EPS; parkinsonism and akathisia) after taking nizatidine for weight loss. Based on a report of another patient who developed EPS after taking higher-than-recommended doses of nizatidine, we reviewed the literature on treatment with H2-receptor antagonists for weight gain and on central nervous system adverse effects of nizatidine. Nizatidine may be effective for reducing weight gain associated with both medical and psychiatric conditions. Its safety profile is usually benign, although some patients may develop serious adverse effects, such as EPS and delirium. Therefore, the drug is recommended for short-term management of weight gain associated with atypical antipsychotic agents. Patients receiving nizatidine therapy should be monitored closely for development of EPS, particularly when high doses are prescribed.

Developmental pharmacokinetics and pharmacodynamics of nizatidine

OBJECTIVES

To characterize the impact of development on the pharmacokinetics and pharmacodynamics of nizatidine.

METHODS

Children (age range, 5 days-18 years) and adults (age range, 18-50 years) were enrolled in four open-label trials. Nizatidine formulation and dose were determined by age: infants received 2 or 4 mg/kg i.v., children 2.5 or 5 mg/kg in one of three oral liquid formulations, and adolescents and adults received a fixed 150-mg capsule. Nizatidine and N-desmethylnizatidine concentrations were measured in serial post-dose plasma samples by a high-performance liquid chromatographic assay with mass spectrometric detection. Intragastric pH was recorded during a 24-hour post-dose interval.

RESULTS

Data on 93 subjects were combined with previous values from 36 individuals to cover an age group not adequately captured and to control for formulation effects. Dose-normalized exposure estimates revealed no apparent age dependence; however, maximum plasma concentration (298.5 +/- 100.7 v 552.8 +/- 152.4 ng/mL per mg/kg dose) and AUC0-infinity (954.4 +/- 379.8 v 1,573.0 +/- 347.4 ng*hour/mL per mg/kg dose) were reduced in extemporaneous formulations in apple juice. The apparent modest age dependence observed for total body clearance (Cl/F) (r = 0.365) and Vss/F (r = 0.221) reflected a formulation-dependent decrease in bioavailability rather than a true age effect. The age-associated changes in lambda z observed for nizatidine and its metabolite were predictable and consistent with developmental acquisition of renal function. Mean and median pH, as well as fraction of time that the dosing interval remained above target pH values, were significantly greater with administration of the drug than without.

CONCLUSIONS

The biodisposition of nizatidine in children and adults is similar; however, response after a comparable weight-based dose is equal and potentially greater in children.

Reduction of ischemia and reperfusion-induced myocardial damage by cytochrome P450 inhibitors

Ischemia and reperfusion both contribute to tissue damage after myocardial infarction. Although many drugs have been shown to reduce infarct size when administered before ischemia, few have been shown to be effective when administered at reperfusion. Moreover, although it is generally accepted that a burst of reactive oxygen species (ROS) occurs at the onset of reperfusion and contributes to tissue damage, the source of ROS and the mechanism of injury is unclear. We now report the finding that chloramphenicol administered at reperfusion reduced infarct size by 60% in a Langendorff isolated perfused rat heart model, and that ROS production was also substantially reduced. Chloramphenicol is an inhibitor of mitochondrial protein synthesis and is also an inhibitor of a subset of cytochrome P450 monooxygenases (CYPs). We could not detect any effect on mitochondrial encoded proteins or mitochondrial respiration in chloramphenicol-perfused hearts, and hypothesized that the effect was caused by inhibition of CYPs. We tested additional CYP inhibitors and found that cimetidine and sulfaphenazole, two CYP inhibitors that have no effect on mitochondrial protein synthesis, were also able to reduce creatine kinase release and infarct size in the Langendorff model. We also showed that chloramphenicol reduced infarct size in an open chest rabbit model of regional ischemia. Taken together, these findings implicate CYPs in myocardial ischemia/reperfusion injury.

Effect of nizatidine on olanzapine-associated weight gain in schizophrenic patients in Korea: a pilot study

A pilot study was conducted to evaluate the effect of nizatidine on olanzapine-associated weight gain (OAWG) in ten patients with schizophrenia and schizophreniform disorder in Korea. Psychometric ratings with positive and negative syndrome scale (PANSS) and brief psychiatric rating scale (BPRS) were measured at baseline, week 4 and week 8; as were weight and body mass index (BMI). A combination of nizatidine for 8 weeks resulted in significant reversal of weight gain without worsening the psychopathology (weight: 3.5% and BMI: 3.7%). In line with studies of Western populations, an add-on therapy of nizatidine could be an effective option for the control of weight gain in olanzapine-treated patients in Korea. Our findings call for further evaluation of the effect of this drug on OAWG, with randomized placebo-controlled studies, in Asian populations.

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

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

Involvement of CYP2C9 and UGT2B7 in the metabolism of zaltoprofen, a nonsteroidal anti-inflammatory drug, and its lack of clinically significant CYP inhibition potential

AIMS

To identify the cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) isoforms responsible for the formation of the primary metabolite(s) of zaltoprofen, and to predict possible drug interactions by investigating the inhibition of CYP isoforms in vitro.

METHODS

The metabolism of zaltoprofen was studied in vitro using recombinant CYP and UGT isoform cDNA-expression systems. The effects of selective isoform inhibitors on zaltoprofen metabolism were studied using human liver microsomes. The inhibitory effects of zaltoprofen on the metabolism of selective probe substrates for CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4 were also determined in human liver microsomes.

RESULTS

Zaltoprofen was extensively metabolized by CYP2C9 and UGT2B7. CYP2C9 catalysed sulphoxidation but not hydroxylation of zaltoprofen. In the human liver microsomal metabolism study, zaltoprofen metabolism was markedly inhibited by sulphaphenazole, a selective inhibitor of CYP2C9. In the drug interaction study, negligible inhibition (< 15%) of the activities of CYP1A2, CYP2C19, CYP2D6, CYP2E1 and CYP3A4 was apparent at 5 micro g ml(-1), the maximum plasma concentration observed in humans after oral administration of an 80 mg zaltoprofen tablet. However, zaltoprofen inhibited CYP2C9 by 26% at 5 micro g ml(-1). At higher concentrations, zaltoprofen produced some inhibition of CYP2C9 (IC50 = 19.2 micro g ml(-1); 64.4 micro m) and CYP3A4 (IC50 = 53.9 micro g ml(-1); 181 micro m). The free drug concentrations in plasma (0.02 micro g ml(-1), 67.0 nm) at the Cmax of the clinically effective doses are much lower than the IC50 values corrected for the nonspecific binding ratio of zaltoprofen to microsomal protein (15.5 micro g ml(-1) for CYP3A4, 49.5 micro g ml(-1) for CYP3A4). Furthermore, the maximum free drug concentrations in the hepatic intracellular was calculated to be 0.068 micro g ml(-1) and the increase in the AUC in the presence of zaltoprofen was estimated to be only 0.4% for CYP2C9 substrates and 0.1% for CYP3A4 substrates, respectively.

CONCLUSIONS

Zaltoprofen is predominantly metabolized by CYP2C9 and UGT2B7, and is considered unlikely to cause significant drug interactions in vivo when coadministered with CYP substrates at clinically effective doses.

Options for pharmacological management of obesity in patients treated with atypical antipsychotics

Obesity is associated with considerable morbidity and decreased life expectancy. Weight gain is a commonly encountered problem associated with antipsychotic treatment. We reviewed the literature regarding the mechanisms of weight gain in response to these agents and eight substances implicated as potential obesity prevention or treatment: orlistat, sibutramine, fluoxetine, topiramate, amantadine, nizatidine and cimetidine, and metformin. Weight gain in response to antipsychotic treatment may be mediated through serotonergic, dopaminergic, adrenergic, cholinergic, histaminergic and glutaminergic receptors. Sex hormone dysregulation and altered insulin sensitivity have also been implicated. Two compounds, orlistat and sibutramine, have been shown to help prevent weight gain following a hypocaloric diet, but orlistat requires compliance with a fat-reduced diet, and sibutramine is unsuitable for patients taking serotonergic agents. The weight reducing effect of fluoxetine, even in conjunction with a hypocaloric diet, is only transient. Topiramate, amantadine and metformin may have adverse side-effects potentially outweighing the weight reducing potential. The effectiveness of cimetidine and nizatedine remains unclear. The hazards of these agents in a psychiatric population are discussed. It is concluded that the current evidence does not support the general use of pharmacological interventions for overweight patients treated with antipsychotic medication, although individually selected patients may benefit.

Interaction of cimetidine with P450 in a mouse model of hepatocarcinogenesis initiation

Many drugs and xenobiotics are lipophilic and they should be transformed into more polar water soluble compounds to be excreted. Cimetidine inhibits cytochrome P450. The aim of this study was to investigate the preventive and/or reversal action of cimetidine on cytochrome P450 induction and other metabolic alterations provoked by the carcinogen p-dimethylaminoazobenzene. A group of male CF1 mice received a standard laboratory diet and another group was placed on dietary p-dimethylaminoazobenzene (0.5% w w(-1). After 40 days of treatment, animals of both groups received p-dimethylaminoazobenzene and two weekly doses of cimetidine (120 mg kg(-1), i.p.) during a following period of 35 days. Cimetidine prevented and reversed delta-aminolevulinate synthetase induction and cytochrome P450 enhancement provoked by p-dimethylaminoazobenzene. However, cimetidine did not restore haem oxygenase activity decreased by p-dimethylaminoazobenzene. Enhancement in glutathione S-transferase activity provoked by p-dimethylaminoazobenzene, persisted in those animals then treated with cimetidine. This drug did not modify either increased lipid peroxidation or diminution of the natural antioxidant defence system (inferred by catalase activity) induced by p-dimethylaminoazobenzene. In conclusion, although cimetidine treatment partially prevented and reversed cytochrome P450 induction, and alteration on haem metabolism provoked by p-dimethylaminoazobenzene AB, it did not reverse liver damage or lipid peroxidation. These results further support our hypothesis on the necessary existence of a multiple biochemical pathway disturbance for the onset of hepatocarcinogenesis initiation.