Omeprazole reduces clozapine plasma concentrations. A case report

Clozapine ultrarapid metabolism during weak induction probably exists but requires careful diagnosis. A literature review, five new cases and a proposed definition

During weak induction (from smoking and/or valproate co-prescription), clozapine ultrarapid metabolizers (UMs) need very high daily doses to reach the minimum therapeutic concentration of 350 ng/ml in plasma; clozapine UMs need clozapine doses higher than: 1) 900 mg/day in patients of European/African ancestry, or 2) 600 mg/day in those of Asian ancestry. Published clozapine UMs include 10 males of European/African ancestry, mainly assessed with single concentrations. Five new clozapine UMs (two of European and three of Asian ancestry) with repeated assessments are described. A US double-blind randomized trial included a 32-year-old male smoking two packages/day with a minimum therapeutic dose of 1,591 mg/day from a single TDM during open treatment of 900 mg/day. In a Turkish inpatient study, a 30-year-old male smoker was a possible clozapine UM needing a minimum therapeutic dose of 1,029 mg/day estimated from two trough steady-state concentrations on 600 mg/day. In a Chinese study, three possible clozapine UMs (all male smokers) were identified. The clozapine minimum therapeutic dose estimated with trough steady-state concentrations >150 ng/ml was: 1) 625 mg/day, based on a mean of 20 concentrations in Case 3; 2) 673 mg/day, based on a mean of 4 concentrations in Case 4; and 3) 648 mg/day, based on a mean of 11 concentrations in Case 5. Based on these limited studies, clozapine UMs during weak induction may account for 1-2% of clozapine-treated patients of European ancestry and <1% of those of Asian ancestry. A clozapine-to-norclozapine ratio <0.5 should not be used to identify clozapine UMs.

Infections associated with clozapine: a pharmacovigilance study using VigiBase®

Introduction: Clozapine is primarily reserved for treatment-resistant schizophrenia due to safety concerns associated with its use. Infections have been reported with clozapine, which may lead to elevated serum levels of the drug. However, the existing literature on this topic is limited. Therefore, we conducted a study using VigiBase® to investigate the potential over-reporting of infections associated with clozapine, to explore the presence of dose-dependency, and to investigate the underlying mechanism. Methods: Disproportionality analyses were performed using VigiBase to assess the association between clozapine and all types of infections, the association between clozapine-associated infections and neutropenia, the association between clozapine-associated infections and agranulocytosis, the dose-effect relationship between clozapine and infections, and the interaction between clozapine and the main strong CYP450 inhibitors using reports carried out until 11 April 2023. Results: A statistically significant signal of infections was observed with clozapine, as indicated by an information component of 0.43 [95% CI: (0.41-0.45)]. The most commonly reported infections were respiratory and gastrointestinal in nature. Neutropenia showed weaker association with clozapine-associated reports of infections compared to other clozapine-associated reports [X2 (1, N = 204,073) = 454; p < 0.005], while agranulocytosis demonstrated a stronger association with clozapine-associated reports of infections [X2 (1, N = 204,073) = 56; p < 0.005]. No evidence of dose-dependency was observed. Among the 17 tested CYP inhibitors, significant drug-drug interactions were found with clarithromycin, metronidazole, valproic acid, lansoprazole, omeprazole, amiodarone, and esomeprazole. Discussion: Our study revealed a significant safety signal between clozapine use and infections, predominantly respiratory and gastrointestinal infections. The co-administration of clozapine with valproic acid or proton pump inhibitors may potentially contribute to an increased risk of infection. Further vigilance is warranted in clinical practice, and consideration of therapeutic drug monitoring of clozapine in cases involving concomitant use of these drugs or in the presence of infections may be beneficial.

Predicting clozapine dose required to achieve a therapeutic plasma concentration - A comparison of a population algorithm and three algorithms based on gene variant models

BACKGROUND

Clozapine is the most effective antipsychotic but requires careful titration to therapeutic blood levels. Methods to predict therapeutic doses are based on population data.

AIMS

We aimed to construct a model based on genetic variants which accurately predicted plasma levels for clozapine.

METHOD

We measured clozapine plasma levels in patients on a stable dose of clozapine who were known to be fully compliant. Measured plasma levels were adjusted for sampling time and dose. Hepatic enzyme variants were analysed and models were constructed to predict the required dose. These predictions were compared with a standard population-based algorithm.

RESULTS

We measured plasma clozapine concentrations in 18 adherent patients on stable doses of clozapine and recorded the exact timing of sampling. For the algorithm-predicted dose, the mean difference was -49.9 mg/day ((SD 155.9), r = 0.36) from the actual dose required to give a plasma concentration of 0.35 ng/ml. The gene variant activity score predicted a dose for which the mean difference was -43.5 mg/day ((SD 140.1), r = 0.55). For the gene variant activity score with omeprazole correction predicted dose, the mean difference was -31.0 mg/day ((SD 140.8), r = 0.54), and with the gene variant CYP1A2 inducibility predicted dose the mean difference was 44.9 mg/day ((SD 160.8), r = 0.32).

CONCLUSION

Our gene variant activity score with omeprazole correction gave the best estimate of the clozapine dose required to achieve a minimum therapeutic plasma concentration. The use of this model will allow safer titration of clozapine and may reduce the need for plasma-level monitoring during titration.

Drug-Drug Interactions With Over-The-Counter Medicines: Mind the Unprescribed

BACKGROUND

This review provides a summary of the currently available clinical data on drug-drug interactions (DDIs) involving over-the-counter (OTC) medicines. It aims to educate and increase awareness among health care providers and to support decisions in daily practice.

METHODS

An extensive literature search was performed using bibliographic databases available through PubMed.gov. An initial structured search was performed using the keywords "drug-drug-interaction AND (over-the-counter OR OTC)," without further restrictions except for the language. The initial results were screened for all described DDIs involving OTC drugs, and further information was gathered specifically on these drugs using dedicated database searches and references found in the bibliography from the initial hits.

RESULTS

From more than 1200 initial hits (1972-June 2021), 408 relevant publications were screened for DDIs involving OTC drugs, leading to 2 major findings: first, certain types of drug regimens are more prone to DDIs or have more serious DDI-related consequences, such as antiretroviral, anti-infective, and oral anticancer therapies. Second, although most DDIs involve OTC drugs as the perpetrators, some prescription drugs (statins or phosphodiesterase-5 inhibitors) that currently have OTC status can be identified as the victims in DDIs. The following groups were identified to be frequently involved in DDIs: nonsteroidal anti-inflammatory drugs, food supplements, antacids, proton-pump inhibitors, H2 antihistamines, laxatives, antidiarrheal drugs, and herbal drugs.

CONCLUSIONS

The most significant finding was the lack of high-quality evidence for commonly acknowledged interactions. High-quality interaction studies involving different phenotypes in drug metabolism (cytochrome P450) and distribution (transporters) are urgently needed. This should include modern and critical drugs, such as oral anticancer medications and direct oral anticoagulants.

Selective Serotonin Reuptake Inhibitors and Clozapine: Clinically Relevant Interactions and Considerations

The monoamine hypothesis of depression attributes the symptoms of major depressive disorders to imbalances of serotonin, noradrenaline, and dopamine in the limbic areas of the brain. The preferential targeting of serotonin receptor (SERT) by selective serotonin reuptake inhibitors (SSRIs) has offered an opportunity to reduce the range of these side effects and improve patient adherence to pharmacotherapy. Clozapine remains an effective drug against treatment-resistant schizophrenia, defined as failing treatment with at least two different antipsychotic medications. Patients with schizophrenia who display a constellation of negative symptoms respond poorly to antipsychotic monotherapy. Negative symptoms include the diminution of motivation, interest, or expression. Conversely to the depressive symptomology of interest presently, supplementation of antipsychotics with SSRIs in schizophrenic patients with negative symptoms lead to synergistic improvements in the function of these patients. Fluvoxamine is one of the most potent inhibitors of CYP1A2 and can lead to an increase in clozapine levels. Similar increases in serum clozapine were detected in two patients taking sertraline. However, studies have been contradictory as well, showing no such increases, which are worrying. Clinicians should be aware that clozapine levels should be monitored with any coadministration with SSRIs.

Mental Health Nurses' Knowledge of Clozapine

Clozapine, sold under the brand name Clozaril, is an antipsychotic medication prescribed since the 1970s as an effective treatment for individuals with treatment resistive schizophrenia. Medical experts have produced extensive literature on the need for close monitoring as this medication can cause life threatening adverse effects. Mental health nurses play a vital role in the management of clozapine, however to date just one study specifically examines psychiatric/mental health nurse's knowledge of this medication. Nurses need to be aware of the adverse effects and be able to provide psychoeducation to help support and inform clients prescribed this medication. This study used a survey questionnaire. A clozapine knowledge questionnaire with 18 questions was developed by De Hert et al. to explore psychiatric/mental health nurse's knowledge on the adverse effects of clozapine. We distributed a shortened version of this knowledge questionnaire (12-multi-choice questions) to 209 mental health nurses working within one Irish Mental Health Service. One hundred and twenty-nine (n = 129) nurses completed the questionnaire with a response rate of 62%. Our results indicated that over 40% of mental nurses had not received sufficient education on clozapine medication during their formal education. We found that just 50% of participants scored over six correct questions out of 12 questions on various areas of knowledge related to clozapine medication. We identified gaps in knowledge on clozapine medication in relation to on myocarditis, epilepsy and metabolic syndrome. We highlighted gaps in mental health nurses' knowledge of clozapine medication that need to be improved in order to help clients. We advocate additional education on clozapine medication at undergraduate education and through educational packages at the service level.

Drug-Drug Interactions Involving Intestinal and Hepatic CYP1A Enzymes

Cytochrome P450 (CYP) 1A enzymes are considerably expressed in the human intestine and liver and involved in the biotransformation of about 10% of marketed drugs. Despite this doubtless clinical relevance, CYP1A1 and CYP1A2 are still somewhat underestimated in terms of unwanted side effects and drug–drug interactions of their respective substrates. In contrast to this, many frequently prescribed drugs that are subjected to extensive CYP1A-mediated metabolism show a narrow therapeutic index and serious adverse drug reactions. Consequently, those drugs are vulnerable to any kind of inhibition or induction in the expression and function of CYP1A. However, available in vitro data are not necessarily predictive for the occurrence of clinically relevant drug–drug interactions. Thus, this review aims to provide an up-to-date summary on the expression, regulation, function, and drug–drug interactions of CYP1A enzymes in humans.

Effects of the Proton Pump Inhibitors Omeprazole and Pantoprazole on the Cytochrome P450-Mediated Metabolism of Venlafaxine

BACKGROUND AND OBJECTIVE: An increasing trend in prescribing proton pump inhibitors (PPIs) inevitably increases the risk of unwanted drug-drug interactions (DDIs). The aim of this study was to uncover pharmacokinetic interactions between two PPIs-omeprazole and pantoprazole-and venlafaxine.

METHODS

A therapeutic drug monitoring database contained plasma concentrations of venlafaxine and its active metabolite O-desmethylvenlafaxine. We considered three groups: a group of patients who received venlafaxine without confounding medications (non-PPI group, n = 906); a group of patients who were comedicated with omeprazole (n = 40); and a group of patients comedicated with pantoprazole (n = 40). Plasma concentrations of venlafaxine, O-desmethylvenlafaxine and active moiety (venlafaxine + O-desmethylvenlafaxine), as well as dose-adjusted plasma concentrations, were compared using non-parametrical tests.

RESULTS

Daily doses of venlafaxine did not differ between groups (p = 0.949). The Mann-Whitney U test showed significantly higher plasma concentrations of active moiety, as well as venlafaxine and O-desmethylvenlafaxine, in both PPI groups [p = 0.023, p = 0.011, p = 0.026, +29% active moiety, +27% venlafaxine, +36% O-desmethylvenlafaxine (pantoprazole); p = 0.003, p = 0.039 and p < 0.001, +36% active moiety, +27% venlafaxine, +55% O-desmethylvenlafaxine (omeprazole)]. Significantly higher concentration-by-dose (C/D) values for venlafaxine and active moiety were detected in the pantoprazole group (p = 0.013, p = 0.006, respectively), while in the omeprazole group, C/D ratios for all three parameters-venlafaxine, O-desmethylvenlafaxine and active moiety-were significantly higher (p = 0.021, p < 0.001 and p < 0.001, respectively).

CONCLUSIONS

Significantly higher plasma concentrations for all parameters (venlafaxine, O-desmethylvenlafaxine, active moiety) suggest clinically relevant inhibitory effects of both PPIs, most likely on the cytochrome P450 (CYP) 2C19-mediated metabolism of venlafaxine. The findings might be the result of different degrees of CYP2C19 involvement, therefore the inhibition of CYP2C19 by both PPIs may lead to an increased metabolism via CYP2D6 to O-desmethylvenlafaxine.

One size fits one: pharmacogenetics in gastroenterology

Individual variability in response and development of adverse effects to drugs is a major challenge in clinical practice. Pharmacogenomics refers to the aspect of personalized medicine where the patient's genetic information instructs the selection and dosage of therapy while also predicting its adverse effects profile. Sequencing of the entire human genome has given us the opportunity to study commonly used drugs as well as newer therapeutic agents in a new light, opening up opportunities for better drug efficacy and decreased adverse effects. This article highlights developments in pharmacogenomics, relates these to practice of gastroenterology, and outlines roadblocks in translation of this knowledge into clinical practice.

Clinically significant drug interactions with atypical antipsychotics

Atypical antipsychotics [also known as second-generation antipsychotics (SGAs)] have become a mainstay therapeutic treatment intervention for patients with schizophrenia, bipolar disorders and other psychotic conditions. These agents are commonly used with other medications--most notably, antidepressants and antiepileptic drugs. Drug interactions can take place by various pharmacokinetic, pharmacodynamic and pharmaceutical mechanisms. The pharmacokinetic profile of each SGA, especially with phase I and phase II metabolism, can allow for potentially significant drug interactions. Pharmacodynamic interactions arise when agents have comparable receptor site activity, which can lead to additive or competitive effects without alterations in measured plasma drug concentrations. Additionally, the role of drug transporters in drug interactions continues to evolve and may effect both pharmacokinetic and pharmacodynamic interactions. Pharmaceutical interactions occur when physical incompatibilities take place between agents prior to drug absorption. Approximate therapeutic plasma concentration ranges have been suggested for a number of SGAs. Drug interactions that markedly increase or decrease the concentrations of these agents beyond their ranges can lead to adverse events or diminished clinical efficacy. Most clinically significant drug interactions with SGAs occur via the cytochrome P450 (CYP) system. Many but not all drug interactions with SGAs are identified during drug discovery and pre-clinical development by employing a series of standardized in vitro and in vivo studies with known CYP inducers and inhibitors. Later therapeutic drug monitoring programmes, clinical studies and case reports offer methods to identify additional clinically significant drug interactions. Some commonly co-administered drugs with a significant potential for drug-drug interactions with selected SGAs include some SSRIs. Antiepileptic mood stabilizers such as carbamazepine and valproate, as well as other antiepileptic drugs such as phenobarbital and phenytoin, may decrease plasma SGA concentrations. Some anti-infective agents such as protease inhibitors and fluoroquinolones are of concern as well. Two additional important factors that influence drug interactions with SGAs are dose and time dependence. Smoking is very common among psychiatric patients and can induce CYP1A2 enzymes, thereby lowering expected plasma levels of certain SGAs. It is recommended that ziprasidone and lurasidone are taken with food to promote drug absorption, otherwise their bioavailability can be reduced. Clinicians must be aware of the variety of factors that can increase the likelihood of clinically significant drug interactions with SGAs, and must carefully monitor patients to maximize treatment efficacy while minimizing adverse events.

Pharmacokinetic drug interaction profile of omeprazole with adverse consequences and clinical risk management

BACKGROUND

Omeprazole, a proton pump inhibitor (PPI), is widely used for the treatment of dyspepsia, peptic ulcer, gastroesophageal reflux disease, and functional dyspepsia. Polypharmacy is common in patients receiving omeprazole. Drug toxicity and treatment failure resulting from inappropriate combination therapy with omeprazole have been reported sporadically. Systematic review has not been available to address the pharmacokinetic drug-drug interaction (DDI) profile of omeprazole with adverse consequences, the factors determining the degree of DDI between omeprazole and comedication, and the corresponding clinical risk management.

METHODS

Literature was identified by performing a PubMed search covering the period from January 1988 to March 2013. The full text of each article was critically reviewed, and data interpretation was performed.

RESULTS

Omeprazole has actual adverse influences on the pharmacokinetics of medications such as diazepam, carbamazepine, clozapine, indinavir, nelfinavir, atazanavir, rilpivirine, methotrexate, tacrolimus, mycophenolate mofetil, clopidogrel, digoxin, itraconazole, posaconazole, and oral iron supplementation. Meanwhile, low efficacy of omeprazole treatment would be anticipated, as omeprazole elimination could be significantly induced by comedicated efavirenz and herb medicines such as St John's wort, Ginkgo biloba, and yin zhi huang. The mechanism for DDI involves induction or inhibition of cytochrome P450, inhibition of P-glycoprotein or breast cancer resistance protein-mediated drug transport, and inhibition of oral absorption by gastric acid suppression. Sometimes, DDIs of omeprazole do not exhibit a PPI class effect. Other suitable PPIs or histamine 2 antagonists may be therapeutic alternatives that can be used to avoid adverse consequences. The degree of DDIs associated with omeprazole and clinical outcomes depend on factors such as genotype status of CYP2C19 and CYP1A2, ethnicity, dose and treatment course of precipitant omeprazole, pharmaceutical formulation of object drug (eg, mycophenolate mofetil versus enteric-coated mycophenolate sodium), other concomitant medication (eg, omeprazole-indinavir versus omeprazole-indinavir-ritonavir), and administration schedule (eg, intensified dosing of mycophenolate mofetil versus standard dosing).

CONCLUSION

Despite the fact that omeprazole is one of the most widely prescribed drugs internationally, clinical professionals should enhance clinical risk management on adverse DDIs associated with omeprazole and ensure safe combination use of omeprazole by rationally prescribing alternatives, checking the appropriateness of physician orders before dispensing, and performing therapeutic drug monitoring.

A review of the use of clozapine levels to guide treatment and determine cause of death

OBJECTIVE

To review the literature to examine the use of clozapine levels to (i) guide therapy and prevent toxicity in clinical care and (ii) determine cause of death in post-mortem examination of patients who were treated with clozapine.

METHODS

MEDLINE was searched in December 2010 using the following keywords: 'clozapine levels', 'clozapine and toxicity', 'clozapine and death', 'clozapine and mortality' and 'post-mortem redistribution'. Data was also collected from the 2010 MIMS Annual.

RESULTS

The literature reported significant variation in clozapine levels attained with any given dose, and considerable variability in the clinical response achieved at any given clozapine level. The lowest effective clozapine levels ranged from 250 to 550 µg/L, while the recommended upper limit to prevent toxicity varied from 600 to 2000 µg/L. There was minimal correlation between clozapine levels and side effects, with the exception of sedation, hypotension and seizure activity. The risk of seizures increased with plasma clozapine levels greater than 600 µg/L or rapid upward titration. In addition to prescribed dose, there are many factors that influence plasma clozapine levels. After death, the process of post-mortem drug redistribution resulted in 3.00 to 4.89 times increases in clozapine levels in central blood vessels and 1.5 fold increases in peripheral vessels compared to ante-mortem levels.

CONCLUSIONS

The exact range of clozapine levels that corresponds to toxicity remains unclear. However, levels between 350 µg/L and 1000 µg/L achieved with gradual upward titration are more likely to be effective and less likely to cause toxicity. Ongoing clozapine level monitoring is indicated, especially when (i) prescribing higher doses (> 600 mg/day) of clozapine, (ii) there has been a change in a patient's concomitant pharmacotherapy or cigarette use and (iii) there has been a suboptimal response to treatment. The use of post-mortem clozapine levels to determine clozapine toxicity as a cause of death is unreliable.

The Value of Desmethylclozapine and Serum CRP in Clozapine Toxicity: A Case Report

Clozapine, an atypical antipsychotic, has proved to be superior to other antipsychotics in treating patients with refractory schizophrenia. An increased plasma clozapine level above the therapeutic window may be associated with serious adverse events including paralytic ileus. Clozapine toxicity may occur in association with infection or after drug overdose. In a medical emergency situation, differentiating between a toxic clozapine ingestion and an infection-induced toxicity might be hindered by associated CNS changes and by the clozapine modulation of the inflammatory process. This may delay prompt initiation of a tailored treatment strategy. Here, we report a case of paralytic ileus developed within the context of clozapine toxicity. Although the underlying cause of toxicity was not clinically obvious, giving antimicrobial therapy resulted in an improvement in the patient's clinical condition. This report indicates the value of serum levels of C-reactive protein and desmethylclozapine, major metabolite of clozapine, in the treatment of aetiologically unclear clozapine toxicity.

Role of CYP pharmacogenetics and drug-drug interactions in the efficacy and safety of atypical and other antipsychotic agents

Cytochrome P450 (CYP) drug oxidases play a pivotal role in the elimination of antipsychotic agents, and therefore influence the toxicity and efficacy of these drugs. Factors that affect CYP function and expression have a major impact on treatment outcomes with antipsychotic agents. In particular, aspects of CYP pharmacogenetics, and the processes of CYP induction and inhibition all influence in-vivo rates of drug elimination. Certain CYPs that mediate the oxidation of antipsychotic drugs exhibit genetic variants that may influence in-vivo activity. Thus, single nucleotide polymorphisms (SNPs) in CYP genes have been shown to encode enzymes that have decreased drug oxidation capacity. Additionally, psychopharmacotherapy has the potential for drug-drug inhibitory interactions involving CYPs, as well as drug-mediated CYP induction. Literature evidence supports a role for CYP1A2 in the clearance of the atypical antipsychotics clozapine and olanzapine; CYP1A2 is inducible by certain drugs and environmental chemicals. Recent studies have suggested that specific CYP1A2 variants possessing individual SNPs, and possibly also SNP combinations (haplotypes), in the 5′-regulatory regions may respond differently to inducing chemicals. CYP2D6 is an important catalyst of the oxidation of chlorpromazine, thioridazine, risperidone and haloperidol. Certain CYP2D6 allelic variants that encode enzymes with decreased drug oxidation capacity are more common in particular ethnic groups, which may lead to adverse effects with standard doses of psychoactive drugs. Thus, genotyping may be useful for dose optimization with certain psychoactive drugs that are substrates for CYP2D6. However, genotyping for inducible CYPs is unlikely to be sufficient to direct therapy with all antipsychotic agents. In-vivo CYP phenotyping with cocktails of drug substrates may assist at the commencement of therapy, but this approach could be complicated by pharmacokinetic interactions if applied when an antipsychotic drug regimen is ongoing.

ABCB1 and cytochrome P450 polymorphisms: clinical pharmacogenetics of clozapine

To examine the genetic factors influencing clozapine kinetics in vivo, 75 patients treated with clozapine were genotyped for CYPs and ABCB1 polymorphisms and phenotyped for CYP1A2 and CYP3A activity. CYP1A2 activity and dose-corrected trough steady-state plasma concentrations of clozapine correlated significantly (r = -0.61; P = 1 x 10), with no influence of the CYP1A2*1F genotype (P = 0.38). CYP2C19 poor metabolizers (*2/*2 genotype) had 2.3-fold higher (P = 0.036) clozapine concentrations than the extensive metabolizers (non-*2/*2). In patients comedicated with fluvoxamine, a strong CYP1A2 inhibitor, clozapine and norclozapine concentrations correlate with CYP3A activity (r = 0.44, P = 0.075; r = 0.63, P = 0.007, respectively). Carriers of the ABCB1 3435TT genotype had a 1.6-fold higher clozapine plasma concentrations than noncarriers (P = 0.046). In conclusion, this study has shown for the first time a significant in vivo role of CYP2C19 and the P-gp transporter in the pharmacokinetics of clozapine. CYP1A2 is the main CYP isoform involved in clozapine metabolism, with CYP2C19 contributing moderately, and CYP3A4 contributing only in patients with reduced CYP1A2 activity. In addition, ABCB1, but not CYP2B6, CYP2C9, CYP2D6, CYP3A5, nor CYP3A7 polymorphisms, influence clozapine pharmacokinetics.

The safety of clozapine in the elderly

BACKGROUND

Clozapine was the first atypical 'broad spectrum' antipsychotic drug to be marketed and the first agent approved for the treatment of schizophrenia refractory to other medications. It is also effective for the treatment of aggressive behaviour in schizophrenic and demented patients and in the management of psychosis and aggression in Parkinson's disease and Lewy body dementia.

OBJECTIVE

The aim of this review is to study the safety of clozapine for use in elderly patients.

METHODS

An extensive Medline search was made. Some studies that were referenced in reports from our pharmacovigilance centre and from regulatory agencies such as the FDA, EMEA and WHO were included.

CONCLUSIONS

Clozapine treatment in the elderly requires a careful geriatric assessment. However, its use is strongly limited by the possibility of onset of severe adverse effects such as potentially fatal agranulocytosis, myocarditis and others such as seizures, weight gain and metabolic adverse effects.

Rational prescription of drugs within similar therapeutic or structural class for gastrointestinal disease treatment: Drug metabolism and its related interactions

AIM: To review and summarize drug metabolism and its related interactions in prescribing drugs within the similar therapeutic or structural class for gastrointestinal disease treatment so as to promote rational use of medicines in clinical practice.

METHODS: Relevant literature was identified by performing MEDLINE/Pubmed searches covering the period from 1988 to 2006.

RESULTS: Seven classes of drugs were chosen, including gastric proton pump inhibitors, histamine H₂-receptor antagonists, benzamide-type gastroprokinetic agents, selective 5-HT₃ receptor antagonists, fluoroquinolones, macrolide antibiotics and azole antifungals. They showed significant differences in metabolic profile (i.e., the fraction of drug metabolized by cytochrome P450 (CYP), CYP reaction phenotype, impact of CYP genotype on interindividual pharmacokinetics variability and CYP-mediated drug-drug interaction potential). Many events of severe adverse drug reactions and treatment failures were closely related to the ignorance of the above issues.

CONCLUSION: Clinicians should acquaint themselves with what kind of drug has less interpatient variability in clearance and whether to perform CYP genotyping prior to initiation of therapy. The relevant CYP knowledge helps clinicians to enhance the management of patients with gastrointestinal disease who may require treatment with polytherapeutic regimens.

The induction of CYP1A2, CYP2D6 and CYP3A4 by six trade herbal products in cultured primary human hepatocytes

The aim of this study was to evaluate the in vitro inductive potential of six commonly used trade herbal products on CYP1A2, CYP2D6 and CYP3A4 metabolic activities. Herbal components were extracted from the trade products in a way that ensured a composition equal to that present in the original product. Primary human hepatocytes and specific CYP substrates were used. Classic inducers were used as positive controls and herbal extracts were added in in vivo-relevant concentrations. Metabolites were determined by high performance liquid chromatography (HPLC). St. John's wort and common valerian were the strongest inducing herbs. In addition to induction of CYP3A4 by St. John's wort, common valerian and Ginkgo biloba increased the activity of CYP3A4 and 2D6 and CYP1A2 and 2D6, respectively. A general inhibitory potential was observed for horse chestnut, Echinacea purpurea and common sage. St. John's wort inhibited CYP3A4 metabolism at the highest applied concentration. Horse chestnut might be a herb with high inhibition potentials in vivo and should be explored further at lower concentrations. We show for the first time that G. biloba may exert opposite and biphasic effects on CYP1A2 and CYP2D6 metabolism. Induction of CYP1A2 and inhibition of CYP2D6 were found at low concentrations; the opposite was observed at high concentrations. CYP2D6 activity, regarded generally as non-inducible, was increased by exposure to common valerian (linear to dose) and G. biloba (highest concentration). An allosteric activation is suggested. From the data obtained, G. biloba, common valerian and St. John's wort are suggested as candidates for clinically significant CYP interactions in vivo.

Metabolic drug interactions with newer antipsychotics: a comparative review

Newer antipsychotics introduced in clinical practice in recent years include clozapine, risperidone, olanzapine, quetiapine, sertindole, ziprasidone, aripiprazole and amisulpride. These agents are subject to drug-drug interactions with other psychotropic agents or with medications used in the treatment of concomitant physical illnesses. Most pharmacokinetic interactions with newer antipsychotics occur at the metabolic level and usually involve changes in the activity of the major drug-metabolizing enzymes involved in their biotransformation, i.e. the cytochrome P450 (CYP) monooxygenases and/or uridine diphosphate-glucuronosyltransferases (UGT). Clozapine is metabolized primarily by CYP1A2, with additional contribution by other CYP isoforms. Risperidone is metabolized primarily by CYP2D6 and, to a lesser extent, CYP3A4. Olanzapine undergoes both direct conjugation and CYP1A2-mediated oxidation. Quetiapine is metabolized by CYP3A4, while sertindole and aripiprazole are metabolized by CYP2D6 and CYP3A4. Ziprasidone pathways include aldehyde oxidase-mediated reduction and CYP3A4-mediated oxidation. Amisulpride is primarily excreted in the urine and undergoes relatively little metabolism. While novel antipsychotics are unlikely to interfere with the elimination of other drugs, co-administration of inhibitors or inducers of the major enzymes responsible for their metabolism may modify their plasma concentrations, leading to potentially significant effects. Most documented metabolic interactions involve antidepressant and anti-epileptic drugs. Of a particular clinical significance is the interaction between fluvoxamine, a potent CYP1A2 inhibitor, and clozapine. Differences in the interaction potential among the novel antipsychotics currently available may be predicted based on their metabolic pathways. The clinical relevance of these interactions should be interpreted in relation to the relative width of their therapeutic index. Avoidance of unnecessary polypharmacy, knowledge of the interaction profiles of individual agents, and careful individualization of dosage based on close evaluation of clinical response and, possibly, plasma drug concentrations are essential to prevent and minimize potentially adverse drug interactions in patients receiving newer antipsychotics.

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.