Change from the CYP2D6 extensive metabolizer to the poor metabolizer phenotype during treatment With bupropion

Optimizing doxepin therapy in dermatology: introducing blood level monitoring and genotype testing

Doxepin, a tricyclic antidepressant, is the most efficacious antipruritic available to dermatologists; however its use is often suboptimal because of significant interindividual variability in doxepin plasma levels and clinical response between patients taking the same dose. As result, the Food and Drug Administration approves a maximum dose of 300 mg of doxepin per day and a 10 mg per cc liquid doxepin concentrate. These allow patients to significantly increase or decrease their dose, due to either a lack of clinical efficacy or side effects at typical dermatologic doses (often 10-25 mg per day). This review initially discusses the unique advantages of doxepin in dermatology. Then, it explores internal and external reasons why doxepin plasma levels and clinical response vary so significantly between patients, including genetic polymorphisms, drug interactions, comorbidities, sex, and ethnicity. Blood level monitoring is introduced, a tool dermatologists can use to optimize doxepin dosing in patients responding subtherapeutically to typical dermatologic doses. Without blood level monitoring, patients initially unresponsive to treatment could be labeled treatment failures when in fact they may be cases of inadequate dosing. Blood level monitoring allows for safe dose adjustments in these individuals to maximize patients' chances of achieving therapeutic success with this agent.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Pharmacokinetic Drug Interactions with Tobacco, Cannabinoids and Smoking Cessation Products

Tobacco smoke contains a large number of compounds in the form of metals, volatile gases and insoluble particles, as well as nicotine, a highly addictive alkaloid. Marijuana is the most widely used illicit drug of abuse in the world, with a significant increase in the USA due to the increasing number of states that allow medical and recreational use. Of the over 70 phytocannabinoids in marijuana, Δ9-tetrahydrocannabinol (Δ9THC), cannabidiol (CBD) and cannibinol are the three main constituents. Both marijuana and tobacco smoking induce cytochrome P450 (CYP) 1A2 through activation of the aromatic hydrocarbon receptor, and the induction effect between the two products is additive. Smoking cessation is associated with rapid downregulation of CYP1A enzymes. On the basis of the estimated half-life of CYP1A2, dose reduction of CYP1A drugs may be necessary as early as the first few days after smoking cessation to prevent toxicity, especially for drugs with a narrow therapeutic index. Nicotine is a substrate of CYP2A6, which is induced by oestrogen, resulting in lower concentrations of nicotine in females than in males, especially in females taking oral contraceptives. The significant effects of CYP3A4 inducers and inhibitors on the pharmacokinetics of Δ9THC/CBD oromucosal spray suggest that CYP3A4 is the primary enzyme responsible for the metabolism of Δ9THC and CBD. Limited data also suggest that CBD may significantly inhibit CYP2C19. With the increasing use of marijuana and cannabis products, clinical studies are needed in order to determine the effects of other drugs on pharmacokinetics and pharmacodynamics.

Addressing phenoconversion: the Achilles' heel of personalized medicine

Phenoconversion is a phenomenon that converts genotypic extensive metabolizers (EMs) into phenotypic poor metabolizers (PMs) of drugs, thereby modifying their clinical response to that of genotypic PMs. Phenoconversion, usually resulting from nongenetic extrinsic factors, has a significant impact on the analysis and interpretation of genotype-focused clinical outcome association studies and personalizing therapy in routine clinical practice. The high phenotypic variability or genotype-phenotype mismatch, frequently observed due to phenoconversion within the genotypic EM population, means that the real number of phenotypic PM subjects may be greater than predicted from their genotype alone, because many genotypic EMs would be phenotypically PMs. If the phenoconverted population with genotype-phenotype mismatch, most extensively studied for CYP2D6, is as large as the evidence suggests, there is a real risk that genotype-focused association studies, typically correlating only the genotype with clinical outcomes, may miss clinically strong pharmacogenetic associations, thus compromising any potential for advancing the prospects of personalized medicine. This review focuses primarily on co-medication-induced phenoconversion and discusses potential approaches to rectify some of the current shortcomings. It advocates routine phenotyping of subjects in genotype-focused association studies and proposes a new nomenclature to categorize study populations. Even with strong and reliable data associating patients' genotypes with clinical outcome(s), there are problems clinically in applying this knowledge into routine pharmacotherapy because of potential genotype-phenotype mismatch. Drug-induced phenoconversion during routine clinical practice remains a major public health issue. Therefore, the principal challenges facing personalized medicine, which need to be addressed, include identification of the following factors: (i) drugs that are susceptible to phenoconversion; (ii) co-medications that can cause phenoconversion; and (iii) dosage amendments that need to be applied during and following phenoconversion.

Bupropion: pharmacology and therapeutic applications

A total of 17 years after its introduction, bupropion remains a safe and effective antidepressant, suitable for first-line use. Bupropion undergoes metabolic transformation to an active metabolite, 4-hydroxybupropion, through hepatic cytochrome P450-2B6 (CYP2B6) and has inhibitory effects on cytochrome P450-2D6 (CYP2D6), thus raising concern for clinically-relevant drug interactions. Common side effects are nervousness and insomnia. Nausea appears slightly less common than with the SSRI drugs and sexual dysfunction is probably the least of any antidepressant. Bupropion is relatively safe in overdose with seizures being the predominant concern. The mechanism of action of bupropion is still uncertain but may be related to inhibition of presynaptic dopamine and norepinephrine reuptake transporters. The activity of vesicular monoamine transporter-2, the transporter pumping dopamine, norepinephrine and serotonin from the cytosol into presynaptic vesicles, is increased by bupropion and may be a component of its mechanism of action. Bupropion is approved for use in major depression and seasonal affective disorder and has demonstrated comparable efficacy to other antidepressants in clinical trials. Bupropion is also useful in augmenting a partial response to selective serotonin reuptake inhibitor antidepressants, although bupropion should not be combined with monoamine oxidase inhibitors. It may be less likely to provoke mania than antidepressants with prominent serotonergic effects. Bupropion is effective in helping people quit tobacco smoking. Anecdotal reports indicate bupropion may lower inflammatory mediators such as tumor necrosis factor-alpha, may lower fatigue in cancer and may help reduce concentration problems.

[Interactions between metoprolol and antidepressants]

BACKGROUND

Metoprolol, the most commonly used beta-receptor antagonist in Norway, is eliminated mainly via the enzyme cytochrome P450 (CYP) 2D6. This enzyme is inhibited to a varying extent by antidepressants. The aim of this article is to provide an overview of the interactions between metoprolol and antidepressants with an emphasis on CYP2D6 inhibition.

MATERIAL AND METHODS

Relevant literature was identified by a PubMed search using the word "metoprolol" combined with generic names of antidepressant drugs.

RESULTS

The potent CYP2D6 inhibitor paroxetine has been shown to increase the biologically available dose of metoprolol about 4- to 6-fold. The same degree of increase is expected for the two other potent CYP2D6 inhibitors in the class, fluoxetine and bupropion. Severe bradycardia and atroventricular block has been reported in patients who have taken metoprolol in combination with these three drugs. Escitalopram, citalopram and duloxetine are less potent CYP2D6 inhibitors, and have been shown to cause 2- to 3-fold increases in biologically available dose of metoprolol. Other antidepressants, such as sertraline, venlafaxine, mianserin and mirtazapine, inhibit CYP2D6 to little or no extent, and are not expected to cause clinically relevant interactions with metoprolol.

CONCLUSION

Metoprolol should not be used concomitantly with paroxetine, fluoxetine or bupropion due to extensive interactions and the risk of serious adverse effects. Dose reductions of metoprolol should be considered for combined treatment with citalopram, escitalopram or duloxetine, while concurrent use with sertraline, venlafaxine, mianserin and mirtazapine should be safe.

High frequency of occurrence of CYP2D6 gene duplication/multiduplication indicating ultrarapid metabolism among suicide cases

In Sweden, about 550 individuals die every year of drug intoxication. Many of these drugs are metabolized by CYP enzymes such as CYP2D6 and CYP2C19. A lack of these enzymes, resulting in poor metabolism, can lead to adverse reactions and even to fatality. On the other hand, an ultrarapid metabolism can lead to insufficient drug plasma concentration, resulting in failure of treatment, or it can lead to high concentrations of active/toxic metabolites. The aim of this project was to study the genetic profile of individuals with regard to the presence of CYP2D6 and CYP2C19 genes, in cases of fatal intoxication (242), suicide (intoxications excluded) (262), and natural death (212). PCR, followed by pyrosequencing, was used for all the analyses. We found that, among those who died of suicide (suicide cases), there was a higher number carrying more than two active CYP2D6 genes (corresponding to the phenotype of ultrarapid metabolizer) as compared with those who died of natural causes (natural-death cases) (P = 0.007).

An in vitro mechanistic study to elucidate the desipramine/bupropion clinical drug-drug interaction

There are documented clinical drug-drug interactions between bupropion and the CYP2D6-metabolized drug desipramine resulting in marked (5-fold) increases in desipramine exposure. This finding was unexpected as CYP2D6 does not play a significant role in bupropion clearance, and bupropion and its major active metabolite, hydroxybupropion, are not strong CYP2D6 inhibitors in vitro. The aims of this study were to investigate whether bupropion's reductive metabolites, threohydrobupropion and erythrohydrobupropion, contribute to the drug interaction with desipramine. In human liver microsomes using the CYP2D6 probe substrate bufuralol, erythrohydrobupropion and threohydrobupropion were more potent inhibitors of CYP2D6 activity (K(i) = 1.7 and 5.4 microM, respectively) than hydroxybupropion (K(i) = 13 microM) or bupropion (K(i) = 21 microM). Furthermore, neither bupropion nor its metabolites were metabolism-dependent CYP2D6 inhibitors. Using the in vitro kinetic constants and estimated liver concentrations of bupropion and its metabolites, modeling was able to predict within 2-fold the increase in desipramine exposure observed when coadministered with bupropion. This work indicates that the reductive metabolites of bupropion are potent competitive CYP2D6 inhibitors in vivo and provides a mechanistic explanation for the clinical drug-drug interaction between bupropion and desipramine.

Review of the pharmacology and clinical profile of bupropion, an antidepressant and tobacco use cessation agent

Bupropion hydrochloride ((+/-)-2-tert-butylamino)-3'-chloropropiophenone x HCl) is a nonselective inhibitor of the dopamine transporter (DAT) and the norepinephrine transporter (NET) and is also an antagonist at neuronal nicotinic acetylcholine receptors (nAChRs). In animal models used commonly to screen for antidepressant activity, bupropion shows a positive response. Also using animal models, bupropion has been shown to attenuate nicotine-induced unconditioned behaviors, to share or enhance discriminative stimulus properties of nicotine and to have a complex effect on nicotine self-administration, i.e., low doses augmenting nicotine self-administration and high doses attenuating self-administration. Current studies show that bupropion facilitates the acquisition of nicotine conditioned place preference in rats, further suggesting that bupropion enhances the rewarding properties of nicotine. Bupropion has been shown to attenuate the expression of nicotine withdrawal symptoms in both animal models and human subjects. With respect to relapse, current studies show that bupropion attenuates nicotine-induced reinstatement in rats, but large individual differences are apparent. Clinically, bupropion is used as a treatment for two indications, as an antidepressant, the indication for which it was developed, and as a tobacco use cessation agent. In clinical trials, bupropion is being tested as a candidate treatment for psychostimulant drug abuse, attention-deficit hyperactivity disorder (ADHD) and obesity. Bupropion is available in three bioequivalent oral formulations, immediate release (IR), sustained release (SR), and extended release (XL). Extensive hepatic metabolism of bupropion produces three pharmacologically active metabolites, which may contribute to its clinical profile.

Antidepressant-drug interactions are potentially but rarely clinically significant

The salient pharmacologic features of the selective serotonin reuptake inhibitors (SSRIs) discovered in the late 1980s included an in vitro ability to inhibit various cytochrome P450 enzymes (CYPs). Differences in potency among the SSRIs for CYP inhibition formed the basis of a marketing focus based largely on predictions of in vivo pharmacokinetic drug interactions from in vitro data, conclusions derived from case reports, and the extrapolation of the results of pharmacokinetic studies conducted in healthy volunteers to patients. Subsequently introduced antidepressants have undergone a similar post hoc scrutiny for potential drug-drug interactions. Concern for the untoward consequences of drug interactions led the FDA to publish guidance for the pharmaceutical industry in 1997 recommending that in vitro metabolic studies be conducted early in the drug development process to evaluate inhibitory properties toward the major CYPs. However, the prevalence of clinically significant enzyme inhibition interactions occurring during antidepressant treatment remains poorly defined despite millions of exposures. Although lack of evidence does not equate to evidence of absence, sparse epidemiological and post-marketing surveillance data do not substantiate a conclusion that widespread morbidity results from antidepressant-induced drug interactions. This commentary discusses points of uncertainty and controversy in the field of drug interactions, notes areas where inadequate data exist, and suggests explanations for a low prevalence of serious interactions. The conclusion is drawn that drug interactions from CYP inhibition caused by the newer antidepressants are potentially, but rarely, clinically significant.

Pharmacogenetics in drug development

Over the last two decades, identification of polymorphisms that influence human diseases has begun to have an impact on the provision of medical care. The promise of genetics lies in its ability to provide insights into an individual's susceptibility to disease, the likely nature of the disease and the most appropriate therapy. For much of its history, pharmacogenetics (PGx-the use of genetic information to impact drug choice) has been limited to comparatively simple phenotypes such as plasma drug levels. Progress in genetics technologies has broadened the scope of PGx efficacy and safety studies that can be implemented, impacting on a broad spectrum of drug discovery and development activities. Recent PGx data show the ability of this approach to generate information that can be applied to dose selection, efficacy determination and safety issues. This in turn will lead to significant opportunities to affect both the approach to clinical development and the probability of success--the latter being an important aspect for pharmaceutical companies and for the patients who will benefit from these new medicines.

Population pharmacokinetic analysis of drug-drug interactions among risperidone, bupropion, and sertraline in CF1 mice

RATIONALE

Accumulating evidence indicates that modulation of the activity of cytochrome P450 (CYP) enzymes and the multidrug resistance transporter P-glycoprotein (P-gp) is responsible for many drug-drug interactions.

OBJECTIVES

The potential interaction of risperidone (RISP), which is metabolized by 2D6 and transported across the blood brain barrier (BBB) by P-gp, was studied in combination with bupropion (BUP) and also with sertraline (SERT).

METHODS

BUP, SERT, and RISP were administered intraperitoneally into CF1 mice at doses of 100, 10, and 1 microg/g mouse, respectively. Plasma and brain samples were collected at timed intervals from 0.5 to 6 h. A pharmacokinetic analysis was performed using both traditional compartmental modeling and a population pharmacokinetic approach.

RESULTS

BUP increased the RISP plasma (5.9-fold, P<0.01) and brain (2.2-fold, P<0.01) area under the drug concentration vs time curve (AUC), but did not alter the brain-to-plasma concentration ratio. SERT did not significantly change the plasma AUC of RISP and 9-hydroxy-RISP, but increased the brain AUC of RISP and 9-hydroxy-RISP 1.5-fold (P<0.05) and 5-fold (P<0.01), respectively. RISP did not produce significant alterations of plasma or brain concentrations of BUP. It increased the plasma AUC and elimination half-life (T1/2e) of desmethyl-SERT 12.5-fold (P<0.01) and 107-fold (P<0.01), respectively.

CONCLUSIONS

These results suggest that pharmacokinetic interactions exist among these three psychoactive drugs involving inhibition of drug metabolizing enzymes and/or P-gp and other drug transporters present in the BBB. The mechanisms and consequences of these interactions require further study in humans to establish clinical relevance.

The impact of Cytochrome P450-2D6 genotype on the use and interpretation of therapeutic drug monitoring in long-stay patients treated with antidepressant and antipsychotic drugs in daily psychiatric practice

PURPOSE

This retrospective follow-up study investigates whether cytochrome P450-2D6 (CYP2D6) genotype explains variability in plasma concentrations of psychotropic drugs in daily psychiatric practice.

METHODS

The study population consisted of 62 hospitalised psychiatric patients genotyped for CYP2D6. Primary endpoint was the normalised plasma concentration ratio which was defined as the [measured concentration]/[mean therapeutic concentration] allowing comparison of plasma concentrations of different substrates. Secondary endpoint was a plasma concentration above the therapeutic range. The determinant was CYP2D6 genotype classified as ultrarapid metaboliser (UM), extensive metaboliser (EM), intermediate metaboliser (IM), or poor metaboliser (PM). The relation between CYP2D6 genotype and the normalised plasma concentration ratio was assessed with a linear mixed-effects model after adjustment for the Prescribed Daily Dose (PDD). The risk of having a plasma concentration above the therapeutic range was assessed with a logistic mixed-effects model.

RESULTS

For antidepressants, CYP2D6 genotype PM (1.68 (95%CI: 1.01-2.28)) and IM (1.09 (95%CI: 0.77-1.29)) were associated with higher normalised plasma concentration ratios of antidepressants compared to EMs (0.56 (95%CI: 0.26-0.74)). In addition, the risk of a plasma concentration above the therapeutic range was increased for PMs (OR 33.1 (95%CI: 2.0-544.6)) and IMs (OR 8.2 (95%CI: 1.1-60.3)) relative to EMs using antidepressants. CYP2D6 genotype could not clearly explain variability in plasma concentrations of antipsychotics possibly due to a low frequency of therapeutic drug monitoring (TDM) in antipsychotics primarily metabolised by CYP2D6 in daily psychiatric practice.

CONCLUSIONS

CYP2D6 genotype contributes to clinically relevant variability in plasma concentrations of antidepressants but probably not antipsychotics in daily clinical practice.

Safety of intravenous methamphetamine administration during treatment with bupropion

RATIONALE

Methamphetamine dependence is a growing problem for which no medication treatments have proven effective.

OBJECTIVES

We evaluated bupropion, an antidepressant with beneficial effects for the treatment of nicotine dependence, in patients with methamphetamine dependence, to assess the safety and tolerability of methamphetamine administration during bupropion treatment.

METHODS

Twenty-six participants entered the study and 20 completed the protocol. Participants received intravenous methamphetamine (0, 15, and 30 mg) before and after randomization to twice-daily bupropion (150 mg SR) or matched placebo. Dependent measures included cardiovascular effects of methamphetamine, methamphetamine and amphetamine pharmacokinetics, and peak and trough plasma concentrations of bupropion and its metabolites.

RESULTS

Bupropion treatment was well tolerated, with bupropion- and placebo-treated groups reporting similar rates of adverse events. Methamphetamine administration was associated with expected stimulant cardiovascular effects, and these were not accentuated by bupropion treatment. Instead, there was a trend for bupropion to reduce methamphetamine-associated increases in blood pressure and a statistically significant reduction in methamphetamine-associated increases in heart rate. Pharmacokinetic analysis revealed that bupropion treatment reduced the plasma clearance of methamphetamine and also reduced the appearance of amphetamine in the plasma. Methamphetamine administration did not alter the peak and trough plasma concentrations of bupropion or its metabolites.

CONCLUSIONS

Methamphetamine administration was well tolerated during bupropion treatment. There was no evidence of additive cardiovascular effects when the drugs were coadministered. This study provides initial evidence for the safety of prescribing bupropion for the treatment of methamphetamine abuse and dependence. The impact of bupropion treatment in patients who abuse larger doses of methamphetamine remains undetermined.

Conversion from sustained-release to immediate-release bupropion: patient tolerability and economic impact

STUDY OBJECTIVE

To assess patient tolerability and impact on institutional drug acquisition costs of converting patients from sustained-release bupropion to generic immediate-release bupropion.

DESIGN

Retrospective medical record review.

SETTING

Veterans Affairs medical center.

PATIENTS

One hundred three outpatients who had been receiving sustained-release bupropion and were converted to immediate-release bupropion at equal individual doses and frequencies.

MEASUREMENTS AND MAIN RESULTS

Medical records were reviewed for documentation of adverse effects thought to be associated with bupropion treatment; reports of seizure occurrence were specifically sought. Patterns of bupropion dosing were also assessed. Institutional drug acquisition costs were evaluated by comparing actual costs of bupropion for each patient before and after conversion. Adverse effects reported with immediate-release bupropion were those commonly associated with this drug (headache, agitation, chest pain, and gastrointestinal complaints) and were neither unusually frequent nor severe. No seizures were reported after the drug conversion. Mean daily doses of bupropion were not significantly different after conversion. Mean single doses of immediate-release bupropion were below 150 mg; however, six patients did receive single doses of 200 mg. After conversion, the annual institutional drug acquisition cost for bupropion decreased by approximately $48,910.

CONCLUSION

Conversion from sustained-release bupropion to immediate-release bupropion appears to be safe. Single 200-mg doses of immediate-release bupropion can apparently be administered to some patients without inducing excessive adverse effects, including seizures. Marked reduction in drug acquisition costs can be achieved with this conversion.

A double-blind placebo-controlled trial of bupropion sustained-release for smoking cessation in schizophrenia

The objective of this study was to examine the efficacy of bupropion for smoking cessation in patients with schizophrenia. Adults with schizophrenia who smoked more than 10 cigarettes per day and wished to try to quit smoking were recruited from community mental health centers, enrolled in a 12-week group cognitive behavioral therapy intervention, and randomly assigned to receive either bupropion sustained-release 300 mg/d or identical placebo. Fifty-three adults, 25 on bupropion and 28 on placebo, were randomized, completed at least 1 postbaseline assessment and were included in the analysis. The primary outcome measures were 7-day point prevalence abstinence in the week after the quit date (week 4) and at the end of the intervention (week 12). Subjects in the bupropion group were significantly more likely to be abstinent for the week after the quit date (36% [9/25] vs. 7% [2/28], P = 0.016) and at end of the intervention (16% [4/25] vs. 0%, P = 0.043). Subjects in the bupropion group also had a higher rate of 4-week continuous abstinence (weeks 8-12) (16% [4/25] vs. 0%, P = 0.043) and a longer duration of abstinence (4.2 [3.2] weeks vs. 1.8 [0.96] weeks, t = 2.30, P = 0.037). The effect of bupropion did not persist after discontinuation of treatment. Subjects in the bupropion group had no worsening of clinical symptoms and had a trend toward improvement in depressive and negative symptoms. We conclude that bupropion does not worsen clinical symptoms of schizophrenia and is modestly effective for smoking cessation in patients with schizophrenia. The relapse rate is high after treatment discontinuation.

Inhibition of CYP2D6 activity by bupropion

The purpose of this study was to assess the effect of bupropion on cytochrome P450 2D6 (CYP2D6) activity. Twenty-one subjects completed this repeated-measures study in which dextromethorphan (30-mg oral dose) was administered to smokers at baseline and after 17 days of treatment with either bupropion sustained-release (150 mg twice daily) or matching placebo. Subjects quit smoking 3 days before the second dextromethorphan administration. To assess CYP2D6 activity, urinary dextromethorphan/dextrorphan metabolic ratios were calculated after an 8-hour urine collection. Thirteen subjects received bupropion, and 8 received placebo. In those receiving active medication, the dextromethorphan/dextrorphan ratio increased significantly at the second assessment relative to the first (0.012 +/- 0.012 vs. 0.418 +/- 0.302; P < 0.0004). No such change was observed in those randomized to placebo (0.009 +/- 0.010 vs. 0.017 +/- 0.015; P = NS). At baseline, all subjects were phenotypically extensive CYP2D6 metabolizers (metabolic ratio <0.3); after treatment, 6 of 13 subjects receiving bupropion, but none of those receiving placebo, had metabolic ratios consistent with poor CYP2D6 metabolizers. Bupropion is therefore a potent inhibitor of CYP2D6 activity, and care should be exercised when initiating or discontinuing bupropion use in patients taking drugs metabolized by CYP2D6.

Co-prescription of cytochrome P450 2D6/3A4 inhibitor-substrate pairs in clinical practice. A retrospective analysis of data from Norwegian primary pharmacies

OBJECTIVE

Inhibition of cytochrome P (P450) (CYP) enzymes, in particular CYP3A4 and CYP2D6, is an important drug-interacting mechanism. The objective of our study was to assess how frequently CYP3A4 and CYP2D6 inhibitors are co-prescribed with substrates of the respective enzymes.

METHODS

Included inhibitors were clarithromycin, erythromycin, fluconazole, itraconazole, ketoconazole and nefazodone (CYP3A4 inhibitors) and bupropion, fluoxetine, paroxetine and terbinafine (CYP2D6 inhibitors). The inhibitors were combined with substrates shown to be pharmacokinetically sensitive towards inhibition (190 drug pairs in total). Lists of patients receiving inhibitors and substrates were drawn from prescription databases (approximately 43,500 patients) of three Norwegian primary pharmacies during a 6-month period (July 2002 to January 2003). The lists were matched on name and date of birth to identify patients using drug pairs. Concurrent use was made probable from dates of purchase and drug profiles.

RESULTS

Inhibitors were prescribed to 2,062 patients. Altogether, 369 events of substrate co-prescription were registered. The highest frequencies of co-prescribed substrates were found for paroxetine (101 events per 267 patients, 38%), fluoxetine (36 events per 110 patients, 33%) and clarithromycin (59 events per 242 patients, 24%). The drugs most often detected in combination with inhibitors were codeine (116 events) and metoprolol (38 events) for CYP2D6 and zopiclone (45 events) and simvastatin (26 events) for CYP3A4.

CONCLUSION

Several commonly used CYP2D6 and CYP3A4 inhibitors are frequently co-prescribed with substrates in Norwegian clinical practice. Alertness when inhibitors are prescribed would aid physicians and pharmacists to detect many drug combinations with potential interaction risk.

Serotonin Syndrome Induced by a Combination of Bupropion and SSRIs

Serotonin syndrome (SS) is a potentially fatal complication of the combined use of agents that enhance serotonin activity. Bupropion inhibits noradrenaline and dopamine reuptake with milder effects on serotonergic activity. Although regarded as a potential causative agent for SS, no cases have been reported in the medical literature. A 62-year-old woman treated with therapeutic dosages of bupropion and sertraline for depression for the previous 3 weeks presented with upper extremity myoclonic jerks, clumsiness, and gait difficulties with fluctuating symptoms of confusion, forgetfulness, and the alternation of agitation and lethargy. Symptoms were interpreted as an aggravation of depression and venlafaxine was added. The clinical picture progressed to alteration of consciousness and dysautonomia. After admission, medications were discontinued and she was started on cyproheptadine and clonazepam with gradual improvement and complete resolution of symptoms. This is a rare report of SS related to the association of bupropion and selective serotonin reuptake inhibitors (SSRIs). It also illustrates the potential for misinterpretation of the earliest manifestations of SS as signs of aggravation of the patient's underlying condition. The role of bupropion in SS is possibly related to its well-established specific inhibition of the cytochrome P450 2D6 pathway, increasing blood levels of SSRIs and tricyclic antidepressants.