CYP2D6 status of extensive metabolizers after multiple-dose fluoxetine, fluvoxamine, paroxetine, or sertraline

Dutch Pharmacogenetics Working Group (DPWG) guideline for the gene-drug interaction between CYP2C19 and CYP2D6 and SSRIs

The Dutch Pharmacogenetics Working Group (DPWG) guideline presented here, presents the gene-drug interaction between the genes CYP2C19 and CYP2D6 and antidepressants of the selective serotonin reuptake inhibitor type (SSRIs). Both genes' genotypes are translated into predicted normal metabolizer (NM), intermediate metabolizer (IM), poor metabolizer (PM), or ultra-rapid metabolizer (UM). Evidence-based dose recommendations were obtained, based on a structured analysis of published literature. In CYP2C19 PM patients, escitalopram dose should not exceed 50% of the normal maximum dose. In CYP2C19 IM patients, this is 75% of the normal maximum dose. Escitalopram should be avoided in UM patients. In CYP2C19 PM patients, citalopram dose should not exceed 50% of the normal maximum dose. In CYP2C19 IM patients, this is 70% (65-75%) of the normal maximum dose. In contrast to escitalopram, no action is needed for CYP2C19 UM patients. In CYP2C19 PM patients, sertraline dose should not exceed 37.5% of the normal maximum dose. No action is needed for CYP2C19 IM and UM patients. In CYP2D6 UM patients, paroxetine should be avoided. No action is needed for CYP2D6 PM and IM patients. In addition, no action is needed for the other gene-drug combinations. Clinical effects (increase in adverse events or decrease in efficacy) were lacking for these other gene-drug combinations. DPWG classifies CYP2C19 genotyping before the start of escitalopram, citalopram, and sertraline, and CYP2D6 genotyping before the start of paroxetine as "potentially beneficial" for toxicity/effectivity predictions. This indicates that genotyping prior to treatment can be considered on an individual patient basis.

The impact of CYP2D6 mediated drug-drug interaction: a systematic review on a combination of metoprolol and paroxetine/fluoxetine

AIM

Metoprolol (a CYP2D6 substrate) is often co-prescribed with paroxetine/fluoxetine (a CYP2D6 inhibitor) because the clinical relevance of this drug-drug interaction (DDI) is still unclear. This review aimed to systematically evaluate the available evidence and quantify the clinical impact of the DDI.

METHOD

Pubmed, Web of Science, Cochrane Library and Embase were searched for studies reporting on the effect of the DDI among adults published until April 2018. Data on pharmacokinetics, pharmacodynamics and clinical outcomes from experimental, observational and case report studies were retrieved. The protocol of this study was registered in PROSPERO (CRD42018093087).

RESULTS

We found nine eligible articles that consisted of four experimental and two observational studies as well as three case reports. Experimental studies reported that paroxetine increased the AUC of metoprolol three to five times, and significantly decreased systolic blood pressure and heart rate of patients. Case reports concerned bradycardia and atrioventricular block due to the DDI. Results from observational studies were conflicting. A cohort study indicated that the DDI was significantly associated with the incidence of early discontinuation of metoprolol as an indicator of the emergence of metoprolol-related side effects. In a case-control study, the DDI was not significantly associated with bradycardia.

CONCLUSION

Despite the contradictory conclusions from the current literature, the majority of studies suggest that the DDI can lead to adverse clinical consequences. Since alternative antidepressants and beta-blockers with comparable efficacy are available, such DDIs can be avoided. Nonetheless, if prescribing the combination is unavoidable, a dose adjustment or close monitoring of the metoprolol-related side effects is necessary.

Discontinuation and dose adjustment of metoprolol after metoprolol-paroxetine/fluoxetine co-prescription in Dutch elderly

Purpose

Co‐prescription of paroxetine/fluoxetine (a strong CYP2D6 inhibitor) in metoprolol (a CYP2D6 substrate) users is common, but data on the clinical consequences of this drug‐drug interaction are limited and inconclusive. Therefore, we assessed the effect of paroxetine/fluoxetine initiation on the existing treatment with metoprolol on the discontinuation and dose adjustment of metoprolol among elderly.

Methods

We performed a cohort study using the University of Groningen IADB.nl prescription database (www.IADB.nl). We selected all elderly (≥60 years) who had ever been prescribed metoprolol and had a first co‐prescription of paroxetine/fluoxetine, citalopram (weak CYP2D6 inhibitor), or mirtazapine (negative control) from 1994 to 2015. The exposure group was metoprolol and paroxetine/fluoxetine co‐prescription, and the other groups acted as controls. The outcomes were early discontinuation and dose adjustment of metoprolol. Logistic regression was applied to estimate adjusted odds ratios (OR) and 95% confidence intervals (CI).

Results

Combinations of metoprolol‐paroxetine/fluoxetine, metoprolol‐citalopram, and metoprolol‐mirtazapine were started in 528, 673, and 625 patients, respectively. Compared with metoprolol‐citalopram, metoprolol‐paroxetine/fluoxetine was not significantly associated with the early discontinuation and dose adjustment of metoprolol (OR = 1.07, 95% CI:0.77‐1.48; OR = 0.87, 95% CI:0.57‐1.33, respectively). In comparison with metoprolol‐mirtazapine, metoprolol‐paroxetine/fluoxetine was associated with a significant 43% relative increase in early discontinuation of metoprolol (OR = 1.43, 95% CI:1.01‐2.02) but no difference in the risk of dose adjustment. Stratified analysis by gender showed that women have a significantly high risk of metoprolol early discontinuation (OR = 1.62, 95% CI:1.03‐2.53).

Conclusion

Paroxetine/fluoxetine initiation in metoprolol prescriptions, especially for female older patients, is associated with the risk of early discontinuation of metoprolol.

The burden and management of cytochrome P450 2D6 (CYP2D6)-mediated drug-drug interaction (DDI): co-medication of metoprolol and paroxetine or fluoxetine in the elderly

PURPOSE

Metoprolol and paroxetine/fluoxetine are inevitably co-prescribed because cardiovascular disorders and depression often coexist in the elderly. This leads to CYP2D6-mediated drug-drug interactions (DDI). Because systematic evaluations are lacking, we assessed the burden of metoprolol-paroxetine/fluoxetine interaction in the elderly and how these interactions are managed in Dutch community pharmacies.

METHOD

Dispensing data were collected from the University of Groningen pharmacy database (IADB.nl, 1999-2014) for elderly patients (≥60 years) starting beta-blockers and/or antidepressants. Based on the two main DDI alert systems (G-Standard and Pharmabase), incidences were divided between signalled (metoprolol-fluoxetine/paroxetine) and not-signalled (metoprolol-alternative antidepressants and alternative beta-blockers-paroxetine/fluoxetine) combinations. Incident users were defined as patients starting at least one signalled or a non-signalled combination. G-Standard signalled throughout the study period, whereas Pharmabase stopped after 2005.

RESULTS

A total of 1763 patients had 2039 metoprolol-paroxetine/fluoxetine co-prescriptions, despite DDI alert systems, and about 57.3% were signalled. The number of metoprolol-alternative antidepressant combinations (incidences = 3150) was higher than alternative beta-blocker-paroxetine/fluoxetine combinations (incidences = 1872). Metoprolol users are more likely to be co-medicated with an alternative antidepressant (incidences = 2320) than paroxetine/fluoxetine users (incidences = 1232) are. The number of paroxetine/fluoxetine users co-prescribed with alternative beta-blockers was comparable to those co-medicated with metoprolol (about 50%). Less than 5% of patients received a substitute therapy after using metoprolol-paroxetine/fluoxetine. Most of the metoprolol users (90%) received a low dose (mean DDD = 0.47) regardless whether they were prescribed paroxetine/fluoxetine.

CONCLUSION

Despite the signalling software, metoprolol-paroxetine/fluoxetine combinations are still observed in the elderly population. The clinical impact of these interactions needs further investigation. Copyright © 2017 John Wiley & Sons, Ltd.

Ultrarapid clozapine metabolism and CYP2D6 gene duplication in a patient with schizophrenia

An optimal clozapine serum level is required for both optimization of clinical response and minimization of troublesome or some of the life-threatening side effects. Serum levels can be influenced by comedication that can cause phenoconversion. When norclozapine/clozapine serum levels and ratios are consistently and significantly lower/higher than expected and there are no concomitant drugs that could account for these findings, further investigation of the genetic variants of CYP1A2, 2D6 and 3A4 are warranted.

CYP450 genotype and pharmacogenetic association studies: a critical appraisal

Despite strong pharmacological support, association studies using genotype-predicted phenotype as a variable have yielded conflicting or inconclusive evidence to promote personalized pharmacotherapy. Unless the patient is a genotypic poor metabolizer, imputation of patient's metabolic capacity (or metabolic phenotype), a major factor in drug exposure-related clinical response, is a complex and highly challenging task because of limited number of alleles interrogated, population-specific differences in allele frequencies, allele-specific substrate-selectivity and importantly, phenoconversion mediated by co-medications and inflammatory co-morbidities that modulate the functional activity of drug metabolizing enzymes. Furthermore, metabolic phenotype and clinical outcomes are not binary functions; there is large intragenotypic and intraindividual variability. Therefore, the ability of association studies to identify relationships between genotype and clinical outcomes can be greatly enhanced by determining phenotype measures of study participants and/or by therapeutic drug monitoring to correlate drug concentrations with genotype and actual metabolic phenotype. To facilitate improved analysis and reporting of association studies, we propose acronyms with the prefixes ‘g’ (genotype-predicted phenotype) and ‘m’ (measured metabolic phenotype) to better describe this important variable of the study subjects. Inclusion of actually measured metabolic phenotype, and when appropriate therapeutic drug monitoring, promises to reveal relationships that may not be detected by using genotype alone as the variable.

Inhibition of Efavirenz Metabolism by Sertraline and Nortriptyline and Their Effect on Efavirenz Plasma Concentrations

Between 22 and 45% of HIV-positive subjects are likely to report symptoms of depression. Considering this background, a potential pharmacokinetic interaction between the nonnucleoside reverse transcriptase inhibitor efavirenz (EFV) and two antidepressants, sertraline (SRT) and nortriptyline (NT), was studied. Rats were administered EFV alone or together with the antidepressants, and changes in the plasma levels and pharmacokinetic parameters of EFV were analyzed. Additional in vitro experiments with rat and human hepatic microsomes were carried out to evaluate the inhibitory effect of SRT and NT on EFV metabolism by determining the formation rate of the major EFV metabolite (8-OH-EFV). In vivo studies showed similar increases in the plasma levels of EFV when it was coadministered with SRT or NT. However, the studies using rat hepatic microsomes showed a more potent inhibitory effect of NT than of SRT on the metabolism of EFV, with values for the 50% inhibition constant (IC50) and inhibitory constant (Ki) for NT about 9-fold lower than those for SRT. An equation was deduced that explains the similar in vivo effects of SRT and NT in spite of the different in vitro performance data. Using human hepatic microsomes, the strongest inhibitory effect was observed with SRT. In summary, pharmacokinetic interactions between EFV, SRT, and NT, associated with the inhibition of hepatic metabolism of EFV, have been detected in rats. Both antidepressants also inhibit EFV metabolism in human hepatic microsomes, but additional in vivo studies in humans are required to evaluate the clinical implication of this interaction.

Therapeutic Drug Monitoring of Antidepressants and Cytochrome P450 Genotyping in General Practice

In the psychiatric setting, therapeutic drug monitoring and genotyping for cytochrome P450 (CYP) polymorphisms help to ensure and maintain therapeutic drug levels. In this study, the authors extended the therapeutic drug monitoring and genotyping protocol routinely used in their psychiatric clinic to primary care patients treated with antidepressants. They examined the variation in serum concentrations and assessed the role of CYP polymorphisms, wrong dosing, and noncompliance in deviating serum concentrations. Of 227 serum concentrations obtained, 127 (56%) were more than 20% outside therapeutic ranges. Of these 127 cases, 64 (50%) were congruous with aberrant CYP2D6 or CYP2C19 genotypes, incorrect dosing, or a pharmacy record suggesting noncompliance. Prevalence of aberrant CYP2D6 and CYP2C19 genotypes did not differ significantly between the investigated primary care patients and 751 secondary care users of antidepressants. The therapeutic drug monitoring and the genotyping findings resulted in recommendations to physicians to alter the medication strategy of 146 (64%) patients. These results strongly suggest that the rationale for therapeutic drug monitoring and CYP genotyping when prescribing antidepressants in secondary care also applies to the primary care setting.

Scientific challenges and implementation barriers to translation of pharmacogenomics in clinical practice

The mapping of the human genome and subsequent advancements in genetic technology had provided clinicians and scientists an understanding of the genetic basis of altered drug pharmacokinetics and pharmacodynamics, as well as some examples of applying genomic data in clinical practice. This has raised the public expectation that predicting patients' responses to drug therapy is now possible in every therapeutic area, and personalized drug therapy would come sooner than later. However, debate continues among most stakeholders involved in drug development and clinical decision-making on whether pharmacogenomic biomarkers should be used in patient assessment, as well as when and in whom to use the biomarker-based diagnostic tests. Currently, most would agree that achieving the goal of personalized therapy remains years, if not decades, away. Realistic application of genomic findings and technologies in clinical practice and drug development require addressing multiple logistics and challenges that go beyond discovery of gene variants and/or completion of prospective controlled clinical trials. The goal of personalized medicine can only be achieved when all stakeholders in the field work together, with willingness to accept occasional paradigm change in their current approach.

Pathophysiology of depression and mechanisms of treatment

Major depression is a serious disorder of enormous sociological and clinical relevance. The discovery of antidepressant drugs in the 1950s led to the first biochemical hypothesis of depression, which suggested that an impairment in central monoaminergic function was the major lesion underlying the disorder. Basic research in all fields of neuroscience (including genetics) and the discovery of new antidepressant drugs have revolutionized our understanding of the mechanisms underlying depression and drug action. There is no doubt that the monoaminergic system is one of the cornerstones of these mechanisms, but multiple interactions with other brain systems and the regulation of central nervous system function must also be taken into account In spite of all the progress achieved so far, we must be aware that many open questions remain to be resolved in the future.

Fetal serotonin reuptake inhibitor antidepressant exposure: maternal and fetal factors

Prenatal serotonin reuptake inhibitor exposure is common and neonatal outcomes vary greatly, often leading to confusion about whether to use or even continue antenatal use of these antidepressants. Importantly, some but not all infants are affected, which raises questions about how maternal drug metabolism contributes to fetal drug exposure. To address this key question, our paper reviews the role of key maternal, fetal, and placental pharmacokinetic, metabolic, and genetic factors that affect the extent of fetal drug exposure. Considering the role of these factors may further our understanding of variables that may assist in optimizing maternal psychopharmacotherapy during pregnancy and neonatal outcomes.

Repeated administration of berberine inhibits cytochromes P450 in humans

PURPOSE

Berberine is a plant alkaloid that is widely used to treat gastrointestinal infections, diabetes, hypertension, and hypercholesterolemia. Many studies have reported interactions between berberine-containing products and cytochromes P450 (CYPs), but little is known about whether berberine alters CYP activities in humans, especially after repeated doses.

METHODS

A two-phase randomized-crossover clinical study in healthy male subjects was performed. After 2 weeks of berberine (300 mg, t.i.d., p.o.) administration, midazolam, omeprazole, dextromethorphan, losartan, and caffeine were used to evaluate enzyme activities of CYP3A4, 2C19, 2D6, 2C9, and CYP1A2, respectively.

RESULTS

A decrease in CYP2D6 activity was observed as the 0-8 h urinary dextromethorphan/dextrorphan increased ninefold (P < 0.01). In addition, losartan/E-3174 ratio doubled (P < 0.01) after BBR administration, indicating a decrease in CYP2C9 activity. CYP3A4 activity was also inhibited, as the C(max), AUC(0-∞), and AUC(0-12) of midazolam were increased 38% (P < 0.05), 40% (P < 0.01), and 37% (P < 0.05) after BBR treatment, respectively. Compared with the placebo period, the T(max) and T(1/2) of midazolam during BBR administration were prolonged from 3.03 ± 0.27 to 3.66 ± 0.37 h and 0.66 ± 0.08 to 0.99 ± 0.09 h, respectively; the oral clearance of midazolam was decreased 27% (P < 0.05); and the phenotypic indices of 1 h midazolam/1'-hydroxymidazolam increased 59% (P < 0.01). There were no statistically significant differences in the pharmacokinetic parameters of the other probe drugs between placebo and the BBR-treated group.

CONCLUSIONS

Repeated administration of berberine (300 mg, t.i.d., p.o.) decreased CYP2D6, 2C9, and CYP3A4 activities. Drug-drug interactions should be considered when berberine is administered.

Safe and effective medicines for all: is personalized medicine the answer?

An improvement in drug treatment and clinical outcome is one of the major challenges in clinical medicine. The development of evidence-based standards of care has led to a significant improvement, but, by definition, strictly standardized cohorts in clinical trials have to ignore individual differences. Personalized medicine is defined as the application of genomic and molecular data to better target the delivery of healthcare, facilitate the discovery and clinical testing of new products, and help determine a person's predisposition to a particular disease or condition. After the deciphering of the human genome, however, the high expectations in individualized medicine were not always fulfilled. However, personalized medicine has become indispensable in the treatment of malignant diseases and there is increasing evidence for its benefit in other areas. This article outlines the impact of pharmacogenetics and pharmacogenomics, especially with regard to personalized medicine, in major medical indications and reflects the obstacles and chances taken in current daily practice.

Selective serotonin reuptake inhibitors: a review of its effects on intraocular pressure

The increase in serotonin (5-HT) neurotransmission is considered to be one of the most efficacious medical approach to depression and its related disorders. The selective serotonin reuptake inhibitors (SSRIs) represent the most widely antidepressive drugs utilized in the medical treatment of depressed patients. Currently available SSRIs include fluoxetine, sertraline, paroxetine, fluvoxamine, citalopram and escitalopram. The primary SSRIs pharmacological action's mechanism consists in the presynaptic inhibition on the serotonin reuptake, with an increased availability of this amine into the synaptic cleft. Serotonin produces its effects as a consequence of interactions with appropriate receptors. Seven distinct families of 5-HT receptors have been identified (5-HT(1) to 5-HT(7)), and subpopulations have been described for several of these. The interaction between serotonin and post-synaptic receptors mediates a wide range of functions. The SSRIs have a very favorable safety profile, although clinical signs of several unexpected pathologic events are often misdiagnosed, in particular, those regarding the eye. In all cases reported in the literature the angle-closure glaucoma represents the most important SSRIs-related ocular adverse event. Thus, it is not quite hazardous to hypothesize that also the other reported and unspecified visual disturbances could be attributed - at least in some cases - to IOP modifications. The knowledge of SSRIs individual tolerability, angle-closure predisposition and critical IOP could be important goals able to avoid further and more dangerous ocular side effects.

Effects of three cytochrome P450 inhibitors, ketoconazole, fluconazole, and paroxetine, on the pharmacokinetics of lasofoxifene

AIMS

Two studies were conduced to assess the effects of ketoconazole, a CYP3A4/5 inhibitor; fluconazole, a CYP2C9 inhibitor; and paroxetine, a CYP2D6 inhibitor, on lasofoxifene pharmacokinetics.

METHODS

The first parallel group study was conducted in 45 healthy postmenopausal women (15 per group) to compare the pharmacokinetics of a single dose of lasofoxifene (0.25 mg) administered alone and in combination with ketoconazole (400 mg daily x 20 days) or fluconazole (400 mg daily x 20 days). Lasofoxifene was administered on day 2 and blood samples were collected serially for up to 456 h postdose (20 days). The second study enrolled 20 healthy postmenopausal women (10 per group) to compare the pharmacokinetics of a single dose of lasofoxifene (0.25 mg) alone and in combination with paroxetine (30 mg qd x 21 days). Lasofoxifene was given on day 8 of paroxetine treatment and blood samples were collected serially for up to 336 h postdose.

RESULTS

All subjects completed the study and the treatments were well tolerated. Lasofoxifene C(max) and AUC ratios [90% confidence interval (CI)] with/without ketoconazole were 111% (98.4, 127) and 120% (105, 136), respectively, and were 91.3% (80.3, 104) and 104% (91.4, 118), respectively, with/without fluconazole. Lasofoxifene C(max) and AUC ratios (90% CI) with/without paroxetine were 118% (95.4, 146) and 135% (120, 152), respectively.

CONCLUSIONS

Coadministration of potent inhibitors of CYP3A4/5 and CYP2D6, but not CYP2C9, resulted in a moderate increase in lasofoxifene exposure. No dosage adjustment should be required when lasofoxifene is coadministered with ketoconazole, fluconazole, paroxetine or other agents that inhibit these CYP enzymes.

Treatment of depression with selective serotonin inhibitors: the role of fluvoxamine

The advent of the selective serotonin reuptake inhibitors (SSRIs) is generally considered to have improved the treatment of depression. Head-to-head trials comparing SSRIs to each other have shown little difference in efficacy among agents. The main differences between the SSRIs relate to safety and tolerability profiles, reflecting the fact that the SSRIs possess significant and variable secondary pharmacological properties. This heterogeneity contributes to clinically relevant differences that clinicians are increasingly using to select antidepressant treatment more closely appropriate to specific patient populations and circumstances. This review assesses the place of fluvoxamine amongst the SSRIs in the context of current issues and concerns with drug therapy. Fluvoxamine has a proven efficacy and safety profile in treating elderly patients with depression. The beneficial effects of fluvoxamine in obsessive-compulsive disorder (OCD) are also well documented. On the other hand, its σ1-receptor binding profile may account for the observed high level of efficacy in psychotic depression and may explain the benefit of fluvoxamine in treating depression comorbid with anxiety/stress. There is no definitive evidence that suicide risk is higher with SSRIs than with other antidepressants or nonpharmacological treatments and postmarketing surveillance indicates that fluvoxamine is not associated with a higher level of suicidality.

The AmpliChip CYP450 genotyping test: Integrating a new clinical tool

The AmpliChip CYP450 Test, which analyzes patient genotypes for cytochrome P450 (CYP) genes CYP2D6 and CYP2C19, is a major step toward introducing personalized prescribing into the clinical environment. Interest in adverse drug reactions (ADRs), the genetic revolution, and pharmacogenetics have converged with the introduction of this tool, which is anticipated to be the first of a new wave of such tools to follow over the next 5-10 years. The AmpliChip CYP450 Test is based on microarray technology, which combines hybridization in precise locations on a glass microarray and a fluorescent labeling system. It classifies individuals into two CYP2C19 phenotypes (extensive metabolizers [EMs] and poor metabolizers [PMs]) by testing three alleles, and into four CYP2D6 phenotypes (ultrarapid metabolizers [UMs], EMs, intermediate metabolizers [IMs], and PMs) by testing 27 alleles, including seven duplications. CYP2D6 is a metabolic enzyme with four activity levels (or phenotypes): UMs with unusually high activity; normal subjects, known as EMs; IMs with low activity; and PMs with no CYP2D6 activity (7% of Caucasians and 1-3% in other ethnic groups). Levels of evidence for the association between CYP2D6 PMs and ADRs are relatively reasonable and include systematic reviews of case-control studies of some typical antipsychotics and tricyclic antidepressants (TCAs). Evidence for other phenotypes is considerably more limited. The CYP2D6 PM phenotype may be associated with risperidone ADRs and discontinuation due to ADRs. Venlafaxine, aripiprazole, duloxetine, and atomoxetine are newer drugs metabolized by CYP2D6 but studies of the clinical relevance of CYP2D6 genotypes are needed. Non-psychiatric drugs metabolized by CYP2D6 include metoprolol, tamoxifen, and codeine-like drugs. CYP2C19 PMs (3-4% of Caucasians and African Americans, and 14-21% of Asians) may require dose adjustment for some TCAs, moclobemide, and citalopram. Other drugs metabolized by CYP2C19 are diazepam and omeprazole. The future of pharmacogenetics depends on the ability to overcome serious obstacles, including the difficulties of conducting and publishing studies in light of resistance from grant agencies, pharmaceutical companies, and some scientific reviewers. Assuming more studies are published, pharmacogenetic clinical applications may be compromised by economic factors and the lack of physician education. The combination of a US FDA-approved test, such as the AmpliChip CYP450 Test, and an FDA definition of CYP2D6 as a 'valid biomarker' makes CYP2D6 genotyping a prime candidate to be the first successful pharmacogenetic test in the clinical environment. One can use microarray technology to test for hundreds of single nucleotide polymorphisms (SNPs) but, taking into account the difficulties for single gene approaches such as CYP2D6, it is unlikely that very complex pharmacogenetic approaches will reach the clinical market in the next 5-10 years.

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.

Tolerability and safety of fluvoxamine and other antidepressants

Selective serotonin [5-hydroxytryptamine (5-HT)] reuptake inhibitors (SSRIs) and the 5-HT noradrenaline reuptake inhibitor, venlafaxine, are mainstays in treatment for depression. The highly specific actions of SSRIs of enhancing serotonergic neurotransmission appears to explain their benefit, while lack of direct actions on other neurotransmitter systems is responsible for their superior safety profile compared with tricyclic antidepressants. Although SSRIs (and venlafaxine) have similar adverse effects, certain differences are emerging. Fluvoxamine may have fewer effects on sexual dysfunction and sleep pattern. SSRIs have a cardiovascular safety profile superior to that of tricyclic antidepressants for patients with cardiovascular disease; fluvoxamine is safe in patients with cardiovascular disease and in the elderly. A discontinuation syndrome may develop upon abrupt SSRI cessation. SSRIs are more tolerable than tricyclic antidepressants in overdose, and there is no conclusive evidence to suggest that they are associated with an increased risk of suicide. Although the literature suggests that there are no clinically significant differences in efficacy amongst SSRIs, treatment decisions need to be based on considerations such as patient acceptability, response history and toxicity.

Clinical Guidelines for Psychiatrists for the Use of Pharmacogenetic Testing for CYP450 2D6 and CYP450 2C19

Pharmacogenetics has arrived in clinical psychiatric practice with the FDA approval of the AmpliChip CYP450 Test that genotypes for two cytochrome P450 2D6 (CYP2D6) and 2C19 (CYP2C19) genes. Other pharmacogenetic tests, including those focused on pharmacodynamic genes, are far from ready for clinical application. CYP2D6 is important for the metabolism of many antidepressants and antipsychotics, and CY2C19 is important for some antidepressant metabolism. Poor metabolizers (PMs), lacking the enzyme, account for up to 7% of Caucasians for CYP2D6 and up to 25% of East Asians for CYP2C19. Patients having three or more active CYP2D6 alleles (up to 29% in North Africa and the Middle East), are called CYP2D6 ultra-rapid metabolizers (UMs). CYP2D6 phenotypes (particularly PMs) are probably important in patients taking tricyclic antidepressants (TCAs), venlafaxine, typical antipsychotics, and risperidone. The CYP2C19 PM phenotype is probably important in patients taking TCAs and perhaps citalopram, escitalopram, and sertraline. On the basis of the literature and the authors' clinical experience, the authors provide provisional recommendations for identifying and treating CYP2D6 PMs, CYP2C19 PMs, and CYP2D6 UMs. The next few years will determine whether CYP2D6 genotyping is beneficial for patients taking the new drugs aripiprazole, duloxetine, and atomoxetine. Practical recommendations for dealing with laboratories offering CYP2D6 and CYP2C29 genotyping are provided.

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.

Mechanism-based inactivation of CYP2D6 by methylenedioxymethamphetamine

The potency of methylenedioxymethamphetamine (MDMA) as a mechanism-based inhibitor of CYP2D6 has been defined using microsomes prepared from yeast expressing the enzyme and from three human livers. The inhibitory effect was increased by preincubation through formation of a metabolic intermediate complex. Inactivation parameters (kinact and KI), defined with respect to the O-demethylation of dextromethorphan, were 0.29 +/- 0.03 (S.E.) min(-1) and 12.9 +/- 3.6 (S.E.) microM for yeast-expressed CYP2D6, and 0.26 +/- 0.02 min(-1) and 14.4 +/- 2.5 microM, 0.15 +/- 0.01 min(-1) and 8.8 +/- 2.6 microM, and 0.12 +/- 0.05 min(-1) and 45.3 +/- 32.1 microM for the liver microsomal preparations. The rate of inactivation of CYP2D6 by MDMA decreased when quinidine, a competitive inhibitor of CYP2D6, was added to the primary incubation mixture. However, inactivation was unaffected by the addition of glutathione. The results indicate that MDMA is a potent mechanism-based inhibitor of CYP2D6, with implications for understanding its in vivo disposition and drug interaction potential.

SSRIs and intraocular pressure modifications: evidence, therapeutic implications and possible mechanisms

SSRIs are the most commonly prescribed antidepressant drugs, in part because of their favourable safety profile compared with older antidepressants. However, the widespread use of SSRIs leads to an increased occurrence of rare adverse effects. This review, based on data from published experimental research, clinical studies and case reports, describes the role of serotonin in the control of intraocular pressure (IOP) and the evidence for IOP modifications in patients receiving SSRIs. In a small percentage of patients with depression, the cause of SSRI withdrawal has been the occurrence of ill-defined visual disturbances. It can be speculated that in some of these patients, the iatrogenic ocular alterations could have been due to changes in IOP. There have also been a limited number of case reports of acute attacks of glaucoma occurring during treatment with SSRIs. Although causality is not exactly specified, the relationship between SSRIs and this ocular adverse event is strongly implied. Nevertheless, in a small clinical study assessing the effect of a single dose of fluoxetine on IOP, the drug was shown to increase this parameter, although the effect was asymptomatic. The clinical signs of unexpected adverse drug effects are often disregarded, with the exception of those characterised by serious symptoms (such as acute angle-closure glaucoma in the case of IOP modifications). Also, the distribution of iridocorneal angle configurations in the general population implies that an adverse effect on IOP will be pauci- or asymptomatic in most patients (intermittent, sub-acute or progressive angle-closure glaucoma). As a result, it is likely that the incidence of SSRI-related IOP modifications is underestimated. Until the involvement of SSRIs in IOP modifications is better understood, ophthalmological consultations should be considered before starting and during treatment with any SSRI in patients with glaucomatous risk factors, especially those who are elderly.

Pharmacogenetics of antidepressants and antipsychotics: the contribution of allelic variations to the phenotype of drug response

Genetic factors contribute to the phenotype of drug response. We systematically analyzed all available pharmacogenetic data from Medline databases (1970-2003) on the impact that genetic polymorphisms have on positive and adverse reactions to antidepressants and antipsychotics. Additionally, dose adjustments that would compensate for genetically caused differences in blood concentrations were calculated. To study pharmacokinetic effects, data for 36 antidepressants were screened. We found that for 20 of those, data on polymorphic CYP2D6 or CYP2C19 were found and that in 14 drugs such genetic variation would require at least doubling of the dose in extensive metabolizers in comparison to poor metabolizers. Data for 38 antipsychotics were examined: for 13 of those CYP2D6 and CYP2C19 genotype was of relevance. To study the effects of genetic variability on pharmacodynamic pathways, we reviewed 80 clinical studies on polymorphisms in candidate genes, but those did not for the most part reveal significant associations between neurotransmitter receptor and transporter genotypes and therapy response or adverse drug reactions. In addition associations found in one study could not be replicated in other studies. For this reason, it is not yet possible to translate pharmacogenetic parameters fully into therapeutic recommendations. At present, antidepressant and antipsychotic drug responses can best be explained as the combinatorial outcome of complex systems that interact at multiple levels. In spite of these limitations, combinations of polymorphisms in pharmacokinetic and pharmacodynamic pathways of relevance might contribute to identify genotypes associated with best and worst responders and they may also identify susceptibility to adverse drug reactions.

Pharmacogenetics and psychiatry

Genetic factors play a significant role in predicting an individual's response to a drug. The response may be the desired therapeutic effect of the drug and also may be the undesirable development of adverse effects. This relationship between genes and drug response interests the pharmacogeneticist. This article aims to give an overview of the exciting discoveries made so far in the field of psychiatry, particularly concerning the response to antidepressants and antipsychotics, as well as to mention some of the more recent findings. The ultimate goal of pharmacogenetics is to provide medication "tailored" to the individual based on their genetic profile, and although this may currently seem a distant target, it has already begun to raise ethical questions, which also are discussed.

Lack of a Fluoxetine Effect on Prednisolone Disposition and Cortisol Suppression

STUDY OBJECTIVE

To evaluate the potential effect of fluoxetine, a cytochrome P450 isoenzyme inhibitor, on prednisolone disposition and cortisol suppression.

DESIGN

Sequential, two-phase, crossover, open-label pharmacokinetic study.

SETTING

General clinical research center.

SUBJECTS

Fourteen healthy volunteers.

INTERVENTION

A single intravenous dose of prednisolone 40 mg before and after 14 days of treatment with fluoxetine 20 mg/day for 5 days followed by 60 mg/day for 9 days to achieve steady-state concentrations.

MEASUREMENTS AND MAIN RESULTS

Pharmacokinetic parameters of the prednisolone and resulting pharmacodynamic effects on the time course of plasma cortisol suppression before and after fluoxetine administration were evaluated. No significant differences were observed for the mean +/- SD area under the prednisolone concentration-time curve (3739 +/- 992 vs 3498 +/- 797 microg x hr/L, respectively), clearance (8.58 +/- 2.62 vs 8.92 +/- 2.05 L/hr, respectively), volume of distribution (39.5 +/- 12.4 vs 38.2 +/- 9.9 L, respectively), elimination half-life (3.32 +/- 0.83 vs 3.05 +/- 0.80 hrs, respectively), or duration of plasma cortisol suppression (23.5 +/- 3.1 vs 22.0 +/- 4.2 hrs, respectively).

CONCLUSION

Fluoxetine administration did not significantly affect prednisolone disposition or prolong cortisol suppression. This finding suggests that coadministration of these agents is unlikely to result in clinically important pharmacokinetic or pharmacodynamic drug interactions. Prednisolone may be a useful alternative for patients who require both glucocorticoid and fluoxetine therapy.

Pharmacogenetics of cytochrome p4502D6: genetic background and clinical implication

Interindividual differences in the pharmacokinetics of a number of drugs are often due to hereditary polymorphisms of drug-metabolizing enzymes. Most important is cytochrome p4502D6 (CYP2D6), also known as debrisoquine/sparteine hydroxylase. It catalyzes hydroxylation or demethylation of more than 20% of drugs metabolized in the human liver, such as neuroleptics, antidepressants, some beta-blockers and many others like codeine. About 7%-10% of Caucasians lack any CYP2D6 activity due to deletions and frame-shift or splice-site mutations of the gene. About 1%-3% of Middle-Europeans, but up to 29% of Ethiopians display gene duplications, leading to elevated so-called ultrarapid metabolization rates. Meanwhile there is now a much better understanding of the genetic background of poor, intermediate, extensive and ultrarapid metabolizers, enabling a more precise DNA genotyping-based prediction of plasma levels. Since there is evidence that deteriorated drug elimination partly accounts for drug side-effects, CYP2D6 genotyping could contribute to an individualized and therefore optimized drug therapy.

Clinical pharmacokinetics of sertraline

Sertraline is a naphthalenamine derivative with the predominant pharmacological action of inhibiting presynaptic reuptake of serotonin from the synaptic cleft. It was initially marketed for the treatment of major depressive disorder and is now approved for the management of panic disorder, obsessive-compulsive disorder and post-traumatic stress disorder. Sertraline is slowly absorbed following oral administration and undergoes extensive first-pass oxidation to form N-desmethyl-sertraline, a weakly active metabolite that accumulates to a greater concentration in plasma than the parent drug at steady state. Sertraline is eliminated from the body by other metabolic pathways to form a ketone and an alcohol, which are largely excreted renally as conjugates. The elimination half-life of sertraline ranges from 22-36 hours, and once-daily administration is therapeutically effective. Steady-state plasma concentrations vary widely, up to 15-fold, in patients receiving usual antidepressant dosages between 50 and 150 mg/day. However, only sparse data have been published that support useful correlations between sertraline plasma concentrations and therapeutic or adverse effects to justify therapeutic drug monitoring. Sertraline has minimal inhibitory effects on the major cytochrome P450 enzymes, and few drug-drug interactions of clinical significance have been documented. Like other selective serotonin reuptake inhibitors, sertraline is well tolerated in therapeutic dosages and relatively safe in overdosage.

Pharmacogenetics of schizophrenia

Patients display significant differences in response to therapeutic agents which may be caused by a variety of factors. Among them, genetic components presumably play a major role. Pharmacogenetics is the field of research that attempts to unravel the relationship between genetic variation affecting drug metabolism (pharmacokinetic level) or drug targets (pharmacodynamic level) and interindividual differences in pharmacoresponse. In schizophrenia, pharmacokinetic studies have shown the role of genetic variants of the cytochrome P450 enzymes CYP2D6, CYP2C19, and CYP2C9 in the metabolism of neuroleptic drugs. At the level of the drug target, variants of the dopamine D3 and D4, and 5-HT2A and 5-HT2C receptors have been examined. A general problem of pharmacogenetic studies in schizophrenia is the high number of controversial findings which may be related to the lack of standardized phenotype definition. Recently, guidelines for an exact and comparable phenotype characterization have been proposed and will aid in designing and evaluating pharmacogenetic studies in the future. The final goal of pharmacogenetic studies-making a prediction of drug response at the level of the individual patient-will require a simultaneous look at a large number of response-determining genetic variants by applying the tools of pharmacogenomics, e.g. large-scale Single Nucleotide Polymorphism (SNP) detection and genotyping.

Apparent mechanism-based inhibition of human CYP2D6 in vitro by paroxetine: comparison with fluoxetine and quinidine

Paroxetine, a selective serotonin reuptake inhibitor, is a potent inhibitor of cytochrome P450 2D6 (CYP2D6) activity, but the mechanism of inhibition is not established. To determine whether preincubation affects the inhibition of human liver microsomal dextromethorphan demethylation activity by paroxetine, we used a two-step incubation scheme in which all of the enzyme assay components, minus substrate, are preincubated with paroxetine. The kinetic parameters of inhibition were also estimated by varying the time of preincubation as well as the concentration of inhibitor. From these data, a Kitz-Wilson plot was constructed, allowing the estimation of both an apparent inactivator concentration required for half-maximal inactivation (K(I)) and the maximal rate constant of inactivation (k(INACT)) value for this interaction. Preincubation of paroxetine with human liver microsomes caused an approximately 8-fold reduction in the IC(50) value (0.34 versus 2.54 microM). Time-dependent inhibition was demonstrated with an apparent K(I) of 4.85 microM and an apparent k(INACT) value of 0.17 min(-1). Spectral scanning of CYP2D6 with paroxetine yielded an increase in absorbance at 456 nm suggesting paroxetine inactivation of CYP2D6 via the formation of a metabolite intermediate complex. This pattern is consistent with the metabolism of the methylenedioxy substituent in paroxetine; such substituents may produce mechanism-based inactivation of cytochrome P450 enzymes. In contrast, quinidine and fluoxetine, both of which are inhibitors of CYP2D6 activity, did not exhibit a preincubation-dependent increase in inhibitory potency. These data are consistent with mechanism-based inhibition of CYP2D6 by paroxetine but not by quinidine or fluoxetine.

Effect of terbinafine on the pharmacokinetics and pharmacodynamics of desipramine in healthy volunteers identified as cytochrome P450 2D6 (CYP2D6) extensive metabolizers

Terbinafine-CYP2D6 inhibition was evaluated by assessing 48-hour concentration-time profiles of the tricyclic antidepressant desipramine in 12 healthy volunteers identified as extensive cytochrome P450 2D6 (CYP2D6) metabolizers by genotyping and phenotyping. Pharmacokinetics was evaluated at baseline (50 mg oral desipramine given alone), steady state (after 250 mg oral terbinafine for 21 days), and 2 and 4 weeks after terbinafine discontinuation. Pharmacodynamics was evaluated before and 2 hours after each desipramine administration, using Mini-Mental Status Examination (MMSE) and EGG. Terbinafine administration inhibited CYP2D6 metabolism, as indicated by the significant increase in desipramine C(max) (19 ng/ml vs. 36 ng/ml) and AUC0-infinity (482 ng.h/ml vs. 2383 ng.h/ml) and decrease in AUC0-24 and C(max) of the CYP2D6-mediated metabolite, 2-hydroxydesipramine. In addition, the C(max) and AGUC0-infinity of desipramine and metabolite were still elevated 4 weeks after terbinafine discontinuation. Caution should be exercised when coprescribing terbinafine and drugs metabolized by CYP2D6, particularly those with a narrow therapeutic index.

Effect of nortriptyline and paroxetine on CYP2D6 activity in depressed elderly patients

This study was performed in elderly patients (1) to assess the degree to which CYP2D6 mediated metabolism of debrisoquine at baseline determines plasma concentration to dose quotients for nortriptyline or paroxetine after 4 weeks of treatment, and (2) to compare the effects of nortriptyline and paroxetine on debrisoquine metabolism after 6 weeks of treatment. CYP2D6 activity was estimated in 66 subjects (71.4 +/- 7.2 years) before initiating treatment and again after 6 weeks of treatment with either nortriptyline or paroxetine under randomized, double-blind conditions according to a standard protocol. CYP2D6 activity was estimated by the debrisoquine recovery ratio in a 6- to 8-hour urine sample collected after oral administration of 10 mg debrisoquine sulfate. Nortriptyline and paroxetine plasma concentrations were obtained weekly. Baseline debrisoquine recovery ratio values were significantly correlated with the plasma concentration to dose quotient at 4 weeks for both nortriptyline ( = -0.75, = 0.0001, N = 29) and paroxetine ( = -0.50, = 0.003, N = 33). Treatment with either nortriptyline or paroxetine was associated with a significant decrease in the median debrisoquine recovery ratio, reflecting inhibition of CYP2D6 metabolism. The percent decrease associated with nortriptyline was significantly smaller than that with paroxetine ( < 0.0001). None of the patients treated with nortriptyline but 19 of the 32 extensive metabolizers treated with paroxetine were converted to phenotypic poor metabolic status. Our observations of CYP2D6 inhibition are consistent with data and results obtained in younger healthy volunteers. The significant correlations between baseline debrisoquine recovery ratio and the plasma concentrations to dose quotients at 4 weeks for both nortriptyline and paroxetine are consistent with CYP2D6 playing a major role in the metabolism of both drugs. CYP2D6 inhibition by paroxetine, which effectively converted 59% of patients to phenotypic PMs, may be especially relevant for elderly patients given their generally higher concentration of paroxetine.

Safety and efficacy of fluoxetine in patients who receive oral contraceptive therapy

OBJECTIVE

Because many women who receive pharmacologic therapy with antidepressants are also prescribed oral contraceptives, it is important to assess the risk of clinically significant drug interactions. We reviewed the United States fluoxetine clinical trial database, specifically analyzing women ages 18 to 45 years, for differences in safety, antidepressant efficacy, and unplanned pregnancies that were associated with oral contraceptive use.

STUDY DESIGN

Data from 17 double-blind, placebo-controlled clinical trials in 1698 women were analyzed retrospectively. A subgroup of women with oral contraceptive use was compared with a subgroup of women with no oral contraceptive use. Differences in treatment-emergent adverse events, unplanned pregnancies, and 17-item Hamilton Depression Scale (HAMD-17) scores were analyzed.

RESULTS

The only treatment-emergent adverse events that showed a statistically significantly different odds ratio for oral contraceptive use versus no oral contraceptive use were headache, asthenia, and pain. There was not a statistically significant interaction in the incidence of unintended pregnancies (P =.111) or in the changes from baseline in HAMDD-17 scores.

CONCLUSION

There is no clinical evidence that concomitant use of oral contraceptives and fluoxetine affects the safety or efficacy of either agent.

CYP2D6 inhibition by selective serotonin reuptake inhibitors: analysis of achievable steady-state plasma concentrations and the effect of ultrarapid metabolism at CYP2D6

STUDY OBJECTIVE

To assess the correlation between plasma concentrations of four commonly administered selective serotonin reuptake inhibitors (SSRIs) and the magnitude of cytochrome P450 (CYP) 2D6 inhibition.

DESIGN

Prospective analysis.

SETTING

University-affiliated research laboratory.

PATIENTS

Thirty-two healthy, drug-free volunteers.

INTERVENTION

Subjects were randomized to four groups and received daily administration of either fluoxetine 60 mg (as a loading dose), fluvoxamine 100 mg, paroxetine 20 mg, or sertraline 100 mg for 8 days.

MEASUREMENTS AND MAIN RESULTS

The urinary concentration ratio of dextromethorphan:dextrorphan (interpreted as an in vivo index of CYP2D6 activity) was determined for each subject before and after the 8 days of receiving SSRIs. Plasma SSRI trough concentrations were measured on days 6-9. The CYP2D6 genotype was determined in a subject with an undetectable paroxetine concentration. Inhibition of CYP2D6 correlated significantly with plasma concentrations of paroxetine and fluoxetine. In contrast, no significant correlations emerged between CYP2D6 inhibition and plasma concentrations of sertraline or fluvoxamine. The subject with an undetectable paroxetine concentration was found to carry at least three functional CYP2D6 genes.

CONCLUSIONS

For paroxetine and fluoxetine, plasma concentrations and dosage strongly influence the magnitude of enzyme inhibition. The potential of paroxetine (a CYP2D6 substrate) as an inhibitor may be affected by the genotypes and metabolic capacities of individual subjects.

Differential time course of cytochrome P450 2D6 enzyme inhibition by fluoxetine, sertraline, and paroxetine in healthy volunteers

The selective serotonin reuptake inhibitors (SSRIs) paroxetine, sertraline, and fluoxetine have varying degrees of potency in inhibiting the hepatic cytochrome P450 (CYP) 2D6 enzyme. However, the time course for maximum inhibition to occur or for inhibition to dissipate when dosing is discontinued, requires clarification. In an open label, parallel group study of 45 healthy volunteers, the time course of CYP2D6 inhibition of the above SSRIs was evaluated. Subjects were randomized to receive paroxetine at 20 mg/day for 10 days; sertraline at 50 mg/day for 3 days, followed by sertraline at 100 mg/day for 10 days; or fluoxetine at 20 mg/day for 28 days. CYP2D6 activity was assessed using the dextromethorphan metabolic ratio (DMR) on antidepressant days 5 and 10 for sertraline and paroxetine and at weekly intervals for fluoxetine. Following SSRI discontinuation, calculation of a CYP2D6 inhibition half-life (t(1/2)inh) revealed the time course of fluoxetine inhibition (t(1/2)inh = 7.0 +/- 1.5 days) to be significantly longer than either paroxetine (t(1/2)inh = 2.9 +/- 1.9) or sertraline (t(1/2)inh = 3.0 +/- 3.0) (p < 0.01), but the latter were not significantly different from each other (p > 0.05). Time for the extrapolated DMR versus time log-linear plots to return to baseline was significantly different between fluoxetine (63.2 +/- 5.6 days) and both paroxetine (20.3 +/- 6.4 days) and sertraline (25.0 +/- 11.0 days) (p < 0.01), making the rank order (from longest to shortest) of time for CYP2D6 inhibition to dissipate: fluoxetine > sertraline >or= paroxetine. Differences between mean baseline DMR values and measured values obtained after drug discontinuation for each drug group became nonsignificant on discontinuation day 5 for both paroxetine and sertraline and on discontinuation day 42 for fluoxetine. These data define the time course of a persistent effect that fluoxetine, sertraline, and paroxetine have on CYP2D6 following drug discontinuation and should be considered when initiating therapy with a CYP2D6 substrate.