The oxidative metabolism of metoprolol in human liver microsomes: inhibition by the selective serotonin reuptake inhibitors

Selective Serotonin Reuptake Inhibitors and Adverse Effects: A Narrative Review

Depression is the most prevalent psychiatric disorder in the world, affecting 4.4% of the global population. Despite an array of treatment modalities, depressive disorders remain difficult to manage due to many factors. Beginning with the introduction of fluoxetine to the United States in 1988, selective serotonin reuptake inhibitors (SSRIs) quickly became a mainstay of treatment for a variety of psychiatric disorders. The primary mechanism of action of SSRIs is to inhibit presynaptic reuptake of serotonin at the serotonin transporter, subsequently increasing serotonin at the postsynaptic membrane in the serotonergic synapse. The six major SSRIs that are marketed in the USA today, fluoxetine, citalopram, escitalopram, paroxetine, sertraline, and fluvoxamine, are a group of structurally unrelated molecules that share a similar mechanism of action. While their primary mechanism of action is similar, each SSRI has unique pharmacokinetics, pharmacodynamics, and side effect profile. One of the more controversial adverse effects of SSRIs is the black box warning for increased risk of suicidality in children and young adults aged 18–24. There is a lack of understanding of the complexities and interactions between SSRIs in the developing brain of a young person with depression. Adults, who do not have certain risk factors, which could be confounding factors, do not seem to carry this increased risk of suicidality. Ultimately, when prescribing SSRIs to any patient, a risk–benefit analysis must factor in the potential treatment effects, adverse effects, and dangers of the illness to be treated. The aim of this review is to educate clinicians on potential adverse effects of SSRIs.

Potentially Inappropriate Prescriptions in Heart Failure with Reduced Ejection Fraction (PIP-HFrEF)

Heart failure (HF) is a chronic debilitating and potentially life-threatening condition. Heart Failure patients are usually at high risk of polypharmacy and consequently, potentially inappropriate prescribing leading to poor clinical outcomes. Based on the published literature, a comprehensive HF-specific prescribing review tool is compiled to avoid medications that may cause HF or harm HF patients and to optimize the prescribing practice of HF guideline-directed medical therapies. Recommendations are made in line with the last versions of ESC guidelines, ESC position papers, scientific evidence, and experts' opinions.

[Effects of CYP2D6*10 on plasma trough concentration of metoprolol in patients with coronary artery disease]

OBJECTIVE

To study the effect of CYP2D6^*10 (c.100 C>T) on plasma trough concentrations of metoprolol and its metabolite α-hydroxy metoprolol, blood pressure and heart rate in patients with coronary artery disease.

METHODS

The patients with coronary artery disease taking metoprolol tablets (n=128) and those taking metoprolol sustained-release tablets (n=126) were genotyped for CYP2D6^*10 using Taqman real-time quantitative PCR. The trough concentrations of metoprolol and α-hydroxy metoprolol were determined with UPLC-MS/MS, and the dose-normalized concentrations (C/D) were compared among the patients with different CYP2D6^*10 genotypes in both groups. Resting blood pressure and heart rate were recorded in all the patients when the concentration of metoprolol reached the steady state and were compared among the patients with different genotypes.

RESULTS

In patients taking metoprolol sustained-release tablets, the plasma trough concentration of α-hydroxy metoprolol was significantly associated with the systolic blood pressure (P=0.0204). The CYP2D6^*10 poor metabolizers showed a significant association with the C/D of metoprolol and α-hydroxy metoprolol (P < 0.01) in patients receiving metoprolol in both formulations, and in both groups, the C/D of metoprolol was significantly higher in the patients with a TT genotype than in those with a CC or CT genotype (P < 0.01); compared with those with the CT genotype, the patients with the TT genotype had a significantly lower C/D of α-hydroxy metoprolol (P < 0.01). In patients taking metoprolol sustained-release tablets, those with the CT (P=0.0281) and TT (P=0.0196) genotypes had lower diastolic blood pressure than patients with the CC genotypes, but the systolic blood pressure or heart rate did not differ significantly among them.

CONCLUSIONS

CYP2D6^*10T allele mutation can reduce the metabolism of metoprolol, increase the C/D of metoprolol and decrease the C/D of α-metoprolol and diastolic blood pressure in patients with coronary artery disease, but CYP2D6^*10 variation does not significantly affect systolic blood pressure or heart rate in the patients when the concentration of metoprolol reaches a steady state.

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.

Metoprolol induces oxidative damage in common carp (Cyprinus carpio)

During the last decade, β-blockers such as metoprolol (MTP) have been frequently detected in surface water, aquatic systems and municipal water at concentrations of ng/L to μg/L. Only a small number of studies exist on the toxic effects induced by this group of pharmaceuticals on aquatic organisms. Therefore, the present study aimed to evaluate the oxidative damage induced by MTP in the common carp Cyprinus carpio, using oxidative stress biomarkers. To this end, indicators of cellular oxidation such as hydroperoxide content (HPC), lipid peroxidation (LPX) and protein carbonyl content (PCC) were determined, as well as the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT). Also, concentrations of MTP and its metabolite O-desmethyl metoprolol were determined in water as well as carp gill, liver, kidney, brain and blood, along with the partial uptake pattern of these compounds. Results show that carp takes up MTP and its metabolite in the different organs evaluated, particularly liver and gill. The oxidative stress biomarkers, HPC, LPX, and PCC, as well as SOD and CAT activity all increased significantly at most exposure times in all organs evaluated. Results indicate that MTP and its metabolite induce oxidative stress on the teleost C. carpio and that the presence of these compounds may constitute a risk in water bodies for aquatic species.

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.

Pharmacogenetics of drug oxidation via cytochrome P450 (CYP) in the populations of Denmark, Faroe Islands and Greenland

Denmark, the Faroe Islands and Greenland are three population-wise small countries on the northern part of the Northern Hemisphere, and studies carried out here on the genetic control over drug metabolism via cytochrome P450 have led to several important discoveries. Thus, CYP2D6 catalyzes the 2-hydroxylation, and CYP2C19 in part catalyzes the N-demethylation of imipramine. The phenomenon of phenocopy with regard to CYP2D6 was first described when Danish patients changed phenotype from extensive to poor metabolizers during treatment with quinidine. It was a Danish extensive metabolizer patient that became a poor metabolizer during paroxetine treatment, and this was due to the potent inhibition of CYP2D6 by paroxetine, which is also is metabolized by this enzyme. Fluoxetine and norfluoxetine are also potent inhibitors of CYP2D6, and fluvoxamine is a potent inhibitor of both CYP1A2 and CYP2C19. The bioactivation of proguanil to cycloguanil is impaired in CYP2C19 poor metabolizers. The O-demethylation of codeine and tramadol to their respective my-opioid active metabolites, morphine and (+)-O-desmethyltramadol was markedly impaired in CYP2D6 poor metabolizers compared to extensive metabolizers, and this impairs the hypoalgesic effect of the two drugs in the poor metabolizers. The frequency of CYP2D6 poor metabolizers is 2%-3% in Greenlanders and nearly 15% in the Faroese population. The frequency of CYP2C19 poor metabolizers in East Greenlanders is approximately 10%. A study in Danish mono and dizygotic twins showed that the non-polymorphic 3-N-demethylation of caffeine catalyzed by CYP1A2 is subject to approximately 70% genetic control.

Effects of the pharmaceuticals diclofenac and metoprolol on gene expression levels of enzymes of biotransformation, excretion pathways and estrogenicity in primary hepatocytes of Nile tilapia (Oreochromis niloticus)

The expression levels of key enzymes of the xenobiotic metabolism and excretion pathways concerning biotransformation phases I (cytochrome P4501A), II (glutathione S-transferase) and III (multidrug resistance protein) and of the estrogenic biomarker vitellogenin (vtg) were investigated in primary hepatocytes isolated from male Nile tilapia (Oreochromis niloticus) after exposure to diclofenac and metoprolol, two pharmaceuticals prevalent in the aquatic environment worldwide. The lowest test concentration (4×10(-9) M) was chosen to reflect an environmentally relevant exposure situation. Furthermore concentration dependent effects were investigated. Therefore a series of concentrations higher than the environmentally relevant range were used (10- and 100-fold). Diclofenac significantly induced all chosen biomarkers already at the environmentally relevant concentration indicating that biotransformation and elimination occur via the pathways under investigation. Estrogenic potential of this substance was demonstrated by VTG up-regulation as well. Metoprolol was either less effective than diclofenac or metabolized using different pathways. Key enzymes of the xenobiotic metabolism were less (CYP1A, GST) or not (MDRP) induced and a mild increase in vtg mRNA was detected only for 4×10(-8) M. No concentration-dependency for metoprolol was found.

Simultaneous determination of metoprolol and its metabolites, α-hydroxymetoprolol and O-desmethylmetoprolol, in human plasma by liquid chromatography with tandem mass spectrometry: Application to the pharmacokinetics of metoprolol associated with CYP2D6 genotypes

A rapid and simple LC with MS/MS method for the simultaneous determination of metoprolol and its two CYP2D6-derived metabolites, α-hydroxy- and O-desmethylmetoprolol, in human plasma was established. Metoprolol (MET), its two metabolites, and the internal standard chlorpropamide were extracted from plasma (50 μL) using ethyl acetate. Chromatographic separation was performed on a Luna CN column with an isocratic mobile phase consisting of distilled water and methanol containing 0.1% formic acid (60:40, v/v) at a flow rate of 0.3 mL/min. The total run time was 3.0 min per sample. Mass spectrometric detection was conducted by ESI in positive ion selected-reaction monitoring mode. The linear ranges of concentration for MET, α-hydroxymetoprolol, and O-desmethylmetoprolol were 2-1000, 2-500, and 2-500 ng/mL, respectively, with a lower limit of quantification of 2 ng/mL for all analytes. The coefficient of variation for the assay's precision was ≤ 13.2%, and the accuracy was 89.1-110%. All analytes were stable under various storage and handling conditions and no relevant cross-talk and matrix effect were observed. Finally, this method was successfully applied to assess the influence of CYP2D6 genotypes on the pharmacokinetics of MET after oral administration of 100 mg to healthy Korean volunteers.

Antidepressants, metoprolol and the risk of bradycardia

Case reports and pharmacologic theory suggest that some antidepressants can interfere with the hepatic metabolism of metoprolol by cytochrome P450 2D6 (CYP2D6), potentially increasing the risk of bradycardia. The objective of this study was to characterize the clinical consequences of this potential drug interaction at the population level. We conducted a population-based, nested case-control study of Ontario residents 66 years of age or older receiving metoprolol. Cases hospitalized for bradycardia were compared with matched controls (4:1) to explore the odds ratio for initiation of antidepressants that inhibit CYP2D6 (fluoxetine and paroxetine) and those that do not inhibit CYP2D6 (fluvoxamine, citalopram, venlafaxine, and sertraline) 30 days before hospitalization. From April 1997 to March 2009, we identified 332,254 older patients continuously receiving metoprolol, of whom 8232 (2.5%) were treated in hospital for bradycardia. The adjusted odds ratio for exposure to fluoxetine or paroxetine compared with other antidepressants 30 days prior to hospitalization for bradycardia was 0.76 (95% confidence interval 0.42-1.37). Among older patients receiving metoprolol, the initiation of antidepressants that inhibit CYP2D6 was not associated with a significant increase in the risk of bradycardia compared with antidepressants that do not inhibit CYP2D6.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Prediction of CYP2D6 drug interactions from in vitro data: evidence for substrate-dependent inhibition

Predicting the magnitude of potential drug-drug interactions is important for underwriting patient safety in the clinical setting. Substrate-dependent inhibition of cytochrome P450 enzymes may confound extrapolation of in vitro results to the in vivo situation. However, the potential for substrate-dependent inhibition with CYP2D6 has not been well characterized. The inhibition profiles of 20 known inhibitors of CYP2D6 were characterized in vitro against four clinically relevant CYP2D6 substrates (desipramine, dextromethorphan, metoprolol, and thioridazine) and bufuralol. Dextromethorphan exhibited the highest sensitivity to in vitro inhibition, whereas metoprolol was the least sensitive. In addition, when metoprolol was the substrate, inhibitors with structurally constrained amino moieties (clozapine, debrisoquine, harmine, quinidine, and yohimbine) exhibited at least a 5-fold decrease in inhibition potency when results were compared with those for dextromethorphan. Atypical inhibition kinetics were observed for these and other inhibitor-substrate pairings. In silico docking studies suggested that interactions with Glu216 and an adjacent hydrophobic binding pocket may influence substrate sensitivity and inhibition potency for CYP2D6. The in vivo sensitivities of the clinically relevant CYP2D6 substrates desipramine, dextromethorphan, and metoprolol were determined on the basis of literature drug-drug interaction (DDI) outcomes. Similar to the in vitro results, dextromethorphan exhibited the highest sensitivity to CYP2D6 inhibition in vivo. Finally, the magnitude of in vivo CYP2D6 DDIs caused by quinidine was predicted using desipramine, dextromethorphan, and metoprolol. Comparisons of the predictions with literature results indicated that the marked decrease in inhibition potency observed for the metoprolol-quinidine interaction in vitro translated to the in vivo situation.

In vitro metabolic stability and metabolite profiling of TJ0711 hydrochloride, a newly developed vasodilatory β-blocker, using a liquid chromatography-tandem mass spectrometry method

In this paper, a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for the simultaneous analysis of metabolic stability and metabolite profiling of 1-[4-(2-methoxyethyl) phenoxy]-3-[[2-(2-methoxyphenoxy) ethyl]amino]-2-propanol hydrochloride (TJ0711 HCl), a new vasodilatory β-blocker. Multiple reaction monitoring (MRM) was used as a survey scan to quantify the parent compound and to trigger the acquisition of enhanced product ions (EPI) for the identification of formed metabolites. In addition, comparison between MRM-only and MRM-information dependent acquisition-EPI (MRM-IDA-EPI) methods was conducted to determine analytical variables, including linearity, limit of detection (LOD), lower limit of quantification (LLOQ), as well as intra-day and inter-day accuracy and precision. Results demonstrated that MRM-IDA-EPI quantitative analysis was not affected by the addition of EPI scans to obtain qualitative information during the same chromatographic run, compared to MRM-only method. Thereafter, metabolic stability and metabolite identification of TJ0711 HCl were investigated using human liver microsomes (HLM) by the MRM-IDA-EPI method. The in vitro metabolic stability parameters were calculated and t(1/2), microsomal intrinsic clearance (CL(int)), as well as hepatic CL, were 13.0 min, 106.5 μL/min/mg microsomal protein, and 1082.2 mL/min, respectively. The major formed metabolites were also simultaneously monitored and the metabolite profiling data demonstrated that this MRM-IDA-EPI method was capable of targeting a large number of metabolites, in which demethylation and hydroxylation were the principle metabolism pathways during the in vitro incubation with HLM.

The beta-receptor blocker metoprolol alters detoxification processes in the non-target organism Dreissena polymorpha

Due to increasing amounts of pharmaceutically active compounds (PhACs) in the aquatic environment, their largely unknown effects to non-target organisms need to be assessed. This study examined physiological changes in the freshwater mussel Dreissena polymorpha exposed to increasing concentrations (0.534, 5.34, 53.4 and 534 microg L(-1)) of the beta-blocker metoprolol in a flow-through system for seven days. The two lower concentrations represent the environmentally relevant range. Surprisingly, metallothionein mRNA was immediately up-regulated in all treatments. For the two higher concentrations mRNA up-regulation in gills was found for P-glycoprotein after one day, and after four days for pi class glutathione S-transferase, demonstrating elimination and biotransformation processes, respectively. Additionally, catalase and superoxide dismutase were up-regulated in the digestive gland indicating oxidative stress. In all treated mussels a significant up-regulation of heat shock protein mRNA was observed in gills after four days, which suggests protein damage and the requirement for repair processes. Metoprolol was 20-fold bioaccumulated for environmentally relevant concentrations.

Effects of selective serotonin reuptake inhibitors on timolol metabolism in human liver microsomes and cryo-preserved hepatocytes

Timolol has been widely used in the treatment of glaucoma. Topically applied, timolol may cause adverse cardiovascular effects due to systemic absorption through the nasolacrimal duct. Timolol is mainly metabolized by cytochrome P450 2D6 (CYP2D6) in the liver. The aim of the present study was to characterize further the metabolism of timolol in vitro. Especially the effect of several drugs such as selective serotonin reuptake inhibitors on the metabolism of timolol was evaluated. In human liver microsomes, four timolol metabolites were identified, in cryo-preserved hepatocytes nine. In both in vitro experiments, the hydroxy metabolite M1 was the main metabolite. The in vivo half-life predicted for timolol was 3.7 hr. in cryo-preserved hepatocytes, corresponding to the half-life of timolol in humans in vivo. Fluoxetine, paroxetine, sertraline, citalopram and fluvoxamine inhibited the formation of M1 in microsomes with IC(50) values of 1.4, 2.0, 3.5, 21 and 20 microM, respectively. In human cryo-preserved hepatocytes, the IC(50) values for fluoxetine, paroxetine and fluvoxamine were 0.7, 0.5 and 5.9 microM, respectively. In conclusion, compounds known to be potent CYP2D6 inhibitors inhibited timolol metabolism in in vitro experiments. The present results strongly suggest that fluoxetine and paroxetine may significantly affect the metabolism of timolol also in vivo and may thus potentiate the adverse cardiovascular effects of topically administered timolol.

Prognosis in Heart Failure and the Value of β-Blockers Are Altered by the Use of Antidepressants and Depend on the Type of Antidepressants Used

Background— Depression worsens the prognosis in patients with cardiac disease, and treatment with antidepressants may improve survival. Guidelines recommend use of selective serotonin reuptake inhibitors (SSRIs), but knowledge of the prognostic effect of different classes of antidepressants is sparse.

Methods and Results— We studied 99 335 patients surviving first hospitalization for heart failure (HF) from 1997 to 2005. Use of HF medication and antidepressants (divided into tricyclic antidepressants [TCA] and SSRI) was determined by prescription claims. Risk of overall and cardiovascular death associated with antidepressants, HF medication, and coadministration of these 2 drug classes was estimated by Cox proportional hazard analyses. Propensity adjusted models were performed as sensitivity analysis. During the study period, there were 53 988 deaths, of which 83.0% were due to cardiovascular causes (median follow-up, 1.9 years; 5, 95% fractiles, 0.04 to 7.06 years). Use of β-blockers was associated with decreased risk of cardiovascular death (hazard ratio [HR], 0.77; 95% CI, 0.75 to 0.79). Antidepressants were prescribed to 19 411 patients, and both TCA and SSRI were associated with increased risk of overall and cardiovascular death (TCA: HR, 1.33; CI, 1.26 to 1.40; and HR, 1.25; CI, 1.17 to 1.32; SSRI: HR, 1.37; CI, 1.34 to 1.40; and HR, 1.34; CI, 1.30 to 1.38, respectively). Coadministration of SSRI and β-blockers was associated with a higher risk of overall and cardiovascular death compared with coadministration of β-blockers and TCA ( P for interaction <0.01).

Conclusions— Use of antidepressants in patients with HF was associated with worse prognosis. Coadministration of SSRIs and β-blockers was associated with increased risk of overall death and cardiovascular death compared with coadministration of TCAs and β-blockers. To further clarify this, clinical trials testing the optimal antidepressant strategy in patients with HF are warranted.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

CYP2D6 genotype and its relationship with metoprolol dose, concentrations and effect in patients with systolic heart failure

The aims of this study were to examine the relationships between CYP2D6 genotype and metoprolol dose, S- and R-metoprolol concentrations and clinical effects in patients with systolic heart failure. Data were obtained for 52 subjects, of which 27 had 2 functional alleles (24/27, CYP2D6*1/*1), 22 had 1 functional allele (18/22, CYP2D6*1/*4) and 3 had no functional alleles (CYP2D6*4/*4). Median dose-adjusted concentrations of S-metoprolol (active) were 6.3- and 3.2-fold higher in subjects with zero or one functional allele (P=0.016 and P=0.006), respectively, compared with subjects with two functional alleles. For the R-enantiomer (inactive), these concentrations were 10.7- and 3.7-fold higher (P=0.013 and P=0.003), respectively. Despite clear gene-concentration differences, no relationships between CYP2D6 genotype and dose or clinical effects could be shown. Although the number with no functional alleles was too small (n=3) to show effects, in patients with 1 functional allele other sources of variance are likely to be obscuring differences in clinical effects.

Screening of drug metabolism by CE

The use of CE for rapid assessment of metabolic stability of drugs with cytochrome P450 (CYP) enzymes, based on relative rates of reduced nicotinamide adenine dinucleotide phosphate (NADPH) consumption and nicotinamide adenine dinucleotide phosphate (NADP) production, was investigated. The separation conditions were as follows: capillary, 80.5 cm (75 microm id, 72 cm effective length for UV detection, 58 cm effective length for fluorescence detection); 25 mM sodium phosphate buffer (pH 8.8); 28 kV (80 microA) applied voltage; UV, 260 nm; fluorescence detection, excitation wavelength, 310 nm, emission wavelength, 418 nm; capillary temperature, 25 degrees C. For UV detection, the incubation conditions were as follows: CYP3A4: 20 pmol/mL; NADPH: 1 mM; EDTA: 1 mM; concentration of the substrate: 5-10 times its reported literature K(m) value; temperature: 37 degrees C; incubation time: 15 min. For fluorescence detection, the concentrations were reduced to CYP3A4: 4 pmol/mL, NADPH: 20 microM, EDTA: 20 microM and substrate: 10 microM. Blank incubations were performed in the absence of substrate. Compared with the blank, significant differences were found for the consumption of NADPH and the production of NADP. The development of this assay system allows rapid assessment of metabolic stability relative to standard compounds, as well as potential identification of the major CYP involved in the metabolism. It would reduce the backlog of compounds that require LC/MS analysis, and thereby expedite the process of metabolic stability screening.

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.

Effect of Amiodarone on the Serum Concentration/Dose Ratio of Metoprolol in Patients with Cardiac Arrhythmia

Amiodarone has pharmacokinetic interactions with a number of therapeutic drugs, including warfarin, phenytoin, flecainide, and cyclosporine. Metoprolol is mainly metabolized by CYP2D6, and desethylamiodarone, a metabolite of amiodarone, has a markedly greater inhibitory effect on CYP2D6 than amiodarone. Therefore, the goal of this study was to evaluate the effect of amiodarone and desethylamiodarone on the serum concentration/dose ratio (C/D) of metoprolol in 120 inpatients with cardiac arrhythmias that received either metoprolol and amiodarone (MET+AMD group, n=30) or metoprolol alone (MET group, n=90). The ratio of administered metoprolol was compared between the MET and the MET+AMD groups. The dose of metoprolol and patient age were significantly higher in the MET group when compared with the MET+AMD group (1.00+/-0.480 versus 0.767+/-0.418 mg/kg/day, p<0.050; 68.6+/-10.6 versus 57.6+/-14.1 years, p<0.001, respectively), but the C/D ratio was significantly lower in the MET group than in the MET+AMD group (90.8+/-64.0 versus 136+/-97.8, p<0.01). Furthermore, a significant correlation was found between the C/D ratio and desethylamiodarone concentration (n=30, r=0.371, p<0.01). The results suggest that there is a significant interaction between amiodarone and metoprolol via desethylamiodarone-induced inhibition of CYP2D6. Therefore, careful monitoring of metoprolol concentrations/bioactivity of CYP2D6 is required in the context of co-administration of amiodarone and metoprolol.

Improvement of Dry Mouth by Replacing Paroxetine with Fluvoxamine

OBJECTIVE:

To present a case of improvement of paroxetine-induced dry mouth by substitution of fluvoxamine and analyze this case based on receptor occupancy theory.

CASE SUMMARY:

A 66-year-old woman with major depressive disorder had been treated with brotizolam 0.5 mg/day, flunitrazepam 2 mg/day, sulpiride 100 mg/day, bromazepam 2 mg/day, trazodone 25 mg/day, and paroxetine hydrochloride 10 mg/day. Although her psychological symptoms improved gradually, she complained of dry mouth. Paroxetine was replaced with fluvoxamine maleate 50 mg/day, and the dryness disappeared within a month.

DISCUSSION:

We calculated the time courses of muscarinic acetylcholine (mACh) receptor occupancy after oral administration of paroxetine and fluvoxamine at the treatment doses by using pharmacokinetic parameters obtained from the literature. The mACh receptor occupancy was estimated to be decreased from 0.22% to 0.020% by replacing paroxetine with fluvoxamine.

CONCLUSIONS:

The improvement of dry mouth observed after the replacement of paroxetine with fluvoxamine in this patient may have been due to a decrease in the mACh receptor occupancy.

Predicting drug clearance from recombinantly expressed CYPs: intersystem extrapolation factors

1. Recombinantly expressed human cytochromes P450 (rhCYPs) have been underused for the prediction of human drug clearance (CL). 2. Differences in intrinsic activity (per unit CYP) between rhCYP and human liver enzymes complicate the issue and these discrepancies have not been investigated systematically. We define intersystem extrapolation factors (ISEFs) that allow the use of rhCYP data for the in vitro-in vivo extrapolation of human drug CL and the variance that is associated with interindividual variation of CYP abundance due to genetic and environmental effects. 3. A large database (n = 451) of metabolic stability data has been compiled and used to derive ISEFs for the most commonly used expression systems and CYP enzymes. 4. Statistical models were constructed for the ISEFs to determine major covariates in order to optimize experimental design to increase prediction accuracy. 5. Suggestions have been made for the conduct of future studies using rhCYP to predict human drug clearance.

Antihypertensive therapy: special focus on drug interactions

Today, the lifetime risk of patients aged 55-65 years to receive antihypertensive drugs approaches 60%. Yet, recent trials suggest that hypertension is not adequately controlled in the majority of patients. The prevalence of hypertension increases with advancing age, as does the prevalence of comorbid conditions and the total number of medications taken. Multi-drug therapy, advancing age and comorbid conditions are also key risk factors for adverse drug reactions and drug interactions. In this review, the authors evaluate the most frequently used antihypertensive drugs (diuretics, beta-adrenergic blockers, angiotensin-converting enzyme inhibitors, calcium channel blockers, angiotensin II receptor Type 1 blockers and alpha-adrenergic blockers) with special reference to pharmacodynamic and pharmacokinetic drug interactions. As the spectrum of drugs prescribed is constantly changing, safety yesterday does not imply safety today and safety today does not imply safety tomorrow. Furthermore, therapeutic efficacy should not be neglected over concerns regarding drug interactions. Many patients are at risk of clinically relevant drug interactions involving antihypertensive drugs but, presently, even more patients may be at risk of suffering from the consequences of their inadequately treated hypertension. In this respect, the authors discuss controversial viewpoints on the overall clinical relevance of drug interactions occurring at the level of cytochrome P450 metabolism.

Comparative molecular field analysis and QSAR on substrates binding to cytochrome P450 2D6

In this study, we utilized comparative molecular field analysis (CoMFA) to gain a better understanding of the steric and electrostatic features of the cytochrome p450 2D6 (CYP2D6) active site. The training set consists of 24 substrates with reported K(M) values from liver microsomal CYP2D6 spanning an activity range of almost three log units. The low energy conformers were fit by root mean square (RMS) to minaprine at the site of metabolism and to the protonated nitrogen. In this manner, we constructed two CoMFA models, one model with a distance constraint and another without. The model with the distance parameter (non-cross-validated R(2)=0.99) was approximately equal to the CoMFA without a distance parameter (non-cross-validated R(2)=0.98). Validation of our CoMFA was accomplished by predicting the K(M) values of 15 diverse CYP2D6 substrates not in the original training set resulting in a predictive R(2)=0.62. Finally, we also pursued correlations of pK(a) and log P with CYP2D6 substrate K(M) in an effort to investigate other physicochemical properties.

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.

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.

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.

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

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

Selective serotonin reuptake inhibitors for late-life depression: a comparative review

Late-life depression is a serious health problem that is challenging to manage but generally responds well to pharmacotherapy. Selective serotonin (5-hydroxytryptamine: 5-HT) reuptake inhibitors (SSRIs), the most commonly prescribed antidepressants, are usually used as first-line agents for elderly patients with depression. Like most drugs, SSRIs have not been widely tested in clinical trials that approximate 'real-life' geriatric situations. However, studies completed to date provide valuable information about the efficacy, safety and tolerability of this class of antidepressants among older patients with depression, including those with depression secondary to stroke or dementia and those with other comorbid physical disorders. Although one SSRI may be more efficacious or better tolerated by elderly patients than another, existing data do not support such claims. However, other distinguishing features may influence the choice of agent. For example, fluoxetine, fluvoxamine and paroxetine are more likely to be involved in significant drug-drug interactions than are citalopram or sertraline. In contrast to the other SSRIs, fluoxetine has a half-life well in excess of 1 day, which can be an advantage when weaning the patient off therapy in that it may reduce the incidence of discontinuation symptoms, but a significant disadvantage if the patient cannot tolerate the drug or experiences an adverse drug-drug interaction.

Serum concentrations of fluoxetine in the clinical treatment setting

This article discusses fluoxetine serum concentrations as displayed in a clinical setting. A racemic serum fluoxetine and norfluoxetine high-performance liquid chromatography method, including ultraviolet light detection, was used for routine therapeutic drug monitoring (TDM) purposes. In all, 508 samples were analyzed. For the scientific investigation, predefined inclusion and exclusion criteria were applied and 150 samples representative of trough values in steady-state conditions with essential clinical information provided on the assay request forms were evaluated. Fluoxetine plus norfluoxetine concentration-to-dose (C/D) ratio showed Gaussian distribution. Interindividual coefficients of variation of fluoxetine and norfluoxetine serum concentrations after different doses were found to be 40-63%. Intraindividual fluoxetine TDM variability was low. The Spearman rank correlation coefficient for fluoxetine and norfluoxetine C/D ratios in first and second samples was 0.68. Minor increases in norfluoxetine C/D and fluoxetine plus norfluoxetine C/D ratios were found in elderly patients compared with younger adult patients. A higher body-mass index was associated with minor decreases in fluoxetine and fluoxetine plus norfluoxetine C/D ratios. New fluoxetine pharmacokinetic data are added to the results from earlier phases of drug development. Moreover, the results of this study support the usefulness of a fluoxetine TDM procedure for individual dose optimization, detection of drug interactions, and assessments of patient compliance.

Selecting a Selective Serotonin Reuptake Inhibitor: Clinically Important Distinguishing Features

Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed to treat depression. Although these drugs presumably have the same mechanism of action, they vary in several clinically important ways, including how long they remain in the body and the extent to which they interfere with the metabolism of other medications. This article reviews the pharmacologic differences among SSRIs and how these differences may affect various aspects of treatment, such as dosing, administration, and discontinuation. Understanding the distinct properties of SSRIs may help primary care physicians to design the most appropriate therapeutic plan for individual patients.

Metoprolol: a review of its use in chronic heart failure

Metoprolol, a relatively selective beta1-blocker, is devoid of intrinsic sympathomimetic activity and possesses weak membrane stabilising activity. The drug has an established role in the management of essential hypertension and angina pectoris, and more recently, in patients with chronic heart failure. The effects of metoprolol controlled-release/extended-release (CR/XL) in patients with stable, predominantly mild to moderate (NYHA functional class II to III) chronic heart failure have been evaluated in the large Metoprolol CR/XL Randomised Intervention Trial in Congestive Heart Failure (MERIT-HF) trial and the much smaller Randomized Evaluation of Strategies for Left Ventricular Dysfunction (RESOLVD) pilot study. Treatment with metoprolol CR/XL was initiated at a low dosage of 12.5 to 25 mg once daily and gradually increased at 2-weekly intervals until the target dosage (200 mg once daily) or maximal tolerated dosage had been attained in patients receiving standard therapy for heart failure. At 12 months, metoprolol CR/XL was associated with a 34% reduction in relative risk of all-cause mortality in patients with chronic heart failure due to ischaemic or dilated cardiomyopathy in the MERIT-HF trial. The incidence of sudden death and death due to progressive heart failure were both significantly decreased with metoprolol CR/XL. Similarly, a trend towards decreased mortality in the metoprolol CR/XL group compared with placebo was observed in the RESOLVD trial. Data from small numbers of patients with severe (NYHA functional class IV) heart failure indicate that metoprolol CR/XL is effective in this subset of patients. However, no firm conclusions can yet be drawn. Improvement from baseline values in NYHA functional class, exercise capacity and some measures of quality of life with metoprolol CR/XL or immediate-release metoprolol were significantly greater than those with placebo. The drug is well tolerated when treatment is initiated in low dosages and gradually increased at intervals of 1 to 2 weeks.

CONCLUSIONS

Metoprolol CR/XL effectively decreases mortality and improves clinical status in patients with stable mild to moderate (NYHA functional class II or III) chronic heart failure due to left ventricular systolic dysfunction, and the drug is effective in patients with ischaemic or dilated cardiomyopathy. Although limited data indicate that metoprolol CR/XL is effective in patients with severe (NYHA functional class IV) chronic heart failure, more data are needed to confirm these findings. Treatment with metoprolol CR/XL significantly reduced the incidence of sudden death and death due to progressive heart failure.

Effect of selective serotonin reuptake inhibitors on the oxidative metabolism of propafenone: in vitro studies using human liver microsomes

Propafenone is mainly metabolized by CYP2D6 to form 5-hydroxypropafenone (5-OHP) and to a minor extent by CYP1A2 and CYP3A4 to form N-depropylpropafenone (N-DPP). The in vitro inhibitory effect of selective serotonin reuptake inhibitors (SSRIs) on the formation of both metabolites was studied, using human liver microsomes. The 5-OHP formation from racemic propafenone and from its individual enantiomers followed one-enzyme Michaelis-Menten kinetics. Incubation with the racemate yielded a mean Vmax of 64 pmol x min(-1) x mg(-1) and a mean Km of 0.12 microM (N = 3). Stereoselectivity in Vmax and Km values was observed, with (S)-propafenone displaying higher Km and Vmax values. N-DPP formation from racemic propafenone followed one-enzyme Michaelis-Menten kinetics and yielded a mean Vmax of 403 pmol x min(-1) x mg(-1) and a mean Km of 116 microM (N = 3). No stereoselectivity in propafenone N-dealkylation was observed. The influence of SSRIs and quinidine, a prototypical CYP2D6 inhbitor, on propafenone 5-hydroxylation was investigated. Quinidine was the most potent inhibitor, followed by fluoxetine, norfluoxetine, and paroxetine. Sertraline, desmethylsertraline, and fluvoxamine had only a moderate inhibitory effect, whereas citalopram displayed slight or no inhibition when racemic propafenone was used as substrate. Mean Ki values of quinidine, fluoxetine, norfluoxetine, and paroxetine were 0.13, 0.33, 0.55, and 0.54 microM, respectively (N = 3). Quinidine and paroxetine were also tested as inhibitors using the individual enantiomers, but no stereoselectivity was observed. Among the SSRIs tested, only fluvoxamine substantially inhbited propafenone N-dealkylation with a mean IC50 of 7.0 microM (N = 3). There was a more pronounced inhibitory effect of fluvoxamine on (R)-propafenone than on (S)-propafenone N-dealkylation. In conclusion, these in vitro data suggest that an in vivo interaction between propafenone and the SSRIs, fluoxetine and paroxetine, can be expected, which can lead to clinically relevant beta-blockade and an increased risk of side effects in the central nervous system. An interaction with fluvoxamine may be of importance in poor metabolizers for CYP2D6.

Effects of the antifungal agents on oxidative drug metabolism: clinical relevance

This article reviews the metabolic pharmacokinetic drug-drug interactions with the systemic antifungal agents: the azoles ketoconazole, miconazole, itraconazole and fluconazole, the allylamine terbinafine and the sulfonamide sulfamethoxazole. The majority of these interactions are metabolic and are caused by inhibition of cytochrome P450 (CYP)-mediated hepatic and/or small intestinal metabolism of coadministered drugs. Human liver microsomal studies in vitro, clinical case reports and controlled pharmacokinetic interaction studies in patients or healthy volunteers are reviewed. A brief overview of the CYP system and the contrasting effects of the antifungal agents on the different human drug-metabolising CYP isoforms is followed by discussion of the role of P-glycoprotein in presystemic extraction and the modulation of its function by the antifungal agents. Methods used for in vitro drug interaction studies and in vitro-in vivo scaling are then discussed, with specific emphasis on the azole antifungals. Ketoconazole and itraconazole are potent inhibitors of the major drug-metabolising CYP isoform in humans, CYP3A4. Coadministration of these drugs with CYP3A substrates such as cyclosporin, tacrolimus, alprazolam, triazolam, midazolam, nifedipine, felodipine, simvastatin, lovastatin, vincristine, terfenadine or astemizole can result in clinically significant drug interactions, some of which can be life-threatening. The interactions of ketoconazole with cyclosporin and tacrolimus have been applied for therapeutic purposes to allow a lower dosage and cost of the immunosuppressant and a reduced risk of fungal infections. The potency of fluconazole as a CYP3A4 inhibitor is much lower. Thus, clinical interactions of CYP3A substrates with this azole derivative are of lesser magnitude, and are generally observed only with fluconazole dosages of > or =200 mg/day. Fluconazole, miconazole and sulfamethoxazole are potent inhibitors of CYP2C9. Coadministration of phenytoin, warfarin, sulfamethoxazole and losartan with fluconazole results in clinically significant drug interactions. Fluconazole is a potent inhibitor of CYP2C19 in vitro, although the clinical significance of this has not been investigated. No clinically significant drug interactions have been predicted or documented between the azoles and drugs that are primarily metabolised by CYP1A2, 2D6 or 2E1. Terbinafine is a potent inhibitor of CYP2D6 and may cause clinically significant interactions with coadministered substrates of this isoform, such as nortriptyline, desipramine, perphenazine, metoprolol, encainide and propafenone. On the basis of the existing in vitro and in vivo data, drug interactions of terbinafine with substrates of other CYP isoforms are unlikely.

Human cytochromes and some newer antidepressants: kinetics, metabolism, and drug interactions

The appearance of selective serotonin reuptake inhibitor antidepressants in the mid-1980s caused the discipline of clinical psychopharmacology to refocus attention to the topics of drug metabolism and drug interactions. This article reviews the metabolic profiles of some newer antidepressants, the clinical implications of metabolic properties, and research methodology that can be applied in determining which specific human cytochromes P450 (CYP) mediate metabolic pathways. Also reviewed are the relative activities of various new antidepressants as inhibitors of CYPs, and the benefits and drawbacks of in vivo and in vitro methodologies for identification and quantitation of drug interactions.

Citalopram and desmethylcitalopram in vitro: human cytochromes mediating transformation, and cytochrome inhibitory effects

BACKGROUND

Biotransformation of citalopram (CT), a newly available selective serotonin reuptake inhibitor antidepressant, to its principal metabolite, desmethycitalopram (DCT), and the capacity of CT and DCT to inhibit human cytochromes P450, were studied in vitro.

METHODS

Formation of DCT from CT was evaluated using human liver microsomes and microsomes from cDNA-transfected human lymphoblastoid cells. Cytochrome inhibition by CT and DCT in liver microsomes was studied using isoform-specific index reactions.

RESULTS

Formation of DCT from CT in liver microsomes had a mean apparent K(m) of 174 mumol/L. Coincubation with 1 mumol/L ketoconazole reduced reaction velocity to 46 to 58% of control values, while omeprazole, 10 mumol/L, reduced velocity to 80% of control. Quinidine produced minimal inhibition. DCT was formed from CT by heterologously expressed human P450-2D6, -2C19, -3A4. After accounting for the relative abundance of individual cytochromes, 3A4 and 2C19 were estimated to make major contributions to net reaction velocity, with a possible contribution of 2D6 at therapeutic CT concentrations. CT and DCT themselves produced negligible inhibition of 2C9, 2E1, and 3A, and only weak inhibition of 1A2, 2C19, and 2D6.

CONCLUSIONS

Formation of DCT from CT is mediated mainly by P450-3A4 and 2C19, with an additional contribution of 2D6. CT at therapeutic doses in humans may produce a small degree of inhibition of P450-1A2, -2C19, and -2D6, but negligible inhibition of P450-2C9, -2E1, and -3A.