Metabolism of some "second"- and "fourth"-generation antidepressants: iprindole, viloxazine, bupropion, mianserin, maprotiline, trazodone, nefazodone, and venlafaxine

1. This review summarizes the major known aspects of the metabolism of second-generation (iprindole, viloxazine, bupropion, mianserin, maprotiline, and trazodone) and fourth-generation (nefazodone and venlafaxine) antidepressants. 2. Discussions about specific enzymes involved and about possible pharmacokinetic drug-drug interactions, particularly as they relate to cytochrome P450 enzymes, are provided.

Metabolism and pharmacokinetics of selective serotonin reuptake inhibitors

1. Five drugs with the predominant pharmacologic effect of inhibiting the neuronal reuptake of serotonin are available worldwide for clinical use. This class of psychoactive drugs, known as selective serotonin reuptake inhibitors (SSRIs), is comprised of fluoxetine, sertraline, paroxetine, fluvoxamine, and citalopram. 2. The SSRIs appear to share similar pharmacodynamic properties which translate to efficacy in the treatment of depression and anxiety syndromes. The drugs are differentiated by their pharmacokinetic properties with regard to stereochemistry, metabolism, inhibition of cytochrome enzymes, and participation in drug-drug interactions. Studies focusing on the relationship of plasma drug concentration to therapeutic and adverse effects have not confirmed the value of plasma concentration monitoring. 3. This review summarizes the metabolism and relevant pharmacokinetic properties of the SSRIs.

[Toxicologic findings in suicide with doxepin and paroxetine]

A young nurse was found dead in her flat. In chemical-toxicological analysis the following femoral blood drug concentrations were determined: paroxetine 0.176 mg/l, doxepine 82.12 mg/l, desmethyldoxepine 0.34 mg/l. Additionally the drug concentrations were determined in various body fluids and organs. The results of the described fatality are discussed. For interpretation of toxicologic results in antidepressant fatalities ratios of parent drug to metabolite and postmortem drug redistribution should be taken into account.

Transport mechanisms for the antidepressant citalopram in brain microvessel endothelium

Blood-brain barrier transport of the selective serotonin reuptake inhibitor and antidepressant, citalopram, was studied using monolayers of bovine brain microvessel endothelial cells (BMECs). This study provides for the first time, evidence of a transport mechanism for a selective serotonin reuptake inhibitor (SSRI). Carrier-mediated transport, efflux mechanisms, as well as inhibition of metabolizing enzymes of citalopram were investigated. Citalopram transport was saturable and temperature-dependent suggesting that passage of the drug across BMECs was mediated by a carrier mechanism. Since the apical to basolateral and basolateral to apical permeability coefficients were similar and cyclosporin A, a P-glycoprotein inhibitor, does not modify the transport of citalopram, it appeared that no active efflux systems were involved in this transport. Citalopram is only available as a racemic drug and its pharmacological effect resides mainly in the S-(+)-enantiomer. However, the passage of citalopram enantiomers across BMEC monolayers was not stereoselective. Finally, inhibition of the metabolizing enzymes of citalopram and monoamine oxidases did not modify the permeation of citalopram across BMECs. Collectively, our results suggested that citalopram crosses the blood-brain barrier via a non-stereoselective, bidirectional and symmetrical carrier-mediated mechanism without influences of active efflux mechanisms or monoamine oxidases.

Pharmacokinetic consequences of a citalopram treatment discontinuation

In this pilot study, the pharmacokinetics of citalopram (CIT) were examined in five hospitalized depressed patients after an abrupt discontinuation of a treatment with 40 mg/d of this selective serotonin reuptake inhibitor (SSRI). During the 8-day study period, clinical ratings were regularly carried out. Between days 5 and 8, the patients were treated with clomipramine (75 mg/d). The enantiomers of CIT and its metabolites, demethyl-CIT (DCIT) and CIT-propionic acid derivative (CIT-PROP), were measured repeatedly from day 0 to day 8 by a stereoselective high-performance liquid chromatography (HPLC) procedure. The following drug plasma half-lives were measured (means +/- SD): R-CIT: 66+/-11 h; S-CIT: 42+/-13 h; R-DCIT: 228+/-148 h; S-DCIT: 93+/-35 h; R-CIT-PROP: 82+/-31 h; S-CIT-PROP: 186+/-93 h.

Paroxetine and its uses in psychiatry

Paroxetine is one of the specific serotonin-reuptake inhibitor antidepressants which is used in a variety of psychiatric disorders. It has recently gained considerable publicity because of its use in social anxiety disorder and its subsequent labelling by the media as a 'lifestyle drug'. This review summarizes current indications for paroxetine and outlines doses and duration of treatment for each condition.

Cardiovascular and neurological toxicity of venlafaxine

1. We report a case of venlafaxine overdose and describe pharmacokinetic data on drug disposition. 2. Case report. Serial venlafaxine levels were measured and drug half-life calculated and compared to data at therapeutic concentrations. Metabolite concentrations were also measured and the potential for toxicity described with reference to individual variation in such metabolism 3. Venlafaxine can cause significant cardiac and neurotoxicity. Its potential for causing such toxicity may be dependent on whether an individual has the extensive metaboliser cytochrome CYP2D6 phenotype or the poor metaboliser phenotype

Effects of carbamazepine coadministration on plasma concentrations of the enantiomers of mianserin and of its metabolites

Concentrations of the enantiomers of unconjugated and of total (unconjugated plus conjugated) mianserin, desmethylmianserin and 8-hydroxymianserin were measured in 12 patients before and after the introduction of carbamazepine. The dose of mianserin was 60 mg/d, carbamazepine was coadministered at 400 mg/d for 4 weeks, and blood samples were taken at weekly intervals after the introduction of carbamazepine. Each week, carbamazepine significantly decreased plasma concentrations of unconjugated and total (S)-mianserin (the more potent enantiomer) and of unconjugated and total (R)-mianserin. On average, plasma concentrations of unconjugated and total (S)-mianserin and of unconjugated and total (R)-mianserin were 55%, 56%, 66%, and 55%, respectively, of the corresponding values before introduction of carbamazepine. These results strongly suggest the involvement of CYP3A4, the major CYP enzyme induced by carbamazepine, in the metabolism of both enantiomers of mianserin. A strong decrease in the concentrations of (S)-8-hydroxymianserin was also measured (on average, the concentrations were 69% of the corresponding values before carbamazepine introduction). Conversely, plasma concentrations of unconjugated and of total (S)-desmethylmianserin, (R)-desmethylmianserin, and (R)-8-hydroxymianserin were only slightly modified by carbamazepine. From a clinical point of view, as a therapeutic window for (S)-mianserin has been recently suggested, the dose of racemic mianserin for a patient whose (S)-mianserin concentrations have been stabilized within this therapeutic window would need to be approximately doubled if carbamazepine, at 400 mg/d, is introduced as a comedication.

Effect of the CYP2D6*10 genotype on venlafaxine pharmacokinetics in healthy adult volunteers

AIMS

Interindividual differences in the pharmacokinetics of venlafaxine, a new antidepressant, were shown during early clinical trials in Japan. Venlafaxine is metabolized mainly by CYP2D6 to an active metabolite, O-desmethylvenlafaxine (ODV). Therefore, the influence of the CYP2D6 genotypes on venlafaxine pharmacokinetics was examined in a Japanese population.

METHODS

Twelve adult Japanese men in good health participated in this study. Genomic DNA was isolated from peripheral lymphocytes, and the CYP2D6 genotypes were determined by codon 188C/T, 1934G/A, 2938G/A and 4268G/C mutations using endonuclease tests based on PCR and by Xba I-RFLP analysis. Subjects were categorized into the following 3 groups (n=4 in each group); Group1: CYP2D6*10/*10, *5/*10, Group2: CYP2D6*1/*10, *2/*10 and Group3: CYP2D6*1/*1, CYP2D6*1/*2. Venlafaxine (25 mg, n=6; 37.5 mg, n=6) was administered orally at 09.00 h following an overnight fast. Plasma concentrations of venlafaxine and ODV were monitored by h.p.l.c. for 48 h.

RESULTS

The Cmax and AUC of venlafaxine were 184% and 484% higher in the group 1 subjects than in the group 3 subjects, and 101% and 203% higher in the group 1 than in the group 2, respectively.

CONCLUSIONS

These results suggest that CYP2D6*10 influences the pharmacokinetics of venlafaxine in a Japanese population.

Effects of trazodone on polysomnography, blood concentration and core body temperature in healthy volunteers

Polysomnography, blood concentration and core body temperature recordings were performed on 12 healthy volunteers with administration of trazodone and placebo. Trazodone increased slow wave sleep (SWS), and decreased the average, the highest and lowest core body temperature significantly compared to placebo. The blood concentration of trazodone correlated positively with amplitude (the difference between the highest and lowest temperature) and %SWS during the first period of a sleep phase divided into three periods, and negatively with the lowest temperature. The appearance time of the lowest temperature correlated negatively with %SWS.

Determination of paroxetine in plasma by high-performance liquid chromatography for bioequivalence studies

A high-performance liquid chromatographic method is described for the determination of paroxetine in human plasma. Dibucaine was used as the internal standard. Paroxetine was isolated by solid phase extraction using a Bond-Elut C18 extraction column. Separation was obtained using a reversed-phase column under isocratic conditions with fluorescence detection. The sample volume was 500 microliters of plasma. The intra- and inter-assay accuracy and precision, determined as relative error and relative standard deviation, respectively, were less than 10%. The lower limit of quantitation, based on standards with acceptable relative error and relative standard deviation, was 10 ng ml-1. No endogenous compounds were found to interfere. The linearity was assessed in the range 5-100 ng ml-1. Stability of paroxetine during processing (autosampler) and in plasma was checked. This method proved suitable for bioequivalence studies following multiple doses in healthy volunteers.

Pharmacokinetic evaluation of co-administration of nefazodone and lithium in healthy subjects

OBJECTIVES

To evaluate the possible pharmacokinetic interaction between nefazodone and lithium.

METHODS

Twelve healthy volunteers received nefazodone 200 mg b.i.d. for 5 days. A 4-day washout phase followed from day 6 to day 9. From day 10 to day 20, escalating doses of lithium 250 mg b.i.d. to 500 mg b.i.d. were given; the daily dose of 1000 mg was obtained on day 13. From day 16 to day 20, nefazodone 200 mg b.i.d. was added to the lithium dosing regimen. Venous blood sampling was performed on days 5, 15 and 20 for 0- to 48-h-pharmacokinetic analysis. Nefazodone and its metabolites, hydroxynefazodone, mCPP and triazoledione were assayed by high-performance liquid chromatography (HPLC). Lithium was assayed by flame photometry.

RESULTS

Co-administration of nefazodone did not modify pharmacokinetic parameters of lithium at steady-state. Comparison of the area under the plasma or serum concentration-versus-time curve calculated from 0-12 h (AUC0-12) of nefazodone and hydroxynefazodone revealed no significant differences when nefazodone was administered alone or with lithium. The mean maximum peak plasma concentration Cmax and AUC0-12 of meta-chlorophenyl-piperazine (mCPP) were significantly reduced by 27% (P < 0.001) and 16% (P < 0.001) with the co-administration. The mean Cmax and AUC0-12 of triazoledione were reduced by 23% (P < 0.005) and 16% (P < 0.01) by the co-administration.

CONCLUSION

Since there were no clinically significant changes in the pharmacokinetics of the parent compounds or metabolites, and the combination was well tolerated, no dosage adjustments of nefazodone or lithium are necessary when they are co-administered.

Zyban: a new aid to smoking cessation treatment--will it work for psychiatric patients?

1. Psychiatric patients who smoke are more highly dependent on nicotine and are less likely to be successful at quitting. When they attempt to quit, depression is more likely to be a part of the withdrawal process. 2. Careful monitoring is required when psychiatric patients abruptly start or stop smoking. Clinicians must be aware that changes in smoking status will affect the actions and side effects of neuroleptic medications. Prescribing clinicians should ask about their patients' smoking patterns on a regular basis. 3. Psychiatric nurses have an ethical responsibility to identify individuals at risk of smoking-related disease, to inform current and future smokers about the risks of cigarette smoking, and to provide counseling and education for those who wish to quit.

Altered [3H]imipramine and 5-HT2 but not [3H]paroxetine binding sites in platelets from depressed patients

BACKGROUND

Serotonergic system alterations were studied in 51 depressed patients classified according to DSM-III-R criteria for major depression with melancholia compared to 31 healthy controls.

METHOD

[3H]Imipramine and [3H]paroxetine binding sites and the 5HT2 receptor were simultaneously determined in blood platelet membranes.

RESULTS

A significantly lower maximum binding in [3H]imipramine binding was observed in depressed patients compared to controls (1134+/-74 vs. 1712+/-106 fmol/mg protein, P<0.0001) without changes in the equilibrium dissociation constant (1.10+0.05 vs. 1.25-/+0.09 nM). [3H]Paroxetine binding did not differ between the two groups (Bmax, 1441+/-55 vs. 1280+/-81 fmol/mg protein; Kd, 0.060+/-0.002 vs. 0.062+/-0.002 nM). The K(d) value of 5HT2 binding was lower in depressed patients than controls (0.95+/-0.04 vs. 1.15+/-0.09 nM, P<0.039) without changes in maximum binding (140+/-11 vs. 127+/-14 fmol/mg protein).

CONCLUSIONS

Taken together, these results suggest that [3H]imipramine and 5HT2 receptors may be good biological markers for serotonergic dysfunction in depressive disorders.

Review of the results from clinical studies on the efficacy, safety and tolerability of mirtazapine for the treatment of patients with major depression

Mirtazapine is a presynaptic alpha-2 antagonist that has dual action by increasing noradrenergic and serotonergic neurotransmission. The enhancement of serotonergic neurotransmission is specifically mediated via 5-HT1 receptors because mirtazapine is a postsynaptic serotonergic 5-HT2 and 5-HT3 antagonist. In addition, mirtazapine has only a weak affinity for 5-HT1 receptors and has very weak muscarinic anticholinergic and histamine (H1) antagonist properties. As a consequence of its unique pharmacodynamic properties, mirtazapine is an effective, safe and well-tolerated addition to the antidepressant armamentarium. Mirtazapine is well absorbed from the gastrointestinal tract following oral administration, and it is extensively metabolized in the liver to four metabolites via demethylation and hydroxylation, followed by glucuronide conjugation. The unconjugated desmethyl metabolite is pharmacologically less active than the parent compound. Mirtazapine lacks auto-induction of hepatic isoenzymes. Although mirtazapine is a substrate of P450 isoenzymes 1A2, 2D6 and 3A4, in vitro studies show that it is not a potent inhibitor or inducer of any of these enzymes. Mirtazapine has been evaluated in a worldwide clinical development program involving approximately 4500 patients. Controlled clinical trials involving almost 2800 mirtazapine-treated patients have demonstrated the compound to be effective for the treatment of moderate-to-serve major depression. Mirtazapine was consistently superior to placebo, and equivalent in efficacy to the tricyclic antidepressants amitriptyline, doxepin and clomipramine, but with an improved tolerability profile. Mirtazapine has shown a rapid onset of action in patients with predominantly severe depressive illness in a comparative study against fluoxetine. Mirtazapine has a unique tolerability profile, since the specific postsynaptic 5-HT2 and 5-HT3 receptor blockade of mirtazapine provides early antidepressant effects without causing unwanted serotonin-related side-effects. Transient somnolence, hyperphagia and weight gain are the most commonly reported adverse events, which may be attributed to the antihistaminic (H1) activity of mirtazapine at low doses. Somnolence, the most commonly reported side-effect, appears to be less frequent at higher dosages. Mirtazapine also demonstrates important anxiolytic and sleep-improving effects, which may be related to its pharmacodynamic properties. In addition, mirtazapine does not appear to be associated with sexual dysfunction. Mirtazapine has shown no significant cardiovascular adverse effects at multiples of 7 to 22 times the maximum recommended dose. Mirtazapine is a unique addition to the antidepressant armamentarium as first-line therapy in patients with major depression and symptoms of anxiety/agitation or anxiety/somatization or complaints of insomnia and as a useful alternative in depressed patients who do not adequately respond to or are intolerant of tricyclic antidepressants or serotonin-specific reuptake inhibitors.

Analysis of the enantiomers of citalopram and its demethylated metabolites using chiral liquid chromatography

A procedure using a chirobiotic V column is presented which allows separation of the enantiomers of citalopram and its two N-demethylated metabolites, and of the internal standard, alprenolol, in human plasma. Citalopram, demethylcitalopram and didemethylcitalopram, as well as the internal standard, were recovered from plasma by liquid-liquid extraction. The limits of quantification were found to be 5 ng/ml for each enantiomer of citalopram and demethylcitalopram, and 7.5 ng/ml for each enantiomer of didemethylcitalopram. Inter- and intra-day coefficients of variation varied from 2.4% to 8.6% for S- and R-citalopram, from 2.9% to 7.4% for S- and R-demethylcitalopram, and from 5.6% to 12.4% for S- and R- didemethylcitalopram. No interference was observed from endogenous compounds following the extraction of plasma samples from 10 different patients treated with citalopram. This method allows accurate quantification for each enantiomer and is, therefore, well suited for pharmacokinetic and drug interaction investigations. The presented method replaces a previously described highly sensitive and selective high-performance liquid chromatography procedure using an acetylated 3-cyclobond column which, because of manufactural problems, is no longer usable for the separation of the enantiomers of citalopram and its demethylated metabolites.

Selective serotonin reuptake inhibitors in affective disorders--I. Basic pharmacology

The selective serotonin reuptake inhibitors (SSRIs), citalopram, fluoxetine, fluvoxamine, paroxetine and sertraline, are the result of rational research to find drugs that were as effective as the tricyclic antidepressants but with fewer safety and tolerability problems. The SSRIs selectively and powerfully inhibit serotonin reuptake and result in a potentiation of serotonergic neurotransmission. The property of potent serotonin reuptake appears to give a broad spectrum of therapeutic activity in depression, anxiety, obsessional and impulse control disorders. However, despite the sharing of the same principal mechanism of action, SSRIs are structurally diverse with clear variations in their pharmacodynamic and pharmacokinetic profiles. The potency for serotonin reuptake inhibition varies amongst this group, as does the selectivity for serotonin relative to noradrenaline and dopamine reuptake inhibition. The relative potency of sertraline for dopamine reuptake inhibition differentiates it pharmacologically from other SSRIs. Affinity for neuroreceptors, such as sigma1, muscarinic and 5-HT2c, also differs widely. Furthermore, the inhibition of nitric oxide synthetase by paroxetine, and possibly other SSRIs, may have significant pharmacodynamic effects. Citalopram and fluoxetine are racemic mixtures of different chiral forms that possess varying pharmacokinetic and pharmacological profiles. Fluoxetine has a long acting and pharmacologically active metabolite. There are important clinical differences among the SSRIs in their pharmacokinetic characteristics. These include differences in their half-lives, linear versus non-linear pharmacokinetics, effect of age on their clearance and their potential to inhibit drug metabolising cytochrome P450 (CYP) isoenzymes. These pharmacological and pharmacokinetic differences underly the increasingly apparent important clinical differences amongst the SSRIs.

Distribution of paroxetine in three postmortem cases

Paroxetine (Paxil) is a selective serotonin reuptake inhibitor, one of a new class of antidepressants used in the treatment of obsessive-compulsive disorder, panic disorder, and depression. Paroxetine potentiates serotonergic activity through the selective inhibition of serotonin reuptake in the central nervous system. There are few reported overdoses in the literature, and of these, three were fatal. Three coroner's cases in which paroxetine was directly associated with the cause of death are reported. In case #1, paroxetine was the only drug detected in significant concentrations. The heart blood paroxetine concentration was 4.0 mg/L. Case #2 was a known suicide in which the decedent herself admitted taking pills and alcohol. The hospital blood sample drawn at admission was analyzed and contained a 0.25% ethanol level and no paroxetine. Death occurred 10 h later. The postmortem heart blood contained ethanol at 0.06%, paroxetine at 3.7 mg/L, fluoxetine at 0.86 mg/L, and norfluoxetine at 0.65 mg/L. In case #3, death was attributed to an apparent adverse drug interaction between paroxetine and imipramine/desipramine. The postmortem heart blood contained paroxetine at 1.4 mg/L, imipramine at 3.0 mg/L, and desipramine at 9.6 mg/L.

Milnacipran. A review of its use in depression

Milnacipran is a cyclopropane derivative which acts by inhibiting noradrenaline (norepinephrine) and serotonin (5-hydroxytryptamine; 5-HT) reuptake at presynaptic sites; no postsynaptic receptor activity has been demonstrated. It is most commonly administered at a dosage of 50 mg twice daily for the treatment of major depressive disorder. Improvement usually occurs within 2 weeks of treatment initiation, but some patients do respond sooner. Most studies which evaluated milnacipran were of short (4 to 8 weeks) duration and results were not published in full with rigorous peer review. Nonetheless, the drug is significantly more effective than placebo for the treatment of in- or outpatients with moderate to severe major depressive disorder. Limited data suggest that it may prevent relapse and be effective for long term use, although this requires confirmation. Milnacipran 200 mg/day is generally not significantly different from amitriptyline 150 mg/day in terms of onset and efficacy. However, when doses are titrated (not a requirement for milnacipran), milnacipran 50 or 100 mg/day has a slower onset than the tricyclic antidepressant. At a dosage of 100 mg/day for 4 to 12 weeks, milnacipran generally has similar efficacy to imipramine and clomipramine 150 mg/day, although milnacipran 50 to 150 mg/day had a faster onset of activity than imipramine 50 to 150 mg/day in Japanese patients. In a 6-month trial, milnacipran was less effective than clomipramine. Milnacipran 50 or 100 mg twice daily was as effective as fluoxetine 20 mg once daily or fluvoxamine 100 mg twice daily in 4- to 12-week studies. At a dosage of 50 then 100 mg daily it was also as effective as mianserin 30 then 60 mg daily in a 4-week study. However, when administered once daily (in the evening), milnacipran 100 mg/day was not as effective as fluoxetine 20 mg/day after 6 weeks. The drug is generally well tolerated, producing no more adverse events (including anticholinergic events) than placebo, selective serotonin reuptake inhibitors or mianserin and fewer adverse events than tricyclic antidepressants in clinical trials. However, dysuria has been reported in 7% of male patients receiving milnacipran.

CONCLUSIONS

Data from predominantly short term trials suggest that milnacipran generally has similar efficacy to tricyclic antidepressants and SSRIs. Although further published data are required to confirm its efficacy, good tolerability profile and pharmacokinetic profile which suggests a low potential for drug interactions, milnacipran should be considered a promising agent for the treatment of patients with major depressive disorder.

Rapid simple high-performance liquid chromatographic determination of paroxetine in human plasma

A rapid, simple method for the measurement of paroxetine in human plasma by reversed-phase high-performance liquid chromatography (HPLC) with fluorescence detection is described. This method includes only one-step extraction of paroxetine and dibucaine, an internal standard, with chloroform. Their recoveries were around 90%. The mobile phase, 10 mM phosphate buffer-acetonitrile (40:60, v/v) was eluted isocratically. Between- and within-day coefficients of variation were in the range of 1.9-9.4% and 2.3-13.3%, respectively. The detection limit was 0.2 ng/ml. The method we describe can be easily applied to the measurement of plasma paroxetine concentration for pharmacokinetic studies as well as for therapeutic drug monitoring in patients taking paroxetine.

[Interaction between sumatriptan and selective serotonin uptake inhibitors]

Migraine is common among patients who suffer from depression, and this category of patients often needs drug treatment for both diseases. A pharmacist consulted the Regional Drug Information Centre in the western part of Norway (RELIS 3) about the combined use of sumatriptan and fluoxetine, as the product information on sumatriptan warns of a possible interaction between these drugs. This possibility was evaluated by the Drug Information Centre, and it was concluded that the combination of sumatriptan and a selective serotonin reuptake inhibitor is not contraindicated. This conclusion was based on both theoretical and clinical considerations.

Nonlinear pharmacokinetics of fluvoxamine and gender differences

This prospective study assessed fluvoxamine serum concentrations under two different fixed doses. The study included 15 male and female patients who met the DSM-III-R criteria for major depression. They were prescribed 50 mg fluvoxamine twice a day for 2 weeks and 100 mg twice a day thereafter. Drug monitoring was carried out on days 14 and 28. Fluvoxamine serum concentrations were highly variable between patients. After the dose was doubled, the serum concentrations of fluvoxamine increased disproportionately (mean, 3.4-fold), and there was a significantly (p < 0.05) more pronounced increase in men (4.6-fold) than in women (2.4-fold). These results provide evidence of nonlinear, sex-dependent pharmacokinetics of fluvoxamine.

The serotonin syndrome scale: first results on validity

As a modification of the diagnostic criteria of the serotonin syndrome proposed by Sternbach, we developed the Serotonin syndrome scale for the operationalized assessment of both the presence and the severity of the core symptoms of the serotonin syndrome. In a first study on the validity of this scale, the relationships between the serotonin syndrome score (SSS) and both the paroxetine plasma levels (n = 42) and the loudness dependence of the auditory evoked potentials (LDAEP; n = 24) were investigated in depressed patients treated with paroxetine. A strong LDAEP is supposed to indicate low central serotonergic neurotransmission, and vice versa. The SSS was positively related to paroxetine plasma levels and negatively to the LDAEP. Both results support the validity of the serotonin syndrome scale. Using a SSS > 6 as diagnostic criterion, mild serotonin syndromes were diagnosed in 5 of our 42 patients. The Serotonin syndrome scale may become a useful tool for clinicians and scientists dealing with the serotonin syndrome.

Minimal interaction between fluoxetine and multiple-dose zolpidem in healthy women

The objective was to evaluate possible pharmacokinetic and pharmacodynamic interactions for repeated nightly zolpidem dosing with fluoxetine. Twenty-nine healthy female volunteers (mean age, 25. 6 years) received zolpidem (10 mg) and fluoxetine (20 mg) in the following open design: zolpidem on night 1 followed by 1 washout day, a daily morning dose of fluoxetine on days 3 through 27, and a morning dose of fluoxetine plus an evening dose of zolpidem on days 28 through 32. Plasma levels of zolpidem, fluoxetine, and norfluoxetine were determined at the transitions from one regimen to the next. Morning psychomotor tests were performed on days 1, 2, 28, 29, and 33. Steady-state plasma concentrations of fluoxetine/norfluoxetine were reached by day 24 of fluoxetine dosing. No significant differences in any pharmacokinetic parameters for fluoxetine and norfluoxetine were observed between day 27 and day 32. There were no significant differences in AUC, maximal plasma concentration, or time to maximal concentration parameters for zolpidem plasma concentrations among nights 1, 28, and 32. There was a statistically significantly increased t1/2 for zolpidem on night 32, compared with night 28 (3.64 and 3.29 hr, respectively). There were no significant differences in the next-morning Digit Symbol Substitution Test performance at any time in the study. Both zolpidem and fluoxetine were well tolerated alone or during coadministration. These findings indicate the absence of clinically significant pharmacokinetic or pharmacodynamic interactions between fluoxetine and zolpidem (five consecutive doses) when the drugs are coadministered to healthy women. Therefore, based on these observations, short-term cotherapy with fluoxetine (20 mg) and zolpidem (10 mg) appears safe.

Magnetic resonance spectroscopy in affective disorders

Magnetic resonance spectroscopy (MRS) is a non-invasive tool for in vivo chemical analysis that has been applied to neurobiological or psychopharmacological studies of affective disorders. Studies employing 31P-MRS and 1H-MRS have indicated possible abnormalities in membrane phospholipid metabolism, high-energy phosphate metabolism, and intracellular pH in affective disorders. They have also suggested that lithium increases the phosphomonoester (possibly inositol-1-phosphate) peak in the brain but does not increase that of choline-containing compounds in the brain. Studies employing 7Li-MRS and 19F-MRS have elucidated the pharmacokinetic properties of lithium, fluoxetine, and fluvoxamine in the brain in patients treated with these drugs.

Bupropion sustained release: a therapeutic overview

Sustained-release bupropion (bupropion SR) represents a new form of an already known effective antidepressant drug. Its pharmacokinetics, mechanism of action, metabolism, and efficacy are reviewed. Benefit relative to placebo has been demonstrated in two large multicenter trials, with low doses (100 or 150 mg) having been shown to have therapeutic efficacy. An overview of all placebo-controlled trials of bupropion SR is given, and the differential properties of bupropion and serotonergic drugs are described. The concept of a catecholamine-indolamine spectrum is presented, along with its implications for possible differential therapeutics of selective antidepressants.

Determination of fluoxetine and its metabolite norfluoxetine in serum and brain areas using high-performance liquid chromatography with ultraviolet detection

A high-performance liquid chromatography (HPLC) method using only 0.1 ml of serum or homogenate from brain areas has been developed for the determination of fluoxetine (FLU) and its metabolite, norfluoxetine (N-FLU), with ultraviolet detection at 227 nm. The small volume of sample required in this method allows studies in small animals, such as mouse. The method provides recoveries of up to 90% for both compounds. Acceptable coefficients of variation were found for both within-run and day-to-day assays. The limit of detection was 5.0 ng/ml. No interferences were found with tricyclic antidepressant drugs and benzodiazepines, which allows this method to be used in clinical studies, Pharmacokinetic parameters for the two compounds are reported in mouse serum, frontal cortex and caudate nucleus. We also report the values of FLU and N-FLU in serum from humans who were treated once daily with 20 mg of FLU, obtained after 1, 14 and 28 days of treatment.

Antidepressant drugs in the elderly

1. In this article some of the most important and tolerated drugs in the elderly are reviewed. 2. Tricyclic antidepressants have to be used carefully because of their important side effects. Nortriptyline and desipramine appear to be the best tolerated tricyclics in old people. 3. Second generation antidepressants are preferred for the elderly and those patients with heart disease as they have milder side effects and are less toxic in overdose. 4. MAO inhibitors are useful drugs in resistant forms of depression in which the above mentioned drugs have no efficacy and the last generation drugs (reversible MAO inhibitors), such as moclobemide, seem to be very successful. 5. Lithium is sometimes used especially to prevent recurrence of depression, even if its use is limited in old patients due to its side effects. 6. Psychotherapy is often used as an adjunct to pharmacotherapy, while electroconvulsant therapy is used only in the elderly patients with severe depression, high risk of suicide, or drug-resistant forms.

Nefazodone in major depression: adjunctive benzodiazepine therapy and tolerability

One hundred sixty-six patients suffering from major depressive disorders were treated for 8 weeks with nefazodone in an open study in dosage ranges from 200 to 600 mg. This report focuses primarily on the first week of therapy and on the concomitant use of several benzodiazepines, one of which is not metabolized by the cytochrome system (temazepam). Triazolam response was further evaluated as a function of two nefazodone dosage regimens provided during the first week of therapy, one group receiving nefazodone 200 mg/day for 7 days, and another group receiving nefazodone 200 mg/day for 3 days, followed by 4 days with 400 mg/day. Finally, a comparison of three different nefazodone dosages, the third being 400 mg from day 1 on, was also carried out. Outcome measures included Hamilton Rating Scale for Depression total and the total of the three Hamilton Rating Scale for Depression insomnia items, as well as global improvement, a daily completed sleep questionnaire, and adverse event assessment. A combination of nefazodone with a benzodiazepine (BZ) caused more sedation than nefazodone alone; triazolam, the BZ with the shortest half-life and the highest dependence on the cytochrome 450 system for its metabolism, caused the least amount of sedation, and alprazolam and diazepam, the two daytime benzodiazepines, caused the most sedation. Triazolam caused significant and identical reduction of insomnia in both nefazodone groups. Compared with nefazodone 200 mg given as monotherapy, insomnia was significantly improved--not only by triazolam, but also alprazolam and diazepam, but not temazepam. The addition of nefazodone raised triazolam plasma levels to almost 500%, the plasma level of desmethyl-diazepam 87%, and that of alprazolam 34%. Temazepam plasma levels remained unchanged. When prescribing nefazodone with a benzodiazepine, one should expect an improved sleep pattern initially, but at the cost of clinically relevant daytime sedation. The prediction that temazepam, the only BZ not dependent on the cytochrome mechanism for metabolism, should be the least sedating, and triazolam, because of its cytochromic metabolism interference with nefazodone should be the most sedating, could not be confirmed. In fact, triazolam 0.25 mg capsules seem to be the safest treatment of choice when one has to combine a benzodiazepine with nefazodone in initial stages of therapy, at least of the four benzodiazepines tested in this study.

Pharmacological profile of venlafaxine, a new antidepressant, given acutely

Pharmacological effects of acute treatment with venlafaxine (VEN), a clinically active antidepressant [a noradrenaline (NA) and 5-hydroxytryptamine (5-HT) reuptake inhibitor without any affinity for neurotransmitter receptors] were studied in mice and rats. VEN inhibited the reserpine- or apomorphine-induced hypothermia and enhanced the L-5-HTP-induced head twitches in mice. It reduced the immobility time in Porsolt's test in mice and rats, but either did not change the locomotor activity (mice) or decreased it (rats). VEN reduced the locomotor hyperactivity induced by amphetamine (AMP), apomorphine (APO) and quinpirole (QUI), as well as the APO-induced stereotypy; the stereotypy induced by AMP in rats was prolonged. VEN neither changed the clonidine-induced aggressiveness in mice nor the behavioral syndrome induced by oxotremorine in rats. The obtained results indicate that VEN, given acutely, shows a pharmacological profile similar to that of tricyclic NA and 5-HT reuptake inhibitors. In contrast to the antidepressants mentioned above, VEN does not exhibit an alpha 1-adrenolytic or a cholinolytic activity (in vivo tests).

[Pharma-Clinics. The drug of the month. Venlafaxine (Efexor)]

Venlafaxine (Efexor) is the first representative of a new class of antidepressants: serotonin noradrenaline reuptake inhibitors. Its usual dose is 75 mg/d in two intakes but can be progressively increased until a maximal daily dose of 375 mg/d in severe or resistant depression, particularly among inpatients. The efficacy of venlafaxine is at least equivalent to reference antidepressants. At high doses, venlafaxine could even exhibit a better efficacy and a shorter latency than current compounds. Its profile of side-effects is quite similar to selective serotonin reuptake inhibitors with mainly nausea, with the exception if an increase in blood pressure which can appear at high doses. In total, venlafaxine represents an interesting innovation in the pharmacological treatment of depression.

[Fluoxetine: relations between plasma concentration and therapeutic effects in 32 patients with major depression and treated with 20 mg/day]

The aim of this clinical study was to investigate 32 melancholic patients treated by fluoxetine (20 mg/day). The clinical examination to evaluate the antidepressant effect of fluoxetine was realized by using the HDS/MES criteria. The patients were divided into three groups (responders, partial responders with or without a relapse, non responders) according to their clinical evolution during treatment. Fluoxetine and norfluoxetine were evaluated by HPLC after 3 weeks of treatment. In the present study, 53% of the patients have a positively reaction to the 21 day's treatment. Our results showed no correlation between the psychiatric scores and the plasma concentrations of fluoxetine and norfluoxetine.

Pharmacokinetics and antidepressant activity of fluoxetine in transgenic mice with elevated serum alpha-1-acid glycoprotein levels

Fluoxetine, a novel selective serotonin reuptake inhibitor utilized in the treatment of depression, is avidly bound to serum albumin and alpha-1-acid glycoprotein (AAG). AAG is an acute phase protein, and its serum levels are elevated in a variety of pathophysiological conditions including inflammation, depression, cancer, and acquired autoimmune deficiency syndrome. Further, the pharmacokinetic disposition and pharmacological activity of several highly bound drugs have been reported to be significantly altered as a result of elevated serum AAG. We investigated the effects of elevated serum AAG levels on the pharmacokinetic disposition, antidepressant activity, and steady state profile of fluoxetine and its demethylated metabolite, norfluoxetine. This was approached utilizing a novel strain of transgenic mice that expressed genetically elevated serum AAG levels severalfold over those of control mice. Serum and brain drug concentrations were determined by HPLC after fluoxetine administration. In transgenic mice, the volume of distribution and the terminal elimination half-life of fluoxetine were significantly reduced. Further, significant reductions in brain-to-serum fluoxetine concentration ratios and antidepressant activity were observed in transgenic mice, despite having higher serum drug levels than control mice. This trend in the serum continued at steady state, and brain fluoxetine levels were significantly lower in transgenic mice. The results of this study provide valuable insights regarding the consequences of elevated serum AAG levels, often seen in several disease states, on the pharmacokinetic disposition of fluoxetine.

Correlation between steady-state plasma concentrations of mianserin and trazodone in depressed patients

OBJECTIVE

The correlations between steady-state plasma concentrations of mianserin and its active metabolite desmethylmianserin and those of trazodone and its active metabolite m-chlorophenylpiperazine (m-CPP) were examined in 19 depressed patients.

METHODS

Ten patients received first mianserin (30 mg per day) and second trazodone (150 mg per day), while 9 patients received these treatments in the opposite sequence, with at least 2-week intervals between the two phases. Blood was sampled at steady state, 1-3 weeks after initiation of each treatment. Plasma concentrations of mianserin, the separate enantiomers S(+)- and R(-)-mianserin, desmethylmianserin, trazodone and m-CPP were measured by means of high-performance liquid chromatography.

RESULTS

There was a significant correlation between steady-state plasma concentrations of trazodone and total mianserin (r = 0.59) or S(+)-mianserin (r = 0.57), but not R(-)-mianserin (r = 0.33).

CONCLUSION

The present study thus suggests that the metabolic capacity of mianserin, especially the more active S(+)-enantiomer, and that of trazodone correlate to each other. This finding supports the previous suggestions that cytochrome P4502D6 is involved in the metabolism of mianserin and trazodone.

Repeated antidepressant drug treatment, time of death and frequency of handling do not affect [3H]paroxetine binding in rat cortex

[3H]Paroxetine binding to the serotonin transporter has been shown to be altered in brain tissue from schizophrenic subjects. Some schizophrenic subjects are treated with antidepressant drugs, some of which bind to the serotonin transporter and their time of death is variable. To determine if these confounding factors could affect [3H]paroxetine binding, [3H]paroxetine binding to cortical membrane from rats treated with the antidepressant drugs for 10 or 28 days and in non-treated rats that were killed at different times of the day was measured. Drug treatment, when compared to injection with 0.9% saline and time of death, did not affect [3H]paroxetine binding. Treatment with imipramine [10 days: mean +/- S.D.: 590 +/- 59 fmol/mg protein (P < 0.05); 28 days: 653 +/- 59 fmol/mg protein (P < 0.01)] or mianserin [10 days: 600 +/- 43 (P < 0.05)] caused a significant decrease in the density of [3H]paroxetine binding compared to that in fluoxetine-treated rats (10 days: 820 +/- 211 fmol/mg protein; 28 days: 764 +/- 100 fmol/mg protein). Thus, overall, these data do not suggest changes in [3H]paroxetine binding reported in the human brain tissue would be due to antidepressant drug treatment or time of death.

Evaluation of the neuropharmacodynamics of paroxetine in vivo utilizing microdialysis

The effect of paroxetine, a selective serotonin reuptake inhibitor used in the treatment of depression, on extracellular serotonin levels was evaluated in freely moving conscious rats. Microdialysis, a powerful in vivo technique to monitor the extracellular levels of neurotransmitters, was used to monitor the baseline changes in the levels of serotonin in rat brain anterior lateral striatum post paroxetine administration, which is a measure of the neuropharmacodynamic effect of the drug. Microdialysis sampling was performed for 210 min prior to and for 240 min after intraperitoneal administration of paroxetine (10 mg/kg). Paroxetine caused a statistically significant increase in the extracellular levels of serotonin in the anterior lateral striatum sampled by microdialysis. The present study demonstrates the utility of microdialysis for studying the in vivo neuropharmacodynamics of paroxetine in conscious rats.

Clinical pharmacokinetics of nefazodone

Nefazodone is a new antidepressant drug, chemically unrelated to the tricyclic, tetracyclic or selective serotonin uptake inhibitors. Nefazodone blocks the serotonin 5-HT2 receptors and reversibly inhibits serotonin reuptake in vivo. Nefazodone is completely and rapidly absorbed after oral administration with a peak plasma concentration observed within 2 hours of administration. Nefazodone undergoes significant first-pass metabolism resulting in an oral bioavailability of approximately 20%. Although there is an 18% increase in nefazodone bioavailability with food, this increase is not clinically significant and nefazodone can be administered without regard to meals. Three pharmacologically active nefazodone metabolites have been identified: hydroxy-nefazodone, triazoledione and m-chlorophenylpiperazine (mCPP). The pharmacokinetics of nefazodone are nonlinear. The increase in plasma concentrations of nefazodone are greater than would be expected if they were proportional to increases in dose. Steady-state plasma concentrations of nefazodone are attained within 4 days of the commencement of administration. The pharmacokinetics of nefazodone are not appreciably altered in patients with renal or mild-to-moderate hepatic impairment. However, nefazodone plasma concentrations are increased in severe hepatic impairment and in the elderly, especially in elderly females. Lower doses of nefazodone may be necessary in these groups. Nefazodone is a weak inhibitor of cytochrome P450 (CYP) 2D6 and does not inhibit CYP1A2. It is not anticipated that nefazodone will interact with drugs cleared by these isozymes. Indeed, nefazodone did not affect the pharmacokinetics of theophylline, a compound cleared by CYP1A2. Nefazodone is metabolised by and inhibits CYP3A4. Clinically significant interactions have been observed between nefazodone and the benzodiazepines triazolam and alprazolam, cyclosporin and carbamazepine. The potential for a clinically significant interaction between nefazodone and other drugs cleared by CYP3A4 (e.g. terfenadine) should be considered before the coadministration of these compounds. There was an increase in haloperidol plasma concentrations when coadministered with nefazodone; nefazodone pharmacokinetics were not affected after coadministration. No clinically significant interaction was observed when nefazodone was administered with lorazepam, lithium, alcohol, cimetidine, warfarin, theophylline or propranolol.

Application of a first-pass effect model to characterize the pharmacokinetic disposition of venlafaxine after oral administration to human subjects

Venlafaxine (VEN), a drug used in the treatment of depression, undergoes significant first-pass metabolism after oral dosing to O-desmethylvenlafaxine (ODV), a metabolite with comparable therapeutic activity to that of parent drug. The pharmacokinetic disposition of VEN was characterized using a "first-pass" model that incorporates a presystemic compartment (liver) to account for the first-pass metabolism of VEN to ODV. A series of differential equations were simultaneously fitted to plasma concentrations of parent and metabolite. A good fit of the model to observed data was demonstrated, generating estimates for the following parameters: ka (1.31 +/- 0.009 hr-1), VVEN (252 +/- 87.6 liters), CLint (65.8 +/- 39.7 liters/hr), RL (liver:plasma partition coefficient, 29.6 +/- 18. 3), VODV (181 +/- 84.1 liters), and CLODV (23.5 +/- 12.5 liters/hr). Parameter estimates correlated closely with those obtained through noncompartmental methods. These results indicate that the time-course disposition of a compound undergoing first-pass hepatic metabolism after oral dosing can be successfully modeled.

Effects of trazodone and fluoxetine in the treatment of major depression: therapeutic pharmacokinetic and pharmacodynamic interactions through formation of meta-chlorophenylpiperazine

It has been suggested that (1) the clinical efficacy of the heterocyclic antidepressant trazodone in depression may, in part, be attributed to its metabolite meta-chlorophenylpiperazine (mCPP); and (2) the enhancement of the efficacy of trazodone by the addition of fluoxetine, a selective serotonin reuptake inhibitor, may, in part, be ascribed to fluoxetine-induced plasma concentrations of trazodone. After a washout period of 10 days, 27 inpatients with major depression were treated with trazodone 100 mg/day (orally). One week later (T0), fluoxetine 20 mg/day, placebo, or pindolol 7.5 mg/day was added. Plasma concentrations of mCPP and trazodone were determined at T0 and 2 and 4 weeks later. Although placebo pindolol had no significant effect on the plasma concentrations of mCPP and trazodone, there was a significant increase of the concentrations of these compounds associated with the combination of trazodone + fluoxetine. The results suggest that fluoxetine-induced increases in plasma mCPP and trazodone concentrations contribute to the clinical efficacy of the combination of fluoxetine + trazodone. It is suggested that desensitization of 5-HT2C receptor function by mCPP as well as fluoxetine may contribute to the antidepressant effects of this combination.

Pharmacokinetics of bupropion and its metabolites in cigarette smokers versus nonsmokers

Bupropion is an antidepressant agent that is also effective as an aid to quit cigarette smoking. A single 150-mg tablet of sustained-release bupropion hydrochloride was administered to two groups of volunteers, cigarette smokers and nonsmokers, who were matched for race, gender, body frame, age, and weight. Pharmacokinetic parameters were calculated for bupropion, and three major metabolites (hydroxybupropion and the aminoalcohol isomers, threohydrobupropion and erythrohydrobupropion, expressed as a composite total). Mean (+/-SD) values of area under the concentration-time curve from time 0 extrapolated to infinity (AUC0-infinity), maximum concentration (Cmax), time to reach Cmax (tmax), and half-life (t1/2) of bupropion in smokers and nonsmokers, respectively, were 1,164 +/- 220 ng.hr/mL and 1,161 +/- 292 ng.hr/mL; 144 +/- 28 ng/mL and 143 +/- 39 ng/mL; 3.00 +/- 0.50 hours and 2.88 +/- 0.49 hours; and 19 +/- 5 hours and 18 +/- 3 hours. No clinically significant differences between smokers and nonsmokers or between male and female volunteers were observed for the pharmacokinetics of bupropion or its metabolites. The absence of pharmacokinetic differences indicates that dosage adjustments are not necessary when bupropion is prescribed to male and female cigarette smokers.

Desipramine pharmacokinetics when coadministered with paroxetine or sertraline in extensive metabolizers

In vitro studies have shown that fluoxetine and paroxetine are more potent inhibitors of cytochrome CYP2D6 than sertraline. The pharmacokinetics of desipramine when coadministered with the selective serotonin reuptake inhibitors (SSRIs) paroxetine and sertraline were studied in 24 healthy male volunteers (CYP2D6 extensive metabolizers). Desipramine (50 mg/day) was administered for 23 days in each phase of the crossover study with a 7-day drug-free period between phases. In addition, subjects were randomly assigned to receive concomitant paroxetine (20 mg/day on days 8 through 17 followed by 30 mg/day on days 18 through 20) or sertraline (50 mg/day on days 8 through 17 and 100 mg/day on days 18 through 20). SSRI treatments were switched between phases. After 10 days of coadministration at the lower dose, mean desipramine maximum concentration in plasma (Cmax) relative to baseline increased from 37.8 to 173 ng/mL (+358%) with paroxetine versus from 36.1 to 51.9 ng/mL (+44%) with sertraline; the mean desipramine 24-hour area under the concentration-time curve (AUC[24]) increased from 634 to 3,305 ng x h/mL (+421%) with paroxetine versus from 611 to 838 ng x h/mL (+37%) with sertraline; and the mean desipramine trough value (C0) increased from 18.5 to 113 ng/mL (+511%) with paroxetine versus from 18.3 to 21.8 ng/mL (+19%) with sertraline (all increases, p < 0.001). An approximately 10-fold increase in the Cmax and AUC(24) of paroxetine and an approximately 2-fold increase in these parameters for sertraline occurred simultaneously with the desipramine concentration changes. Thus, when coadministered with 50 mg/day desipramine, sertraline had significantly less pharmacokinetic interaction than paroxetine with desipramine at the recommended starting dosages of 50 mg/day and 20 mg/day, respectively.

Venlafaxine: in vitro inhibition of CYP2D6 dependent imipramine and desipramine metabolism; comparative studies with selected SSRIs, and effects on human hepatic CYP3A4, CYP2C9 and CYP1A2

AIMS

In order to anticipate drug-interactions of potential clinical significance the ability of the novel antidepressant, venlafaxine, to inhibit CYP2D6 dependent imipramine and desipramine 2-hydroxylation was investigated in human liver microsomes. The data obtained were compared with the selective serotonin re-uptake inhibitors, fluoxetine, sertraline, fluvoxamine and paroxetine. Venlafaxine's potential to inhibit several other major P450 s was also studied (CYP3A4, CYP2D6, CYP1A2).

METHODS

Ki values for venlafaxine, paroxetine, fluoxetine, fluvoxamine and sertraline as inhibitors of imipramine and desipramine 2-hydroxylation were determined from Dixon plots of control and inhibited rate data in human hepatic microsomal incubations. The inhibitory effect of imipramine and desipramine on liver microsomal CYP2D6 dependent venlafaxine O-demethylation was determined similarly. Venlafaxine's IC50 values for CYP3A4, CYP1A2 CYP2C9 were determined based on inhibition of probe substrate activities (testosterone 6 beta-hydroxylation, ethoxyresorufin O-dealkylase and tolbutamide 4-hydroxylation, respectively).

RESULTS

Fluoxetine, paroxetine, and fluvoxamine were potent inhibitors of imipramine 2-hydroxylase activity (Ki values of 1.6 +/- 0.8, 3.2 +/- 0.8 and 8.0 +/- 4.3 microM, respectively; mean +/- s.d., n = 3), while sertraline was less inhibitory (Ki of 24.7 +/- 8.9 microM). Fluoxetine also markedly inhibited desipramine 2-hydroxylation with a Ki of 1.3 +/- 0.5 microM. Venlafaxine was less potent an inhibitor of imipramine 2-hydroxylation (Ki of 41.0 +/- 9.5 microM) than the SSRIs that were studied. Imipramine and desipramine gave marked inhibition of CYP2D6 dependent venlafaxine O-demethylase activity (Ki values of 3.9 +/- 1.7 and 1.7 +/- 0.9 microM, respectively). Venlafaxine did not inhibit ethoxyresorufin O-dealkylase (CYP1A2), tolbutamide 4-hydroxylase (CYP2C9) or testosterone 6 beta-hydroxylase (CYP3A4) activities at concentrations of up to 1 mM.

CONCLUSIONS

It is concluded that venlafaxine has a low potential to inhibit the metabolism of substrates for CYP2D6 such as imipramine and desipramine compared with several of the most widely used SSRIs, as well as the metabolism of substrates for several of the other major human hepatic P450s.

Venlafaxine and nefazodone, two pharmacologically distinct antidepressants

Venlafaxine, a phenylethylamine, and nefazodone, a phenylpiperazine compound, are the newest antidepressants to receive approval of the Food and Drug Administration and to be marketed in the United States. Both strongly inhibit serotonin (5-HT) reuptake; venlafaxine also inhibits norepinephrine reuptake, and nefazodone also exhibits 5-HT2-receptor antagonism. Venlafaxine inhibits the cytochrome P-450 2D6 isozyme to a lesser extent than the selective serotonin reuptake inhibitors (SSRIs) and is 27% protein bound. Structurally, the drugs are unrelated to SSRIs and have some clinically important differences in side effect profiles. Nausea, headache, somnolence, and dry mouth are the most frequently reported side effects with both. Sustained hypertension was reported by a limited number of venlafaxine-treated patients.