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.

Biotransformation of protriptyline by filamentous fungi and yeasts

1. The potential of various fungi to metabolize protriptyline (an extensively used antidepressant) was studied to investigate similarities between mammalian and microbial metabolism. 2. Metabolites produced by each organism were isolated by high-pressure liquid chromatography and identified by nuclear magnetic resonance and mass spectrometry. The metabolites identified in one or more fungi were 2-hydroxyprotriptyline, N-desmethylprotriptyline, N-acetylprotriptyline, N-acetoxyprotriptyline, 14-oxo-N-desmethylprotriptyline, 2-hydroxy-acetoxyprotriptyline and 3-(5-hydrodibenzo[bf][7]annulen-5-yl)propanoic acid. 3. Among 27 filamentous fungi and yeast species screened, Fusarium oxysporum f. sp. pini 2380 metabolized 97% of the protriptyline added. Several other fungi screened gave significant metabolism of protriptyline, including Cunninghamella echinulata ATCC 42616 (67%), C. elegans ATCC 9245 (17%), C. elegans ATCC 36112 (22%), C. phaeospora ATCC 22110 (50%), F. moniliforme MRC-826 (33%) and F. solani 3179 (12%). 4. F. oxysporum f. sp. pini produced phase I and phase II metabolites and thus is a suitable microbial model for protriptyline metabolism.

Pharmacokinetic and pharmacodynamic characterization of OROS and immediate-release amitriptyline

AIMS

To characterize the pharmacokinetics of amitriptyline and its metabolite nortriptyline following OROS and IR treatments, and to correlate them with anticholinergic side-effects.

METHODS

The pharmacokinetics and safety of amitriptyline following administration of an osmotic controlled release tablet (OROS and an immediate release (IR) tablet were evaluated in 14 healthy subjects. In this randomized, open label, three-way crossover feasibility study, the subjects received a single 75 mg OROS tablet, three 25 mg IR tablets administered every 8 h, or 3x25 mg IR tablets administered at nighttime. In each treatment arm serial blood samples were collected for a period of 84 h after dosing. The plasma samples were analysed by gas chromatography for amitriptyline and its metabolite nortriptyline. Anticholinergic effects such as saliva output, visual acuity, and subject-rated drowsiness and dry mouth were measured on a continuous scale during each treatment period.

RESULTS

Following dosing with OROS (amitriptyline hydrochloride), the mean maximal plasma amitriptyline concentration Cmax (15.3 ng ml-1 ) was lower and the mean tmax (25.7 h) was longer than that associated with the equivalent IR dose administered at nighttime (26.8 ng ml-1 and 6.3 h, respectively). The bioavailability of amitriptyline following OROS dosing was 95% relative to IR every 8 h dosing, and 89% relative to IR nighttime dosing. The metabolite-to-drug ratios after the three treatment periods were similar, suggesting no change in metabolism between treatments. The relationships between plasma amitriptyline concentration and anticholinergic effects (e.g. reduced saliva weight, dry mouth, and drowsiness) were similar with all three treatments. Of the anticholinergic effects, only decreased saliva weight and dry mouth correlated well with plasma amitriptyline concentrations; drowsiness did not. There was no apparent correlation between anticholinergic effects and the plasma nortriptyline concentration.

CONCLUSIONS

The bioavailability of OROS (amitriptyline hydrochloride) was similar to that of the IR treatments and the pharmacokinetics of amitriptyline after OROS dosing may decrease the incidence of anticholinergic effects compared with that seen with nighttime dosing of the IR formulation. Therefore, this controlled-release formulation of amitriptyline may be appropriate for single daily administration.

Adverse effects in a newborn infant breast-fed by a mother treated with doxepin

OBJECTIVE

To report adverse effects in a newborn infant whose mother had been treated with doxepin during pregnancy and while breast-feeding.

CASE SUMMARY

The nine-day-old white boy was admitted because of poor sucking and swallowing, with muscle hypotonia and vomiting. He was drowsy and had lost 150 g. At the time of admission, he was breast-fed by his mother who was being treated with doxepin 35 mg/d. Samples of plasma and breast milk were taken and analyzed by HPLC and fluorescence polarization immunoassay. The amount of doxepin and N-desmethyldoxepin (DDP) ingested via breast-feeding was approximately 10-20 micrograms/kg/d (i.e., only 2.5% of the weight-adjusted dose of the mother). Doxepin was detectable in small amounts in the infant's plasma (approximately 10 micrograms/L); DDP was below the lower limit of detection of 10 micrograms/L. All adverse effects subsided within 48 hours after breast-feeding was stopped.

DISCUSSION

Despite the small doses of doxepin and its active metabolite ingested by breast-fed babies, there is a risk of accumulation and resultant adverse effects. In newborns, the metabolic activity is considerably decreased and may be further reduced by hyperbilirubinemia.

CONCLUSIONS

Available data suggest that women treated with doxepin should breast-feed their infants with great caution, if at all, although much larger databases are needed to confirm this.

Metabolism of tricyclic antidepressants

1. Despite the considerable advances in the treatments available for mood disorders over the past generation, tricyclic antidepressants (TCAs) remain an important option for the pharmacotherapy of depression. 2. The pharmacokinetics of TCAs are characterized by substantial presystemic first-pass metabolism, a large volume of distribution, extensive protein binding, and an elimination half-life averaging about 1 day (up to 3 days for protriptyline). 3. Clearance of tricyclics is dependent primarily on hepatic cytochrome P450 (CYP) oxidative enzymes. Although the activities of some P450 isoenzymes are largely under genetic control, they may be influenced by external factors, such as the concomitant use of other medications or substances. Patient variables, such as ethnicity and age, also affect TCA metabolism. The impact of gender and related reproductive issues is coming under increased scrutiny. 4. Metabolism of TCAs, especially their hydroxylation, results in the formation of active metabolites, which contribute to both the therapeutic and the adverse effects of these compounds. 5. Renal clearance of the polar metabolites of TCAs is reduced by normal aging, accounting for much of the increased risk of toxicity in older patients. 6. Knowledge of factors affecting the metabolism of TCAs can further the development and understanding of newer antidepressant medications.

Absence of effect of stimulants on the phamacokinetics of desipramine in children

We conducted a retrospective chart review to examine the pharmacokinetic interaction between desipramine and the stimulants methylphenidate and dexedrine using routinely monitored desipramine serum concentrations in children receiving desipramine either alone or with a stimulant. Subjects were 142 children and adolescents (age 6-17 yrs; 113 taking desipramine, 29 taking desipramine-stimulants) in whom 401 dose- and weight-normalized serum concentrations were evaluated (333 desipramine, 68 desipramine-stimulants). Desipramine pharmacokinetic parameters were similar for both groups, including mean weight-corrected dose (3.66+/-1.36 mg/kg, desipramine; 3.66+/-1.09 mg/kg, desipramine-stimulants; p=0.97), weight- and dose-normalized serum concentrations (47.26+/-39.26 [microg/L]/[mg/kg], desipramine, 39.02+/-19.92 [microg/L]/[mg/kg], desipramine-stimulants; p=0.09), and clearance (0.690+/-0.913 [L/kg]/hr, desipramine; 0.613+/-0.514 [L/kg]/hr, desipramine-stimulants; p=0.499). When stratified by age, gender, and type of stimulant, no difference was detected (p>0.05) between groups. Our findings indicate the absence of a clinically significant interaction between desipramine and stimulants.

A puzzling case of seizures and visual hallucinations during clomipramine treatment with a high dose but causing a low serum concentration

We present a puzzling case of a 25-year-old depressive man suffering from seizures and visual hallucinations during clomipramine treatment with a high dose but causing a low serum concentration. We examined alleles of cytochrome P450 (CYP) isozymes. It was revealed that he was not an ultrarapid metabolizer for CYP2D6, and that the genotypes were homozygous for CYP2D6J and heterozygous for CYP2C19m1. Throughout the treatment period, his compliance was good. Since he was a smoker, it seems likely that his low clomipramine level was due to smoking-induced CYP1A2 activity. These findings suggest that smoking-induced CYP1A2 activity overcomes the possibly inhibiting effects of homozygosity for CYP2D6J and heterozygosity for CYP2C19m1, and that high-dose clomipramine is not always a direct cause of seizures.

Mirtazapine: a review of its use in major depression

Mirtazapine is a noradrenergic and specific serotonergic antidepressant (NaSSA) which has predominantly been evaluated in the treatment of major depression. The drug had equivalent efficacy to tricyclic antidepressants and it was at least as effective as trazodone in the majority of available short term trials in patients with moderate or severe depression, including those with baseline anxiety symptoms or sleep disturbance and the elderly. A continuation study also showed that sustained remission rates were higher with mirtazapine than with amitriptyline and that the drugs had similar efficacy for the prevention of relapse. There is some evidence for a faster onset of action with mirtazapine than with the selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitors (SSRIs). Mirtazapine was more effective than the SSRI fluoxetine at weeks 3 and 4 of therapy and it was also more effective than paroxetine and citalopram at weeks 1 and 2, respectively, in short term assessments (6 or 8 weeks). Preliminary data suggest that the drug may be effective as an augmentation or combination therapy in patients with refractory depression. Anticholinergic events and other events including tremor and dyspepsia are less common with mirtazapine than with tricyclic antidepressants. There was a greater tendency for SSRI-related adverse events with fluoxetine than with mirtazapine, but, overall, mirtazapine had a similar tolerability profile to the SSRIs. Increased appetite and bodyweight gain appear to be the only events that are reported more often with mirtazapine than with comparator antidepressants. In vitro and in vivo data have suggested that mirtazapine is unlikely to affect the metabolism of drugs metabolised by cytochrome P450 (CYP)2D6, although few formal drug interaction data are available.

CONCLUSIONS

Mirtazapine is effective and well tolerated for the treatment of patients with moderate to severe major depression. Further research is required to define the comparative efficacy of mirtazapine in specific patient groups, including the elderly and those with severe depression. Clarification of its efficacy as an augmentation therapy and in patients with refractory depression and its role in improving the efficacy and reducing the extrapyramidal effects of antipsychotic drugs would also help to establish its clinical value. The low potential for interaction with drugs that are metabolised by CYP2D6, including antipsychotics, tricyclic antidepressants and some SSRIs, may also make mirtazapine an important option for the treatment of major depression in patients who require polytherapy. Mirtazapine also appears to be useful in patients with depression who present with anxiety symptoms and sleep disturbance.

Chronopharmacokinetics of amitriptyline in rats

The circadian changes in absorption, tissue distribution and elimination of amitriptyline after single intravenous (i.v.) and intragastric (i.g.) administration, as well as the differences in pharmacokinetic profile after multiple i.g. administration (at 10:00 and 22:00 h) during a 12 h dosing interval, were investigated. The circadian changes of pharmacokinetic parameters of amitriptyline such as AUC (serum and tissues), clearance (i.v. and i.g.), volume of distribution, biological half-life and bioavailability were estimated. Acrophases for clearance appeared between 19:00 and 21:00 h; the bioavailability was highest during the dark phase at around 04:00 h. Higher values of AUC in serum were observed at the beginning of the light phase. A circadian rhythm of tissue distribution (AUC, K(D)) of amitriptyline with acrophase in the dark phase was observed for brain (12 h period), lung and liver (24 h), but not for heart or kidney. After single (i.v. and i.g.) amitriptyline administration, concentrations of its major metabolite, nortriptyline, were negligible; however, after ten doses, nortriptyline serum and tissue levels were similar to the concentrations of the parent drug with higher values during the day (light phase).

Optimal sampling times for Bayesian estimation of the pharmacokinetic parameters of nortriptyline during therapeutic drug monitoring

Sampling times for Bayesian estimation of the pharmacokinetic parameters of an antidepressant drug, nortriptyline, during its therapeutic drug monitoring were optimized. Our attention was focused on designs including a limited number of measurements: one, two, and three sample designs in which sampling times had to be chosen between 0 and 24 hr after the last intake of a test-dose study. The optimization was conducted in four groups of patients defined by their gender and the administration or not of concomitant drugs inhibiting the metabolism of nortriptyline. The Bayesian design criterion was defined as the expected information provided by an experiment. A stochastic approximation algorithm, the Kiefer-Wolfowitz algorithm, was used for the criterion maximization under experimental constraints. Results showed that optimal Bayesian sampling times differ between patients in monotherapy and polytherapy. For one-sample designs the measurements have to be performed either at the lower (0 hr) or at the upper (24 hr) bound of the admissible interval. Replications were often found for 2- and 3-point designs. Other sampling designs can lead to criterion close to the optimum and can therefore be performed without great loss of information. In contrast, we found that several designs lead to low values of the information criterion, which justifies the approach.

High-performance liquid chromatographic assay with fluorescence detection for the routine monitoring of the antidepressant mirtazapine and its demethyl metabolite in human plasma

A validated HPLC method for the simultaneous quantitative analysis of the antidepressant mirtazapine and its demethyl metabolite in human plasma is described. The active constituents including internal standard were extracted from 1 ml of plasma with hexane and separated on a muBondapak Phenyl column with fluorescence detection. The lower limit of quantification was 0.5 ng/ml, without significant interferences with endogenous or exogenous components. Inter- and intra-assay accuracy determined at quality control levels of 2, 10 and 80 ng/ml were, respectively, 104.6-113.7% and 105.1-117.7% for mirtazapine, and 91.7-99.3% and 89.9-103.7% for demethylmirtazapine. In all cases the precision was below 6.8%.

Redistribution of basic drugs into cardiac blood from surrounding tissues during early-stages postmortem

The objective of this study was to elucidate the mechanism(s) responsible for increases in the concentrations of basic drugs in cardiac blood of bodies in a supine position during early-stages postmortem. The concentrations of basic drugs in cardiac blood and other fluids and tissues of three individuals who had used one or more basic drugs were examined. The results were compared with those obtained in experiments using rabbits. In the first case, autopsy of whom was performed approximately 12 h after death, methamphetamine was detected and its concentrations were in the order: lung >> pulmonary venous blood > blood in the left cardiac chambers (left cardiac blood) >> pulmonary arterial blood > blood in the right cardiac chambers (right cardiac blood). In the second case, autopsy of whom was performed approximately 9 h after death, methamphetamine and morphine were detected and their concentrations in the left cardiac blood were roughly twice those in the right cardiac blood. The methamphetamine and morphine concentrations in the lung were 2 to 4 times higher than those in cardiac blood samples. In the third case, autopsy of whom was performed approximately 2.5 days after death, the pulmonary veins and arteries were filled with chicken fat clots. Toxicological examination revealed the presence of four basic drugs: methamphetamine, amitriptyline, nortriptyline and promethazine. Their concentrations in the lung were 5 to 300 times higher than those in cardiac blood, but postmortem increases in the concentrations of these drugs in the cardiac blood were not observed. In the animal experiments, rabbits were given 5 mg/kg methamphetamine intravenously or 20 mg/kg amitriptyline subcutaneously and sacrificed 20 min or 1 h later, respectively. The carcasses were left in a supine position at the ambient temperature for 6 h after or without ligation of the large vessels around the heart. For the groups with ligated vessels, the mean ratios of the drug concentrations in both left and right cardiac blood samples 6 to 0 h postmortem were about 1, whereas in those without ligated vessels, these ratios were about 2 and 1, respectively. The order of the methamphetamine and amitriptyline concentrations in blood and tissue samples were roughly: lungs > myocardium and pulmonary venous blood > cardiac blood, inferior vena caval blood and liver. Our results demonstrate that when bodies are in a supine position, (1) basic drugs in the lungs diffuse rapidly postmortem into the left cardiac chambers via the pulmonary venous blood rather than simply diffusing across concentration gradients, and (2) basic drugs in the myocardium contribute little to the increases in their concentrations in cardiac blood during the early postmortem period.

[Grapefruit juice as a contraindication? An approach in psychiatry]

The authors investigated in this preliminary study the influence of grapefruit juice on the metabolism of two tricyclic antidepressants. An increase of plasma concentrations is observed indeed for many drugs when administered concomitantly with grapefruit juice. This effect was mainly attributed to inhibition of cytochrome P450 1A2 and 3A4 enzymes by naringenin. These isoenzymes are involved too in the metabolism of many psychotropic drugs. Only two benzodiazepines (midazolam and triazolam) were studied in the conditions of grapefruit juice association. All these studies are performed in healthy subjects and with a study design very different from the clinical conditions. On the basis of these considerations, the authors hypothesized that grapefruit juice should inhibit tricyclic antidepressant metabolism and thus increase the bioavailability of these drugs. They want to precise if this possible drug plasma level increase could be clinically important for depressed patients. Fourteen depressed inpatients were selected for the study. Seven of them received amitriptyline (100 to 150 mg/d) and the seven others clomipramine (112.5 to 225 mg/d). Tricyclic antidepressant and desmethylated metabolite plasma levels were determined on four occasions. The first and second day samples were obtained to determined the plasma level intraindividual variability of antidepressants. On the third and fourth days, plasma levels were determined after an oral coadministration of the antidepressant and 250 ml of pure and fresh grapefruit juice. One patient was excluded from the study due to the coadministration of clomipramine and fluvoxamine. There is indeed a major drug-interaction between these two drugs, and the tricyclic antidepressant plasma levels of this patient were in the toxic range, without side effect. In this group of patients, there was no metabolic interaction between amitriptyline and grapefruit juice. But the mean plasma levels of clomipramine and desmethylclomipramine increased after coadministration of this juice (+4.5% and +10.5% respectively). The authors concluded that with these preliminary results, the potential clinical relevance of this interaction cannot be estimated.

Desipramine clearance in children and adolescents: absence of effect of development and gender

OBJECTIVE

To examine the influence of development and gender on the pharmacokinetics of desipramine (DMI) in the pediatric population.

METHOD

DMI pharmacokinetic parameters were calculated from 407 routinely drawn, dose- and weight-normalized serum concentrations in 173 youths receiving DMI (90 children, 83 adolescents; 29 were female, 144 were male).

RESULTS

Mean pharmacokinetic parameters for the entire population included dose (3.78 +/- 1.51 mg/kg), weight- and dose-normalized serum concentration (45.41 +/- 47.39 [micrograms/L]/[mg/kg]), and DMI clearance (0.68 +/- 1.51 [L/kg]/hr). No between-group differences for children and adolescents were detected in dose (child, adolescent) (3.73 +/- 1.40 mg/kg, 3.83 +/- 1.68 mg/kg), weight- and dose-normalized serum concentrations (44.52 +/- 39.6 [micrograms/L]/[mg/kg], 46.34 +/- 34.89 [micrograms/L]/[mg/kg]; p = .62), and clearance (0.680 +/- 0.890 [L/kg]/hr, 0.695 +/- 1.05 [L/kg]/hr; p = .103). No between-group gender differences were detected in dose (male, female) (3.83 +/- 1.55 mg, 3.39 +/- 1.84 mg), weight- and dose-normalized serum concentrations (45.15 +/- 37.76 [micrograms/L]/[mg/kg], 47.14 +/- 34.96 [micrograms/L]/mg/kg]; p = .720), and clearance (0.699 +/- 0.89 [L/kg]/hr, 0.606 +/- 0.535 [L/kg]/hr; p = .390).

CONCLUSIONS

These results suggest that age and gender do not significantly influence DMI clearance or dose-normalized serum concentrations in the pediatric population.

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.

[The bioequivalence of a new amitriptyline tablet formulation in comparison with a reference preparation]

An investigation of the bioequivalence of a new tablet formulation (amitriptylin 25 von ct) with 28.3 mg amitriptyline hydrochloride (CAS 549-18-8) was performed in a two-way cross-over study with 18 subjects. The relative bioavailability with respect to a reference preparation for AUC related to amitriptyline (CAS 50-48-6) was 99.3% and for Cmax 100.4%. A positive decision for bioequivalence derived from the usual confidence intervals for both parameters related to amitriptyline and the metabolite nortriptyline (CAS 72-69-5), respectively tmax showed no difference. The new formulation was bioequivalent to the reference.

Pharmacokinetics of nortriptyline and its 10-hydroxy metabolite in Chinese subjects of different CYP2D6 genotypes

OBJECTIVES

To study the impact of the CYP2D6*10 allele on the disposition of nortriptyline in Chinese subjects.

METHODS

A single dose of 25 mg nortriptyline was given orally to 15 healthy Chinese volunteers who were classified as extensive metabolizers after phenotyping with debrisoquin (INN, debrisoquine) and who were genotyped by allele-specific polymerase chain reaction. Five subjects were homozygous for CYP2D6*1, 5 subjects were homozygous for CYP2D6*10, and 5 subjects were heterozygous for these 2 alleles. Plasma concentrations of nortriptyline and its main metabolite 10-hydroxynortriptyline were measured by liquid chromatography-mass spectrometry, and the pharmacokinetics were studied during 168 hours after the dose.

RESULTS

Subjects who were homozygous for CYP2D6*10 had significantly higher total areas under the plasma concentration-time curve (AUC), lower apparent oral clearances, and longer mean plasma half-life of nortriptyline than subjects in the CYP2D6*1/*1 and the heterozygous groups. For 10-hydroxynortriptyline, the AUC was lower and the plasma half-life was longer in subjects who were homozygous for CYP2D6*10 than in subjects in the other 2 groups.

CONCLUSION

The CYP2D6*10 allele in Chinese subjects was associated with significantly higher plasma levels of nortriptyline compared with the CYP2D6*1 allele because of an impaired metabolism of nortriptyline to 10-hydroxynortriptyline, particularly in the subjects with the CYP2D6*10/*10 genotype. The results suggest that genotyping of CYP2D6 may be a useful tool in predicting the pharmacokinetics of nortriptyline.

Inhibition of desipramine hydroxylation (Cytochrome P450-2D6) in vitro by quinidine and by viral protease inhibitors: relation to drug interactions in vivo

Pharmacokinetic drug interactions with viral protease inhibitors are of potential clinical importance. An in vitro model was applied to the quantitative identification of possible interactions of protease inhibitors with substrates of cytochrome P450-2D6. Biotransformation of desipramine (DMI) to hydroxydesipramine (OH-DMI), an index reaction used to profile activity of human cytochrome P450-2D6, was studied in vitro using human liver microsomes. Quinidine and four viral protease inhibitors currently used to treat human immunodeficiency virus infection were tested as chemical inhibitors in this system. Formation of OH-DMI from DMI was consistent with Michaelis-Menten kinetics, having a mean Km value of 11.7 microM (range: 9.9-15.3 microM). Quinidine, a highly potent and relatively selective inhibitor of P450-2D6, strongly inhibited OH-DMI formation with an apparent competitive mechanism, having a mean inhibition constant of 0.16 microM (range: 0.13-0.18 microM). All four protease inhibitors impaired OH-DMI formation; the pattern was consistent with a mixed competitive-noncompetitive mechanism. Mean inhibition constants (small numbers indicating greater inhibiting potency) were as follows: ritonavir, 4.8 microM; indinavir, 15.6 microM; saquinavir, 24.0 microM; nelfinavir, 51.9 microM. In a clinical pharmacokinetic study, coadministration of ritonavir with DMI inhibited DMI clearance by an average of 59%. The in vitro findings, together with observed plasma ritonavir concentrations, provided a reasonable quantitative forecast of this interaction, whereas estimated unbound plasma or intrahepatic ritonavir concentrations yielded poor quantitative forecasts. Thus the in vitro model correctly identifies ritonavir as a potent and clinically important inhibitor of human P450-2D6. Other protease inhibitors may also inhibit 2D6 activity in humans, but with lower potency than ritonavir.

[Interaction between paroxetine and clomipramine as a possible reason for admission to a department of internal medicine]

A 79-year-old woman was admitted due to increasing dizziness during the past six days. She had been treated with clomipramine 75 mg daily for years, and in addition had received paroxetine 20 mg daily for the last eight days. Both drugs were terminated and the symptoms disappeared. Thirty-six hours after taking the last drugs, the S-clomipramine plus S-desmethylclomipramine were 790 mmol/l. The symptoms were probably related to this high serum level caused by interaction with paroxetine.