[Tricyclic antidepressants as inhibitors of brain glutathione-S-transferase]

Tricyclic antidepressants, amitriptyline, doxepin--derivatives of cycloheptadiene as well as imipramine and clomipramine, derivatives of dibenzazepine inhibit the activity of glutathione-S-transferase pi isolated from different regions of human brain (parietal cortex, frontal cortex, brain stem). The inhibitory effect of studied drugs depends more on their chemical structure than on brain localization of the enzyme. All tricyclics bind nonspecifically to the effector site of glutathione-S-transferase (GST). The obtained results indicate that the inhibitory effect of tricyclic antidepressants on brain GST may decrease the efficiency of the enzymatic barrier, which protects brain against toxic electrophiles, and contribute in their adverse effects. On the other hand, brain GST may decrease the therapeutic effects of tricyclic antidepressants by binding them as ligands.

CYP2D6 genotyping with oligonucleotide microarrays and nortriptyline concentrations in geriatric depression

Recent advances in oligonucleotide microarray technology ("gene chips") permit rapid screening for DNA sequence variation. The CYP2D6 gene encodes debrisoquine hydroxylase, which metabolizes the antidepressant nortriptyline and other psychotropic medications. Nortriptyline plasma concentrations were obtained after at least three weeks of treatment in 36 geriatric patients with major depression who were taking a mean of 8.6 other medications besides nortriptyline. Oligonucleotide microarrays were used to detect 16 CYP2D6 alleles that affect debrisoquine hydroxylase activity. Subjects carrying alleles encoding impaired debrisoquine hydroxylase activity had significantly greater nortriptyline concentrations and lower nortriptyline doses than did other subjects. Significant correlations were found between the numbers of alleles encoding decreased metabolism and nortriptyline plasma concentration, nortriptyline dose, and nortriptyline plasma concentration standardized for dose, indicating a gene dosage effect. These results demonstrate that CYP2D6 genotyping on a microarray platform can be used to predict plasma antidepressant concentrations despite advanced patient age and numerous concurrent medications.

Tianeptine: a review of its use in depressive disorders

Tianeptine is an antidepressant agent with a novel neurochemical profile. It increases serotonin (5-hydroxytryptamine; 5-HT) uptake in the brain (in contrast with most antidepressant agents) and reduces stress-induced atrophy of neuronal dendrites. Like the selective serotonin reuptake inhibitors (SSRIs) and in contrast with most tricyclic antidepressant agents, tianeptine does not appear to be associated with adverse cognitive, psychomotor, sleep, cardiovascular or bodyweight effects and has a low propensity for abuse. Tianeptine has a comparatively favourable pharmacokinetic profile. It is not subject to first-pass hepatic metabolism, has high bioavailability and limited distribution, and is rapidly eliminated. While this offers advantages for tianeptine over the tricyclic antidepressant agents in terms of dose titration, treatment changes and potential drug interactions, its rapid elimination makes adherence to dosage schedules more important. Tianeptine differs from most antidepressants in that it is not primarily metabolised by the hepatic cytochrome P450 system, indicating less likelihood of drug-drug interactions; this is of particular interest for elderly patients. Tianeptine, in dosages of 25 to 50 mg/day, has been investigated in patients with major depression, depressed bipolar disorder, dysthymia or adjustment disorder. It has equivalent antidepressant efficacy to several classical antidepressant agents (amitriptyline, clomipramine, imipramine, mianserin) and the SSRIs fluoxetine (in most patients), paroxetine and sertraline. Comparison with maprotiline indicated superior efficacy for tianeptine but dothiepin appeared superior in another study. Extended treatment with tianeptine decreases the incidence of relapse/recurrence of depression. Tianeptine appears to be as effective as fluoxetine, sertraline, amitriptyline, clomipramine and mianserin and more effective than maprotiline in improving associated anxiety in patients with depressive disorders. Depression and anxiety symptoms in alcohol dependant patients also respond well to tianeptine. The adverse effects associated with tianeptine are similar in many respects to those of the SSRIs and minimal in comparison with the tricyclic antidepressants. The most common adverse effects are nausea, constipation, abdominal pain, headache, dizziness and changes in dreaming. Anticholinergic effects occur less often with tianeptine than with tricyclic agents. Hepatoxicity is rare. The dosage should be decreased in elderly patients and those with severe renal failure, but adjustment is not necessary in patients with alcoholism or hepatic impairment, or those undergoing haemodialysis.

CONCLUSIONS

The antidepressant efficacy and favourable tolerability and pharmacokinetic profiles of tianeptine in patients with depression, including those with associated anxiety, have been proven; the data indicate that it may have additional potential in specific subgroups of depressed patients such as the elderly and those with chronic alcoholism.

Relative contribution of CYP3A to amitriptyline clearance in humans: in vitro and in vivo studies

The relative contribution of cytochrome P450 3A (CYP3A) to the oral clearance of amitriptyline in humans has been assessed using a combination of in vitro approaches together with a clinical pharmacokinetic interaction study using the CYP3A-selective inhibitor ketoconazole. Lymphoblast-expressed CYPs were used to study amitriptyline N-demethylation and E-10 hydroxylation in vitro. The relative activity factor (RAF) approach was used to predict the relative contribution of each CYP isoform to the net hepatic intrinsic clearance (sum of N-demethylation and E-10 hydroxylation). Assuming no extrahepatic metabolism, the model-predicted contribution of CYP3A to net intrinsic clearance should equal the fractional decrement in apparent oral clearance of amitriptyline upon complete inhibition of the enzyme. This hypothesis was tested in a clinical study of amitriptyline (50 mg, p.o.) with ketoconazole (three 200 mg doses spaced 12 hours apart) in 8 healthy volunteers. The RAF approach predicted CYP2C19 to be the dominant contributor (34%), with a mean 21% contribution of CYP3A (range: 8%-42% in a panel of 12 human livers). The mean apparent oral clearance of amitriptyline in 8 human volunteers was decreased from 2791 ml/min in the control condition to 2069 ml/min with ketoconazole. The average 21% decrement (range: 2%-40%) was identical to the mean value predicted in vitro using the RAF approach. The central nervous system (CNS) sedative effects of amitriptyline were slightly greater when ketoconazole was coadministered, but the differences were not statistically significant. In conclusion, CYP3A plays a relatively minor role in amitriptyline clearance in vivo, which is consistent with in vitro predictions using the RAF approach.

Inhibition of cytochrome P4502D6 activity with paroxetine normalizes the ultrarapid metabolizer phenotype as measured by nortriptyline pharmacokinetics and the debrisoquin test

BACKGROUND

The ultrarapid metabolizer phenotype of the cytochrome P4502D6 (CYP2D6) enzyme has been considered a relevant cause of nonresponse to antidepressant drug therapy. Prescribing high doses of antidepressants to such patients leads to high concentrations of potentially toxic metabolites and an increased risk for adverse reactions. Normalization of the metabolic status of ultrarapid metabolizers by inhibition of CYP2D6 activity could offer a clinically acceptable method to successfully treat such patients with antidepressants.

METHODS

Five ultrarapid metabolizers with a CYP2D6 gene duplication or triplication were treated with 25 mg nortriptyline twice a day for 3 consecutive weeks, alone during the first week and concomitantly with the CYP2D6 inhibitor paroxetine 10 mg or 20 mg twice a day, respectively, during the second and third weeks. After the third week, nortriptyline was discontinued and the subjects were treated with paroxetine 20 mg twice a day during the fourth study week. At the end of each study week, the steady-state pharmacokinetic parameters of nortriptyline or paroxetine were determined within the dose interval. In addition, the CYP2D6 phenotype was determined by debrisoquin (INN, debrisoquine) test at baseline and at the end of each study phase. Treatment-related adverse events were recorded during drug administration and for 1 week thereafter.

RESULTS

All 5 subjects had very low (subtherapeutic) nortriptyline concentrations after 7 days' treatment with nortriptyline only. Addition of paroxetine 10 mg twice a day to the nortriptyline regimen resulted in a change in all individuals to the "normal" extensive debrisoquine metabolizer phenotype, and therapeutic plasma nortriptyline concentrations were achieved in 4 of 5 subjects after a 3 times mean increase in nortriptyline trough concentration (P =.0011). Doubling the paroxetine dose caused a 15 times mean increase in paroxetine trough concentration (P <.001), indicating strong inhibition by paroxetine of its own metabolism. The high paroxetine concentrations in 2 subjects caused them to have the poor debrisoquine metabolizer phenotype and resulted in a further increase in plasma nortriptyline trough concentration (P =.0099). A strong correlation (rank correlation coefficient [r(s)] = 0.89; P <.0001) was observed between paroxetine and nortriptyline trough concentrations. Paroxetine also significantly decreased the fluctuation of nortriptyline concentrations within the dose interval. One subject discontinued the study after the second study week because of adverse effects; otherwise, the study drugs were well tolerated.

CONCLUSIONS

Paroxetine, with a daily dosage from 20 to 40 mg, is an effective tool in normalizing the metabolic status of CYP2D6 ultrarapid metabolizers.

[Pharmacokinetics and drug interactions of antidepressive agents]

Tricyclic antidepressive agents(TCAs) are conventional antidepressant. Cytochrome P450(CYP) 2D6 is involved in the hydroxylation of TCAs, while N-demethylation of TCAs is mediated by other such as CYP2C19, 3A4 and 1A2. The elimination of TCAs is impaired by CYP2D6 inhibitors such as quinidine. Newer antidepressants, selective serotonin uptake inhibitors(SSRIs), are also metabolized in the liver. Fluvoxamine, an SSRI, is a potent inhibitors for CYP1A2 and CYP2C19, moderate for CYP3A4 and weak for CYP 2D6. Paroxetine, another SSRI, causes substantial inhibition of CYP2D6 activity. Milnacipran, a serotonin and noradrenaline reuptake inhibitor, is mainly excreted unchanged in urine and some part as its glucronide conjugate. In contrast to many SSRIs, milnacipran is devoid of metabolic inhibition.

Tricyclic antidepressant overdose: a review

Overdoses of tricyclic antidepressants are among the commonest causes of drug poisoning seen in accident and emergency departments. This review discusses the pharmacokinetics, clinical presentation and treatment of tricyclic overdose.

Relative bioavailability of imipramine (Tofranil) coated tablets in healthy volunteers

OBJECTIVES

Imipramine is a tricyclic antidepressant drug with a considerable hepatic first-pass metabolism resulting in highly variable pharmacokinetic characteristics and desipramine as active major metabolite. This study describes the bioavailability of 3 formulations of imipramine.

METHODS

In a randomized, three-period crossover study, 18 healthy male Caucasian subjects received single oral doses of Tofranil 25, Tofranil mite (10 mg) and an aqueous solution containing 25 mg imipramine-HCl. Serum concentrations of imipramine-HCl and its main metabolite desipramine were measured. The pharmacokinetic characteristics, Cmax, AUC, t1/2 and tmax were determined and the relative bioavailability of the two coated tablet formulations was calculated with the aqueous solution as reference. Safety and tolerability were assessed using vital signs, ECG, clinical laboratory and adverse event recording.

RESULTS

The relative bioavailabilities of Tofranil 25 and Tofranil mite were 97% and 81%, respectively. The study medication was well tolerated.

CONCLUSIONS

A sufficiently high extent of absorption was found for the test formulations ensuring therapeutic efficacy.

Chronic effects of triiodothyronine in combination with imipramine on 5-HT transporter, 5-HT(1A) and 5-HT(2A) receptors in adult rat brain

Triiodothyronine (T3) has been shown to accelerate and potentiate the clinical response to tricyclic antidepressant (TCA) treatment in depressive disorders. The neurobiological mechanisms underlying these therapeutic effects of T3 are still unknown. Since brain serotonin (5-HT) changes have been implicated in the mode of action of TCA drugs, the effects of a chronic (7 or 21 days) administration of imipramine (10 mg/kg/day) and of a low dose of T3 (4 microg/kg/day), given alone or in combination, were investigated on the density of midbrain 5-HT transporters and of hippocampal 5-HT(1A) and cortical 5-HT(2A) receptors in adult Wistar rats. Neither single nor combined administration of imipramine and T3 for 7 days modified the density of 5-HT transporters and of 5-HT(1A) receptors. On day 21, the combination did not change imipramine- or T3-induced decrease in 5-HT transporter density whereas it prevented imipramine-induced increase in 5-HT(1A) receptor density. Whatever the treatment duration, imipramine-T3 combination potentiated imipramine-induced decrease in 5-HT(2A) receptor density. On both day 7 and day 21, T3 given alone had no effects on the density of 5-HT(1A) and 5-HT(2A) receptors. These data indicate that T3 is able to modulate the long-term adaptive changes which occur at the postsynaptic level of 5-HT neurotransmission after antidepressant treatment.

Bioavailability investigation of two different oral formulations of doxepin

Two different oral doxepin (CAS 1668-195) formulations (Doxepin-ratiopharm 25 mg film-coated tablets as test preparation and 25 mg dragées of the reference preparation) were investigated in 30 healthy male and female volunteers in order to prove bioequivalence between these preparations. A single 75 mg oral dose (3 units of test or reference preparation) was given according to a randomised two-way cross-over design in the fasted state. Blood samples for determination of plasma concentration of doxepin and its metabolite N-desmethyldoxepin were collected at pre-defined time points up to 168 h following drug administration. A wash-out period of three weeks separated both treatment periods. Doxepin and N-desmethyldoxepin plasma concentrations were determined by means of a validated LC-MS/MS method. Values of 193,463 pgh/ml (test preparation) and 197,988 pg h/ml (reference preparation) for doxepin as well as values of 313,298 pg h/ml (test preparation) and 306,663 pgh/ml (reference preparation) for N-desmethyldoxepin for the parameter AUC0-infinity demonstrate a nearly identical extent of drug absorption. Maximum concentrations (Cmax) for doxepin/N-desmethyldoxepin of 15,960.06/6,883.69 pg/ml and 18,614.73/6,846.62 pg/ml were achieved for test and reference preparation. Time to reach doxepin maximum plasma concentration (tmax) was 1.98 h for both preparations and for N-desmethyldoxepin tmax was 4.52 h (test preparation) and 4.15 h (reference preparation). Cmax and AUC0-infinity values were tested for statistically significant differences by means of the Two One-Sided T-Tests procedure following ln-transformation of data. Bioequivalence was assumed if the 90% confidence intervals of the T/R-ratios were in the range of 80%-125% for ln-transformed AUC0-infinity and 70%-143% for ln-transformed Cmax. Based on the results obtained in this study, bioequivalence between the test and the reference preparation was demonstrated.

Duplication of CYP2D6 predicts high clearance of desipramine but high clearance does not predict duplication of CYP2D6

OBJECTIVE

Duplication of CYP2D6 causes very rapid metabolism of CYP2D6 substrates such as desipramine. However, we have previously shown that in the Danish population, only about 15% of very rapid metabolisers, defined as subjects with a metabolic ratio of sparteine of 0.15 or less, carried a duplicated allele. The question is whether gene duplication is a relatively rare cause (perhaps predictor) of very rapid metabolism or whether a low metabolic ratio is a poor predictor of this.

METHODS

After measuring metabolic ratios anew, we selected six volunteers with duplication of CYP2D6 and metabolic ratios ranging from 0.07 to 0.17 and six volunteers without duplication with metabolic ratios ranging from 0.08 to 0.21. Each subject took 100 mg of desipramine. Blood and urine were collected for 48 h.

RESULTS

The median total oral clearance of desipramine was 372 l/h and 196 l/h [median difference 108 l/h (95.9% c.i., -304-598 l/h)] and the median partial clearance of desipramine by 2-hydroxylation was 155 l/h and 87 l/h [median difference 47 l/h (95.9% c.i., -124-141 l/h)] for the group with duplication and the group without duplication, respectively.

CONCLUSION

The predictive value of duplication of CYP2D6 is poor; there must be other causes (or predictors) of very rapid metabolism and with much higher frequency than duplication of CYP2D6.

Pharmacokinetics of imipramine in narcoleptic horses

OBJECTIVE

To validate use of high-performance liquid chromatography (HPLC) in determining imipramine concentrations in equine serum and to determine pharmacokinetics of imipramine in narcoleptic horses.

ANIMALS

5 horses with adult-onset narcolepsy.

PROCEDURE

Blood samples were collected before (time 0) and 3, 5, 10, 15, 20, 30, and 45 minutes and 1, 2, 3, 4, 6, 8, 12, and 24 hours after IV administration of imipramine hydrochloride (2 or 4 mg/kg of body weight). Serum was analyzed, using HPLC, to determine imipramine concentration. The serum concentration-versus-time curve for each horse was analyzed separately to estimate pharmacokinetic values.

RESULTS

Adverse effects (muscle fasciculations, tachycardia, hyperresponsiveness to sound, and hemolysis) were detected in most horses when serum imipramine concentrations were high, and these effects were most severe in horses receiving 4 mg of imipramine/kg. Residual adverse effects were not apparent. Value (mean +/- SD) for area under the curve was 3.9 +/- 0.7 h X microg/ml, whereas volume of distribution was 584 +/- 161.7 ml/kg, total body clearance was 522 +/- 102 ml/kg/h, and mean residence time was 1.8 +/- 0.6 hours. One horse had signs of narcolepsy 6 and 12 hours after imipramine administration; corrresponding serum imipramine concentrations were less than the therapeutic range.

CONCLUSIONS AND CLINICAL RELEVANCE

Potentially serious adverse effects may be seen in horses administered doses of imipramine that exceed a dosage of 2 mg/kg. Total body clearance of imipramine in horses is slower than that in humans; thus, the interval between subsequent doses should be longer in horses.

Determination of opipramol in human plasma by high-performance liquid chromatography with photometric detection using a cyanopropyl column

A high-performance liquid chromatographic method is described for the determination of opipramol in human plasma. Opipramol was extracted into tert.-butylmethyl ether, separated on a cyanopropyl silica column and detected at 254 nm. Imipramine was used as internal standard. The limit of quantitation was 250 pg/ml using 1.5 ml plasma. Precision was better than 9%, inaccuracy less than 8%. The assay is more sensitive than previously published methods, and it has been applied to the analysis of plasma samples from a pharmacokinetic study.

Selective imidazoline I2 ligands do not show antidepressant-like activity in the forced swim test in mice

Clonidine is an adrenergic agonist with high affinity for alpha2 adrenoceptors that also has affinity for imidazoline receptors. Clonidine has previously been shown to reduce immobility in the forced swim test (FST) in mice. In the present study, this effect was blocked by idazoxan (0.06 mg/kg s.c.) and by yohimbine (1.0 mg/kg s.c.) suggesting that clonidine's effects in this test are mediated via its action at alpha2 sites. Imidazoline I2 site ligands have been shown to inhibit monoamine oxidase and thus may also have antidepressant activity. Three compounds with selective affinity for I2 receptors (BU224, BU239, BDF 8082) were also tested in the FST. These compounds showed no activity either alone or in combination with a subthreshold dose of imipramine in the FST. These results suggest that I2 receptor ligands do not show antidepressant-like activity in the FST in mice. Furthermore the activity of the mixed alpha2/I1 agonist clonidine is most likely to be due to its action at alpha2 sites.

Radiopacity of clomipramine conglomerations and unsuccessful endoscopy: report of 4 cases

BACKGROUND

The radiopacity of ingested substances may serve as a clue to the presence of particular compounds, as this characteristic varies considerably among medications and household products. Tablet conglomerations are also variably radiopaque. We report 4 cases of clomipramine poisoning associated with formation of radiopaque masses, believed to be clomipramine, in the area of the stomach.

CASE REPORTS

Four patients were admitted to the Toxicological Intensive Care Unit after ingestions of, respectively, 8.5 g (180 tablets of mixed strength), 7.5 g (100 tablets), 10.5 g (140 tablets), and 4.5 g (60 tablets) of clomipramine, along with other sedatives and antipsychotics. In each case, a rounded density was observed in the gastric area on plain chest radiograph. The hospital courses of each patient were marked by tachycardia, hypotension, QRS and QT prolongation, seizures, and decreased mental status. Three of 4 patients underwent unsuccessful endoscopy to remove tablet fragments and subsequently suffered gastrointestinal hemorrhage requiring transfusion. All patients were discharged recovered from the hospital.

DISCUSSION

Clomipramine, a potent tricyclic antidepressant, has been previously reported to be nonradiopaque, and has not been reported to induce formation of concretions. These cases suggest that massive ingestions of clomipramine may form bezoars which are radiopaque and may be associated with serious toxicity. Careful consideration should be given prior to the use of gastric endoscopy for the retrieval of tablet fragments since significant hemorrhage, attributed to the procedure itself rather than to clomipramine toxicity, may ensue.

Transplacental transfer of amitriptyline and nortriptyline in isolated perfused human placenta

RATIONALE

Although tricyclic antidepressants (TCAs) have gained wide acceptance for use in the treatment of depression in pregnant women, their pharmacokinetics during pregnancy have been poorly characterized. The aim of the present study was to investigate the transplacental transfer of amitriptyline (AMI) and its main active metabolite nortriptyline (NOR) in isolated perfused human placenta.

METHODS

Nine term human placentae were obtained immediately after delivery with maternal consent and a 2-h non-recirculating perfusion of a single placental cotyledon was performed. AMI (200 ng/ml) and NOR (150 ng/ml), with antipyrine as a reference compound, were added to the maternal reservoir and their appearance to the fetal circulation was followed for 2 h. AMI and NOR concentrations were measured by high performance liquid chromatography (HPLC) and antipyrine concentrations spectrophotometrically.

RESULTS

The mean (SD) transplacental transfers (TPT(SS)%) for AMI and NOR were 8.2 (2.3)% and 6.5 (1.8)%, respectively, calculated as the ratio between the steady-state concentrations in fetal venous and maternal arterial sides. The TPTs of AMI and NOR were 81% and 62% of the freely diffusable antipyrine. The absolute fraction of the dose that crossed the placenta (TPT(A)) was moderately, but significantly higher for AMI (7.7%) than for NOR (5.7%) (P=0.037). In all perfusions, steady state at the fetal side was reached by 30 min for AMI and by 50 min for NOR in the fetal side. The viability of the placentae was retained during the 2-h perfusion, as evidenced by unchanged pH of the perfusate and by stable perfusion pressures in fetal artery and stable antipyrine transfer.

CONCLUSIONS

Both AMI and NOR cross the human placenta. However, the fetal exposure with NOR may be somewhat smaller compared with AMI, probably due to the higher lipophilicity of AMI.

Pharmacokinetic interaction between imipramine and carbamazepine in patients with major depression

RATIONALE

Despite the fact that carbamazepine (CBZ) is frequently added to the existing tricyclic antidepressant (TCA) therapy, to date little is known about serum levels of pharmacologically active hydroxy metabolites of TCAs, as well as about possible changes in free (non-protein-bound) concentrations of these drugs and their metabolites during such combination treatment of depression.

OBJECTIVE

The aim of this study was to evaluate the effect of CBZ on steady-state total and free serum concentrations of imipramine (IMI) and its metabolites, desipramine (DMI), 2-hydroxyimipramine and 2-hydroxydesipramnine, in depressed patients. In addition, the free and total serum concentrations of CBZ and 10,11-epoxycarbamazepine were measured.

METHOD

Thirteen patients with DSM-III-R diagnosis of major depression were enrolled in the study. All patients hospitalised at the Department of Psychiatry, Collegium Medicum, Jagiellonian University were treated with IMI at a dose of 2 mg/kg per day for 3 weeks, after which CBZ at a dose of 400 mg/day was added. Steady-state serum concentrations of IMI, CBZ and their metabolites were assayed by HPLC. Free drug concentrations were measured by ultrafiltration.

RESULTS

After 2 weeks of combination therapy a significant decrease in mean steady-state total serum concentrations of IMI (from 168.84 +/- 102.18 to 98.12 +/- 43.79 ng/ml) and DMI (from 293.89 +/- 171.93 to 221.85 +/- 153.21 ng/ml) was observed. Simultaneously, steady-state serum concentrations of total hydroxy metabolites and free IMI and its metabolites, measured just before and 2 weeks after CBZ were started, did not differ significantly. In consequence, a significant increase in free fraction of the parent drug was observed (3.36 +/- 3.24% vs 5.75 +/- 3.60%). Also free fraction of DMI tended to be higher after CBZ addition.

CONCLUSION

CBZ affects not only the metabolism of IMI and its metabolites, but also their protein binding. Therefore, despite considerable reductions in total serum levels of IMI and DMI, but when the unchanged free fraction concentration of these compounds is maintained, a dosage elevation of IMI does not seem to be necessary after CBZ addition to TCA therapy.

Pharmacokinetics of a sustained-release dosage form of clomipramine

In this study, the pharmacokinetic parameters of the sustained-release (SR) dosage form of clomipramine (CMI) were compared with those obtained after the administration of the immediate-release (IR) dosage form. Two studies were performed. In the first study, a single oral dose of both products was administered in 12 healthy volunteers, and in the second study, multiple doses of both products were administered in 6 patients with depression in which steady-state plasma levels (Css) were determined. Plasma levels were assayed using an HPLC method. In the single-dose study, the mean Cmax and AUC values of CMI were 63.37 ng/mL-1 and 1375.38 ng.h/mL-1 for the reference product and 32.55 ng/mL-1 and 1285.26 ng.h/mL-1 for the test product, respectively. The mean beta and MRT values of CMI were 0.030 h-1 and 47.21 h for the reference product and 0.026 h-1 and 55.63 h for test product, respectively. Results showed that after the multiple-dose study, the mean clomipramine plus demethylclomipramine values of Cavss were 406.2 ng/mL-1 for the reference and 328.6 ng/mL-1 for the sustained-release dosage form. This product also presented less fluctuation in steady-state plasma levels (1.08 vs. 1.23). The values of MRT and tmax were higher for the SR dosage form, showing that the drug was maintained longer in the body. The mean values for the ratio metabolite/drug were 2.06 and 2.41 for the IR and SR dosage forms, respectively. Neither AUC nor elimination half-life was affected significantly after the administration of the sustained-release dosage form.

Population pharmacokinetics of clomipramine, desmethylclomipramine, and hydroxylated metabolites in patients with depression receiving chronic treatment: model evaluation

Because metabolites play a major role in the clinical response to clomipramine, the objective of the current study was to develop a population model and evaluate its performance to describe the pharmacokinetic profiles of clomipramine (C) and its active metabolites desmethylclomipramine (DC), 8-hydroxy-clomipramine (OHC) and 8-hydroxy-desmethylclomipramine (OHDC). A first sample of 14 patients served for development of a 2-molecule C and DC model, which was shown to provide reasonable estimates of AUC-based clearances, as well as precise estimation of interindividual variability. Simulated data, generated to mimic a semi-rich sampling design and chronic treatment with clomipramine, indicated that clearance estimation was feasible under routine treatment conditions. A second sample of 30 patients, recruited prospectively and followed for a median 4-week period, was used to extend the 2-molecule model to a 4-molecule model. Goodness-of-fit assessment revealed that model-predicted concentrations were reasonably close to observed concentrations for a majority of patients. Interindividual variability was 50% to 60% for hydroxylation and desmethylation clearances, and residual variability was 30%. The proposed model incorporates much of what is known about the metabolism of clomipramine and may valuably integrate the influence of genetic and environmental factors on each metabolic pathway.

Effect of venlafaxine on imipramine metabolism

The present study was designed to determine the effect of venlafaxine on imipramine metabolism in an attempt to elucidate the potential for cytochrome P450 drug-drug interactions with venlafaxine. We examined the metabolism of a single 100-mg dose of imipramine before and after treatment with venlafaxine, 50 mg three times a day. Eight male subjects were phenotyped for CYP2D6 activity. Two subjects were poor metabolizers of dextromethophan, and data from the remaining six subjects (mean age=45.3+/-15) were analyzed. Venlafaxine increased imipramine C(max) and elevated AUC by 40%. Desipramine clearance and volume of distribution were reduced by 20% and 25%, respectively. These findings are consistent with a statistically significant, but clinically modest impact of venlafaxine on CYP2D6-metabolized substrates.

The role of metabolites in bioequivalency assessment. III. Highly variable drugs with linear kinetics and first-pass effect

PURPOSE

Simulated pharmacokinetic (PK) studies were done to determine the effect of intrinsic clearance (CL(INT)) on the probability of meeting bioequivalence criteria for extent (AUC) and rate (Cmax) of drug absorption when the absorption rate and fraction absorbed (F) were formulated either to be equivalent or to differ by 25%.

METHODS

Simulated PK studies were done using a linear first-pass model with CL(INT) values ranging from 15 L/HR to 900 L/HR. Test/Reference absorption rate constants (Ka) and fraction absorbed (Fa) ratios of 1.0 or 1.25 were used for all simulations. The impact of the value of CL(INT) and its intrasubject variation upon the probability of concluding bioequivalence at the two different Ka and F ratios was studied. Additionally, the effect of fraction metabolized i.v., (Fm) on the probabilities of concluding equivalence was studied at values of 0.25 and 0.75.

RESULTS

When CL(INT) values were raised above those for liver blood flow, the frequency of trials in which bioequivalence was correctly declared decreased when parent AUC was used as a bioequivalence criterion. Only when CL(INT) exceeded liver blood flow did the metabolite become important in assessing extent of absorption.

CONCLUSIONS

The Cmax for the parent drug provided the most accurate assessment of bioequivalence. The Cmax for the metabolite was insensitive to changes related to rate of input, and when CL(INT) exceeded liver blood flow, evaluation of the metabolite Cmax data may lead to a conclusion of bioequivalence for products that were not.

High-performance liquid chromatographic determination of tianeptine in plasma applied to pharmacokinetic studies

An improved analytical method for the quantitative measurement of tianeptine and its main metabolite MC5 in human plasma was designed. Extraction involved ion-paired liquid-liquid extraction of the compounds from 1.0 ml of human plasma adjusted to pH 7.0. HPLC separation was performed using a Nucleosil C18, 5 microm column (150x4.6 mm I.D.) and a mixture of acetonitrile and pH 3, 2.7 g l(-1) solution of sodium heptanesulfonate in distilled water (40:60, v/v) as mobile phase. UV detection was performed using a diode array detector in the 200-400 nm passband, and quantification of the analytes was made at 220 nm. For both tianeptine and MC5 metabolite, the limit of quantitation was 5 microg l(-1) and the calibration curves were linear from 5 to 500 microg l(-1). Intra- and inter-assay precision and accuracy fulfilled the international requirements. The recovery of tianeptine and its metabolite from plasma was, respectively, 71.5 and 74.3% at 20 microg l(-1), 71.2 and 70.8% at 400 microg l(-1). The selectivity of the method was checked by verifying the absence of chromatographic interference from pure solutions of the most commonly associated therapeutic drugs. This method, validated according to the criteria established by the Journal of Chromatography B, was applied to the determination of tianeptine and MC5-metabolite in human plasma in pharmacokinetic studies.

Concomitant use of mirtazapine and cimetidine: a drug-drug interaction study in healthy male subjects

OBJECTIVE

The objective of this study was to examine the pharmacokinetics and the tolerability/safety of mirtazapine and cimetidine separately and in combination following oral administration of multiple doses.

METHODS

This was a double-blind, placebo-controlled, two-period cross-over, multiple-dose pharmacokinetic interaction study in 12 healthy male subjects. They received either cimetidine (800 mg b.i.d.) or placebo in combination with (commercially available, racemic) mirtazapine (30 mg nocte). Cimetidine and placebo were administered for 14 days, with mirtazapine added during days 6-12 of each period. Serial blood samples for kinetic profiling were taken on day 5 and day 12 for cimetidine and on days 12-14 for mirtazapine.

RESULTS

The co-administration of cimetidine resulted in a statistically significant increase in the area under the curve (AUC(0-24)) and Cmax of mirtazapine (54% and 22% respectively). The AUC(0-24) of demethylmirtazapine increased only slightly, and there was no effect on Cmax. The elimination half-lives for both mirtazapine and its demethyl metabolite were unaffected by cimetidine co-administration. The trough and average plasma concentrations during the steady state were elevated during cimetidine treatment (62% and 54%, respectively). Mirtazapine had no effect on the pharmacokinetics of cimetidine.

CONCLUSION

Co-administration of cimetidine (800 mg b.i.d.) and mirtazapine (30 mg nocte) resulted in increased steady-state plasma levels of mirtazapine (C(ss,min) = +61%, P < 0.05; C(ss,av) = +54%, P < 0.05), probably as a result of increased bio-availability. The Cmax (+22%, P < 0.05) and AUC(0-24) (+54%, P < 0.05) also increased. Due to the variability of the mirtazapine plasma levels in patients, the clinical meaning of these increases is probably limited. Co-administration of mirtazapine did not alter cimetidine pharmacokinetics.