Richard F. Bergstrom

Quantification and mechanism of the fluoxetine and tricyclic antidepressant interaction

Clinical reports of concurrent use of fluoxetine and tricyclic antidepressant agents suggest that tricyclic concentrations increase upon coadministration with fluoxetine. This study was conducted to confirm the clinical reports, to quantify the degree of change in tricyclic kinetics, and to establish the mechanism of interaction. Twelve male subjects were given 50 mg desipramine (six subjects) or 50 mg imipramine (six subjects) on three occasions: alone, after a 60 mg dose of fluoxetine, and after eight daily 60 mg doses of fluoxetine. Fluoxetine significantly reduced oral clearance of both imipramine and desipramine as much as tenfold and prolonged half‐life as much as fourfold. Desipramine oral clearance values were 289, 112, and 27 L/hr alone, after a single fluoxetine dose, and after multiple fluoxetine doses, respectively. Correspondingly, imipramine oral clearance values were 181, 87, and 51 L/hr. These kinetic changes resulted in significantly higher plasma tricyclic concentrations after fluoxetine administration. The amount of parent drug excreted unchanged in urine increased and imipramine or desipramine clearance to their respective 2‐hydroxy metabolites decreased. Metabolic conversion of imipramine to desipramine appeared to be unaffected. The findings indicate that fluoxetine causes an inhibition of tricyclic 2‐hydroxylation and may decrease first‐pass and systemic metabolism. When imipramine or desipramine are to be coadministered with fluoxetine, a lower dosage may be needed to maintain steady‐state concentrations and to avoid undesirable side effects caused by excessive tricyclic concentrations.

The effect of fluoxetine on the pharmacokinetics and psychomotor responses of diazepam

To determine the effect of fluoxetine on diazepams pharmacokinetic and psychomotor responses, single oral doses of 10 mg diazepam were administered to six normal subjects on three occasions, either alone or in combination with 60 mg fluoxetine. Diazepam was given alone, after a single dose of fluoxetine, and after eight daily doses of fluoxetine. Psychometric data showed that fluoxetine had no significant effect on the psychomotor responses to diazepam. However, the pharmacokinetic data indicated a change in diazepam disposition after fluoxetine administration. Diazepam AUC was larger, the half‐life was longer, and the plasma clearance was lower after fluoxetine administration, suggesting that fluoxetine inhibited the metabolism of diazepam. The reduced formation of an active metabolite, N‐desmethyldiazepam, also suggested that fluoxetine inhibited diazepams metabolism. The clinical implications of this pharmacokinetic drug‐drug interaction are minor because psychomotor responses were unaffected and offsetting changes in the kinetics of diazepam and its metabolite occurred. Dosage modification of either fluoxetine or diazepam is unlikely to be necessary.

Fluoxetine in Depressed Patients on Dialysis

Objective: To test the safety and efficacy of fluoxetine in patients with renal failure on dialysis. Method: Fourteen patients with major depression and end stage renal disease on hemodialysis were randomized into two groups for an eight-week study. Subjects as well as investigators were blinded as to which subject received fluoxetine and which placebo. Patients were carefully monitored concerning adverse events, serum fluoxetine and norfluoxetine levels, and psychological measurements of degree of depression. Results: No patients discontinued treatment because of adverse events, all of which were minor. All psychological tests showed improvement in depression at the four-week and eight-weeks point, although statistical significance could only be demonstrated at the fourth week of this study. All patients in the active group had serum plasma concentrations of fluoxetine and norfluoxetine less than 250 ng/ml at eight weeks, similar to levels in patients with normal renal function in a previous open label study. Conclusions: This study confirms the relative safety of fluoxetine in depressed patients in renal failure on hemodialysis. It also suggests that fluoxetine may be efficacious in depressed patients on dialysis.

Duloxetine: clinical pharmacokinetics and drug interactions.

Duloxetine, a potent reuptake inhibitor of serotonin (5-HT) and norepinephrine, is effective for the treatment of major depressive disorder, diabetic neuropathic pain, stress urinary incontinence, generalized anxiety disorder and fibromyalgia. Duloxetine achieves a maximum plasma concentration (C(max)) of approximately 47 ng/mL (40 mg twice-daily dosing) to 110 ng/mL (80 mg twice-daily dosing) approximately 6 hours after dosing. The elimination half-life of duloxetine is approximately 10-12 hours and the volume of distribution is approximately 1640 L. The goal of this paper is to provide a review of the literature on intrinsic and extrinsic factors that may impact the pharmacokinetics of duloxetine with a focus on concomitant medications and their clinical implications. Patient demographic characteristics found to influence the pharmacokinetics of duloxetine include sex, smoking status, age, ethnicity, cytochrome P450 (CYP) 2D6 genotype, hepatic function and renal function. Of these, only impaired hepatic function or severely impaired renal function warrant specific warnings or dose recommendations. Pharmacokinetic results from drug interaction studies show that activated charcoal decreases duloxetine exposure, and that CYP1A2 inhibition increases duloxetine exposure to a clinically significant degree. Specifically, following oral administration in the presence of fluvoxamine, the area under the plasma concentration-time curve and C(max) of duloxetine significantly increased by 460% (90% CI 359, 584) and 141% (90% CI 93, 200), respectively. In addition, smoking is associated with a 30% decrease in duloxetine concentration. The exposure of duloxetine with CYP2D6 inhibitors or in CYP2D6 poor metabolizers is increased to a lesser extent than that observed with CYP1A2 inhibition and does not require a dose adjustment. In addition, duloxetine increases the exposure of drugs that are metabolized by CYP2D6, but not CYP1A2. Pharmacodynamic study results indicate that duloxetine may enhance the effects of benzodiazepines, but not alcohol or warfarin. An increase in gastric pH produced by histamine H(2)-receptor antagonists or antacids did not impact the absorption of duloxetine. While duloxetine is generally well tolerated, it is important to be knowledgeable about the potential for pharmacokinetic interactions between duloxetine and drugs that inhibit CYP1A2 or drugs that are metabolized by CYP2D6 enzymes.

Olanzapine pharmacokinetics in pediatric and adolescent inpatients with childhood-onset schizophrenia.

Well-designed studies investigating how pediatric or adolescent patients with mental disorders respond to and metabolize the newer antipsychotic drugs are practically nonexistent. Without such data, clinicians have difficulty designing appropriate dosage regimens for patients in these age groups. The results from a study of olanzapine pharmacokinetics in children and adolescents are described. Eight inpatients (ages 10-18 years) with treatment-resistant childhood-onset schizophrenia received olanzapine (2.5-20 mg/day) over 8 weeks. Blood samples, collected during dose titration and at a steady state provided pharmacokinetic data. The final evaluation (week 8) included extensive sampling for 36 hours after a 20-mg dose. Olanzapine concentrations in these eight pediatric patients were of the same magnitude as those for nonsmoking adult patients with schizophrenia but may be as much as twice the typical olanzapine concentrations in patients with schizophrenia who smoke. Olanzapine pharmacokinetic evaluation gave an apparent mean oral clearance of 9.6 +/- 2.4 L/hr and a mean elimination half-life of 37.2 +/- 5.1 hours in these young patients. The determination of the initial olanzapine dose for adolescent patients should take into consideration factors such as the patients size. In general, however, the usual dose recommendation of 5 to 10 mg once daily with a target dose of 10 mg/day is likely a good clinical guideline for most adolescent patients on the basis of our pharmacokinetics results.

Olanzapine plus carbamazepine v. carbamazepine alone in treating manic episodes

BACKGROUND Combinations of olanzapine and carbamazepine are often used in clinical practice in the management of mania. AIMS To assess the efficacy and safety of olanzapine plus carbamazepine in mixed and manic bipolar episodes. METHOD Randomised, double-blind, 6-week trial of olanzapine (10-30 mg/day) plus carbamazepine (400-1200 mg/day; n=58) v. placebo plus carbamazepine (n=60) followed by open-label, 20-week olanzapine (10-30 mg/day) plus carbamazepine (400-1200 mg/day, n=86), with change in manic symptoms as main outcome measure. Safety and pharmacokinetics were also evaluated. RESULTS There were no significant differences (baseline to endpoint) in efficacy measures between treatment groups, but at 6 weeks triglyceride levels were significantly higher (P=0.008) and potentially clinically significant weight gain (>or=7%) occurred more frequently (24.6% v. 3.4%, P=0.002) in the combined olanzapine and carbamazepine group. Carbamazepine reduced olanzapine concentrations but olanzapine had no effect on carbamazepine concentrations. CONCLUSIONS The combination of olanzapine and carbamazepine did not have superior efficacy to carbamazepine alone. The increases in weight and triglycerides observed during combination treatment are a matter of concern.

The effect of sertraline on the pharmacokinetics of desipramine and imipramine

To examine the pharmacokinetic interaction between the selective serotonin reuptake inhibitor sertraline and the tricyclic antidepressants desipramine or imipramine in 12 healthy male subjects.

In vitro and in vivo evaluations of cytochrome P450 1A2 interactions with duloxetine.

AbstractObjective: To determine whether duloxetine is a substrate, inhibitor or inducer of cytochrome P450 (CYP) 1A2 enzyme, using in vitro and in vivo studies in humans. Methods: Human liver microsomes or cells with expressed CYP enzymes and specific CYP inhibitors were used to identify which CYP enzymes catalyse the initial oxidation steps in the metabolism of duloxetine. The potential of duloxetine to inhibit CYP1A2 activity was determined using incubations with human liver microsomes and phenacetin, the CYP1A2 substrate. The potential for duloxetine to induce CYP1A2 activity was determined using human primary hepatocytes treated with duloxetine for 72 hours. Studies in humans were conducted using fluvoxamine, a potent CYP1A2 inhibitor, and theophylline, a CYP1A2 substrate, as probes. The subjects were healthy men and women aged 18–65 years. Single-dose duloxetine was administered either intravenously as a 10-mg infusion over 30 minutes or orally as a 60-mg dose in the presence or absence of steady-state fluvoxamine (100 mg orally once daily). Single-dose theophylline was given as 30-minute intravenous infusions of aminophylline 250 mg in the presence or absence of steady-state duloxetine (60 mg orally twice daily). Plasma concentrations of duloxetine, its metabolites and theophylline were determined using liquid chromatography with tandem mass spectrometry. Pharmacokinetic parameters were estimated using noncompartmental methods and evaluated using mixed-effects ANOVA. Safety measurements included vital signs, clinical laboratory tests, a physical examination, ECG readings and adverse event reports. Results: The in vitro results indicated that duloxetine is metabolized by CYP1A2; however, duloxetine was predicted not to be an inhibitor or inducer of CYP1A2 in humans. Following oral administration in the presence of fluvoxamine, the duloxetine area under the plasma concentration-time curve from time zero to infinity (AUC∞) and the maximum plasma drug concentration (Cmax) significantly increased by 460% (90% CI 359, 584) and 141% (90% CI 93, 200), respectively. In the presence of fluvoxamine, the oral bioavailability of duloxetine increased from 42.8% to 81.9%. In the presence of duloxetine, the theophylline AUC∞ and Cmax increased by only 13% (90% CI 7, 18) and 7% (90% CI 2,14), respectively. Coadministration of duloxetine with fluvoxamine or theophylline did not result in any clinically important safety concerns, and these combinations were generally well tolerated. Conclusion: Duloxetine is metabolized primarily by CYP1A2; therefore, coadministration of duloxetine with potent CYP1A2 inhibitors should be avoided. Duloxetine does not seem to be a clinically significant inhibitor or inducer of CYP1A2; therefore, dose adjustment of CYP1A2 substrates may not be necessary when they are coadministered with duloxetine.

Olanzapine Plasma Concentrations After Treatment With 10, 20, and 40 mg/d in Patients With Schizophrenia: An Analysis of Correlations With Efficacy, Weight Gain, and Prolactin Concentration

Objectives of the study were to evaluate the relationship between olanzapine plasma concentrations and efficacy, prolactin, and weight and to assess effects of smoking, sex, and race on the pharmacokinetic characteristics of oral olanzapine up to 40 mg/d. Patients were randomly allocated to olanzapine 10, 20, or 40 mg/d for 8 weeks. Olanzapine concentrations in 634 samples from 380 patients were analyzed. Mean sample collection time was approximately 15 hours after dose for all groups. Mean olanzapine concentrations were 19.7 ± 11.4, 37.9 ± 22.8, and 74.5 ± 43.7 ng/mL for 10-, 20-, and 40-mg doses, respectively. Olanzapine concentration and Positive and Negative Syndrome Scale improvement were not significantly correlated. Change in both weight and prolactin showed significant dose response. Prolactin concentration was correlated with olanzapine concentration (r = 0.46, P < 0.001). No significant correlation between olanzapine concentration and weight change was observed. Olanzapine concentrations were lower in self-reported smokers (16.5 ± 9.6, 34.2 ± 20.8, and 60.9 ± 34.6 ng/mL) than in self-reported nonsmokers (25.6 ± 12.3, 43.4 ± 24.7, and 113.2 ± 44.0 ng/mL) for 10-, 20-, and 40-mg doses, respectively (P ≤ 0.022). In the 40-mg group only, African Americans had a lower mean olanzapine concentration than whites (65.6 ± 44.1 and 84.8 ± 44.1 ng/mL, respectively, P = 0.048). Women had numerically but not significantly higher mean olanzapine concentrations than men. In conclusion, olanzapine pharmacokinetics of doses up to 40 mg/d was generally consistent with prior findings in studies with fewer subjects and/or lower doses.

Single‐Dose and Steady‐State Pharmacokinetics of Tomoxetine in Normal Subjects

A pharmacokinetic profile of tomoxetine, a selective norepinephrine uptake inhibitor, was developed in human volunteers following single and multiple oral administrations. Following the administration of a single 90‐mg oral dose of tomoxetine to four normal volunteers, the plasma half‐life was 4.3 ± 0.5 hours. Mean plasma clearance was 0.60 ± 0.14 L/Kg/hr, and the mean volume of distribution was 3.7 4pL 0.9 L/kg. Multiple doses of tomoxetine (20 mg bid and 40 mg bid) for seven days were administered to an additional seven subjects. The data appeared to have a bimodal distribution. The mean plasma half‐life determined following the last dose was 4.6 ± 0.5 hours in five subjects. The other two subjects, one at each dose level, demonstrated accumulation of tomoxetine occurring from the first to last dose where tomoxetine disappeared from plasma with a mean half‐life of 19 hours.