Joseph C. Fleishaker

Kinetic Characterization and Identification of the Enzymes Responsible for the Hepatic Biotransformation of Adinazolam and N‐Desmethyladinazolam in Man

The kinetics of the N‐demethylation of adinazolam to N‐desmethyladinazolam (NDMAD), and of NDMAD to didesmethyladinazolam (DDMAD), were studied with human liver microsomes using substrate concentrations in the range 10–1000 μm. The specific cytochrome P450 (CYP) isoforms mediating the biotransformations were identified using microsomes containing specific recombinant CYP isozymes expressed in human lympho‐blastoid cells, and by the use of CYP isoform‐selective chemical inhibitors.

Absolute bioavailability of reboxetine enantiomers and effect of gender on pharmacokinetics

The absolute bioavailability of reboxetine enantiomers was assessed in six male and six female volunteers. In a two-way crossover study, subjects received 1.0 mg reboxetine orally and 0.3 mg reboxetine as an intravenous bolus. The R,R(-) and S,S(+) enantiomers in serial plasma and urine samples were determined by a validated LC-MS-MS method. There were no significant differences between treatments for clearance or dose-corrected AUC(0-infinity) values. The absolute bioavailability was 0.919 and 1.02 for R,R(-) reboxetine and S,S(+) reboxetine, respectively. A secondary objective of the study was to assess gender effects on pharmacokinetics of the enantiomers. Significant differences in volume of distribution between genders were observed, but differences in weight-corrected volumes were not significant. Weight-corrected systemic clearance and oral clearance tended to be lower in males, but this difference reached statistical significance only for weight-corrected oral clearance of R,R(-) reboxetine. C(max) after oral administration was 40 and 48% higher in women than men for R,R(-) reboxetine and S,S(+) reboxetine, respectively. These results indicate that reboxetine enantiomers are well absorbed after oral administration and that little first-pass metabolism occurs. There are no clinically significant effects of gender on the pharmacokinetics of reboxetine enantiomers.

Pharmacokinetics of single-dose reboxetine in volunteers with renal insufficiency

Reboxetine is a new selective norepinephrine reuptake inhibitor (selective NRI) for the short‐ and long‐term treatment of depression that is effective and well tolerated at a dose of 8 to 10 mg/day. This study assessed the pharmacokinetics of reboxetine in volunteers with renal impairment. A single 4 mg dose of reboxetine was administered to a total of 18 volunteers with mild (n = 6), moderate (n = 6), or severe (n = 6) renal impairment (creatinine clearance: 56–64, 26–51, and 9–19 ml/min, respectively), and reboxetine concentrations were measured in plasma by HPLC. Mean AUC∞ increased by 43% (mild vs. severe; p < 0.01) as renal function declined, while renal clearance and total urinary excretion of unchanged reboxetine decreased by 67% and 62%, respectively (mild vs. severe; p < 0.01 for both parameters). tmax and t1/2 were not significantly different between groups. In comparison with historical data from young healthy volunteers, AUC∞ and t1/2 are at least doubled in volunteers with renal impairment, while CL is halved. This pharmacokinetic study has shown that increasing renal dysfunction leads to increasing systemic exposure to reboxetine, particularly in severe renal insufficiency, although reboxetine was well tolerated by all volunteers. Thus, a reduction of the starting dose of reboxetine to 2 mg twice daily would be prudent in patients with renal dysfunction.

Kinetics and Dynamics of Intravenous Adinazolam, N‐Desmethyl Adinazolam, and Alprazolam in Healthy Volunteers

The pharmacokinetics and pharmacodynamics of adinazolam mesylate (10 mg), N‐desmethyl adinazolam mesylate (NDMAD, 10 mg), and alprazolam (1 mg) were investigated in 9 healthy male subjects in a randomized, blinded, single‐dose, 4‐way crossover study. All drugs were intravenously infused over 30 minutes. Plasma adinazolam, NDMAD, and alprazolam concentrations, electroencephalographic (EEG) activity in the β (12–30 Hz) range, performance on the Digit Symbol Substitution Test (DSST), and subjective measures of mood and sedation were monitored for 12 to 24 hours. Mean pharmacokinetic parameters for adinazolam, NDMAD, and alprazolam, respectively, were as follows: volume of distribution (L), 106, 100, and 77; elimination half‐life (hours), 2.9, 2.8, and 14.6; and clearance (mL/min), 444, 321, and 84. More than 80% of the total infused adinazolam dose was converted to systemically appearing NDMAD. All 3 benzodiazepine agonists significantly increased β EEG activity, with alprazolam showing the strongest agonist activity and adinazolam showing the weakest activity. Alprazolam and NDMAD significantly decreased DSST performance, whereas adinazolam had no effect relative to placebo. Adinazolam, NDMAD, and alprazolam all produced significant observer‐rated sedation. Plots of EEG effect versus plasma alprazolam concentration demonstrated counterclockwise hysteresis, consistent with an effect site delay. This was incorporated into a kinetic‐dynamic model in which hypothetical effect site concentration was related to pharmacodynamic EEG effect via the sigmoid Emax model, yielding an effect site equilibration half‐life of 4.8 minutes. The exponential effect model described NDMAD pharmacokinetics and EEG pharmacodynamics. The relation of both alprazolam and NDMAD plasma concentrations to DSST performance could be described by a modified exponential model. Pharmacokinetic‐dynamic modeling was not possible for adinazolam, as the data did not conform to any known concentration‐effect model. Collectively, these results indicate that the benzodiazepine‐like effects occurring after adinazolam administration are mediated by mainly NDMAD.

Hormonal Effects on Tirilazad Clearance in Women: Assessment of the Role of CYP3A

This study assessed whether the previously reported difference in tirilazad clearance between pre‐ and postmenopausal women is reversed by hormone replacement and whether this observation can be explained by differences in CYP3A4 activity. Ten healthy women from each group were enrolled: premenopausal (ages 18–35), postmenopausal (ages 50–70), postmenopausal receiving estrogen, and postmenopausal women receiving estrogen and progestin. Volunteers received 0.0145 mg/kg midazolam and 3.0 mg/kg tirilazad mesylate intravenously on separate days. Plasma tirilazad and midazolam were measured by HPLC/dual mass spectrophotometry (MS/MS) assays. Tirilazad clearance was significantly higher in premenopausal women (0.51 ± 0.09 L/hr/kg) than in postmenopausal groups (0.34 ± 0.07, 0.32 ± 0.06, and 0.36 ± 0.08 L/hr/kg, respectively) (p = 0.0001). Midazolam clearance (0.64 ± 0.12 L/hr/kg) was significantly higher in premenopausal women compared to postmenopausal groups (0.47 ± 0.11, 0.49 ± 0.11, and 0.53 ± 0.19 L/hr/kg, respectively) (p = 0.037). Tirilazad clearance was weakly correlated with midazolam clearance (r2 = 0.129, p = 0.02). Tirilazad clearance is faster in premenopausal women than in postmenopausal women, but the effect of menopause on clearance is not reversed by hormone replacement. Tirilazad clearance in these women is weakly related to midazolam clearance, a marker of CYP3A activity.

Evaluation of the Potential Pharmacokinetic/Pharmacodynamic Interaction between Fluoxetine and Reboxetine in Healthy Volunteers

AbstractObjective: This study was performed to assess the tolerability of combined administration of reboxetine, a selective noradrenaline (norepinephrine) reuptake inhibitor, and fluoxetine, a selective serotonin reuptake inhibitor, relative to administration of each drug separately. Design: The following treatments were administered for 8 days according to a randomised, double-blind, placebo-controlled parallel design: (a) oral reboxetine 4mg twice daily, (b) oral fluoxetine 20mg once daily, or (c) oral reboxetine 4mg twice daily and fluoxetine 20mg once daily. Participants: Thirty healthy, nonsmoking volunteers (27 male, three female), aged between 20 and 55 years and within 15% of normal bodyweight were included in the study. Target Parameters: Plasma reboxetine enantiomers were quantified using HPLC-MS-MS. Fluoxetine and norfluoxetine concentrations were determined using high performance liquid chromatography Pharmacokinetic parameters were compared by unpaired t-test. Clinical laboratory data were analysed as the change from baseline, and adverse events were tabulated by treatment. Vital sign and Digit Symbol Substitution Test (DSST) data were analysed by repeated measures analysis of variance. Results: The adverse event profiles were similar for combined reboxetine and fluoxetine relative to administration of each drug separately. Reboxetine significantly increased mean standing and supine heart rate versus baseline, whereas heart rate was not modified by fluoxetine. No statistically significant treatment effects were seen for DSST scores or oral temperature. The area under the plasma concentration-time curve from 0 to 12 hours for S,S(+) reboxetine was approximately 23% higher with fluoxetine coadministration than with reboxetine alone, but this effect, as well as effects on other pharmacokinetic parameters for either reboxetine enantiomer, was not statistically significant. In addition, no statistically significant effects of reboxetine on fluoxetine or norfluoxetine pharmacokinetics were observed. Conclusion: Combined administration of reboxetine and fluoxetine was well tolerated in healthy volunteers. These results suggest minimal clinical impact when these drugs are administered concomitantly to depressed patients.

Induction of tirilazad clearance by phenytoin.

Tirilazad is a membrane lipid peroxidation inhibitor being studied for the management of subarachnoid hemorrhage; phenytoin is used for seizure prophylaxis in the same disorder. The induction of tirilazad clearance by phenytoin was assessed in 12 volunteers (6 male, 6 female). Subjects received phenytoin orally every 8 h for 7 days (200 mg for nine doses and 100 mg for 13 doses) in one phase of a crossover study. In both study phases, 1.5 mg kg−1 tirilazad mesylate was administered by IV infusion every 6 h for 29 doses. Tirilazad mesylate and U‐89678 (an active metabolite) in plasma were quantified by HPLC. After the final dose, tirilazad clearance was increased by 91.8% in subjects receiving phenytoin+tirilazad versus tirilazad alone. AUC0–6 for U‐89678 after the last tirilazad dose was reduced by 93.1% by concomitant phenytoin. These effects were statistically significant. The time course of induction was consistent with that of phenytoins effect on the ratio of urinary 6β ‐hydroxycortisol to cortisol, a measure of hepatic CYP3A activity. The results show that phenytoin induces metabolism of tirilazad and U‐89678 in healthy subjects and that, under these conditions, tirilazad clearance approaches liver blood flow.

Pharmacokinetics of Reboxetine in Volunteers with Hepatic Impairment

AbstractObjective: Reboxetine is a unique selective norepinephrine (noradrenaline) reuptake inhibitor (selective NRI) that is effective and well tolerated at a dosage of 8 to 10 mg/day in the short- and long-term treatment of depression. The objective of the study was to assess reboxetine pharmacokinetics in patients with moderate to severe hepatic impairment. Design: 12 volunteers with alcoholic liver disease received a single 4mg dose of reboxetine, and plasma reboxetine concentrations were measured by high performance liquid chromatography. Results: Reboxetine was well tolerated by all recipients. Compared with the pharmacokinetics of reboxetine 4mg determined in a similar study in young healthy volunteers, maximum concentration (Cmax) was up to 22% lower in patients with liver disease, the area under the plasma reboxetine concentration-time curve extrapolated to infinity (AUC∞) was up to 92% higher, and the terminal elimination half-life (t1/2z) was up to 150% longer. When comparing volunteers with moderate (Child-Pugh score 7 to 8) and severe hepatic (Child-Pugh score 10 to 13) impairment, mean Cmax was 21% lower, mean AUC∞ was 10% greater and t1/2,z was 23% longer in the severely impaired compared with the moderately impaired patients. Although none of these differences was statistically significant, the trend is of clinical relevance. Conclusions: The severity of alcoholic liver disease does not appear to affect reboxetine pharmacokinetics in a statistically significant manner, but there is a clinically relevant trend in changes to reboxetine pharmacokinetics with increasing hepatic impairment. Although reboxetine 4mg was well tolerated in the present study, comparisons with historical data from young healthy volunteers suggest that a lower starting dosage of reboxetine (4 mg/day in divided doses) should be used in patients with hepatic impairment.

Pharmacokinetics of Tirilazad and U-89678 in Ischemic Stroke Patients Receiving a Loading Regimen and Maintenance Regimen of 10 mg/kg/day of Tirilazad

The pharmacokinetics of tirilazad mesylate and its active reduced metabolite, U‐89678, were evaluated in ischemic stroke patients receiving 2.5 mg/kg tirilazad every 3 hours for the first 12 hours of dosing followed by 2.5 mg/kg every 6 hours for a total of 22 doses (5 days). Trough and serial samples drawn during the 6 hours after administration of the last dose were analyzed for plasma levels of tirilazad and U‐89678 by means of high‐performance liquid chromatography. Complete concentration—time profiles were available for 20 patients, including 12 men (mean age, 68.0 years) and 8 women (mean age, 75.0 years). Trough concentrations of tirilazad and U‐89678 were consistent with the loading regimen used. The mean area under the concentration—time curve from time 0 to 6 hours (AUC0–6) of tirilazad was 8181 ± 2398 ng · hr/mL in men and 8135 ± 3671 ng · hr/mL in women. The mean AUC0–6 of U‐89678 was 2761 ± 1834 ng · hr/mL in men and 1477 ± 903 ng · hr/mL in women. These results show that gender has a modest effect on the pharmacokinetics of U‐89678 but little effect on the pharmacokinetics of tirilazad in elderly ischemic stroke patients. These observations are consistent with previous findings in healthy young and elderly subjects.

Comparative pharmacodynamics of intravenous alprazolam, adinazolam, desmethyladinazolam, and placebo

Clinical Pharmacology & Therapeutics (1996) 59, 178–178; doi: 10.1038/sj.clpt.1996.213