C. Eugene Wright

Midazolam Hydroxylation by Human Liver Microsomes In Vitro: Inhibition by Fluoxetine, Norfluoxetine, and by Azole Antifungal Agents

Biotransformation of the imidazobenzodiazepine midazolam to its α‐hydroxy and 4‐hydroxy metabolites was studied in vitro using human liver microsomal preparations. Formation of α‐hydroxy‐midazolam was a high‐affinity (Km = 3.3 μmol/L) Michaelis‐Menten process coupled with substrate inhibition at high concentrations of midazolam. Formation of 4‐hydroxy‐midazolam had much lower apparent affinity (57 μmol/L), with minimal evidence of substrate inhibition. Based on comparison of Vmax/Km ratios for the two pathways, α‐hydroxy‐midazolam formation was estimated to account for 95% of net intrinsic clearance. Three azole antifungal agents were inhibitors of midazolam metabolism in vitro, with inhibition being largely consistent with a competitive mechanism. Mean competitive inhibition constants (Ki) versus α‐hydroxy‐midazolam formation were 0.0037 μmol/L for ketoconazole, 0.27 μmol/L for itraconazole, and 1.27 μmol/L for fluconazole. An in vitro‐in vivo scaling model predicted inhibition of oral midazolam clearance due to coadministration of ketoconazole or itraconazole; the predicted inhibition was consistent with observed interactions in clinical pharmacokinetic studies. The selective serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine and its principal metabolite, norfluoxetine, also were inhibitors of both pathways of midazolam biotransformation, with norfluoxetine being a much more potent inhibitor than was fluoxetine itself. This finding is consistent with results of other in vitro studies and of clinical studies, indicating that fluoxetine, largely via its metabolite norfluoxetine, may impair clearance of P450‐3A substrates.

Clinical pharmacokinetics of alprazolam. Therapeutic implications.

SummaryAlprazolam is a triazolobenzodiazepine that is extensively prescribed in the Western world for the treatment of anxiety and panic disorders. Its benzodiazepine receptor binding characteristics are qualitatively similar to those of other benzodiazepines. The drug is metabolised primarily by hepatic microsomal oxidation, yielding α-hydroxy- and 4-hydroxy-alprazolam as principal initial metabolites. Both have lower intrinsic benzodiazepine receptor affinity than alprazolam and appear in human plasma at less than 10% of the concentrations of the parent drug. Plasma concentrations of the 4-hydroxy metabolite exceed those of the α-hydroxy derivative, but urinary recovery of α-hydroxy-alprazolam greatly exceeds that of 4-hydroxy-alprazolam. This may be explained by chemical instability of 4-hydroxy-alprazolam in vitro.After single 1mg oral doses in humans, typical pharmacokinetic variables for alprazolam are: a peak plasma concentration 12 to 22 μg/L occurring 0.7 to 1.8h postdose, a volume of distribution of 0.8 to 1.3 L/kg, elimination half-life of 9 to 16h and clearance of 0.7 to 1.5 ml/min/kg. Absolute bioavailability of oral alprazolam averages 80 to 100%. Pharmacokinetics are dose-independent and are unchanged during multiple-dose treatment. On average, mean steady-state plasma alprazolam concentrations change by 10 to 12 μg/L for each daily dosage change of 1 mg/day.Most studies show that alprazolam pharmacokinetics are not significantly influenced by gender. Clearance of alprazolam is reduced in many elderly individuals, even those who are apparently healthy. Clearance is significantly reduced in patients with cirrhosis. Renal disease causes reduced plasma protein binding of alprazolam (increased free fraction) and some data suggest reduced free clearance of alprazolam in such patients. Pharmacokinetics of alprazolam are not significantly altered in abstinent alcoholics or patients with panic disorder, and are not influenced by the phase of the menstrual cycle in women.Coadministration of Cimetidine, fluoxetine, fluvoxamine or propoxyphene significantly impairs alprazolam clearance. However, alprazolam clearance is not altered by coadministration of propranolol, metronidazole, disulfiram, oral contraceptives or ethanol. Imipramine clearance may be impaired if alprazolam is coadministered. Alprazolam does not alter the pharmacokinetics of digoxin.Although a therapeutic concentration range is not clearly established, some studies indicate that optimal reduction of anxiety associated with panic disorder occurs at steady-state plasma alprazolam concentrations of 20 to 40 μg/L. Concentrations higher than this may be needed for suppression of the actual panic attacks. Side effects associated with alprazolam (drowsiness, sedation, etc.) are consistent with its primary benzodiazepine agonist action and increase in frequency with higher steady-state plasma concentrations. As with other benzodiazepines, tolerance develops to the central depressant effects of alprazolam. Side effects diminish over time with continuous administration.Because of its relatively short half-life, abrupt termination of alprazolam treatment can be followed by one or more discontinuation syndromes (recurrence, rebound or withdrawal). For this reason, alprazolam should be tapered rather than abruptly discontinued when treatment is stopped. Clinical consequences of alprazolam discontinuation are no different than those occurring after discontinuation of any benzodiazepine with a short half-life.

Ketoconazole inhibition of triazolam and alprazolam clearance: Differential kinetic and dynamic consequences

Kinetic and dynamic consequences of metabolic inhibition were evaluated in a study of the interaction of ketoconazole, a P4503A inhibitor, with alprazolam and triazolam, two 3A substrate drugs with different kinetic profiles.

Steady-state pharmacokinetic properties of pramipexole in healthy volunteers

Pramipexole is a dopamine receptor agonist that has proved effective in the treatment of Parkinsons disease. The pharmacokinetic properties of pramipexole at steady‐state concentrations were studied in 16 healthy men and women at four dose levels throughout the range recommended for Parkinsons patients. Plasma and urine samples collected within the four dose intervals were assayed for concentrations of pramipexole, using high‐performance liquid chromatography. The total oral clearance for all participants was 419 mL/min. The mean volume of distribution and elimination half‐life for all participants was 486 ± 93.2 L and 12.9 ± 3.27 hours. Concentrations of pramipexole were proportional to dose, although the drugs pharmacokinetic properties differed between men and women. The area under the concentration—time curve for each dose level was 35% to 43% greater in women, mainly because of a 24% to 27% lower oral clearance. The mean creatinine clearance in men and women was 112 ± 12.8 mL/min/1.73 m2 and 80.9 ± 15.6 mL/min/1.73 m2, respectively. The renal clearance of pramipexole accounts for approximately 80% of oral clearance, and there was a significant correlation between renal and creatinine clearances. The influence of gender could not be distinguished from the influence of age and the resulting reduced creatinine clearance, but the measurement of pharmacokinetic properties produced linear results in both men and women.

Inhibition of triazolam clearance by macrolide antimicrobial agents: In vitro correlates and dynamic consequences

Macrolide antimicrobial agents may impair hepatic clearance of drugs metabolized by cytochrome P4503A isoforms. Potential interactions of triazolam, a substrate metabolized almost entirely by cytochrome P4503A in humans, with 3 commonly prescribed macrolides were identified using an in vitro metabolic model. The actual interactions, and their pharmacodynamic consequences, were verified in a controlled clinical study.

Age and gender effects on the pharmacokinetics and pharmacodynamics of triazolam, a cytochrome P450 3A substrate.

Sixty‐one healthy men and women, aged 20 to 75 years, received single 0.25‐mg doses of triazolam, a cytochrome P450 (CYP) 3A substrate benzodiazepine, and placebo in a double‐blind crossover study. Among women, age had no significant effect on area under the triazolam plasma concentration curve (AUC) (Spearman r = 0.14, P = .44) or clearance (r = −0.09, P = .62). Among men, AUC increased (r = 0.43, P < .02) and clearance declined (r = −0.42, P < .02) with increasing age. Gender differences in triazolam kinetics were not apparent. Compared with placebo, triazolam impaired digit‐symbol substitution test performance, increased observer‐rated sedation, impaired delayed recall of information learned at 1.5 hours after dosing, and increased electroencephalographic β amplitude. Among men, mean values of relative digit‐symbol substitution test decrement (P < .002) and observer‐rated sedation (P < .05) were significantly greater in elderly subjects compared with young subjects. Age‐dependent differences among women reached significance for observer‐rated sedation (P < .02). A combination of higher plasma levels and increased intrinsic sensitivity explained the greater pharmacodynamic effects of triazolam in elderly subjects. Although the findings are consistent with reduced clearance of triazolam in elderly men, individual variability was large and was not explained by identifiable demographic or environmental factors.

Human cytochromes mediating N-demethylation of fluoxetine in vitro

Abstract Biotransformation of the selective serotonin reuptake inhibitor antidepressant, fluoxetine, to its principal metabolite, norfluoxetine, was evaluated in human liver microsomes and in microsomes from transfected cell lines expressing pure human cytochromes. In human liver microsomes, formation of norfluoxetine from R,S-fluoxetine was consistent with Michaelis-Menten kinetics (mean Km = 33 μM), with evidence of substrate inhibition at high substrate concentrations in a number of cases. The reaction was minimally inhibited by coincubation with chemical probes inhibitory for P450-2D6 (quinidine), -1A2 (furafylline, α-naphthoflavone), and -2E1 (diethyldithiocarbamate). Substantial inhibition was produced by coincubation with sulfaphenazole (Ki = 2.8 μM), an inhibitory probe for P450-2C9, and by ketoconazole (Ki = 2.5 μM) and fluvoxamine (Ki = 5.2 μM). However, ketoconazole, relatively specific for P450-3A isoforms only at low concentrations, reduced norfluoxetine formation by only 20% at 1 μM, and triacetyloleandomycin (≥ 5 μM) reduced the velocity by only 20–25%. Microsomes from cDNA-transfected human lymphoblastoid cells containing human P450-2C9 produced substantial quantities of norfluoxetine when incubated with 100 μM fluoxetine. Smaller amounts of product were produced by P450-2C19 and -2D6, but no product was produced by P450-1A2, -2E1, or 3A4. Cytochrome P450-2C9 appears to be the principal human cytochrome mediating fluoxetine N-demethylation. P450-2C19 and -3A may make a further small contribution, but P450-2D6 is unlikely to make an important contribution.

Influence of route of administration on the pharmacokinetics of methylprednisolone

This study was conducted to evaluate the influence of route of administration upon the bioavailability and pharmacokinetics of methylprednisolone sodium succinate. Fourteen healthy adult male volunteers received 40 mg doses of methylprednisolone as the following treatments after an overnight fast in a 4-way crossover design: (a) as a 1 ml i.v. bolus;(b) as a 1 ml i.m. injection;(c) administered as an oral solution;and (d) as 5×8 mg oral tablets. Both the ester and free methylprednisolone were determined in plasma and urine. Study results indicate that the ester is rapidly and extensively converted to free methylprednisolone after all routes. The extent of methylprednisolone absorption was equivalent after i.v. and i.m. administration. Both orally administered treatments resulted in a lower extent of absorption attributed to a first-pass effect. Although a slightly lower extent of absorption was demonstrated following the oral administration of the methylprednisolone sodium succinate solution relative to the methylprednisolone oral tablets, this average difference of 9% would probably be of minimal therapeutic importance.

Ibuprofen and acetaminophen kinetics when taken concurrently

We evaluated kinetics of ibuprofen and acetaminophen taken concurrently by 20 healthy adults in a randomized crossover design. Steady‐state blood levels of ibuprofen and acetaminophen were measured by gas‐liquid chrornatography and HPLC. There were no significant differences in any of the ibuprofen serum concentrations, but there were differences in acetaminophen serum concentrations in 5 of 19 sampling times. When bioavailability and kinetic parameters for both drugs were compared, there were no significant differences. Our data demonstrate that steady‐state kinetics of ibuprofen and acetaminophen are not changed when taken concurrently.

A comparative pharmacokinetic and dynamic evaluation of alprazolam sustained-release, bromazepam, and lorazepam.

Sustained-release (SR) alprazolam may facilitate compliance with oral benzodiazepine treatment of panic disorders that currently requires doses administered three or four times daily. To compare the pharmacokinetic, psychomotor performance, and subjective effects of alprazolam SR (1.5 mg), bromazepam (3 mg taken three times daily), and lorazepam (1 mg taken three times daily), 13 male volunteers (aged 20-45 years) randomly received on four separate occasions one of these medications or placebo. Once before and 11 times after drug administration, the subjects were tested using psychomotor performance tests (manual tracking and digit-symbol substitution test [DSST]) and computerized questionnaires (such as the Tufts University Benzodiazepine Scale [TUBS], the Addiction Research Center Inventory, and the visual analog scales) to determine the subjective effects of the drugs. Blood samples for the determination of the plasma levels of the drugs were collected before and 17 times after the drug was administered. A peak plateau of plasma alprazolam began approximately 6 hours after the dose, which was later than the initial peaks for lorazepam and bromazepam (1-2 hours after the dose). Once this plateau had begun, alprazolam SR sustained that concentration better than did the other two formulations. Of the 10 measures on which the response averaged for the first 14 hours differed among drugs (p < 0.05), bromazepam differed from placebo on two measures, lorazepam on four (including DSST Performance and TUBS Sedation), and alprazolam SR on nine (including all four affected by lorazepam). Lorazepam and alprazolam, but not bromazepam, produced significantly more sedation than placebo. The doses of the three drugs were not equipotent in sedation and mood effects. None of the drugs tested differed from placebo on measures relevant to abuse liability.