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Biomedical subjects

R I Shader

Publications and source records attributed to R I Shader.

At least 109 records · Page 6Linked to original sources
PubMed1996

Triazolam biotransformation by human liver microsomes in vitro: effects of metabolic inhibitors and clinical confirmation of a predicted interaction with ketoconazole.

Biotransformation of the triazolobenzodiazepine triazolam to its hydroxylated metabolites, alpha-hydroxy (OH)- and 4-OH-triazolam, was studied in vitro using microsomal preparations of human liver. Mean values of Vmax (10.3 nM/min/mg of protein) and Km (304 microM) for the 4-OH pathway exceeded values for the alpha-OH pathway (2.4 and 74, respectively). However the mean Vmax/Km ratios for the two pathways were nearly identical, indicating that both contribute approximately equally to intrinsic clearance. Ketoconazole was a powerful inhibitor of triazolam biotransformation, having mean competitive Ki values of 0.006 and 0.023 microM for the alpha-OH and 4-OH pathways. This is consistent with the role of P450-3A isoforms in mediating triazolam biotransformation. The serotonin2 antagonist antidepressant nefazodone inhibited the alpha-OH and 4-OH pathways (Ki = 0.6 and 1.7 microM, respectively), but with considerably less activity than ketoconazole. Among six selective serotonin reuptake-inhibitor antidepressants, norfluoxetine was the most potent inhibitor (Ki = 2.7 and 8.0 microM) and fluoxetine itself was the weakest (Ki = 7.0 and 44.3 microM). In a double-blind clinical pharmacokinetic-pharmacodynamic study, administration of triazolam (0.125 mg) preceded by ketoconazole, compared to triazolam preceded by placebo, produced a nearly 9-fold reduction in apparent oral clearance of triazolam (41 vs. 337 ml/min) and a 4-fold prolongation of half-life (13.5 vs. 3.4 hr). Pharmacodynamic testing indicated enhancement of electroencephalographic beta activity, and enhanced decrements in digit-symbol substitution test performance, attributable to coadministration of ketoconazole. Plasma ketoconazole concentrations measured in the clinical study ranged from 0.02 microgram/ml (projected minimum) to 4.95 micrograms/ml (maximum). An in vitro-in vivo scaling model, using these plasma ketoconazole concentrations together with liver partition ratios and the in vitro Ki values, predicted a decrement of triazolam clearance due to ketoconazole coadministration that was consistent with the 88% decrement in clearance actually observed in vivo.

Adult↗
PubMed1995

Inhibition of alprazolam and desipramine hydroxylation in vitro by paroxetine and fluvoxamine: comparison with other selective serotonin reuptake inhibitor antidepressants.

In vitro preparations of human liver microsomes were used to study the inhibiting effects of two selective serotonin reuptake inhibitor (SSRI) antidepressants, paroxetine and fluvoxamine, on metabolism via hydroxylation of alprazolam and of desipramine. These reactions are mediated by Cytochromes P450-3A4 and P450-2D6, respectively. Paroxetine was a highly potent inhibitor of desipramine hydroxylation; the inhibition constant (Ki) value of 2.0 microM indicated greater inhibiting potency than fluoxetine or norfluoxetine. The in vitro data predicted in vivo impairment of desipramine clearance by coadministration of paroxetine which was in the same range as observed in a clinical study. Fluvoxamine, by contrast, was a much weaker inhibitor of desipramine hydroxylation, having a Ki value (16.6 microM) similar to those of sertraline and desmethylsertraline. For hydroxylation of alprazolam, paroxetine was a relatively weak inhibitor, approximately comparable to fluoxetine, whereas fluvoxamine showed inhibiting capacity similar to that of norfluoxetine. The in vitro data predicted the degree of impairment of alprazolam clearance observed in vitro model can therefore provide clinically relevant data on prediction of potential drug interactions with SSRIs.

Alprazolam↗
PubMed1995

Metabolism of drugs by cytochrome P450 3A isoforms. Implications for drug interactions in psychopharmacology.

Members of the P450 3A subfamily are the most abundant of the human hepatic cytochromes. CYP3A isoforms mediate the biotransformation of many drugs, including a number of psychotropic, cardiac, analgesic, hormonal, immunosuppressant, antineoplastic, and antihistaminic agents. Activity of CYP3A in humans is variable among individuals, but there is no evidence of genetic polymorphism. Significant amounts of CYP3A are present in the gastrointestinal tract, and may contribute to presystemic extraction of drugs such as cyclosporin. The azole antifungal agents ketoconazole and itraconazole are potent inhibitors of human CYP3A isoforms. Selective serotonin reuptake inhibitor (SSRI) antidepressants are also CYP3A inhibitors, but much less potent than ketoconazole or itraconazole. In vitro models can provide important information on the qualitative and quantitative activity of potential inhibitors of human cytochromes. However, in vitro inhibition constant (Ki) values alone do not predict the magnitude of an in vivo interaction, nor whether an interaction will be of clinical importance. For example, SSRIs are predicted to impair clearance of the antihistamine terfenadine in humans. However, the magnitude of this effect is much less than would be associated with a pharmacokinetic interaction of clinical importance.

Animals↗
PubMed1995

N-demethylation of amitriptyline in vitro: role of cytochrome P-450 3A (CYP3A) isoforms and effect of metabolic inhibitors.

Biotransformation of amitriptyline (AMI) to its demethylated product nortriptyline (NT) was studied in vitro with human liver microsomes from four different donors, preselected to reflect a range of metabolic rates. Reaction velocity versus substrate concentration was consistent with a sigmoid Vmax model. Vmax varied from 0.42 to 3.42 nmol/mg/min, Km from 33 to 89 microM AMI. Ketoconazole was a highly potent inhibitor of N-demethylation, with a mean Ki value of 0.11 +/- 0.013 microM (+/- S.D.), whereas quinidine (up to 50 microM), a CYP2D6 inhibitor, and alpha-naphthoflavone (up to 5 microM), a CYP1A2 inhibitor only at low concentrations, showed no effect. All selective serotonin reuptake inhibitors (SSRIs) tested had an inhibitory effect on the formation of NT, with mean Ki values of 4.37 (+/- 3.38) microM for sertraline, 5.46 (+/- 1.95) microM for desmethylsertraline, 9.22 (+/- 3.69) microM for fluvoxamine, 12.26 (+/- 5.67) microM for norfluoxetine, 15.76 (+/- 5.05) microM for paroxetine, and 43.55 (+/- 18.28) microM for fluoxetine. A polyclonal rabbit antibody against rat liver CYP3A1, in antibody/microsomal protein ratios varying from 1:1 to 10:1, inhibited N-demethylation of AMI to an asymptotic maximum of 60%. These results are consistent with several case reports describing impairment of AMI metabolism by SSRIs. Inhibition of AMI demethylation by low concentrations of ketoconazole and by anti-3A antibody supports an important role for CYP3A isoforms in mediating this reaction.

1-Naphthylamine↗
PubMed1994

In vitro prediction of the terfenadine-ketoconazole pharmacokinetic interaction.

Biotransformation of the peripherally acting H-1 histamine antagonist, terfenadine, to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations from six separate human livers. These metabolic reactions are mediated by the specific cytochrome P450-3A4. Addition of ketoconazole to the reaction mixtures reduced the rate of formation of both metabolites in a manner consistent with competitive inhibition. Ketoconazole inhibition constants (Ki) averaged 0.024 microM for the desalkyl terfenadine pathway, and 0.237 microM for the hydroxy terfenadine pathway. A mathematical model, based on the in vitro Ki values and the usual clinical range of plasma ketoconazole concentrations (1-5 micrograms/mL; 1.88-0.94 microM), predicted that plasma terfenadine levels during coadministration of ketoconazole would increase by a factor ranging from 13-fold to 59-fold relative to the same dose of terfenadine given without ketoconazole. Actual plasma terfenadine levels during terfenadine-ketoconazole coadministration in a clinical pharmacokinetic study were close to those predicted by the model. These plasma levels were associated with prolongation of the corrected QT interval, thereby explaining the potentially life-threatening ventricular arrhythmias reportedly associated with terfenadine-ketoconazole cotherapy. Thus, data from studies of drug metabolism in vitro can be used to predict and thereby possibly avoid clinically important drug interactions.

Biotransformation↗
PubMed1994

Distinguishing a benzodiazepine agonist (triazolam) from a nonagonist anxiolytic (buspirone) by electroencephalography: kinetic-dynamic studies.

BACKGROUND AND OBJECTIVES: Benzodiazepine agonists and azaperone derivatives are used clinically as anxiolytics but have different neuroreceptor mechanisms of action. This study evaluated clinical pharmacodynamic approaches to distinguishing these two classes of compounds. METHODS: Healthy volunteers received single oral doses of placebo, the benzodiazepine agonist triazolam (0.25 mg) or the azaperone anxiolytic buspirone (20 mg), in a double-blind, three-way crossover study. Ratings of mood and sedation, performance on the digit symbol substitution test (DSST), and quantitative measures of electroencephalographic (EEG) beta activity (13 to 31.75 cycles/sec) determined by fast-Fourier transform were obtained at multiple times after dosage. RESULTS: Triazolam significantly increased self- and observer-rated sedation, impaired DSST performance, impaired recall, and increased EEG beta activity. Pharmacodynamic changes were significantly intercorrelated; all effects were maximal 1 to 2 hours after dosage but were indistinguishable from placebo by 8 hours. Buspirone did not alter the EEG or DSST performance but did increase self-ratings of sedation and feeling "spacey" and impaired memory function; these effects generally were quantitatively less than with triazolam. Peak plasma triazolam concentrations preceded maximum pharmacodynamic effects; the mean plasma effect site equilibration half-life was 9.4 minutes. Kinetic-dynamic modeling procedures yielded significant relationships between hypothetical effect site triazolam concentrations and pharmacodynamic changes. CONCLUSIONS: Quantitative analysis of the EEG clearly distinguishes a typical benzodiazepine agonist from a nonagonist anxiolytic, in clinically relevant dosage, whose pharmacodynamic actions do not involve benzodiazepine receptor occupancy. EEG effects associated with triazolam are intercorrelated with other pharmacodynamic measures.

Administration, Oral↗
PubMed1994

Inhibitors of alprazolam metabolism in vitro: effect of serotonin-reuptake-inhibitor antidepressants, ketoconazole and quinidine.

1. The biotransformation of the triazolobenzodiazepine alprazolam (ALP) to its hydroxylated metabolites (4-OH-ALP and alpha-OH-ALP) was evaluated in human, monkey, rat, and mouse liver microsomes. 2. In all species 4-OH-ALP was the principal metabolite, accounting for 84% of clearance in human microsomes compared with 16% for alpha-OH-ALP. 3. Among the serotonin-specific reuptake inhibitors fluoxetine (FLU) and sertraline (SERT), and their respective demethylated metabolites norfluoxetine (NOR) and desmethylsertraline (DES), NOR was the most potent inhibitor (mean Ki for 4-OH-ALP formation in humans: 11 microM), FLU the weakest (Ki = 83 microM), with SERT and DES falling in between (Ki = 24 and 20 microM). 4. The in vitro data predict 29% inhibition of ALP clearance at mean FLU and NOR plasma concentrations of 77 ng ml-1 and 72 ng ml-1, respectively, after correction for liver:water partition ratios in the range of 12-14. The observed mean degree of inhibition in a previous in vivo study was 21%. 5. Ketoconazole was a potent inhibitor of ALP metabolism in vitro (Ki = 0.046 microM), suggesting that ALP hydroxylation is mediated by the cytochrome P450-3A sub-family. Quinidine was a weak inhibitor (Ki = 626 microM).

Adult↗