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.

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.

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.

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.

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).

Inhibition of desipramine hydroxylation in vitro by serotonin-reuptake-inhibitor antidepressants, and by quinidine and ketoconazole: a model system to predict drug interactions in vivo

Biotransformation of the tricyclic antidepressant desipramine (DMI) to its metabolite 2-hydroxy-desipramine (2-OH-DMI) was studied in vitro using microsomal preparations from human, monkey, mouse and rat liver. In all species 2-OH-DMI was the principal identified metabolite. Mean (+/- S.E.) reaction parameters in six human liver samples were: Vmax, 0.11 +/- .02 nmol/ml/min/mg protein; Km, 16.1 +/- 4.2 microM. Quinidine was a highly potent inhibitor of 2-OH-DMI formation (mean Ki = 0.053 microM), consistent with the presumed role of Cytochrome P450-2D6 in mediating this reaction. Ketoconazole was a much less potent inhibitor (mean Ki = 10.3 microM). Two serotonin-specific reuptake inhibitor (SSRI) antidepressants, and their respective metabolites, were evaluated as potential inhibitors of 2-OH-DMI formation. Fluoxetine (FLU) and norfluoxetine (NOR) were the most potent inhibitors (mean Ki values: 3.0 and 3.5 microM, respectively). Sertraline (SERT) and its metabolite desmethylsertraline (DES) also inhibited the reaction (mean Ki: 22.7 and 16.0 microM), but were significantly less potent than FLU or NOR. Values of Ki and Km measured in vitro were used to generate a theoretical prediction of the degree of clearance inhibition in vivo at any given concentration of substrate and inhibitor. The model was applied to a clinical study in which DMI clearance in humans was impaired by coadministration of FLU (yielding FLU and NOR in plasma) or by SERT (yielding SERT and DES in plasma). Use of plasma SSRI concentrations in the predictive model underestimated the actual impairment of DMI clearance.(ABSTRACT TRUNCATED AT 250 WORDS)

Alprazolam metabolism in vitro: studies of human, monkey, mouse, and rat liver microsomes

Biotransformation of the triazolobenzodiazepine alprazolam (ALP) was studied in vitro using hepatic microsomal preparations from human, monkey, mouse, and rat liver tissue. Two principal hydroxylated metabolites were identified: 4-hydroxy- and alpha-hydroxy-alprazolam (4-OH-ALP and alpha-OH-ALP). In all species, rates of 4-OH-ALP formation exceeded those of alpha-OH-ALP. In human liver microsomes, ratios of 4-OH-ALP/alpha-OH-ALP reaction velocities calculated at clinically relevant plasma concentrations of ALP ranged from 7 to 17, qualitatively consistent with, but numerically larger than, the ratio of the plasma levels of the two metabolites during clinical use of ALP in humans. Km values for both 4-OH-ALP (170-305 microM) and alpha-OH-ALP (63-441 microM) considerably exceeded the usual maximum plasma concentration observed in humans (200 ng/ml, 0.65 microM), consistent with the linear (dose-independent) pharmacokinetic characteristics of ALP observed in humans. Thus formation of 4-OH-ALP via hydroxylation is the major route of ALP metabolism. This pathway is probably mediated by the cytochrome P-450-3A subfamily. Factors that impair the activity of this cytochrome subtype are likely to impair clearance of ALP in vivo.

Alprazolam pharmacokinetics, metabolism, and plasma levels: clinical implications

The triazolobenzodiazepine alprazolam is biotransformed by hepatic microsomal oxidation, yielding two hydroxylated metabolites (4-hydroxy- and a-hydroxy-alprazolam) as the principal metabolic products. Both metabolites have lower benzodiazepine receptor affinity than the parent compound and at steady state appear in plasma at concentrations considerably lower than intact alprazolam. Thus, clinical activity during treatment with alprazolam is essentially entirely attributable to intact alprazolam. The cytochrome P450 IIIA subfamily appears to mediate alprazolam metabolism in humans. This cytochrome subfamily is not subject to variation due to genetic polymorphism. Ketoconazole, cimetidine, macrolide antibiotics, and serotonin-reuptake-inhibitor antidepressants impair alprazolam biotransformation in vitro. Reduced clearance of alprazolam in vivo has been demonstrated for drugs in this group that have been studied in humans; for those not yet studied, impaired alprazolam clearance should be anticipated during coadministration. Studies of plasma alprazolam concentration versus clinical response during short-term treatment of panic disorder indicate that therapeutic response at steady-state plasma levels of 20 to 40 ng/mL is significantly greater than at levels less than 20 ng/mL. Substantial additional benefit from plasma levels greater than 40 ng/mL is not consistently demonstrated. However, side effects attributable to benzodiazepine agonist activity (e.g., drowsiness, sedation) increase in frequency with increasing steady-state plasma levels. Concentration-response data indicate that monitoring of alprazolam plasma levels can be of considerable clinical value during treatment of panic disorder.

Plasma alprazolam concentrations. Relation to efficacy and side effects in the treatment of panic disorder

A series of 237 patients with DSM-III-diagnosed panic disorder, or agoraphobia with panic attacks, received alprazolam as part of the placebo-controlled Cross-National Collaborative Panic Study. After a 1-week drug-free period, alprazolam dosage was titrated upward with the objective of reaching 6.0 mg/d in all patients. At week 3 of treatment, alprazolam plasma levels were significantly correlated with daily dosage (regression slope: 11.7 ng/mL per milligram per day) but with considerable individual variation. Among patients with spontaneous panic attacks, 70% of those with plasma alprazolam levels greater than 20 ng/mL achieved complete remission vs 31% of those with levels less than 20 ng/mL. Situational panic attack remission increased in frequency with increasing plasma levels, but the relationship was not significant. Patient- and physician-rated global improvement and Hamilton Anxiety and Depression Scale score reductions were maximal at 20 to 39 ng/mL, with no further benefit at higher levels. Central nervous system-depressant side effects increased in frequency with higher plasma levels. Between weeks 3 and 8 of treatment, physicians were permitted to adjust dosage (maximum: 10 mg/d) to optimize response. At week 8, the dose-concentration relationship was essentially identical (regression slope: 10.8 ng/mL per milligram per day), but plasma levels were no longer related to efficacy or side effects. Thus, monitoring of plasma alprazolam concentrations may have a clinically useful role during short-term treatment of panic disorder.

Cognitive effects of beta-adrenergic antagonists after single doses: pharmacokinetics and pharmacodynamics of propranolol, atenolol, lorazepam, and placebo

The behavioral effects of two beta-adrenergic receptor antagonists, selected to represent differing lipophilicity, were evaluated in a double-blind, single-dose, parallel-group study. A group of 55 healthy volunteers (mean age, 28 years) received single oral doses of placebo, atenolol (50 mg), propranolol (40 mg), or lorazepam (2 mg). Plasma drug concentrations, self-ratings of sedation and mood, observer ratings of sedation, and performance on the Digit Symbol Substitution Test (DSST) were assessed at multiple times during 24 hours after drug administration. Information acquisition and recall were tested at 3 and 24 hours after drug administration. Lorazepam significantly increased sedation and fatigue, impaired DSST performance, and impaired memory. The time course of these changes was highly consistent with plasma lorazepam concentrations. In contrast, atenolol and propranolol produced at most small changes in self-ratings and observer ratings and did not alter DSST performance or memory. Under experimental conditions that are sensitive to the depressant effects of a typical benzodiazepine, single doses of atenolol and propranolol produced no meaningful changes, compared with placebo.

Chronic benzodiazepine administration. XI. Concurrent administration of PK11195 attenuates lorazepam discontinuation effects

Benzodiazepine discontinuation is associated with alterations in motor activity and gamma-aminobutyric acid-A receptor upregulation in a mouse model. Prior studies indicate that concurrent administration of the compound N-methyl-N-(methyl-1-propyl)chloro-2-phenyl-1-isoquinoline-3- carboxamide (PK1195), a "peripheral" site benzodiazepine antagonist, can attenuate the effects of lorazepam on tolerance and receptor alterations. To evaluate the effects of PK11195 administration on benzodiazepine discontinuation, we administered lorazepam (2 mg/kg per day), PK 11195 (1 to 10 mg/kg per day) or the combination to mice for 7 days, and then evaluated pentylenetetrazole-induced seizure threshold and benzodiazepine binding at days 1, 4, and 7 after discontinuation. Seizure threshold was reduced at 4 days after lorazepam discontinuation; this effect was attenuated by coadministration of PK11195 at 5 mg/kg per day. Lorazepam discontinuation effects were not altered by PK11195 at 1 mg/kg per day, whereas the 10-mg/kg dose was not different from 5 mg/kg per day. The competitive ligand Ro5-4864 at 10 mg/kg per day, blocked the effects of PK11195 on lorazepam discontinuation. Benzodiazepine receptor binding in vivo was increased in the cortex and hippocampus at 4 days postlorazepam discontinuation. This effect was attenuated in the hippocampus but not in the cortex by concurrent administration of PK1195. These data indicate that concurrent administration of PK11195 may attenuate discontinuation effects of lorazepam.

Possible interaction between fluoxetine and pimozide causing sinus bradycardia

Fluoxetine has been reported to have interactions with antipsychotics and other antidepressants with symptoms of toxicity of these drugs. We report a case of a possible interaction with pimozide which led to a potentially life threatening bradycardia. Careful monitoring should be done when these two drugs are prescribed at the same time, especially in the elderly and in patients with cardiac disease.

Clinical pharmacokinetics of antidepressants in the elderly. Therapeutic implications

The prevalence of depression in the elderly suggests that a substantial number of older patients will be treated with an antidepressant medication such as one of the tricyclics, trazodone, fluoxetine or lithium. The physiological changes that accompany aging raise the possibilities of altered pharmacokinetics, patterns of efficacy and adverse effect profiles. The literature addressing the subject of antidepressant use in the elderly has not provided a clear, consistent picture of how these drugs behave in this population in comparison with younger patients. Particularly in the case of the tricyclic antidepressants (TCAs), a large degree of interindividual variation in drug clearance (CL) confounds attempts to find differences attributable to age per se. Study design, however, is also a problem in that very few investigators include a young control group, choosing instead to compare their data with previously reported outcomes. Designations of statistical significance and positive correlation also differ among investigators, and the clinical significance of any finding is not always addressed. The available data suggest that imipramine CL is reduced in the elderly and that amitriptyline CL may be reduced. Desipramine CL does not appear to be affected by age, although decreased renal function in the elderly may lead to accumulation of the hydroxylated metabolite, the clinical importance of which is not known. Nortriptyline is the most thoroughly studied TCA in the elderly. CL seems decisively lower only in elderly patients with concurrent medical illness. The hydroxylated metabolite probably accumulates with diminishing renal function. Not enough data are available on doxepin to make a conclusion. Trazodone CL is diminished somewhat in elderly men. Lithium CL appears to diminish with the declining renal function associated with aging. Fluoxetine data are sparse. Available data do not show any decrease in CL of the parent drug; more information is needed on the metabolite norfluoxetine. Although knowledge of CL changes with aging can help the clinician more accurately achieve the desired steady-state concentration of a drug during long term therapy, much work is still needed to evaluate the relationships among drug concentrations at steady-state, efficacy and adverse effects in the elderly.

Chronic benzodiazepine administration. XII. Anticonvulsant cross-tolerance but distinct neurochemical effects of alprazolam and lorazepam

Tolerance to the sedative and anticonvulsant effects of benzodiazepines has been reported, but cross-tolerance among benzodiazepines is poorly characterized. To evaluate cross-tolerance between lorazepam and alprazolam in a reliable anticonvulsant pharmacodynamic model, we treated mice with either drug for 14 days, and with the two drugs sequentially for 7 days each. Pentylenetetrazole-induced seizure thresholds were similar in mice treated for 14 days with lorazepam or alprazolam, 2 mg/kg/day. For both compounds, a discontinuation effect characterized by reduced seizure threshold occurred at 4 days after discontinuation. Substitution of alprazolam for lorazepam after 1 week, and vice versa, did not interrupt tolerance. [3H]flumazenil binding in vivo was downregulated in cortex after 14 days of either drug. However, binding was also reduced in hippocampus for lorazepam but not for alprazolam. Substitution of alprazolam for lorazepam resulted in downregulation in cortex only, similar to lorazepam alone. Conversely, substitution of lorazepam for alprazolam led to binding changes similar to lorazepam alone. These data demonstrate cross-tolerance to the convulsant effects of pentylenetetrazole between lorazepam and alprazolam. However, effects of the two compounds on benzodiazepine receptor binding in hippocampus remain distinct.

Caffeine-induced behavioral stimulation is dose-dependent and associated with A1 adenosine receptor occupancy

Caffeine's psychomotor stimulant effects may relate to its blockade of central adenosine receptors. We examined acute caffeine effects on motor activity, adenosine receptor occupancy in vivo, and receptor affinity and density ex vivo. Acute doses of caffeine-sodium benzoate (0, 20, 40, and 60 mg/kg, intraperitoneally [0, 0.10, 0.21, 0.31 mu mol/kg]) were given to CD-1 mice and their activity was measured in an animal activity monitor over a 1-hour period. Adenosine receptor occupancy in vivo was quantified in mice 1 hour postdosage, using the high-affinity, A1 receptor selective adenosine antagonist [3H]-8-cyclopentyl-1,3-dipropylxanthine. Adenosine receptor binding affinities and densities were determined from analyses of binding studies in cortical, hippocampal, and brainstem membranes from treated mice (0 and 40 mg/kg caffeine). Caffeine doses of 20 and 40 mg/kg, corresponding to mean brain concentrations of 5 and 17 micrograms/g, increased all horizontal and vertical motor activity measures and stereotypy counts, as compared to doses of 0 and 60 mg/kg. Additionally, all acute caffeine doses significantly altered specific A1 binding in vivo (decreasing binding between 55% and 73% versus vehicle), presumably as it occupied A1 receptors. Therefore, at doses of 20 and 40 mg/kg, caffeine stimulated motor activity as it occupied A1 receptors; at a dose of 60 mg/kg (mean brain concentration of 26 micrograms/g) caffeine had no stimulant effect even though it appeared to occupy A1 receptors. Acute caffeine dosage did not alter ex vivo adenosine receptor binding affinity or density in any brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

The pharmacokinetics and pharmacodynamics of sublingual and oral alprazolam in the post-prandial state

We gave 12 healthy male volunteers 1 mg of alprazolam or placebo on three occasions after a standard breakfast in a double-blind, randomized, single-dose, three-way crossover study. The three trials were: (a) oral alprazolam and sublingual placebo; (b) oral placebo and sublingual alprazolam; (c) placebo by both routes. Plasma alprazolam concentrations during 24 h after each dose were measured by electron-capture gas-liquid chromatography. Peak plasma concentrations were reached later after sublingual than oral dosage (2.8 vs 1.8 h, P less than 0.01). Other kinetic variables were not significantly different: peak plasma concentration, 11.3 vs 12.0 ng.ml-1; elimination half-life, 12.5 vs 11.7 h; and total area under the plasma concentration versus time curve, 197 vs 186 h.ng.ml-1. Pharmacodynamic measures showed that sublingual and oral alprazolam both produced sedation, fatigue, impaired digit symbol substitution, slowing of reaction time, and impairment of the acquisition and recall of information. These changes were initially observed at 0.5 h after dosage and lasted up to 8 h. In general the two routes were significantly different from placebo but not from each other.

Chronic benzodiazepine administration. X. Concurrent administration of the peripheral-type benzodiazepine ligand PK11195 attenuates chronic effects of lorazepam

Chronic administration of benzodiazepine active at the tau-aminobutyric acidA receptor ("central" benzodiazepine sites) is associated with behavioral tolerance and receptor downregulation. Recent reports indicate possible interactions between central sites and benzodiazepines active at "peripheral-type" sites located primarily on non-neuronal cells. To evaluate these interactions during chronic administration, we treated mice with lorazepam for 1 to 14 days alone or in combination with the peripheral-type site ligand PK11195 [N-methyl-N-(methyl-1-propyl)chloro-2-phenyl-1-isoquinoline-3-carboxamid e]. Lorazepam was associated with tolerance at 7 days, but tolerance was not observed during concurrent administration of PK11195. Lorazepam was also associated with benzodiazepine receptor down-regulation in cortex and hippocampus at 7 days. With concurrent administration of PK11195, this effect remained in cortex but was absent in hippocampus. tau-Aminobutyric acid-dependent chloride uptake was reduced in both cortex and hippocampus with lorazepam, but not with concurrent lorazepam and PK11195. PK11195 administration alone did not affect behavior or neurochemical parameters, or did it alter brain lorazepam concentrations. These data indicate that concurrent PK11195 administration attenuates behavioral and neurochemical effects of chronic lorazepam administration.

Chronic low-dose alprazolam augments gamma-aminobutyric acid(A) receptor function

After acute administration of low doses, alprazolam displays unusual behavioral and neurochemical characteristics. To determine whether chronic low-dose alprazolam has unique effects, we treated mice for 1-14 days with alprazolam 0.2 mg/kg per day and evaluated open-field activity, benzodiazepine receptor binding, t-butylbicyclophosphorothionate binding, and muscimol-stimulated chloride uptake. Open-field activity in treated mice was similar to that of control mice at each timepoint during alprazolam administration. Similarly, benzodiazepine receptor binding in vivo was unchanged in five brain regions. Benzodiazepine receptor binding in vivo was unchanged in five brain regions. Benzodiazepine binding in vitro in the cortex was unaffected by alprazolam treatment, as was t-butylbicyclophosphorothionate binding in the cortex. However, muscimol-stimulated chloride uptake was increased after 2 and 4 days of alprazolam compared with results after 1, 7, and 14 days. These results are consistent with prior reports of unusual effects of low-dose alprazolam and extend these findings to chronic administration.

Chronic benzodiazepine administration. IX. Attenuation of alprazolam discontinuation effects by carbamazepine

Clinical studies suggest that carbamazepine may attenuate effects of alprazolam discontinuation. Since discontinuation of chronic alprazolam in a mouse model is associated with behavioral alterations and upregulation at the gamma-aminobutyric acidA (GABAA) receptor, we studied the effects of carbamazepine administration after alprazolam (2 mg/kg/day) discontinuation. Open-field activity was increased in mice 4 days after alprazolam discontinuation, but this effect was reduced significantly by continuous infusion of carbamazepine, 25 or 100 mg/kg/day. Benzodiazepine receptor binding in vivo was increased in cortex at 2 and 4 days after alprazolam discontinuation, and in hypothalamus at 4 days; with carbamazepine, 100 mg/kg/day, binding in both regions at these time points was similar to control values. Similar results were observed in cortex with benzodiazepine receptor binding in vitro. GABA-dependent chloride uptake was also increased at 4 days alprazolam administration. Treatment with carbamazepine attenuated (P less than 0.10) this increase. Carbamazepine alone after vehicle did not alter benzodiazepine binding or GABA-dependent chloride uptake. These results indicate that carbamazepine administration after alprazolam discontinuation attenuates behavioral and neurochemical alterations associated with discontinuation.

Prenatal benzodiazepine administration. II. Lorazepam exposure is associated with decreases in [35S]TBPS binding but not benzodiazepine binding

Prenatal benzodiazepine exposure has been associated with neurobehavioral alterations in humans and animals. To determine effects of prenatal benzodiazepine exposure on binding at the benzodiazepine and t-butylbicyclophosphorothionate (TBPS) sites on the GABAA receptor in mature offspring, we treated mice with lorazepam, 2 mg/kg/day, during days 13-20 of gestation. Binding was assessed at 6 weeks of age. There were no differences among controls, vehicle- or lorazepam-exposed mice in benzodiazepine receptor binding determined in vivo or in vitro. However, receptor density for [35S]TBPS binding sites was decreased in lorazepam-exposed offspring compared to the other groups. These data are consistent with prior neurochemical results indicating decreased TBPS binding and GABAA receptor function in several systems.

Clinical pharmacokinetics of anxiolytics and hypnotics in the elderly. Therapeutic considerations (Part II)

Part I of this article, which appeared in the previous issue of the Journal, discussed the scope of and scientific basis for special pharmacokinetic studies of anxiolytic and hypnotic drugs in the elderly, and examined the methodology and results of such studies and the prediction of pharmacokinetic changes. In Part II the authors continue their review, focusing on age-related pharmacodynamic changes in the effects of these drugs, the attempts to correlate pharmacokinetic with pharmacodynamic findings, and the clinical applications of these data.

Clinical pharmacokinetics of anxiolytics and hypnotics in the elderly. Therapeutic considerations (Part I)

Anxiolytic and hypnotic drugs are extensively prescribed for elderly individuals throughout Western society. Old age may be associated with an altered clinical response to this class of compounds, and there is a considerable ethical and economic stake in understanding these changes so that therapy may be approached with a maximum likelihood of therapeutic benefit and a minimum risk of side effects. Old age may lead to altered pharmacokinetics of sedative-anxiolytic drugs, causing higher plasma concentrations (relative to young individuals) after single or multiple doses. By far the majority of the available scientific data refer to the benzodiazepines, which have become the most widely prescribed class of sedative-anxiolytic drugs. Although there is not complete consistency in the available data, the weight of evidence indicates that old age is associated with impaired clearance of the benzodiazepines which are biotransformed by microsomal oxidation (such as diazepam, desmethyldiazepam, desalkylflurazepam, bromazepam, alprazolam, triazolam and others). For those benzodiazepines metabolised mainly by glucuronide conjugation (oxazepam, lorazepam, temazepam) or nitroreduction (nitrazepam), there are minimal, if any, age-related decrements in clearance. Only in the case of triazolam is there direct evidence linking impaired clearance to enhanced clinical effects in the elderly. The logical suggestion that benzodiazepines biotransformed by conjugation or by nitroreduction may be safer for the elderly than those biotransformed by oxidation has not yet been directly validated. Reasonable epidemiological evidence has linked the use of long (versus short) half-life benzodiazepines (regardless of the specific metabolic pathway) with an increased incidence of adverse reactions such as confusion, falls and hip fractures in elderly persons. However, the decreased clearance and increased accumulation of the benzodiazepines in question are not clearly validated as the cause of the increased frequency of adverse reactions. Old age may also be associated with an increased intrinsic sensitivity to benzodiazepines; that is, enhanced pharmacodynamic response, relative to young individuals, at any given plasma or target organ concentration. This change in sensitivity may coexist with, or be independent of, alterations in pharmacokinetics. Altered benzodiazepine sensitivity has been documented both in the course of clinical use of benzodiazepines prior to endoscopy or cardioversion, and in placebo-controlled laboratory trials. Animal models of aging have validated an enhanced response to benzodiazepines as a consequence of impaired clearance, increased intrinsic sensitivity or both. However, many studies directly assessing benzodiazepine receptor affinity, density and function in aging animals have failed to identify significant age-related changes.(ABSTRACT TRUNCATED AT 400 WORDS)

Sensitivity to triazolam in the elderly

BACKGROUND

Elderly persons frequently appear to be sensitive to the effects of many drugs that depress the central nervous system. We studied the effect of age on the pharmacokinetics and pharmacodynamics of the benzodiazepine hypnotic agent triazolam, now the most frequently prescribed hypnotic drug in the United States.

METHODS

Twenty-six healthy young subjects (mean age, 30 years) and 21 healthy elderly subjects (mean age, 69 years) participated in a four-way crossover study. After a single-blind adaptation trial with placebo, each subject received, in random order and in double-blind fashion, single doses of placebo, 0.125 mg of triazolam, and 0.25 mg of triazolam. For 24 hours after the administration of each of the three study medications, plasma triazolam levels were determined and psychomotor performance, memory, and degree of sedation were assessed.

RESULTS

Plasma triazolam concentrations increased in proportion to the dose, but the elderly subjects had higher plasma concentrations due to reduced clearance of the drug. The degree of sedation as rated by an observer and the reduction in the subjects' performance on the digit-symbol substitution test were both greater in the elderly than in the young subjects after they were given the same doses. The relation of the plasma triazolam concentration to the degree of impairment was similar for the two groups. As part of the study, information was presented 1 1/2 hours after the administration of the drugs; the subjects' ability to recall the information 24 hours later was impaired by both doses of triazolam, and the percent decrease was similar in the young and elderly groups.

CONCLUSIONS

Triazolam caused a greater degree of sedation and greater impairment of psychomotor performance in healthy elderly persons than in young persons who received the same dose. These effects resulted from reduced clearance and higher plasma concentrations of triazolam rather than from an increased intrinsic sensitivity to the drug. On the basis of these results, the dosage of triazolam for elderly persons should be reduced on average by 50 percent.

The effect of benztropine on haloperidol-induced dystonia, clinical efficacy and pharmacokinetics: a prospective, double-blind trial

Twenty-nine inpatients with major psychotic disorders were treated for 14 days with a clinician-determined dose of haloperidol and with either benztropine or placebo given by double-blind random assignment on days 1 through 7. No differences were noted in haloperidol mean dose, haloperidol blood levels, or BPRS scores during the first seven days between benztropine (N = 14) and placebo (N = 15) groups. Benztropine-treated patients demonstrated increased dry mouth and diminished sweat and a non-significantly lower rate of dystonia compared to placebo (14% vs. 33%). Dystonic patients were significantly younger than nondystonic patients, but did not differ in haloperidol mean dose or plasma concentration. The effect of benztropine on the incidence of dystonia was consistent with other studies, which, when analyzed together, demonstrate the efficacy of anticholinergic prophylaxis. The relatively low incidence of anticholinergic side effects, coupled with the lack of effect on haloperidol blood levels or antipsychotic efficacy, suggest that moderate doses of benztropine in conjunction with haloperidol are a rational approach for the treatment of acute psychosis in young patients.

Modification of gamma-aminobutyric acidA receptor binding and function by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline in vitro and in vivo: effects of aging

The irreversible protein-modifying reagent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was used to investigate binding site characteristics on the gamma-aminobutyric acidA (GABAA) receptor complex. In vitro, preincubation with EEDQ led to a concentration-dependent decrease in receptor number for benzodiazepine, t-butylbicyclophosphorothionate (TBPS), and GABA binding sites in cerebral cortex. The effect was maximal at the highest concentration of EEDQ used (10(-4) M) and was greatest for the benzodiazepine site. Pretreatment of membranes with the benzodiazepine antagonist Ro 15-1788, 1 or 10 microM, or the agonist lorazepam, 10 microM, largely prevented the effects of EEDQ. Scatchard analysis indicated no effect of EEDQ, 10(-4) M, on apparent affinity, but a decrease in receptor density for each site. Administration of EEDQ to mice, 12.5 mg/kg i.p., led to a substantial (55-65%) decrease in number of benzodiazepine binding sites in cortex after 4 h. Slightly smaller changes were observed for TBPS and GABA binding. No changes were observed in apparent affinity at any site. Prior administration of Ro 15-1788, 5 mg/kg, prevented the effect of EEDQ on benzodiazepine binding. Density of benzodiazepine binding sites gradually recovered over time, and receptor density returned to control values by 96 h after EEDQ injection. Number of binding sites in cortex for TBPS and GABA also increased over time after EEDQ. Benzodiazepine sites in cerebellum were decreased proportionally to cortex after EEDQ, and increased over a similar time course. Function of the GABAA receptor in chloride uptake in cortex was markedly reduced (65%) by EEDQ.(ABSTRACT TRUNCATED AT 250 WORDS)