Enzyme kinetic modelling as a tool to analyse the behaviour of cytochrome P450 catalysed reactions: application to amitriptyline N-demethylation

1. To determine kinetic parameters (Vmax, K(m)) for cytochrome P450 (CYP) mediated metabolic pathways, nonlinear least squares regression is commonly used to fit a model equation (e.g., Michaelis Menten [MM]) to sets of data points (reaction velocity vs substrate concentration). This method can also be utilized to determine the parameters for more complex mechanisms involving allosteric or multi-enzyme systems. Akaike's Information Criterion (AIC), or an estimation of improvement of fit as successive parameters are introduced in the model (F-test), can be used to determine whether application of more complex models is helpful. To evaluate these approaches, we have examined the complex enzyme kinetics of amitriptyline (AMI) N-demethylation in vitro by human liver microsomes. 2. For a 15-point nortriptyline (NT) formation rate vs substrate (AMI) concentration curve, a two enzyme model, consisting of one enzyme with MM kinetics (Vmax = 1.2 nmol min-1 mg-1, K(m) = 24 microM) together with a sigmoidal component (described by an equation equivalent to the Hill equation for cooperative substrate binding; Vmax = 2.1 nmol min-1 mg-1, K' = 70 microM; Hill exponent n = 2.34), was favoured according to AIC and the F-test. 3. Data generated by incubating AMI under the same conditions but in the presence of 10 microM ketoconazole (KET), a CYP3A3/4 inhibitor, were consistent with a single enzyme model with substrate inhibition (Vmax = 0.74 nmol min-1 mg-1, K(m) = 186 microM, K1 = 0.0028 microM-1). 4. Sulphaphenazole (SPA), a CYP2C9 inhibitor, decreased the rate of NT formation in a concentration dependent manner, whereas a polyclonal rat liver CYP2C11 antibody, inhibitory for S-mephenytoin 4'-hydroxylation in humans, had no important effect on this reaction. 5. Incubation of AMI with 50 microM SPA resulted in a curve consistent with a two enzyme model, one with MM kinetics (Vmax = 0.72 nmol min-1 mg-1, K(m) = 54 microM) the other with 'Hill-kinetics' (Vmax = 2.1 nmol min-1 mg-1, K' = 195 microM; n = 2.38). 6. A fourth data-set was generated by incubating AMI with 10 microM KET and 50 microM SPA. The proposed model of best fit describes two activities, one obeying MM-kinetics (Vmax = 0.048 nmol min-1 mg-1, K(m) = 7 microM) and the other obeying MM kinetics but with substrate inhibition (Vmax = 0.8 nmol min-1 mg-1, K(m) = 443 microM, K1 = 0.0041 microM-1). 7. The combination of kinetic modelling tools and biological data has permitted the discrimination of at least three CYP enzymes involved in AMI N-demethylation. Two are identified as CYP3A3/4 and CYP2C9, although further work in several more livers is required to confirm the participation of the latter.

Effect of diazepam on plasma gamma-aminobutyric acid in sons of alcoholic fathers

A subgroup of abstinent alcoholics, display low levels of plasma gamma-aminobutyric acid (GABA). Two previous studies of plasma GABA in sons of alcoholic fathers (SOAs) have yielded conflicting results. The aim of the current study was to measure plasma GABA both at baseline and after challenge with diazepam, a GABAA receptor agonist, in a group of SOAs already shown to display decreased eye movement, memory, and sedative effects of diazepam. Twenty-seven SOAs and 23 male control subjects received four logarithmically increasing doses of diazepam or placebo in randomized order on 2 days at least 1 week apart. Plasma GABA was measured at baseline and after the last dose. There were no significant differences between SOAs and controls in baseline plasma GABA levels. In the whole sample, there were significant correlations between baseline plasma GABA and both high novelty-seeking and low-harm avoidance scores on the Tridimensional Personality Questionnaire. Both SOAs and controls displayed decreases in plasma GABA over time on both testing days, but there was no effect of diazepam on plasma GABA and no significant difference between groups in plasma GABA response to diazepam. These results suggest that neither low plasma GABA at baseline nor altered plasma GABA response to diazepam is associated with increased genetic risk for alcoholism.

Inhibition of terfenadine metabolism in vitro by azole antifungal agents and by selective serotonin reuptake inhibitor antidepressants: relation to pharmacokinetic interactions in vivo

Biotransformation of the H-1 antagonist terfenadine to its desalkyl and hydroxy metabolites was studied in vitro using microsomal preparations of human liver. These metabolic reactions are presumed to be mediated by Cytochrome P450-3A isoforms. The azole antifungal agent ketoconazole was a highly potent inhibitor of both reactions, having mean inhibition constants (Ki) of 0.037 and 0.34 microM for desalkyl- and hydroxy-terfenadine formation, respectively. Itraconazole also was a potent inhibitor, with Ki values of 0.28 and 2.05 microM, respectively. Fluconazole, on the other hand, was a weak inhibitor. Six selective serotonin reuptake inhibitor antidepressants tested in this system were at least 20 times less potent inhibitors of terfenadine metabolism than was ketoconazole. An in vitro-in vivo scaling model used in vitro Ki values, typical clinically relevant plasma concentrations of inhibitors, and presumed liver:plasma partition ratios to predict the degree of terfenadine clearance impairment during coadministration of terfenadine with these inhibitors in humans. The model predicted a large and potentially hazardous impairment of terfenadine clearance by ketoconazole and, to a slightly lesser extent, by itraconazole. However, fluconazole and the six selective serotonin reuptake inhibitors (SSRIs) at usual clinical doses were not predicted to impair terfenadine clearance to a degree that would be of clinical importance. Caution is nonetheless warranted with the coadministration of SSRIs and terfenadine when high doses of SSRIs (particularly fluoxetine) are administered. Also, some individuals may be unusually susceptible to metabolic inhibition for a variety of reasons.

Inhibition of cytochrome P450 by nefazodone in vitro: studies of dextromethorphan O- and N-demethylation

Nefazodone (NEF), a 5-HT2A/2C antagonist antidepressant, is extensively metabolized in the human body to hydroxy NEF (OH-NEF), p-hydroxy NEF (pOH-NEF), a dione metabolite, and via cleavage of the molecule to m-chlorophenyl-piperazine (mCPP) and BMY-33604. The latter is further metabolized to BMS-183695-01 (BMSa) and BMS-183562-01 (BMSb). To investigate the potential of NEF and its metabolites to interfere with the metabolism of other drugs, we tested these compounds for their ability to alter dextromethorphan (DMO) O-demethylation to dextrorphan (DOP; an index reaction for CYP2D6) and N-demethylation to 3-methoxy morphinan (MEM, a recently proposed index reaction of CYP3A3/4). The assay was performed in an in vitro system with human liver microsomes from three different donors. NEF, OH-NEF, pOH-NEF, mCPP and BMSb were weak inhibitors of DMO O and N-demethylation, with average Ki values ranging from 18 to 50 microM for DOP formation, and from 21 to > 200 microM for MEM formation. The dione metabolite and BMSa did not produce detectable inhibition of either pathway. The findings for DMO O-demethylation, well-established as a CYP2D6-mediated reaction, indicate that NEF and metabolites are weak inhibitors of this reaction, with Ki values at least 100 times higher than fluoxetine (Ki = 0.1 microM +/- 0.09). The implications of results on DMO N-demethylation are not clear. In vivo data, as well as in vitro data based on "pure' CYP3A3/4 substrates, provide evidence for clinically relevant CYP3A3/4 inhibition by NEF, OH-NEF, and pOH-NEF. Thus, formation of MEM by N-demethylation of DMO may not constitute a suitable index reaction to probe CYP3A3/4 activity.

Characterization of six in vitro reactions mediated by human cytochrome P450: application to the testing of cytochrome P450-directed antibodies

The identification of cytochrome P450 (CYP) isozymes mediating metabolic pathways of drugs has become increasingly important in anticipating pharmacokinetic drug interactions. The activity of individual CYPs can be monitored in vitro with human liver microsomes by means by relatively specific metabolic reactions: for CYP1A1/2, phenacetin O-deethylation; for CYP2C9, phenytoin 4-hydroxylation; for CYP2C19, S-mephenytoin 4'-hydroxylation; for CYP2D6, dextromethorphan O-demethylation; for CYP3A3/4, alprazolam 4-hydroxylation, and for CYP2E1, chlorzoxazone 6-hydroxylation. We determined the kinetic parameters (Vmax, Km) of these reactions and utilized them to test a monoclonal rat liver CYP1A1 antibody, a monoclonal rat liver CYP2E1 antibody, a polyclonal rabbit anti-rat liver CYP3A1 antibody, and a polyclonal goat anti-rat liver CYP2C11 antibody for their specificity and inhibitory capacity. The CYP1A1 monoclonal antibody (MAb), the CYP2E1 MAb, and the CYP3A1 polyclonal antibody (PAb) inhibited only their respective index reactions. The CYP2C11 PAb inhibited both phenytoin 4-hydroxylation and S-mephenytoin 4'-hydroxylation. At a microsomal versus antibody protein mass ratio of 1:15, 4-hydroxyalprazolam formation was reduced by 73.4% of control with the CYP3A1 PAb; 4'-hydroxymephenytoin formation decreased by 66.3% and 4-hydroxyphenytoin formation by 43.4% with the CYP2C11 PAb; phenacetin O-deethylation was reduced by 39.7% with the CYP1A1 MAb, and 6-hydroxychlorzoxazone formation decreased by 30.0% with the CYP2E1 MAb. Thus, all antibodies tested are at least CYP subfamily specific. The PAbs exhibited greater than 60% inhibition versus the CYP3A3/4- and the CYP2C19-mediated reactions, whereas the MAbs produced less than 50% inhibition for their respective index reactions. Because of their limited inhibitory capacity, MAbs may be correspondingly limited as tools for identification of human CYP enzymes via immunoinhibition studies.

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.

Pharmacokinetic-pharmacodynamic relationships for benzodiazepines

This article reviews the literature on the plasma concentration-effect relationships for benzodiazepines, in humans and in experimental animals. Only literature that explicitly links pharmacokinetics to pharmacodynamics is included. The following questions are evaluated. Can concentration-effect relationships be demonstrated? If so, are these relations stable? Are the influences of specific factors such as age and disease on these relationships established? It is clear that, when studies are conducted and interpreted appropriately, relations can be found for a wide range of benzodiazepine effects. These include objective measures such as electroencephalography, semisubjective measures such as psychomotor performance, and subjective measures such as mood/sedation scales. A generally applicable model of the relationship which will allow prediction of effect is, however, not yet established. The relationship appears to be dependent on route and rate of administration, because of factors such as distributional delay, formation of active metabolites and, probably, acute tolerance. Furthermore, intra- and interindividual variability is considerable, probably due to varying experimental conditions and intrinsic interindividual differences. The limited data available on factors influencing the plasma concentration-effect relationships for benzodiazepines demonstrate clear changes in the pharmacodynamics after multiple doses, suggesting the development of tolerance, and a subsensitivity in patients with panic disorder. The influence of factors such as age, disease and drug interactions on the pharmacokinetic-pharmacodynamic relationship remains less clear.

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.

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.

Benzodiazepine sensitivity in panic disorder: effects of chronic alprazolam treatment

The aim of the current study was to determine the degree to which patients with panic disorder develop tolerance to subjective and physiological effects of benzodiazepine after chronic treatment with alprazolam. Response to acute administration of diazepam was assessed in 19 panic disorder patients receiving chronic treatment with alprazolam and 23 untreated panic disorder patients. At baseline in the laboratory, the two groups did not differ in peak saccadic eye movement velocity, saccade latency, short-term memory, plasma cortisol and growth hormone concentrations, heart rate, and self-rated levels of sedation and anxiety. Compared with untreated patients, alprazolam-treated patients displayed significantly less diazepam-induced change in peak saccadic velocity, saccade latency, growth hormone secretion, memory, and self-rated levels of sedation. There was no difference between groups in diazepam effects on plasma cortisol concentrations or self-rated anxiety. Within alprazolam-treated patients, diazepam-induced slowing of peak saccade velocity was significantly inversely correlated with illness severity, as measured by reported panic attacks per week and severity of phobic avoidance, but not with alprazolam dose, blood level, or duration of treatment. Because the alprazolam-treated group reported more panic attacks per week than the untreated panic patients, treated patients were divided into those who were asymptomatic versus those with continuing panic attacks. The subgroup of nine alprazolam-treated subjects who were asymptomatic also showed significantly less diazepam effects than the group of untreated panic disorder patients, suggesting that overall group differences were at least partially attributable to the development of tolerance to selected benzodiazepine effects with chronic alprazolam treatment.

Effects of diphenhydramine on human eye movements

Peak saccadic eye movement velocity (SEV) and average smooth pursuit gain (SP) are reduced in a dose-dependent manner by diazepam and provide reliable, quantitative measures of benzodiazepine agonist effects. To evaluate the specificity of these eye movement effects for agents acting at the central GABA-benzodiazepine receptor complex and the role of sedation in benzodiazepine effects, we studied eye movement effects of diphenhydramine, a sedating drug which does not act at the GABA-benzodiazepine receptor complex. Ten healthy males, aged 19-28 years, with no history of axis I psychiatric disorders or substance abuse, received 50 mg/70 kg intravenous diphenhydramine or a similar volume of saline on separate days 1 week apart. SEV, saccade latency and accuracy, SP, self-rated sedation, and short-term memory were assessed at baseline and at 5, 15, 30, 45, 60, 90 and 120 min after drug administration. Compared with placebo, diphenhydramine produced significant SEV slowing, and increases in saccade latency and self-rated sedation. There was no significant effect of diphenhydramine on smooth pursuit gain, saccade accuracy, or short-term memory. These results suggest that, like diazepam, diphenhydramine causes sedation, SEV slowing, and an increase in saccade latency. Since the degree of diphenhydramine-induced sedation was not correlated with changes in SEV or saccade latency, slowing of saccadic eye movements is unlikely to be attributable to sedation alone. Unlike diazepam, diphenhydramine does not impair smooth pursuit gain, saccadic accuracy, or memory. Different neurotransmitter systems may influence the neural pathways involved in SEV and smooth pursuit again.

Benzodiazepine receptors mediate regional blood flow changes in the living human brain

We studied the effects of a high-affinity gamma-aminobutyric acid (GABA)-benzodiazepine-receptor agonist (lorazepam) and an antagonist (flumazenil) in humans, using H2(15)O positron-emission tomography. Administration of lorazepam to healthy volunteers caused time- and dose-dependent reductions in regional cerebral blood flow and self-reported alterations in behavioral/mood parameters. Flumazenil administration reversed these changes. These observations indicated that benzodiazepine-induced effects on regional cerebral blood flow and mood/behavior are mediated at some level through GABA-benzodiazepine receptors, although the specific mechanism remains unclear. The approach described here provides a method for quantifying GABA-benzodiazepine-receptor-mediated neurotransmission in the living human brain and may be useful for studying the role of these receptors in a variety of neuropsychiatric disorders.

Progesterone co-administration in patients discontinuing long-term benzodiazepine therapy: effects on withdrawal severity and taper outcome

Since recent research has suggested that the major metabolites of progesterone are barbiturate-like modulators of GABAergic function, we undertook a pilot study of the efficacy of micronized progesterone in attenuating withdrawal and facilitating discontinuation in benzodiazepine-dependent patients with a minimum of 1 year of continuous daily use. Forty-three patients taking a mean daily dose of 16.2 mg of diazepam (or its equivalent) were assigned, doubleblind, to treatment with either placebo (n = 13) or progesterone (n = 30). Progesterone was titrated to a mean daily dose of 1983 mg, and was co-administered for 3 weeks, after which the benzodiazepine was tapered by 25% per week. Progesterone (or placebo) was then continued for 4 weeks before being discontinued. There was no progesterone versus placebo difference in the severity of taper withdrawal. Withdrawal checklist change scores were 17.3 for progesterone and 16.5 for placebo (F 0.63; df 2.31; n.s.), and the Hamilton rating scale for anxiety change scores were 7.8 for progesterone and 6.3 for placebo (F 0.22; df 2.30; n.s.). There was no difference in ability to remain drug-free at 12 weeks post-taper, with 57% of progesterone-treated patients, and 58% of placebo-treated patients having a successful outcome.

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).

Eye movement effects of diazepam in sons of alcoholic fathers and male control subjects

Both animal and human studies suggest that the GABA-benzodiazepine receptor complex may be involved in the acute effects of ethanol, as well as the development of tolerance and dependence with chronic ethanol use. The current study was performed to assess sensitivity to benzodiazepines, and thus the functional sensitivity of the GABA-benzodiazepine receptor system, in subjects at high risk for alcoholism. Sons of alcoholic fathers (SOAs; n = 27) were compared with male controls without a family history of alcoholism (n = 23) in response to diazepam versus placebo. SOAs and controls received four logarithmically increasing doses of intravenous diazepam or placebo in randomized order on 2 days at least 1 week apart. Effects of diazepam were assessed using two eye movement tasks, peak saccadic eye movement velocity, and average smooth pursuit eye movement gain, which provide reliable, quantitative measures of benzodiazepine effects. In addition, memory, self-rated sedation, and pleasurable drug effects were measured. In comparison with control subjects, SOAs displayed significantly less diazepam effects on peak saccade velocity, average smooth pursuit gain, memory, and self-rated sedation, but significantly greater pleasurable drug effects. Differences in response to diazepam between SOAs and male controls may reflect altered functional sensitivity of the central GABA-benzodiazepine receptor system or a more general difference between groups in the effects of CNS active or sedating drugs.

Cocaine toxicity in cultured chicken hepatocytes: role of cytochrome P450

Cocaine (COC) causes liver damage in several species, including man. Chicken embryo hepatocyte cultures were evaluated as a model system to investigate the mechanism of cocaine-mediated hepatotoxicity. Parameters used to assess toxicity were: (1) release of lactate dehydrogenase (LDH); (2) decreased induction of 5-aminolevulinic acid synthase (ALAS), measured as porphyrin accumulation; and (3) decreased protein synthesis. Exposure of untreated cultures to COC or norcocaine (NOR) caused dose-dependent increases in LDH release, decreased protein synthesis, and eventual cell death. Pretreatment with 2-propyl-2-isopropylacetamide (PIA), a phenobarbital-like inducer of cytochrome P450, accelerated toxicity and lowered the threshold dose at which toxicity occurred. PIA pretreatment also increased rates of elimination of both COC and NOR and increased rates of formation of NOR from COC. The toxicity of COC and NOR could also be detected as decreased porphyrin accumulation. Addition of the P450 inhibitor SKF-525A concurrently with COC or NOR decreased their rates of elimination. SKF-525A also prevented the increase in LDH release as well as the decrease in protein synthesis caused by treatment with COC or N-hydroxynorcocaine (N-OH). Addition of SKF-525A up to 3 hr after COC resulted in partial prevention of the LDH increase. Exposure of the cultures to COC induced cytochrome P450 2H protein. We conclude that this hepatocyte culture system is highly sensitive to COC toxicity and that constitutive as well as induced cytochrome P450 isoforms are involved in the production of liver damage from COC.

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)

Rapid and sensitive gas chromatographic determination of estazolam

The triazolobenzodiazepine estazolam can be quantitated by gas chromatography with electron-capture detection. After addition of a suitable internal standard, plasma samples are extracted into toluene-isoamyl alcohol or benzene-isoamyl alcohol. The organic extract is separated, evaporated to dryness, reconstituted, and chromatographed using a 50:50 methyl-phenyl column (SP-2250). The sensitivity limit is approximately 1 ng of estazolam in a 1-ml sample. The method is suitable for clinical or experimental pharmacokinetic studies.

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.

Basic pharmacokinetic principles and their application to psychotropic drugs

The application of pharmacokinetic principles to the administration of psychotropic medications provides a rational approach to understanding factors influencing the time course and intensity of drug action. When plasma concentrations of a given medication can be linked to levels at receptors in the brain, the pharmacodynamic effects of that drug can then be predicted. The pharmacokinetic parameter of most interest to clinicians is clearance, which describes the rate of drug removal per unit of plasma concentration. For any given drug, this parameter varies widely from person to person and may be markedly altered both in elderly patients and in patients with either kidney or liver disease. The clearance of a given medication can also be either increased or reduced by coadministration of other drugs. Such drug:drug interactions can potentially result in either reduced therapeutic effect or toxicity. Elimination half-life is determined by clearance and by another pharmacokinetic parameter, volume of distribution. Elimination half-life will be related to both the time necessary for plasma concentrations to reach steady state with repeated dosing and for the drug to be washed out after it is discontinued.

Caffeine treatment and withdrawal in mice: relationships between dosage, concentrations, locomotor activity and A1 adenosine receptor binding

Relationships between caffeine dose, methylxanthine tissue concentrations, adenosine receptor binding and locomotor activity were examined in CD-1 mice. A method of caffeine infusion via s.c. pumps provided constant steady-state methylxanthine concentrations. Mice receiving caffeine doses of 97 mg/kg/day (with mean plasma concentration of 2.7 micrograms/ml) demonstrated motor activity depression for 6 days after pump implantation (vs. vehicle-treated controls). Mice receiving caffeine doses of 194 mg/kg/day (mean plasma concentration of 7.1 micrograms/ml) demonstrated motor stimulation 4 and 24 hr after implantation. Mice receiving this dose for 6 days developed motor depression. A reduction in the stimulant effects of acute caffeine (20 mg/kg i.p.) was found in mice receiving caffeine infusions (194 mg/kg/day for 6 days) as compared to those receiving vehicle infusions, suggestive of drug tolerance. These dose- and time-dependent behavioral effects during caffeine-infusion were associated with decreases between 20 and 69% in specific binding of A1 adenosine radioligand 1,3-[3H]dipropyl-8-cyclopentylxanthine in vivo. Behavioral alterations during caffeine infusion appear to be mediated by A1 adenosine receptor occupancy. Increasing motor depression developed on days 1 and 2 after pump removal with values returning to control levels by days 4 and 6. Behavioral alterations were associated with in vivo binding increases of 98 and 324%, respectively, and a return to control values on days 4 and 6. In vivo binding alterations were not associated with ex vivo A1 receptor binding changes. Caffeine tolerance and withdrawal effects in this animal model appear to be mediated by chronic occupancy of A1 adenosine receptors. The behavioral and in vivo receptor binding alterations observed after caffeine discontinuation follow a time course similar to caffeine withdrawal in humans.

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.

Presystemic extraction: mechanisms and consequences

Many drugs have incomplete systemic availability after oral dosage. This can be attributed to incomplete absorption from the gastrointestinal tract, or to presystemic extraction, in which a fraction of an orally administered dose is biotransformed before reaching the systemic circulation. Presystemic extraction can occur either via biotransformation by gastrointestinal mucosa or enteric flora, or via metabolism during the "first-pass" through the liver. For drugs with low oral bioavailability due to high presystemic extraction, impaired clearance leads to increased peak plasma levels and greater area under the concentration-time curve, but minimal change in elimination half-life.

Clinical pharmacokinetics of alprazolam. Therapeutic implications

Alprazolam 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 alpha-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 alpha-hydroxy derivative, but urinary recovery of alpha-hydroxy-alprazolam greatly exceeds that of 4-hydroxy-alprazolam. This may be explained by chemical instability of 4-hydroxy-alprazolam in vitro. After single 1 mg oral doses in humans, typical pharmacokinetic variables for alprazolam are: a peak plasma concentration 12 to 22 micrograms/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 micrograms/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 micrograms/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.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Personality and benzodiazepine sensitivity in anxious patients and control subjects

Cloninger has recently proposed a model of personality variability that is based on three independent heritable traits of harm avoidance, novelty seeking, and reward dependence, each of which is thought to be mediated by a separate neurochemical and neuroanatomic mechanism. The current study tested hypotheses generated on the basis of this theory in anxious patients and control subjects. Eighteen patients with panic disorder, 12 patients with generalized anxiety disorder, and 21 control subjects underwent both personality testing and assessment of their sensitivity to diazepam, as measured by slowing of saccadic eye movement velocity. As expected, anxious patients displayed higher harm avoidance scores than controls. Although an inverse correlation between harm avoidance and benzodiazepine sensitivity was predicted, no relationship between these variables was found in any diagnostic group. However, a significant correlation was found between novelty-seeking scores and sensitivity to diazepam. This finding, although not predicted by Cloninger's theory, is consistent with prior preclinical and human studies.

Norcocaine and N-hydroxynorcocaine formation in human liver microsomes: role of cytochrome P-450 3A4

Cocaine was metabolized to norcocaine by microsomes prepared from lymphoblastoid cells expressing transfected human P-450 3A4. The specific activities of norcocaine formation by microsomes prepared from three human liver samples correlated with the amount of P-450 3A immunoreactive protein detected by immunoblot. Triacetyloleandomycin, a specific inhibitor of P-450 3A isoforms, inhibited formation of norcocaine from cocaine, but not formation of N-hydroxynorcocaine from norcocaine. The chemical identity of the norcocaine and N-hydroxynorcocaine produced by human liver microsomes was established by combination of gas chromatography and mass spectrometry. Thus, human P-450 3A4 is a cocaine demethylase, and P-450 isoforms of the 3A family are responsible for the majority of norcocaine production by human hepatic microsomes.

Inhibition of acetaminophen and lorazepam glucuronidation in vitro by probenecid

The effect of probenecid on glucuronidation of acetaminophen and lorazepam in hepatic microsomes from various species was studied to see if in vitro results were consistent with previous in vivo observations. Mouse, rat, and human microsomes were incubated with acetaminophen and probenecid while monkey microsomes were incubated with lorazepam and probenecid. Glucuronidation rates in all species varied with substrate, protein, and detergent concentrations. Mice exhibited faster rates of glucuronidation than rats or humans. All species showed inhibition of glucuronidation of acetaminophen or lorazepam when probenecid was added. Analysis suggested competitive inhibition. Thus, in vitro studies support in vivo results and confirm that the inhibition takes place at the hepatic level.

Benzodiazepine receptor binding of nonbenzodiazepines in vivo: alpidem, zolpidem and zopiclone

Several classes of nonbenzodiazepine compounds, including imidazopyridines such as alpidem and zolpidem and cyclopyrrolones, e.g., zopiclone, have effects similar to benzodiazepines and may act at the benzodiazepine receptor in brain. We characterized the binding of these compounds to the benzodiazepine site in three brain regions using specific uptake of the high-affinity ligand [3H]Ro15-1788 (flumazenil). For alpidem, benzodiazepine binding was decreased in cortex and hippocampus with increasing drug dose. For zolpidem, receptor binding was reduced in cortex without a dose-response effect and no effect was observed on cerebellar binding. Zopiclone did not alter binding except for a decrease in binding at the lowest dose evaluated and an increase in binding above control at the highest dose. These data corroborate prior studies indicating that the imidazopyridines appear to act at the benzodiazepine receptor, but do not support receptor subtype selectivity of zolpidem. The limited effect of zopiclone except for increased binding at high doses is also consistent with prior studies suggesting that zopiclone acts at a site distinct from the benzodiazepine receptor.

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.

Antipyrine kinetics in patients with primary biliary cirrhosis

Fourteen antimitochrodrial antibody-positive patients (13 women, 1 man) with biopsy-proven primary biliary cirrhosis, aged 40 to 71 years (mean, 57 years) weighing 43 to 102 kg (mean, 63 kg), along with 14 age- and sex-matched healthy controls, received a single 1.0- to 1.2-g dose of intravenous antipyrine. Plasma antipyrine levels were determined during a 12- to 24-hour period. Patients' mean serum chemistry values were: albumin, 3.9 g/dL (range, 3.1-4.4) and total bilirubin, 1.9 mg/dL (range, 0.3-10.9). Seven of the fourteen patients had cirrhosis. Mean kinetic variables for antipyrine in controls and primary biliary cirrhosis patients were: Vd, .54 versus .49 L/kg; half-life, 12.0 versus 15.1 hours (P < .07); clearance, .55 versus .41 mL/min/kg (P < .04). Within the primary biliary cirrhosis group, there was no correlation between total bilirubin and clearance (r = .09), nor did clearance vary significantly among histologic categories. Clearance of antipyrine in primary biliary cirrhosis patients is reduced by an average of 25%, but the clinical prognosticators of serum bilirubin levels and histologic grade do not correlate with or predict the degree of clearance impairment.