Nighttime dosing of triazolam in patients with liver disease and normal subjects: kinetics and daytime effects

Effects of liver disease on pharmacokinetics. An update

Liver disease can modify the kinetics of drugs biotransformed by the liver. This review updates recent developments in this field, with particular emphasis on cytochrome P450 (CYP). CYP is a rapidly expanding area in clinical pharmacology. The information currently available on specific isoforms involved in drug metabolism has increased tremendously over the latest years, but knowledge remains incomplete. Studies on the effects of liver disease on specific isoenzymes of CYP have shown that some isoforms are more susceptible than others to liver disease. A detailed knowledge of the particular isoenzyme involved in the metabolism of a drug and the impact of liver disease on that enzyme can provide a rational basis for dosage adjustment in patients with hepatic impairment. The capacity of the liver to metabolise drugs depends on hepatic blood flow and liver enzyme activity, both of which can be affected by liver disease. In addition, liver failure can influence the binding of a drug to plasma proteins. These changes can occur alone or in combination; when they coexist their effect on drug kinetics is synergistic, not simply additive. The kinetics of drugs with a low hepatic extraction are sensitive to hepatic failure rather than to liver blood flow changes, but drugs having a significant first-pass effect are sensitive to alterations in hepatic blood flow. The drugs examined in this review are: cardiovascular agents (angiotensin converting enzyme inhibitors, angiotensin II receptor antagonists, calcium antagonists, ketanserin, antiarrhythmics and hypolipidaemics), diuretics (torasemide), psychoactive and anticonvulsant agents (benzodiazepines, flumazenil, antidepressants and tiagabine), antiemetics (metoclopramide and serotonin antagonists), antiulcers (acid pump inhibitors), anti-infectives and antiretroviral agents (grepafloxacin, ornidazole, pefloxacin, stavudine and zidovudine), immunosuppressants (cyclosporin and tacrolimus), naltrexone, tolcapone and toremifene. According to the available data, the kinetics of many drugs are altered by liver disease to an extent that requires dosage adjustment; the problem is to quantify the required changes. Obviously, this requires the evaluation of the degree of hepatic impairment. At present there is no satisfactory test that gives a quantitative measure of liver function and its impairment. A critical evaluation of these methods is provided. Guidelines providing a rational basis for dosage adjustment are illustrated. Finally, it is important to consider that liver disease not only affects pharmacokinetics but also pharmacodynamics. This review also examines drugs with altered pharmacodynamics.

Anticonvulsant pharmacodynamics and disposition of triazolam in rats

Triazolam (TZ) is a triazolobenzodiazepine used in the treatment of insomnia that possesses significant anticonvulsant properties. Despite the widespread use of this drug, detailed pharmacokinetic-pharmacodynamic information is lacking, especially with respect to inhibition of seizure activity. TZ disposition has been described previously by methods with limited specificity, and the concentration-anticonvulsant effect relationship has not been characterized. The current studies were undertaken to examine TZ disposition with a specific HPLC method, and to evaluate the relationship between anticonvulsant effect and concentration in Sprague-Dawley rats. TZ pharmacokinetics were characterized after bolus or infusion administration; in a separate experiment, TZ pharmacodynamics were assessed with pentylenetetrazol-induced seizures. The systemic disposition of TZ could be described with a two-compartment model; systemic clearance ranged from 2.45 to 5.30 L/h/ kg, steady-state volume of distribution ranged from 2.10 to 4.02 L/kg, and mean residence time ranged from 47 to 65 min. The concentration-effect relationship was well described by a simple Emax model: Emax, expressed as the ratio of post-TZ to pre-TZ threshold convulsant doses of pentylenetetrazol, was 9.9 +/- 0.7, and the EC50 values were 10.0 +/- 4.6 ng/mL and 34.8 +/- 9.0 ng/g in serum and whole brain tissue, respectively. Under single-dose conditions, TZ is a very potent anticonvulsant in the rat pentylenetetrazol seizure model.

Triazolam pharmacokinetics after intravenous, oral, and sublingual administration

This study was designed to evaluate the relative and absolute bioavailability of triazolam, 0.25 mg, after the administration of the marketed oral tablet and a sublingual prototype wafer; an intravenous dose was used as a reference. Twelve men were evaluated in a three-way crossover study; study days were separated by 1 week. A single dose was administered to each subject at approximately 8 a.m.; serial blood samples were obtained for the determination of triazolam concentration. The fraction absorbed relative to intravenous was 20% higher in the sublingual than in the oral treatment (p = 0.0128); the difference between treatments was greatest in the first 2 hours as indicated by the area under the curve from 0 to 2 hours (p < 0.05). The extraction ratio ranged from 0.05 to 0.25, and the predicted availability after oral administration was 86% with a range of 75 to 95%. In contrast, the observed mean absolute availability was 44% (oral) and 53% (sublingual). A potential explanation for this discrepancy between predicted and observed bioavailability is that after oral administration, a fraction of triazolam may be metabolized by cytochrome P450IIIA4 in the gut wall, with a separate fraction subject to first-pass metabolism in the liver. Although this study was not designed to identify sites of triazolam metabolism, the proposed explanation is consistent with the occurrence of P450IIIA4 in the stomach, small intestine, and liver. Doses administered sublingually avoid first-pass metabolism, producing earlier and higher peak concentrations than do doses administered orally.

Simultaneous modeling of the pharmacokinetic and pharmacodynamic properties of benzodiazepines. II. Triazolam

This study compares the time course of triazolam effects on psychomotor and cognitive skills with triazolam plasma concentrations in a combined pharmacokinetic-pharmacodynamic (sigmoid-Emax) model. Ten male subjects received a single oral dose (1 mg) of triazolam or placebo. The CNS impairment effects were measured by using computerized tracking, body sway, and digit symbol substitution tests, and triazolam plasma concentration was measured by gas chromatography. The drug-induced effect changes lagged behind the plasma drug level changes. The magnitude of the time lag was quantified by the half-time of equilibration between concentrations in the hypothetical effect compartment and the plasma triazolam levels (t 1/2 keo). Essentially the same t 1/2 keo (approximately 6 min) was found for subcritical tracking, body sway, and digit symbol substitution tests. When using the predicted drug concentrations at the effect site, the hysteresis of the plasma concentration-effect disappears, suggesting that the hysteresis is not caused by drug induced tolerance. Moreover, the model allows for estimation of the effect site concentration that causes one-half of the maximal predicted effect (EC50, approximately 5 ng/ml) which is a measure of an individual's sensitivity to triazolam. On the basis of the EC50 values of the effect measures, body sway was slightly less sensitive to triazolam than subcritical tracking and digit symbol substitution tests.

Psychopharmacology in patients with hepatic and gastrointestinal disease

Psychotropic drugs often need to be prescribed to patients who also have pre-existing gastrointestinal (GI) and/or hepatic disease. This paper addresses the effect of GI and hepatic disease on the pharmacokinetics of psychotropic drugs, the effect of psychotropic drugs on pre-existing GI and hepatic diseases, the adverse GI and hepatic effects of psychotropic medications, the effects of GI medications on mental status, and the potential drug interactions between commonly prescribed GI medications and psychotropic drugs. Drug selection and dosage modification based on these considerations should allow safe and effective psychotropic treatment for patients with pre-existing GI and/or hepatic disease.

Pharmacokinetics of the newer benzodiazepines

The assay methods used to determine the concentrations of the newer benzodiazepines include electron-capture gas-liquid chromatography, high performance liquid chromatography with ultraviolet detection, gas chromatography-mass spectrometry, radioassay and radioreceptor assay. The method used frequently is the highly sensitive and specific electron-capture gas-liquid chromatography. Other methods are associated with limitations. The triazolo- and imidazolebenzodiazepines differ structurally from the 'classical' benzodiazepines such as diazepam, and offer distinct differences in pharmacological activity and in time-course of effect. Alprazolam and triazolam, both 1,4-triazolobenzodiazepines, have high affinities for the benzodiazepine receptor as do midazolam and loprazolam, which are 1,4-imidazolebenzodiazepines. Absorption is characteristically rapid, with peak alprazolam and triazolam concentrations occurring within 1 hour after oral administration. Sublingual administration results in peak alprazolam and triazolam concentrations that are higher and occur earlier than with the oral route. The volume of distribution of alprazolam and triazolam is approximately 1L. Alprazolam is 70% bound to plasma proteins and the extent of binding is independent of concentration. Similarly, triazolam is approximately 85% bound to plasma proteins, variability in binding being explained by variations in alpha 1-acid glycoprotein concentration. The 1,4-triazolo ring prevents the oxidative metabolism of the classical benzodiazepines which results in formation of active metabolites with long elimination half-lives. Alprazolam is extensively metabolised: 29 metabolites have been identified in the urine, and its major metabolite, alpha-hydroxyalprazolam, has pharmacological activity. alpha-Hydroxyalprazolam and 4-hydroxyalprazolam are detectable in plasma in amounts which account for less than 10% of the administered dose. Mean alprazolam elimination half-life in healthy adult subjects ranges from 9.5 to 12 hours; liver disease prolongs alprazolam elimination, but renal insufficiency does not. Triazolam also undergoes oxidation and subsequent glucuronidation. alpha-Hydroxytriazolam is the major metabolite, in addition to which 4-hydroxyalprazolam and alpha-4-hydroxytriazolam have been identified in plasma and urine. The elimination half-life of triazolam ranges between 1.8 and 5.9 hours, while that of the conjugated metabolites is short, approximately 3.8 hours. Accumulation of triazolam or its metabolites after multiple doses does not occur. Liver disease prolongs triazolam elimination from the body, but renal disease does not.(ABSTRACT TRUNCATED AT 400 WORDS)