Clearance of diazepam can be impaired by its major metabolite desmethyldiazepam

Brain-plasma distribution of free and total benzodiazepines in dogs physically dependent on different doses of diazepam

Steady-state levels of oxazepam (OX), nordiazepam (ND), and diazepam (DZ) in plasma, brain tissue, cerebrospinal fluid (CSF), and intracranial microdialysis perfusate were determined in dogs dependent on 0.56, 4.5, 9, and 36 mg/kg per day of DZ. There was a linear relationship between the total plasma and brain levels of DZ, ND, and OX and the chronic dose of DZ. Levels of free benzodiazepines in plasma and CSF and levels in microdialysis perfusates from plasma and brain were significantly correlated. With increasing dependence on DZ there was progressively more free ND and OX and less free DZ in plasma, CSF, and brain. There was a correlation between several signs of precipitated abstinence and free ND in the brain interstitial fluid, whereas convulsions emerged only when free metabolites exceeded free DZ. The changes in contribution of free DZ, ND, and OX to the overall levels of benzodiazepines present in the CNS may explain differences in signs of abstinence for different levels of dependence on DZ.

Concentration-dependent metabolism of diazepam in mouse liver

Previous mouse liver studies with diazepam (DZ), N-desmethyldiazepam (NZ), and temazepam (TZ) confirmed that under first-order conditions, DZ formed NZ and TZ in parallel. Oxazepam (OZ) was generated via NZ and not TZ despite that preformed NZ and TZ were both capable of forming OZ. In the present studies, the concentration-dependent sequential metabolism of DZ was studied in perfused mouse livers and microsomes, with the aim of distinguishing the relative importance of NZ and TZ as precursors of OZ. In microsomal studies, the Kms and Vmaxs, corrected for binding to microsomal proteins, were 34 microM and 3.6 nmole/min per mg and 239 microM and 18 nmole/min per mg, respectively, for N-demethylation and C3-hydroxylation of DZ. The Kms and Vmaxs for N-demethylation and C3-hydroxylation of TZ and NZ, respectively, to form OZ, were 58 microM and 2.5 nmole/min per mg and 311 microM and 2 nmole/min per mg, respectively. The constants suggest that at low DZ concentrations, NZ formation predominates and is a major source of OZ, whereas at higher DZ concentrations, TZ is the important source of OZ. In livers perfused with DZ at input concentrations of 13 to 35 microM, the extraction ratio of DZ (E[DZ]) decreased from 0.83 to 0.60. NZ was the major metabolite formed although its appearance was less than proportionate with increasing DZ input concentration. By contrast, the formation of TZ increased disproportionately with increasing DZ concentration, whereas that for OZ decreased and paralleled the behavior of NZ. Computer simulations based on a tubular flow model and the in vitro enzymatic parameters provided a poor in vitro-organ correlation. The E[DZ], appearance rates of the metabolites, and the extraction ratio of formed NZ (E[NZ, DZ]) were poorly predicted; TZ was incorrectly identified as the major precursor of OZ. Simulations with optimized parameters improved the correlations and identified NZ as the major contributor of OZ. Saturation of DZ N-demethylation at higher DZ concentrations increased the role of TZ in the formation of OZ. The poor aqueous solubility (limiting the concentration range of substrates used in vitro), avid tissue binding and the coupling of enzymatic reactions in liver, favoring sequential metabolism, are possible explanations for the poor in vitro-organ correlation. This work emphasizes the complexity of the hepatic intracellular milieu for drug metabolism and the need for additional modeling efforts to adequately describe metabolite kinetics.

Metabolism of xenobiotics by Beauveria bassiana

1. Diazepam, warfarin and testosterone were metabolized by whole resting cells of the fungus Beauveria bassiana IMI 12939 via oxidative reactions such as hydroxylation and N-demethylation. 2. Metabolism of each substrate was inhibited by the cytochrome P450 inhibitors SKF-525A and metyrapone, consistent with the involvement of this enzyme system in the metabolism of these drugs by B. bassiana. 3. Substrate concentration-dependent inhibition was observed during diazepam metabolism by this organism, as has been observed in some mammalian systems. 4. Unlike most mammalian P450 systems, the warfarin-metabolizing activity of B. bassiana could not be induced by growing the organism in the presence of phenobarbitone, beta-naphthoflavone, 3-methylcholanthrene, 1-benzylimidazole or warfarin. 5. Overall findings indicate that B. bassiana possesses an oxidative metabolizing system capable of producing metabolites found in mammalian systems.

Pharmacokinetics of nordiazepam in physical dependence and precipitated abstinence in dogs

Previous studies suggested that the extensive accumulation of benzodiazepines is an important factor in the induction of physical dependence. The mechanistic basis for accumulation of nordiazepam (ND) and its metabolite, oxazepam (OX), have been examined in crossover studies in drug-naive and in ND-dependent dogs that exhibited a flumazenil-precipitated abstinence syndrome. ND and parent OX have similar pharmacokinetic profiles. Steady-state plasma levels of ND and OX cannot be predicted from single-dose pharmacokinetics. Reduced plasma clearance of ND and altered plasma protein binding were observed in dogs physically dependent upon ND. The benzodiazepine antagonist, flumazenil, significantly reduces steady-state plasma levels of total and free ND.

In vitro drug metabolism and pharmacokinetics of diazepam in cynomolgus monkey hepatocytes during culture for six days

Diazepam (DZ), N-desmethyl diazepam (NOR) and temazepam (TEM) were used as substrates in drug metabolism studies to characterize the changes in cytochrome P-450 mono-oxygenase pathways in hepatocytes isolated from cynomolgus monkeys, during culture for 6 days. Hepatocytes were incubated with DZ (20 microM), NOR (6 microM) or TEM (20 microM) for 3 hr at 3, 24, 48, 96 and 144 hr post-isolation in culture, and the profiles of disappearance of DZ, as substrate, and appearance of its metabolites determined. Major metabolites were NOR, TEM and oxazepam (OX). The kinetic profiles for the disappearance of DZ and the accumulation of metabolite were analysed using a four-compartment model and constants for the rates of formation of the metabolites were derived. There were significant changes during the period in culture for the rate constants of DZ demethylation, but no alteration in the 3-hydroxylation activities. Rates of DZ metabolism were unchanged during the initial 2 days in culture and well maintained for the subsequent 4 days, despite a fall in total cytochrome P-450 to 23% of initial values after 6 days. Cynomolgus monkey hepatocytes produce similar metabolite profiles for DZ to those found in man, both in vitro and in vivo, indicating that cynomolgus monkey hepatocytes may represent a relatively stable and valuable model of human drug metabolism.

Parallel pathway interactions in imipramine metabolism in rats

The in vitro metabolic inhibitions between imipramine and its metabolites were investigated in rat liver microsomes. A type of precursor-metabolite interaction similar to that shown with lidocaine was observed in imipramine metabolism. Desipramine competitively inhibited the formation of 2-hydroxyimipramine from imipramine. Similarly, imipramine inhibited the formation of 2-hydroxydesipramine from desipramine. As in the cases of those 2-hydroxylations, a competitive inhibitory relationship also existed in the N-demethylation pathways of imipramine and 2-hydroxyimipramine. Studies on age-associated alterations of the metabolic rates of imipramine and its metabolites in rats demonstrated that N-demethylation activities of imipramine and of 2-hydroxyimipramine, which showed a large sex difference (male greater than female) in young rats, decreased markedly only in old male rats, while 2-hydroxylation activities of imipramine and desipramine, with no sex difference at any age, did not show a marked alteration in either sex. These data strongly suggest that the hydroxylation pathways of imipramine and desipramine and the demethylation pathways of imipramine and 2-hydroxyimipramine are each sharing the same species of cytochrome P-450. The in vivo metabolic inhibition between imipramine and desipramine was examined by simultaneous intraportal infusion of imipramine (25 nmol/min) and desipramine (175 nmol/min). The steady-state concentration of imipramine after simultaneous infusion was increased twofold over that after infusion of imipramine alone, without any change in the free fraction in blood.

Plasma and brain pharmacokinetics of mianserin after single and multiple dosing in mice

Pharmacokinetics parameters describing the time course of concentrations of mianserin (MIA) in plasma and brain and the relationship between plasma and brain concentrations were studied after acute and chronic administration of increasing doses of MIA in adult mice. There was a linear relationship between the area under the curve (AUC), the maximum concentration (Cmax) and doses, in plasma and brain, both during acute and chronic experiments (p less than 0.05). A five-fold variation in plasma and brain terminal half-life (t 1/2) after chronic administration of the drug was observed, possibly due to a reduction in plasma drug clearance (CL). The values of Cmax and AUC in plasma and brain showed an increase of respectively about three and twelve times after chronic treatment. A very good correlation was observed between plasma and brain Cmax in both acute and chronic experiments; brain Cmax was 10.2 (+/- 0.16) times higher than plasma Cmax after acute administration and 12.08 (+/- 1.33) times higher after chronic administration.

Acute tolerance to diazepam induced by benzodiazepines

It was observed that the effectiveness of diazepam in causing sleep, as defined by the loss of righting reflex, was significantly decreased after a single exposure to either diazepam or lorazepam. RO 15-1788, a benzodiazepine antagonist, in contrast did not induce tolerance to diazepam. The mechanism for this acute tolerance is unclear. The rapidity in its development may exclude metabolic tolerance while alterations in brain sensitivity to diazepam remain a possibility.

Inhibition of diazepam metabolism in microsomal- and perfused liver preparations of the rat by desmethyldiazepam, N-methyloxazepam and oxazepam

Hydroxylated metabolites of diazepam can be conjugated and are therefore generally thought not to affect the metabolism of diazepam. Liver microsomes, obtained from phenobarbital-pretreated rats, showed an inhibition of diazepam (10(-5) M) metabolism by desmethyldiazepam as well as by N-methyloxazepam or oxazepam (5 X 10(-5) M). In a single-pass perfusion of the rat liver an inhibition of diazepam disposition by exogenously administered desmethyldiazepam and by hydroxylated diazepam metabolites was also demonstrated. No oxazepam glucuronides were found after oxazepam infusion. However, infusion with N-methyloxazepam resulted in large amounts of oxazepam-glucuronides. The results indicate that administration of N-demethylated as well as hydroxylated metabolites may result in inhibition of the metabolism of their precursor. If hydroxylated metabolites are formed in situ they become more easily conjugated in comparison with administered hydroxylated metabolites and are therefore less effective as inhibitor.

Development of tolerance to the anticonvulsant effect of diazepam in dogs

In dogs the development of tolerance to the anticonvulsant effect of diazepam was followed by weekly determinations of the convulsive threshold for pentetrazole, 10-15 min after intravenous (i.v.) injection of 0.25 or 0.5 mg/kg diazepam. As soon as after 1 week of oral treatment with diazepam, 0.25 or 0.5 mg/kg three times daily (t.i.d.), the pentetrazole threshold showed a decline or even a fall to the control level in spite of unaltered or rising concentrations of diazepam and its active metabolites. Tolerance also developed when the dogs were treated with chlorazepate, 2 mg/kg t.i.d., between the weekly diazepam injections for threshold determination. The results favor a functional type of tolerance since there was no indication of a more rapid inactivation of diazepam. Treatment with desmethyldiazepam (2 mg/kg i.v. for threshold determination and oral treatment with the desmethyldiazepam precursor chlorazepate, 2 mg/kg t.i.d.) did not produce tolerance. In further experiments in dogs anesthetized, relaxed with suxamethonium and ventilated, a spike-wave activity in the EEG was induced and maintained by an injection and subsequent infusion of pentetrazole. Out of 6 dogs, receiving 4-5 i.v. injections of 0.25-0.5 mg/kg diazepam, 2 showed the phenomenon of acute tolerance, i.e. the effect of the drug on the spiking activity in the EEG became less from one injection to the next, and thus paralleled a situation which may be observed during treatment of clinical status epilepticus. No acute tolerance was observed in corresponding experiments with desmethyldiazepam.

Prediction of diazepam disposition in the rat and man by a physiologically based pharmacokinetic model

A physiologically based pharmacokinetic model for diazepam disposition was developed in the rat, incorporating anatomical, physiological, and biochemical parameters, i.e., tissue volume, blood flow rate, serum free fraction, distribution of diazepam into red blood cells, drug metabolism and tissue-to-blood distribution ratio. The serum free fraction of diazepam was determined by equilibrium dialysis at 37 degrees C and was constant over a wide concentration range. Partition of diazepam between plasma and erythrocytes was determined in vitro at 37 degrees C, and the resultant blood-to-plasma concentration ratio was constant over a wide concentration range. The enzymatic parameters (Km, Vmax) of the eliminating organs, i.e., liver, kidney, and lung, previously determined using microsomes, were used for the prediction. The tissue-to-blood distribution ratios inferred by inspection of the data when pseudoequilibrium is reached after i.v. bolus injection of 1.2 mg/kg diazepam were corrected according to the method of Chen and Gross. Predicted diazepam concentration time-course profiles in plasma and various organs or tissues, using an 11-compartmental model, were compared with those observed. Prediction was successful in all compartments including brain, the target organ of diazepam. Scale-up of the disposition kinetics of diazepam from rat to man was also successful.

Development of tolerance and cross-tolerance to the psychomotor actions of lorazepam and diazepam in man

1 Development of tolerance and cross-tolerance to lorazepam and diazepam in man was assessed in a double-blind and cross-over trial where eight pretrained healthy students volunteered for four 1 week treatment periods started at 1 month intervals. 2 In each period acute psychomotor responses to oral lorazepam 3 mg and diazepam 15 mg were recorded on day 1, as well as on day 8 after 1 week's treatment twice daily with diazepam 5 mg, lorazepam 1 mg, and placebo. At each session several objective psychomotor tests and subjective assessments were done before the drug intake and 1, 2.5, and 4 h after it. 3 In general, the effects of lorazepam were stronger and of longer duration than those of diazepam at the doses used. When comparing the single-dose responses on days 1 and 8, tolerance to lorazepam effects and some cross-tolerance developed on several functions measured. Tolerance but not cross-tolerance developed on choice reaction errors whereas the opposite was found on flicker fusion. No definite tolerance was found on subjective effects. 4 The results tally with an assumption that tolerance to benzodiazepine actions develops at different rates on various parameters measured.