Effect of low-dose phenobarbitone on five indirect indices of hepatic microsomal enzyme induction and plasma lipoproteins in normal subjects

Quantitative evaluation of hepatic and intestinal induction of CYP3A in clinical practice

This is the first report quantitatively evaluating the clinical induction of CYP3A in the liver and the intestine.To evaluate hepatic induction, we collected literature data on endogenous biomarkers of hepatic CYP3A induction which we then used to calculate the fold-induction (inducer-mediated change in biomarker level). Literature data on decreases in the area under the curve (AUC) of alfentanil, a CYP3A substrate, caused by CYP3A inducers were also collected. We used the hepatic intrinsic clearance of alfentanil to calculate the hepatic induction ratio (inducer-mediated change in intrinsic clearance). For intestinal induction, the intestinal bioavailability (F_g) of alfentanil was used to calculate the intestinal induction ratio. We determined in vivo maximum induction (E_max) and the average unbound plasma concentration (C_av,u) required for half the maximum induction (EC₅₀) for inducers using an E_max model analysis.In our results, fold-induction was comparable to the induction ratio at several inducer concentrations, and almost the maximum induction was achieved by a therapeutic dose. Induction ratios in the intestine were similar to the liver.Our findings suggest that, by knowing only hepatic induction ratios for common inducers, we can quantitatively predict the decreases in the AUC of substrates by CYP3A induction.

The Quantitative Prediction of In Vivo Enzyme-Induction Caused by Drug Exposure from In Vitro Information on Human Hepatocytes

There have been no reports of the quantitative prediction of induction for drug-metabolizing enzymes in humans. We have tried to predict such enzyme induction in humans from in vitro data obtained using human hepatocytes. The in vitro and in vivo data on enzyme induction by inducers, such as rifampicin, phenobarbital and omeprazole, were collected from the published literature. The degree of enzyme induction in humans was compared with that predicted from in vitro data on human hepatocytes. Using the in vivo data, we calculated the hepatic intrinsic clearance of typical CYP substrates, such as midazolam and caffeine, before and after inducer treatment and estimated the induction ratios of hepatic intrinsic clearance following treatment. In the in vitro studies, the amount of mRNA or enzyme and enzyme activity in human hepatocytes, with or without an inducer, were compared and the induction ratios were estimated. The unbound mean concentration was taken as an index of drug exposure and the induction ratios in the in vivo and in vitro studies were compared. The unbound mean concentrations of inducers used in the in vitro studies were higher than those in the in vivo studies. The maximum induction ratios by inducers in the in vitro studies were higher than those in the in vivo studies. The induction ratio for rifampicin, omeprazole, troglitazone, dexamethasone and phenobarbital increased as the unbound mean concentration increased to reach a constant value. The induction of CYP3A and 1A was analyzed by the Emax model. The maximum induction ratio (Emax) and the concentration at half maximum induction (EC50) for rifampicin, omeprazole, troglitazone, dexamethasone and phenobarbital were 12.3, 0.847 micromol/L, 2.36, 0.225 micromol/L, 6.86, 0.002 micromol/L, 8.30, 9.32 micromol/L, and 7.62, 58.4 micromol/L, respectively. The Emax and EC50 of omeprazole for CYP1A were 12.02 and 0.075 micromol/L, respectively. The predicted induction ratio of all those inducers, except for omeprazole, based on the Emax and EC50 values obtained from the in vitro data were similar to the observed values. On the whole, a good correlation between the observed and predicted induction ratio of omeprazole was observed (r=0.768, p<0.05), although the predicted induction ratio was higher than the observed value. In conclusion, the present study suggests that it is possible to predict quantitatively the CYP3A enzyme induction from hepatocyte data.

Lack of effect of phenobarbital on ex vivo leukocyte oxidative metabolism in healthy volunteers

Oxidative metabolism in activated human polymorphonuclears catabolizes leukotriene B4 by a cytochrome P450 omega-hydroxylase and procainamide by a myeloperoxidase. The percentage of leukotriene B4 metabolized by activated human polymorphonuclears and the apparent enzymatic parameters of procainamide metabolism were studied ex vivo in six healthy volunteers before and after phenobarbital intake (100 mg/day) for 10 days and in six healthy control volunteers. No differences were found between groups for the difference in percentage of leukotriene B4 metabolized between day 11 and day 1. The apparent enzymatic parameters, Km and Vm, of procainamide oxidation did not differ significantly between the groups both on day 1 and day 11. These results do not show any evidence of inducibility of leukotriene B4 and procainamide metabolism in human polymorphonuclears. However, a positive correlation between 6 beta OH-cortisol excretion and percentage of leukotriene B4 metabolized was observed on day 11. This study suggests that human polymorphonuclears cytochrome P450 leukotriene B4 omega-hydroxylase and procainamide metabolism is not a useful method to study cytochrome P450 induction in clinical pharmacology.

Enzyme induction and inhibition

The rate and extent of drug metabolism significantly influences drug effect. Enzyme induction by increasing the metabolism of drugs may result in important drug interactions. Other implications of enzyme induction include alterations in the metabolism of endogenous substrates, vitamins and activity of extrahepatic enzyme systems. Similarly a wide range of drugs may produce clinically significant drug interactions following enzyme inhibition. Assessment of enzyme induction and inhibition in man involves diverse methods including the use of model drugs.

Low-dose phenobarbitone as an indicator of compliance with drug therapy

1 To assess the potential value of low-dose phenobarbitone (PB) as a marker of compliance we studied the relationship between plasma level of PB and dose (2-16 mg daily) following 3 or 4 weeks treatment in healthy volunteers (n = 26) and in-patient volunteers (n = 7). 2 Also, to simulate poor compliance, PB levels were measured in some volunteers following alternate-day (n = 6) or short-term (n = 5) treatment with similar doses. These levels, expressed as the level: dose ratios (LDRs), did not overlap with those obtained following 3 or 4 weeks of daily PB intake. 3 To evaluate the efficacy of this marker in patients taking other drugs we gave a group of out-patients (n = 24) compound tablets containing B vitamins and a small dose (16 mg) of PB; their compliance over 2-5 weeks was assessed both by measuring plasma levels of PB and residual tablet counting. 4 In the latter study, as well as providing absolute evidence of good compliance by many patients, the plasma levels of PB proved particularly valuable when non-compliant individuals 'forgot' to bring their residual tablets. 5 We suggest that phenobarbitone, in doses low enough to be non-sedative and non-enzyme inducing, is potentially useful as a pharmacological indicator of compliance with drug therapy.

Comparison of maternal and fetal toxic dose responses in mammals

The role of maternal toxicity in adverse developmental outcome and the importance of maternal toxicity as a factor in developmental risk assessment have received increasing attention in the recent literature. This paper reviews these concepts and some of the experimental approaches that have been used to assess their importance. The often non-parallel nature of maternal and fetal toxic dose response curves makes specific comparisons of maternal and fetal toxic doses quite difficult. The use of specific ratios of maternal to fetal toxic doses, such as the A/D ratio and the Relative Teratogenic Index is discussed, including some of the difficulties encountered in assigning such ratios, and a compilation of A/D ratios for compounds for which this ratio could be estimated in more than one mammalian species is included. Results of our studies on the fungicide, dinocap, for which the A/D ratio does not seem to be consistent across species, are briefly reviewed. Maternal toxicity is usually evaluated only on the basis of significant mortality, dose-related weight loss, or obvious external observations. Some examples of other more specific indications of maternal toxicity that may be involved in teratogenesis are presented.

The effect of low-dose phenobarbitone on three indices of hepatic microsomal enzyme induction

The effects of low-dose phenobarbitone on three indices of hepatic enzyme induction were studied. Eight healthy volunteers took phenobarbitone 7.5 mg daily for 4 weeks followed by 15 mg daily for 4 weeks; five subjects took 30 mg daily for a further 2 weeks. Phenobarbitone 15 mg daily produced a significant rise in antipyrine clearance (P less than 0.05). Phenobarbitone 30 mg daily produced a further rise, but probably because of the reduced numbers of subjects, this did not achieve significance (P = 0.06). Urinary 6-beta-hydroxycortisol and D-glucaric acid levels did not change significantly and remained within the range seen in subjects not taking enzyme-inducing drugs. We conclude that phenobarbitone 7.5 mg daily produces little (if any) enzyme induction whereas 15 mg, or more, may have the potential to produce drug interactions through enzyme induction.

Enzyme induction with rifampicin; lipoproteins and drug binding to alpha 1-acid glycoprotein

Hepatic enzyme induction has been reported to increase lignocaine binding, alpha 1-acid glycoprotein concentration and high density lipoprotein (HDL) cholesterol. In eight volunteers treated with rifampicin for 3 weeks there was no significant alteration in these three variables although their antipyrine clearance was significantly increased. In 10 patients with pulmonary tuberculosis treated with rifampicin (mean 5 months) the degree of serum protein binding of lignocaine, alpha 1-acid glycoprotein and HDL-cholesterol concentration was not different from that of matched control patients. These results suggest that differential induction of these variables may occur.

Hepatic enzyme induction and leucocyte delta-aminolaevulinic acid synthase activity: studies with carbamazepine

Antipyrine metabolism, daily urinary 6-beta-hydroxycortisol excretion, carbamazepine (CBZ) half-lives and leucocyte delta-aminolaevulinic acid synthase (ALA.S) activities were measured following 2 weeks' treatment with CBZ 400 mg and 600 mg once daily in eight healthy male volunteers. Dose-dependent induction of antipyrine metabolism was demonstrated but cortisol hydroxylation appeared maximally induced by the 400 mg dose. CBZ half-lives fell significantly in both studies (P less than 0.01 in each case) but a greater fall was seen with the higher dose (P less than 0.01). Plasma CBZ concentrations were higher following the eighth doses (P less than 0.01) in both studies. Leucocyte ALA.S activity increased by a mean of 657% following 1 week's treatment with 400 mg CBZ and 1145% on the 600 mg dose. In both studies ALA.S activities fell towards baseline during the second week of treatment. CBZ possesses potent dose-dependent hetero- and auto-inducing properties. Leucocyte ALA.S activity may represent a novel in vivo index of extrahepatic enzyme induction in man.

Free fraction of valproic acid: in vitro time-dependent increase and correlation with free fatty acid concentration in human plasma and serum

The effect of sample incubation and storage on the protein binding of the antiepileptic drug valproic acid (VPA) and on the concentration of free fatty acids (FFA) was investigated in serum and plasma collected from four normal volunteers. Both the free fraction of VPA and the concentration of FFA increased progressively with time when samples were incubated at 4 to 37 degrees C. These effects occurred to the same extent in both serum and heparinized plasma. At 4 degrees C and at room temperature, the increase in free drug fraction was relatively small (18 and 25% respectively at 24 h), whereas at 37 degrees C it was quite considerable (24% at 8 h and 40% at 24 h). At room temperature, FFA rose on average by 22% at 4 h, 34% at 8 h, and 86% at 24 h, whereas at 37 degrees C the increases at the same incubation times were 59, 90, and 160%, respectively. There was a strong positive relationship between changes in free VPA fraction and FFA content of the samples. The time-dependent changes in VPA binding capacity described in this study may lead to overestimation of the actual free concentration in vivo, especially when this is estimated by equilibrium dialysis or ultracentrifugation techniques requiring long incubation (centrifugation) times at 37 degrees C.