Drug metabolizing capacity in vitro and in vivo--II. Correlations between hepatic microsomal monooxygenase markers in phenobarbital-induced rats

Pharmacokinetic Interactions Between Isoniazid and Theophylline in Rats

Pharmacokinetic interactions between isoniazid and theophylline were studied in male Wistar rats, 206±17 g. Concomitant oral administration of 2 × 5 mg kg−1 isoniazid accelerated slightly the disposition of theophylline (10 mg kg−1, i.v.) whereas 2 × 25 mg kg−1 isoniazid slowed it marginally. The differences in distribution volume, systemic clearance and area under the concentration-time curve (AUC) between the high and the low dose, however, were statistically significant. One week pretreatment with 10 mg kg−1 isoniazid tended towards inhibition (significant decrease of systemic clearance, increase of AUC) and 50 mg kg−1 to acceleration (decrease of half-life, mean residence time and AUC, increase of systemic clearance) of theophylline disposition. After oral pretreatment with 20 mg kg−1 theophylline, neither the kinetics of free isoniazid (50 mg kg−1, i.v.) and the amount acetylated nor the acetylation indices differed from the controls. There was no evidence that concomitant or subacute administration of different doses of isoniazid affects major metabolic pathways of theophylline or that prolonged theophylline treatment interacts with the N-acetylation capacity.

COMPARISON OF FRESH AND CRYOPRESERVED RAT HEPATOCYTE SUSPENSIONS FOR THE PREDICTION OF IN VITRO INTRINSIC CLEARANCE

Freshly isolated hepatocytes are currently regarded as the most superior in vitro model for use in prediction studies, in particular to provide estimates of in vivo intrinsic clearance (CL(int)). However, due to their loss of viability within 4 h and a decrease in cytochrome P450-dependent metabolism upon culture, newer cellular models are being developed. Cryopreserved hepatocytes have several potential advantages, but to date evaluation of the utility of this model for estimating in vitro CL(int) has been limited to the substrate depletion approach. We have incubated eight compounds with suspensions of freshly isolated and cryopreserved rat hepatocytes and obtained in vitro CL(int) via metabolite formation kinetics (for 14 pathways). A substantial range of in vitro CL(int) values (0.1-98 microl/min/10(6)cells) was obtained in both models, and the freshly isolated suspension data were in good agreement with the literature. Cryopreserved suspensions were able to give a comparable estimation (within 2-fold) of in vitro CL(int) to fresh cells for six pathways, namely tolbutamide, three diazepam metabolites, propranolol, and 7-hydroxylation of warfarin. A higher estimation of in vitro CL(int) was obtained for the three other metabolites of warfarin due to a decrease in the K(M) values. Lower estimations of in vitro CL(int) were observed for four compounds (six pathways), and this was particularly pronounced (4-16%) for pathways showing atypical Michaelis-Menten kinetic profiles (dextromethorphan, nordiazepam) but less so (25-45%) for pathways showing biphasic Michaelis-Menten kinetics (7-ethoxycoumarin and phenytoin).

The effect of sepsis during parenteral nutrition on hepatic microsomal function in rats

STUDY OBJECTIVE

To investigate the effect of sepsis during parenteral nutrition on hepatic cytochrome P450 (CYP) activity in rats.

DESIGN

Prospective, randomized, controlled study.

SETTING

University-based animal research laboratory.

ANIMALS

Twenty adult male Sprague-Dawley rats.

INTERVENTION

The animals were cannulated intravenously and randomized to receive parenteral nutrition (PN), intravenous live Escherichia coli 4 x 10(8) colony-forming units/100 g body weight for 2 consecutive days with PN (PNEC), or chow (CH).

MEASUREMENTS AND MAIN RESULTS

Both PN alone and PNEC resulted in a progressive decline in hepatic CYP concentration compared with CH (0.53 +/- 0.10, 0.41 +/- 0.17, and 0.35 +/- 0.14 nmol/mg microsomal protein, respectively, p < 0.05). Parenteral nutrition alone was associated with a 57% decrease in isoenzyme ethoxycoumarin-O-deethylase activity (ECOD) compared with CH, but sepsis did not further decrease ECOD activity any more than PN alone (0.103 +/- 0.049, 0.044 +/- 0.018, and 0.050 +/- 0.020 nmol/mg microsomal protein/min, respectively, p < 0.05).

CONCLUSION

Hepatic CYP concentration declines with PN and is further decreased when compounded by sepsis. The disproportional decrease in ECOD activity relative to CYP concentration with PN is unchanged by sepsis, indicating a selective alteration in hepatic isoenzymes by PN.

Interaction of drugs and Chinese herbs: pharmacokinetic changes of tolbutamide and diazepam caused by extract of Angelica dahurica

The inhibitory effects of Angelica dahurica root extract on rat liver microsomal cytochrome P450 and drug-drug interactions were studied. The 2alpha- and 16alpha-hydroxylase activity of testosterone were most strongly inhibited, with 17.2% and 28-5% of their activity remaining, respectively, after oral administration of A. dahurica extract at a 1 g kg(-1) dose. 6beta-Hydroxylase activity was also inhibited, with 70% of its activity remaining, under the same conditions. In addition, treatment with the extract inhibited the metabolism of tolbutamide, nifedipine and bufuralol. These results showed that the extract inhibited the various isoforms of cytochrome P450 such as CYP2C, CYP3A and CYP2D1. The A. dahurica extract delayed elimination of tolbutamide after intravenous administration at a 10 mg kg(-1) dose to rats. Thus, the extract altered the liver intrinsic clearance. It had little effect, however, on the pharmacokinetic parameters of diazepam after intravenous administration at 10 mg kg(-1). Since diazepam showed high clearance, it underwent hepatic blood flow rate-limited metabolism. Therefore, the change of intrinsic clearance had little effect on hepatic clearance. However, the Cmax value after oral administration of diazepam with extract treatment was four times that with non-treatment. It was suggested that the first-pass effect was changed markedly by the extract. High-dose (1 g kg(-1)), but not low dose (0.3 g kg(-1)), administration of A. dahurica extract increased significantly the duration of rotarod disruption following intravenous administration of diazepam at 5 mg kg(-1). It was concluded that administration of A. dahurica extract has the potential to interfere with the metabolism, by liver cytochrome P450, of other drugs.

In vitro to in vivo extrapolation for trichloroethylene metabolism in humans

The use of in vitro systems in the assessment of xenobiotic metabolism has distinct advantages and disadvantages. While isolated hepatocytes and microsomes prepared from human liver may be used to generate data for comparisons among species and in vitro systems, such comparisons are generally performed on the basis of microsomal protein or million (viable) hepatocytes. Recently, in vitro data have been investigated for their value as quantitative predictors of in vivo metabolic capacity. Because of the existence of large amounts of trichloroethylene (TRI) data in the human, we have examined the metabolism of TRI as a case study in the development of a method to compare metabolism across species using in vitro systems and for extrapolation of metabolic rates from in vitro to in vivo. TRI is well metabolized by human hepatocytes in culture with a K(m) of 266 +/- 202 ppm (mean +/- SD) in headspace and a Vmax of 16.1 +/- 12.9 nmol/h/10(6) viable hepatocytes. We determined that human liver contains approximately 116 x 10(6) hepatocytes and 20.8 mg microsomal protein/g, based on DNA recovery and glucose-6-phosphatase activity, respectively. Thus, the microsomal protein content of hepatocytes is 179 micrograms microsomal protein/10(6) isolated hepatocytes. The microsomal apparent Vmax value of 1589 pmol/min/mg microsomal protein extrapolates to 17.07 nmol/h/10(6) hepatocytes. The combination of protein recovery and metabolic rate predicted a Vmax of approximately 1400 nmol/h/g human liver, which, when extrapolated and incorporated into an existing physiologically based pharmacokinetic (PBPK) model for TRI, slightly underpredicted TRI metabolism in the intact human. The quantitation, extrapolation, and inclusion of extrahepatic and cytochrome P450 (CYP)-independent TRI metabolism may increase the predictive value of this approach.

Metabolism of theophylline and its inhibition by fluoroquinolones in rat hepatic microsomes

1. The effects of beta-naphthoflavone, dexamethasone, phenobarbitone and isosafrole on the metabolism of theophylline by rat liver microsomes have been studied. Only beta-naphthoflavone, a known P4501A inducer, increased the rate of 1-methylxanthine formation (3-fold), whereas all the inducers studied increased the rate of 1,3-dimethyluric acid production (2.5-3-fold). 2. To study the effects of a range of fluoroquinolones on theophylline metabolism, beta-naphthoflavone-induced microsomes were used, as the ratio for metabolite production rates was similar to that of untreated microsomes (4:1,3-dimethyluric acid: 1-methylxanthine at 2 mM theophylline). High concentrations of fluoroquinolones (0.5-1.5 mM) were required to affect microsomal theophylline metabolism. 1-Methylxanthine was more sensitive to fluoroquinolone inhibition by enoxacin, ciprofloxacin, norfloxacin and pipemidic acid than 1,3-dimethyluric acid; CP67015, had a significant effect on 1,3-dimethyluric acid production only; binfloxacin had no effect on either pathway. 3. Ethoxycoumarin, a rapidly metabolized substrate, was also investigated as a surrogate for theophylline in in vitro experiments. Fluoroquinolone inhibition of ethoxycoumarin O-de-ethylation in beta-naphthoflavone-induced microsomes was quantitatively greater but qualitatively similar to theophylline metabolism (IC50s 440-870 microM at 2 microM 7-ethoxycoumarin). 4. These data are comparable with previous rat experiments in vivo, indicating that enoxacin, ciprofloxacin and norfloxacin have similar intrinsic activity in the inhibition of theophylline metabolism.

Circadian time-dependent kinetics of theophylline and its modulation by phenobarbital pretreatment in rats

The pharmacokinetics of theophylline (TPH, 10 mg/kg i.v.) were assessed in rats (natural light-dark span, June 9-10) after i.p. pretreatment with saline and 80 mg/kg phenobarbital (PB), respectively, for 3 consecutive days at either 07:00 h or at 19:00 h. Serum concentrations of TPH were assayed by high-performance liquid chromatography. No significant differences of the elimination rates of TPH could be found between the times of TPH administration (clearance: 1.17 +/- 0.17 ml/kg/min at 07:00 h vs. 1.23 +/- 0.17 ml/kg/min at 19:00 hours). PB premedication markedly accelerated TPH elimination. The increase in clearance values was more expressed when TPH was injected at 07:00 h than at 19:00 h (2.48 +/- 0.67 vs. 2.06 +/- 0.41 ml/kg/min, p < 0.01).

Simplified approach for evaluation of hepatic drug-oxidizing capacity with a simultaneous measurement of caffeine and its primary demethylated metabolites in carbon tetrachloride-intoxicated rats

1. Prediction of hepatic injury from changes in blood concentrations of caffeine (CA) and its three primary metabolites (theobromine: TB; paraxanthine: PX; theophylline; TP) after CA administration has been studied in carbon tetrachloride (CCl4)-intoxicated rats. 2. The plasma half-life (t1/2) of CA (10 mg/kg, oral) was increased, and total body clearance (CL) decreased, in CCl4-treated rats; the apparent volume of distribution (Vd) was relatively unchanged. 3. The ratios of CA to the three metabolites (TB/CA, PX/CA, TP/CA) were significantly decreased compared to those of controls. 4. There were good correlations between the ratios of TB/CA, PX/CA, and TP/CA and CL of CA (TB/CA; r = 0.944 at 1 h, r = 0.942 at 2 h: PX/CA; r = 0.974 at 1 h, r = 0.923 at 2 h: TP/CA; r = 0.866 at 1 h, r = 0.962 at 2 h). 5. Results indicate that it is possible to evaluate hepatic drug-oxidizing capacity by measuring the ratio of CA to its metabolites, using single blood sampling 1 or 2 h after CA administration in CCl4-intoxicated rats.

Drug metabolizing capacity in vitro and in vivo--I. Correlations between hepatic microsomal monooxygenase markers in beta-naphthoflavone-induced rats

Relationships between and within in vivo and in vitro markers of drug oxidative metabolism have been investigated in rats displaying a wide range of hepatic microsomal monooxygenase activity due to prior treatment with various doses of the inducing agent beta-naphthoflavone (BNF). BNF induction produced large dose-related changes in the in vivo clearance (CL) of theophylline (TH), antipyrine (AP) and the individual AP metabolite formation clearances, 4-hydroxyantipyrine (4H) and norantipyrine, and the in vitro parameters, 7-ethoxycoumarin O-deethylase, 7-ethoxyresorufin and P450. No trends were observed with the formation clearance of 3-hydroxymethylantipyrine and 7-methoxycoumarin O-demethylase whilst a negative response was observed with aldrin epoxidase. The selectivity of the markers towards BNF induction was coincident with the degree of covariance observed between these parameters. Strong correlations were observed in particular between CL(TH) and CL(4H) and ECOD and EROD indicating the high predictive value of these parameters. These studies demonstrate that under the well controlled conditions which may be imposed in animal environments predictively useful relationships (r2 greater than 0.8) can be established between in vitro and in vivo markers of hepatic microsomal monooxygenase activity.