Route- and dose-dependent pharmacokinetics of hexobarbitone in the rat: a re-evaluation of the use of sleeping times in metabolic studies

Utility of Minimal Physiologically Based Pharmacokinetic Models for Assessing Fractional Distribution, Oral Absorption, and Series-Compartment Models of Hepatic Clearance

Minimal physiologically based pharmacokinetic (mPBPK) models are physiologically relevant, require less information than full PBPK models, and offer flexibility in pharmacokinetics (PK). The well-stirred hepatic model (WSM) is commonly used in PBPK, whereas the more plausible dispersion model (DM) poses computational complexities. The series-compartment model (SCM) mimics the DM but is easier to operate. This work implements the SCM and mPBPK models for assessing fractional tissue distribution, oral absorption, and hepatic clearance using literature-reported blood and liver concentration-time data in rats for compounds mainly cleared by the liver. Further handled were various complexities, including nonlinear hepatic binding and metabolism, differing absorption kinetics, and sites of administration. The SCM containing one to five (n) liver subcompartments yields similar fittings and provides comparable estimates for hepatic extraction ratio (ER), prehepatic availability (Fg ), and first-order absorption rate constants (ka ). However, they produce decreased intrinsic clearances (CLint ) and liver-to-plasma partition coefficients (Kph ) with increasing n as expected. Model simulations demonstrated changes in intravenous and oral PK profiles with alterations in Kph and ka and with hepatic metabolic zonation. The permeability (PAMPA P) of the various compounds well explained the fitted fractional distribution (fd ) parameters. The SCM and mPBPK models offer advantages in distinguishing systemic, extrahepatic, and hepatic clearances. The SCM allows for incorporation of liver zonation and is useful in assessing changes in internal concentration gradients potentially masked by similar blood PK profiles. Improved assessment of intraorgan drug concentrations may offer insights into active moieties driving metabolism, biliary excretion, pharmacodynamics, and hepatic toxicity. SIGNIFICANCE STATEMENT: The minimal physiologically based pharmacokinetic model and the series-compartment model are useful in assessing oral absorption and hepatic clearance. They add flexibility in accounting for various drug- or system-specific complexities, including fractional distribution, nonlinear binding and saturable hepatic metabolism, and hepatic zonation. These models can offer improved insights into the intraorgan concentrations that reflect physiologically active moieties often driving disposition, pharmacodynamics, and toxicity.

Chronic physical activity: hepatic hypertrophy and increased total biotransformation enzyme activity

Does chronic voluntary physical activity alter hepatic or intestinal capacities for xenobiotic biotransformation? This question was investigated by comparing biotransformation enzyme activities in liver and small intestine of active and sedentary rats. Male rats allowed unlimited access to a running wheel and fed ad lib. for 6 weeks were weight-matched to sedentary controls; the active rats ate 22% more food than the sedentary rats (P less than 0.05). Active rats ran 2.8 +/- 0.6 miles/day. Liver weights were higher in the active rats (11.2 +/- 0.2 vs 9.8 +/- 0.2 g; P less than 0.05), as were total liver protein, and liver microsomal and cytosolic protein (P less than 0.05). As a result of liver hypertrophy, the active rats showed higher total liver activity of several biotransformation enzymes, including 2-naphthol sulfotransferase, styrene oxide hydrolase, benzphetamine N-demethylase, ethacrynic acid glutathione S-transferase and morphine UDP-glucuronosyltransferase (P less than 0.05). In contrast, there was no detectable difference in total liver N-acetyltransferase activity toward p-aminobenzoic acid, 2-naphthylamine, and 2-amino-fluorene as well as, relative hepatic enzyme activity (expressed per g liver or per mg protein) and total and relative intestinal enzyme activity. We conclude that chronic voluntary physical activity, accompanied by an increased food intake, results in liver hypertrophy and potentially increases total hepatic capacity to biotransform certain xenobiotic chemicals.

High-performance liquid chromatographic method for the simultaneous determination of R-(-)- and S-(+)-hexobarbital in rat plasma

The enantiospecific determination of R- and S-hexobarbital in rat plasma is described. The method involves liquid-liquid extraction of racemic hexobarbital from plasma, separation of the underivatized enantiomers by high-performance liquid chromatography on an alpha 1-acid glycoprotein column and ultraviolet detection. The mobile phase consists of a phosphate buffer (pH 5.4) containing 0.4% 2-propanol as organic modifier. An alpha 1-acid glycoprotein guard column is used to increase the lifetime of the analytical column. Heptabarbital is the achiral internal standard. With detection limits of ca. 0.05 microgram/ml for both R- and S-hexobarbital, the assay is suitable for pharmacokinetic studies of the enantiomers in rats.

Disposition of hexobarbitone and antipyrine in DOCA-hypertensive rats

The disposition of antipyrine and hexobarbitone, and their effects on drug metabolizing hepatic enzymes have been investigated in DOCA-hypertensive rats. Antipyrine pharmacokinetic parameters were the same in hypertensive and control animals. Hexobarbitone sleeping time was longer in hypertensive rats compared with controls, while the activity of hepatic hexobarbitone hydroxylase was the same in both groups. Hepatic aminopyrine-N-demethylase activity was elevated in hypertensive rats while aniline hydroxylase and aryl hydrocarbon hydroxylase were lower. Glucuronyl transferase was the same in both groups. The sensitivity of the central nervous system of hypertensive rats to hexobarbitone was not altered, as determined by hexobarbitone concentration in blood and in brain. The total hepatic blood flow (arterial and portal) was significantly increased. Thus it is suggested that the difference in the disposition of the two drugs is probably not due to drug metabolizing enzyme activity. It is likely that the increase in total hepatic blood flow and rapid saturation of hepatic hexobarbitone metabolizing enzymes have significant roles in the slower metabolism and increased activity of hexobarbitone in hypertensive rats as compared with control rats.

Pharmacokinetics of hexobarbital in young and old rats

Hexobarbital is a drug widely used to study the capacity of the liver to metabolize drugs. The pharmacokinetics of hexobarbital in 3- and 30-month-old male BN/BiRij rats were studied. The half-life of hexobarbital in 30-month-old rats (39.9 +/- 4.1 min) was significantly higher than that of 3-month-old ones (21.3 +/- 3.8 min). The volume of distribution (ml.kg-1 body weight) did not change with age. The intrinsic clearance, expressed as ml.min-1.kg-1 body weight, of hexobarbital in 30-month-old rats (20.2 +/- 6.6) was half that of the 3-month-old ones (39.5 +/- 7.6). Further studies will be performed to investigate the effect of age on the intrinsic clearance of S(+)- and R(-)- hexobarbital.