High capacity for pulmonary first-pass elimination of propranolol in rats

Intra-arterial pharmacokinetics and pulmonary first-pass of levamisole in rabbits

The pharmacokinetics of levamisole after intra-arterial administration of 12.5, 16 and 20 mg kg (-1) was investigated in rabbits. After compartmental analysis, the disposition of levamisole was well described by a two-compartment open model with mean values +/- sd of: alpha= 0.1650 +/- 0.0839, 0.1611 +/- 0.0298, 0.2312 +/- 0.0540 min (-1), and beta= 0.0118 +/- 0.0022, 0.0125 +/- 0.0026, 0.0120 +/- 0.0024 min (-1), for the three doses studied, respectively. There were no dose-related differences (one-way analyses of variance (ANOVA), P<or= 0.05) in alpha, beta, total body clearance (Cl) and volume of distribution at steady state (V(ss)). The AUC increased significantly with the doses (249.7, 376.7 and 562.5 microg min ml (-1)). After non-compartmental analysis there were no significant differences in plasma elimination rate constant ( lambda), MRT and V(ss) as a function of dose, but these differences were significant for Cl, between 16 and 20 mg kg (-1), and AUC (one-way ANOVA, P<or= 0.05). The two-way ANOVA showed no significant differences between the values obtained for the three doses when lambda- beta, Cl, V(ss)and V(a)were compared while AUC showed significant changes. On the other hand, the pharmacokinetic analysis (compartmental and non-compartmental) showed significant differences in AUC, Cl, V(ss) and V(a), but there were no significant differences when lambda- beta were compared. The slow clearance of levamisole by rabbit lung compared to a high pulmonary blood flow rate makes the possibility of significant first-pass lung metabolism unlikely in this animal species.

Toxicokinetics of a single intravenous dose of rac-propranolol versus optically pure propranolol in the rat

Conscious male Wistar SPF Riv:TOX rats were dosed intravenously with 2.5, 5, or 10 mg/kg rac-propranolol.HCl, or with 5 mg/kg of either (-)-(S)- or (+)-(R)-propranolol.HCl. Disposition of (-)-(S)- and (+)-(R)-propranolol after dosing of rac-propranolol was linear in the dose range examined. Total plasma clearance was not changed in animals dosed with the individual enantiomers compared to the animals that were dosed with rac-propranolol. However, for (-)-(S)-propranolol both volume of distribution and elimination half-life decreased, whereas for (+)-(R)-propranolol increases were observed for these characteristics, in animals dosed with the individual enantiomers. Our observations suggest that the (+)-(R)-enantiomer competes with (-)-(S)-propranolol for plasma protein binding sites, resulting in lower plasma protein binding of the (-)-(S)-enantiomer when the racemate is administered. From recent toxicological experiments, it was concluded that rac-propranolol is more toxic than the individual enantiomers in the rat, when dosed iv at the same total mass. It is concluded that the observed potentiation of toxic effects of propranolol enantiomers when administered as a racemate can at least partly be explained by a pharmacokinetic interaction.

Individual variation in first-pass metabolism

Individual variation in pharmacokinetics has long been recognised. This variability is extremely pronounced in drugs that undergo extensive first-pass metabolism. Drug concentrations obtained from individuals given the same dose could range several-fold, even in young healthy volunteers. In addition to the liver, which is the major organ for drug and xenobiotic metabolism, the gut and the lung can contribute significantly to variability in first-pass metabolism. Unfortunately, the contributions of the latter 2 organs are difficult to quantify because conventional in vivo methods for quantifying first-pass metabolism are not sufficiently specific. Drugs that are mainly eliminated by phase II metabolism (e.g. estrogens and progestogens, morphine, etc.) undergo significant first-pass gut metabolism. This is because the gut is rich in conjugating enzymes. The role of the lung in first-pass metabolism is not clear, although it is quite avid in binding basic drugs such as lidocaine (lignocaine), propranolol, etc. Factors such as age, gender, disease states, enzyme induction and inhibition, genetic polymorphism and food effects have been implicated in causing variability in pharmacokinetics of drugs that undergo extensive first-pass metabolism. Of various factors considered, age and gender make the least evident contributions, whereas genetic polymorphism, enzymatic changes due to induction or inhibition, and the effects of food are major contributors to the variability in first-pass metabolism. These factors can easily cause several-fold variations. Polymorphic disposition of imipramine and propafenone, an increase in verapamil first-pass metabolism by rifampicin (rifampin), and the effects of food on propranolol, metoprolol and propafenone, are typical examples. Unfortunately, the contributions of these factors towards variability are unpredictable and tend to be drug-dependent. A change in steady-state clearance of a drug can sometimes be exacerbated when first-pass metabolism and systemic clearance of a drug are simultaneously altered. Therefore, an understanding of the source of variability is the key to the optimisation of therapy.

Analysis of arterial-venous blood concentration difference of drugs based on recirculatory theory with fast inverse Laplace transform (FILT)

An arterial and venous blood (or plasma) concentration difference of drugs across the lung of rats was evaluated based on the recirculatory concept. The recirculatory system is given by the combination of the transfer functions for the pulmonary and the systemic circulations and is described by a Laplace-transformed equation, i.e., an image equation. For the manipulation of the image equations, the fast inverse Laplace transform (FILT) was adopted and MULTI(FILT) was used for the simultaneous curve fitting to estimate the pharmacokinetic parameters in the recirculatory model. Metoprolol as a test drug and cephalexin as a control drug were infused respectively into the femoral vein for 30 min, and arterial and venous blood samples were collected simultaneously through the cannula at the femoral artery and at right atrium during and after the infusion. Exponential functions were assumed for the weight functions through both the pulmonary and systemic circulations. Results of the curve fitting showed that the single-pass extraction ratio through the pulmonary circulation (Ep) of metoprolol was about 0.2, whereas that of cephalexin was negligible. The mean transit times through the pulmonary circulation (tp) of metoprolol and cephalexin were both about 0.5 min, which is small. The single-pass extraction ratios through the systemic circulation (Es) of metoprolol and cephalexin were both about 0.1, and the mean transit times through the systemic circulation (ts) were 11.5 min and 8.2 min, respectively.

Propofol: assay and regional mass balance in the sheep

1. Pharmacokinetic data for propofol, a new intravenous anaesthetic agent, indicate that there may be extensive extrahepatic clearance. This was investigated during intravenous infusions of propofol in adult merino ewes with chronic intravascular cannulae using a newly developed simple and rapid assay for propofol in blood and other biological samples. 2. The assay was based on organic solvent extraction of pH 4.5 buffered blood, urine or tissue homogenate, followed by reverse-phase h.p.l.c. with fluorescence detection. 3. A mean total body clearance of propofol of 3.15 l/min, (SD 0.87 l/min; n = 8) was found, consistent with a high hepatic extraction ratio (overall mean 0.87, SD 0.19; n = 8) and clearance (overall mean 1.12, SD 0.25 l/min; n = 7). The difference between total and hepatic clearances consisted principally of pulmonary clearance, but its extent was variable. 4. Other regional pharmacokinetic data were consistent with propofol distribution into muscle and brain tissues and propofol 'production' by the kidney, probably from a propofol metabolite formed elsewhere. 5. If these data are confirmed in humans then clinical pharmacokinetic data so far derived from peripheral venous blood sampling will require re-evaluation.

Effects of metabolic inhibitors and incubation temperature on the saturable uptake of propranolol by isolated rat lung tissue

The effects of several metabolic inhibitors (50 microM) on the initial uptake rate of propranolol (0.5 to 500 micrograms mL-1) by the minced lungs (0.4 g) isolated from 7-week-old rats has been investigated in oxygenated, pH 7.4 Krebs-Ringer bicarbonate buffer solution (20 mL) containing 3% BSA at 37 degrees C for 5 min. The effect of the incubation temperature was also examined. Metabolism of propranolol was almost insignificant (i.e. less than 1.3% of the initial load). The overall initial uptake rate was considered to be a combination of apparent linear transport and saturable processes. For the control uptake rate, the linear transport rate constant was 1.26 +/- 0.16 g-1 mL-1 min-1, while Vmax and Km' of the capacity limited uptake process were estimated as 0.727 +/- 0.074 mg g-1 min-1 and 24.8 +/- 2.71 micrograms mL-1, respectively. No metabolic inhibitor tested had an effect on the linear transport rate of propranolol but 2,4-dinitrophenol and potassium cyanide inhibited saturable uptake rate (i.e. Vmax) of propranolol significantly (P less than 0.01) while ouabain, phloridine and iodoacetic acid did not do so significantly. Reduction of the incubation temperature to 15 degrees C decreased and at 25 degrees C tended to decrease, both linear transport and saturable uptake rates.

Specific age-dependence in capacity-limited uptake of propranolol by isolated rat lung

To investigate the effect of age on the pulmonary uptake of propranolol, minced tissue (0.4 g) of lungs isolated from 3- to 104-week-old rats was incubated with the drug (1 to 500 micrograms mL-1) prepared in oxygenated, pH 7.4 Krebs-Ringer bicarbonate buffer solution (20 mL) containing 3% BSA for 60 min at 37 degrees C. In any age-group the metabolism of propranolol was not significant (i.e. less than 0.6% of any initial load) under the present in-vitro conditions. The extent of uptake after the incubation with 2.5 micrograms mL-1 of the drug was largest in the 7 weeks (i.e. 82% of the initial load) and relatively small in the 3(64%), 24(61%), 52(51%) and 104(48%) week old rats. Similar, specific age-dependence was observed in the tissue-to-medium concentration ratio of the drug. In any age-group, the initial uptake rate obtained in the first 5 min of the incubation was found to be a combination of apparently linear transport and saturable (capacity-limited) processes. There was a marked, specific age-dependence in the capacity-limited uptake rate. Although Km' value was almost equivalent in any age-group (i.e. 24.4 to 25.4 micrograms mL-1), Vmax exhibited a specific age-dependence by yielding the highest value in 7 weeks (0.726 +/- 0.101 mg g-1 min-1) and relatively low values in 3 (0.501 +/- 0.082 mg g-1 min-1), 52 (0.410 +/- 0.088 mg-1 g-1 min-1) and 104 (0.397 +/- 0.074 mg g-1 min-1) weeks.

Effects of bovine serum albumin and recirculation rate on the uptake of propranolol by rat perfused lung

Lungs isolated from 7-week-old rats were perfused with pH 7.4, oxygenated Krebs-Ringer bicarbonate buffer solution containing 2.5 micrograms mL-1 of propranolol and 3 to 5% BSA at the recirculation rate of 4 to 16 mL min-1, at 37 degrees C for 60 min. The extent of propranolol metabolism after 60 min was less than 2.3% of the initial load under any in-vitro perfusion condition. Therefore, the amount disappearing from the perfusion medium was considered as being predominantly that taken up by tissue. Under all experimental conditions, perfusate drug level declined bi-exponentially with time. Apparent in-vitro pulmonary clearance of propranolol was not affected by the increase of BSA level from 3 to 5%. When the perfusate BSA level was fixed at 3%, the lowest recirculation rate (4 mL min-1) yielded the smallest clearance (about 0.15 mL min-1 g-1) but almost constant clearance value (about 0.40 mL min-1 g-1) was obtained at the rate ranging from 8 to 16 mL min-1. The tissue to medium concentration ratio of propranolol, after the perfusion with 3 to 5% BSA at the rate of 8 to 16 mL min-1, was approximately 35, whereas that with 3% BSA at 4 mL min-1 was reduced to about 20. The findings suggest evidence for flow-dependent in-vitro pulmonary clearance of propranolol.

Age-dependent pulmonary first-pass elimination of propranolol in rats

Plasma levels of propranolol after 2.5 mg kg-1 given i.v. and i.a. have been compared in 3- to 4-week-old rats to evaluate the effect of age on pulmonary first-pass elimination of the drug. In 5- to 52-week-old rats, the area under the plasma concentration-time curve (AUC) after an intra-arterial dose was always larger than that after the i.v. dose. The plasma elimination half-lives after both routes of administration were almost identical, but tended to increase with age between weeks 7 and 104. First-pass pulmonary clearance and extraction ratio tended to decrease with age between weeks 7 and 52.