The in vitro uptake and metabolism of lignocaine, procainamide and pethidine by tissues of the hindquarters of sheep

A Complete Extension of Classical Hepatic Clearance Models Using Fractional Distribution Parameter fd in Physiologically Based Pharmacokinetics

The classical organ clearance models have been proposed to relate the plasma clearance CLp to probable mechanism(s) of hepatic clearance. However, the classical models assume the intrinsic capability of drug elimination (CLu,int) that is physically segregated from the vascular blood but directly acts upon the unbound drug concentration in the blood (fubCavg), and do not handle the transit-time delay between the inlet/outlet concentrations in their closed-form clearance equations. Therefore, we propose unified model structures that can address the internal blood concentration patterns of clearance organs in a more mechanistic/physiological manner, based on the fractional distribution parameter fd operative in PBPK. The basic partial/ordinary differential equations for four classical models are revisited/modified to yield a more complete set of extended clearance models, i.e., the Rattle, Sieve, Tube, and Jar models, which are the counterparts of the dispersion, series-compartment, parallel-tube, and well-stirred models. We demonstrate the feasibility of applying the resulting extended models to isolated perfused rat liver data for 11 compounds and an example dataset for in vitro-in vivo extrapolation of the intrinsic to the systemic clearances. Based on their feasibilities to handle such real data, these models may serve as an improved basis for applying clearance models in the future.

Anatomical-physiological approaches in pharmacokinetics and pharmacodynamics

Use of an anatomical-physiological approach allows an investigator an alternative to regarding the whole body as a 'black box' producing biofluid specimens for drug assay, and then blindly applying a formula-driven mathematical approach to determine the pharmacokinetics and pharmacodynamics of the drug of interest. Instead, it means the investigator can consider that the body is the sum of interacting parts or regions connected anatomically by blood flow carrying the drug of interest, that the regions as well as the carrier blood are not homogeneous because each has a physiological role, and that the parts or regions are connected neurally and humorally so that the response in any region or part of the system may be modified by and/or modulate effects at another region or part. Such an approach is difficult to institute experimentally because a complicated (and often expensive) preparation is usually required in animal studies, and is rarely possible in research with humans because of ethical constraints. Despite these restrictions, there are many examples of the use of an anatomical-physiological approach allowing greater insight into pharmacological problems than would have been possible with a conventional 'whole body' approach alone. This paper takes a number of examples from the discipline of anaesthesia and pain management and groups them to illustrate the principles of the approach regarding drug arterio-venous equality and tissue distribution, multiple sites of clearance and multiple sites of action.

Prediction of in vivo tissue distribution from in vitro data 1. Experiments with markers of aqueous spaces

PURPOSE

The aim of this study was to evaluate the ability of an in vitro method of tissue distribution to accurately predict total water and extracellular aqueous spaces using marker compounds urea and inulin.

METHODS

Slices (50-200 mg) of all the major tissues in the rat were incubated with Hanks/HEPES pH7.4 buffer containing 14C-urea and 3H-inulin for 2 h at 37 degrees C. Tissue weight was noted before and after incubation and the tissue-to-buffer ratios determined.

RESULTS

14C-Urea Kp estimates were generally greater than total tissue water due to tissue swelling, which varied widely among the tissues, up to 41% in muscle. In most cases, Kp values were much closer to in vivo values after correcting for the 14C-urea in the imbibed media (Kpcorr). The method was able to distinguish between 14C-urea and 3H-inulin Kp values and indicated that inulin occupied a smaller space than urea, which for the majority of tissues corresponded to the extracellular space.

CONCLUSIONS

The Kp(corr) values for 14C-urea and Kp for 3H-inulin were consistent with total tissue water and extracellular space for the majority of tissues studied, indicating their suitability as marker compounds for checking the viability of this in vitro method for estimating tissue distribution.

Use of a single-pass in situ perfused rat hindlimb to study tissue distribution kinetics. Method development and experiences with Evans blue

A single-pass in situ rat hindlimb preparation has been developed as an alternative to the isolated perfused preparation to study tissue drug distribution kinetics with minimal disturbance of the animal physiology. Evans blue (EB), a vascular marker, was administered (in saline or plasma) as a bolus into the femoral artery, and the total outflow blood was collected at timed intervals from the corresponding femoral vein. Donor blood was infused through the jugular vein at a rate that matched the loss. No changes in the blood flow or development of edema were observed. The outflow profile was characterized using statistical moments. Regardless of the injected solution, the estimated vascular volume (10% of the hindlimb wet weight) was higher than that reported for the isolated rat hindlimb (4.1%) but closer to the in vivo blood volume (6.8-8.1% of body weight for 250-g rat) and sum of vascular volumes of its composite tissues (6.9% of tissue weights). The large normalized variance (CV(2)) of the outflow (2.2+/-0.1) confirms the known heterogeneity of the hindlimb. Entrapment in the limb (approximately 4%) and escape to the rest of the body could explain the incomplete recovery (79-89%) of the dye.

The in vivo blood, fat and muscle concentrations of lignocaine and bupivacaine in the hindquarters of sheep

1. A method was developed for sampling muscle and fat from the hindquarters of sheep undergoing spinal anaesthesia. The method was used to measure the concentrations of lignocaine and bupivacaine in the blood, muscle and fat of the hindquarters of sheep during and after 180 min constant-rate infusions of the drugs. 2. For both drugs the muscle drug concentrations were a relatively constant ratio of the simultaneous arterial blood drug concentrations during and after the infusion. 3. There was uptake of both lignocaine and bupivacaine into subcutaneous fat during the infusions. At the end of the infusion the ratio of the fat: arterial blood drug concentrations were 1.54 (SD = 0.57, n = 4) and 3.1 (SD = 1.4, n = 4) for lignocaine and bupivacaine, respectively. 4. The drug concentrations in fat declined relatively slowly after the infusion. The ratio of the fat: arterial blood drug concentrations 180 min after the end of the infusion was 21.5 (SD 4.0, n = 3) and for lignocaine, and 120 min after the end of the infusion was 9.54 (SD 5.2, n = 3) for bupivacaine. 5. It was concluded that the concentrations of lignocaine and bupivacaine in muscle were essentially in equilibrium with the arterial concentrations during and after the infusion. However, the concentrations of lignocaine and bupivacaine in fat were not in equilibrium with the arterial concentrations in the post-infusion period.