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

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.

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.

Systemic and regional pharmacokinetics of levobupivacaine and bupivacaine enantiomers in sheep

Commercially available bupivacaine is an equimolar mixture of R(+)- and S(-)-bupivacaine. S(-)-bupivacaine (levobupivacaine) is the subject of current clinical evaluation. We conducted partial cross-over systemic and regional pharmacokinetic studies of i.v. bupivacaine (12.5-200 mg) and levobupivacaine (6.25-200 mg) in ewes. Enantiospecific analysis of blood drug concentration-time data and of regional myocardial and brain drug mass balance data indicated that (a) there was a higher mean total body clearance of R(+)-bupivacaine than of S(-)-bupivacaine (as previously reported); (b) there were no differences in the systemic pharmacokinetics of S(-)-bupivacaine whether administered alone or as a component of bupivacaine; (c) there was no evidence of dose-dependent pharmacokinetics with either enantiomer; (d) for both enantiomers, mean calculated myocardial tissue concentrations of 1%-4% dose occurred between 3 and 5 min. Mean brain concentrations of 0.2%-1% dose occurred between 2 and 4 min after the administration of bupivacaine but between 4 and 5 min after the administration of levobupivacaine. There was no evidence that systemic toxicity induced by these local anesthetics significantly modified their pharmacokinetics, and there was no evidence of an enantiomer-enantiomer pharmacokinetic interaction for bupivacaine.

IMPLICATIONS

Levobupivacaine comprises 50% of commercially available bupivacaine and is being considered for use in its own right. As a part of its preclinical evaluation, this study considered whether levobupivacaine behaved kinetically in the body in the same way as when administered as a component of bupivacaine.

Discrepancies in pharmacokinetic parameter estimation between bolus and infusion studies in the perfused rat hindlimb

Isolated, perfused rat hindlimb consists of skeletal muscle, skin, bone, and adipose. Hence, it is a heterogeneous preparation composed of slowly equilibrating tissues of different characteristics and fractional flow rates. This paper shows how caution should be exercised in interpreting the results following bolus administration and subsequent statistical moment analysis of intravascular markers (51Cr-erythrocytes and 125I-albumin) and lipophilic barbiturates. For the intravascular markers, the events in the hindlimb are overshadowed by events in the connecting tubing and cannulas, due to their comparable volumes. For the barbiturates, these estimates appear to apply to short-term effects as the volume estimates obtained following infusion to steady state are greater than after bolus administration. For the extravascular markers, 14C-sucrose, 14C-urea, and 3H-water, no such time dependency was shown. However, it is only from the outflow profiles following bolus administration that events in the tissue beds can be elucidated.

Bupivacaine enantiomer pharmacokinetics after intercostal neural blockade in liver transplantation patients

Bupivacaine, being a racemic local anesthetic, exists as an equal mixture of its component enantiomers R(+)- and S(-)-bupivacaine, which behave pharmacokinetically as independent drugs after injection into the body. Intercostal neural blockade using bupivacaine was performed for postoperative analgesia in 12 patients after orthotopic liver transplantation. Arterial blood, sampled serially, was assayed by enantioselective high-performance liquid chromatography for R(+)- and S(-)-bupivacaine. The average of the simultaneous R(+):S(-) ratios of blood bupivacaine concentrations in the 12 patients was 0.74 (SD 0.11); however, the use of a population mean value or a mean value for any patient denies the time-dependence of this entity. The blood enantiomer concentration difference was reflected in the maximum measured concentrations which, after the first dose, were, respectively, 0.38 (SD 0.19) and 0.52 (SD 0.28) mg.L-1.100 mg-1 RS-bupivacaine administered (P = 0.0003). The difference in blood concentrations between the enantiomers, reflected by the R(+):S(-) ratio being less than unity, could be explained by a greater mean total body clearance and a larger apparent volume distribution of R(+)-bupivacaine. Elimination of both enantiomers was prolonged in these patients after liver transplantation compared to data from the literature, but there was no tendency for either enantiomer to accumulate selectively, even upon repeated dosing. We conclude that this demonstration of differences in pharmacokinetics (and, in laboratory studies, also in pharmacodynamics) between the bupivacaine enantiomers points to the need for future studies to recognize the enantiomeric duality of this local anesthetic.

An analysis of errors arising from the direct use of mass balance principles to describe regional drug uptake and elution

Errors occurring during the direct application of mass balance principles to describe the uptake and elution of a drug in an organ during and after a constant rate infusion were analyzed. The uptake of lignocaine in the hindquarters of sheep was used as an example--the net mass of lignocaine was calculated from the arterial and inferior vena cava blood lignocaine concentrations and hindquarter blood flow using an integrated form of the Fick equation. The general strategy was to generate a continuous time course of arterial and inferior vena cava drug concentrations that closely resembled the data obtained from in vivo experiments (the "true" blood concentrations). These were used to calculate the time course of the "true" net mass of lignocaine in the hindquarters by numerical integration with a small step size. The true blood concentrations were then used to generate data sets that simulated different blood sample intervals and random, normally distributed errors added to the blood concentration and blood flow measurements. Simulated data sets were also used to compare different numerical integration methods. There were significant absolute errors in the calculated net mass in the period after the start and end of the constant rate infusion due to numerical integration, but the error resulting from the latter to some extent canceled the error from the former. These errors did not greatly change the time course of the calculated net mass. Decreasing the interval between regular blood samples from 30 to 10 min reduced this absolute error, but greater reductions in error were achieved by optimizing the time interval between blood samples to give an approximate constant error due to numerical integration. There was no advantage in using numerical integration methods other than the linear trapezoidal method. Random noise added to the blood concentration and blood flow terms of the net mass equation added a small bias to the mean value of the calculated net mass. More important, such noise rapidly increased the number of studies required to characterize the calculated mean net mass to a given level of accuracy. It is concluded best results are obtained by minimizing the variability of blood concentration and blood flow measurements, and by using an optimized blood sampling regimen. The direct mass balance calculations and an analysis of their errors are simple enough to be performed using a spreadsheet program on a personal computer.

Dermal and underlying tissue pharmacokinetics of lidocaine after topical application

The deep-tissue penetration of lidocaine below a dermally applied site was quantified in a rat model. The concentrations of lidocaine in tissues below the applied site were measured and compared with plasma concentrations and concentrations in similar tissues on the contralateral side. The direct penetration of lidocaine was predominant for the first 2 h up to a depth of about 1 cm below the applied site. A physiologically based pharmacokinetic model based on apparent tissue-tissue clearances and local blood flow to tissues is presented which adequately describes the concentration-time profiles of lidocaine in underlying tissues after dermal application. The apparent tissue-tissue clearances were estimated by nonlinear regression assuming first-order diffusional mass transfer of lidocaine between the various tissue compartments below the applied site in anesthetized rats. Tissue levels of lidocaine were estimated using simulations from the model with and without direct penetration and tissue blood supply. Dermal microcirculation is not a perfect sink for lidocaine.

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

1. In vitro studies using tissue slices or tissue homogenates of liver, skeletal muscle, fat skin and blood were conducted to determine whether the uptake of procainamide, lignocaine and pethidine into the hindquarters of sheep was due to distribution or metabolism. Both homogenates and slice preparations of liver showed significant metabolism or uptake, confirming the viability of the preparations. 2. None of the drugs was metabolized in blood and there was minimal uptake of the drugs into the skin. 3. There was metabolism of pethidine in skeletal muscle and substantial uptake of pethidine into fat, indicating that the rapid rate of uptake and prolonged elution of pethidine in the hindquarters was due to both distribution and metabolism. 4. No metabolism of lignocaine in muscle was found, but there was substantial uptake into fat, indicating that the rapid rate of uptake and prolonged elution of lignocaine in the hindquarters was due to its distribution into fat. 5. There was negligible uptake of procainamide into either muscle or fat, presumably due to its relatively low lipophilicity.