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

Drug structure-transport relationships

Malcolm Rowland has greatly facilitated an understanding of drug structure–pharmacokinetic relationships using a physiological perspective. His view points, covering a wide range of activities, have impacted on my own work and on my appreciation and understanding of our science. This overview summarises some of our parallel activities, beginning with Malcolm’s work on the pH control of amphetamine excretion, his work on the disposition of aspirin and on the application of clearance concepts in describing the disposition of lidocaine. Malcolm also spent a considerable amount of time developing principles that define solute structure and transport/pharmacokinetic relationships using in situ organ studies, which he then extended to involve the whole body. Together, we developed a physiological approach to studying hepatic clearance, introducing the convection–dispersion model in which there was a spread in blood transit times through the liver accompanied by permeation into hepatocytes and removal by metabolism or excretion into the bile. With a range of colleagues, we then further developed the model and applied it to various organs in the body. One of Malcolm’s special interests was in being able to apply this knowledge, together with an understanding of physiological differences in scaling up pharmacokinetics from animals to man. The description of his many other activities, such as the development of clearance concepts, application of pharmacokinetics to the clinical situation and using pharmacokinetics to develop new compounds and delivery systems, has been left to others.

Wash-in methodology and modeling to determine hepatocellular D-glucose transport in the perfused rat liver

Wash-in experiments, which may be useful for the study of the disposition of substrates in the liver, have not been well described. To investigate physiological models for wash-in curves, we performed experiments on the perfused livers of male Wistar rats anesthetized with pentobarbital. Test perfusate contained (14)C-sucrose as the extracellular marker and (3)H-glucose. Liver perfusions were performed with background glucose concentrations of 5.7, 10.7, 51.2, or 108.5 mM. Outflow time-activity curves were analyzed with the use of four models. The V(max) and K(m) for the influx of glucose were 1.1 +/- 0.03 micromol/s/g and 41 +/- 3 mM with the Crone-Renkin early extraction model; 1.4 +/- 0.04 micromol/s/g and 36 +/- 3 mM with dispersion model analysis; 1.8 +/- 0.1 micromol/s/g and 25 +/- 4 mM with the Goresky distributed model to fit differentiated wash-in curves; and 2.7 +/- 0.6 micromol/s/g and 28 +/- 21 mM with compartmental analysis. There was reasonable agreement between the four models, and they yielded results similar to those reported for glucose uptake in other preparations.

Carbon monoxide wash-in method to determine gas transfer in vascular beds: application to rat hindlimb

The vascular barrier to gas transfer is an important physiological parameter; however, no readily applicable technique exists to quantitate the process. A simple technique to measure the permeability-surface area (PS) product for gas transfer in vascular beds is proposed using wash in of carbon monoxide (CO) and Crone-Renkin analysis. Wash-in experiments were performed on the perfused hindlimbs of male Wistar rats (n = 15) by using CO as a surrogate marker for oxygen and technetium-99m-labeled albumin as the vascular marker. The use of CO and erythrocyte-free perfusate and the collection of outflow samples into tubes preloaded with erythrocytes obviated the need for an anaerobic collection device or consideration of Hb binding in the analysis. The PS product for CO was determined from the early extraction as 0.013 +/- 0.006 ml. s(-1). g(-1). Compartmental analysis revealed that the fractional recovery of CO was 0.45 +/- 0.14 and the volume of distribution was 2.31 +/- 0.76 ml/g. This technique detected a small measurable barrier to the transfer of CO across the hindlimb vasculature and is potentially applicable to other vascular beds in health and disease.

Kinetics of propranolol uptake in muscle, skin, and fat of the isolated perfused rat hindlimb

The tissue distribution kinetics of a highly bound solute, propranolol, was investigated in a heterogeneous organ, the isolated perfused limb, using the impulse-response technique and destructive sampling. The propranolol concentration in muscle, skin, and fat as well as in outflow perfusate was measured up to 30 min after injection. The resulting data were analysed assuming (1) vascular, muscle, skin and fat compartments as well mixed (compartmental model) and (2) using a distributed-in-space model which accounts for the noninstantaneous intravascular mixing and tissue distribution processes but consists only of a vascular and extravascular phase (two-phase model). The compartmental model adequately described propranolol concentration-time data in the three tissue compartments and the outflow concentration-time curve (except of the early mixing phase). In contrast, the two-phase model better described the outflow concentration-time curve but is limited in accounting only for the distribution kinetics in the dominant tissue, the muscle. The two-phase model well described the time course of propranolol concentration in muscle tissue, with parameter estimates similar to those obtained with the compartmental model. The results suggest, first that the uptake kinetics of propranolol into skin and fat cannot be analysed on the basis of outflow data alone and, second that the assumption of well-mixed compartments is a valid approximation from a practical point of view (as, e.g., in physiological based pharmacokinetic modelling). The steady-state distribution volumes of skin and fat were only 16 and 4%, respectively, of that of muscle tissue (16.7 ml), with higher partition coefficient in fat (6.36) than in skin (2.64) and muscle (2.79).

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.

Pharmacokinetics in organs and the intact body: model validation and reduction

Physiological pharmacokinetic models are based on the structure of the circulatory system reflecting the convective transport of drug by blood flow to the various organs and tissues. Distribution kinetics at the organ level is mostly simplified as transfer between well-stirred compartments neglecting a priori the effects of intravascular dispersion and diffusion within tissue parenchyma. Recirculatory models based on residence time theory overcome these structural limitations since they allow in a most general way the decomposition of the body into its natural subsystems. Because of the unidentifiability of the global multi-organ model on the basis of plasma concentration-time curves the following methods/experimental designs will be discussed which provide quantitative information regarding the subsystems under in vivo conditions: (i) determination of tissue concentration-time profiles (destructive sampling), (ii) estimation of the organ transit time density from input/output profiles and (iii) application of a recirculatory model with reduced complexity to clinical pharmacokinetic data.

A physiologically based pharmacokinetic model incorporating dispersion principles to describe solute distribution in the perfused rat hindlimb preparation

A physiologically based pharmacokinetic model incorporating dispersion principles has been developed to describe outflow data from the isolated perfused rat hindlimb preparation, for the three reference markers 14C-sucrose, 14C-urea, and 3H-water and three 14C-labeled 5-n-alkyl-5-ethyl barbiturates; the methyl, butyl, and nonyl homologues. Also 51Cr-RBC and 125I-albumin were studied. The model consists of four parallel components representing each of the tissues comprising the hindlimb: skeletal muscle, skin, bone, and adipose. Attempts to simplify the model by using the principle of tissue lumping were made by examining the tissue equilibration rate constant k tau for each of respective tissues for each compound. It was found that simplification was only possible in the case of 3H-water data. The model took into account a possible shunting component in the skin tissue and incomplete mass but not volumetric recovery from the system. The dispersion model characterizes the relative spreading of solute on transit through a tissue bed by a dimension-less parameter DN. The estimated dispersion numbers (DN) obtained were in the region of 2.7-4.72, 8.39-15.54, 0.61-2.74, and 6.02-14.0 for skeletal muscle, skin, bone, and adipose, respectively, and were independent of the compound studied. These values are much larger than the range reported in the literature for hepatic outflow data, DN = 0.2-0.5, and suggest a greater heterogeneity of vascular flow in the different component tissues of the rat hindlimb.