Nonlinear pharmacokinetic model of intravenous anesthesia

Drug pharmacokinetics in the obese population: challenging common assumptions on predictors of obesity-related parameter changes

INTRODUCTION

Obesity is associated with many physiological changes. We review available evidence regarding five commonly accepted assumptions to a priori predict the impact of obesity on drug pharmacokinetics (PK).

AREAS COVERED

The investigated assumptions are: 1) lean body weight is the preferred descriptor of clearance and dose adjustments; 2) volume of distribution increases for lipophilic, but not for hydrophilic drugs; 3) CYP-3A4 activity is suppressed and UGT activity is increased, implying decreased and increased dose requirements for substrates of these enzyme systems, respectively; 4) glomerular filtration rate is enhanced, necessitating higher doses for drugs cleared through glomerular filtration; 5) drug dosing information from obese adults can be extrapolated to obese adolescents.

EXPERT OPINION

Available literature contradicts, or at least limits the generalizability, of all five assumptions. Clinical studies should focus on quantifying the impact of duration and severity of obesity on drug PK in adults and adolescents, and also include oral bioavailability and pharmacodynamics in these studies. Physiologically-based PK approaches can be used to predict PK changes for individual drugs, but can also be used to define in general terms based on patient characteristics and drug properties, when certain assumptions can or cannot be expected to be systematically accurate.

Drug Disposition in Subjects with Obesity: The Research Work of Darrell R. Abernethy

In 1979, the late Dr. Darrell R. Abernethy and colleagues began a series of clinical studies aimed at understanding the pertinent determinants of drug distribution, elimination, and clearance in obesity, and how those variables are interconnected. The studies confirmed that volume of distribution (Vd) and clearance are the principal independent biological variables, which conjointly determine elimination half-life as a dependent variable. For drugs distributed by passive diffusion, their pharmacokinetic Vd - after correcting for plasma protein binding - was increased in obesity, depending in part on the physicochemical lipophilicity of the individual drugs, and the quantitative extent of obesity in overweight individuals. Across all studies, the ratio of mean clearance in obese divided by control groups had an overall median value of 1.21 (range: 0.75 to 3.11), indicating a small and variable effect of obesity on clearance, without clear directionality. Since drug clearance was not clearly related to lipophilicity or degree of obesity, the prolonged half-life of lipophilic drugs in obese patients was largely explained by the increased Vd. Dr. Abernethy further identified delayed attainment of steady-state after initiation of multiple-dose treatment, and delayed washout after termination of dosage, as potential clinical consequences of the extended half-life in obese persons. These consequences for specific drugs have been recently emphasized in contemporary studies of chronic dosage in subjects with obesity. Without data identifying an obesity-related change in clearance for a specific drug, maintenance doses (in milligrams) should be based on ideal weight rather than adjusted upward based on total weight. This article is protected by copyright. All rights reserved.

In vivo relationships between the cerebral pharmacokinetics and pharmacodynamics of thiopentone in sheep after short-term administration

The cerebral kinetics and dynamics of thiopentone after infusions of 250, 500, and 750 mg over 2 min were examined in chronically instrumented sheep (6, 6, and 5 sheep per dose, respectively). The cerebral kinetics were studied by rapid sampling of arterial and dorsal sagittal sinus blood (afferent and efferent blood for the brain, respectively) for 40 min, and could be described by a single flow-limited compartment when arterial concentrations and cerebral blood flow were used as forcing input functions. The half-lives of equilibration between blood and the brain were estimated to be 0.67 (SEM = 0.07), 0.57 (0.03) and 0.74 (0.05) min for the 250-, 500- and 750-mg doses, respectively, showing that the cerebral concentrations of thiopentone rapidly equilibrate with the afferent blood concentration. Simultaneous pharmacodynamic measurements included cerebral blood flow via a Doppler flowmeter on the sagittal sinus, and an index of the depth of anesthesia based on an algesimetry method. Thiopentone transiently reduced cerebral blood flow to 82 (SEM = 3), 80% (7), and 74% (10) of baseline for the 250-, 500-, and 750-mg doses, respectively, and failure to account for drug-induced changes in cerebral blood flow in the model overestimated the apparent volume of the brain by 12% for the 500-mg dose. For the 500-mg dose, the changes in cerebral blood flow could be accounted for by an effect compartment with a half-life of 0.82 min for arterial blood, and 0.00 min for sagittal sinus blood, showing the effluent brain concentrations were in equilibrium with this drug effect. The time course of the depth of anesthesia for the 250-mg dose could be accounted for by an effect compartment with a half-life of 1.33 min for arterial blood, and 0.41 min for sagittal sinus blood. Thus, the rate of equilibration between blood and brain could not account for all of this delay. It is concluded that after short-term administration thiopentone equilibrated rapidly with the brain, and that this is consistent with the observation that the magnitude of its clinically relevant effects closely follow the time course of the arterial blood concentrations.

Extravascular transport in normal and tumor tissues

The transport characteristics of the normal and tumor tissue extravascular space provide the basis for the determination of the optimal dosage and schedule regimes of various pharmacological agents in detection and treatment of cancer. In order for the drug to reach the cellular space where most therapeutic action takes place, several transport steps must first occur: (1) tissue perfusion; (2) permeation across the capillary wall; (3) transport through interstitial space; and (4) transport across the cell membrane. Any of these steps including intracellular events such as metabolism can be the rate-limiting step to uptake of the drug, and these rate-limiting steps may be different in normal and tumor tissues. This review examines these transport limitations, first from an experimental point of view and then from a modeling point of view. Various types of experimental tumor models which have been used in animals to represent human tumors are discussed. Then, mathematical models of extravascular transport are discussed from the prespective of two approaches: compartmental and distributed. Compartmental models lump one or more sections of a tissue or body into a "compartment" to describe the time course of disposition of a substance. These models contain "effective" parameters which represent the entire compartment. Distributed models consider the structural and morphological aspects of the tissue to determine the transport properties of that tissue. These distributed models describe both the temporal and spatial distribution of a substance in tissues. Each of these modeling techniques is described in detail with applications for cancer detection and treatment in mind.

Prediction of the concentrations in rat tissues of the intravenous anaesthetic methohexital from a nonlinear pharmacokinetic model

A nonlinear pharmacokinetic model that describes the tissue distribution of intravenous anaesthetics was evaluated against experimental values for methohexital in the rat. There was excellent agreement between experimental and theoretical values for brain tissue, and good agreement for blood and adipose tissue. Agreement for lean tissue was good if it was assumed that some adipose tissue was present in skeletal muscle. Agreement was poor for all other visceral tissues. The experimental results justify further development of this mathematical model for use in accounting for differences in tissue distribution of anaesthetics, especially under various physiological conditions.

Physiologically based pharmacokinetic models for anticancer drugs

The rationale and history of the development of physiologically based pharmacokinetic models are briefly reviewed in this paper. The methods of model construction and the previous application of this type of model to anticancer drugs are discussed. Future research should be focused on the following areas: (1) interspecies scaling, (2) the effects of disease states on the pharmacokinetics of anticancer drugs, and (3) the applications of pharmocokinetics to the studies of growth behavior of cancer cells. The ultimate goal will be to utilize this basic information to design an optimal dosage regimen and treatment schedule for the safe and effective cancer chemotherapy of each individual patient.