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W F Ebling

Publications and source records attributed to W F Ebling.

At least 37 records · Page 2Linked to original sources
PubMed1994

From piecewise to full physiologic pharmacokinetic modeling: applied to thiopental disposition in the rat.

Physiologically based pharmacokinetic modeling procedures employ anatomical tissue weight, blood flow, and steady tissue/blood partition data, often obtained from different sources, to construct a system of differential equations that predict blood and tissue concentrations. Because the system of equations and the number of variables optimized is considerable, physiologic modeling frequently remains a simulation activity where fits to the data are adjusted by eye rather than with a computer-driven optimization algorithm. We propose a new approach to physiological modeling in which we characterize drug disposition in each tissue separately using constrained numerical deconvolution. This technique takes advantage of the fact that the drug concentration time course, CT(t), in a given tissue can be described as the convolution of an input function with the unit disposition function (UDFT) of the drug in the tissue, (i.e., CT(t) = (Ca(t)QT)*UDFT(t) where Ca(t) is the arterial concentration, Q tau is the tissue blood flow and * is the convolution operator). The obtained tissue until disposition function (UDF) for each tissue describes the theoretical disposition of a unit amount of drug infected into the tissue in the absence of recirculation. From the UDF, a parametric model for the intratissue disposition of each tissue can be postulated. Using as input the product of arterial concentration and blood flow, this submodel is fit separately utilizing standard nonlinear regression programs. In a separate step, the entire body is characterized by reassembly of the individuals submodels. Unlike classical physiologic modeling the fit for a given tissue is not dependent on the estimates obtained for other tissues in the model. Additionally, because this method permits examination of individual UDFs, appropriate submodel selection is driven by relevant information. This paper reports our experience with a piecewise modeling approach for thiopental disposition in the rat.

Algorithms↗
PubMed1994

Comparative physiological pharmacokinetics of fentanyl and alfentanil in rats and humans based on parametric single-tissue models.

The objectives of this investigation were to characterize the disposition of fentanyl and alfentanil in 14 tissues in the rat, and to create physiological pharmacokinetic models for these opioids that would be scalable to man. We first created a parametric submodel for the disposition of either drug in each tissue and then assembled these submodels into whole-body models. The disposition of fentanyl and alfentanil in the heart and brain and of fentanyl in the lungs could be described by perfusion-limited 1-compartment models. The disposition of both opioids in all other examined tissues was characterized by 2- or 3-compartment models. From these models, the extraction ratios of the opioids in the various tissues could be calculated, confirming the generally lower extraction of alfentanil as compared to fentanyl. Assembly of the single-tissue models resulted in a wholebody model for fentanyl that accurately described its disposition in the rat. A similar assembly of the tissue models for alfentanil revealed non-first-order elimination kinetics that were not apparent in the blood concentration data. Michaelis-Menten parameters for the hepatic metabolism of alfentanil were determined by iterative optimization of the entire model. The parametric models were finally scaled to describe the disposition of fentanyl and alfentanil in humans.

Alfentanil↗
PubMed1993

Tissue distribution of fentanyl and alfentanil in the rat cannot be described by a blood flow limited model.

Traditionally, physiological pharmacokinetic models assume that arterial blood flow to tissue is the rate-limiting step in the transfer of drug into tissue parenchyma. When this assumption is made the tissue can be described as a well-stirred single compartment. This study presents the tissue washout concentration curves of the two opioid analgesics fentanyl and alfentanil after simultaneous 1-min iv infusions in the rat and explores the feasibility of characterizing their tissue pharmacokinetics, modeling each of the 12 tissues separately, by means of either a one-compartment model or a unit disposition function. The tissue and blood concentrations of the two opioids were measured by gas-liquid chromatography. The well-stirred one-compartment tissue model could reasonably predict the concentration-time course of fentanyl in the heart, pancreas, testes, muscle, and fat, and of alfentanil in the brain and heart only. In most other tissues, the initial uptake of the opioids was considerably lower than predicted by this model. The unit disposition functions of the opioids in each tissue could be estimated by nonparametric numerical deconvolution, using the arterial concentration times tissue blood flow as the input and measured tissue concentrations as the response function. The observed zero-time intercepts of the unit disposition functions were below the theoretical value of one, and were invariably lower for alfentanil than for fentanyl. These findings can be explained by the existence of diffusion barriers within the tissues and they also indicate that alfentanil is less efficiently extracted by the tissue parenchyma than the more lipophilic compound fentanyl. The individual unit disposition functions obtained for fentanyl and alfentanil in 12 rat tissues provide a starting point for the development of models of intratissue kinetics of these opioids. These submodels can then be assembled into full physiological models of drug disposition.

Adipose Tissue↗
PubMed1992

Plasma concentration clamping in the rat using a computer-controlled infusion pump.

We have developed a computer-controlled infusion pump to achieve rapidly and then maintain stable plasma thiopental concentrations in rats. Initially we derived the parameters of a triexponential pharmacokinetic model for thiopental, administered as a brief infusion to 10 rats, using nonlinear regression and standard pharmacokinetic equations. These parameters were incorporated into the pharmacokinetic model of a computer-controlled infusion pump. In a second group of animals this device was used to maintain three consecutive target thiopental concentrations ranging from 5 to 100 micrograms/ml in a stepwise fashion. Arterial blood gases were kept normal through controlled ventilation when necessary. The plasma thiopental concentrations in this second group of animals were generally higher than the target concentrations. The bias in pump performance (median prediction error) was +25%, and the inaccuracy (median absolute prediction error) was 26%. We fit the parameters of a three-compartment model to the plasma thiopental concentrations observed in the second group of animals. This produced a second set of thiopental pharmacokinetic parameters with the unique characteristic of having been derived from a computer controlled infusion study. These parameters were tested prospectively with a computer-controlled infusion pump in a third group of animals. This second set of thiopental pharmacokinetic parameters performed better, with a median prediction error of 0% and a median absolute prediction error of 15%. This study shows that it is possible to achieve rapidly and maintain steady plasma thiopental concentrations in the rat. Our results suggest that it is feasible to derive robust pharmacokinetic parameters from unusual drug dosing approaches, such as employed by a computer-controlled infusion pump.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Thiopental pharmacodynamics. I. Defining the pseudo-steady-state serum concentration-EEG effect relationship.

To assess depth of anesthesia for intravenous anesthetics using clinical stimuli and observed responses, it is necessary to achieve constant serum concentrations of drug that result in constant biophase or central nervous system concentrations. The goal of this investigation was to use a computer-controlled infusion pump (CCIP) to obtain constant serum thiopental concentrations and use the electroencephalogram (EEG) as a measure of thiopental's central nervous system drug effect. The number of waves per second obtained from aperiodic waveform analysis was used as the EEG measure. A CCIP was used in six male volunteers to attain rapidly and then maintain for 6-min time periods the following pseudo-steady-state constant serum thiopental target concentrations: 10, 20, 30, and 40 micrograms/ml. The median performance error (bias) of the CCIP using 149 measurements of thiopental serum concentrations in six subjects was +5%, and the median absolute performance error (accuracy) was 16%. Following the step change in serum thiopental concentration, the EEG number of waves per second stabilized within 2-3 min and the remained constant until the target serum thiopental concentration was changed. When the constant serum thiopental concentration was plotted against the number of waves per second for each subject, a biphasic serum concentration versus EEG effect relationship was seen. This biphasic concentration:response relationship was characterized with a nonparametric pharmacodynamic model. The awake, baseline EEG was 10.6 waves/s; at peak activation the EEG was 19.1 waves/s and occurred at a serum thiopental concentration of 13.3 micrograms/ml. At a serum thiopental concentration of 31.2 micrograms/ml the EEG had slowed to 10.6 waves/s (back to baseline) and at 41.2 micrograms/ml was 50% below the baseline, awake value. Zero waves per second occurred at serum thiopental concentrations greater than 50 micrograms/ml. Using a CCIP it is possible to establish constant serum thiopental concentration rapidly and characterize the concentration versus EEG drug effect relationship.

Adult↗
PubMed1991

Pharmacodynamic characterization of the electroencephalographic effects of thiopental in rats.

We have developed a chronically instrumented rat model that uses changes in electroencephalographic wave forms to estimate continuously the degree of central nervous system (CNS) depression induced by thiopental. Such changes were subject to aperiodic signal analysis, a technique that breaks down the complex EEG into a series of discreet neurologic "events" which are then quantitated as waves/sec. We thus obtained a continuous measure of CNS drug effect. In addition we continuously recorded central arterial blood pressure and heart rate and monitored ventilatory status using arterial blood gas determinations. We also determined, with frequent arterial blood sampling, the distribution and elimination of thiopental in individual animals. The time lag occurring in the curve representing arterial concentration of thiopental vs. EEG effect suggests that arterial plasma is not kinetically equivalent to the EEG effect site. Application of semiparametric pharmacodynamic modeling techniques enabled us to estimate equilibration rate constant (Keo) for concentrations of thiopental between arterial plasma and the effect site. The half-life for equilibration of thiopental with the EEG (CNS) effect was less than 80 sec. Knowledge of the rate of equilibration permitted characterization of the relationship between the steady state plasma concentrations and CNS effect of thiopental, as measured by activation and slowing of the EEG. At concentrations of thiopental below 5 micrograms/ml, EEG activity was 180% higher than during the baseline awake state. Thiopental produced an activated EEG over more than 20% of the concentration-effect relationship. Further increases in the concentration of thiopental at the site of effect depressed EEG activity progressively until complete suppression of the EEG signal occurred (at which time, the concentration was approximately 80 micrograms/ml). This report describes our model and its application to the assessment of the pharmacodynamics of thiopental as manifested by changes on the EEG.

Animals↗
PubMed1991

A comparative study of the pharmacokinetics of thiopental in the rabbit, sheep and dog.

The central arterial pharmacokinetics of thiopental were studied in six rabbits, six sheep and six dogs after a short infusion at approximately 10 mg/kg min. Thiopental was infused to a defined electro-encephalographic endpoint (EEG burst suppression). The time to reach early burst suppression was longer in the dog (3.9 +/- 0.5 min) compared with the sheep (3.0 +/- 0.6 min) and the rabbit (2.5 +/- 0.5 min). The total dose required to produce the same level of EEG activity was higher in the dog (35.9 +/- 6.8 mg/kg) compared with the sheep (24.3 +/- 5.3 mg/kg) and the rabbit (21.6 +/- 6.8 mg/kg). The plasma concentration-time data for each animal was fitted using non-linear regression to a bi- or tri-exponential function. In all animals, the plasma-time profile was best described as a tri-exponential decay. The initial volume of distribution was similar in all three species (rabbit, 38.6 +/- 10.0 mg/kg; sheep, 44.5 +/- 9.1 ml/kg; dog, 38.1 +/- 18.4 ml/kg). The maximum arterial plasma thiopental concentration achieved at EEG burst suppression was higher in the sheep (221.8 +/- 27.9 micrograms/ml) than the dog (164.7 +/- 29.9 micrograms/ml) or the rabbit (112.3 +/- 15.1 micrograms/ml). Thiopental distribution clearance was slower in the sheep (43.6 +/- 15.1 ml/min/kg) compared with the rabbit (110.5 +/- 18.7 ml/min kg) and the dog (97.2 +/- 47.2 ml/min kg). Elimination half-life was extended in the sheep (251.9 +/- 107.8 min) and dog (182.4 +/- 57.9 min) relative to the rabbit (43.1 +/- 3.4 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Understanding pharmacokinetics and pharmacodynamics through computer stimulation: I. The comparative clinical profiles of fentanyl and alfentanil.

The authors have used computer simulation to examine the time course of the plasma concentration, estimated effect site concentration, and the intensity of the central nervous system (CNS) effect of fentanyl and alfentanil. The simulations were performed over a range of clinically equivalent doses. Simulations of the changes in the processed electroencephalogram (EEG) was used as a reflection of drug induced CNS effect. The simulations reveal that the rate of equilibration between effect site and plasma concentrations can explain differences in the clinical time course of drug effect between these opioids. The onset of fentanyl EEG drug effect is delayed relative to alfentanil and the duration of action is longer. Pharmacokinetic differences do not explain the disparity seen in the time courses of EEG drug effect. Alfentanil and fentanyl have similar plasma disposition curves during the first 90 min. The concentrations at the effect site are, however, quite different. The simulations illustrate how fentanyl's slow blood:brain equilibration can dampen the rate of rise and fall of effect site concentrations. As a mechanism for terminating effect, redistribution of opioid from effect site to other body regions is less relevant for fentanyl compared with that for alfentanil. The evanescent clinical effects of alfentanil can be explained by the rapid blood:brain equilibration. Computer simulation is a useful tool for revealing relevant determinants of the complex relationship between dose and the time course of effect.

Alfentanil↗
PubMed1989

High-performance liquid chromatographic method for determining thiopental concentrations in twelve rat tissues: application to physiologic modeling of disposition of barbiturate.

A sensitive, selective and reproducible high-performance liquid chromatographic assay of thiopental concentrations in twelve rat tissues was developed using thiamylal as the internal standard. Samples were homogenized in phosphate buffer, extracted into pentane and chromatographed on a microparticulate octadecyl reversed-phase column using ultraviolet detection at 290 nm. A simple digestive step with collagenase prior to homogenization facilitated analysis of thiobarbiturate in skin. Thiopental extraction recovery from fat exceeded 90%. Assay sensitivity was greater than 1 microgram/ml for tissue and plasma samples as small as 50 microliters. This assay has been applied to physiologic pharmacokinetic studies. The paper also presents typical concentration-time profiles of thiopental in four tissues taken from 74 rats given 20 mg/kg thiopental.

Adipose Tissue↗
PubMed1989

A semiparametric approach to physiological flow models.

By regarding sampled tissues in a physiological model as linear subsystems, the usual advantages of flow models are preserved while mitigating two of their disadvantages, (i) the need for assumptions regarding intratissue kinetics, and (ii) the need to simultaneously fit data from several tissues. To apply the linear systems approach, both arterial blood and (interesting) tissue drug concentrations must be measured. The body is modeled as having an arterial compartment (A) distributing drug to different linear subsystems (tissues), connected in a specific way by blood flow. The response (CA, with dimensions of concentration) of A is measured. Tissues receive input from A (and optionally from other tissues), and send output to the outside or to other parts of the body. The response (CT, total amount of drug in the tissue (T) divided by the volume of T) from the T-th one, for example, of such tissues is also observed. From linear systems theory, CT can be expressed as the convolution of CA with a disposition function, F(t) (with dimensions 1/time). The function F(t) depends on the (unknown) structure of T, but has certain other constant properties: The integral integral infinity0 F(t) dt is the steady state ratio of CT to CA, and the point F(0) is the clearance rate of drug from A to T divided by the volume of T. A formula for the clearance rate of drug from T to outside T can be derived. To estimate F(t) empirically, and thus mitigate disadvantage (i), we suggest that, first, a nonparametric (or parametric) function be fitted to CA data yielding predicted values, CA, and, second, the convolution integral of CA with F(t) be fitted to CT data using a deconvolution method. By so doing, each tissue's data are analyzed separately, thus mitigating disadvantage (ii). A method for system simulation is also proposed. The results of applying the approach to simulated data and to real thiopental data are reported.

Models, Biological↗
PubMed1986

6 alpha-Methylprednisolone and 6 alpha-methylprednisone plasma protein binding in humans and rabbits.

The binding of 6 alpha-methylprednisolone and 6 alpha-methylprednisone to proteins of rabbit and human plasma was studied in vitro by equilibrium dialysis. Steroid binding was determined using radiolabeled compounds and HPLC analysis methods. Both methods produced equivalent results. Plasma protein binding of 6 alpha-methylprednisolone and 6 alpha-methylprednisone averaged 75-82% and was independent of steroid concentration, suggestive of low affinity, nonspecific protein binding. A positive linear correlation of the log octanol-water partition coefficient with the nonspecific binding affinities of a homologous series of steroids, including 6 alpha-methylprednisolone and 6 alpha-methylprednisone, was demonstrated. This correlation suggests that hydrophobic binding is a major determinant of nonspecific steroid-protein interactions.

Animals↗
PubMed1986

Methylprednisolone versus prednisolone pharmacokinetics in relation to dose in adults.

The disposition and plasma binding of methylprednisolone were examined in seven normal volunteers following the administration of 5, 20 and 40 mg of intravenous methylprednisolone sodium succinate. Methylprednisolone exhibits linear plasma protein binding averaging 77%. The mean plasma methylprednisolone clearance of 337 ml X h-1 X kg-1 was independent of dose. The steroid appears to moderately distribute into tissue spaces with a mean volume of distribution of 1.41 X kg-1. Methylprednisolone disposition parameters were compared with the non-transcortin bound parameters for prednisolone. The prednisolone plasma clearance based on the transcortin free-drug is similar to methylprednisolone total plasma clearance. However, the corrected volume of distribution of prednisolone is only one-half that of methylprednisolone. The disposition rate of these two steroids is thus similar, in spite of their metabolic control by different enzymatic pathways and major influence of saturable transcortin binding on prednisolone elimination.

Adult↗
PubMed1986

The determination of essential clearance, volume, and residence time parameters of recirculating metabolic systems: the reversible metabolism of methylprednisolone and methylprednisone in rabbits.

Methods based on moment analysis are described which permit the calculation of the fundamental parameters of reversible drug/metabolite systems. These parameters include the four essential clearances of reversible and irreversible elimination, the central and steady-state distributional volumes, and the sojourn times or turnover rates of the metabolic pair. Additional parameters unique to interconversion systems are developed which describe the properties of metabolic entrapment ("recycled fraction"), conservation ("exposure enhancement"), and equilibrium resulting from reversible metabolism ("Percent parent drug at steady-state"). Parameters obtained by these methods are compared to those generated by conventional mammillary analysis. The influence of perturbation of essential parameters on the response of mammillary descriptors and the state of the interconversion system are simulated. The interconversion analysis is applied to disposition data for methylprednisolone and methylprednisone in the rabbit. Mammillary methods underestimate the metabolic clearance of these two steroids by 30%, while steroid turnover is underestimated by 100%. The steady-state volumes of distribution of the two steroids are overestimated by 10 and 61%. Additional literature data for disposition of several corticosteroids in various species and disease states are reanalyzed. Examination of cortisol/cortisone disposition in thyroid disorders reveals that mammillary methods detect the overall acceleration of steroid elimination in hyperthyroidism, but fail to reveal a 50% reduction in metabolite backconversion and decreased metabolic cycling. These moment analysis methods should facilitate characterization of the pharmacokinetics of the increasing array of reversible drug/metabolite systems.

Animals↗
PubMed1984

Analysis of cortisol, methylprednisolone, and methylprednisolone hemisuccinate. Absence of effects of troleandomycin on ester hydrolysis.

A sensitive, selective, and reproducible high-performance liquid chromatographic assay for the simultaneous measurement of cortisol and methylprednisolone using dexamethasone as the internal standard is presented. Samples are extracted with methylene chloride, washed with sodium hydroxide and then water, and chromatographed on a microparticle silica gel column with ultraviolet detection at 254 nm. Sensitivity is greater than 10 ng/ml and the intra-day coefficient of variation is less than 5% for both steroids. The use of porcine liver esterase allows the quantitation of the hemisuccinate ester of methylprednisolone. This assay has been applied in pharmacokinetic studies including investigations of troleandomycin--methylprednisolone interactions. A typical plasma concentration--time profile for methylprednisolone and its ester prodrug is presented for one subject before and after receiving troleandomycin therapy. Although methylprednisolone elimination is reduced in the presence of troleandomycin therapy, there is no effect on the pharmacokinetics of methylprednisolone sodium succinate.

Adult↗
PubMed1984

Cimetidine-methylprednisolone-theophylline metabolic interaction.

The pharmacokinetics of theophylline and methylprednisolone were examined before and during cimetidine treatment in an asthmatic woman who required long-term administration of these drugs. Cimetidine reduced theophylline plasma clearance by 30 percent, and measurement of urinary metabolites showed that 3-methylxanthine formation was inhibited more strongly than that of the methylated uric acid metabolites. Assessment of methylprednisolone disposition following oral and intravenous doses revealed no effect of cimetidine on the bioavailability (74 to 81 percent absorption) or plasma clearance (22 to 24 liters per hour) of the steroid. Thus, cimetidine exhibits variable and selective effects on the biotransformation pathways of drugs important in asthma therapy.

Adult↗
PubMed1981

Chlorothiazide absorption from solution and tablet dosage forms in dogs.

The bioavailability of an aqueous solution of chlorothiazide and three commercially available chlorothiazide tablets was assessed in adult mongrel dogs. In two crossover urinary excretion studies, six fasting dogs received single 500-mg doses of chlorothiazide as an aqueous solution, one 500-mg originator tablet on two separate occasions (Tablets A-1 and A-2), two 250-mg originator tablets (Tablet B), or one 500-mg generic tablet (Tablet C). 6-Amino-4-chlorobenzene-1,3-disulfonamide )chloraminophenamide), a pharmacologically active hydrolysis product of chlorothiazide was not detected in any urine sample. Urinary recoveries of chlorothiazide after oral administration, expressed as the mean (range) percent of the dose, was only 22.0 (8.41-33.9), 15.7 (10.2-25.0), 20.7 (7.25-31.0), 18.5 (8.72-33.2), and 21.9% (6.69-41.1%) for the aqueous solution and Tablets A-1, A-2, B, and C, respectively. Considerable interindividual variation and some intraindividual variation were observed. No statistically significant difference in bioavailability existed among the aqueous solution and Tablets A-2 and B, between Tablets A-1 and C, and between Tablets A-1 and A-2.

Animals↗
PubMedDate not supplied

Troleandomycin effects on methylprednisolone and methylprednisone interconversion and disposition in the rabbit.

A study of the effects of troleandomycin (TAO) on the disposition of intravenous methylprednisolone in rabbits was performed in order to develop an animal model to further evaluate the mechanism of TAO/steroid beneficial effects in severe asthma. The plasma concentration-time profiles of methylprednisolone and methylprednisone were determined in the presence and absence of single and multiple dose TAO regimens. Pharmacokinetic analysis revealed a significant decrease in total plasma clearance of methylprednisolone in the presence of multiple dose TAO. Alterations in the disposition of the reversible metabolite, methylprednisone, were also observed. The TAO-methylprednisolone interaction may involve decreasing the degree of interconversion between the steroid and its reversible metabolite. TAO also decreases metabolite turnover more than three-fold. The antibiotic does not cause marked deviation from linear biexponential elimination of methylprednisolone as observed in man. The rabbit may serve as a useful animal model for further studies of the TAO/methylprednisolone interaction.

Animals↗