Physiologically based pharmacokinetic model for digoxin distribution and elimination in the rat

The trip of a drug inside the body: From a lipid-based nanocarrier to a target cell

To date, enormous investigations have been conducted to enhance medicines' target-oriented delivery to improve their therapeutic index. In this regard, lipid-based carrier system might have been regarded as prime delivery systems that are very close to the naturally cell-derived vesicles used for biomolecular communication among cells from occasionally remote tissues. Upon examination of the literature, we found a chasm between groups of investigations in drug pharmaceutics and thought that maybe holistic research could provide better information with respect to drug delivery inside the body, especially when they are going to be injected directly into the bloodstream for systemic distribution. While a collection of older research in most cases dealt with the determination of drug partition coefficient between the aqueous and cell membrane compartments, the link has been overlooked in newer investigations that were mostly focused on drug formulation optimization and their association with particle biodistribution. This gap in the literature motivated us to present the current opinion paper, in which drug physicochemical properties like drug lipophilicity/hydrophilicity is considered as an important element in designing drug-carrying liposomes or micelles. How a hypothetical high throughput cell-embedded chromatographic technique might help to investigate a nanocarrier tissue distribution and to design 'multi-epitope grafted lipid-based drug carrier systems' are discussed. Whenever we would need support for our opinions, we have provided analogy from hydrophobic biomolecules like cholesterol, steroid hormones, and sex hormones and encouraged readers to consider our principle hypothesis: If these molecules could reach their targets far away from the site of production, then a large list of hydrophobic drugs could be delivered to their targets using the same principles.

Physiologically based pharmacokinetic modeling of tea catechin mixture in rats and humans

Although green tea (Camellia sinensis) (GT) contains a large number of polyphenolic compounds with anti-oxidative and anti-proliferative activities, little is known of the pharmacokinetics and tissue dose of tea catechins (TCs) as a chemical mixture in humans. The objectives of this study were to develop and validate a physiologically based pharmacokinetic (PBPK) model of tea catechin mixture (TCM) in rats and humans, and to predict an integrated or total concentration of TCM in the plasma of humans after consuming GT or Polyphenon E (PE). To this end, a PBPK model of epigallocatechin gallate (EGCg) consisting of 13 first-order, blood flow-limited tissue compartments was first developed in rats. The rat model was scaled up to humans by replacing its physiological parameters, pharmacokinetic parameters and tissue/blood partition coefficients (PCs) with human-specific values. Both rat and human EGCg models were then extrapolated to other TCs by substituting its physicochemical parameters, pharmacokinetic parameters, and PCs with catechin-specific values. Finally, a PBPK model of TCM was constructed by linking three rat (or human) tea catechin models together without including a description for pharmacokinetic interaction between the TCs. The mixture PBPK model accurately predicted the pharmacokinetic behaviors of three individual TCs in the plasma of rats and humans after GT or PE consumption. Model-predicted total TCM concentration in the plasma was linearly related to the dose consumed by humans. The mixture PBPK model is able to translate an external dose of TCM into internal target tissue doses for future safety assessment and dose-response analysis studies in humans. The modeling framework as described in this paper is also applicable to the bioactive chemical in other plant-based health products.

Physiologically-based Simulation Modelling for the Reduction of Animal Use in the Discovery of Novel Pharmaceuticals

The global pharmaceutical industry is estimated to use close to 20 million animals annually, in in vivo studies which apply the results of fundamental biomedical research to the discovery and development of novel pharmaceuticals, or to the application of existing pharmaceuticals to novel therapeutic indications. These applications of in vivo experimentation include: a) the use of animals as disease models against which the efficacy of therapeutics can be tested; b) the study of the toxicity of those therapeutics, before they are administered to humans for the first time; and c) the study of their pharmacokinetics —i.e. their distribution throughout, and elimination from, the body. In vivo pharmacokinetic (PK) studies are estimated to use several hundred thousand animals annually. The success of pharmaceutical research currently relies heavily on the ability to extrapolate from data obtained in such in vivo studies to predict therapeutic behaviour in humans. Physiologically-based modelling has the potential to reduce the number of in vivo animal studies that are performed by the pharmaceutical industry. In particular, the technique of physiologically-based pharmacokinetic (PBPK) modelling is sufficiently developed to serve as a replacement for many in vivo PK studies in animals during drug discovery. Extension of the technique to incorporate the prediction of in vivo therapeutic effects and/or toxicity is less well-developed, but has potential in the longer-term to effect a significant reduction in animal use, and also to lead to improvements in drug discovery via the increased rationalisation of lead optimisation.

Influence of method of systemic administration of adenovirus on virus-mediated toxicity: focus on mortality, virus distribution, and drug metabolism

INTRODUCTION

Doses of 2 x 10(12) virus particles/kilogram (vp/kg) and higher of recombinant human adenovirus serotype 5 (HAdV-5) given via the tail vein induce significant toxicity and mortality in the rat. This was not observed when doses of 5.7 x 10(12) vp/kg were given through a surgically implanted jugular catheter. Here we assess how the manner by which HAdV-5 is introduced into the systemic circulation affects biodistribution, transgene expression, toxicity and mortality 0.25, 1, and 4 days after treatment in the rat. Animals were given 5.7 x 10(12) vp/kg of HAdV-5 expressing beta-galactosidase or saline through a jugular catheter or by direct tail vein injection.

RESULTS

All animals survived after jugular vein dosing. Tail vein injection of HAdV-5 increased the mortality rate to 42% (p< or =0.01). All deaths occurred within 4 h. Animals dosed through the jugular vein had significantly higher levels of transgene expression in the liver and spleen and significantly more viral genomes in these tissues and kidney and lung within the first 24 h of viral infection compared to those dosed by tail vein injection (p< or =0.01). There was no significant difference between the groups thereafter. Samples from animals that died contained even higher levels of viral genomes and serum transaminases were elevated on average by a factor of 4 at the time of death. There was no significant difference between the two dosing methods with respect to changes in hepatic cytochrome P450 expression and activity throughout the study.

CONCLUSION

These findings suggest that the method of systemic administration should be carefully considered when assessing toxicity data and other parameters at early time points after virus administration in the rat and possibly other animal models.

Physiologically based pharmacokinetic modelling 2: Predicting the tissue distribution of acids, very weak bases, neutrals and zwitterions

A key component of whole body physiologically based pharmacokinetic (WBPBPK) models is the tissue-to-plasma water partition coefficients (Kpu's). The predictability of Kpu values using mechanistically derived equations has been investigated for 7 very weak bases, 20 acids, 4 neutral drugs and 8 zwitterions in rat adipose, bone, brain, gut, heart, kidney, liver, lung, muscle, pancreas, skin, spleen and thymus. These equations incorporate expressions for dissolution in tissue water and, partitioning into neutral lipids and neutral phospholipids. Additionally, associations with acidic phospholipids were incorporated for zwitterions with a highly basic functionality, or extracellular proteins for the other compound classes. The affinity for these cellular constituents was determined from blood cell data or plasma protein binding, respectively. These equations assume drugs are passively distributed and that processes are nonsaturating. Resultant Kpu predictions were more accurate when compared to published equations, with 84% as opposed to 61% of the predicted values agreeing with experimental values to within a factor of 3. This improvement was largely due to the incorporation of distribution processes related to drug ionisation, an issue that is not addressed in earlier equations. Such advancements in parameter prediction will assist WBPBPK modelling, where time, cost and labour requirements greatly deter its application.

Route-dependent metabolism of morphine in the vascularly perfused rat small intestine preparation

PURPOSE

1. To compare the disposition of tracer morphine ([3H]M) following systemic and intraduodenal administration in the recirculating, rat small intestine preparation in absence or presence of verapamil (V), an inhibitor of P-glycoprotein. 2. To develop a physiological model to explain the observations.

METHODS

A bolus dose of [3H]M was added to the reservoir or injected into the duodenum of the rat small intestine preparation. V (200 microM in reservoir) was either absent (control studies) or present. Intestinal microsomal, incubation studies were performed to evaluate the effect of V on morphine glucuronidation.

RESULTS

After systemic administration, [3H]M was not metabolized but was exsorbed into lumen. By contrast, both [3H]M and the 3beta-glucuronide metabolite, [3H]M3G, appeared in reservoir and lumen after intraduodenal administration. A physiologically-based model that encompassed absorption, metabolism and secretion was able to describe the route-dependent glucuronidation of M. The presence of V resulted in diminished levels of M3G in perfusate and lumen and mirrored the observation of decreased glucuronidation in microsomal incubations. Verapamil appeared to be an inhibitor of glucuronidation and not secretion of M.

CONCLUSIONS

M was secreted and absorbed by the rat small intestine. Route-dependent glucuronidation of M was explained by physiological modeling when M was poorly partitioned in intestinal tissue, with a low influx clearance from blood and a even poorer efflux clearance from tissue. The poor efflux rendered a much greater metabolism of M that was initially absorbed from the lumen. V increased the extent of M absorption through inhibition of M glucuronidation.

A priori prediction of tissue:plasma partition coefficients of drugs to facilitate the use of physiologically-based pharmacokinetic models in drug discovery

The tissue:plasma (P(t:p)) partition coefficients (PCs) are important drug-specific input parameters in physiologically based pharmacokinetic (PBPK) models used to estimate the disposition of drugs in biota. Until now the use of PBPK models in early stages of the drug discovery process was not possible, since the estimation of P(t:p) of new drug candidates by using conventional in vitro and/or in vivo methods is too time and cost intensive. The objectives of the study were (i) to develop and validate two mechanistic equations for predicting a priori the rabbit, rat and mouse P(t:p) of non-adipose and non-excretory tissues (bone, brain, heart, intestine, lung, muscle, skin, spleen) for 65 structurally unrelated drugs and (ii) to evaluate the adequacy of using P(t:p) of muscle as predictors for P(t:p) of other tissues. The first equation predicts P(t:p) at steady state, assuming a homogenous distribution and passive diffusion of drugs in tissues, from a ratio of solubility and macromolecular binding between tissues and plasma. The ratio of solubility was estimated from log vegetable oil:water PCs (K(vo:w)) of drugs and lipid and water levels in tissues and plasma, whereas the ratio of macromolecular binding for drugs was estimated from tissue interstitial fluid-to-plasma concentration ratios of albumin, globulins and lipoproteins. The second equation predicts P(t:p) of drugs residing predominantly in the interstitial space of tissues. Therefore, the fractional volume content of interstitial space in each tissue replaced drug solubilities in the first equation. Following the development of these equations, regression analyses between P(t:p) of muscle and those of the other tissues were examined. The average ratio of predicted-to-experimental P(t:p) values was 1.26 (SD = 1.40, r = 0.90, n = 269), and 85% of the 269 predicted values were within a factor of three of the corresponding literature values obtained under in vivo and in vitro conditions. For predicted and experimental P(t:p), linear relationships (r > 0.9 in most cases) were observed between muscle and other tissues, suggesting that P(t:p) of muscle is a good predictor for the P(t:p) of other tissues. The two previous equations could explain the mechanistic basis of these linear relationships. The practical aim of this study is a worthwhile goal for pharmacokinetic screening of new drug candidates.

Physiologically based pharmacokinetics of digoxin in mdr1a knockout mice

To determine the contribution of the mdr1a gene product to digoxin pharmacokinetics, we constructed a physiologically based pharmacokinetic model for digoxin in mdr1a (-/-) and mdr1a (+/+) mice. After intravenous administration, total body clearance and tissue-to-plasma concentration ratios for muscle and heart were decreased in mdr1a (-/-) mice as compared with mdr1a (+/+) mice, and in particular, the digoxin concentration in the brain was 68-fold higher than that in mdr1a (+/+) mice at 12 h. On the other hand, mdr1a gene disruption did not change the contributions of renal and bile clearances to total clearance, the plasma protein binding, or the blood-to-plasma partition coefficient. Brain concentration-time profiles in mdr1a (+/+) and mdr1a (-/-) mice showed a different pattern from those in plasma and other tissues, indicating digoxin accumulation in the brain tissue. Because there was no difference in the uptake or release of digoxin by brain tissue slices from the two types of mice, we assumed the brain tissue compartment to consist of two parts (a well-stirred part with influx and efflux clearance and an accumulative part). Simulation with this model gave excellent agreement with observation when active efflux clearance across the blood-brain barrier was assumed to be zero in mdr1a (-/-) mice. The observations in other tissues in both types of mice were also well simulated.

Kinetic modeling of ouabain tissue distribution based on slow and saturable binding to Na,K-ATPase

The significance of the binding to Na,K-ATPase in the tissue distribution of ouabain was previously documented (Harashima et al., Pharm. Res. 9:474-479, 1992). The purpose of this study was to obtain a kinetic model of ouabain tissue distribution. In most tissues, the ouabain concentration continued to rise after the termination of infusion (5 min), with the peak tissue concentration at approximately 20 min. This delay could not be explained by the rapid equilibrium model (RE model), nor could the kinetics of ouabain be explained by an RE model modified for saturable binding. Since ouabain binding to Na,K-ATPase is slow, the association and dissociation processes were incorporated into a model that can accurately fit the observed time courses of ouabain. The obtained binding parameters corresponded well with the observed values in the in vitro binding experiments, except for muscle. These results quantitatively support the role of the slow and saturable binding of ouabain to Na,K-ATPase in its tissue distribution.

Significance of binding to Na,K-ATPase in the tissue distribution of ouabain in guinea pigs

Ouabain binds specifically to Na,K-ATPase on the plasma membrane and therefore serves to measure the tissue concentration of Na,K-ATPase. We examined the role of ouabain binding to Na,K-ATPase in its overall tissue distribution. The tissue-to-plasma concentration ratio (Kp,vivo) was defined in each tissue after intravenous administration of 3H-ouabain in guinea pigs, and specific binding of ouabain to Na,K-ATPase was measured in tissue homogenate to obtain the dissociation constant and binding capacity in each tissue. A predicted tissue-to-plasma concentration ratio (Kp,vitro) was calculated using the obtained binding parameters and the volume of extracellular space in each tissue. The absolute values of Kp,vitro were comparable to those of Kp,vivo, except in brain. Regression analysis showed that the specific binding capacity of Na,K-ATPase in each tissue is the main factor in the tissue variation of Kp,vivo. Therefore, the binding of ouabain to Na,K-ATPase plays a significant role in the tissue distribution of ouabain.

Reevaluation of digoxin-encainide interactions using an animal model

The effects of intravenous encainide on digoxin-induced atrial ectopic tachycardia (AET) were investigated in the rat using 3-channel simultaneous limb-lead electrocardiography. Pentobarbital-anesthetized (35 mg/kg, intraperitoneal) adult male rats were given digoxin subcutaneously, 30 mg/kg. After onset of AET, rats received either saline (0.5 ml/kg) or encainide; 0.25, 0.5, 1.0, or 2.0 mg/kg intravenously in repeated doses at 15-min intervals. At all doses, encainide converted digoxin-induced AET to ventricular arrhythmias, prolonged recovery time, and increased mortality in comparison to saline-treated animals. An additional group of anesthetized rats was not given digoxin. These animals received encainide (2.0 mg/kg, intravenously) in repeated doses at 15-min interval and developed dose-related increase in the P-R interval only. Blood samples were obtained by cardiac puncture from 12 additional anesthetized, digoxin-treated rats 5 min after the fourth intravenous dose of saline (0.5 ml/kg, n = 6) or encainide (1.0 mg/kg, n = 6). Serum was prepared and analyzed by affinity column-mediated immunoassay. Digoxin levels were the same in both groups. These results suggest that encainide may exacerbate digoxin-induced arrhythmias (proarrhythmic effect) in this species. In view of our findings of digoxin-encainide interactions in the rat, we recommend caution if these drugs are coadministered in humans.

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.

Determination of gallium concentration in "blood-free" tissues using a radiolabeled blood marker

Radioiodinated serum albumin has been used as a blood marker to define and quantitate physiological volumes for 12 organs and tissue types. The concentration of gallium-67 in "blood-free" tissues of rats was also determined at various times after intravenous administration. Tissues were divided into two kinetically distinguishable types based on reported nonuniform distribution of the blood marker and the gallium distribution observed in the present study. Gallium distribution into the liver and spleen was observed to be slow, with a discernable accumulation phase followed by monoexponential elimination. In contrast, gallium accumulation into the stomach, small and large intestines, heart, lung, skin/adipose tissue, and muscle was rapid and elimination was monophasic.

A physiologically based description of the inhalation pharmacokinetics of styrene in rats and humans

A physiologically based pharmacokinetic model which describes the behavior of inhaled styrene in rats accurately predicts the behavior of inhaled styrene in humans. The model consists of a series of mass-balance differential equations which quantify the time course of styrene concentration within four tissue groups representing (1) highly perfused organs, (2) moderately perfused tissues such as muscle, (3) slowly perfused fat tissue, and (4) organs with high capacity to metabolize styrene (principally liver). The pulmonary compartment of the model incorporates uptake of styrene controlled by ventilation and perfusion rates and the blood:air partition coefficient. The metabolizing tissue group incorporates saturable Michaelis-Menten metabolism controlled by the biochemical constants Vmax and Km. With a single set of physiological and biochemical constants, the model adequately simulates styrene concentrations in blood and fat of rats exposed to 80, 200, 600, or 1200 ppm styrene (data from previously published studies). The simulated behavior of styrene is particularly sensitive to changes in the constants describing the fat tissue group, and to the maximum metabolic rate described by Vmax. The constants used to simulate the fate of styrene in rats were scaled up to represent humans. Simulated styrene concentrations in blood and exhaled air of humans are in good agreement with previously published data. Model simulations show that styrene metabolism is saturated at inhaled concentrations above approximately 200 ppm in mice, rats, and humans. At inhaled concentrations below 200 ppm, the ratio of styrene concentration in blood to inhaled air is controlled by perfusion limited metabolism. At inhaled concentrations above 200 ppm, this ratio is controlled by the blood:air partition coefficient and is not linearly related to the ratio attained at lower (nonsaturating) exposure concentrations. These results show that physiologically based pharmacokinetic models provide a rational basis with which (1) to explain the relationship between blood concentration and air concentration of an inhaled chemical, and (2) to extrapolate this relationship from experimental animals to humans.

Effect of quinidine on digoxin distribution and elimination in guinea pigs

The effect of quinidine on the distribution and elimination of digoxin was examined by comparing the change in the steady-state volume of distribution (Vdss), determined both from in vivo plasma elimination and tissue distribution and in vitro serum binding studies, with that in the total body clearance (CLtot) determined from biliary, renal, and metabolic clearances in guinea pigs. The plasma disappearance of digoxin after a 250-micrograms/kg iv dose followed a triexponential decline in both the control and quinidine-treated guinea pigs. In the quinidine-treated guinea pigs, the pharmacokinetic parameters Vdss and CLtot significantly decreased to approximately half of that for the control guinea pigs. The tissue-to-plasma partition coefficients (Kp) of all tissues studied, i.e. liver, heart, muscle, and brain, at 6 hr after bolus injection of digoxin decreased in the presence of quinidine. The serum free fraction and the plasma-to-blood concentration ratio of digoxin in the therapeutic range did not show a significant alteration in the presence of quinidine. This suggested that the decrease of Kp is due mainly to the inhibition of tissue distribution of digoxin by quinidine. The biliary clearance (CLB) and renal clearance (CLR) also significantly decreased in the presence of quinidine. It was concluded that quinidine caused a inhibition of digoxin in the tissue binding or uptake, which significantly decreased the Kp values of digoxin; this result may explain the significant decrease of Vdss. Moreover quinidine may be the cause of a reduction of biliary, renal, and metabolic clearances, which significantly decrease the CLtot of digoxin.

Variability in the human drug response

Variations in response to drugs may be pharmacodynamic, implying inter-individual differences in the response of receptors in equal concentrations of drug, or pharmacokinetic, implying that individuals receiving the same dose of drug will have different concentrations of drug in their body fluids. Either type of variation can be inherited or acquired. Variations in receptor sensitivity do occur but few instances, inherited or acquired, have well documented clinical relevance. If the dose response relationship for the drug in question is not steep, or if the therapeutic index is low, drug concentration in the region of the receptor will not be critical and causes of kinetic variation are unlikely to be clinically significant. However, it is the many causes of kinetic variation which are best described. These include effects due to drug formulation and changes in the absorption, distribution, metabolism and excretion of drugs. If a consideration of dynamics suggests that drug concentration will determine therapeutic efficacy, analysis and prediction of variability due to these factors is desirable. Prediction requires an accurate description of the system but commonly used pharmacokinetic models may fail when prediction is a goal. The variables, volume of distribution (Vd) and rate constant of elimination (Ke) are hybrid in that they arise from the interaction of patient and drug characteristics. Important events including macromolecular binding and altered blood flow may not be represented. More data is required to determine the clinical significance of pharmacodynamic variation but better analytical tools are required to deal with kinetic variation when this is important. Specifically, pharmacokinetic models should represent physiological variables and levels of unbound drug in body fluids should receive greater emphasis.

Physiological pharmacokinetics of ethoxybenzamide based on biochemical data obtained in vitro as well as on physiological data

Ethoxybenzamide (EB) concentrations in plasma and various tissues were simulated using a physiological pharmacokinetic model. The biochemical parameters, such as plasma and tissue binding constants and Michaelis-Menten constants for EB deethylation, which were needed for these simulations, were, however, obtained from in vitro data. The simulations predicted well the observed data in plasma and various tissues of the rat. Furthermore, animal scale-up predicted reasonably well the concentrations of EB in plasma and various tissues of the rabbit from data gathered in rats.

In vitro and in vivo evaluation of the tissue-to-blood partition coefficient for physiological pharmacokinetic models

An important parameter used in physiologically based pharmacokinetic models is the partition coefficient (Kp), which is defined as the ratio of tissue drug concentration to the concentration of drug in the emergent venous blood of the tissue. Since Kp is governed by reversible binding to protein and other constituents in blood and tissue, an attempt was made here to estimate the Kp values for a model drug ethoxybenzamide (EB) by means of in vitro binding studies and to compare these Kp values to those obtained from in vivo kinetic parameters observed following the administration of EB by two different routes, i.e., i.v. bolus injection and constant rate infusion. The Kp values obtained by using these three different methods were in reasonably good agreement suggesting that binding data obtained in vitro can successfully be used to estimate in vivo distribution.

Dynamic simulation of pharmacokinetic systems using the electrical circuit analysis program SPICE2

The electrical circuit simulation program SPICE2 is used to perform computer simulations of linear and non-linear pharmacokinetic systems. This is achieved by applying novel network thermodynamic principles which make use of the analogy between the conservation laws of chemical reactions and mass transport and Kirchoff's laws of current and voltage balance for electrical circuits. A simple description of program input for general pharmacokinetic simulation as well as simulation of complex pharmacokinetic and physiologic phenomena such as single and multiple divided daily dosing, Michaelis--Menten kinetics, gastric emptying cycle, drug resorption and linear and non-linear drug protein binding is provided. Drug concentrations or amounts in different compartments are graphically obtained or tabulated as time functions. The economy of time and effort afforded by this program is illustrated by simulating the metabolism and accumulation kinetics of salicylic acid on single and repeated divided dosing. The advantages of SPICE2 over other available simulation packages and its educational value as a teaching and research tool are discussed.

Physiologically based pharmacokinetics of drug-drug interaction: a study of tolbutamide-sulfonamide interaction in rats

A blood flow rate-limited pharmacokinetic model was developed to study the effect of sulfonamide on the plasma elimination and tissue distribution of 14C-tolbutamide (TB) in rats. The sulfonamides (SA) used were sulfaphenazole (SP), sulfadimethoxine (SDM), and sulfamethoxazole (SMZ). The tissue-to-plasma partition coefficients (Kp) of all tissues studied, i.e., lung, liver, heart, kidney, spleen, G.I. tract, pancreas, brain, muscle, adipose tissue, and skin, increased in the presence of SA, but except for brain, liver, and spleen, the tissue-to-plasma unbound concentration ratio (Kp,f) of other tissues did not show a significant alteration. This suggested that the tissue binding of TB is not affected by SA and that the increase of Kp is due mainly to the displacement of plasma protein-bound TB by SA. The concentrations of TB in several tissues and plasma were predicted by a physiologically based pharmacokinetic model using in vitro plasma binding and metabolic parameters, the plasma-to-blood concentration ratio and the tissue-to-plasma unbound concentration ratios having been determined from both the tissue and plasma concentrations of TB at the beta-phase after intravenous administration of TB and the plasma free fraction. The predicted concentration curves of TB in each tissue and in plasma showed good agreement with the observed values except for the brain, for which the predicted concentrations were lower than the observed values in the early time period. In the SP- and SDM-treated rats, the predicted free concentration of TB in the target organ, the pancreas, at 6 h was six times higher than that of the control rats. From these findings, it is suggested that physiologically based pharmacokinetic analysis could be generally useful to predict approximate plasma and tissue concentrations of a drug in the presence of drug-drug interaction.

Presystemic elimination of drugs: theoretical considerations for quantifying the relative contribution of gut and liver

From a consideration of the basic processes involved in drug elimination, the fraction of drug cleared by the gut and by the liver were described as functions of availability and hepatic clearance. For a drug given orally, a plot of the fraction of drug cleared by the gut or liver against alpha, a proportionality constant relating gut elimination following intravenous administration to that following oral administration, allowed an estimate of the possible contribution of gut and liver to presystemic elimination. This method was dependent only on the measurement of peripheral blood drug concentrations and urine levels. Application of the theory to published data for several drugs known to have a reduced availability after oral administration was used to illustrate the procedure.

Determination of tissue to blood partition coefficients in physiologically-based pharmacokinetic studies

The partition coefficient between tissue and blood used in physiologically-based pharmacokinetic modeling analysis was investigated using the concept of clearance. New equations were derived and compared with previously reported equations in constant intravenous infusion and bolus injection methods. The importance of differentiating arterial from venous blood is discussed.

Pharmacokinetic evidence for the occurrence of extrahepatic conjugative metabolism of p-nitrophenol in rats

p-Nitrophenol (PNP), as a model compound for the study of conjugative metabolism, was administered intravenously to rats. PNP and its conjugated metabolites, i.e. PNP-glucuronide (PNP-Glu) and PNP-sulfate (PNP-Sul), were determined in body fluids by reversed-phase high-performance liquid chromatography using ion-pair systems. Linear pharmacokinetics was applicable in the dose range of 1.6 to 8 mg/kg. The metabolic clearance which was obtained from the area under the PNP blood concentration curve (AUCiv) and from the excretion ratio of the total conjugates as PNP-Glu and PNP-Sul was so close to the hepatic blood flow that the PNP conjugation reactions seemed to be limited by the hepatic blood flow, that is the hepatic extraction ratio (EH) was expected to be 1. However, AUCpv, following portal vein administration of PNP (4 mg/kg), was not zero but was significantly different from AUCiv after the same dosing (P less than 0.05). Consequently, comparison between the AUC values from both dosing routes and the excretion ratio of PNP-Glu and PNP-Sul gave and EH of 0.43. Such a difference in EH obtained by the two methods suggested a contribution by extrahepatic conjugative metabolism. It was shown that the intrinsic hepatic clearance obtained, assuming exclusively hepatic conjugative metabolism, was certainly overestimated. Furthermore, the results of the conjugation reaction in tissue homogenates suggested a contribution by extrahepatic glucuronidation.

Pharmacokinetics of 2-butanol and its metabolites in the rat

A pharmacokinetic model is presented to describe the biotransformation of 2-butanol (2-OL) and its metabolites (2-butanone, 3-hydroxy-2-butanone, and 2, 3-butanediol) using in vivo experimental blood concentration. A flow limited model is developed to simulate 2-OL, 2-butanone (2-ONE), 3-hydroxy-2-butanone (3H-2B), and 2,3-butanediol (2,3-RD) blood concentrations in rats after oral administration of 2-OL. Assuming the only important site of 2-OL biotransformation is the liver, the tissues included are the liver and a volume of distribution, essentially body water in the case of 2-OL and its metabolites. A distribution coefficient is found to be necessary to describe the low concentration of 3H-2B in blood after administration of 2-OL. The need for this coefficient may be due to partitioning, binding, or altered transport rates from the liver. Inhibition of 2-ONE metabolism to 3H-2B by 2-OL has been included to explain a time delay in the appearance of 3H-2B after administration of 2-OL. Subsequent experimental verification confirms the mixed function oxidase inhibitory properties of 2-OL. The model is able to simulate blood concentrations and elimination of all four compounds after the oral administration of 2-OL. Additionally, the model also simulates the results obtained after i.v. administration of 3H-3B and 2,3-BD.

Saturable metabolism and its relationship to toxicity

Metabolism plays a central role in regulating the toxicity of a variety of chemicals. Relatively innocuous substances can be converted to highly toxic metabolites. Conversely, toxic substances can be biotransformed to less harmful metabolites or be excreted, thus limiting their duration of biological action. Virtually all metabolism and many excretory processes utilize specific binding proteins, i.e., enzymes and carrier proteins. These metabolic and carrier-mediated excretory clearance pathways are capacity-limited, becoming saturated at sufficiently high substrate concentrations. Saturable metabolic clearance processes lead to dose-dependent pharmacokinetics for many chemicals. When dose-dependent pharmacokinetics prevail, internally significant parameters, such as area under the curve for concentration of toxicant at active sites and the amount of metabolite formed during inhalation exposure, are not linearly related to externally significant parameters such as administered dose or inspired concentration. Dose-response curves should relate observed effects to some internally significant parameter. Toxic response should often be indexed to area under the curve relationships or total amount metabolized, instead of dose or inspired concentration. The former parameters are complexly related to the latter. The nature of the relationship depends on the kinetic constants for metabolic and excretory clearance. Pharmacokinetic analyses of dose-dependent clearance mechanisms provide an understanding of how one transforms externally significant parameters to internally significant parameters under various exposure conditions. Consideration of metabolic clearance at the organ level illuminates the importance of physiological factors, showing unequivocally that blood flow may be rate-limiting for metabolism under many exposure conditions. Recognition of the potential for this behavior is essential to the proper design and evaluation of certain toxicological experimentation. Development of comprehensive pharmacokinetic descriptions of the influence of saturable clearance on delivery of active chemical to target sites augurs well for improving both intraspecies and interspecies extrapolation of toxicity data. This is a critical area of contemporary toxicology. Dose selection for chronic studies could also be improved by knowledge of the dose-dependence of pharmacokinetic parameters in proposed test species. The field of toxicology reviewed here represents an interface between pharmacokinetic research and studies on basic mechanisms of toxic action. It entails utilization of quantitative concepts to better understand the physiological and biochemical controls which regulate the expression of the toxicity of various chemicals. Much work remains to be accomplished in this exciting area of toxicological research. Some of the predictions of the pharmacokinetic analyses are still tentative and require more definitive experimentation...

'MacDope': a simulation of drug disposition in the human body: applications in clinical pharmacokinetics

1 We have described a novel approach to absorption, distribution, metabolism and elimination of drugs in which the patient is described using 23 Patient Factors and drugs by up to 50 Drug Factors. Kinetic behavior of a drug results from the interaction of patient and drug factors according to equations describing an eight compartment model. In this model non-linear processes (protein binding, hepatic drug metabolism and renal tubular transport) are handled by derivations of the law of mass action which have been generalised to permit the consequences of multiple drugs interacting at single macromolecular sites to be correctly calculated. 2 The mathematical description of this model is provided in a companion paper and solution of the equations is only possible using a digital computer. The computer programme is provided with an interactional format which makes operation independent of mathematical skills. Patients are defined by age, sex, height and weight with, or without, organ dysfunction; the programme then generates appropriate factors. Drug enters the system when a prescription is type on the keyboard and required Drug Factors are then retrieved from the disc flies. Drug concentrations in plasma or body fluids are given as simple graphs as a function of time, or in tabular form. 3 Any of the Patient or Drug Factors may be altered before, or during a run and up to three drugs may be simulated at one time thus permitting certain kinetic interactions to be examined. The scope of the simulator is illustrated using aspirin: pH dependent gastric absorption, first-order conversion of aspirin to salicylate, partly first order and partly saturable hepatic metabolism of salicylate, and the complex renal handling of this drug are all represented. Interaction of phenytoin with salicylate has been examined quantitatively to suggest limited clinical relevance for the observed displacement of phenytoin from serum albumin. The use of the simulator in a short course of pharmacokinetics is briefly described.

Intestinal secretion of digoxin in the rat. Augmentation by feeding activated charcoal

Secretion of tritium-labeled digoxin was studied in rats using noneverted gut sacs, in ligated and perfused intestinal preparations in bile duct ligated rats, and by measuring 4 day fecal excretion of total radioactivity in bile duct ligated rats. Serosal to mucosal transfer was proportional to substrate concentration in vitro. In ligated intestinal loops radioactivity was concentrated in the lumen relative to serum. In perfused intestinal preparations the fraction of dose increased with time and was similar over a 100-fold range of doses. Bile duct ligated rats excreted 13.4 +/- 5.8 (S.D.) % of parenterally administered label in 4 day stool collections. Bile duct ligated rats treated with p.o. activated charcoal excreted significantly more radioactivity (33.4 +/- 7.9%). The results suggest that net nonbiliary intestinal secretion of digoxin and metabolites can be augmented by intraluminal binding. A role for this phenomenon in accounting for some effects of diseases and drug interactions is suggested.

Propranolol in conscious spontaneously hypertensive rats. II. Disposition after subcutaneous and intracerebroventricular administration

The disposition of dl-propranolol was studied in spontaneously hypertensive rats (SHR), both after subcutaneous (s.c.) and intracerebroventricular (i.c.v.) injection of 1 mg/kg. 1. Upon s.c. injection propranolol appeared rapidly in plasma. A maximum concentration of 374 +/- 33 ng/ml (N = 10) was reached 5 min after injection. After a distribution phase with a half-life of t 1/2 alpha = 17 min propranolol was eliminated with a t 1/2 beta = 59 min. 2. Both propranolol and its metabolites were taken up rapidly into all tissues studied. Highest concentrations (10.4 +/- 1.5 micrograms/g, N = 5) were found in lungs 30 min after injection. 3. Neither propranolol nor its metabolites accumulated in any of the tissues examined. 4. Upon i.c.v. injection of propranolol, a maximal concentration of 573 +/- 47 ng/ml (N = 3) was reached in plasma already 2 min after injection. In this case t 1/2 alpha was 13 min and t 1/2 beta was 80 min. 5. Dialysis experiments indicated that propranolol is bound to plasma proteins for 92% in the concentration range of 20--100 ng/ml. With increasing concentrations binding diminishes progressively. At the highest concentration tested (345 ng/ml) only 76% was bound. It is concluded that s.c. and i.c.v. injection of an identical dose of propranolol gives a similar plasma concentration-time profile. Moreover, it is suggested that the pharmacokinetic behaviour of propranolol in SHR does not explain the delayed antihypertensive effect of this drug.

A pharmacokinetic model to differentiate preabsorptive, gut epithelial, and hepatic first-pass metabolism

A combined perfusion/compartmental pharmacokinetic model has been developed to describe the time course of drugs that are subject to preabsorptive, intestinal epithelial, and hepatic first-pass metabolism. Equations are derived to estimate the fraction of the administered dose which is metabolized at each of the three sites and to establish the limits of the true absorption rate constant. The model is tested using literature data for phenacetin.

Estimation of tissue-to-plasma partition coefficients used in physiological pharmacokinetic models

An important parameter in the development of pharmacokinetic models is the ratio of tissue drug concentration to the concentration of the drug in the arterial plasma or the effluent plasma. The relationship between these two tissue/plasma ratios is derived analytically for different routes of drug administration. The two are equal only in compartments with no elimination when the drug is infused at constant rate. For other routes of administration, the two ratios are identical in all compartments only when there is no elimination process. The tissue/plasma concentration ratios for infusion equilibrium are not equal to the corresponding values for the postdistribution phase after an intravenous bolus injection. When the plasma concentration for infusion and injection are the same, more drug will appear in the lung during infusion steady state than during the postdistribution equilibrium. The reverse is true for the other organs. The importance of properly defining the tissue/plasma ratio and its implication for pharmacokinetic modeling are discussed. The results may have important therapeutic implications for the availability of drugs using different routes of administration.

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.

Physiologically based pharmacokinetic model for digoxin disposition in dogs and its preliminary application to humans

A physiologically based pharmacokinetic model for digoxin disposition developed in the rat was modified to account for the interspecies differences in tissue-to-plasma digoxin concentration ratios and applied to the dog. The model provided a quantitative assessment of the time course of digoxin concentrations in dog plasma, various tissues, and urine. It also predicted the effect of renal failure on digoxin pharmacokinetics in the dog. An attempt to scale the dog model to humans by simply considering differences in organ volumes, organ flow rates, and digoxin clearances was partially successful. Good predictions of plasma digoxin concentration and urinary digoxin excretion after a single dose and of steady-state plasma, heart, and skeletal muscle digoxin concentrations were obtained. However, the model predicted considerably higher kidney digoxin concentrations than are actually found. Although the model adequately characterized the time course of digoxin concentrations in patients with moderate renal impairment, it provided a relatively poor fit to that observed in anuric patients.