Integrated equation to evaluate accumulation profiles of drugs eliminated by Michaelis-Menten kinetics

Using an equation for the calculation of plasma profiles of drug based on the zero-order input and Michaelis-Menten kinetic output in a one-compartment open model system, the times required to reach various degrees of several steady-state plasma concentrations of phenytoin are calculated. The effect of the apparent volume of distribution (due to intersubject variation or change in protein and/or tissue binding, etc.) on the time required to reach various fractions of steady-state plasma concentrations is discussed.

Role of P-glycoprotein in the hepatic metabolism of tacrolimus

The main objective was to determine the potential effect of P-glycoprotein (P-gp) modulation on hepatic metabolism of tacrolimus, a P-gp and cytochrome P450(CYP)3A4 substrate, and to investigate various potential factors that may contribute to the interaction between P-gp and CYP. An isolated perfused rat liver system was used to study the hepatic disposition of tacrolimus in the presence of a P-gp inhibitor, GF120918, and a comprehensive pharmacokinetic analysis was conducted. GF120918 significantly decreased mean intrinsic metabolic clearance (by 86 and 41% based on the well-stirred and tube models, respectively) as well as hepatic clearance (from 47.3 to 44.2 ml min(-1)). Potential factors that might contribute to these observations, such as the effects of GF120918 on hepatic metabolism or on distribution of tacrolimus, were investigated and found to be negligible. In conclusion, it was shown that P-gp inhibition by GF120918 reduces hepatic clearances of its substrate drugs although the mechanism is yet to be determined.

Causes and consequences of high osmotic potentials in epiphytic higher plants

Past reports of the water relations of epiphytes, particularly bromeliads, indicate that tissue osmotic potentials in these tropical and subtropical plants are very high (close to zero) and are similar to values for aquatic plants. This is puzzling because several ecophysiological studies have revealed a high degree of drought stress tolerance in some of these epiphytes. The goal of this study was two-fold: (1) to increase the number of epiphytic taxa sampled for tissue osmotic potentials; and (2) to explain the apparent discrepancy in the significance of the tissue water relations and tolerance of drought stress in epiphytes. Tissue osmotic potentials of 30 species of epiphytic ferns, lycophytes, and orchids were measured in a subtropical rain forest in northeastern Taiwan. Nearly all values were less negative than -1.0 MPa, in line with all previous data for epiphytes. It is argued that such high osmotic potentials, indicative of low solute concentrations, are the result of environmental constraints of the epiphytic habitat on productivity of these plants, and that low rates of photosynthesis and transpiration delay the onset of turgor loss in the tissues of epiphytes such that they appear to be very drought-stress tolerant. Maintenance of photosynthetic activity long into drought periods is ascribed to low rates of transpiration and, hence, delayed tissue desiccation, and hydration of the photosynthetic tissue at the expense of water from the water-storage parenchyma.

A comprehensive account on the role of efflux transporters in the gastrointestinal absorption of 13 commonly used substrate drugs in humans

BACKGROUND

The potential absorption-limiting effect of intestinal efflux transporters such as P-glycoprotein (P-gp) has been well recognized, primarily based on results of numerous Caco-2 cell studies showing that flux, permeability, or transport clearance of drugs from the basolateral to the apical (B --> A) compartment is greater than that from the apical to the basolateral (A --> B) compartment. Except for very limited examples such as celiprolol, talinolol, pafenolol and paclitaxel, the potential clinical impact of these transporters on oral absorption of the vast number of commonly prescribed drug substrates in humans has not been closely examined to date.

OBJECTIVE

To evaluate whether these efflux transporters may play a significant role in limiting oral absorption of 13 commonly used drugs (digoxin, etoposide, felodipine, fexofenadine, furosemide, indinavir, losartan, nadolol, propranolol, ritonavir, saquinavir, tacrolimus, and verapamil) in humans.

METHODS

Drug absorption properties such as the rate (as judged by the Cmax and tmax) and extent (as judged by AUC or urinary excretion of drugs) of absorption as a function of dose, as well as the completeness of oral absorption were obtained from the literature.

RESULTS

The absorption properties of these 13 drugs are not consistent with absorption-retarding expectations from in vitro studies because they all show apparent dose-independent kinetics in absorption or bioavailability and completeness of oral absorption is shown for most of the drugs evaluated.

CONCLUSIONS

In spite of being substrates of intestinal efflux transporters such as P-gp, the in vivo oral absorption of 13 drugs examined apparently is not significantly impeded by efflux transporters. Thus, there may exist an apparent discrepancy between in vitro "expectations" and in vivo results; potential reasons for this are discussed. The present findings, however, do not de-emphasize potential in vivo importance of efflux transporters in affecting (increasing or decreasing) oral absorption of certain substrate drugs, especially those with low to moderate intestinal permeability and with low therapeutic index, or the importance of efflux transporters in the study of mechanisms of drug absorption and some potentially clinically significant drug-drug and drug-food interactions.

The rate and extent of oral bioavailability versus the rate and extent of oral absorption: clarification and recommendation of terminology

In the literature, the meanings of the terms oral absorption and oral bioavailability of drugs vary greatly. Absorption has been considered to take place at the mucosal membrane of the gastrointestinal (GI) tract. It has also been defined as the process from the site of drug administration to the site of measurement. In the latter definition, the extent of oral absorption depends on the extent of first-pass elimination in the gut wall and liver even though a drug may be completely absorbed from the GI tract. Moreover, these two terms have also been used interchangeably. Inconsistency in the definition of these two terms has led to varying interpretations of these terms among students, researchers and laymen, and such an inconsistency seems undesirable. Apparently because of these inconsistencies, improper correlations between the Caco-2 permeability or intestinal permeability and the oral bioavailability of drugs subject to extensive first-pass effect may have occurred. It is suggested that absorption be defined as movement of drug across the outer mucosal membranes of the GI tract, while bioavailability be defined as availability of drug to the general circulation or site of pharmacological actions. Since transit times (this may range from about 1 min to several hours) across enterocytes, liver, lungs, and the peripheral venous sampling tissue are virtually unknown for all drugs, this factor alone would favor the use of "oral bioavailability rate" rather than "oral absorption rate" in all routine studies.

Similarity in the linear and non-linear oral absorption of drugs between human and rat

OBJECTIVE

The main aim of this study was to provide a simple, pharmacokinetic rationale for great similarity in the extent (Fab) of gastrointestinal absorption of about 100 different, diverse compounds between human and the rat in linear dosing range, and to test the general applicability of a novel empirical method to correlate the non-linear Fab between the human and the rat by normalizing doses by body surface area (BSA) or body weight 0.67.

METHOD

The mean small intestinal transit time (t) of 36 rats was estimated from the reported study, and this was used to compare with that in humans. The reported great similarity in apparent first-order absorption rate constants (k) of seven structurally diverse compounds between the two species were obtained. Extensive computer search was made and non-linear Fab data for the two species were obtained for chlorothiazide, acyclovir, miglitol and pafenolol.

RESULTS

The mean t for rats was estimated to be 3.32 h which is almost identical to that reported in humans. The great similarity in Fab between human and rat in linear absorption range can be rationalized by similar t and k between the two species. The markedly different Fab vs dose/kg of body weight profiles between human and rat for the four drugs showing dose-dependent Fab were found to collapse when doses were normalized by BW0.67.

CONCLUSION

For Fab not limited by the solubility problem, the great similarity in Fab between human and rat in linear absorption range can be rationalized by the similar t and k. For non-linear Fab drugs, great similarity in Fab can also be obtained between human and rat when doses are normalized by BSA or BW0.67. Regardless of absorption properties (active, passive or facilitated), similar Fab between the two species may be generally obtained when doses used in humans are about 5 to 7 times lower than that in rats. The above findings may be valuable in drug development.

Evaluation of using dog as an animal model to study the fraction of oral dose absorbed of 43 drugs in humans

PURPOSE

To conduct a retrospective evaluation of using dog as an animal model to study the fraction of oral dose absorbed (F) of 43 drugs in humans and to briefly discuss potential factors that might have contributed to the observed differences in absorption.

METHODS

Mean human and dog absorption data obtained under fasted state of 43 drugs with markedly different physicochemical and pharmacological properties and with mean F values ranging from 0.015 to 1.0 were obtained from the literature. Correlation of F values between humans and dogs was studied. Based on the same references, additional F data for humans and rats were also obtained for 18 drugs.

RESULTS

Among the 43 drugs studied, 22 drugs were virtually completely absorbed in both dogs and humans. However, the overall correlation was relatively poor (r2 = 0.5123) as compared to the earlier rat vs. human study on 64 drugs (r2 = 0.975). Several drugs showed much better absorption in dogs than in humans. Marked differences in the nonliner absorption profiles between the two species were found for some drugs. Also, some drugs had much longer Tmax values and prolonged absorption in humans than in dogs that might be theoretically predicted. Data on 18 drugs further support great similarity in F between humans and rats reported earlier from our laboratory.

CONCLUSIONS

Although dog has been commonly employed as an animal model for studying oral absorption in drug discovery and development, the present study suggests that one may need to exercise caution in the interpretation of data obtained. Exact reasons for the observed interspecies differences in oral absorption remain to be explored.

Apparent lack of effect of P-glycoprotein on the gastrointestinal absorption of a substrate, tacrolimus, in normal mice

PURPOSE

To study the contribution of P-glycoprotein (P-gp) to the oral absorption of a substrate, tacrolimus, by comparing the extent and rate of bioavailability in normal and mdr1a knockout mice. Methods. Intravenous and oral (2 mg/kg) blood concentration data of tacrolimus in normal and knockout mice were obtained from a study by K. Yokogawa et al. in Pharm. Res. 16:1213-1218 (1999). Mean bioavailability (F), mean hepatic first-pass extraction ratio (Fh), mean bioavailability rates, mean oral clearance, and mean total hepatic intrinsic clearance were calculated using standard pharmacokinetic methods.

RESULTS

The mean F of tacrolimus (an apparently highly permeable compound) was increased from 0.22 in normal mice to 0.72 in knockout mice. These values were consistent with mean predicted Eh (based on intravenous data) of 0.77 and 0.27 in normal and knockout mice, respectively. Great similarity in the relative bioavailability profile (such as short Tmax) between normal and knockout mice was also found. Mean oral clearance and mean total or unbound hepatic intrinsic clearance of tacrolimus in knockout mice were found to be about 10 times lower compared to those in normal mice.

CONCLUSIONS

The above results suggest an apparent lack of effect of P-gp on the gastrointestinal absorption of tacrolimus in normal mice under the study condition. It is postulated that the effect of P-gp on the rate and extent of oral absorption should be more pronounced for those more slowly or incompletely absorbed drugs (i.e., drugs with relatively low permeabilities) as illustrated by talinolol in humans. The clearance data also suggest a very dominant role of P-glycoprotein in controlling the rate of hepatic metabolism of tacrolimus in normal mice, and P-glycoprotein may serve as an effective efflux pump for direct transport of metabolites formed in hepatocytes into the blood circulation.

Elucidation of human amphotericin B pharmacokinetics: identification of a new potential factor affecting interspecies pharmacokinetic scaling

PURPOSE

To elucidate the pharmacokinetics of amphotericin B in rats, mice and humans, and to perform interspecies scaling to humans using allometry.

METHODS

Plasma concentrations following intravenous bolus administration in rats, and mice were determined by HPLC. Human pharmacokinetic parameters elucidated from literature data were validated in a preliminary study involving a patient receiving daily infusion dose for 27 days. A critical literature review was conducted to identify appropriate pharmacokinetic parameter values in other species for interspecies scale-up. Interspecies allometric scale-up was performed across mice, rats, rabbits and dogs and the resulting predictions in humans were compared to observed values.

RESULTS

A triexponential decline in rat, mouse and human plasma concentrations were observed. No gender differences in rat pharmacokinetics were observed. In contrast to allometry, mouse CL was smaller (82 vs 116 ml/h/kg) and T0.5 (33 vs 20 h) was longer compared to rat. In the preliminary human study, Cpeak and Cmin values remained relatively constant over the duration of therapy, and a CL, MRT, T0.5, Vss and Vdarea of 26 ml/h/kg, 10 and 23 days, 6.2 and 20 L/kg, respectively, were estimated. The relative contributions of the terminal phase area in rat, mouse and human were 75%, 92% and 31%, respectively. Interspecies allometric scale-up predictions of human CL (41 ml/h/kg), CLu (467 ml/h/kg) and Vss (3.3 L/kg) were similar to reported values, whereas poor predictions of human Vuss (33 L/kg), Vdarea (4.1 L/kg) and T0.5 (3 days) were obtained.

CONCLUSIONS

Insignificant accumulation in humans inspite of the long terminal T0.5 was rationalized to be due to the small terminal-phase area contribution. While human CL and Vss were successfully predicted in the interspecies scaling, poor predictions of human Vdarea and T0.5 were obtained, which was attributed to disposition pattern differences between humans and other species, a potential new critical factor affecting interspecies scale-up.

Correlation of plasma clearance of 54 extensively metabolized drugs between humans and rats: mean allometric coefficient of 0.66

PURPOSE

To evaluate the distribution of allometric exponents for relationship of total plasma clearance of 54 extensively metabolized drugs, with wide-ranging linear clearance values, between humans and rats, to provide a rationale for the observed data, and to discuss potential significance of the findings.

METHODS

Human and rat plasma clearance values of 54 drugs with markedly different physicochemical properties were obtained from the literature. Standard allometric analysis was performed for each drug using both rat and human data. Unbound vs. total plasma clearances were obtained for 15 out of 54 drugs and their correlations between humans and rats were compared.

RESULTS

The mean+/-SD of the allometric exponent for the 54 drugs studied is 0.660+/-0.190. The median clearance ratio based on unit body weight is 7.41 and the median exponent is 0.645. Excluding two outliers the correlation coefficient of plasma clearance between humans and rats was 0.745 (p < 0.0001). For the 15 drugs, use of unbound plasma clearance approach seems to significantly improve the correlation coefficient compared to total plasma clearance (0.940 vs. 0.841).

CONCLUSIONS

The present study indicates that on average, humans and rats may eliminate extensively metabolized drugs at a rate similar to that expected from the allometric or body surface area relationship of basal metabolic rate between the two species. A simple statistical distribution hypothesis is used to rationalize the species difference in plasma drug clearance. Rat may serve as an useful animal model to predict (unbound) plasma clearance of drugs in humans.

Mechanism of ascorbic acid enhancement of the bioavailability and diuretic effect of furosemide

The following possible explanations for the significant increases in the oral bioavailability and the diuretic and natriuretic effects of orally administered furosemide observed when ascorbic acid was coadministered to dogs were investigated: ascorbic acid might enhance the gastrointestinal (GI) absorption of furosemide, might inhibit GI wall metabolism of furosemide, might enhance the reabsorption of furosemide from the renal tubules, and might increase the unionized fraction of furosemide at the receptor sites. The significant increase in the oral bioavailability with coadministration of ascorbic acid seemed to result from reduced gastric first-pass metabolism of furosemide and not enhanced GI absorption of furosemide. This might be supported by rat studies; the percentages of the oral doses of furosemide recovered from the GI tract at 8 hr after oral administration were similar (p < 0.583) without (39.5%) and with (44.7%) coadministration of ascorbic acid, and the amounts of furosemide remaining per gram of stomach after 30-min incubations of 50 micrograms of furosemide with 9000g supernatant fractions of stomach homogenates were increased significantly (48.5 vs. 42.4 micrograms) by the addition of 100 micrograms of ascorbic acid. The significant increases in the diuretic and natriuretic effects of furosemide with ascorbic acid could be the result of increases in the reabsorption of furosemide from renal tubules and increases in the unionized fraction of furosemide at the renal tubular receptor sites. This was supported by 1.5-4.2-fold increases in urine output and approximately 20% decreases in the time-averaged renal clearance of furosemide when the urine pH was decreased by 1.5-2.5 units by oral administration of ammonium chloride.

Similarity or discrepancy in pharmacokinetic parameter estimation between bolus and infusion studies

A recent study by Heatherington and Rowland showing discrepancies in steady-state volume of distribution (Vss) estimation of two barbiturates between bolus and infusion studies in rat hindlimb preparations was reviewed. Their rationale is that increasing the duration of administration may increase the accessibility for tissue distribution and thus increase Vss for compounds showing slow tissue uptake. Such a dosing-duration-dependent distribution concept is, however, inconsistent with the principle in linear kinetics that the fate or disposition function of any drug molecules is independent of time of administration and presence of other molecules. When their well-designed bolus studies were reanalyzed by including extrapolated outflow data from the last sampling time to infinity, the Vss values for the two barbiturates were found to be very similar to those obtained by the infusion method. Our analysis seems to validate a theoretical concept that parameter estimation is independent of the duration of administration in linear kinetics. A potential complication of using the bolus method to study Vss is presented.

New perspectives on the theory of permeability and resistance in the study of drug transport and absorption

Permeability coefficient (P) expressed as distance per unit time has been commonly interpreted to represent the velocity of drug movement across a heterogeneous medium such as skin and intestinal epithelium. The basis of such an interpretation is questioned on several grounds. For example, the basic assumption for calculating P (as defined conventionally) according to the Fick's law of diffusion requires the entire medium to be homogeneous and rate-limiting in transport. The theoretical basis of the widely used total resistance or resistance additivity concept is reviewed. Such a concept is shown to be applicable to the study of total transit time across the medium but may not be applicable to the study of steady-state flux or absorption across the medium under normal conditions. Based on the diffusional, compartmental, absorptive clearance or carrier-mediated-transport analysis it is shown that only the first transport resistance from the bulk medium across the surface (such as cellular membranes) of the permeation medium (such as a cell or a tissue) is usually the deciding factor in drug transport or absorption. Resistances on the other side of the surface barrier usually only affect the drug accumulation vs. time profile in the medium, but not the steady-state flux or absorption. The role of unstirred water layer adjacent to the internal capillary wall is postulated to play an important role in causing the blood-flow dependency in absorption, a phenomenon that cannot be rationalized by the conventional effective permeability concept. The conventional concept of sink conditions on the serosal side is questioned. The present analysis further supports the use of the absorptive or transport clearance concept in absorption or transport study. Effective permeability is regarded as a mathematical operator for transport across a barrier or a tissue, and may be unrelated to the Fick's law of diffusion under most conditions. The velocity unit for P is regarded simply as a "collapsed" unit based on the absorptive or transport clearance per unit gross surface area. It is hoped that this commentary will stimulate further research and discussions in this general area of drug transport and absorption. It appears that there is a need to experimentally confirm the total resistance theory in biological systems.

First-pass accumulation of salicylic acid in gut tissue after absorption in anesthetized rat

PURPOSE

The purpose of this paper is to report the study on the first-pass accumulation kinetics of salicylic acid (SA) in gut tissue after absorption by simultaneously analyzing drug contents in the lumen, gut tissue, and blood in anesthetized rats.

METHODS

Sodium salicylate (5.4 mg as SA) in 0.4 ml normal saline was administered into a closed 10-cm jejunal loop. Drained mesenteric blood from the loop area was collected every minute, while lost blood was replaced through infusion of oxygenated blood from donor rats. At 3, 10, 20, 40, or 60 min after dosing, SA remaining in lumen, accumulating in gut tissue, and appearing in blood were analyzed by HPLC. All the data were fitted into a linear two-consecutive (lumen and gut tissue) first-order kinetic model.

RESULTS

After absorption, significant amounts of SA accumulated in gut tissue before appearing in blood, e.g., at 3 or 20 min after dosing, 74.4 or 54.4% of absorbed SA accumulated in gut tissue, respectively. Practically all administered SA was recovered. The estimated mean absorption time from the lumen and mean transit time in gut tissue of SA were 20.4 and 18.5 min, respectively.

CONCLUSIONS

The above results indicate that gut tissue may act as a reservoir for drug accumulation during the first pass after oral absorption. Thus, the rate of transport of drug into blood circulation after oral administration may significantly differ from the true rate of absorption through the gut membrane. The potential transport resistance from gut tissue to blood should probably be considered in the modeling of GI absorption.

We may not measure the correct intestinal wall permeability coefficient of drugs: alternative absorptive clearance concept

It is shown that the conventional method (based on the thin-wall membrane theory) of studying the rate of disappearance or absorption from the lumen alone may not yield the correct intestinal wall (membrane) permeability coefficient of drugs. Potential reasons for causing this problem such as first-pass metabolism and accumulation of drugs in gut wall or tissue as well as back diffusion of drugs from the gut tissue to the lumen (i.e., a part of exsorption phenomenon) and potential major transport barriers across the protoplasm, basal membrane, and basement membrane are discussed. Contrary to the conventional concept, basal and basement membranes should also be considered major barriers for absorption into the blood for compounds with low intestinal permeability, and the protoplasm or cytoplasma should also be considered a major absorption barrier for compounds with high intestinal permeability. Strictly speaking, the conventional experimental method cannot be considered a bona fide method for studying drug permeability that deals with the movement of drug molecules from one side to the other side of a membrane, cell, medium, or device. The wall permeability coefficient thus obtained may therefore not represent the true wall permeability coefficient. "Intestinal absorptive clearance per unit gross surface area" is advocated as the best alternative term because it should more accurately reflect the true meaning of an experimental result for any compounds studied. In contrast to conventional cylindrical, unstirred tube models for the determination of wall permeability coefficients, the absorptive clearance calculation can be made based on a physiologically more realistic model-independent, "flat" or "distended," stirred (not well-stirred) intestinal model. Two model-independent terms, "effective intestinal permeability coefficient" and "effective absorptive permeability coefficient," are recommended as the second alternatives. These terms are theoretically valid for compounds that are not metabolized in the intestinal tissue; they represent the overall permeability across the intestinal tissue (from lumen to blood) under given experimental conditions. Potential shortcomings of using dimensionless wall permeability in the conventional absorption modeling are also discussed.

Effect of 'unstirred' water layer in the intestine on the rate and extent of absorption after oral administration

The presence of an aqueous diffusion layer or 'unstirred' water layer adjacent to the intestinal membrane has long been regarded as a potential barrier for intestinal absorption of compounds. Theoretical analyses were performed in the present study to quantitatively determine the effect of this layer on the rate and extent of absorption of passively absorbed compounds with different membrane absorption half-lives (10 to 300 min) in humans, dogs, rabbits, rats and mice. Diffusion half-lives across this (40 microns thick) layer were estimated to be 5.8, 2.5, 1.1, 0.65 and 0.32 min, respectively, in the distended intestine of the above five species. These half-lives are reduced by about 5-fold when the intestine is about 80% 'flat' in fasting state. The results of extensive analysis indicate that the presence of such a layer is generally expected to have a relatively mild or insignificant effect on the rate of absorption and an insignificant effect on the extent of absorption. The study also indicates that an aqueous layer of 708 microns has practically no effect on the extent of absorption of progesterone, a highly lipophilic compound, in rats. For prediction of or correlation with the fraction of oral dose absorbed after oral administration, the present study indicates that use of apparent or effective permeability rather than unbiased or true wall (membrane) permeability, as advocated earlier by others, should generally suffice.

Determination of drug permeability in a flat or distended stirred intestine. Prediction of fraction dose absorbed in humans after oral administration

Equations published earlier to calculate intestinal permeability (P) from in situ or in vivo studies and to predict fraction of dose absorbed (F) after oral administration may often be based on some physiologically and pharmacokinetically unrealistic assumptions; for example, (a) the intestine behaving like a "fully" distended, rigid, cylindrical tube without any intrinsic motility or stirring, (b) no accumulation of drug in gut wall or tissue, (c) lack of blood flow effect on permeability, (d) lack of back diffusion from absorptive cell to lumen, (e) a smooth surface area for absorption, and (f) a well-defined luminal concentration profile. An alternative, simple, realistic organ clearance approach is described here to re-define and calculate P as apparent absorptive permeability or absorptive clearance per unit gross surface area and to predict F based on either human or animal data. The approach may prove particularly valuable in view of the insignificant impact of "unstirred" water layer on F. The small intestine in humans and animals is found to be very flat overall (only about 10 of 30% distension in fasting state). The human intestinal luminal flow after normal dosing is estimated at about 3 ml/min, sixfold higher than 0.5 ml/min proposed by others. The direct consequence of an early assumption that dimensionless P is the same between human and rat is pointed out; the permeability coefficient being 7-10 times higher in rats than in humans, that is, however, not supported by experimental data. The apparent success of a widely used approach to estimate F based on rat data is rationalized.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrahepatic distribution of hydrochlorothiazide and quinidine in rats: implications in pharmacokinetics

The intrahepatic distributions of a nonmetabolized drug, hydrochlorothiazide (HCTZ) and a highly metabolized drug, quinidine (QD), were studied separately in five anesthetized rats during steady-state intravenous infusion. Liver samples (20-24) from various parts (0.1-0.2 g each) at the end of infusion were collected from each rat and analyzed for drug concentration by HPLC. The distribution of HCTZ in various macro regions appears quite "homogeneous," with a CV of drug concentration for each rat ranging from 2.6 to 4.7% (grand mean, 3.7 +/- 0.8%), whereas that of QD seems less "homogeneous," with a CV ranging from 8.5 to 28.3% (grand mean, 15.6 +/- 7.7%). The above results indicate that drugs that are not complicated by hepatic metabolism may tend to show more "homogeneous" distribution and those that are highly metabolized and/or known to strongly bind to hepatic tissue component(s) may show less "homogeneous" distribution. The results from the present QD study are in contrast with the general, much more heterogeneous distribution found in an isolated in situ perfusion study reported earlier. The implication of the present study in physiological pharmacokinetic and hepatic modeling is discussed.

Intravenous verapamil kinetics in rats: marked arteriovenous concentration difference and comparison with humans

The pharmacokinetics of verapamil, a calcium channel blocker, were studied in male Sprague-Dawley rats following i.v. administration at a dose of 1 mg kg-1. Both arterial and venous blood were collected and the plasma drug concentrations were determined by reversed-phase high-performance liquid chromatography. Verapamil was distributed to the extravascular tissues very rapidly as indicated by the large Vdss (2.99 +/- 0.57 l kg-1) and Vd beta (5.08 +/- 0.54 l kg-1). The apparent terminal plasma T1/2, MRTiv, and CLp were 1.59 +/- 0.46, 1.26 +/- 0.12 h, and 40.4 +/- 9.73 ml min-1 kg-1, respectively. Marked arterial/venous differences were found with a considerable influence on the MRT and Vdss, and the terminal phase venous levels were higher than arterial levels by 103, 69, and 90%, respectively, for the three rats studied. The distribution of verapamil between plasma and erythrocytes occurred very rapidly and was identical in vitro and in vivo. The average blood to plasma and plasma to blood cell concentration ratios were 0.85 and 1.47, respectively. In contrast to propranolol, blood data rather than plasma data should be used to predict the hepatic extraction ratio of verapamil (0.87). The plasma protein binding of verapamil in humans (90%) and rats (95%) were quite similar and constant over the wide concentration range studied. A comparison of some pharmacokinetic parameters between rats and humans is presented and the potential shortcomings of using T1/2 or CLp and the advantage of using CLu (unbound plasma clearance) in interspecies scaling is also discussed.

Erythrocytes as a total barrier for renal excretion of hydrochlorothiazide: slow influx and efflux across erythrocyte membranes

The potential barrier effect of erythrocytes (RBC) on renal excretion (mainly by tubular secretion) of hydrochlorothiazide (HCTZ) was evaluated in nine anesthetized rats during steady-state iv infusion. Drug concentrations in plasma and blood from the carotid artery and renal vein were assayed by a simple modified HPLC method. Renal extraction ratios were concentration-independent with a mean of 0.17 +/- 0.05 (SD). The renal excretion was found to occur primarily from the drug in plasma; the mean net fractional removal from plasma was 0.57 +/- 0.12, while that from RBC was less than 0.008 +/- 0.041. The virtual total unavailability of HCTZ from RBC (containing approximately 70% of drug in arterial blood) for renal excretion is attributed to relatively slow efflux of drug from RBC to plasma during each passage through the kidney compared with the blood transit time (in seconds). Preliminary in vitro influx and efflux kinetics of HCTZ across RBC membranes were studied using rat and human blood. The flux data could be adequately described by a linear, reversible, closed two-component system model, and the mean equilibration half-times (ET1/2) in rat and human blood were 10.9 and 20.5 min, respectively. The mean residence time of drug in blood circulation of rats was estimated to be 8.32 +/- 1.06 min, which is shorter than the ET1/2. This is consistent with data indicating that distribution equilibrium of HCTZ in arterial blood might not be reached in vivo even at steady state. Other implications of slow transport kinetics of drugs across RBC membranes are discussed.

Large differences in the biological half-life and volume of distribution of hydrochlorothiazide in normal subjects from eleven studies. Correlation with their last blood sampling times

In spite of more than three decades of wide use of hydrochlorothiazide as a diuretic, its mean biological half-life (t1/2) and apparent volume of distribution (Vdarea) in normal subjects were found to vary greatly among 11 studies reported between 1976 and 1986. For example, the mean t1/2 values ranged from 3.2 to 13.1 h and Vdarea values from 1.53 to 4.19 l/kg. Furthermore, a t1/2 of only 2.5 h and a Vdarea of only 0.83 l/kg were cited in a recent textbook. The present analyses show a positive correlation between the length of the last blood sampling time (tlast) employed and the t1/2 or Vdarea. The smaller t1/2 and Vdarea values reported were attributed to the insufficient length of time used in blood sampling for a drug exhibiting polyexponential disposition function. The kinetic and dynamic significance of the findings were briefly described.

Analysis of drugs and other toxic substances in biological samples for pharmacokinetic studies

The importance of the role of analysis of drugs and other toxic substances in biological samples (bioanalysis) in medicine, toxicology, pharmacology, forensic science, environmental research and other biomedical disciplines is self-evident. Among these disciplines, bioanalysis plays a special pivotal role in pharmacokinetics. The pharmacokinetic parameters, such as half-life, volume of distribution, clearance and bioavailability, of drugs and other compounds are derived from the concentrations of these analytes assayed in the biological samples collected at specified time points. The capability of analysts to develop sensitive and specific analytical methods for the assay of low concentrations of drugs and other toxic compounds in small amounts of biological samples has contributed significantly to the theoretical advances in pharmacokinetics and its applications in clinical pharmacology and the management of drug therapy in patients. The increased demands for pharmacokinetic applications in turn have stimulated the innovation and improvement in bioanalytical technologies. The reliability of the pharmacokinetic conclusions depends on the accuracy and precision of the analytical methods employed to assay the biological samples. Factors that affect the integrity of the bioanalytical data should therefore be controlled in analysis of biological samples for pharmacokinetics studies. The biological samples for drug concentration determination should be collected as specified in the study protocol with respect to the time and site of sampling. These samples should be processed to avoid extraneous interactions between the analytes and sampling devices or additives resulting in the redistribution of the analytes between components of the biological samples, such as displacement of drug binding and changes in the distribution of the analytes between plasma and red blood cells. The stability of the drugs and other analytes in the samples should also be evaluated to establish the conditions suitable for the transportation and storage of the samples to avoid chemical, photochemical and enzymatic degradation of the analytes. Various technologies have been utilized to assay biological samples for pharmacokinetic studies. The most frequently used are chromatography (high-performance liquid chromatography, gas chromatography and thin-layer chromatography), immunoassays and mass spectrometry.(ABSTRACT TRUNCATED AT 400 WORDS)

Marked heterogeneity in the intrahepatic distribution of quinidine in rats: implications in pharmacokinetics

Approximately 20 to 24 samples (0.1-0.2 g each) were obtained from each of six isolated rat livers following steady-state infusion of quinidine. The concentrations of quinidine, analyzed by an HPLC method, were found to vary markedly within each lobe (up to approximately 52-fold) or between lobes (up to approximately 25-fold) from the same liver. The maximum intrahepatic concentration difference in the six livers studied was 208-fold. Implications of the present study in the determination of liver drug concentration, and of the partition coefficient between liver and venous drug concentration in physiological pharmacokinetic modeling, as well as in hepatic modeling, are discussed.

Effect of a change in the luminal perfusion rate on intestinal drug absorption studied by a simple unified organ clearance approach

A recently proposed simple, noncompartmental, unified organ clearance approach was employed to study the effect of change in luminal perfusion rate (Q) on the steady-state intestinal absorption extraction ratio (E) of drugs. The equation used for correlation or prediction is E = H/(Q + H), where H is the apparent intrinsic intestinal absorption clearance of drug and assumed to be constant under linear conditions. Reported experimental data from intestinal perfusion studies in rats were used to evaluate the applicability or accuracy of the above equation. It was found that the mean difference between the predicted and the reported E values for seven steroids with a very wide range of partition coefficients between n-octanol and water (P) was 4.37% as Q was reduced from 0.497 to 0.247 ml/min. Reported changes in E due to multiple variation in Q (up to about 10 times) for hydrocortisone, progesterone, and iopanoic acid were satisfactorily predicted. The relationship between H and log P and the potential limitation of the present absorption approach are discussed. The present limited study also indicates that the absorption of the steroids with higher lipophilicity is usually less flow dependent.

Erythrocytes as barriers for drug elimination in the isolated rat liver. II. Propranolol

The potential of erythrocytes (RBC) to serve as "barriers" of hepatic elimination of propranolol, a drug with rapid equilibration in blood, was studied in rats under two conditions: (I) the drug was preequilibrated in blood before infusion into the liver, and (II) the drug was directly infused into the liver. The mean fractions of dose escaping elimination during each pass under conditions I and II were 0.0561 +/- 0.040 and 0.0290 +/- 0.024, respectively (P less than 0.02). Contrary to the early study on doxorubicin, most drug molecules in RBC were found to be available for elimination. Implications of the present findings in the prediction of hepatic first-pass effect after oral administration, on the basis of intravenous data, are discussed. Marked underestimation of oral bioavailability of propranolol in humans is consistent with the RBC "barrier" effect hypothesis.

Erythrocytes as barriers for drug elimination in the isolated rat liver. I. Doxorubicin

The effect of doxorubicin (Dx) equilibration between plasma and erythrocytes (RBC), prior to entering the liver, on hepatic elimination was evaluated under two conditions: (I) the drug being first equilibrated for 30 min in the perfusate (containing 27% human RBC) before infusion into the liver and (II) the drug being directly infused into the liver. Mean (N = 6) steady-state hepatic extraction ratios (E) under conditions I and II were 0.286 +/- 0.131 (SD) and 0.592 +/- 0.147, respectively. The marked difference in E was attributed mainly to the initial difference in plasma/RBC Dx distribution ratio of the inlet blood, the slow efflux of Dx from RBC into plasma under condition I, and the slow influx of Dx from plasma to RBC under condition II. The results indicate that most Dx molecules in RBC are not available for elimination. Drug equilibration between plasma and RBC may therefore represent an important factor in hepatic first-pass metabolism.

The phenomenon and rationale of marked dependence of drug concentration on blood sampling site. Implications in pharmacokinetics, pharmacodynamics, toxicology and therapeutics (Part I)

At least 42 compounds have been reported to exhibit significant or marked blood sampling site dependence in concentration after dosing in humans and animals. The very high efficiency of uptake of drug by the poorly perfused sampling tissue (e.g. arm or leg) during its very short transit through the capillary (1 to 3 seconds) is mainly responsible for such a universal phenomenon. When marked arteriovenous concentration differences exist, their entire plasma (blood or serum) concentration-time profiles may resemble those obtained from completely different drugs or from different dosing rates. After an intravenous bolus injection, the reported maximal arteriovenous differences were 3240-fold for griseofulvin during the early distribution phase (arterial concentration being higher than venous, and 234% for procainamide during the terminal phase (venous concentration being higher). The reported maximal steady-state arteriovenous difference during infusion was 3.8-fold for glyceryl trinitrate (nitroglycerin), with the arterial level higher, due to metabolism and possible strong binding by sampling tissue. Interestingly, peak arterial plasma concentrations were usually achieved at about 0.5 minutes, while peak venous plasma concentrations generally occurred at 1 to 5 minutes after injection. Thus, the plasma concentration-time profile after an intravenous bolus injection actually resembles that predicted for a short term intravenous infusion, according to the classical instantaneous input hypothesis. Potential factors that may affect the degree of arteriovenous difference are here reviewed in detail. The implications of potential marked arteriovenous differences in pharmacokinetics, in pharmacokinetic/pharmacodynamic correlations or modelling, in toxicology, and in drug therapy are extensively discussed. Clinicians or scientists dealing with the determination and/or use of plasma concentration data should be fully aware of this problem. Many previous studies, based on the commonly accepted assumption that immediately or shortly after dosing plasma (blood) concentrations are essentially uniform throughout the blood circulation or the central (plasma) compartment, may require a reexamination. This is particularly important since the 'driving force' for distribution of a drug to various parts of the body for elimination, for accumulation or for producing a pharmacological or toxic effect, is its concentration in arterial blood, and not in venous blood drained from a poorly perfused tissue (venous blood may more accurately reflect drug concentrations in the poorly perfused sampling tissue itself). The present review probably represents the first of its kind ever reported in the literature. It is hoped that the review will increase the awareness of this very fundamental and important subject matter among our readers, and may also stimulate further studies or discussions.

Effect of change in blood flow on hemodialysis clearance studied by a simple unified organ clearance approach

The applicability of a simple noncompartmental organ clearance approach was extended to evaluate the effect of change in blood flow (Q) on hemodialysis clearance (CL) using patient data reported in the literature. Hemodialysis blood or plasma clearance data of vitamin B12, urea, creatinine and uric acid were fitted into the equation, CL = HQ/(Q + H), where H is the apparent intrinsic hemodialysis clearance for total (bound and unbound) compound. The Q values varied up to 16-fold. The extraction ratios ranged from about 0.4 to 1.0 (indicating existence of marked concentration gradient) and 0.1 to 0.6 for urea and vitamin B12, respectively. The H only serves as an "operator" in a given system, and the H value per unit length of the dialyzer coil can be shown to increase markedly with the length of coil, which is different from the basic assumption in the well-stirred model. For vitamin B12 the present clearance approach appears to be slightly superior to the "parallel-tube model".

Correlation of unbound plasma clearances of fifteen extensively metabolized drugs between humans and rats

Unbound plasma clearances (CLu) in humans and rats of 15 extensively metabolized drugs (phenytoin, hexobarbital, pentobarbital, phenylbutazone, warfarin, tolbutamide, valproate, phenobarbital, amobarbital, quinidine, chlorpromazine, propranolol, pentazocin, antipyrine, and diazepam), studied earlier by Sawada et al. (J. Pharmacokin, Biopharm. 13:477-491, 1985), were calculated. It was found that the ratio of CLu per square meter of body surface area between human and rat ranged from 0.38 for pentobarbital to 2.34 for tolbutamide, with a mean ratio of 1.07. When body weight (BW) was used for correlation, the mean CLu was proportional to BW 0.657 +/- 0.0935. A rationale for the above empirical findings is postulated. The present study seems to indicate the existence of a general similarity or predictability in the CLu of drugs between rats and humans. Low correlations were generally obtained when total (bound and unbound) plasma clearances were used for comparison.

The phenomenon and cause of the dose-dependent oral absorption of chlorothiazide in rats: extrapolation to human data based on the body surface area concept

The reported incomplete and dose-dependent absorption of chlorothiazide in humans was demonstrated in six rats after five oral solutions at doses of 0.93, 2.55, 9.23, 25.6, and 70.2 mg/kg. Mean 48-hr urinary recoveries of intact drug were 57.3, 50.4, 36.7, 22.8, and 15.3%, respectively. A similar degree of dose dependency in absorption was found in rat, dog, and human when the doses were related to unit body surface area (BSA) but not on unit body weight, indicating similar interspecies absorptive capacity in terms of unit BSA. This finding may be partly rationalized by marked similarities in the reported solution transit time (2-3 hr) in the small intestine as well as in the calculated gross surface area of the small intestine per unit BSA (0.163 for rat and 0.132 for human). Contrary to the previous postulation of a specific absorption site, the drug was absorbed from different regions of the GI tract with apparent 1-hr absorption rates, studied by the in situ closed-loop method, in the following rank order: jejunum (34.6%) greater than duodenum (32.7%) greater than large intestine (20.1%) greater than ileum (18.0%) greater than stomach (12.4%). Different from the commonly assumed first-order absorption process, the intestinal loop absorption was concentration-dependent, suggesting a saturable mechanism. For example, the absorption rate at 0.008 mg/mL was higher than that at 0.2 mg/mL in ileal loops (61%, p less than 0.01) and jejunal loops (22%, p less than 0.1). In addition, the absorption rates at pH 6 and 7.4 were statistically identical, indicating a lack of ionization effect that is important in the passive absorption process. The solubility-limited absorption could probably be ruled out at doses below 2.55 mg/kg for rat and 125 mg for human in view of higher aqueous solubilities at 37 degrees C (e.g., 1.3 mg/mL at pH 7) found in the present study. Contrary to the previous hypothesis of low membrane permeability as a limiting factor for absorption, the "intrinsic" partition coefficient in 1-octanol/aqueous buffer was moderate, 0.6. Furthermore, the absorption in ileal and jejunal loops was enhanced by an apparent increase in mesenteric blood flow by caffeine. The existence of prolonged oral absorption in rats and humans is discussed.

Albumin as a potential membrane transport barrier for hepatic clearance of diazepam studied by a universal organ clearance approach

Published data on the effect of albumin binding on hepatic extraction of diazepam in isolated perfused rat livers were analyzed by a simple universal organ clearance approach proposed earlier by this author. Apparent hepatic intrinsic clearance of free (unbound) diazepam was found to decrease 39-fold from 334 ml/min in the absence of albumin to only 8.6 ml/min when the fraction unbound was reduced to 0.052. This interesting finding is rationalized by adsorption of albumin onto the surface of liver cells which might temporarily bind the drug molecules and slow down their diffusion across the membrane. A term "relative permeability" was introduced.

Effect of intravenous infusion time on the pharmacokinetics and pharmacodynamics of the same total dose of furosemide

The pharmacokinetics and pharmacodynamics of furosemide were evaluated after intravenous administration of the same total dose of furosemide in different lengths of infusion time (10 s, 30 min, 2 h, and 8 h) to 6 dogs. The fluid loss in urine was immediately replaced volume for volume with intravenous infusion of Lactated Ringer's solution. The pharmacokinetic parameters such as per cent of the dose excreted in urine, total body and renal clearances, and terminal half-life were not significantly different with four different infusion times. The volume of distribution at steady state and mean residence time based on venous data, on the other hand, appeared to increase with increasing infusion time. The mean values for Vss were 0.334, 0.478, 0.499, and 0.708 1 kg-1 for 10 s, 30 min, 2 h, and 8 h of infusion, respectively, and the corresponding values for MRT were 17.5, 22.2, 24.8, and 38.1 min. The diuretic effects (urine output and urinary excretion of sodium) were generally found to increase with increasing infusion times; the total mean 24 h urine outputs were 1102, 1464, 2190, and 3470 ml for 10 s, 30 min, 2 h, and 8 h of infusion, respectively, and the corresponding values for sodium excretion were 170, 175, 272, and 440 mmol. Furosemide plasma concentrations and hourly urinary excretion rates of furosemide, sodium, and potassium during the apparent steady state (between 2 and 8 h) in the 8 h infusion study were fairly constant.

A new simple approach to study the effect of changes in urine flow and/or urine pH on renal clearance and its applications

After entering the renal tubular lumen either through glomerular filtration or tubular secretion, drug molecules are considered to be removed from there by two competing processes: tubular fluid flow for urinary excretion and tubular reabsorption into blood circulation. The relative overall magnitude of the "force" or "efficiency" of these two processes will determine the fraction of drug molecules to be either excreted or reabsorbed, and hence their renal clearance (CLr). Urine flow rate (Q) is assumed to be proportional to the mean flow rate of tubular fluid at reabsorption sites (mainly in distal tubules), and used as an index to indirectly estimate the relative reabsorption "force" or "efficiency" (this may also be called apparent intrinsic reabsorption clearance). With the above assumptions, a plot of 1/CLr vs. 1/Q should yield a straight line under apparent first-order conditions. This has been confirmed for urea, theophylline, ethanol, chloramphenicol, amobarbital, riboflavin and fluoride based on human and dog (riboflavin only) data reported in the literature. Assuming that tubular reabsorption occurs only through the diffusion of unionized molecules, a plot of 1/CLr vs. fn/Q should also yield a straight line for weak acids and weak bases under linear conditions; the fn is the mean fraction of drug present in the unionized form at reabsorption sites whose mean pH is approximated or reflected by the urinary pH. The above straight line plot has been confirmed with human data for phenobarbital as well as human and rat data for salicylic acid. The effect of urine pH on biological half-life of pseudoephedrine in humans has also been successfully characterized.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the rate and composition of fluid replacement on the pharmacokinetics and pharmacodynamics of intravenous furosemide

Effects of differences in the rate and composition of intravenous fluid replacement for urine loss on the pharmacokinetics and pharmacodynamics of furosemide were evaluated using the dog as a model animal. Each of six dogs received 8-hr constant intravenous infusion of 20 mg (15 mg used in one dog) of furosemide with 0% replacement (treatment I), 50% replacement (treatment II), and 100% replacement (treatment III) with lactated Ringer's solution, as well as with 100% replacement with 5% dextrose in water (treatment IV). Most pharmacokinetic parameters, such as plasma clearance, steady-state volume of distribution, mean residence time, and terminal half-life, were essentially the same in all four treatments. Renal clearances and urinary excretion rates of the drug in treatments II-IV were essentially the same, but about 20% higher than those in treatment I. In spite of the similarities in kinetic properties, diuretic and/or natriuretic effects from furosemide were markedly different among the four treatments. For example, mean 10-hr urine outputs were 646, 1046, 3156, and 1976 ml and mean 10-hr sodium excretions were 87.0, 142, 383, and 97.2 mmole for treatments I-IV, respectively. Except for treatment III, diuresis and/or natriuresis were found to be time-dependent, generally decreasing with time until reaching a low plateau during later hours of infusion. The present findings also showed that no fluid replacement and 100% replacement with 5% dextrose solution both produced the same degree of severe acute tolerance in natriuresis, indicating the insignificance of water compensation in tolerance development; in treatment II, where neutral sodium balance was achieved, the development of acute tolerance in diuresis and natriuresis can mainly be attributed to negative water balance under this special condition; at steady state the hourly diuresis and natriuresis could differ up to about ten times between treatments. Some implications for the kinetic/dynamic relationship or modeling, in the clinical use, and in the bioequivalence evaluation of dosage forms are discussed.