Partition Coefficient Of Drug Research Articles

Drug dissolution profiles from polymeric matrices: Data versus numerical solution of the diffusion problem and kinetic models

This paper presents a comparative study between the data collected in a drug dissolution experiment and the predictions obtained from simple mathematical approaches of drug diffusion in the delivery device and also with the results achieved from available kinetic models for dissolution processes. The controlled release of timolol maleate from a hydrogel disc, obtained by thermal copolymerization of hydroxyethyl methacrylate and methacrylic acid, was used as the case study.The equilibrium parameter (drug partition coefficient) used to model the mass transfer process dictates the predictions’ accuracy. When this parameter is calculated from the drug release experiment, the diffusion equation with a Robin boundary condition type gives good predictions of the dissolution process. Predictions obtained with zero-sink condition in the release medium resulted in an overestimation of data.Several kinetic models available in the literature to describe drug release were used to correlate data. All the models tested describe the data adequately, but the Weibull model was the one that had the best correlation performance.

Zone fluidics for measurement of octanol–water partition coefficient of drugs

A novel zone fluidics (ZF) system for the determination of the octanol–water partition coefficient (Pow) of drugs was developed. The ZF system consisted of a syringe pump with a selection valve, a holding column, a silica capillary flow-cell and an in-line spectrophotometer. Exact microliter volumes of solvents (octanol and phosphate buffer saline) and a solution of the drug, sandwiched between air segments, were sequentially loaded into the vertically aligned holding column. Distribution of the drug between the aqueous and octanol phases occurred by the oscillation movement of the syringe pump piston. Phase separation occurred due to the difference in densities. The liquid zones were then pushed into the detection flow cell. In this method, absorbance measurements in only one of the phase (octanol or aqueous) were employed, which together with the volumes of the solvents and pure drug sample, allowed the calculation of the Pow. The developed system was applied to the determination of the Pow of some common drugs. The log (Pow) values agreed well with a batch method (R2=0.999) and literature (R2=0.997). Standard deviations for intra- and inter-day analyses were both less than 0.1log unit. This ZF system provides a robust and automated method for screening of Pow values in the drug discovery process.

Erythrocyte membranes from slaughterhouse blood as potential drug vehicles: Isolation by gradual hypotonic hemolysis and biochemical and morphological characterization

The present study was aimed at investigating the effect of isolation process-gradual hypotonic hemolysis on chosen parameters of the erythrocyte membranes (ghosts) originating from bovine and porcine slaughterhouse blood. The estimation of the gradual hypotonic hemolysis as a drug loading procedure for the erythrocyte ghosts was performed as well.Based on the results derived from analysis of the osmotic properties of the erythrocytes, the gradual hemolysis was performed with high volume of erythrocytes and 35mM hypotonic sodium-phosphate/NaCl, enabling >90% of hemolysis for both types of erythrocytes. Detailed insight into ghosts’ morphology by field emission-scanning electron microscopy revealed a distortion from erythrocyte shape and an altered surface texture with increased bilayer curvature for both samples. Compared to erythrocytes, an average diameter of ghosts from both type of erythrocytes decreased for only about 10%. The reported unidispersity of the isolated ghosts is of great importance for their potential application as vehicles of active compounds. Gradual hemolysis did not lead to substantial loss of cholesterol and membrane/cytoskeleton proteins. This result indicated the ghosts’ possibility to mimic the chemical and structural anisotropic environment of in vivo cell membranes, which is of significance for drug diffusion and partition coefficients. Induced shift of phosphatidylserine to external surface of the ghosts demonstrated their potential application as vehicles for targeted drug delivery to cells of reticuloendothelial system. Ultra high-performance liquid chromatography and Fourier transform infrared spectroscopy revealed the presence of a drug model – dexamethasone-sodium phosphate, and its interaction with structural components in both types of erythrocyte ghosts.

Novel lutein loaded lipid nanoparticles on porcine corneal distribution.

Topical delivery has the advantages including being user friendly and cost effective. Development of topical delivery carriers for lutein is becoming an important issue for the ocular drug delivery. Quantification of the partition coefficient of drug in the ocular tissue is the first step for the evaluation of delivery efficacy. The objectives of this study were to evaluate the effects of lipid nanoparticles and cyclodextrin (CD) on the corneal lutein accumulation and to measure the partition coefficients in the porcine cornea. Lipid nanoparticles combined with 2% HPβCD could enhance lutein accumulation up to 209.2 ± 18 (μg/g) which is 4.9-fold higher than that of the nanoparticles. CD combined nanoparticles have 68% of drug loading efficiency and lower cytotoxicity in the bovine cornea cells. From the confocal images, this improvement is due to the increased partitioning of lutein to the corneal epithelium by CD in the lipid nanoparticles. The novel lipid nanoparticles could not only improve the stability and entrapment efficacy of lutein but also enhance the lutein accumulation and partition in the cornea. Additionally the corneal accumulation of lutein was further enhanced by increasing the lutein payload in the vehicles.

Hepatocyte Composition-Based Model as a Mechanistic Tool for Predicting the Cell Suspension: Aqueous Phase Partition Coefficient of Drugs in In Vitro Metabolic Studies

This study is an extension of a previously published microsome composition-based model by Poulin and Haddad (Poulin and Haddad. 2011. J Pharm Sci 100:4501-4517), which was converted to the hepatocyte composition-based model. The first objective was to investigate the ability of the composition-based model to predict nonspecific binding of drugs in hepatocytes suspended in the incubation medium in in vitro metabolic studies. The hepatocyte composition-based model describes the cell suspension-aqueous phase partition coefficients, which were used to estimate fraction unbound in the incubation medium (fuinc ) for each drug. The second objective was to make a comparative analysis between the proposed hepatocyte composition-based model and an empirical regression equation published in the literature by Austin et al. (Austin RP, Barton P, Mohmed S, Riley RJ. 2004. Drug Metab Dispos 33:419-425). The assessment was confined by the availability of experimentally determined in vitro fuinc values at diverse hepatocyte concentrations for 92 drugs. The model that made use of hepatocyte composition data provides comparable or superior prediction performance compared with the regression equation that relied solely on physicochemical data; therefore, this demonstrates the ability of predicting fuinc also based on mechanisms of drug tissue distribution. The accuracy of the predictions differed depending on the class of drugs (neutrals vs. ionized drugs) and species (rat vs. human) for each method. This study for hepatocytes corroborates a previous study for microsomes. Overall, this work represents a significant first step toward the development of a generic and mechanistic calculation method of fuinc in incubations of hepatocytes, which should facilitate rational interindividual and interspecies extrapolations of fuinc by considering differences in lipid composition of hepatocytes, for clearance prediction in the physiologically-based pharmacokinetics (PBPK) models.

Transdermal Delivery of Lercanidipine Hydrochloride: Effect of Chemical Enhancers and Ultrasound

The effects of permeation enhancers and sonophoresis on the transdermal permeation of lercanidipine hydrochloride (LRDP) across mouse skin were investigated. Parameters including drug solubility, partition coefficient, drug degradation and drug permeation in skin were determined. Tween-20, dimethyl formamide, propylene glycol, poly ethylene glycol (5% v/v) and different concentration of ethanol were used for permeation enhancement. Low frequency ultrasound was also applied in the presence and absence of permeation enhancers to assess its effect on augmenting the permeation of drug. All the permeation enhancers, except propylene glycol, increased the transdermal permeation of LRDP. Sonophoresis significantly increased the cumulative amount of LRDP permeating through the skin in comparison to passive diffusion. A synergistic effect was noted when sonophoresis was applied in presence of permeation enhancers. The results suggest that the formulation of LRDP with an appropriate penetration enhancer may be useful in the development of a therapeutic system to deliver LRDP across the skin for a prolonged period (i.e., 24 h). The application of ultrasound in association with permeation enhancers could further serve as non-oral and non-invasive drug delivery modality for the immediate therapeutic effect.

A novel approach to formulation factor of aceclofenac eye drops efficiency evaluation based on physicochemical characteristics of in vitro and in vivo permeation

The purpose of the present study was to evaluate aceclofenac eye drop through excised goat cornea. Raising pH of the formulation from 6.0 to 8.0, effect of different preservatives or effect of viscosity enhancer decreases apparent permeability coefficient. Topical ophthalmic NSAID are used to treat ocular surface and anterior segment inflammation as well as post operative management of pain and inflammation. Aceclofenac’s unique chemical structure makes it both a potent anti inflammatory drug and lipophilic molecule that penetrates ocular tissue, ensuring relief of pain in cataract and refractive surgery and corneal abrasion. The octanol/water partition coefficient of aceclofenac drug is 1.86±0.75. Permeation characteristics of the drug were evaluated by putting 1ml formulation on freshly excised goat cornea fixed between donor and receptor compartments of an all-glass modified Franz diffusion cell and measuring the drug permeated in the receptor by spectrophotometry at 275nm, after 120min. The results suggest that aceclofenac ophthalmic solution (pH 7) containing BAC provides increased in vitro ocular availability through goat corneas. The combination of methyl paraben and propyl paraben MP–PP preservative in aceclofenac ophthalmic eye drop 0.1% formulated in phosphate buffer increases transcorneal permeation. The developed formulations were evaluated for their pharmacodynamics in albino rabbits, by measuring in-vivo study and were compared to a marketed voltrane ophthalmic solution.

In Vitro Permeation Characteristics of Itraconazole from Oil Drops and Ophthalmic Ointment Through Excised Goat And Sheep Corneas

In vitro transcorneal permeation of itraconazole from oil drops and ophthalmic ointments was studied using freshly excised goat and sheep corneas. Permeation of itraconazole from castor oil was found maximum and minimum with kardi oil formulation. The higher permeation of itraconazole from castor oil formulation could be attributed to the lower partition coefficient of drug between oil and aqueous phase. The addition of benzyl alcohol, a preservative, in oil drops, increased the permeation of itraconazole. Partition experiments indicated increased partitioning of itraconazole in the aqueous phase in the presence of benzyl alcohol. The addition of phenyl mercuric acetate, phenyl mercuric nitrate and thiomersal as preservative, in castor oil drop decreased the permeation of itraconazole. Corneal hydration was within the desired limit below 80 %. Permeation of itraconazole from ointment containing dissolved drug was higher than the ointment containing solid drug. In antifungal study, the castor oil formulation showed maximum zone of inhibition against Candida albicans. Stability study conducted at 40 0 C and 70% RH indicated the formulations as quite

The contribution of the hydrogen bond acidity on the lipophilicity of drugs estimated from chromatographic measurements

The influence of the hydrogen bond acidity when the 1-octanol/water partition coefficient (log Po/w) of drugs is determined from chromatographic measurements was studied in this work. This influence was firstly evaluated by means of the comparison between the Abraham solvation parameter model when it is applied to express the 1-octanol/water partitioning and the chromatographic retention, expressed as the solute polarity p.Then, several hydrogen bond acidity descriptors were compared in order to determine properly the log Po/w of drugs. These descriptors were obtained from different software and comprise two-dimensional parameters such as the calculated Abraham hydrogen bond acidity A and three-dimensional descriptors like HDCA-2 from CODESSA program or WO1 and DRDODO descriptors calculated from Volsurf+software. The additional HOMO–LUMO polarizability descriptor should be added when the three-dimensional descriptors are used to complement the chromatographic retention.The models generated using these descriptors were compared studying the correlations between the determined log Po/w values and the reference ones. The comparison showed that there was no significant difference between the tested models and any of them was able to determine the log Po/w of drugs from a single chromatographic measurement and the correspondent molecular descriptors terms. However, the model that involved the calculated A descriptor was simpler and it is thus recommended for practical uses.

An in vitro spectrometric method for determining the partition coefficients of non-steroidal anti-inflammatory drugs into human erythrocyte ghost membranes

Usefulness of second derivative spectrophotometry for determining the partition coefficients (Kps) of four non-steroidal anti-inflammatory drugs (NSAIDs) between human erythrocyte ghost (HEG) membranes and buffer at simulated physiological conditions (pH=7.4, 37°C) has been adequately emphasized. In the absorption spectra for each of the investigated NSAIDs, λmax was red-shifted in presence of HEG membranes, indicating that NSAIDs have the nature of metachromasy between lipid bilayer and water. Further quantitative spectral data for calculating Kps could not be obtained from the absorption spectra because of the presence of background signal impacts of HEG lipid bilayers. Second derivative spectra were calculated from absorption spectra and fortunately showed three isosbestic derivative points for each NSAID, indicating without doubt that the background signals were entirely eliminated. From the relation between the derivative intensity change (ΔD) induced by addition of HEG membranes, Kps were calculated and obtained with RSD of below 6%. Fractions of partitioned NSAIDs are in well-harmony with that derived from the experimental values. Moreover, validity of the proposed method was confirmed. Conclusively, the second derivative spectrometry has proven to be a facile, reliable and more expeditious method to obtain in vitro Kps of drugs to HEG without previous separation.

Modeling of drug release from biodegradable triple‐layered microparticles

Numerous models that predict drug release from nonerodible reservoir-membrane sphere systems have been presented. Most of these models cater only to a phase of drug release from a constant reservoir. All these models, however, are not applicable to drug release from biodegradable triple-layered microparticle system, in which the drug-loaded core (reservoir) is surrounded by nondrug holding outer layers (membrane). In this article, a mathematical model was developed for ibuprofen release from degradable triple-layered microparticles made of poly(D,L-lactide-co-glycolide, 50:50) (PLGA), poly(L-lactide) (PLLA), and poly(ethylene-co-vinyl acetate, 40 wt % vinyl acetate) (EVA), where ibuprofen was localized within the nonconstant reservoir (EVA core). The model postulated that the drug release through the bulk-degrading PLLA and PLGA layers consisted of two mechanisms: simple diffusional release followed by a degradation-controlled release through a rate-limiting membrane. The proposed model showed very good match with the experimental data of release from microparticles of various layer thicknesses and particle sizes. The underlying drug release mechanisms are dictated by three parameters determined by the model, including constant characteristic of diffusion, end time point of simple diffusion-controlled release and partition coefficient of drug. The presented model is effective for understanding the drug release mechanisms and for the design of this type of dosage form.

Transdermal absorption of memantine – Effect of chemical enhancers, iontophoresis, and role of enhancer lipophilicity

The transdermal administration of memantine may have advantages with respect to oral therapy when treating advanced stages of Alzheimer’s disease. With the ultimate objective of administrating memantine through a transdermal patch, the absorption of the drug across skin was evaluated by means of in vitro permeation studies. The effect of several chemical enhancers was studied in order to enhance percutaneous absorption of the memantine. The iontophoretic transdermal transport of memantine hydrochloride using a current density of 0.5mA/cm2 was also investigated. Results demonstrated that pre-treatment of the skin with R-(+)-limonene, laurocapram, decenoic acid, or oleic acid produced a statistically significant increment in the transdermal flux of memantine hydrochloride with respect to the control. Iontophoresis exhibited the greatest ability to enhance the flux of drug with respect to the control; nevertheless, the results obtained with R-(+)-limonene indicate that this compound could be of great use as a percutaneous enhancer in a memantine transdermal delivery system. In this study, the relationship between enhancement activity and lipophilicity was also studied. Satisfactory correlations have been obtained between the optimum lipophilicity of the enhancer and n-octanol/water partition coefficients of drugs. This relationship is a very useful tool that could allow to reduce time and to optimize the selection of appropriate enhancers for transdermal formulations.

Physiologically Based Pharmacokinetic Tissue Compartment Model Selection in Drug Development and Risk Assessment

A well-stirred tank (WST) has been the predominant flow-limited tissue compartment model in physiologically based pharmacokinetic (PBPK) modeling. Recently, we developed a two-region asymptotically reduced (TAR) PBPK tissue compartment model through an asymptotic approximation to a two-region vascular-extravascular system to incorporate more biophysical detail than the WST model. To determine the relevance of a flow-limited TAR (F-TAR) approach, 75 structurally diverse drugs were evaluated herein using a priori predicted tissue:plasma partition coefficients along with hybrid and whole-body PBPK of eight rat tissues to determine the impact of model selection on simulation and optimization. Simulations showed that the F-TAR model significantly improved the ability to predict drug exposure, with hybrid and whole-body WST model error approaching 50% for tissues with larger vascular volumes. When optimization was used to fit F-TAR and WST models to pseudo data, WST-optimized drug partition coefficients more appropriately represented curve-fitting parameters rather than biophysically meaningful partition coefficients. Median F-TAR-optimized error ranged from -0.4% to +0.3%, whereas WST-optimized median error ranged from -22.2% to +1.8%. These studies demonstrated that the use of F-TAR represents a more accurate, biophysically realistic PBPK tissue model for predicting tissue exposure to drug and that it should be considered for use in drug development and regulatory review.

Study on partition and release of molecules in superabsorbent thermosensitive nanocomposites

Nanocomposites are synthesized by in-situ polymerization of conducting polymers: polyaniline (PANI) or poly(N-methylaniline) (PNMANI) inside thermosensitive hydrogels based on poly(N-isopropylacrylamide) (PNIPAM) homopolymer and copolymers with 2-acrylamido-2-methyl propane sulfonic acid (AMPS). Large swelling capacities (up to 25,000%) are measured for a nanocomposite based on a hydrogel containing only 2% of anionic monomer units. Dynamic swelling experiences show that water intake follows a non-Fickian mechanism. Drug release kinetics and partition coefficients ( K) of model compounds (Methyl Orange, Ruthenium-tris(2,2′-bipyridyl) ion, Dansyl Chloride) and pharmaceuticals (Tryptophan, Propranolol Chloride, Riboflavin) were measured. Log K values were determined by UV–visible or fluorescence spectroscopy, below and above the phase transition temperature of the materials. Large values of log K (>3) were measured for tryptophan while the same material shows small log K (1.5) for riboflavin. Depending on the nanocomposite/solute interactions, the partition increases or decreases upon phase transition. Several nanocomposites studied show potential for application in the separation of solutes or water purification. Additionally, it is possible to synthesize nanocomposites which show different partition coefficients for the same molecule. In that way, a specific pattern could be measured for each molecule allowing its identification.

A Physiologically Based Pharmacokinetic Model for Absorption and Distribution of Imatinib in Human Body

E-mail: dohyun.kim@kaist.ac.krReceived August 27, 2011, Accepted September 13, 2011A whole body physiologically based pharmacokinetic (PBPK) model was applied to investigate absorption,distribution, and physiologic variations on pharmacokinetics of imatinib in human body. Previously publishedpharmacokinetic data of the drug after intravenous (i.v.) infusion and oral administration were simulated by thePBPK model. Oral dose absorption kinetics were analyzed by adopting a compartmental absorption and transitmodel in gut section. Tissue/plasma partition coefficients of drug after i.v. infusion were also used for oraladministration. Sensitivity analysis of the PBPK model was carried out by taking parameters that werecommonly subject to variation in human. Drug concentration in adipose tissue was found to be higher thanthose in other tissues, suggesting that adipose tissue plays a role as a storage tissue for the drug. Variations ofmetabolism in liver, body weight, and blood/plasma partition coefficient were found to be important factorsaffecting the plasma concentration profile of drug in human body.Key Words : Physiologically based pharmacokinetic model, Imatinib, Compartmental absorption and transitmodel, Plasma concentration profileIntroductionImatinib (Glivec) is a tyrosin kinase inhibitor regulatingsignal transduction and is approved for the treatment ofchronic myeloid leukemia (CML) and gastrointestinalstromal tumor.

Influence of drug partition coefficient and pH value of sink solution on permeation from porous thick-walled ethyl cellulose microcapsules.

The permeation of barbitone sodium, benzoic acid, and salicylic acid from microcapsules into aqueous medium has been examined at different pH values. The apparent diffusion coefficients of drugs were linearly proportional to the ethyl cellulose/water partition coefficient of drugs, and the straight line parameters were dependent upon volume fractions of water-filled pores (i.e. capsule size), testifying to a previously proposed mechanism of drug permeation. The rate of drug permeation was also a function of the pH-value of the surrounding sink solution; the period of zero order release was longer at low pH because of the change of drug partition or solubility or both.

Effect of a second solubilizate on the partition coefficient of drugs in micellar solution and their permeation rate across an artificial membrane.

The distribution of a solubilizate between micelles and the aqueous phase does not obey a simple partition law when a second solubilizate species is present. Alterations of the apparent partition coefficient cannot be explained in terms of a simple displacement mechanism, following the interaction of both solubilizates with the same site of the micelle. A non linear increase in solubilizate association to micelles following an increase in surfactant concentration is observed in the presence of a second solubilizate. A depression in the cloud point temperature follows the addition of a second species and is such that cannot be interpreted as a simple additive effect. Alteration of the apparent partition coefficient in a miscellar solution has an effect on the permeation rate of the solubilizate across an artificial membrane. Biopharmaceutical implications are discussed.

A membrane model of the human oral mucosa as derived from buccal absorption performance and physicochemical properties of the beta-blocking drugs atenolol and propranolol.

The buccal absorption characteristics and physicochemical properties of the beta-adrenoceptor blocking agents propranolol and atenolol have been investigated to evaluate their permeation properties across biological lipid membranes. The dissociation constants, solubilities of free base, and n-heptane partition coefficients show that propranolol in its unionized form is much more lipophilic than atenolol, both drugs being bases with a similar pKa. Buccal absorption was studied under conditions of varying drug concentration, contact time, and pH, and controlled through the use of a non-absorbable marker. The absorption findings are in general agreement with the pH-partition theory. A new compartmental diffusional model that includes membrane storage and a hypothetical "aqueous pH-buffering surfaces system" allowed a more exhaustive interpretation to be made. A method for the estimation of the intrinsic pH and buffer capacity of the postulated surface system from drug pH-absorption data and partition coefficients alone is described. With human oral mucosa the intrinsic pH was near 6.7, and the buffering capacity of the system (expressed as the ratio deltapH/deltapH eff) about 2.86. The method was validated using published absorption data from the rat small intestine. Absorption of unionized drug through pores is shown to be negligible in the buccal absorption situation. The time course of absorption suggests membrane storage of lipophilic compounds; the in vivo partition coefficient of unionized propranolol relative to the mucous membrane could be calculated for the peusdo-steady state of absorption, i.e. the partition equilibrium between mouth content and membrane, to be approximately 776; this value is of the same order as the in vitro partition coefficient for the erythrocyte/plasma system. The lipid biophase of the buccal membrane is estimated semiquantitatively to be of intermediate polarity (epsilon = 3-4).

Penetration of β-Blockers through Ocular Membranes in A1bino Rabbits

Abstract The purpose of this study was to investigate the barrier properties of ocular membranes for controlling the extent and pathway of ocular absorption of instilled β-blockers. The penetration of β-blockers was measured across the isolated corneal, conjunctival and scleral membranes of the albino rabbit using a two-chamber glass diffusion cell. β-Blockers tested were atenolol, carteolol, tilisolol, timolol and befunolol. Corneal penetration of befunolol was much higher than that of atenolol. Scraping the epithelium increased corneal penetration of β-blockers. Conjunctival membranes showed higher permeability than corneal and scleral membranes. The penetration parameters were estimated according to Fick's equation. The corneal permeability coefficient showed an apparent linear relationship with penetrant lipophilicity. The lipophilic character of the corneal barrier was determined by the partition coefficient of drug to corneal surface, not by the diffusion coefficient. Conjunctival and scleral permeability coefficients were not determined by the lipophilicity of β-blockers. These results indicate that the conjunctiva, sclera and cornea of the rabbit eye are sufficiently different in permeation character to control the extent and pathway for ocular absorption.

Morphine Brain Pharmacokinetics at Very Low Concentrations Studied with Accelerator Mass Spectrometry and Liquid Chromatography-Tandem Mass Spectrometry

Morphine has been predicted to show nonlinear blood-brain barrier transport at lower concentrations. In this study, we investigated the possibility of separating active influx of morphine from its efflux by using very low morphine concentrations and compared accelerator mass spectrometry (AMS) with liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a method for analyzing microdialysis samples. A 10-min bolus infusion of morphine, followed by a constant-rate infusion, was given to male rats (n = 6) to achieve high (250 ng/ml), medium (50 ng/ml), and low (10 ng/ml) steady-state plasma concentrations. An additional rat received infusions to achieve low (10 ng/ml), very low (2 ng/ml), and ultralow (0.4 ng/ml) concentrations. Unbound morphine concentrations from brain extracellular fluid and blood were sampled by microdialysis and analyzed by LC-MS/MS and AMS. The average partition coefficient for unbound drug (K(p,uu)) values for the low and medium steady-state levels were 0.22 ± 0.08 and 0.21 ± 0.05, respectively, when measured by AMS [not significant (NS); p = 0.5]. For the medium and high steady-state levels, K(p,uu) values were 0.24 ± 0.05 and 0.26 ± 0.05, respectively, when measured by LC-MS/MS (NS; p = 0.2). For the low, very low, and ultralow steady-state levels, K(p,uu) values were 0.16 ± 0.01, 0.16 ± 0.02, and 0.18 ± 0.03, respectively, when measured by AMS. The medium-concentration K(p,uu) values were, on average, 16% lower when measured by AMS than by LC-MS/MS. There were no significant changes in K(p,uu) over a 625-fold concentration range (0.4-250 ng/ml). It was not possible to separate active uptake transport from active efflux using these low concentrations. The two analytical methods provided indistinguishable results for plasma concentrations but differed by up to 38% for microdialysis samples; however, this difference did not affect our conclusions.