'Albumin-mediated transport phenomenon' observed for ligands with high membrane permeability. Effect of the unstirred water layer in the Disse's space of rat liver

Exploring the Boundaries for In Vitro-In Vivo Extrapolation: Use of Isolated Rat Hepatocytes in Co-culture and Impact of Albumin Binding Properties in the Prediction of Clearance of Various Drug Types

Prediction of hepatic clearance of drugs (via uptake or metabolism) from in vitro systems continues to be problematic, particularly when plasma protein binding is high. The following work explores simultaneous assessment of both clearance processes, focusing on a commercial hepatocyte-fibroblast co-culture system (HμREL) over a 24-hour period using six probe drugs (ranging in metabolic and transporter clearance and low-to-high plasma protein binding). A rat hepatocyte co-culture assay was established using drug depletion (measuring both medium and total concentrations) and cell uptake kinetic analysis, both in the presence and absence of plasma protein (1% bovine serum albumin). Secretion of endogenous albumin was monitored as a marker of viability, and this reached 0.004% in incubations (at a rate similar to in vivo synthesis). Binding to stromal cells was substantial and required appropriate correction factors. Drug concentration-time courses were analyzed both by conventional methods and a mechanistic cell model prior to in vivo extrapolation. Clearance assayed by drug depletion in conventional suspended rat hepatocytes provided a benchmark to evaluate co-culture value. Addition of albumin appeared to improve predictions for some compounds (where fraction unbound in the medium is less than 0.1); however, for high-binding drugs, albumin significantly limited quantification and thus predictions. Overall, these results highlight ongoing challenges concerning in vitro hepatocyte system complexity and limitations of practical expediency. Considering this, more reliable measurement of hepatically cleared compounds seems possible through judicious use of available hepatocyte systems, including co-culture systems, as described herein; this would include those compounds with low metabolic turnover but high active uptake clearance. SIGNIFICANCE STATEMENT: Co-culture systems offer a more advanced tool than standard hepatocytes, with the ability to be cultured for longer periods of time, yet their potential as an in vitro tool has not been extensively assessed. We evaluate the strengths and limitations of the HμREL system using six drugs representing various metabolic and transporter-mediated clearance pathways with various degrees of albumin binding. Studies in the presence/absence of albumin allow in vitro-in vivo extrapolation and a framework to maximize their utility.

Plasma Protein-Mediated Uptake and Contradictions to the Free Drug Hypothesis: A Critical Review

According to the free drug hypothesis (FDH), only free, unbound drug is available to interact with biological targets. This hypothesis is the fundamental principle that continues to explain the vast majority of all pharmacokinetic and pharmacodynamic processes. Under the FDH, the free drug concentration at the target site is considered the driver of pharmacodynamic activity and pharmacokinetic processes. However, deviations from the FDH are observed in hepatic uptake and clearance predictions, where observed unbound intrinsic hepatic clearance (CL_int,u) is larger than expected. Such deviations are commonly observed when plasma proteins are present and form the basis of the so-called plasma protein-mediated uptake effect (PMUE). This review will discuss the basis of plasma protein binding as it pertains to hepatic clearance based on the FDH, as well as several hypotheses that may explain the underlying mechanisms of PMUE. Notably, some, but not all, potential mechanisms remained aligned with the FDH. Finally, we will outline possible experimental strategies to elucidate PMUE mechanisms. Understanding the mechanisms of PMUE and its potential contribution to clearance underprediction is vital to improving the drug development process.

Consideration of albumin-mediated hepatic uptake for highly protein-bound anionic drugs: Bridging the gap of hepatic uptake clearance between in vitro and in vivo

The accuracy in predicting in vivo hepatic clearance of drugs from in vitro data (often termed as in vitro-to-in vivo extrapolation, IVIVE) has improved in part by applying the extended-clearance concept that considers the interplay between hepatic metabolism and uptake/efflux processes. However, the IVIVE-based prediction performs poorly in predicting the hepatic uptake clearance of highly albumin-bound anionic drugs. Their hepatic uptake clearances tend to be much higher than expected based on the free-drug theory. Such observation can be attributable to a phenomenon called albumin-mediated hepatic uptake, for which various models have been thus far proposed. Our group has been applying a facilitated-dissociation model, which assumes the enhanced dissociation of the drug-albumin complex upon interaction with the cell surface. By considering the albumin-mediated hepatic uptake (using the facilitated-dissociation model or alternative kinetic models), a number of investigations demonstrated the improvement in the prediction accuracy for the hepatic clearance of highly protein-bound anionic drugs that are substrates for hepatic uptake transporters. This review summarizes the reported kinetic analyses of the albumin-mediated hepatic uptake of highly albumin-bound drugs concerning the IVIVE and the clinical and physiological relevance.

Impact of Plasma Protein Binding in Drug Clearance Prediction: A Data Base Analysis of Published Studies and Implications for In Vitro-In Vivo Extrapolation

Plasma protein-mediated uptake (PMU) and its effect on clearance (CL) prediction have been studied in various formats; however, a comprehensive analysis of the overall impact of PMU on CL parameters from hepatocyte assays (routinely used for IVIVE) has not previously been performed. The following work collated data reflecting the effect of PMU for 26 compounds with a wide variety of physicochemical, drug, and in vivo CL properties. PMU enhanced the unbound intrinsic clearance in vitro (CLint,u in vitro) beyond that conventionally calculated using fraction unbound and was correlated with the unbound fraction of drug in vitro and in plasma (fup) and absolute unbound intrinsic clearance in vivo (CLint,u in vivo) in both rat and human hepatocytes. PMU appeared to be more important for highly bound (fup < 0.1) and high CLint,u in vivo drugs. These trends were independent of species, assay conditions, ionization, and extended clearance classification system group, although the type of plasma protein used in in vitro assays may require further investigation. Such generalized trends (spanning fup 0.0008-0.99) may suggest a generic mechanism behind PMU; however, multiple drug-dependent mechanisms are also possible. Using the identified relationship between the impact of PMU on CLint,u in vitro and fup, PMU-enhanced predictions of CLint,u in vivo were calculated for both transporter substrates and metabolically cleared drugs. PMU was accurately predicted, and incorporation of predicted PMU improved the IVIVE of hepatic CL, with an average fold error of 1.17 and >50% of compounds predicted within a 2-fold error for both rat and human data sets (n ≥ 100). SIGNIFICANCE STATEMENT: Current strategies for prediction of hepatic clearance from in vitro data are recognized to be inaccurate, but they do not account for PMU. The impact of PMU on CLint,u in vitro is wide ranging and can be predicted based on fraction unbound in plasma and applied to CLint,u in vitro values obtained by standard procedures in the absence of plasma protein. Such PMU-enhanced predictions improved IVIVE, and future studies may easily incorporate this PMU relationship to provide more accurate IVIVE.

Hepatic transporter-mediated pharmacokinetic drug-drug interactions: Recent studies and regulatory recommendations

Transporter-mediated drug-drug interactions are one of the major mechanisms in pharmacokinetic-based drug interactions and correspondingly affecting drugs' safety and efficacy. Regulatory bodies underlined the importance of the evaluation of transporter-mediated interactions as a part of the drug development process. The liver is responsible for the elimination of a wide range of endogenous and exogenous compounds via metabolism and biliary excretion. Therefore, hepatic uptake transporters, expressed on the sinusoidal membranes of hepatocytes, and efflux transporters mediating the transport from hepatocytes to the bile are determinant factors for pharmacokinetics of drugs, and hence, drug-drug interactions. In parallel with the growing research interest in this area, regulatory guidances have been updated with detailed assay models and criteria. According to well-established preclinical results, observed or expected hepatic transporter-mediated drug-drug interactions can be taken into account for clinical studies. In this paper, various methods including in vitro, in situ, in vivo, in silico approaches, and combinational concepts and several clinical studies on the assessment of transporter-mediated drug-drug interactions were reviewed. Informative and effective evaluation by preclinical tools together with the integration of pharmacokinetic modeling and simulation can reduce unexpected clinical outcomes and enhance the success rate in drug development.

An examination of protein binding and protein-facilitated uptake relating to in vitro-in vivo extrapolation

As explained by the free drug theory, the unbound fraction of drug has long been thought to drive the efficacy of a molecule. Thus, the fraction unbound term, or f_u, appears in equations for fundamental pharmacokinetic parameters such as clearance, and is used when attempting in vitro to in vivo extrapolation (IVIVE). In recent years though, it has been noted that IVIVE does not always yield accurate predictions, and that some highly protein bound ligands have more efficient uptake than can be explained by their unbound fractions. This review explores the evolution of f_u terms included when implementing IVIVE, the concept of protein-facilitated uptake, and the mechanisms that have been proposed to account for facilitated uptake.

The Phenomenon of Albumin-Mediated Hepatic Uptake of Organic Anion Transport Polypeptide Substrates: Prediction of the In Vivo Uptake Clearance from the In Vitro Uptake by Isolated Hepatocytes Using a Facilitated-Dissociation Model

The effects of bovine serum albumin and human serum albumin on the unbound hepatic uptake clearance (PSu,inf) of the organic anion-transporting polypeptide substrates 1-anilino-8-naphthalene sulfonate (ANS) and pitavastatin (PTV) were determined using primary cultured rat hepatocytes and isolated human hepatocytes, respectively. The PSu,inf value of hepatocytes was estimated by dividing the initial uptake rate of these anions by their unbound concentrations. The PSu,inf values for ANS and PTV were enhanced in the presence of albumin, thereby demonstrating the phenomenon of "albumin-mediated" hepatic uptake. We previously constructed a "facilitated-dissociation" model, in which the interaction of the ligand-albumin complex with the cell surface enhanced the dissociation of that complex to provide unbound ligand for uptake to the hepatocytes [J Pharmacokinet Biopharm 16:165-181 (1988)]. That model was able to describe accurately the relationship between the enhancement of the PSu,inf values and the albumin concentration. By considering the enhancement of hepatic uptake clearance by albumin using this facilitated-dissociation model, we could predict accurately the PSu,inf in vivo from that obtained in isolated hepatocytes. In the light of these findings, we suggest that the facilitated-dissociation model is applicable to describing the phenomenon of albumin-mediated hepatic uptake via organic anion transporters and to evaluating hepatic uptake clearance in vivo.

Importance of the Unstirred Water Layer and Hepatocyte Membrane Integrity In Vitro for Quantification of Intrinsic Metabolic Clearance

Prediction of clearance-a vital component of drug discovery-remains in need of improvement and, in particular, requires more incisive assessment of mechanistic methodology in vitro, according to a number of recent reports. Although isolated hepatocytes have become an irreplaceable standard system for the measurement of intrinsic hepatic clearance mediated by active uptake transport and metabolism, the lack of prediction reliability appears to reflect a lack of methodological validation, especially for highly cleared drugs, as we have previously shown. Here, novel approaches were employed to explore fundamental experimental processes and associated potential limitations of in vitro predictions of clearance. Rat hepatocytes deemed nonviable by trypan blue staining showed undiminished metabolic activity for probe cytochrome P450 (P450) substrates midazolam and propranolol; supplementation with NADPH enhanced these activities. Extensive permeabilization of the plasma membrane using saponin showed either full or minimal P450 activity, depending on the presence or absence of 1 mM NADPH, respectively. The shaking of incubations facilitated P450 metabolic rates up to 5-fold greater than static incubation, depending on intrinsic clearance, indicating the critical influence of the unstirred water layer (UWL). Permeabilization allowed static incubation metabolic rates to approach those of shaking for intact cells, indicating an artificially induced breakdown of the UWL. Permeabilization combined with shaking allowed an increased metabolic rate for saquinavir, resolving the membrane permeability limitation for this drug. These findings advance the interpretation of the rate-limiting processes involved in intrinsic clearance measurements and could be critical for successful in vitro prediction.

Comparison of Methods for Estimating Unbound Intracellular-to-Medium Concentration Ratios in Rat and Human Hepatocytes Using Statins

It is essential to estimate concentrations of unbound drugs inside the hepatocytes to predict hepatic clearance, efficacy, and toxicity of the drugs. The present study was undertaken to compare predictability of the unbound hepatocyte-to-medium concentration ratios (Kp,uu) by two methods based on the steady-state cell-to-medium total concentration ratios at 37°C and on ice (Kp,uu,ss) and based on their initial uptake rates (Kp,uu,V0). Poorly metabolized statins were used as test drugs because of their concentrative uptake via organic anion-transporting polypeptides. Kp,uu,ss values of these statins provided less interexperimental variation than the Kp,uu,V0 values, because only data at longer time are required for Kp,uu,ss Kp,uu,V0 values for pitavastatin, rosuvastatin, and pravastatin were 1.2- to 5.1-fold Kp,uu,ss in rat hepatocytes; Kp,uu,V0 values in human hepatocytes also tended to be larger than corresponding Kp,uu,ss To explain these discrepancies, theoretical values of Kp,uu,ss and Kp,uu,V0 were compared with true Kp,uu (Kp,uu,true), considering the inside-negative membrane potential and ionization of the drugs in hepatocytes and medium. Membrane potentials were approximately -30 mV in human hepatocytes at 37°C and almost abolished on ice. Theoretical equations considering the membrane potentials indicate that Kp,uu,ss values for the statins are 0.85- to 1.2-fold Kp,uu,true, whereas Kp,uu,V0 values are 2.2- to 3.1-fold Kp,uu,true, depending on the ratio of the passive permeability of the ionized to nonionized forms. In conclusion, Kp,uu,ss values of anions are similar to Kp,uu,true when the inside-negative membrane potential is considered. This suggests that Kp,uu,ss is preferable for estimating the concentration of unbound drugs inside the hepatocytes.

First observation of diffusion-limited plant root phosphorus uptake from nutrient solution

Diffusion towards the root surface has recently been shown to control the uptake of metal ions from solutions. The uptake flux of phosphorus (P) from solutions often approaches the maximal diffusion flux at low external concentrations, suggesting diffusion-controlled uptake also for P. Potential diffusion limitation in P uptake from nutrient solutions was investigated by measuring P uptake of Brassica napus from solutions using P-loaded Al(2) O(3) nanoparticles as mobile P buffer. At constant, low free phosphate concentration, plant P uptake increased up to eightfold and that of passive, diffusion-based samplers up to 40-fold. This study represents the first experimental evidence of diffusion-limited P uptake by plant roots from nutrient solution. The Michaelis constant of the free phosphate ion obtained in unbuffered solutions (K(m) = 10.4 µmol L(-1) ) was 20-fold larger than in the buffered system (K(m) ∼0.5 µmol L(-1) ), indicating that K(m) s determined in unbuffered solutions do not represent the transporter affinity. Increases in the P uptake efficiency of plants by increasing the carrier affinity are therefore unlikely, while increased root surface area or exudation of P-solubilizing compounds are more likely to enhance P uptake. Furthermore, our results highlight the important role natural nanoparticles may have in plant P nutrition.

Transmembrane diffusion of gemcitabine by a nanoparticulate squalenoyl prodrug: an original drug delivery pathway

We have designed an amphiphilic prodrug of gemcitabine (dFdC) by its covalent coupling to a derivative of squalene, a natural lipid. The resulting bioconjugate self-assembled spontaneously in water as nanoparticles that displayed a promising in vivo anticancer activity. The aim of the present study was to provide further insight into the in vitro subcellular localization and on the metabolization pathway of the prodrug. Cells treated with radiolabelled squalenoyl gemcitabine (SQdFdC) were studied by differential detergent permeation, and microautography coupled to fluorescent immunolabeling and confocal microscopy. This revealed that the bioconjugate accumulated within cellular membranes, especially in those of the endoplasmic reticulum. Radio-chromatography analysis proved that SQdFdC delivered dFdC directly in the cell cytoplasm. Mass spectrometry studies confirmed that gemcitabine was then either converted into its biologically active triphosphate metabolite or exported from the cells through membrane transporters. To our knowledge, this is the first description of such an intracellular drug delivery pathway. In vitro cytotoxicity assays revealed that SQdFdC was more active than dFdC on a transporter-deficient human resistant leukemia model, which was explained by the subcellular distribution of the drugs and their metabolites. The squalenoylation drug delivery strategy might, therefore, dramatically improve the efficacy of gemcitabine on transporter-deficient resistant cancer in the clinical context.

Structural and ligand-binding properties of serum albumin species interacting with a biomembrane interface

In the process of drug development, preclinical testing using experimental animals is an important aspect, for verification of the efficacy and safety of a drug. Serum albumin is a major binding protein for endogenous and exogenous ligands and regulates their distribution in various tissues. In this study, the structural and drug-binding properties of albumins on a biomembrane surface were investigated using reverse micelles as a model membrane. In reverse micelles, the secondary structures of all albumins were found, to varying degrees, to be intermediate between the native and denatured states. The tertiary structures of human and bovine albumin were similar to those of the native and intermediate states, respectively, whereas those of the dog, rabbit, and rat were in a denatured state. Thus, bovine albumin is an appropriate model for studying structural changes in human albumin in a membrane-water phase. Binding studies also showed the presence of species difference in the change in binding capacity of albumins during their interaction with reverse micelles. Among the albumins, rat albumin appears to be a good model for the protein-mediated drug uptake of human albumin in a biomembrane environment. These findings are significant in terms of the appropriate extrapolation of pharmacokinetics and pharmacodynamics data in various animals to humans.

Human red blood cells as a natural flavonoid reservoir

Quercetin is rapidly and avidly taken up by human red blood cells (RBC) via a passive diffusion mechanism, driven by flavonoid binding to haemoglobin and resulting in an almost quantitative accumulation of the flavonoid. Heamoglobin-free resealed ghosts accumulated quercetin exclusively in the membrane fraction. Cell-associated quercetin was biological active and could be quantitatively utilised to support the reduction of extracellular oxidants mediated by a transplasma-membrane oxido-reductase. Additional experimental evidence revealed that quercetin uptake declined in the presence of albumin and that, under these conditions, the amount of cell-associated quercetin is enhanced by increasing the RBC number. Quercetin release from flavonoid-preloaded RBC was observed only in the presence of albumin (or in human plasma) and this response was progressively inhibited upon incubation in solutions containing albumin previously exposed to increasing concentrations of quercetin and cleared of the unbound fraction of the flavonoid. Furthermore, exposure to quercetin pre-saturated albumin promoted accumulation of the flavonoid in fresh RBC and this response was a direct function of the extent of albumin saturation. These results, indicating a flow of quercetin from albumin to haemoglobin, and vice versa, are therefore consistent with the possibility that human RBC play a pivotal role in the distribution and bioavailability of circulating flavonoids.

An equivalent length model of microdialysis sampling

One of the critical issues in microdialysis sampling is how to predict the extraction fraction (E(d)), based on transport properties of analytes in both tissues and probes. A one-dimensional (1-D) model has been used widely in previous studies to predict E(d) at the steady state. However, this model is valid only for long probes. To this end, an equivalent length (EL) model was developed for probes with any length used in experiments. The key idea in the model was to replace the probe length (L) in the 1-D model with an equivalent length (L(E)) when calculating transport resistance in surrounding tissues. The length difference, (L(E)-L), was assumed to be proportional to the penetration depth of analytes (Gamma). The proportionality constant (lambda) was determined through minimizing the errors in predicted E(d). We found that, the EL model could accurately predict E(d) when lambda=0.369. The maximum error in EL model predictions was <6%, for model constants varying in the same ranges as those in microdialysis experiments. This error was one order of magnitude smaller than that in 1-D model predictions.

Effects of Serum Albumin and Liver Cytosol on CYP2C9- and CYP3A4-mediated Drug Metabolism

To investigate whether the free-drug theory is accurate in that only unbound drug is available for drug metabolism or enzyme inhibition. The effect of addition of rat liver cytosol to an in vitro system using human liver microsomes was examined by measuring the catalytic activities of CYP2C9 (tolbutamide and diclofenac) and CYP3A4 (terfenadine). And, the results were compared with those obtained when human serum albumin (HSA) was added to microsomes as far as unbound drug concentrations were concerned. After addition of rat liver cytosol, the unbound Km value (Km,u) for terfenadine metabolism by CYP3A4, and the unbound Ki value of miconazole (Ki,u) for CYP2C9 were smaller than for the controls. Addition of HSA resulted in smaller Km,u values for diclofenac and terfenadine metabolism by CYP2C9 and CYP3A4, respectively, and the Ki,u value for ketoconazole inhibition of CYP3A4 was also reduced. These results suggest protein-facilitated effects on drug metabolism and enzyme inhibition for both CYP2C9 and CYP3A4. However, no protein-facilitated drug metabolism was observed for tolbutamide in the presence of HSA or cytosol, or for diclofenac in the presence of cytosol. Protein-facilitated enzyme inhibition did not occur with miconazole in the presence of HSA or with ketoconazole in the presence of rat liver cytosol. Protein-facilitated metabolism and enzyme inhibition were observed for CYP2C9 and CYP3A4 in five cases but there was no obvious pattern of enzyme, substrate, or binding protein specificity. Further investigations are necessary to clarify the relevance of these results to in vivo observations.

Flavonoids as antioxidants

Flavonoids are phenolic substances isolated from a wide range of vascular plants, with over 8000 individual compounds known. They act in plants as antioxidants, antimicrobials, photoreceptors, visual attractors, feeding repellants, and for light screening. Many studies have suggested that flavonoids exhibit biological activities, including antiallergenic, antiviral, antiinflammatory, and vasodilating actions. However, most interest has been devoted to the antioxidant activity of flavonoids, which is due to their ability to reduce free radical formation and to scavenge free radicals. The capacity of flavonoids to act as antioxidants in vitro has been the subject of several studies in the past years, and important structure-activity relationships of the antioxidant activity have been established. The antioxidant efficacy of flavonoids in vivo is less documented, presumably because of the limited knowledge on their uptake in humans. Most ingested flavonoids are extensively degraded to various phenolic acids, some of which still possess a radical-scavenging ability. Both the absorbed flavonoids and their metabolites may display an in vivo antioxidant activity, which is evidenced experimentally by the increase of the plasma antioxidant status, the sparing effect on vitamin E of erythrocyte membranes and low-density lipoproteins, and the preservation of erythrocyte membrane polyunsaturated fatty acids. This review presents the current knowledge on structural aspects and in vitro antioxidant capacity of most common flavonoids as well as in vivo antioxidant activity and effects on endogenous antioxidants.

Differences in the hepatobiliary transport of two quinolone antibiotics, grepafloxacin and lomefloxacin, in the rat

The biliary excretion of grepafloxacin (GPFX) was compared with that of lomefloxacin (LFLX) in rats. The biliary clearances (Cl(plasma)(bile)) of GPFX was 2.9 times greater than LFLX based on the plasma concentration reached during constant intravenous (i.v.) infusion. The liver-plasma unbound concentration ratio, K(pu), of GPFX (1.7) was also higher than that of LFLX (0.7). The hepatic uptake clearance, assessed from an integration plot analysis, of GPFX was comparable with the hepatic blood flow rate, and 1.5 times that of LFLX, indicating that membrane transport in the uptake process is more efficient for GPFX. This was also supported by the difference between the uptake clearance of GPFX and LFLX in isolated rat hepatocytes. The bile-liver unbound concentration ratio of GPFX and LFLX was approximately 6 and 3, respectively, and the biliary clearance based on the unbound liver concentration of GPFX was 1.8 times that of LFLX. These results suggest that the concentrative transport of GPFX also across the canalicular membrane was more efficient than that of LFLX. Thus, the membrane transport activity via both sinusoidal and canalicular membranes determines the net excretion of each compound.

The size of the unstirred layer as a function of the solute diffusion coefficient

By monitoring the concentration distribution of several solutes that are diffusing at the same time under given mixing conditions, it was established that the unstirred layer (USL) has no clearly defined boundary. For the cases of solute permeation and water movement across planar bilayer lipid membranes, respectively, experiments carried out with double-barreled microelectrodes have shown that the thickness of the USL depends on which species is diffusing. Small molecules with a larger diffusion coefficient encounter an apparently thicker USL than larger molecules with a smaller diffusion coefficient. The ratio of the USL thicknesses of two different substances is equal to the third root of the ratio of the respective diffusion coefficients. This experimental finding is in good agreement with theoretical predictions from the theory of physicochemical hydrodynamics.

Factors that influence microdialysis recovery. Comparison of experimental and theoretical microdialysis recoveries in rat liver

Inhibition of metabolic processes was used to assess the possible change in the recovery of material from a microdialysis probe implanted in vivo in rat liver. Phenacetin and antipyrine were perfused through a microdialysis probe implanted in the liver. Inhibition of phenacetin and antipyrine metabolism was achieved through an iv bolus dose of the cytochrome P450 suicide substrate 1-aminobenzotriazole (1-ABT). 1-ABT inhibited phenacetin clearance by 90%, thus also inhibiting metabolism by 90%. There was no statistical difference in the recovery of phenacetin and antipyrine across the microdialysis membrane in the liver between the control and metabolically inhibited animals. Partial differential equations were developed that describe the transport of analyte from the microdialysis probe and solved by an implicit finite-difference method to aid in the understanding of the above-mentioned microdialysis experiments. Predictions of microdialysis recovery obtained from the numerical model are compared with those found experimentally. The model could predict trends in the data, but not the actual experimental values. This suggests that predictions from this microdialysis model are essentially heuristic and as presently formulated can be used only to show mechanisms that affect recovery, but they cannot be used to accurately predict recovery. Prediction of actual recovery requires knowledge of the values of the parameters that describe chemical properties such as the in vivo diffusion coefficient, metabolism rate constant, and capillary exchange rate constant. For microdialysis experiments performed in the liver, capillary exchange and the rate of liver blood flow appear to be the dominant processes that facilitate net transport from a microdialysis probe rather than metabolic processes. These results indicate that microdialysis recoveries measured after inhibition of a concentration-dependent kinetic process via pharmacological challenge will change only when the kinetic process that is being challenged is large compared to the contribution of all concentration-dependent kinetic processes, including other metabolism routes, capillary exchange, or uptake that remove the analyte from the tissue space. It is concluded that the microdialysis recovery of a substance from the liver is not generally affected by liver metabolism.

The disposition of aspirin and salicylic acid in the isolated perfused rat liver: the effect of normal and retrograde flow on availability and mean transit time

The effect of changing the direction of perfusate flow from anterograde to retrograde on the disposition of acetylsalicylic acid (aspirin) and salicylic acid was studied in the single pass in-situ perfused rat liver. Mixtures of aspirin, [14C]salicylic acid and the inert reference solute [3H]sucrose were administered as boluses into the liver using red blood cell and albumin-free perfusate media at a flow rate of 30 mL min-1/liver. Hepatic availability (F), mean transit time (MTT) and normalized variance (CV2) for aspirin, preformed [14C]salicylic acid, salicylic acid produced from aspirin in the liver and [3H]sucrose were deduced from the outflow concentration profiles using statistical moment analysis. The values for F, MTT and CV2 for the solutes under anterograde perfusion were: aspirin (0.73 +/- 0.04, 15.13 +/- 2.01 s, 0.33 +/- 0.09, n = 5), preformed [14C]salicylic acid (1.05 +/- 0.06, n = 12, 43.19 +/- 2.21 s, 1.08 +/- 0.08, n = 5), salicylic acid from aspirin (0.33 +/- 0.05, 42.82 +/- 9.16 s, 0.73 +/- 0.10, n = 5) and [3H]sucrose (1.05 +/- 0.05, 16.88 +/- 0.77 s, 0.74 +/- 0.10, n = 5). The corresponding values for retrograde perfusions were: aspirin (0.73 +/- 0.02, 17.41 +/- 3.06 s, 0.32 +/- 0.09, n = 5), preformed [14C]salicylic acid (1.14 +/- 0.02, 44.42 +/- 3.16 s, 0.95 +/- 0.07, n = 5), salicylic acid from aspirin (0.33 +/- 0.09, 36.47 +/- 10.28 s, 0.58 +/- 0.05, n = 5) and sucrose (1.01 +/- 0.04, 18.08 +/- 1.61 s, 0.76 +/- 0.15, n = 5). No significant differences in F or MTT were apparent between anterograde and retrograde perfusions for all solutes. The MTT and CV2 data for [14C]salicylic acid and salicylic acid produced from aspirin is suggestive of a permeability limitation for salicylic acid transport.

[Role of substitution albumin therapy in drugs, hormones, electrolytes and miscellanous substances transport]

Human serum albumin (HSA) is quantitatively the most important non specific transport protein. HSA binds a wide variety of both endogenous and exogenous ligands. Hypoalbuminaemia may lead to a decreased plasma binding capability of some compounds. Biological/pharmacological consequences depend on the ligand and the target tissue. Many experimental studies suggest that hypoalbuminaemia may influence the metabolism and toxicity of endogenous ligands (bilirubin, metallic ions, oxygen radicals) and the pharmacological effect of some drugs (among others: furosemide, phenytoin, warfarin). The relevance of such information for human surgical situations remains unclear. Clinical studies are scarce and inconclusive. There is a lack of pertinent data supporting the necessity of HSA infusions in order to maintain a minimal plasma concentration and a convenient plasma transport. However, experimental data indicate that major hypoalbuminaemia should be considered with caution.

Applications and prospects for physiologically based pharmacokinetic (PB-PK) models involving pharmaceutical agents

Because of the increasing availability of human liver samples, we are now able to predict in vivo drug disposition in man from in vitro metabolic and binding studies. In this report, we summarize successful attempts to predict in vivo metabolic clearances in animals and humans from in vitro biochemical parameters, using physiologically based pharmacokinetic models. There are still some problems, however, in extrapolating in vivo hepatic metabolism in man from in vitro data obtained using human liver specimens, due to (1) large interindividual differences resulting from genetic polymorphism and/or (2) differences in enzyme activities depending upon the conditions under which liver specimens may have been kept. We propose a possible method to overcome these difficulties by applying the concept of a 'scaling factor'. In addition, we also review several additional factors which should be considered to help achieve more reliable predictions.

Discrepancies in pharmacokinetic parameter estimation between bolus and infusion studies in the perfused rat hindlimb

Isolated, perfused rat hindlimb consists of skeletal muscle, skin, bone, and adipose. Hence, it is a heterogeneous preparation composed of slowly equilibrating tissues of different characteristics and fractional flow rates. This paper shows how caution should be exercised in interpreting the results following bolus administration and subsequent statistical moment analysis of intravascular markers (51Cr-erythrocytes and 125I-albumin) and lipophilic barbiturates. For the intravascular markers, the events in the hindlimb are overshadowed by events in the connecting tubing and cannulas, due to their comparable volumes. For the barbiturates, these estimates appear to apply to short-term effects as the volume estimates obtained following infusion to steady state are greater than after bolus administration. For the extravascular markers, 14C-sucrose, 14C-urea, and 3H-water, no such time dependency was shown. However, it is only from the outflow profiles following bolus administration that events in the tissue beds can be elucidated.

Distribution kinetics of salicylic acid in the isolated perfused rat liver assessed using moment analysis and the two-compartment axial dispersion model

The distribution kinetics of salicylic acid in the single-pass isolated perfused rat liver has been investigated under varying conditions of perfusate flow (15 to 30 ml min-1) and of salicylate perfusate concentration (0, 100, 200 mg l-1) using statistical moment analysis and the two-compartment axial dispersion model. Salicylic acid was not metabolised during the experiment. The perfusate did not contain binding protein. As flow rate was increased, the maximum fraction output per second (f(t)max) increased and the mean transit time (MTTH) decreased, while tmax became shorter for both tritiated water and 14C-salicylic acid. Increasing the salicylate perfusate concentration profoundly affected the frequency outflow profile of 14C-salicylic acid, but not that of tritiated water. The one-compartment axial dispersion model adequately described the frequency outflow profile for tritiated water, whereas the two-compartment form, which incorporates a cellular permeability barrier, provided a better description of the 14C-salicylic acid outflow data. The estimated two-compartment axial dispersion model parameters for 14C-salicylic acid, DN, the dispersion number (0.08 +/- 0.03), k12, the influx rate constant (0.56 +/- 0.04 sec-1) and k21, the efflux rate constant (0.095 +/- 0.01 sec-1) were independent of perfusate flow rate. The in situ permeability-surface area product for 14C-salicylic acid (4.6 +/- 0.7 ml min-1g-1 liver) was in good agreement with literature estimates obtained from in vitro hepatocyte experiments, suggesting that the permeability barrier is at the hepatocyte membrane. Whereas DN and k12 were uninfluenced by, k21 displayed a positive correlation with, salicylate perfusate concentration. This correlation was most likely due to decreased intracellular salicylate binding.

The sinusoidal efflux of dibromosulfophthalein from rat liver is stimulated by albumin, ligandin and fatty acid binding protein but not by other dibromosulfophthalein binding proteins

Organic anions can be excreted from the liver into the bile or back into the general circulation (sinusoidal efflux). It has previously been shown that the net sinusoidal efflux rate of dibromosulfophthalein from the perfused liver into the perfusate is the result of actual efflux from and reuptake into the liver, and can be strongly influenced by the presence of bovine serum albumin in the perfusion medium. The present study investigated whether the influence of albumin on the net sinusoidal efflux process is albumin-specific or whether other binding proteins could have a similar effect on the sinusoidal efflux. Using a single-pass liver perfusion technique and short-lasting (pulse) protein infusions, the stimulatory effect of a wide range of dibromosulfophthalein binding proteins on the sinusoidal efflux process were determined. These experiments showed that all the serum albumins tested as well as the liver cytosolic binding proteins fatty acid binding protein and ligandin (glutathione S-transferase) stimulated this process. The other proteins tested, bovine beta lactoglobulin-b, human gamma globulin and chicken egg lysozyme showed no stimulatory effect, despite relatively high equilibrium binding of dibromosulfophthalein. No clear-cut relationship was found between the equilibrium unbound ligand concentration as measured in perfusate and the stimulatory effect, suggesting absence of equilibrium binding in the sinusoids. Equilibrium binding of dibromosulfophthalein to chicken serum albumin and ligandin as well as the dissociation rate constants were determined in vitro with rapid filtration techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic modeling of the sinusoidal efflux of anionic ligands from the isolated perfused rat liver: the influence of albumin

This study contains a pharmacokinetic analysis on the efflux of organic anions from the liver into the bloodstream (sinusoidal efflux) with specific reference to the influence of albumin. The net sinusoidal efflux rate of dibromosulfophthalein (DBSP) from preloaded livers, being the resultant of sinusoidal efflux and reuptake of ligand by hepatocytes downstream the sinusoid, can be strongly increased by the presence of bovine serum albumin (BSA), a protein having multiple binding sites for DBSP. We previously attributed this effect to a reduction of reuptake through extracellular binding of the organic anion to the protein, rather than to an intrinsic stimulatory effect on the actual membrane transport process from the cells. In the present study we tested this hypothesis using a pharmacokinetic multicompartment liver model. This model resembles the parallel tube model in that the liver is described by several compartments placed in series instead of a single well-stirred compartment and it takes into account rates of dissociation and association in binding to proteins in the sinusoidal space. The model parameters were fitted from the sinusoidal efflux and biliary excretion data from efflux experiments measuring the stimulatory effect of various concentrations of BSA. Equilibrium binding of DBSP to albumin as well as the dissociation rate constant (koff) were determined in vitro with rapid filtration techniques. The experimental data could not be fitted satisfactorily when using the experimentally obtained values of the protein association and dissociation rate constants (kon and koff). However, they could be simulated accurately assuming 16 times higher values for the association and dissociation rate constant compared to those determined in vitro. Time constants of the perfusate flow, liver (re)uptake, and protein association and dissociation indicate that binding equilibrium does not exist within the sinusoids and that, in particular at low protein concentrations, the net sinusoidal efflux rate is association rate-limited: A large fraction of the ligand effluxed from the cell into the median is taken up by the hepatocyte before binding to the proteins occurs. Higher kon and koff values predicted by the model might indicate altered DBSP-albumin binding characteristics upon passage through the liver but alternatively can be explained by an intrinsic effect of albumin on the carrier-mediated efflux process. Efflux experiments showed a marked stimulatory effect of the protein on sinusoidal efflux but only a moderate effect on biliary excretion, despite a strong decrease in liver content. These patterns indicate that sinusoidal efflux and biliary excretion occur from two different intracellular compartments that equilibrate slowly.

Comparison of the hepatic uptake clearances of fifteen drugs with a wide range of membrane permeabilities in isolated rat hepatocytes and perfused rat livers

The hepatic uptake clearances of 15 ligands with a wide range of permeabilities were determined in rats using two techniques: centrifugal filtration with isolated hepatocytes and the multiple indicator dilution (MID) method with isolated perfused livers. Some of the uptake clearance values were taken from the literature. Uptake clearance values obtained from isolated hepatocytes were extrapolated to that per gram liver (PSinf.cell), assuming that 1 g of liver has 1.3 x 10(8) cells. The values of PSinf.cell varied from approximately 0.1 to 72 (mL/min/g liver). The values of PSinf.cell were similar to those (PSinf.MID) determined by the MID method for ligands with uptake clearances below approximately 1 mL/min/g liver. However, for the ligands with larger uptake clearances, the PSinf.MID values were lower than the PSinf.cell values and appeared to reach an upper limit (approx. 15-20 mL/min/g liver). The PSinf.cell values of 1-propranolol, tetraphenylphosphonium (TPP+), and diazepam were 72, 43, and 22 mL/min/g liver, respectively, whereas their uptake clearances (PSinf.MID) determined by the MID method were 4 to 10 times lower. One of the possible mechanisms for this discrepancy is that an unstirred water layer, which may exist in Disse's space in isolated perfused livers (and probably under in vivo condition), limits the hepatic uptake rate of ligands with extremely high membrane permeabilities.