Methods of determining plasma and tissue binding of drugs. Pharmacokinetic consequences

Modeling Drug Albumin Binding Affinity with E-State Topological Structure Representation

The binding affinity to human serum albumin for 94 drugs was modeled with topological descriptors of molecular structure, using as experimental data the HPLC chromatographic retention index [logk(HSA)] on immobilized albumin. The electrotopological state (E-State) along with the molecular connectivity chi indices provided the basis for a satisfactory model: r(2) = 0.77, s = 0.29, q(2) = 0.70, s(press) = 0.33. The 10% leave-group-out (LGO) cross-validation method yielded q(2) (= r(2)(press)) = 0.69. Further, the model was tested on a 10 compound external validation set, yielding a mean absolute error, MAE = 0.31; q(2) (= r(2)(press)) = 0.74. MDL QSAR software was used for setting up the data set, creation of combination descriptors, modeling, and database management. All the statistical tests indicate that the topological model is useful for property estimation. Internal and external validation methods were used, and the results indicate that the model is useful for prediction. Randomizations of the activity values also indicate statistically sound models are very different from random statistics. The model indicates that positive factors for binding affinity include electron accessibility and the number of aromatic rings, aliphatic CH groups (-CH(3), -CH(2)-, >CH-), halogens (fluorine and chlorine), and -OH groups. Five-membered heteroatomic rings present a negative factor, whereas six-membered heteroatomic rings present a positive factor. The specific information described can be used as an aid to the drug design process.

Modulation of nonspecific binding in ultrafiltration protein binding studies

PURPOSE

The aim of this study was to reduce or prevent nonspecific binding (NSB) of compounds to ultrafiltration (UF) protein binding (PB) testing units.

METHODS

UF units (regenerated cellulose, MWCO 10K) were used for PB and NSB measurements with or without pretreatment with 5% tween 80 (TW 80) or 5% benzalkonium chloride (BAK) on the filter membrane. Dosing solutions (10 microM) in human serum and pH 7.4 phosphate-buffered saline were centrifuged at 3,000 g and room temperature after 1-h incubation in UF testing units. In parallel, a 96-well equilibrium dialyzer was used for PB and NSB measurements in equilibrium dialysis (ED) at 37 degrees C for 4 h. Samples of UF and ED were analyzed by LC/MS or LSC.

RESULTS

Severe NSB was observed for etoposide, hydrocortisone, propranolol, and vinblastine in UF. In contrast, TW 80 or BAK pre-treatment on the filter membrane decreased the NSB from 87-95% to 13-64% without causing a significant change in membrane integrity. When NSB was below 50% as a result of pretreating agents, PB data of marker compounds were comparable to those of ED.

CONCLUSIONS

The pretreated membrane with TW 80 or BAK showed significantly less NSB for compounds that had a tendency toward high membrane binding. A modified UF method with pretreatment improved the performance of UF and was able to produce comparable PB results to ED.

Pharmacokinetic study of the digoxin-acenocoumarol interaction in rabbits

A study was carried out to evaluate the potential pharmacokinetic interaction between digoxin and acenocoumarol. The binding of digoxin to rabbit cardiac tissue homogenates was assessed in vitro, using the equilibrium dialysis technique. An increase in the first-order constant (p<0.05) and a reduction in the partition coefficient in the equilibrium situation (p<0.001) of digoxin were observed when the cardiac homogenates were previously treated with acenocoumarol. In the in vivo study, the kinetics of digoxin administered in single and multiple dosage regimens were compared in control rabbits and in rabbits treated simultaneously with acenocoumarol. Kinetic analysis of the results was performed using Non-linear Mixed Effects Modeling (NONMEM). In the presence of acenocoumarol, the population distribution volume (Vd) of digoxin was increased by 40-60%, no differences being found as regards the elimination clearance. Also, joint administration of both drugs led to a reduction in digoxin concentrations in the heart (p<0.01) at the end of the dosage regimen. Both sets of results point to the hypothesis of a hitherto unreported possible pharmacokinetic interaction between the two drugs affecting the distribution process. This interaction could lead to lower plasma digoxin levels, in view of the increased Vd, and a possible reduction in the therapeutic effect, owing to the decrease in affinity and in concentration in heart tissue.

Influence of age and cytochrome P450 2C9 genotype on the steady-state disposition of diclofenac and celecoxib

OBJECTIVE

To analyse the influence of age and cytochrome P450 (CYP) 2C9 genotype on the steady-state disposition of the standard NSAID diclofenac and the new COX-2 selective inhibitor celecoxib, both of which are metabolised by the polymorphically expressed CYP2C9.

DESIGN

Double-blind randomised crossover study under steady-state conditions.

SUBJECTS

12 young (age 32 +/- 5 years, bodyweight 71 +/- 12kg; mean +/- SD) and 12 elderly (68 +/- 2 years, 82 +/- 15kg) healthy, drug-free, nonsmoking Caucasians of both sexes.

METHODS

All subjects received oral celecoxib (200mg twice daily) and diclofenac (75mg twice daily) for 15 days separated by a drug-free interval of at least 3 weeks. Following the last morning dose, multiple blood samples were taken for 25 hours. Concentrations of celecoxib and diclofenac were measured by specific and sensitive high performance liquid chromatography. Identification of CYP2C9 genotype was performed by genomic DNA sequencing. Pharmacokinetic parameters for total and unbound drugs were individually analysed by noncompartmental techniques.

RESULTS

For diclofenac, area under the concentration-time curve over the dosage interval (AUC(tau)) was larger in young subjects (3.2 +/- 1.0 mg * h/L) than in older individuals (2.4 +/- 0.4 mg * h/L; p < 0.05). As the terminal half-life (t((1/2)Z)) was very similar in both groups (3.9 +/- 4.4 vs 3.5 +/- 3.3 hours), either less complete absorption in the elderly or their higher bodyweight could account for the difference. For celecoxib, AUC(tau) (5.8 +/- 1.7 vs 5.6 +/- 2.3 mg * h/L) and t((1/2)z) (11.8 +/- 8.7 vs 11.2 +/- 2.9 hours) were almost identical in young and older subjects. Plasma protein binding of both NSAIDs was unaffected by age, and apparent oral clearances for unbound drugs were not different between the two groups of healthy subjects. When considering the genotype of all individuals (CYP2C9*1/*1, n = 10; CYP2C9*1/*2, n = 6; CYP2C9*2/*2, n = 2; CYP2C9*1/*3, n = 4; CYP2C9*3/*3, n = 1), no association with any pharmacokinetic parameter of either drug was apparent. Moreover, there was no significant correlation between the AUC values of celecoxib and diclofenac.

CONCLUSIONS

Age and CYP2C9 genotype do not significantly affect the steady-state disposition of celecoxib and diclofenac. This would indicate that both drugs need no dosage reduction in the elderly (at least up to 75 years) and that, besides CYP2C9, additional CYP species contribute to the elimination of both agents.

Development and validation of a 96-well equilibrium dialysis apparatus for measuring plasma protein binding

A 96-well equilibrium dialysis block was designed and constructed that is compatible with most standard 96-well format laboratory supplies and instruments. The unique design of the dialysis apparatus allows one to dispense and aspirate from either or both the sample and dialysate sides from the top of the apparatus, which is not possible with systems currently on the market. This feature permits the investigator to analyze a large number of samples, time points, or replicates in the same experiment. The novel alignment of the dialysis membrane vertically in the well maximizes the surface-to-volume ratio, eliminates problems associated with trapped air pockets, and allows one to add or remove samples independently or all at once. Furthermore, the design of the apparatus allows both the sample and dialysate sides of the dialysis well to be accessible by robotic systems, so assays can be readily automated. Teflon construction is used to minimize nonspecific binding of test samples to the apparatus. The device is reusable, easily assembled, and can be shaken in controlled temperature environments to decrease the time required to reach equilibrium as well as facilitate dissolution of test compounds. Plasma protein binding values obtained for 10 diverse compounds using standard dialysis equipment and the 96-well dialysis block validates this method.

In silico prediction of drug-binding strengths to human serum albumin

Drug binding to Human Serum Albumin (HSA) is an area of intense research. The pharmacokinetics and pharmacodynamics of drugs are strongly affected by their binding to this protein. In this article, the field is reviewed, as well as our models to predict drug-binding affinities to HSA from drug structure. The physiological role of HSA is described, as well as its influence in drug action. The crystal structures of this protein are discussed, emphasizing the two drug-binding sites and the fatty acids binding sites observed therein. The advantages of using high-performance affinity chromatography to rapidly screen drugs for HSA binding are explained. The different QSAR models for HSA binding of restricted families of drugs (both from other groups and our group) are enumerated. Finally, a detailed description of our general models to predict drug-binding strengths to HSA from structure is given. It is expected for these models to be useful in drug design and pharmaceutical research.

HPLC investigation of free and bound propofol in human plasma and cerebrospinal ?uid

The paper compares the total propofol concentration in the cerebrospinal fluid (CSF) with the free drug concentration in plasma measured in 35 humans scheduled for elective neurosurgical procedures during propofol anaesthesia. The concentrations of total and free propofol in the blood and CSF samples were measured by means of HPLC using liquid-liquid extraction and ultrafiltration in the sample preparation procedure. The arterial blood and CSF samples (collected from intraventricular drainage) were taken at the same time. According to the obtained results, the usually expected equality between free drug concentration in plasma and its total concentration in CSF is not valid for propofol: the unbound propofol concentration in plasma is not equal to its total concentration in CSF (p < 0.05). This difference suggests a substantial contribution of active transport in propofol transfer from blood into CSF. Moreover, the paper shows the presence of bound propofol in CSF, which is a novel finding.

Negligible depletion solid-phase microextraction with radiolabeled analytes to study free concentrations and protein binding: an example with [3H]estradiol

A new method is presented that enables sensitive measurement of free concentrations of radiolabeled ligands. Additionally, protein binding of radiochemicals in complex matrixes can be determined with this new technique that combines negligible depletion solid-phase microextraction (nd-SPME) with liquid scintillation counting (LSC) as detection. [3H]Estradiol was taken as an example compound. Possible matrix effects of protein on fiber uptake kinetics were studied. No matrix effect was found, either by fouling of the fiber, or by changed uptake kinetics. The validity of the method was shown in the determination of the affinity constant (Ka) of estradiol for human serum albumin (HSA). The Ka was estimated at 8.9 x 10(4) M(-1), which corresponds well with literature values. This study shows that nd-SPME is suitable to study the free concentration and protein binding of [3H]estradiol. The method described in this paper combines the advantages of nd-SPME with the advantages of radiolabeled analytes, creating a timesaving, simple, and sensitive analytical tool that will be particularly useful in complex matrixes containing many potential interferences for chromatographic methods.

Evalution of capillary electrophoresis-frontal analysis for the study of low molecular weight drug-human serum albumin interactions

Capillary electrophoresis frontal analysis was applied to 12 low molecular weight compounds including 8 drug substances displaying a range of different properties with respect to binding affinity, binding location, structure, lipophilicity, charge at physiological pH, and electrophoretic mobility. It was found that capillary electrophoresis frontal analysis can be used as a general method to study and quantify drug-human serum albumin interactions. The binding parameters obtained were consistent with literature values. Dextran was in some cases added to the run buffer to improve separation of the drug and human serum albumin plateau peaks. Results indicate that mobility differences between free and complexed human serum albumin give rise to only minor errors. Capillary electrophoresis frontal analysis was also found applicable to the study of human serum albumin drug displacement reactions. Low sensitivity of the UV-detection system was found to be the major limitation of capillary electrophoresis frontal analysis. The method is simple, and minimal effort has to be put into method development, which makes it well suited for screening in early drug development.

SPME-LC with UV detection to study delorazepam-serum albumin interactions

Solid phase microextraction coupled to high performance liquid chromatography with UV detection (SPME/LC-UV) has been employed to study the binding of delorazepam to human serum albumin (HSA) and bovine serum albumin (BSA). The procedure could also be potentially extended to the measurement of partition coefficients between a wide variety of semi- or non-volatile compounds and matrices. The method is solvent free, simple, fast, and drawbacks of the conventional analytical techniques are avoided. Moreover, the matrix did not interfere with the measurement by binding to the fibre and the amount extracted by the fibre was negligibly small; thus it did not disturb the delorazepam-protein binding.

The applicability of in vitro-derived data in hazard identification and characterisation of chemicals

Toxicological hazard and risk assessments for chemicals presently are mainly based on highly standardised protocols for animal experimentation and exposure assessment. In this paper the possibilities are being discussed of developing systems in which the systemic (acute and chronic) toxicity of chemicals can be quantified, without the heavy reliance on animal experiments. On the basis of a chemical's structure, in vitro data on its toxicity, and biokinetic modelling a decision/flow scheme is presented. Key elements are the evaluation of chemical functionalities representing structural alerts for toxic actions, the construction of biokinetic models on the basis of non-animal data (e.g. tissue-blood partition coefficients (PCs), in vitro biotransformation parameters), tests or batteries of tests for determining basal cytotoxicity and more specific tests for evaluating tissue- or organ toxicity. It is concluded that such a flow chart is a useful tool for different steps in the toxicological hazard and risk assessment, especially for those forms of toxicity for which validated in vitro and other non-animal tests have already been developed.

Monitoring and kinetic parameter estimation for the binding process of berberine hydrochloride to bovine serum albumin with piezoelectric quartz crystal impedance analysis

A new method for monitoring, in real time, the drug-binding process to protein with piezoelectric quartz crystal impedance (PQCI) is proposed. The method was used to monitor the binding process of berberine hydrochloride to bovine serum albumin (BSA). BSA was immobilized on the silver electrode surface of a piezoelectric quartz crystal and the optimized experimental conditions were established. The BSA-coated piezoelectric sensor was in contact with berberine solution. The time courses of the resonant frequency and equivalent circuit parameters of the sensor during the protein-drug binding were simultaneously obtained. On the basis of the analysis of the multidimensional information provided by PQCI, it was concluded that the observed frequency decrease was mainly ascribed to the mass increase of the sensor surface resulting from the binding. According to the frequency decrease with time, the kinetics of the binding process were quantitatively studied. A piezoelectric response model for the binding was theoretically derived. Fitting the experimental data to the model, the kinetic parameters, such as the binding and dissociation rate constants (k(1) and k(-1)) and the binding equilibrium constant (K(a)), were determined. The k(1), k(-1), and K(a) values obtained at 25 degrees C were 67.5 (+/-0.1) (mol liter(-1))(-1) s(-1), 1.7 (+/- 0.1) x 10(-3) s(-1), and 3.97 (+/- 0.06) x10(4) (mol liter(-1))(-1), respectively.

Effect of albumin on phenytoin and tolbutamide metabolism in human liver microsomes: an impact more than protein binding

The cytochrome P450 (P450)-dependent conversion of phenytoin (PHT) to p-hydroxy phenytoin (pHPPH), and tolbutamide (TLB) to 4-hydroxy tolbutamide (hydroxy-TLB), in human liver microsomes was studied in the presence of increasing concentrations (0-4%) of bovine serum albumin (BSA). Therefore, the free fraction (f(u)) of PHT and TLB varied. Whereas the f(u) of PHT (5 microM) decreased, an increase (3-fold), rather than a decrease in the pHPPH formation rate was observed when BSA (<1%) was present. The stimulation was attributed to a significant decrease in apparent K(m). The change, however, was diminished as the BSA concentration reached 4% (PHT f(u) = 0.2), in which the reaction velocity remained the same as that measured in the absence of BSA. Therefore, unchanged K(m) (16.2 +/- 0.7 microM) and V(max) (9.4 +/- 0.2 pmol/min/mg of protein) values were determined based on total PHT concentrations, whereas correction for f(u) led to an unbound K(m) (K(mu)) of approximately 3.2 microM. Similarly, the metabolism of TLB (50 microM) was enhanced (approximately 2-fold) in the presence of 0.25% BSA but remained only 35% of the control activity (no BSA) at 1% BSA. However, the remaining activity was higher (3-fold) than that determined with an equivalent free concentration of TLB (4 microM) calculated according to its f(u) (0.08). The difference became less significant when BSA concentration was 4% (f(u) < 0.02). Collectively, the results suggest a 2-fold effect of BSA on PHT and TLB hydroxylation: first, facilitation of the reactions via a decrease in K(m); second, a decrease in f(u) leading to a drop in reaction rate. For a given P450 reaction, therefore, the effect of BSA may depend upon enzyme affinity, catalytic capacity, and the extent of protein binding.

Synthesis and biological evaluation of structurally highly modified analogues of the antimitotic natural product curacin A

Structure-activity relationship analysis of synthetic analogues of curacin A revealed the lack of activity of traditional heterocyclic replacements of the thiazoline ring or cyclopropyl analogues of the core diene segment. The significance of the C(3)-C(4)-(Z)-alkene geometry was established, and a novel oxime analogue was designed that displays biological properties that are a close match of the natural product lead. The much less lipophilic, structurally simplified oxime 50 was only slightly weaker at inhibiting the growth of cultured human tumor cells than the natural product and was found to be more potent than curacin A at inhibiting the assembly of purified tubulin. Accordingly, the oxime moiety is likely to serve as a novel bioisostere of the (Z)-alkene group.

SB-242235, a selective inhibitor of p38 mitogen-activated protein kinase. I: preclinical pharmacokinetics

1. SB-242235 (1-(4-piperidinyl)-4-(4-fluorophenyl)-5-(2-methoxy-4-pyrimidinyl) imidazole) is a potent and selective p38 MAP kinase inhibitor that may be an effective therapy for cytokine-mediated diseases such as autoimmune or inflammatory diseases. The present studies were conducted to evaluate the pharmacokinetics of SB-242235 in several preclinical species, including rat, dog and monkey. 2. SB-242235 demonstrates generally favourable pharmacokinetic properties in all species examined. Systemic plasma clearance was high in rat, but in the non-rodent species SB-242235 demonstrated low to moderate clearance with plasma half-lives > 4h. Oral bioavailability in each preclinical species was high. In rat and monkey, SB-242235 demonstrated non-linear elimination kinetics that manifested as a decrease in clearance with increasing dose and apparent oral bioavailability > 100% at high oral doses. Furthermore, SB-242235 displayed concentration-dependent plasma protein binding over a concentration range of 1000-10,000 ng ml(-1). 3. In conclusion, SB-242235 demonstrates high oral bioavailability across the major preclinical species, and may thus be a useful tool compound for investigation of the role of p38 inhibition in various disease states. However, the observations of non-linear protein binding and disposition also suggest the need for caution in the design of and data interpretation from such studies.

Methods of in vitro toxicology

In vitro methods are common and widely used for screening and ranking chemicals, and have also been taken into account sporadically for risk assessment purposes in the case of food additives. However, the range of food-associated compounds amenable to in vitro toxicology is considered much broader, comprising not only natural ingredients, including those from food preparation, but also compounds formed endogenously after exposure, permissible/authorised chemicals including additives, residues, supplements, chemicals from processing and packaging and contaminants. A major promise of in vitro systems is to obtain mechanism-derived information that is considered pivotal for adequate risk assessment. This paper critically reviews the entire process of risk assessment by in vitro toxicology, encompassing ongoing and future developments, with major emphasis on cytotoxicity, cellular responses, toxicokinetics, modelling, metabolism, cancer-related endpoints, developmental toxicity, prediction of allergenicity, and finally, development and application of biomarkers. It describes in depth the use of in vitro methods in strategies for characterising and predicting hazards to the human. Major weaknesses and strengths of these assay systems are addressed, together with some key issues concerning major research priorities to improve hazard identification and characterisation of food-associated chemicals.

Preclinical pharmacokinetics of SB-203580, a potent inhibitor of p38 mitogen-activated protein kinase

1. SB-203580 (4-(4-fluorophenyl)-2-(4-methylsulphinylphenyl)-5-(4-pyridyl)imidazole) is a potent, selective inhibitor of p38 MAP kinase used extensively as a tool inhibitor in various pharmacological and toxicological models. This study was designed to evaluate the pharmacokinetics of SB-203580 in several preclinical species, both to assist with the interpretation of existing studies and to aid in the design of future studies with this inhibitor. 2. In vitro, SB-203580 was stable in mouse, rat, dog, monkey and human plasma over 24 h. However, species differences in plasma protein binding were observed; SB-203580 was 96-97% bound in human plasma and 78-92% bound in other species. These data suggest that protein binding may influence the results of in vitro studies using SB-203580, particularly when comparing results from different in vitro systems that incorporate plasma components. In vivo, SB-203580) demonstrated moderate to high clearance in all species tested, with non-linear elimination observed in the rat at plasma concentrations > 1,000 ngml(-1). Although good solution bioavailability was observed in non-rodents (78% in dog, 32% in monkey), lower and more variable bioavailability was observed in the rat and mouse (3 -48%). 3. These interspecies differences in bioavailability, and the non-linear pharmacokinetics observed in rat, highlight the importance of monitoring SB-203580 systemic exposure in parallel witb the pharmacological endpoint during in vivo pharmacology

Correlation between in vitro and in vivo activities of GM 237354, a new sordarin derivative, against Candida albicans in an in vitro pharmacokinetic-pharmacodynamic model and influence of protein binding

The antifungal effect of GM 237354, a sordarin derivative, was studied in an in vitro pharmacokinetic (PK)-pharmacodynamic dynamic system (bioreactor) which reproduces PK profiles observed in a previously described model of drug efficacy against murine systemic candidiasis. Immunocompetent mice infected intravenously with 10(5) CFU of Candida albicans were treated with GM 237354 at 2.5, 10, and 40 mg/kg of body weight every 8 h subcutaneously for 7 days. Free concentrations in serum were calculated by multiplying total concentrations measured in vivo by 0.05, the free fraction determined in vitro by equilibrium dialysis. In the bioreactor the inoculum was approximately 10(6) CFU/ml; and a one-compartment PK model was used to reproduce the PK profiles of free and total GM 237354 in serum obtained in mice, and clearance of C. albicans was measured over 48 h. A good correlation was observed when the in vivo fungal kidney burden and the area under the survival time curve were compared with the in vitro broth "burden," although only when free in vivo levels in serum were reproduced in vitro. GM 237354 displayed a 3-log decrease effect both in vivo and in vitro. The very few reports available on in vitro-in vivo correlations have been obtained with antibiotics. The good in vitro-in vivo correlation obtained with an antifungal agent shows that the in vitro dynamic system could constitute a powerful investigational tool prior to assessment of the efficacy of an anti-infective agent in animals and humans.

An evaluation of risperidone drug interactions

Risperidone, an atypical antipsychotic drug, is widely used in the treatment of psychoses associated with schizophrenia, Alzheimer's disease, and other psychiatric disorders. Polypharmacology is a necessary condition for the optimal treatment of many patients with comorbid psychiatric and medical illness. One concern raised by the widespread use of multiple concurrent pharmacotherapies is the potential for drug-drug interactions to adversely affect patient outcome. Accordingly, the biomedical literature was reviewed for reports of drug interactions involving risperidone, and the clinical significance of each report was evaluated. Additionally, the potential for risperidone to participate in drug interactions was evaluated by considering the drug's pharmacokinetic properties. Controlled studies and case reports indicate that risperidone has a low potential for metabolic drug interactions. Drugs that inhibit cytochrome P450 (CYP) 2D6 or induce or inhibit CYP3A4 may alter risperidone plasma concentrations, but the clinical significance of such interactions seems to be minimal. Adherence to a few guidelines for the design of dosage regimens should limit the effect of drug-drug interactions on patient status and contribute to optimal pharmacotherapy with risperidone.

High-throughput screening approaches for investigating drug metabolism and pharmacokinetics

1. High-throughput screening approaches have been adopted throughout the pharmaceutical industry to aid in the rapid discovery of new chemical entities. Because it is now well recognized that the selection of a robust candidate requires a balance of potency, safety and pharmacokinetics, the role of drug metabolism departments has widened from their traditional one of supporting drug development to include the screening of compounds during the discovery process. To put drug metabolism and pharmacokinetic (DMPK) studies in context, the evolving role of DMPK screening in the drug discovery strategy of pharmaceutical companies will be discussed and a generalized approach will be presented. 2. With the increasing numbers of compounds requiring screening, DMPK optimization methods have had to be adapted for high throughput. There have been many developments in this field over the past decade and this review will focus on the high-throughput DMPK screening methodologies used today and in the recent past. 3. In vitro and in silico (computer-based) methods have proven most amenable to high-throughput approaches and these will firm the bulk of the review, but some advances with in vivo methods will also be discussed. As there has been a vast increase in published material on the topic of high-throughput DMPK methodologies in the past 10 years, it would be impossible to cover every method in detail, so this review will concentrate on the key areas and refer the reader to other, more detailed reviews wherever possible. 4. Most high-throughput methods would not be possible without the enabling technologies of computing, automation, new sample preparation technologies, and highly sensitive and selective detection systems, and these will also be reviewed. 5. The advantages and disadvantages of the screening methods will be presented, in particular the issue of handling the false-positives and -negatives that arise. 6. In concluding the review, future developments in this field will be discussed along with key issues that will need to be addressed.

Stability of a cisplatin-chondroitin sulfate A complex in plasma and kidney in terms of protein binding

To assess the stability of a cisplatin (CDDP) complex prepared with chondroitin sulfate A (CSA) relative to protein binding in the circulation and kidney, a trichloroacetic acid (TCA) precipitation method was developed to measure the protein-unbound species of CDDP and the CDDP-CSA complex in plasma and kidney homogenates. The total and unbound drug concentrations were determined up to 3 h following a 2 mg/kg bolus injection of CDDP or CDDP-CSA complex to rats. The stability against plasma binding was evaluated by a determination of the area under concentration-time curve from time 0 to infinite time (AUC(0-infinity)); the ratio of unbound drug AUC(0-infinity) to total drug AUC(0-infinity) was employed to estimate the availability of the unbound drug in the circulation. The results showed that a competitive reaction to platinum existed between plasma protein and the CDDP-CSA complex, but the complex accounted for more than 60% of the protein-unbound species for a dosage, compared to 30% obtained by an administration of uncomplexed CDDP. The tissue binding kinetics in kidney for CDDP and the CDDP-CSA complex was investigated by the use of homogenates. The binding rate constants of CDDP and CDDP-CSA in a kidney homogenate were 0.0040 min(-1) and 0.0014 min(-1), respectively. The results indicate that the CDDP-CSA complex could effectively retard the binding of CDDP to protein in the kidney. These data provide evidence that endogenous protein is able to compete for platinum from the CDDP-CSA complex, but the complex effectively retarded the protein binding reaction with CDDP in plasma and kidney as compared to native CDDP, which has the potential for reducing the accumulation of CDDP in plasma and kidney.

Binding of cosalane--a novel highly lipophilic anti-HIV agent--to albumin and glycoprotein

Cosalane is a potent inhibitor of HIV replication with multiple sites of action. The purposes of this study were to (a) determine the extent and nature of cosalane binding to mucin, alpha(1)-acid glycoprotein (AAG), plasma, and human (HSA) and bovine serum (BSA) albumin, and (b) determine the primary site(s) of cosalane binding to HSA. Plasma protein binding of cosalane was studied by a gel filtration technique. Cosalane binding to HSA was also determined in the presence of salicylic acid. Competitive inhibition studies were conducted using warfarin, digitoxin, and diazepam to determine the primary HSA binding site(s) of cosalane. The drug was bound extensively to HSA and BSA and required 500-550 moles to saturate 1 mole of protein. Stoichiometries of cosalane binding to alpha(1)-acid glycoprotein (AAG) and mucin were between 30 and 50 mol/mol of either glycoprotein. The binding isotherm deviated from a rectangular hyperbola, suggesting self-association of the ligand. Salicylic acid decreased cosalane binding to HSA by one order of magnitude. Inhibition studies of cosalane to HSA revealed that the compound binds primarily to warfarin site with a K(i) of 1.24 +/- 0.24 nM. In summary, cosalane binds extensively to serum albumins and to a lesser extent to both AAG and mucin.

Development of a miniaturized 384-well high throughput screen for the detection of substrates of cytochrome P450 2D6 and 3A4 metabolism

The identification of a large number of biologically active chemical entities during high throughput screening (HTS) necessitates the incorporation of new strategies to identify compounds with druglike properties early during the lead prioritization and development process. One of the major steps in lead prioritization is the assessment of drug metabolism mediated by the cytochrome P(450) (CYP) enzymes to evaluate the potential drug-drug interactions. CYP2D6 and CYP3A4 comprise the main human CYP enzymes involved in drug metabolism. The recent availability of specific CYP cDNA expression systems and the development of specific fluorescent probes have accelerated the ability to develop robust in vitro assays in HTS format. The aim of this study was to optimize conditions for the CYP2D6 and CYP3A4 HTS assays and subsequently adapt those assays to a miniaturized 384-well format. Assay conversion to a miniaturized format presents certain difficulties, such as robustness of the signal and of compound delivery. Thus the assay optimization involved the comparison of different substrates to identify those most suitable for use in a miniaturized format. Because of current technical limitations in liquid dispensing of nanoliter volumes, assay sensitivity to organic solvents also provides a main concern during assay miniaturization. Therefore, compound activity from redissolved dry films and from DMSO stocks directly delivered into assay buffer was compared. The data indicate that compound activity was comparable in both formats. The data support the conclusion that CYP2D6 and CYP3A4 in vitro metabolism assays can be successfully performed in 384-well plate format and the substrate potencies, as evaluated by the IC(50) values, determined.

Binding profile of spiramycin to oviducal proteins of laying hens

In vitro protein binding of spiramycin (SP) in the plasma and oviducts of laying hens was studied. The data for SP were compared with those for oxytetracycline (OTC), sulphadimidine (SDD), sulphamonomethoxine (SMM) and sulphaquinoxaline (SQ). The two oviduct segments, magnum (M) and isthmus plus shell gland (IS), were collected. The soluble (cell sap) fractions from the magnum (M-S9) and the isthmus plus shell gland (IS-S9) were used as samples. Plasma protein binding was highest for SQ (81.4%) (P < 0.01), and lowest for SDD (30.9%) (P < 0.01). No M-S9 protein binding of OTC was found. The IS-S9 protein binding of SP (60.4%) was very much higher than those of OTC (0.8%), SDD (4.1%), SMM (4.0%) and SQ (12.3%) (P < 0.01). Biological half-lives of these drugs in egg albumen were directly correlated to the extent of their binding to IS proteins. Of plasma, M-S9 and IS-S9, variation in SP concentration in the ranges from 1 to 20 micrograms/ml did not alter the binding properties of the drug.

Determination of the free fraction and relative free fraction of drugs strongly bound to plasma proteins

This report describes a new method for the determination of protein binding and relative protein binding (ratio of f(u) for different species) for compounds strongly bound to proteins. The method used is based on the distribution of the drug in plasma water, plasma proteins, and blood cells. Incubations were performed in diluted plasma. In diluted plasma, the erythrocyte/plasma distribution was determined with greater precision than in undiluted plasma. Formulae were derived for calculating f(u) in undiluted plasma based on the f(u) values determined in diluted plasma. These formulae are also valid in the event of more than one independent binding site in plasma. All incubations with plasma of different species were performed using rat erythrocyte suspensions, thereby making it possible for relative f(u) values in different species to be calculated without knowing the absolute free fractions. This method avoids the determination of the erythrocyte/buffer distribution in cases where it is sufficient to know relative f(u) values (e.g., exposure comparisons). Relative protein binding can also be quantified for compounds that tend to adsorb to surfaces of vials or test tubes, thus avoiding errors caused by adsorption when quantifying the drug in a protein-free aqueous solution. This method was validated by making comparisons of free fraction values obtained by the method herein described with those obtained by either ultrafiltration or equilibrium dialysis for two compounds that bind predominantly to albumin and another compound that binds to alpha(1)-acid glycoprotein. The results confirm our method produces identical free fractions in comparison with other established techniques. In addition, the range of applications of our method is much wider.

Validation of negligible depletion solid-phase microextraction as a tool to determine tissue/blood partition coefficients for semivolatile and nonvolatile organic chemicals

In the current study, negligible depletion solid-phase microextraction (nd-SPME) as a technique to determine tissue/blood (tissue/water and blood/water) partition coefficients was validated. With this method the free fraction of chemicals in water in the presence of different tissues was determined and, subsequently, the partition coefficients were calculated. Liver, blood, muscle, and fat tissues obtained from male Wistar-derived rats (U:Wu) were used without homogenization. Data obtained were compared with literature data for lindane, parathion, and paraoxon. The results show that the data in the present work differ by less than a factor of two from those reported in literature. In addition, the standard deviations obtained show that the technique is accurate. Therefore, we conclude that this accurate and automated method can be used to determine tissue/blood partition coefficients for semivolatile and nonvolatile chemicals.

Sulphation of resveratrol, a natural product present in grapes and wine, in the human liver and duodenum

1. Resveratrol, a polyphenolic compound present in grapes and wine, has beneficial effects against cancer and protective effects on the cardiovascular system. It is present in the diet, and the hepatic and duodenal sulphation might limit the bioavailability of this compound. The aim was to study the sulphation of resveratrol in the human liver and duodenum. 2. A simple and reproducible radiometric assay for resveratrol sulphation was developed. It employed 3'-phosphoadenosine-5'-phosphosulphate-[35S] as the sulphate donor and the rates of resveratrol sulphation (mean +/- SD, pmol/min/mg cytosolic protein) were 90 +/- 21 (liver, n = 10) and 74 +/- 60 (duodenum, n = 10, p = 0.082). 3. Resveratrol sulphotransferase followed Michaelis-Menten kinetics and Km (mean +/- SD; microM) was 0.63 +/- 0.03 (liver, n = 5) and 0.50 +/- 0.26 (duodenum, n = 5, p = 0.39) and Vmax (mean +/- SD, pmol/min/mg cytosolic protein) were 125 +/- 31 (liver, n = 5) and 129 +/- 85 (duodenum, n = 5, p = 0.62). 4. Resveratrol sulphation was inhibited by the flavonoid quercetin, by mefenamic acid and salicylic acid, two commonly used non-steroidal anti-inflammatory drugs. IC50 of resveratrol sulphation for quercetin was 12 +/- 2 pM (liver) and 15 +/- 2 pM (duodenum), those for mefenamic acid were 24 +/- 3 nM (liver) and 11 +/- 0.6 nM (duodenum), and those for salicylic acid were 53 +/- 9 microM (liver) and 66 +/- 4 microM (duodenum). 5. The potent inhibition of resveratrol sulphation by quercetin, a flavonoid present in wine, fruits and vegetables, suggests that compounds present in the diet may inhibit the sulphation of resveratrol, thus improving its bioavailability.

Fenamates and the potent inhibition of human liver phenol sulphotransferase

1. The inhibition of the human liver phenol sulphotransferase (HL-PST) and catechol sulphotransferase (HL-CST) by five fenamates has been studied and the activities of HL-PST and HL-CST were measured with 4-nitrophenol and dopamine as substrates, respectively. 2. The IC50 for inhibition of HL-PST were 0.02 microM (mefenamic acid); 0.12 microM (tolfenamic acid); 0.28 microM (niflumic acid); 0.87 microM (meclofenamic acid) and 1.50 microM (flufenamic acid). 3. HL-CST was less susceptible than HL-PST to the inhibition by fenamates and the IC50 for HL-CST were 36 microM (tolfenamic acid); 70 microM (flufenamic acid); 76 microM (mefenamic acid); 180 microM (niflumic acid) and 185 microM (meclofenamic acid). 4. The ratios of the IC50 for HL-CST:HL-PST were drug-dependent and ranged from 47 (flufenamic acid) to 3800 (mefenamic acid). Mefenamic acid is a relatively potent and selective inhibitor of HL-PST. 5. The IC50 for HL-PST obtained with mefenamic acid was three orders of magnitude lower than the peak plasma concentration of this drug after an oral dose of 0.5 g. Accordingly, mefenamic acid should impair sulphation in vivo.

A displacement approach for competitive drug-protein binding studies using the potentiometric 1-anilino-8-naphthalene-sulfonate probe technique

A displacement approach for competitive binding studies was developed. The method utilizes the potentiometric 1-anilino-8-naphthalene-sulfonate (ANS) probe technique and is applied to the binding study of several non-steroidal anti-inflammatory drugs (NSADs) to bovine serum albumin (BSA). A home-made ANS electrode was used to monitor the displaced free ANS probe from its binding sites on the protein molecule by the stepwise addition of the studied drug. To assess and compare quantitatively the displacing ability of the various drugs, the 'ANS Displacement Index' is used. The possible interference of 19 ionizable drugs (NSADs, sulfonamides, etc.) to the ANS selective electrode at pH 7.4 was studied and their potentiometric selectivity coefficients (K(pot)(ANS,D)) were determined. Correction procedures for the determination of the free ANS concentration are proposed in the case of interfering ionic drugs. A blank binding experiment in conjunction with the incorporation of K(pot)(ANS,D) values in the 'general competitive site oriented model' allows one to derive estimates for the drug binding parameters, i.e. the number of binding sites and association constants.

Simultaneous determination of enantioselective plasma protein binding of aminohydantoins by ultrafiltration and chiral high-performance liquid chromatography

Chiral HPLC methods were developed and utilized for the simultaneous determination of plasma protein binding of enantiomers of two racemic aminohydantoin compounds. Reversed-phase HPLC with the use of a polysaccharide-type chiral stationary phase column was employed for the separation and quantitation of the enantiomers of the two compounds with detection limits in the range 5-10 ng/ml in the plasma matrix. The chiral HPLC methods were selective, sensitive and reproducible. The R and S enantiomers of both compounds were baseline-resolved under the chromatographic conditions employed. Ultrafiltration techniques were applied to determining the plasma protein binding for each enantiomer in rat, dog and human plasma. The results clearly show stereoselective binding of the two enantiomers of each compound with higher protein binding of the R enantiomer than the S enantiomer in rat, dog and human plasma. Binding association constants were also determined to be in the range 1.01-14.0 x 10(4) M(-1) at 37 degrees C. Both the protein binding percentage and binding association constant were enantioselective and species-dependent. Such information is important for a clear understanding of the differences in biological activity as well as in pharmacokinetic and pharmacodynamic properties between the two enantiomers of each compound in the drug discovery and development process.

Substituted analogues of GV150526 as potent glycine binding site antagonists in animal models of cerebral ischemia

A series of analogues of the indole-2-carboxylate GV150526, currently in clinical trials as a potential neuroprotective agent for the control of the cerebral damage after stroke onset, was designed based on previous studies dealing with the electronic features of the north-east region of the glycine binding site associated with the NMDA receptor. In particular, the substitution of the para position of the terminal phenyl ring of GV150526 with suitable hydrophilic groups resulted in the identification of a new class of glycine antagonists. These compounds exhibited nanomolar in vitro affinity to the glycine binding site, high receptor selectivity, and outstanding in vivo potency. In particular, 3-[(E)-2-[(4-ureidomethylphenyl)aminocarbonyl]ethenyl]-4, 6-dichloroindole-2-carboxylic acid was found to be highly effective in the middle cerebral artery occlusion (MCAo) model in the rat, an animal model of focal ischemia, when given both prior to and after the occlusion of the middle cerebral artery. Notably, a significant neuroprotective effect was seen in this model postischaemia, when the administration of this compound was delayed up to 6 h from the occlusion of the middle cerebral artery, further confirming the wide therapeutic window seen for GV150526A.

Bioavailability, biodegradation, and acclimation of Tetrahymena pyriformis to 1-octanol

Previous work has indicated that the ubiquitous freshwater ciliate Tetrahymena pyriformis acclimates to the presence of hydrophobic chemicals acting by nonpolar narcosis. Four explanations have been identified to explain this apparent acclimation: (1) genetic adaptation occurs resulting in a resistant population, (2) T. pyriformis quickly biodegrades hydrophobic chemicals resulting in a perceived acclimation response, (3) hydrophobic chemicals are not bioavailable, and (4) T. pyriformis contain an endogenous biochemical adaptation system which can quickly cause cellular changes resulting in acclimation. Results of biodegradation experiments indicated that the total extractable 1-octanol did not change over the duration of the experiments. Bioavailability experiments were performed using the solid-phase microextraction technique. Although there is a decrease in freely available concentrations of 1-octanol over a 2.5 log unit range of Tetrahymena population density, the freely available concentration is constant for the population densities used for population growth experiments. Genetic change is highly unlikely since acclimation occurs in less than the time required for one population division. It is hypothesized that the acclimation response seen in Tetrahymena results from partitioning of the chemical into the membrane followed by active changes in the membrane structure to restore homeostasis.

Plasma protein binding of celecoxib in mice, rat, rabbit, dog and human

The plasma protein binding of celecoxib was determined for animals and humans using in vitro and ex vivo methods. Eight, healthy, human volunteers (three male, five female, 20-39 years) received celecoxib (600 mg) BID for 7 days, blood samples were collected and concentrations of bound and unbound celecoxib determined. The fraction of bound drug in the volunteers was constant (97.4 +/- 0.1%) at total celecoxib plasma concentrations ranging from 0.01 to 4.02 microg/mL. The ex vivo plasma protein binding of celecoxib in the animals was concentration-independent up to approximately 12, 8 and 10 microg/mL for mouse, rat and dog, respectively. The plasma protein binding of celecoxib after a single oral dose of 10 and 300 mg/kg to mice was 98.3 +/- 0.2%, of 1 and 400 mg/kg to rats was 98.3 +/- 0.2% and of 1 and 100 mg/kg to dogs was 98.5 +/- 0.1%. The percent binding of celecoxib to plasma proteins in vitro was slightly lower than those values determined ex vivo. The in vitro binding of celecoxib to plasma protein was constant over the concentrations of 0.1-10 microg/mL for all species, except rat.

Extrapolating in vitro data on drug metabolism to in vivo pharmacokinetics: evaluation of the pharmacokinetic interaction between amitriptyline and fluoxetine

Recently, models have been proposed to extrapolate in vitro data on the influence of inhibitors on drug metabolism to in vivo decrement in drug clearance. Many factors influence drug clearance such as age, gender, habits, diet, environment, liver disease, heredity, and other drugs. In vitro investigation of hepatic cytochrome P450 activity has generally centered on genetic influences and interactions with other drugs. This group of enzymes is involved in many, although not all, drug interactions. The interaction of amitriptyline and fluoxetine is an example. Of the different in vitro paradigms, interaction studies utilizing human liver microsomal preparations have proved to be the most generally applicable for in vitro scaling models. Assuming Michaelis-Menten conditions and applying nonlinear regression, a hybrid inhibition constant (Ki) can be generated that allows classification of the inhibitory potency of an inhibitor toward a specific reaction. This constant is largely independent of the substrate concentration, but in vivo relevance is critically dependent on the inhibitor concentration in the site of metabolic activity, the liver cell cytosol. Many lipophilic drugs are extensively bound to plasma protein but, nonetheless, demonstrate extensive partitioning into liver tissue. This is not compatible with diffusion only of the unbound drug fraction into liver cells. The introduction of a partition factor, based on data from a number of possible sources, provided a reasonable basis for the scaling of in vitro data to in vivo conditions. Many interactions could be reconstructed or predicted with greater accuracy and clinical relevance for interactions such as terfenadine or midazolam and ketoconazole. Even for less marked interactions such as amitriptyline and fluoxetine, this model provides a forecast consistent with the clinically observed range of 22-45% reduction in oral clearance, although this interaction is complicated by the presence of two inhibitors, fluoxetine and norfluoxetine. The concept of in vitro-in vivo scaling is promising and might ultimately yield a fast and more cost-effective screening for drug interactions with reduced human drug exposure and risk.

Protein binding of aspirin and salicylate measured by in vivo ultrafiltration

OBJECTIVE

Methods for measuring protein binding of drugs generally require direct measurement of the concentration of unbound drug and thus may require a highly sensitive assay. In vivo ultrafiltration has been used to determine protein binding of endogenous substances. We have examined its value for measuring protein binding of drugs because it requires measurement of only the concentration of total drug, not unbound drug, in plasma.

METHODS

The protein binding of aspirin and its metabolite salicylate was measured in 29 healthy subjects 20 minutes after a single oral dose of 600 mg soluble aspirin, by the new method, in vivo ultrafiltration, as well as by a standard method, in vitro ultracentrifugation.

RESULTS

The data for salicylate were examined systematically to determine the optimal method of determining estimates of protein binding by in vivo ultrafiltration. Estimates of protein binding of salicylate were 81.7% +/- 10.1% (mean +/- SD) by the in vivo method and 81.6% +/- 11.3% by in vitro ultracentrifugation. Bland-Altman analysis of agreement showed that within-individual differences in percentage of protein binding determined by the two methods did not differ significantly from zero (mean difference, 0.07%; 95% confidence interval, -2.33 to +2.46). There was a highly significant correlation between estimates of protein binding by the two methods (r = 0.82; p = 0.001). Protein binding of aspirin was estimated of protein binding by the two methods (r = 0.82; p = 0.001). Protein binding of aspirin was estimated at 58.3% +/- 9.6% by in vivo ultrafiltration and could not be estimated by in vitro ultracentrifugation because the concentration of unbound aspirin in plasma was below the limit of detection for the assay.

CONCLUSION

In vivo ultrafiltration can be used to measure protein binding of drugs and has potential advantages over conventional methods. A sensitive assay may not be required because the unbound drug need not be measured, measurement in vivo may maintain more physiologic conditions, and it may be useful in measuring protein binding of drugs that are degraded rapidly in vitro.

Effects of probenecid and cimetidine on renal disposition of ofloxacin in rats

The renal handling of ofloxacin in rats which were given ofloxacin either alone or in combination with probenecid or cimetidine was studied. In the presence of cimetidine or probenecid, ofloxacin's total and renal clearances were reduced and its half-life was prolonged. This suggests that ofloxacin is secreted by both the anionic and cationic transport systems.

In vitro approaches to predicting drug interactions in vivo

In vitro metabolic models using human liver microsomes can be applied to quantitative prediction of in vivo drug interactions caused by reversible inhibition of metabolism. One approach utilizes in vitro Ki, values together with in vivo values of inhibitor concentration to forecast in vivo decrements of clearance caused by coadministration of inhibitor. A critical limitation is the lack of a general scheme for assigning intrahepatic exposure of enzyme to inhibitor or substrate based only on plasma concentration; however, the assumption that plasma protein binding necessarily restricts hepatic uptake is not tenable. Other potential limitations include: flow-dependent hepatic clearance, "mechanism-based" chemical inhibition, concurrent induction, or a major contribution of gastrointestinal P450-3A isoforms to presystemic extraction. Nonetheless, the model to date has provided reasonably accurate forecasts of in vivo inhibition of clearance of several substrates (desipramine, terfenadine, triazolam, alprazolam, midazolam) by coadministration of selective serotonin reuptake-inhibitor antidepressants and azole antifungal agents. Such predictive models deserve further evaluation, since they may ultimately yield more cost-effective and expeditious screening for drug interactions, with reduced human drug exposure and risk.

Serum protein binding of nonsteroidal antiinflammatory drugs: a comparative study

The unbound fraction in serum, fu, is a critical parameter in describing and understanding the pharmacokinetics of NSAIDs. We compared fu for 6 different NSAIDs using ultrafiltration of pooled serum at pH 7.4 and 24C. Measurements covered a wide concentration range in order to define binding affinity and number of binding sites. HPLC was used to measure drug concentrations in serum and ultrafiltrate. Direct injection of ultrafiltrate and serum (diluted 250 x) permitted quantitation down to approximately 70 nM for most of the NSAIDs, i.e., approximately 15-20 ng/ml. Assuming binding only to albumin, the data were fitted to a model of two classes of binding sites with dissociation constants K1 and K2. The lowest K1 (highest affinity) was found with flurbiprofen, 0.0658 microM, the highest with ketoprofen, 5.23 microM, an 80-fold difference. At low drug concentrations, fu becomes virtually constant and approaches a lower limit, fumin. The following fumin values were calculated: diclofenac 0.21%; fenoprofen 0.25%, flurbiprofen 0.022%, ketoprofen 0.52%, naproxen 0.039%, and tolmetin 0.37%. Thus the least bound NSAID, ketoprofen, had a value 24-fold that of the most highly bound, flurbiprofen. The NSAIDs also differed widely with regard to the extent of variation in fu within the range of therapeutic concentrations, and hence with regard to their potential as displacers of other drugs.

Plasma protein binding of phenylacetate and phenylbutyrate, two novel antineoplastic agents

Phenylacetate and phenylbutyrate, two novel inducers of tumor cytostasis and differentiation, are currently in clinical trials for the treatment of cancer in adults. The purpose of our study was to evaluate the plasma protein-binding characteristics of phenylacetate and phenylbutyrate in the plasma of normal volunteers and that of patients with cancer. Drug plasma protein-binding analysis was examined using three separate devices: a micropartition system and two equilibrium dialysis systems, all of which exhibited similar results. Phenylacetate and phenylbutyrate concentrations were determined by high-performance liquid chromatography. Both drugs exhibited concentration-dependent binding. Our results showed sodium phenylacetate to have a higher free fraction than sodium phenylbutyrate at corresponding concentrations (> 0.442 +/- 0.008 and > 0.188 +/- 0.001, respectively). Plasma pH did not greatly affect protein binding of either drug. As albumin concentration decreased, an increase in free fraction of both drugs was observed, however alpha 1-acid glyco-protein showed no change in free fraction as its concentration increased. Patients with cancer with lower levels of albumin showed an increase in free fraction with both phenylacetate and phenylbutyrate. When phenylacetate and phenylbutyrate were added together in plasma, the free fraction of phenylacetate increased, whereas the phenylbutyrate free fraction slightly decreased. We conclude that phenylacetate and phenylbutyrate have high free fractions that change with varying albumin levels and when both phenylacetate and phenylbutyrate are present together in plasma.

Biokinetics and biokinetic models in risk assessment

Risk assessment of xenobiotics using animal data involves extrapolation from high doses to low ones, and from animal species to humans. In some cases it also involves extrapolation from one route of exposure to another. To assess the risk of exposure to xenobiotics, information on both biokinetics and biodynamics are needed. The contribution of biokinetics to risk assessment is the subject of this review. The review includes the general aspects of biokinetics of chemicals, the models available to describe the biokinetic behaviour of a chemical and a discussion of the class of biokinetic models that is considered most suited for application to risk assessment: the physiologically-based biokinetic (PBBK) models. The power of PBBK models is illustrated with a few examples.

Measurement and analysis of unbound drug concentrations

The plasma protein binding of drugs has been shown to have significant effects on numerous aspects of clinical pharmacokinetics and pharmacodynamics. In many clinical situations, measurement of the total drug concentration does not provide the needed information concerning the unbound fraction of drug in plasma which is available for distribution, elimination, and pharmacodynamic action. Thus, accurate determination of unbound plasma drug concentrations is essential in the therapeutic monitoring of drugs. Many methodologies are available for determining the extent of plasma protein binding of drugs, however, in the clinical evaluation of drug therapy, equilibrium dialysis and ultrafiltration are the most routinely utilised methods. Both of these methods have been proven to be experimentally sound and to yield adequate protein binding data. Furthermore, the characterisation of the interactions between drug and protein molecules is essential for the assessment of the pharmacokinetic implications of drug-protein binding. Protein binding parameters which characterise the affinity of the drug-protein association, the number of classes of binding sites, the number of binding sites per class or protein and the binding capacity are useful for predicting unbound drug concentrations. Simple graphical methods have often been used to obtain protein binding parameters, but these methods have limitations and are not useful for drugs with more than 1 class of binding site. Therefore, the fitting of protein binding models which characterise the drug-protein binding interaction for experimental data is the preferred method of calculating binding parameters. Using the appropriate model, values for binding parameters are typically estimated by using nonlinear least-squares regression analysis.

Protein binding of methohexital. Study of parameters and modulating factors using the equilibrium dialysis technique

This paper describes the parameters that characterize methohexital-albumin binding and the influence of physiological or analytical factors on this binding. Two useful and reproducible methods for measuring the free concentration-equilibrium dialysis (ED) and ultrafiltration (UF)-are described and their performances are compared. Methohexital binds exclusively to albumin according to a two-class binding model. The first is a saturable class site of high affinity constant (KA = 11 200 M-1) and a number of sites per albumin molecule of 1. The second is a non-saturable site of poorer affinity (nKA = 810 M-1). The bound fraction of methohexital in the therapeutic range and at physiological albumin concentration is 86.7 +/- 0.9% in isolated albumin solution. In serum, it ranges from 80 to 84.5%, according to subjects (n = 6). Binding is inhibited by the presence of endogenous compounds of serum (for a given albumin concentration the bound fraction decreases from 90.3% in isolated albumin solution to 82.6% in serum), probably by free fatty acids. An increase in the bound fraction is observed when the pH is increased from 7 to 9. This phenomenon may be explained by a higher affinity of the drug towards the basic (B-form) conformation of the albumin molecule, in analogy with the close barbiturate thiopental. A decrease in the bound fraction against temperature is shown, as though binding forces diminished with increase in temperature. Indeed, the binding modification is less pronounced in the presence of serum endogenous compounds. As expected, there is no evidence of any effect of heparin anticoagulant on the bound fraction. Methohexital binding is strongly modified by the albumin concentration; the bound fractions change from 67 to 91% in the albumin range 150-900 microM.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

Binding of endothelin to plasma proteins and tissue receptors: effects on endothelin determination, vasoactivity, and tissue kinetics

In vitro binding of (3-[125I]Tyr)-endothelin-1 ([125I]ET-1) and (3-[125I]Tyr)-big ET-1(1-38) ([125I]big ET-1) to plasma proteins of healthy humans, cardiac patients and normotensive and hypertensive rats was investigated by equilibrium dialysis. Binding of both tracers was similar in plasma from healthy humans, patients with congestive heart failure, and following myocardial infarction (approximately 60%), and marginally higher in rat plasmas (approximately 70%). Binding of [125I]ET-1 to human plasma could be explained by binding to human serum albumin. Endogenous plasma ET-1 levels were approximately 9 pg/ml in healthy humans, and approximately 12-16 pg/ml in cardiac patients; big ET-1 concentrations were approximately two- to threefold higher. ET-1 bound to plasma protein was partly lost in column extraction. In rat isolated perfused hearts, the coronary dilator and constrictor potency of exogenous free and albumin-bound ET-1 was similar, whereas the kinetics of endogenous ET-1 was impeded by tight binding to ET receptors. The data indicate that binding of ET-1 to plasma proteins is without effect on peptide vasoactivity, but binding to tissue receptors greatly impedes its tissue kinetics.

Glyburide protein binding and the effect of albumin glycation in children, young adults, and older adults with diabetes

Because glyburide is a weak acid that is more than 98% bound to albumin, the authors evaluated the binding in vitro and the influence of albumin glycation in children, young adults, and older adults with diabetes mellitus. Glyburide binding to non-glycated albumin was greater than 98%, and remained constant over a total concentration range of 50 to 1000 ng/mL. Increasing the albumin concentration from 0.5 to 5.0 g/dL was logarithmically related to the free fraction of glyburide. After the in vitro glycation of albumin (range, 5.7-15.6%), mean (+/- SD) glyburide binding was 99.05 +/- 0.082%, a value in agreement with that obtained from control serum. Serum samples from 57 subjects with type I and 16 patients with type II diabetes were incubated with 300 ng/mL of unlabeled glyburide and 200 ng/mL of 14C-glyburide. The extent of albumin glycation varied from 5 to 22% for the type I subjects and 5 to 14% for the type II subjects. The free fraction from these groups ranged from 1.07 to 1.75% and 0.66 to 0.88% for the type I and type II subjects, respectively. Although these values did not differ significantly from those of the control samples, the glyburide free fraction in patients with type I diabetes (1.39 +/- 0.85%) was significantly greater than that found for the 16 elderly patients with type II diabetes (0.78 +/- 0.05%). A significant linear relationship was not found between glyburide free fraction and the degree of albumin glycation for either group. Glyburide protein binding did not appear to be influenced by the extent of albumin glycation.