Free drug measurements: methodology and clinical significance

Recent developments in chiral capillary electrophoresis and applications of this technique to pharmaceutical and biomedical analysis

This paper provides an overview of the current status of chiral capillary electrophoresis (CE). The emphasis is placed on the application of CE in chiral separation of various racemic compounds. During the last two years about 280 papers, several review articles, and two entire issues, edited by S. Fanali (Electrophoresis 1999, 20, 2577-2798, and H. Nishi and S. Terabe (J. Chromatogr. A 2000, 879, 1-471.) have been devoted to chiral CE. Enantiomeric separations of various compounds, e.g., pharmaceuticals, drug candidates, drugs and related metabolites in biological fluids, amino acids, di- and tri peptides, pesticides and fungicides, have been performed using different chiral selectors. Native and derivatized cyclodextrins continue to be the most widely used chiral selectors. Other chiral selectors such as natural and synthetic chiral micelles, crown ethers, chiral ligands, proteins, oligo- and polysaccharides, and macrocyclic antibiotics have also been applied to chiral CE separations.

Analysis of free drug fractions by ultrafast immunoaffinity chromatography

A chromatographic method was developed for measuring the nonbound (or free) fraction of drugs by using millisecond-scale extractions on small immunoaffinity columns. The design of this system was developed by considering the dissociation rates of (R)- and (S)-warfarin from the binding protein human serum albumin (HSA) and by performing computer simulations of the immunoaffinity extraction of these drugs. The final system was tested by using it to measure the free fractions of (R)- or (S)-warfarin in samples with known concentrations of these agents and HSA. The free warfarin fraction was extracted in 180 ms by a 2.1-mm-i.d. sandwich microcolumn that contained a 1.1-mm layer of an anti-warfarin antibody support. The nonretained peaks from this immunoaffinity column were passed through a series internal surface reversed-phase columns and a fluorescence detector for the analysis of any protein-bound warfarin that remained in the sample. The experimental results were found to have good agreement with those predicted from the known equilibrium constants for the binding of (R)- and (S)-warfarin with HSA. This approach can be modified for other analytes by changing the types of antibodies that are used in the immunoaffinity column and by using an appropriate detector for the nonretained drug fraction.

Interaction of plasma proteins with cytochromes P450 mediated metabolic reactions: inhibition by human serum albumin and alpha-globulins of the debrisoquine 4-hydroxylation (CYP2D) in liver microsomes of human, hamster and rat

The effect of human serum albumin (HSA), alpha1-acid glycoprotein (alpha1-AGP), and alpha- and gamma-globulins on the in vitro metabolism of debrisoquine in human, hamster and rat liver microsomes was studied. Interaction of albumin with cytochrome P450 mediated phenytoin metabolism has been reported. Since plasma protein binding of phenytoin is high, in the present study a weakly protein bound drug, debrisoquine, was studied. Debrisoquine is a substrate of CYP2D6. The debrisoquine 4-hydroxylation was measured using a radio-TLC method. Among the four plasma proteins, alpha-globulins had the strongest inhibitory effect on the debrisoquine 4-hydroxylase activity. The inhibition with 2% alpha-globulins was 42+/-18% for human and higher for hamster and rat liver microsomes (65-71%). HSA had less effect than alpha-globulins. In the presence of HSA, the decrease in activity was between 18 and 35% for all liver microsomes studied. The debrisoquine 4-hydroxylase activity was not significantly changed by alpha1-AGP or gamma-globulins. Using an ultra-filtration method, the protein binding of debrisoquine to 4% HSA, 0.5% alpha1-AGP, 2% alpha-globulins and 2% gamma-globulins was found to be 22, 20, 22 and 5%, respectively. Since the observed inhibition is inconsistent with level of protein binding, it appears, particularly in the case of alpha-globulins, that the plasma proteins interact with CYP2D directly.

Analysis of bioactive components in traditional Chinese medicines by molecular biochromatography with alpha1-acid glycoprotein stationary phase

Molecular biochromatography with alpha1-acid glycoprotein (AGP) stationary phase was proposed to screen and analyse the biologically active components in traditional Chinese medicines (TCMs) with extracts from Radix Salviae miltiorrhizae as a tested sample. More than ten peaks were resolved based on their affinity to AGP. The effects of concentrations of acetonitrile, pH, concentration of inorganic salt and temperature on the retention behaviors of several major active components were also investigated, and it was found that the hydrophobic effect is the major contributor to retention. Tanshinone IIA was identified as one of the principal bioactive components, which is the marker for the quality control of Radix Salviae miltiorrhizae and a complicated remedy named YiXiTongMai. The amount of tanshinone IIA in Radix Salviae miltiorrhizae and YiXiTongMai determined by this method was 2.9 mg/g (net weight, RSD 4.9%, n=5) and 0.078 mg/g (net weight, RSD 2.5%, n=3), respectively. The possibility for fast differentiation of the TCM sources was also studied by the comparison of the fingerprint of chromatograms for eight typical TCMs on the AGP column. It was observed that different TCMs showed different fingerprint characteristics. Even for the same plant, Rhizoma cimicifugae from three different geographical sources, although there were common characteristics, distinct differences in types and concentrations of biologically active components were clearly observed. It was shown that molecular biochromatography was an effective and fast way for the analysis and screening of biologically active compounds in traditional Chinese medicines.

Role of biantennary glycans and genetic variants of human alpha1-acid glycoprotein in enantioselective binding of basic drugs as studied by high performance frontal analysis/capillary electrophoresis

PURPOSE

To establish a clear understanding of the role of biantennary branching glycans and genetic variants of alpha1-acid glycoprotein (AGP) in enantioselective bindings of basic drug.

METHODS

Human native AGP was separated using concanavalin A affinity chromatography into two subfractions, the unretained fraction (UR-AGP, defect of biantennary glycan) and the retained fraction (R-AGP, possessing biantennary glycan(s)). Imminodiacetate-copper (II) affinity chromatography was used to separate human native AGP into A variant and a mixture of F1 and S variants (F1*S variants). The mixed solutions of the (R)- or (S)-isomer of the model drugs (15 microM disopyramide (DP) or 30 microM verapamil (VER)) and 40 microM of respective AGP species were subjected to high-performance frontal analysis/capillary electrophoresis (HPFA/CE) to determine the unbound drug concentrations.

RESULTS

The unbound concentrations (Cu) of DP in UR-AGP solutions were lower than those in R-AGP solutions, whereas there was no significant difference in the enantiomeric ratios (Cu(R)/Cu(S)) of DP between UR- and R-AGP solutions. In case of genetic variant, the Cu(R)/Cu(S) values of DP in F1*S and A solutions were 1.07 and 2.37, respectively. On the other hand, the enantiomeric ratio of VER in F1*S and A variant solutions were 0.900 and 0.871, respectively.

CONCLUSIONS

The biantennary glycan structures are related to binding affinity of DP to AGP, but not responsible for the enantioselectivity. Genetic variants give significant effect on the enantioselectivity in DP binding, but not in VER binding.

Antiretroviral drug concentrations in semen of HIV-1 infected men

Because semen is a major vehicle for the sexual transmission of HIV-1, control of viral replication within the sanctuary of the male genital tract should be a goal of antiretroviral therapy. Local immune responses, virus specific factors, and the degree of viral and cellular trafficking all appear to be important in controlling viral replication and evolution. However, the most important factor influencing viral replication and evolution within the male genital tract may be the disposition of antiretroviral agents into genital tissues and fluids. This review proposes possible mechanisms of antiretroviral distribution into the male genital tract by using other sanctuary barriers; such as the placenta, renal tubules, and blood-brain barrier; as models. In addition, this review summarises recent clinical studies regarding the disposition of currently available antiretroviral drugs into the seminal plasma and discusses some of the difficulties in interpreting drug concentration in the genital tract.

Chromatographic and electrophoretic studies of protein binding to chiral solutes

Protein interactions are important in determining the transport, metabolism and/or activity of many chiral compounds within the body. This review examines data that have been obtained on these interactions by various chromatographic and electrophoretic methods, especially those based on either high-performance liquid chromatography or capillary electrophoresis. Zonal elution, frontal analysis and vacancy methods are each considered, as are approaches that employ either soluble or immobilized proteins. There are a variety of different items that can be learned about a solute-protein system through these techniques. This includes information on the binding constants and number of binding sites for a solute-protein system, as well as the thermodynamic parameters, rate constants, interaction forces and binding site structure for the protein and solute. Numerous examples are provided throughout this review, as taken from the literature and from work performed within the author's laboratory.

Determination of free concentrations of ropivacaine and bupivacaine in plasma from neonates using small-scale equilibrium-dialysis followed by liquid chromatography-mass spectrometry

Attempts to determine a safe plasma concentration of ropivacaine and bupivacaine in neonates have not been consistent. This might be due to an underestimation of free drug in small plasma samples by currently used techniques, e.g., ultrafiltration. We describe a simple microscale equilibrium-dialysis technique for the separation of free and bound ropivacaine and bupivacaine. The free drug in the dialysate was determined using solid-phase extraction and liquid chromatography with mass spectrometry. Pentycaine was used as an internal standard and added to the dialysates prior to extraction. The method is very selective and sensitive, as no compounds other than the analyte and internal standard were observed in the resulting chromatograms at low ng/ml levels. The limit of quantitation was 2.5 ng/ml. The calibration curve was linear in the range of 2 to 1000 ng/ml. The precision of the whole procedure was 8.1% (n=10) and 6.5% (n=7) for ropivacaine and bupivacaine, respectively. The method was tested in the analysis of plasma samples taken from neonates who had received epidural injections.

Determination of free levels of phenytoin in human plasma by liquid chromatography/tandem mass spectrometry

A liquid chromatography combined with tandem mass spectrometry assay for the determination of free levels of the highly protein bound drug phenytoin (5,5-diphenylhydantoin) in human plasma is described. The assay was demonstrated to be reliable, accurate and precise, and specific for phenytoin. The procedure involves isolation of the unbound drug from the drug/protein complex by ultrafiltration. Liquid-liquid extraction was employed to extract the resultant ultrafiltrate. PHT was separated on a 50 x 3 mm reversed-phase column using isocratic mobile phase conditions that yielded a run time of 1.5 min, enabling high throughput sample analysis. Linearity was obtained over the range 5.00 to 500 ng/ml. Both between-run and within-run coefficients of variation were less than 15% and accuracy's across the assay range were all within 100 +/- 10%. The assay was successfully implemented to support a clinical interaction study with phenytoin.

Evidence-based Implementation of Free Phenytoin Therapeutic Drug Monitoring

Background: The majority of laboratories measure total phenytoin concentration for therapeutic drug monitoring. However, there are substantial interindividual variations in free phenytoin concentrations, the pharmacologically active component.

Methods: We describe the process and data used to implement monitoring of free phenytoin only in an urban medical center. Over a 6-week period, total and free phenytoin concentrations were measured, clinical charts reviewed, and indications for alterations in the percentage of free phenytoin fraction were determined.

Results: Of the 189 phenytoin requests from 139 patients, 136 data points were analyzed. Free phenytoin concentrations were 6.8–35.3%, with 50% outside the expected range of 8–12%. Clinical indications likely responsible for variations were hypoalbuminemia, drug interactions, uremia, pregnancy, and age. Overall, 30% of patients demonstrated a discrepancy between therapeutic, subtherapeutic, or supratherapeutic concentrations between free and total phenytoin concentrations. The largest discordance (53%) occurred in the patient group with free phenytoin <8% or >12%.

Conclusions: This study supports previous clinical findings that monitoring total phenytoin is not as reliable as free phenytoin as a clinical indicator for therapeutic and nontherapeutic concentrations. Thus, we recommend that therapeutic monitoring should use free phenytoin concentrations only.

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.

Enantioselective binding analysis of verapamil to plasma lipoproteins by capillary electrophoresis-frontal analysis

Capillary electrophoresis coupled with frontal analysis was applied to the study of enantioselective binding of verapamil (VER) to plasma lipoproteins. The drug-lipoprotein mixed solution, which had been in the binding equilibrium, was hydrodynamically introduced into a non-coated fused-silica capillary. Since VER is positively charged in the neutral run buffer (pH 7.4), the unbound VER enantiomers migrated toward the cathodic end much faster than negatively charged lipoproteins and their bound forms. Once unbound VER migrated apart from lipoprotein, the bound VER was quickly released from the protein to maintain the binding equilibrium. Thus, VER migrated as a zone through the capillary and gave a trapezoidal peak with a plateau region on the electropherogram. The VER concentration in this plateau region was equal to the unbound VER concentration in the initial sample solution. It was found that the bindings of VER to high-density lipoprotein (HDL), low-density lipoprotein (LDL) and oxidized LDL were not site-specific and not enantioselective. Partition-like binding to lipid part of these lipoproteins seemed to be dominant. The total binding affinities of LDL to VER were about seven-times stronger than those of HDL, and the oxidation of LDL by copper ion enhanced the binding affinities significantly.

High-performance affinity chromatography and immobilized serum albumin as probes for drug- and hormone-protein binding

The binding of drugs and hormones to proteins within the blood is an important process in determining the transport, excretion, metabolism and activity of such agents. This paper discusses the combined use of immobilized serum albumin and high-performance affinity chromatography (HPAC) as tools for the study of such binding processes. The general approaches that are used in such work and are illustrated by several examples taken from previous work in the author's laboratory. The type of qualitative and quantitative information that can be obtained by such work is described, including the comparison of relative binding affinities, competitive displacement by other agents or the measurement of equilibrium and rate constants based on immobilized albumin columns. A comparison is also provided between the results that are obtained by these methods and those that are provided by solution-phase albumin. Some newer advances that are highlighted include use of HPAC to examine the binding of non-polar compounds to albumin, the effects of binding site heterogeneity on HPAC measurements and the use of chemically-modified albumin as a tool to examined the site-specific interactions of solutes with albumin.

Fractionation and analysis of Artemisia capillaris Thunb. by affinity chromatography with human serum albumin as stationary phase

A method for the screening and analysis of biologically active compounds in traditional Chinese medicine is proposed. Affinity chromatography using a human serum albumin (HSA) stationary phase was applied to separate and analyze the bioactive compounds from Artemisia capillaris Thunb. Five major peaks and several minor peaks were resolved based on their affinity to HSA, two of them were identified as scoparone (SCO, 6,7-dimethoxycoumarin) and capillarisin (CAP). CAP shows a much higher affinity to HSA than SCO. The effects of acetonitrile concentration, eluent pH, phosphate concentration and temperature on the retention behaviors of several major active components were also investigated, and it was found that hydrophobicity and eluent pH play major roles in changing retention values. The results demonstrate that the affinity chromatography with a HSA stationary phase is an effective way for analyzing and screening biologically active compounds in traditional Chinese medicine.

Binding of centchroman with human serum as determined by charcoal adsorption method

Protein binding of drugs is an important factor influencing both pharmacokinetic and pharmacodynamic parameters. Thus, knowing the extent of protein binding of drugs is crucial. Centchroman is a non-steroidal once a week oral contraceptive. It has been reported to be useful for the treatment of breast cancer and osteoporosis. Ample data has been generated on pharmacokinetics of centchroman in animals and humans. The extent of protein binding of centchroman has not been established so far. Non-specific adsorption of the drug limits the use of conventional methods like ultrafiltration and equilibrium dialysis. A method of charcoal adsorption as reported by Yuan et al. (method I) was used after modification (method II) to determine its binding to human serum. The extent of protein binding (%) is estimated from decline of percent drug remaining in the supernatant after adding the charcoal. Study was carried out at 1- and 10-microg/ml concentrations in drug free human serum samples and an HPLC assay was used to determine concentration-time data. The percentage of centchroman remaining in serum versus time data was analysed using non-linear fitting programs on WinNonlin software. Method II was found to give higher estimates of protein binding than the former method by preventing the dilution effect. Using this method, the extent of protein binding of centchroman was found to be 101.83+/-1.28 and 94.87+/-3.59% at 1 and 10 microg/ml, respectively. However, it was approximately 90% in the individual serum samples showing intersubject variability in protein binding of centchroman.

Binding analysis of nilvadipine to plasma lipoproteins by capillary electrophoresis-frontal analysis

Capillary electrophoresis coupled with frontal analysis (HPCE/FA) was applied to the ultramicro analysis of enantioselective binding of nilvadipine (NV), a calcium channel blocker, to plasma lipoproteins. The drug lipoprotein mixed solution was hydrodynamically introduced into a non-coated fused silica capillary for capillary electrophoresis. Since NV has no electric charge in the run buffer (pH 7.4), the unbound NV moved towards the cathodic end by electroosmotic flow, which was faster than the electrophoretic migrations of negatively charged lipoproteins and the bound NV. Once unbound NV migrated apart from lipoprotein, and bound NV was quickly released from the protein to maintain the binding equilibrium. Thus, NV migrated as a zone with a plateau region. The concentration of NV in this plateau region appearing on the electrophorogram was the same as the unbound NV concentration in the initial sample solution. It was found that the binding of NV to high-density lipoprotein (HDL), low-density lipoprotein (LDL) and oxidized LDL was non-specific and not enantioselective. Partition-like binding to the lipid part of these lipoproteins seemed to occur dominantly. The total binding affinities of NV to LDL were about seven times stronger than those to HDL, and the oxidation of LDL enhanced the binding affinity significantly.

Binding study of semotiadil and levosemotiadil with alpha(1)-acid glycoprotein using high-performance frontal analysis

High-performance frontal analysis (HPFA) was used to investigate the binding properties of human alpha(1)-acid glycoprotein (AGP) with semotiadil ((R)-isomer, Ca-channel blocker) and its antipode levosemotiadil ((S)-isomer, Ca- and Na-channel blockers). An on-line HPLC system consisting of a HPFA column, an extraction column, and an analytical HPLC column was used to determine the unbound concentrations of these enantiomers, and the experimental data were subsequently subjected to the Scatchard analyses to estimate their binding parameters. The binding affinity of the (R)-isomer (K = 3.17 x 10(7) M, n = 0.74) is approximately 1.2 times stronger than that of (S)-isomer (K = 2.59 x 10(7) M, n = 0.74). An enantioselective competitive binding study indicated that both enantiomers are bound at the same site on AGP molecules.

Affinity Chromatography: A Review of Clinical Applications

Affinity chromatography is a type of liquid chromatography that makes use of biological-like interactions for the separation and specific analysis of sample components. This review describes the basic principles of affinity chromatography and examines its use in the testing of clinical samples, with an emphasis on HPLC-based methods. Some traditional applications of this approach include the use of boronate, lectin, protein A or protein G, and immunoaffinity supports for the direct quantification of solutes. Newer techniques that use antibody-based columns for on- or off-line sample extraction are examined in detail, as are methods that use affinity chromatography in combination with other analytical methods, such as reversed-phase liquid chromatography, gas chromatography, and capillary electrophoresis. Indirect analyte detection methods are also described in which immunoaffinity chromatography is used to perform flow-based immunoassays. Other applications that are reviewed include affinity-based chiral separations and the use of affinity chromatography for the study of drug or hormone interactions with binding proteins. Some areas of possible future developments are then considered, such as tandem affinity methods and the use of synthetic dyes, immobilized metal ions, molecular imprints, or aptamers as affinity ligands for clinical analytes.

The role of branching glycan of human alpha1-acid glycoprotein in enantioselective binding to basic drugs as studied by capillary electrophoresis

The role of the branching glycan structure of human alpha1-acid glycoprotein (AGP) in the interaction with basic drugs was investigated in terms of enantioselectivity in binding ability. AGP was separated by concanavalin A lectin affinity chromatography into two subfractions, the unretained AGP (UR-AGP) which has no biantennary glycan chain and the retained AGP (R-AGP) which possesses biantennary oligosaccharide chain(s). The unbound concentrations of propranolol (PRO) enantiomers and verapamil (VER) enantiomers in UR-AGP solution and R-AGP solution were determined by high-performance frontal analysis combined with capillary electrophoresis. It was found that (S)-PRO is bound to UR-AGP and R-AGP more strongly than (R)-PRO, whereas the reverse applies to VER enantiomers, and that such enantioselectivity is common to these proteins. This suggests that the branching type of glycan chains of AGP does not play significant role in the chiral recognition in binding these basic drugs.

High-performance frontal analysis for drug-protein binding study

High-performance frontal analysis (HPFA) is a novel analytical method which enables simultaneous determination of total and unbound drug concentrations under drug-plasma protein binding condition. HPFA can be achieved using separation systems such as HPLC and CE. This paper deals with the principle and feature of HPFA method and its application to the stereoselective protein binding study. HPFA allows a simple analysis following direct sample injection, and does not suffer from undesirable drug adsorption on membrane nor leakage of bound drug through the membrane which are often encountered in conventional ultrafiltration and dialysis methods. HPFA can be easily incorporated into on-line HPLC system. By coupling HPFA with a chiral HPLC column, the unbound concentration of a racemic drug can be determined enantioselectively. The detection limit can be improved by coupling of HPFA with a preconcentration column. High-performance capillary electrophoresis/frontal analysis (HPCE/FA) enables to determine unbound concentrations enantioselectively with ultramicro injection volume, and is hence useful especially for the binding study of proteins which are scarce and difficult to obtain.

Column switching in capillary liquid chromatography-tandem mass spectrometry for the quantitation of pg/ml concentrations of the free basic drug tolterodine and its active 5-hydroxymethyl metabolite in microliter volumes of plasma

A capillary column switching system was developed for the determination of low, unbound concentrations of the basic drug tolterodine and its active 5-hydroxymethyl (5-HM) metabolite in human plasma. Free concentrations of tolterodine and 5-HM at pM and nM (pg/ml and ng/ml) levels were obtained by ultrafiltration of 40-400 microliters plasma at 37 degrees C. The free fraction (%) was independent of the plasma concentrations of the analytes. Detection of the analytes was performed by sheathless electrospray tandem mass spectrometry in the multiple-reaction monitoring mode. The selectivity of the mass spectrometric detection and the additional clean-up on the pre-column allowed direct injection of the ultrafiltrated plasma samples. Tolterodine and 5-HM were pre-concentrated on a reversed-phase capillary pre-column (1 cm x 200 microns) and subsequently backflushed onto the separation column (25 cm x 200 microns). The stability of the chromatographic system was good; a large number of ultrafiltrated plasma samples could be injected and the relative standard deviation of the retention times was typically < or = 1% (within-day). The accuracy was between 86 and 105% and the precision was between 1 and 7% without the use of an internal standard. Linear calibration curves were obtained between 100 pM and 100 nM.

Multi-site binding of fenoprofen to human serum albumin studied by a combined technique of microdialysis with high performance liquid chromatography

A simple and fast method for the determination of the multi-site binding of fenoprofen (FP) to human serum albumin (HSA) has been developed by utilizing microdialysis sampling techniques combined with high performance liquid chromatography (HPLC). The drug and protein were mixed in different molar ratios in 0.067 Mol potassium phosphate buffer, pH 7.4, and incubated at 37 degrees C in a water-bath. Then the microdialysis probe was put in the FP-HSA solution and sampled at the perfusion rate of 1 microL/min. The concentrations of FP in microdialysates were determined by the reversed-phase high performance liquid chromatography. Relative recovery (R) was also determined in vitro on similar condition, R is about 56.03 +/- 1.11% (n = 3). Fenoprofen was found to bind to two classes of sites, the association constant (K1) and the number of the binding sites on primary binding sites of a HSA molecule (n1) for fenoprofen are 3.4 x 10(5)/M and 2.5, respectively, and those for secondary binding are 1.0 x 10(4)/M and 10.0, respectively. The competitive interaction of ibuprofen (IP) and palmitic acid with fenoprofen to HSA were also studied, both compounds significantly decrease the binding degree of fenoprofen to HSA.

Recent advances in chromatographic and electrophoretic methods for the study of drug-protein interactions

Drug-protein binding is an important process in determining the activity and fate of a pharmaceutical agent once it has entered the body. This review examines various chromatographic and electrophoretic methods that have been developed to study such interactions. An overview of each technique is presented along with a discussion of its strengths, weaknesses and potential applications. Formats that are discussed include the use of both soluble and immobilized drugs or proteins, and approaches based on zonal elution, frontal analysis or vacancy peak measurements. Furthermore, examples are provided that illustrate the use of these methods in determining the overall extent of drug-protein binding, in examining the displacement of a drug by other agents and in measuring the equilibrium or rate constants for drug-protein interactions. Examples are also given demonstrating how the same methods, particularly when used in high-performance liquid chromatography or capillary electrophoresis systems, can be employed as rapid screening tools for investigating the binding of different forms of a chiral drug to a protein or the binding of different proteins and peptides to a given pharmaceutical agent.

Highly sensitive method for the determination of tamsulosin hydrochloride in human plasma dialysate, plasma and urine by high-performance liquid chromatography-electrospray tandem mass spectrometry

A highly sensitive method for the determination of tamsulosin hydrochloride, a structurally new type of sulphamoile derivative, in human plasma dialysate, plasma and urine has been developed by using liquid chromatography-electrospray tandem mass spectrometry (LC-MS-MS). Plasma dialysate, plasma and urine samples were extracted by brief liquid-phase extraction and analyzed using an HPLC system coupled to a mass spectrometer via an electrospray ionization interface. Selected reaction monitoring was used for the detection of tamsulosin and its internal standard. This method was validated in the concentration range 10-1000 pg/ml in plasma dialysate, 0.5-50 ng/ml in plasma, and 1-100 ng/ml in urine with sufficient specificity, accuracy and precision. The in vivo protein binding study demonstrated that the unbound tamsulosin in human plasma obtained by the equilibrium dialysis after 0.4-mg oral dosing was measurable. In addition, the percentage of unbound tamsulosin in an in vitro study (0.71-0.91%) obtained by using spiked 14C-labelled tamsulosin was slightly larger than that of the in vivo study (0.68-0.86%), indicating that the unbound concentration calculated by the product of the plasma concentration and the in vitro unbound fraction (fu) was unfavorably overestimated. These results suggest that the combination of LC-MS-MS and equilibrium dialysis method has enough sensitivity to determine the unbound concentration in clinical use and gives the concentration more exactly than the in vitro fu.

Enantioselective protein binding of semotiadil and levosemotiadil determined by high-performance frontal analysis

An on-line frontal analysis HPLC system was developed for the determination of the unbound concentrations of semotiadil, a new calcium antagonist with non-dihydropyridine structure, and its antipode (Levosemotiadil), and was applied to the enantioselective investigation of their plasma protein binding properties. This system consists of a high-performance frontal analysis (HPFA) column, an extraction column, and an analytical column, which are connected via two switching valves. After the direct injection of the sample solution into the HPFA column, the drug was eluted as a zonal peak with a plateau region. The unbound drug concentration was determined as the drug concentration in the plateau. As low as 1.04 nM of the unbound drug was determined with good reproducibility. Semotiadil (R-isomer) and levosemotiadil (S-isomer) are bound strongly and enantioselectively to human serum albumin (HSA) and human alpha 1-acid glycoprotein (AGP), and the enantioselectivity was reversed between these plasma proteins. While HSA binds S-isomer more strongly than the antipode, human AGP binds R-isomer more strongly. In human plasma, the unbound drug fraction was less than 1%, and the enantioselectivity was similar to that observed in AGP solution.

Valproic acid-ketoconazole interaction in normal, hypoalbuminemic, and uremic sera: lack of interaction in uremic serum caused by the presence of inhibitor

Ketoconazole is an antifungal agent widely used in the management of patients with fungal infection, especially in patients with acute acquired immuno-deficiency syndrome (AIDS). Ketoconazole is 99% bound to serum albumin and may interact with valproic acid, an anticonvulsant with 90% to 95% binding to serum albumin. The interaction may be more significant in hypoalbuminemia, a common finding in patients with AIDS. However, valproic acid-ketoconazole interaction has not been reported. The authors prepared two serum pools from patients receiving valproic acid with normal serum albumin and another pool from patients with hypoalbuminemia. Another serum pool was prepared from uremic patients not receiving valproic acid. The aliquots of serum pool were supplemented with various concentrations of ketoconazole, representing therapeutic and slightly higher therapeutic concentrations. The concentrations of free valproic acid were determined in protein-free ultrafiltrates (prepared by centrifuging specimens at 25 degrees C with the Centrifree Micropartition System at 1500 g for 20 minutes) using fluorescence polarization immunoassay. In the serum pool with normal albumin concentration, the authors observed statistically significant displacement of valproic acid only at higher ketoconazole concentrations (10 and 20 micrograms/ml) whereas, in the serum pool with hypoalbuminemia, they observed statistically significant displacement of valproic acid by ketoconazole with lower and higher concentrations of ketoconazole. The magnitude of displacement was more significant at high valproic acid concentrations (95 and 150 mg/ml, respectively) probably because of the concentration-dependent binding of valproic acid to serum albumin. The authors observed no displacement of valproic acid by ketoconazole in the uremic serum pool. On the other hand, the free valproic acid concentrations were decreased in the presence of ketoconazole in the uremic serum pool.

Determination of the binding of a beta 2-blocker drug, frusemide and ceftriaxone to serum proteins by capillary zone electrophoresis

A modified Hummel-Dreyer method was used to study the binding of drugs with serum proteins by high performance capillary electrophoresis. The study was carried out to check the possible interaction between serum proteins and a highly selective beta 2-blocker, ICI 118551 (ICI). To prove the suitability of the method the protein binding of frusemide and ceftriaxone, drugs previously investigated, was also studied. The analyses were carried out by injecting a solution of s alpha(1)-acidic glycoprotein (alpha(1)-AGP) or human serum albumin in 70 mM NaH2PO4/Na2HPO4(pH 7.4) buffer into an uncoated fused silica capillary filled with the same buffer. In the capillary, maintained at a working temperature of 35 degrees C, a known amount of the ICI, frusemide or ceftriaxone was added. The method allows the bound drug to be determined directly.

Protein-binding high-performance frontal analysis of (R)- and (S)-warfarin on HSA with and without phenylbutazone

Applicability of high-performance frontal analysis (HPFA) to the stereoselective study of drug-drug interaction upon plasma protein binding has been investigated. Racemic warfarin and phenylbutazone were used as model drugs. An on-line HPFA/HPLC system consisting of a HPFA column (diol-silica column), an extraction column, and a chiral separation column was developed, and human serum albumin solution containing racemic warfarin and/or phenylbutazone was injected directly to the HPFA column. When the injection volume was large enough, the binding equilibrium in the sample solution was reproduced in the column, and consequently a plateau region appeared on the chromatogram. This plateau region contains unbound drug(s). A given volume of eluent in the plateau part was transferred into the extraction column by column-switching. The concentrated drug(s) was then transferred to the chiral separation column to determine the unbound concentrations of the enantiomers and/or the competitor. The results agreed with those obtained by a conventional ultrafiltration-HPLC method. The influence of phenylbutazone upon the protein binding of warfarin is enantioselective. In warfarin and human serum albumin mixed solution, the unbound concentration of (R)-warfarin was 1.22 times higher than that of the S-isomer. By addition of phenylbutazone, the unbound concentration of (S)-warfarin increased more than that of (R)-warfarin, resulting in the reversed enantioselectivity, i.e., the unbound concentration of (S)-warfarin became 1.19 times larger than that of (R)-warfarin. The present method was also applicable to human plasma samples.

Effect of sialic acid residues of human alpha 1-acid glycoprotein on stereoselectivity in basic drug-protein binding

The function of sialic acid groups at the terminal of sugar chains of human alpha 1-acid glycoprotein (AGP) was investigated with respect to chiral discrimination between optical isomers of basic drugs, using high-performance capillary electrophoresis/frontal analysis (HPCE/FA), a novel analytical method developed for the determination of unbound drug concentration with ultramicrosample volume (100-200 nl). Native human AGP and desialylated AGP were used as test proteins, and propranolol (PRO) and verapamil (VER) were used as model drugs. The unbound concentration of (S)-VER was 1.31 times higher than that of (R)-VER in native AGP solution. This selectivity was not affected by desialylation. Further, enzymatic elimination of galactose residues, which neighbored sialic acid groups, did not change the binding of either isomer of VER. On the other hand, the unbound concentration of (R)-PRO was 1.27 times higher than that of (S)-PRO in native AGP solution. Desialylation caused the unbound concentration of (S)-PRO to rise to the same level of (R)-PRO, resulting in loss of enantioselectivity. Thus, it follows that sialic acid groups of AGP, as a whole, are not responsible for chiral recognition between enantiomers of VER but are involved in enantioselectivity toward the isomers of PRO.

Pharmacokinetics of tamsulosin hydrochloride in patients with renal impairment: effects of alpha 1-acid glycoprotein

The pharmacokinetics of tamsulosin hydrochloride in patients with renal impairment were compared with those in healthy volunteers, and the factors that influenced plasma levels of tamsulosin were elucidated. A single oral dose of 0.2 mg of tamsulosin was given and blood and urine samples were obtained for 36 hours after administration. Unbound plasma concentration of tamsulosin was measured by a combination of equilibrium dialysis and liquid chromatography tandem mass spectrometry methods to examine the effect of protein binding on the pharmacokinetics of tamsulosin. Mean values for maximum concentration (Cmax) and area under the concentration-time curve (AUC) of total drug (Cmax,t and AUC1) in patients with renal impairment were 73% and 211% greater, respectively, than those in healthy volunteers. Mean Cmax and AUC of unbound drug (Cmax,u and AUCu), however, were almost the same in the two groups. A high correlation was found between alpha 1-acid glycoprotein (alpha 1-AGP) concentration and AUCt, but no correlation was found between alpha 1-AGP concentration and AUCu,0-36) or between creatinine clearance (ClCR) and AUCu,0-36). These results show that in patients with renal impairment, the pharmacokinetics of tamsulosin are affected by the change in protein binding that is associated with alteration of plasma alpha 1-AGP concentration, but are not largely affected by the decrease in the renal excretion. Although total tamsulosin levels increased as plasma protein binding increased, unbound tamsulosin levels (which are directly associated with the pharmacologic effects) remained unchanged in these patients.

Solid-phase microextraction for the determination of the free concentration of valproic acid in human plasma by capillary gas chromatography

The potential of solid-phase microextraction in the bioanalysis of drugs is demonstrated. The free concentration of valproic acid in human plasma was determined by equilibrium dialysis at room temperature. To the dialysate was added an internal standard and the pH was adjusted to 2.5. The polymethylsiloxane-coated fused-silica fibre of the solid-phase microextraction device was inserted into the dialysate for 3 min. The sorbed analytes were then thermally desorbed at 210 degrees C in the split-splitless injection port of the gas chromatograph, separated on a Nukol capillary column and detected with a flame ionization detector. The method was shown to be highly reproducible with a detection limit of 1 microgram/ml of free valproic acid in human plasma.

High-performance frontal analysis for the study of protein binding of troglitazone (CS-045) in albumin solution and in human plasma

An on-line frontal analysis HPLC system was developed for the determination of the unbound concentration of troglitazone (CS-045), a new oral antidiabetic agent, in human serum albumin (HSA) solution and in human plasma. This system consists of a high-performance frontal analysis (HPFA) column, an extraction column, and an analytical column, which are connected via two switching valves. After the direct injection of the sample solution into the HPFA column, the drug was eluted as a zonal peak with a plateau region. The unbound drug concentration was determined as the drug concentration in the plateau. As low as 0.3 nM unbound CS-045 was determined with good reproducibility. It was found that CS-045 strongly binds with HSA, and the bound fraction in the 550 microM HSA solution was 99.93%, which was very close to that in human plasma (99.89%). The bound fractions were constant within the total drug concentration range of 1-10 microM in the HSA solution and 250 nM-10 microM in human plasma.

Study of the enantioselective binding between BOF-4272 and serum albumins by means of high-performance frontal analysis

High-performance frontal analysis (HPFA) was incorporated in an on-line HPLC system for the study of the enantioselective binding of BOF-4272, a new xanthine oxidase inhibitor, with human, bovine and rat serum albumins. This HPLC system consists of a HPFA column (diol-silica column), an extraction column (C4 column) and a chiral separation column (beta-cyclodextrin immobilized silica column), which were connected in series via two column switching valves. After the direct injection of a solution of 0.5-400 microM racemic BOF-4272 and 550 microM serum albumin onto the HPFA column, BOF-4272 was eluted, under a mild mobile phase condition (phosphate buffer, pH 7.4, ionic strength 0.17), as a zonal peak containing a plateau region. The drug concentration in the plateau region is the same as that for the unbound drug concentration in the sample solution. A given volume of this plateau region was transferred into the extraction column, and subsequently the extracted BOF-4272 was transferred into the chiral separation column to determine the unbound concentration of each enantiomer. The binding between BOF-4272 and the serum albumins was enantioselective and species dependent. The unbound concentration of the (+)-isomer in rat serum albumin solution was 1.04-1.14 times larger than that of the antipode, while the unbound concentration of the (-)-isomer in bovine serum albumin solution was 1.04-1.16 times larger than that of the antipode. The enantioselectivity of the binding between BOF-4272 and human serum albumin was concentration dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Disposition kinetics of HEPP in rats after intravenous, oral, and intraperitoneal administration. Correlation of plasma and brain levels with the anticonvulsant effect

D, L-3-hydroxy-3-ethyl-3-phenylpropanamide (HEPP) is a synthetic drug with anticonvulsant effects in a variety of seizure models. HEPP pharmacokinetics was studied after single 50 mg kg-1 intravenous (i.v.), intraperitoneal (i.p.), and oral (PO) administration in male albino Wistar rats. The plasma concentration against time curves showed a biphasic decay pattern with a similar distribution phase and the same terminal rate constant (beta = 0.22 h-1) by all three routes. The apparent volume of distribution at steady state (Vss = 0.80 L kg-1) indicates that HEPP is extensively distributed in extracellular tissues. This finding agrees very well with its low binding to plasma protein (mean bound fraction = 19.3 +/- 1.1%). The systemic clearance (Cl) was very low (3.30 mL min-1 kg-1). The bioavailability after IP and PO administration was 0.80 and 0.60 respectively. In the pharmacokinetic-pharmacodynamic studies a direct relationship was found between the protective effect of HEPP against pentylenetetrazole (PTZ) induced seizures and its concentration in plasma and/or brain. The concentrations at half-maximal effect (EC50) with 95% confidence interval (Cl) were 70.6 (66-75.5) micrograms mL-1 in serum and 60.1 (55.4-65.1) micrograms g-1 in brain. There was a rapid uptake of HEPP into the brain, and after the distributive phase, the disappearances in plasma and brain were almost parallel [C(serum) = 109 e-0.25t, r2 = 0.95; C(brain) = 38 e-2.53t + 91 e-0.21t, r2 = 0.93], with a C(brain)/C(plasma) ratio of 1.1.

Evaluation of ultrafiltration for the free-fraction determination of single photon emission computed tomography (SPECT) radiotracers: beta-CIT, IBF, and iomazenil

An ultrafiltration system was evaluated for the free-fraction measurement of SPECT radiotracers (beta-CIT, IBF, and iomazenil) used in functional brain imaging. The effect of temperature, storage, centrifugal force, tracer concentration, and percentage filtered demonstrated a relative error of < 9%. As a result of the minimal temperature effect, 25 degrees C was employed for all measurements. A comparison of the ultrafiltration system with equilibrium dialysis revealed < 5% difference for beta-CIT and iomazenil, but 16% for IBF. Additionally, the time and ease of operation considerably favored the ultrafiltration system. The precision quantitated by repetition was < 6% for between-run and within-run variability. In conclusion, ultrafiltration provided rapid results, demonstrated minor analytical errors, revealed generally good correlation with equilibrium dialysis, and allowed excellent precision.

Determination of the unbound concentration of hydrophobic drugs in albumin solutions by high-performance frontal analysis using a diol-silica column

High-performance frontal analysis (HPFA) using diol-silica column allows the determination of the unbound concentration of hydrophobic drugs under protein binding equilibrium, which is often difficult by the conventional methods. After the direct injection of a drug-protein mixed solution onto a diol-silica column, the drug is eluted as a zonal peak containing a plateau region under a mild mobile phase condition (phosphate buffer, pH 7.4, ionic strength = 0.17). The unbound drug concentration was determined as the drug concentration in the plateau region. The reliability of this method was confirmed by comparison with the results obtained by the conventional ultrafiltration-HPLC method. The versatility of the HPFA method using a diol-silica column was demonstrated by developing a novel on-line HPFA-chiral HPLC system for a simple and easy determination of the unbound concentration of nilvadipine (NV) enantiomers. The direct injection of 1.33 mL of a sample solution containing 20 nM-1 microM NV and 550 microM HSA gave the plateau region due to unbound NV, which was heart-cut and transferred on-line into a preconcentration and chiral HPLC separation system. As low as a few hundred picomolar level of unbound NV enantiomers was determined by use of UV detection (at 244 nm). The binding between NV and HSA is enantioselective; (R)-NV binds more strongly than (S)-NV.

High-performance capillary electrophoresis/frontal analysis for the study of protein binding of a basic drug

A simple high-performance capillary electrophoresis (HPCE) method based on the principle of frontal analysis was applied to the determination of the concentration of unbound basic drug in protein binding equilibrium. A small volume of sample solution (approximately 80 nL) containing 113-340 microM of verapamil (VER) and 100-550 microM of human serum albumin was introduced into the fused silica capillary (effective length, 22 cm; 50-microns i.d.) by suction. Because the silanol groups on the inner surface of the capillary were bound with linear polyacrylamide through Si-C bonds, electroosmotic flow was not generated even at pH 7.4 with an applied voltage of +10 kV. The unbound drug bearing positive charge migrated electrophoretically from the drug-protein mixed zone toward the detection end, whereas human serum albumin did not co-migrate because of its negative charge. The bound drug migrated after it was released from the protein. As a result of an 80-nL injection of the sample solution, VER was eluted as a zonal peak with a plateau region. The VER concentration calculated from the plateau height agreed well with the unbound VER concentration determined by the conventional ultrafiltration-HPLC method, with good reproducibility (CV, < 6.23%, n = 15). The present HPCE/FA system was applied to the Scatchard analysis of VER and alpha 1-acid glycoprotein binding, and the estimated binding parameters agreed well with literature values.

Liquid chromatographic analysis, stability and protein binding studies of the anti-HIV agent benzoic acid, 2-chloro-5[[(1-methylethoxy)thioxomethyl]amino]-,1-methylethyl ester

A method was developed for the assay of benzoic acid, 2-chloro-5-[[(1-methylethoxyl)thioxomethyl]amino]-,1-meth yle thyl ester (NSC 629243) in hamster, mouse, human and, to a limited extent, dog plasma. Protein in 0.5 ml of plasma was precipitated with four volumes of methanol and the supernatant was analysed for NSC 629243 by LC. Liquid chromatography was carried out on a reversed-phase Nova-Pak C18 column, with a mobile phase of 60% acetonitrile in water at 1 ml min-1, and the compound was quantified with a UV detector set at 283 nm. Two standard curves of NSC 629243 were needed to cover a concentration range of 0.05-100 micrograms ml-1. All standard curves had correlation coefficients > 0.999. In practice, the minimum quantifiable concentration was approximately 0.05 microgram ml-1 in 0.5 ml of plasma. NSC 629243 appeared to have good stability at 37 degrees C in hamster, human and dog plasma at concentrations of 1 and 50 micrograms ml-1 (at least 80% remained in plasma after a 4 h incubation). Breakdown occurred in mouse plasma after 1 h at 37 degrees C, with extensive breakdown occurring after 24 h. NSC 629243 was extensively bound to plasma proteins of Syrian hamsters and humans. The extent of binding ranged from a minimum of 88.6% to a maximum of 99.9% over a concentration range of ca 1-100 micrograms ml-1.

Liposolubility and protein binding of oxycodone in vitro

The liposolubility and protein-binding of oxycodone were studied in vitro and compared with other opioids. Liposolubility was assessed by three different methods: 1) the shake-flask method with n-octanol at pH 4-9, 2) measuring the retention time in reversed-phase high-performance liquid chromatography (RP-HPLC) with a LiChrosorb RP-18 and 3) studying the solubility in human epidural and subcutaneous fat. Human fat was obtained from patients undergoing surgery for herniated intervertebral disc. After incubation, pieces of fatty tissue immersed in a buffer solution containing oxycodone, morphine, pethidine or fentanyl for 10-40 min.; tissue pieces were homogenated, opioids extracted and opioid concentrations measured by gas- and high-performance liquid chromatography. The binding of oxycodone, morphine and fentanyl in plasma proteins was studied by ultrafiltration (Amicon-kit). The mean apparent partition coefficients Papp of oxycodone, morphine, pethidine and fentanyl in n-octanol at pH 7 were 0.7, 0.5, 10.5 and 399, respectively. The retention times in RP-HPLC for oxycodone, morphine, pethidine, fentanyl and buprenorphine were 0.6 min., 0.2 min., 2.4 min., 2.3 min. and 10.5 min., respectively. Only buprenorphine and fentanyl appeared to be highly lipophilic in the human fat tissue experiments; no difference was found between epidural or subcutaneous fat in this respect. The in vitro protein binding of oxycodone was 38%, of morphine 31% and of fentanyl 87% in average. It is concluded that, in terms of physiochemical properties, liposolubility and protein-binding, oxycodone resembles morphine more than it does fentanyl.

A naturally occurring furan fatty acid enhances drug inhibition of thyroxine binding in serum

We studied the thyroxine (T4)-displacing effects of a naturally occurring, highly albumin-bound furanoid acid that accumulates in serum in renal failure to concentrations in excess of 0.2 mmol/L. This substance, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), has been shown to displace acidic drugs from albumin binding. The effects of CMPF on ligand binding were assessed in the following systems: (1) T4 binding to T4-binding globulin (TBG) and transthyretin (TTR), (2) T4 binding in undiluted serum, (3) T4-displacing potency of fenclofenac, furosemide, diflunisal, and aspirin in undiluted serum, (4) serum binding of [14C]-drug preparations, and (5) serum binding of [14C]-oleic acid. CMPF had a minor direct effect on T4 binding to TBG comparable in relative affinity to that of aspirin, ie, almost 7 orders of magnitude less than T4 itself. CMPF alone at a concentration of 0.3 mmol/L, which produced only a 10% to 14% increase in free T4 augmented the T4-displacing effects of high therapeutic concentrations of the various drugs in undiluted serum as follows: furosemide by 180%, fenclofenac by 160%, diflunisal by 130%, and aspirin by 40%. In the presence of fenclofenac, increments of CMPF from 0.075 to 0.3 mmol/L progressively augmented the T4-displacing effect of this drug, associated with a progressive increase in its calculated free concentration. CMPF also inhibited the binding of [14C]-oleic acid, suggesting that in some situations CMPF could also indirectly influence thyroid hormone binding by increasing the unbound concentration of nonesterified fatty acids (NEFA), as previously described.(ABSTRACT TRUNCATED AT 250 WORDS)

Simultaneous determination of carbamazepine, phenytoin, phenobarbital, primidone and their principal metabolites by high-performance liquid chromatography with photodiode-array detection

We have established a precise and accurate high-performance liquid chromatographic method for the simultaneous assay of carbamazepine, phenytoin, phenobarbital, primidone and their principal metabolites. This method has been used for the analysis of these drugs and the metabolites in serum, saliva and urine samples. Acetonitrile is used for the deproteinization of serum and saliva samples while solid-phase extraction is utilized for urine sample pretreatment. Samples of 2 microliters are injected onto a 3-microns ODS-Hypersil column (250 mm x 2 mm I.D.) with a column temperature of 40 degrees C. The drugs and metabolites are eluted with a mobile phase containing potassium phosphate buffer-acetonitrile-methanol (110:50:30, v/v/v) at a flow-rate of 0.2 ml/min. Signals are monitored by a photodiode-array detector at a sample wavelength of 200 nm with a bandwidth of 10 nm. These four commonly used antiepileptic drugs and their six metabolites are well separated from one another within 15 min. Within-day coefficients of variation (C.V.) are within 5% in most cases and between-day C.V. are from 2.32 to 4.75%. The recovery rates range from 95.12 to 104.42%. This method has the necessary sensitivity and linearity for routine therapeutic monitoring of both total and free drug levels and may be employed for pharmacokinetics studies of drug interactions and metabolism as well.