Identification of drug-binding sites on human serum albumin using affinity capillary electrophoresis and chemically modified proteins as buffer additives

Analysis of drug interactions with serum proteins and related binding agents by affinity capillary electrophoresis: A review

Biomolecules such as serum proteins can interact with drugs in the body and influence their pharmaceutical effects. Specific and precise methods that analyze these interactions are critical for drug development or monitoring and for diagnostic purposes. Affinity capillary electrophoresis (ACE) is one technique that can be used to examine the binding between drugs and serum proteins, or other agents found in serum or blood. This article will review the basic principles of ACE, along with related affinity-based capillary electrophoresis (CE) methods, and examine recent developments that have occurred in this field as related to the characterization of drug-protein interactions. An overview will be given of the various formats that can be used in ACE and CE for such work, including the relative advantages or weaknesses of each approach. Various applications of ACE and affinity-based CE methods for the analysis of drug interactions with serum proteins and other binding agents will also be presented. Applications of ACE and related techniques that will be discussed include drug interaction studies with serum agents, chiral drug separations employing serum proteins, and the use of CE in hybrid methods to characterize drug binding with serum proteins.

Enantioresolution and Binding Affinity Studies on Human Serum Albumin: Recent Applications and Trends

The interaction between proteins and drugs or other bioactive compounds has been widely explored over the past years. Several methods for analysis of this phenomenon have been developed and improved. Nowadays, increasing attention is paid to innovative methods, such as high performance affinity liquid chromatography (HPALC) and affinity capillary electrophoresis (ACE), taking into account various advantages. Moreover, the development of separation methods for the analysis and resolution of chiral drugs has been an area of ongoing interest in analytical and medicinal chemistry research. In addition to bioaffinity binding studies, both HPALC and ACE al-low one to perform other type of analyses, namely, displacement studies and enantioseparation of racemic or enantiomeric mixtures. Actually, proteins used as chiral selectors in chromatographic and electrophoretic methods have unique enantioselective properties demonstrating suitability for the enantioseparation of a large variety of chiral drugs or other bioactive compounds. This review is mainly focused in chromatographic and electrophoretic methods using human serum albumin (HSA), the most abundant plasma protein, as chiral selector for binding affinity analysis and enantioresolution of drugs. For both analytical purposes, updated examples are presented to highlight recent applications and current trends.

Characterization of drug binding with alpha1-acid glycoprotein in clinical samples using ultrafast affinity extraction

Many drugs bind to serum transport proteins, which can affect both drug distribution and activity in the body. α₁-Acid glycoprotein (AGP) is a key transport protein for basic and neutral drugs. Both elevated levels and altered glycosylation patterns of AGP have been seen in clinical conditions such as systemic lupus erythematosus (SLE). This study developed, optimized, and used the method of ultrafast affinity extraction (UAE) to examine whether these changes in AGP are associated with changes in the binding by some drugs to this transport protein. This approach used affinity microcolumns to capture and measure, in serum, the free fractions of several drugs known to bind AGP. These measurements were made with pooled normal control serum and serum samples from individuals with SLE. Immunoaffinity chromatography was used to obtain the content of AGP and HSA in these samples, and CE was used to examine the glycoform pattern for AGP in each serum sample. The free drug fractions measured for normal control serum ranged from 3.5 to 29.1%, in agreement with the results of ultrafiltration, and provided binding constants of ~10⁵-10⁶ M^-1 for the given drugs with AGP at 37⁰C. Analysis of a screening set of SLE serum samples by UAE gave decreased free fractions (relative change, 12-55%) vs normal serum when spiked with the same types and amounts of drugs. These changes were related in some cases to AGP content, with some SLE samples having AGP levels 1.3- to 2.1-fold above the upper end of the normal range. In other cases, the changes in free fractions appeared to be linked to alterations in the glycoforms and binding constants of AGP, with some affinities differing by 1.2- to 1.5-fold vs normal AGP. This approach can be employed with other solute-protein systems and to investigate binding by other drugs or transport proteins directly in clinical samples.

Characterization of solution-phase drug-protein interactions by ultrafast affinity extraction

A number of tools based on high-performance affinity separations have been developed for studying drug-protein interactions. An example of one recent approach is ultrafast affinity extraction. This method has been employed to examine the free (or non-bound) fractions of drugs and other solutes in simple or complex samples that contain soluble binding agents. These free fractions have also been used to determine the binding constants and rate constants for the interactions of drugs with these soluble agents. This report describes the general principles of ultrafast affinity extraction and the experimental conditions under which it can be used to characterize such interactions. This method will be illustrated by utilizing data that have been obtained when using this approach to measure the binding and dissociation of various drugs with the serum transport proteins human serum albumin and alpha₁-acid glycoprotein. A number of practical factors will be discussed that should be considered in the design and optimization of this approach for use with single-column or multi-column systems. Techniques will also be described for analyzing the resulting data for the determination of free fractions, rate constants and binding constants. In addition, the extension of this method to complex samples, such as clinical specimens, will be considered.

Analysis of biomolecular interactions using affinity microcolumns: a review

Affinity chromatography has become an important tool for characterizing biomolecular interactions. The use of affinity microcolumns, which contain immobilized binding agents and have volumes in the mid-to-low microliter range, has received particular attention in recent years. Potential advantages of affinity microcolumns include the many analysis and detection formats that can be used with these columns, as well as the need for only small amounts of supports and immobilized binding agents. This review examines how affinity microcolumns have been used to examine biomolecular interactions. Both capillary-based microcolumns and short microcolumns are considered. The use of affinity microcolumns with zonal elution and frontal analysis methods are discussed. The techniques of peak decay analysis, ultrafast affinity extraction, split-peak analysis, and band-broadening studies are also explored. The principles of these methods are examined and various applications are provided to illustrate the use of these methods with affinity microcolumns. It is shown how these techniques can be utilized to provide information on the binding strength and kinetics of an interaction, as well as on the number and types of binding sites. It is further demonstrated how information on competition or displacement effects can be obtained by these methods.

CHROMATOGRAPHIC ANALYSIS OF DRUG INTERACTIONS IN THE SERUM PROTEOME

The binding of drugs with serum proteins and binding agents such as human serum albumin, α1-acid glycoprotein, and lipoproteins is an important process in determining the activity and fate of many pharmaceuticals in the body. A variety of techniques have been used to study drug interactions with serum proteins, but there is still a need for faster or better methods for such work. High-performance liquid chromatography (HPLC) is one tool that has been utilized in many formats for these types of measurements. Advantages of using HPLC for this application include its speed and precision, its ability to be automated, its good limits of detection, and its compatibility with a wide range of assay formats and detectors. This review will discuss various approaches in which HPLC can be employed for the study of drug-protein interactions. These techniques include the use of soluble proteins in zonal elution and frontal analysis methods or vacancy techniques such as the Hummel-Dreyer method. Zonal elution and frontal analysis methods that make use of immobilized proteins and high-performance affinity chromatography will also be presented. A variety of applications will be examined, ranging from the determination of free drug fractions to the measurement of the strength or rate of a drug-protein interaction. Newer developments that will be discussed include recent work in the creation of novel mathematical approaches for HPLC studies of drug-protein binding, the use of HPLC methods for the high-throughput screening of drug-protein binding, and the creation and use of affinity monoliths or affinity microcolumns for examining drug-protein systems.

Evaluation of indole-based probes for high-throughput screening of drug binding to human serum albumin: Analysis by high-performance affinity chromatography

There has been growing interest in the use of rapid and selective separation methods such as high-performance affinity chromatography (HPAC) or affinity capillary electrophoresis (ACE) for the characterization of drug-protein interactions. L-Tryptophan is commonly used in these and other methods as a site-selective probe for examining the binding of small solutes and drugs at Sudlow site II on the protein HSA. However, solutions of L-tryptophan can be unstable and are generally prepared fresh daily for these studies. In this report, HPAC was used to examine other indole compounds as possible replacements for L-tryptophan as a site-selective probe for use in the high-throughput screening of drug binding to HSA; the implications of these results in the use of such compounds in ACE were also considered. The probe candidates that were tested included indole-3-acetic acid, indole-3-carboxylic acid, indole-3-butyric acid, indole-3-propionic acid, indole-3-methanol, 3-acetylindole, and 3-methylindole. All of these compounds were found by (1)H NMR and UV-Vis spectroscopy to be stable for up to 3 wk at room temperature when kept in a pH 7.4, 0.067 M phosphate buffer. The binding of these compounds was examined by using columns that contained immobilized HSA. 3-Acetylindole was found to be the best candidate in this group for use as an alternative probe to L-tryptophan for Sudlow site II. This probe had the same binding site and a similar affinity to L-tryptophan but was more stable in aqueous solution, making it suitable for high-throughput screening of drug-HSA binding in both HPAC and ACE.

Characterization of drug-protein interactions in blood using high-performance affinity chromatography

The binding of drugs with proteins in blood, serum, or plasma is an important process in determining the activity, distribution, rate of excretion, and toxicity of drugs in the body. High-performance affinity chromatography (HPAC) has received a great deal of interest as a means for studying these interactions. This review examines the various techniques that have been used in HPAC to examine drug-protein binding and discusses the types of information that can be obtained through this approach. A comparison of these techniques with traditional methods for binding studies (e.g., equilibrium dialysis and ultrafiltration) will also be presented. The use of HPAC with specific serum proteins and binding agents will then be discussed, including HSA and alpha(1)-acid glycoprotein (AGP). Several examples from the literature are provided to illustrate the applications of such research. Recent developments in this field are also described, such as the use of improved immobilization techniques, new data analysis methods, techniques for working directly with complex biological samples, and work with immobilized lipoproteins. The relative advantages and limitations of the methods that are described will be considered and the possible use of these techniques in the high-throughput screening or characterization of drug-protein binding will be discussed.

Rapid analysis of the interactions between drugs and human serum albumin (HSA) using high-performance affinity chromatography (HPAC)

This study used a combination of zonal elution and frontal affinity chromatography on immobilized human serum albumin (HSA) high-performance affinity chromatography (HPAC) column to examine the association constants of various compounds that have been studied by equilibrium dialysis or ultra filtration. A standard plot was generated from retention factors of reference compounds using zonal elution chromatography against association constants of reference compounds using frontal affinity chromatography. The linear relationship was established (r2=0.9993) between retention factors and association constants of reference compounds. This standard plot was later used for rapid determination of association constants of various drugs which show low to medium binding affinity to HSA. Association constants of those drugs from this study were compared to that of more generally used methods (i.e., equilibrium dialysis or ultra filtration) from literature and resulted in a relatively high correlation (r2=0.945) value. This combination of zonal elution and frontal affinity chromatography method for determining association constants showed several advantages against traditional methods. Depending on drugs of interest, an association constant of drug to HSA can be measured as fast as 1.5 min. Other notable advantages include an ease of automation and its ability to distinguish association constants of chiral compounds at the same time. The same approach could be used for studying interaction of other drugs and proteins and should further improve overall drug screening process.

Development of sulfhydryl-reactive silica for protein immobilization in high-performance affinity chromatography

Two techniques were developed for the immobilization of proteins and other ligands to silica through sulfhydryl groups. These methods made use of maleimide-activated silica (the SMCC method) or iodoacetyl-activated silica (the SIA method). The resulting supports were tested for use in high-performance affinity chromatography by employing human serum albumin (HSA) as a model protein. Studies with normal and iodoacetamide-modified HSA indicated that these methods had a high selectivity for sulfhydryl groups on this protein, which accounted for the coupling of 77-81% of this protein to maleimide- or iodoacetyl-activated silica. These supports were also evaluated in terms of their total protein content, binding capacity, specific activity, nonspecific binding, stability, and chiral selectivity for several test solutes. HSA columns prepared using maleimide-activated silica gave the best overall results for these properties when compared to HSA that had been immobilized to silica through the Schiff base method (i.e., an amine-based coupling technique). A key advantage of the supports developed in this work is that they offer the potential of giving greater site-selective immobilization and ligand activity than amine-based coupling methods. These features make these supports attractive in the development of protein columns for such applications as the study of biological interactions and chiral separations.

Immunoaffinity CE in clinical analysis of body fluids and tissues

This paper reviews immunoaffinity CE procedures developed since 1998 for drug, hormone, and disease marker analyses of body fluids and tissues. Immunoaffinity CE and related techniques are described. Examples of clinical applications are included.

Chromatographic analysis of carbamazepine binding to human serum albumin

Recent studies with carbamazepine on human serum albumin (HSA) columns have noted an appreciable degree of non-specific binding on supports prepared by the Schiff base immobilization method. This work examines an alternative immobilization method for HSA based on N-hydroxysuccinimide (NHS)-activated silica. This support was prepared by reacting HPLC-grade silica directly with disuccinimidyl carbonate. The resulting material was compared to an HSA support prepared by the Schiff base method in terms of its activity for carbamazepine and non-specific interactions with this drug. When examined by frontal analysis, both supports gave comparable association equilibrium constants for carbamazepine interactions with HSA ((0.53-0.55) x 10(4)M(-1) at 37 degrees C). However, columns prepared by the Schiff base method gave greater non-specific binding. These columns, as well as control columns prepared using the carbonyldiimidazole (CDI) immobilization method, were also evaluated for their non-specific binding to a variety of other solutes known to interact with HSA. From these results it was concluded that the NHS method was an attractive alternative to the Schiff base technique in the preparation of immobilized HSA for HPLC-based binding studies for carbamazepine. However, it was also noted that non-specific binding varies from one drug to the next in these immobilization methods, indicating that such properties should be evaluated on a case-by-case basis in the use and development of HSA columns for binding studies.

Poly(ethylene oxide) facilitates the characterization of an affinity between strongly basic proteins with DNA by affinity capillary electrophoresis

In order to study kin17 protein-DNA affinity, we have developed a fast and reproducible capillary electrophoresis (CE) analysis of a strongly basic protein: kin17 protein, using a nonpermanent coating based on poly(ethylene oxide) (PEO) to avoid adsorption of kin17. The coating procedure was optimized to provide a residual and stable electroosmotic flow (EOF = 5 x 10(-5) cm(2)/V x s), exhibiting RSD of 0.3% and excellent long-term stability. Good intraday and interday reproducibility of kin17 migration times (0.8 and 0.3% relative standard deviation (RSD), respectively) enabled us to consider that the recovery percentage obtained for kin17 protein was satisfactory (79%). The potential of this PEO-based coating procedure was evaluated for affinity CE method in order to study the affinity of kin17 protein for two single-stranded DNA (ssDNA) models: polydeoxyadenylic acid and polydeoxycytidilic acid (pdA and pdC). Binding constants (1.5 x 10(7) +/- 17% and 1.7 x 10(7) + 25%M(-1)) were evaluated assuming a 1:1 affinity between kin17 and pdA or pdC, respectively.

Chromatographic analysis of carbamazepine binding to human serum albumin

In this study, high-performance affinity chromatography was used to characterize the binding of carbamazepine to an immobilized human serum albumin (HSA) column. Frontal analysis was first used to determine the association equilibrium constant and binding capacity for carbamazepine on this column at various temperatures. The non-specific binding of carbamazepine within the column was also considered. The results indicated that carbamazepine had a single binding site on HSA with an association equilibrium constant of 5.3 x 10(3)M(-1) at pH 7.4 and 37 degrees C. This was confirmed through zonal elution self-competition studies. The value of DeltaG for this reaction was -5.35 kcal/mol at 37 degrees C, with an associated change in enthalpy (DeltaH) of -6.45 kcal/mol and a change in entropy (DeltaS) of -3.56 cal/molK. The location of this binding region was examined by competitive zonal elution experiments using probe compounds with known sites on HSA. It was found that carbamazepine had direct competition with l-tryptophan, a probe for the indole-benzodiazepine site of HSA, but allosteric interactions with probes for the warfarin, tamoxifen and digitoxin sites. Changes in the pH, ionic strength, and organic modifier content of the mobile phase were used to identify the predominant forces in the carbamazepine-HSA interaction.

Using capillary electrophoresis with laser-induced fluorescence to study the interaction of green fluorescent protein-labeled calmodulin with Ca2+- and calmodulin-binding protein

A separation using capillary electrophoresis with laser-induced fluorescence (CE-LIF) was applied to the study of green fluorescent protein tagged calmoldulin (GFP-CaM) that was expressed from Escherichia coli and purified with Ni(2+)-nitrilotriacetate (Ni-NTA) resin column. It was found that GFP-CaM not only has good fluorescence properties under various conditions similar to GFP, but also retains its calcium-binding ability as the native CaM. GFP-CaM was separated and detected by CE-LIF within 10 min with a limit-of-detection (LOD) of 2 x 10(-10) M for an injection volume of 3 nl, higher than that of common chemical fluorescent-tagged protein method. The results indicated that, as a fluorescence probe, GFP could overcome the drawback of inefficient derivatization of chemical fluorescence probes. The interaction between the GFP-CaM and Ca(2+) was studied in detail using affinity capillary electrophoresis with laser-induced fluorescence and the dissociation constant (K(d)) between GFP-CaM and Ca(2+) was determined to be 1.2 x 10(-5), which is in good agreement with the literature values of untagged CaM (10(-6) to 10(-5)M) obtained by conventional method. As a preliminary application, the interaction between GFP-CaM and OsCBK was also investigated. The method makes it possible to screen the trace amounts of target proteins in crude extracts interacting with CaM under physiological conditions.

Drug affinity to immobilized target bio-polymers by high-performance liquid chromatography and capillary electrophoresis

This review addresses the use of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) as affinity separation methods to characterise drugs or potential drugs-bio-polymer interactions. Targets for the development of new drugs such as enzymes (IMERs), receptors, and membrane proteins were immobilized on solid supports. After the insertion in the HPLC system, these immobilized bio-polymers were used for the determination of binding constants of specific ligands, substrates and inhibitors of pharmaceutical interest, by frontal analyses and zonal elution methods. The most used bio-polymer immobilization techniques and methods for assessing the amount of active immobilized protein are reported. Examples of increased stability of immobilized enzymes with reduced amount of used protein were shown and the advantages in terms of recovery for reuse, reproducibility and on-line high-throughput screening for potential ligands are evidenced. Dealing with the acquisition of relevant pharmacokinetic data, examples concerning human serum albumin binding studies are reviewed. In particular, papers are reported in which the serum carrier has been studied to monitor the enantioselective binding of chiral drugs and the mutual interaction between co-administered drugs by CE and HPLC. Finally CE, as merging techniques with very promising and interesting application of microscale analysis of drugs' binding parameters to immobilized bio-polymers is examined.

Separation and detection methods for covalent drug–protein adducts

Covalent binding of reactive metabolites of drugs to proteins has been a predominant hypothesis for the mechanism of toxicity caused by numerous drugs. The development of efficient and sensitive analytical methods for the separation, identification, quantification of drug-protein adducts have important clinical and toxicological implications. In the last few decades, continuous progress in analytical methodology has been achieved with substantial increase in the number of new, more specific and more sensitive methods for drug-protein adducts. The methods used for drug-protein adduct studies include those for separation and for subsequent detection and identification. Various chromatographic (e.g., affinity chromatography, ion-exchange chromatography, and high-performance liquid chromatography) and electrophoretic techniques [e.g., sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and capillary electrophoresis], used alone or in combination, offer an opportunity to purify proteins adducted by reactive drug metabolites. Conventionally, mass spectrometric (MS), nuclear magnetic resonance, and immunological and radioisotope methods are used to detect and identify protein targets for reactive drug metabolites. However, these methods are labor-intensive, and have provided very limited sequence information on the target proteins adducted, and thus the identities of the protein targets are usually unknown. Moreover, the antibody-based methods are limited by the availability, quality, and specificity of antibodies to protein adducts, which greatly hindered the identification of specific protein targets of drugs and their clinical applications. Recently, the use of powerful MS technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight) together with analytical proteomics have enabled one to separate, identify unknown protein adducts, and establish the sequence context of specific adducts by offering the opportunity to search for adducts in proteomes containing a large number of proteins with protein adducts and unmodified proteins. The present review highlights the separation and detection technologies for drug-protein adducts, with an emphasis on methodology, advantages and limitations to these techniques. Furthermore, a brief discussion of the application of these techniques to individual drugs and their target proteins will be outlined.

An NMR spectroscopy study of bendaline-albumin interactions

The complete assignment of the 1H and 13C NMR spectra of bendaline (BNDL) was performed by mono-dimensional and homo- and hetero-correlated two-dimensional NMR experiments. The interaction between bendaline and albumin was also studied by the analysis of the motional parameters spin-lattice relaxation times, allowing the motional state of the BNDL free and bound with albumin to be defined. In absence of albumin the indazolacetic and benzylic moieties are characterized by roughly the same mobility and by positive sigma (cross-relaxation rates) values. In the presence of the macromolecule, the indazolacetic and benzylic moieties and the lysine change their motional behaviour to different extents, as indicated by correlation times. Data obtained in absence and in presence of the protein show that the molecular moiety of the bendaline most involved in the binding with albumin is the fragment H-4 H-5. The binding constant was evaluated at 2.4x10(3)M(-1).

Molecular interaction in capillary electrophoresis

At the present time, affinity capillary electrophoresis is a well-established method for the measurement of binding constants involving 1:1 interactions. Many recent applications of this technique have illustrated its versatility. If affinity capillary electrophoresis is to be widely applied to the study of all types of interactions it is imperative that we consider that higher order interactions may be present. Applying proper equations or developing new models appropriate to the description of such systems is essential to obtaining reliable binding information.

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.