Analysis of free drug fractions by ultrafast affinity extraction: interactions of sulfonylurea drugs with normal or glycated human serum albumin

Prediction of the binding interactions between rosmarinic acid and cysteinyl leukotriene receptor type 1 by molecular docking and immobilized receptor chromatography

Drug-protein interaction analysis is still at the center of research efforts to illustrate binding mechanisms and provide valuable information for selecting drug candidates with ideal properties in the early drug discovery stage. We present the prediction of the binding of rosmarinic acid (RA) to cysteinyl leukotriene receptor type1 (CysLTR1) by molecular docking. According to our findings, CysLTR1 is a potential anti-inflammatory target of RA. Under this assumption, we prepared the immobilized CysLTR1 column via a one-step method and characterized the immobilized CysLTR1 by fluorescent and chromatographic analyses. Furthermore, we used the immobilized CysLTR1 column to evaluate the binding interactions between RA and the immobilized receptor. Molecular docking showed that Tyr 249, Phe 174, Thr 280, Pro 177, and Thr 100 are the main sites for RA to interact with CysLTR1. The main forces that drive the findings are hydrogen bonds and hydrophobic interactions. Characterization results show that CysLTR1 is successfully immobilized with high specificity and stability. Almost no non-specific binding is observed on the immobilized CysLTR1 gels. The association constant and the binding sites are calculated to be 7.268 × 105 L mol-1 and 1.237 × 10-8 mol L-1 by injection amount-dependent method. These results, taken together, confirm the potential target of RA on the anti-inflammatory effect. We believe that it can provide valuable reference information on the in-depth exploration of drug-protein interaction mechanisms, and lead compound screening by this method.

Determination of binding constants by ultrafast affinity extraction: Theoretical and experimental studies of optimum conditions for analysis

Ultrafast affinity extraction (UAE) is a form of microscale affinity HPLC that can be employed to quickly measure equilibrium constants for solute-binding agent interactions in solution. This study used chromatographic and equilibrium theory with universal plots to examine the general conditions that are needed in UAE to obtain accurate, precise, and robust measurements of equilibrium constants for such interactions. The predicted results were compared to those obtained by UAE in studies that examined the binding of various drugs with two transport proteins: human serum albumin and α1-acid glycoprotein. The most precise and robust conditions for these binding studies occurred for systems with intermediate values for their equilibrium free fraction for the solute (F0 ≈ 0.20-0.80). These trends showed good agreement with those seen in prior studies using UAE. It was further determined how the apparent free fraction of a solute was related to the dissociation rate of this solute, the time allowed for solute dissociation during UAE, and the equilibrium free fraction for the solute. These results also agreed with experimental results, as obtained for the binding of warfarin and gliclazide with human serum albumin. The final section examined how a change in the apparent free fraction, as caused by solute dissociation, affected the accuracy of an equilibrium constant that was measured by UAE. In addition, theoretical plots were generated to allow the selection of conditions for UAE that provided a given level of accuracy during the measurement of an equilibrium constant. The equations created and trends identified for UAE were general ones that can be extended in future work to other solutes and binding agents.

Analysis of the binding of warfarin to glyoxal- and methylglyoxal-modified human serum albumin by ultrafast affinity extraction

Ultrafast affinity extraction (UAE) and affinity microcolumns containing immobilized human serum albumin (HSA) were employed to evaluate the effect of advanced stage glycation on HSA and its binding to warfarin, a common site-specific probe for Sudlow site I of this protein. The modification of HSA by glyoxal (GO) and methylglyoxal (MGO) was considered, where GO and MGO are known to be important in the formation of many types of advanced glycation end products. Free drug fractions were measured by UAE for warfarin in solutions containing normal HSA or HSA that had been modified by GO or MGO at levels seen in serum during diabetes. The free fractions measured with the GO-modified HSA gave association equilibrium constants that ranged from 2.42-2.63 × 105 M-1 at pH 7.4 and 37 °C. These values were not significantly different from a value of 2.33 (±0.15) × 105 M-1 that was determined by the same method for warfarin with normal HSA. Similar studies using MGO-modified HSA gave association equilibrium constants for warfarin in the range of 3.07-3.31 × 105 M-1, which were 1.32- to 1.42-fold higher than the value seen for normal HSA (differences that were significant at the 95% confidence level). These results will be valuable in future binding studies based on affinity chromatography or other methods that employ warfarin as a probe to examine drug interactions at Sudlow site I of HSA and modified forms of this protein. This work also illustrates how UAE can be used, with analysis times of only minutes, to detect and measure small changes in the binding by drugs with unmodified or modified forms of a soluble binding agent or protein.

Evaluation of microcolumn stability in ultrafast affinity extraction for binding and rate studies

Ultrafast affinity extraction (UAE) has recently been developed and employed for measuring non-bound (or free) fractions and binding or rate constants for drugs and other targets with soluble binding agents such as serum proteins. This study examined the long-term stability of 10 mm × 2.1 mm i.d. affinity microcolumns when used in UAE at both low and high flow rates (e.g., 0.5 and 3.5 mL/min) over an extended series of injections. This stability was investigated by using immobilized human serum albumin (HSA) and samples containing the drug warfarin with or without soluble HSA as a model system. The free warfarin fractions measured at 0.5 mL/min in the presence of soluble HSA were stable up to 150 injections and changed by <10% at 3.5 mL/min. The association equilibrium constant for warfarin with HSA that was estimated by UAE at 3.5 mL/min had no significant change over 50 injections and a change of only ∼18-22% over 100-150 injections. The dissociation rate constant for warfarin from HSA was found by combining UAE results at 0.5 and 3.5 mL/min and employing a new two-point approach, with no significant changes in this value being seen even after 200 injections. The effects of extended microcolumn use on the retention time, peak width, and peak asymmetry for warfarin, and on the backpressure of the microcolumn, were also considered. These results indicated that UAE and HSA microcolumns could be used to provide consistent values for free solute fractions, binding constants, and rate constants over a large series of injections. These results should be useful in future work by providing guidelines for the assessment, further development, and use of UAE in characterizing interactions involving other drugs and binding agents in solution-based samples.

[Advances in chromatography in the study of drug-plasma protein interactions]

After entering human blood circulation, small-molecule drugs interact extensively with various plasma proteins, such as human serum albumin and α1-acid glycoprotein. These interactions profoundly affect the distribution of drugs in vivo and the binding of drugs to targets, thus affecting the efficacy of drugs. In-depth investigation of drug-plasma protein interactions is of great significance for the optimization of drug properties, the development of new drugs, risk assessment, and combination therapy of drugs. Therefore, it is essential to develop highly efficient, sensitive, and accurate methods for elucidating drug-plasma protein interactions. Chromatography is a powerful tool with high throughput, high separation performance, and high sensitivity in the characterization of drug-protein interactions. High-performance affinity chromatography (HPAC) and capillary electrophoresis (CE) have been widely utilized in this field. These methods include the determination of the effects of the posttranslational modification of proteins on binding and the competitive binding of multiple drugs. In addition, various chromatographic methods are used to obtain interaction information such as the binding constant, binding-site number, and dissociation rate constant. In this review, the common strategies and recent advances in HPAC and CE in the study of drug-plasma protein interactions are briefly reviewed. The immobilization methods of proteins, the principles and applications of frontal analysis, zonal elution, ultrafast affinity extraction, peak profiling, and peak decay analysis are discussed for HPAC and affinity capillary electrophoresis (ACE) and capillary electrophoresis frontal analysis (CE-FA) for CE. HPAC relies on the fixation of proteins on the surfaces of chromatographic stationary phases by covalent linking or physical adsorption, followed by obtaining the drug-protein interaction information through a variety of chromatographic methods. In the frontal chromatography analysis, mobile phases with different concentrations of drugs are passed through the HPAC column to obtain different breakthrough times. The process can determine the number of drug binding sites and the binding constant of each site in the affinity protein with high accuracy. The zonal elution method can detect the drug binding sites on proteins using site-specific probes to determine whether there is competition between drugs and probes. The sample consumption and analysis time of the zonal elution method are much less than those in frontal chromatography analysis. The ultrafast affinity extraction method can inject complex samples, such as serum, into affinity columns to determine the free drug components. It can measure the combination and dissociation constants of drug-protein interactions by changing the chromatography flow rate. Peak profiling and peak decay analyses are both effective methods for investigating the dissociation of drugs and proteins. In CE analysis, the drug and protein samples are dissolved in an electrophoresis buffer, and their interactions are measured during electrophoresis with high accuracy and low sample consumption. However, the adsorption of proteins on the capillary wall can compromise CE performance. Common CE methods in drug-protein interaction analysis are ACE and CE-FA. ACE is usually performed by changing the effective mobility of drugs via the addition of different concentrations of proteins. This method has been widely used, and several variant techniques have been developed recently. CE-FA involves the sampling of a drug premixed at a known concentration with a target protein. Compared with other CE methods, CE-FA exhibits the unique advantages of high throughput, automatic online analysis, and the ability to determine high-order drug-protein interactions. Finally, the shortcomings of current chromatography methods are summarized, and the application prospects and development direction of chromatography technology in the field of drug-plasma protein interaction research are discussed.

Development of an on-line immunoextraction/entrapment system for protein capture and use in drug binding studies by high-performance affinity chromatography

An on-line purification and entrapment system was developed that could extract a protein from a sample such as serum and entrap this protein within a small column for use in high-performance affinity chromatography. Human serum albumin (HSA) was employed as a model protein for this work. Immunoextraction columns containing polyclonal anti-HSA antibodies were developed to capture and isolate HSA from applied samples. This was followed by the use of a strong cation-exchange column to recapture and focus HSA as it eluted from the immunoextraction columns. The recaptured HSA was entrapped within 1.0 cm × 2.1 mm I.D. columns containing hydrazide-activated silica and in the presence of oxidized glycogen as a capping agent. The binding and elution properties of HSA on the various components of this system were examined and optimized. The entrapped columns produced by this system were then evaluated for their use in binding studies with several sulfonylurea drugs. The HSA columns created by this approach typically contained 0.3-0.6 nmol HSA and were stable over several weeks and more than 50-60 sample injections. Drug binding constants could be determined with these columns in 8 min or less by zonal elution and gave good agreement with literature values. The same system could be used for the capture and entrapment of other proteins by utilizing antibodies against the given target for immunoextraction.

Biological evaluations of newly-designed Pt(II) and Pd(II) complexes using spectroscopic and molecular docking approaches

To perform biological evaluations of newly-designed Pt(II) and Pd(II) complexes, the present study was conducted with targeted protein human serum albumin (HSA) and HCT116 cell line as model of human colorectal carcinoma. The binding of Pt(II) and Pd(II) complexes to HSA was analyzed using fluorescence spectroscopy and molecular docking. The thermal stability and alterations in the secondary structure of HSA in the presence of Pt(II) and Pd(II) complexes were investigated using the thermal denaturation method and circular dichroism (CD) spectroscopy. The cytotoxicity of the Pt(II) and Pd(II) complexes was studied against the HCT116 cell line using MTT assay. The binding analysis revealed that the fluorescence findings were well in agreement with docking results such that there is only one binding site for each complex on HSA. Binding constants of 8.7 × 10³ M^-1, 2.65 × 10³ M^-1, 0.3 × 10³ M^-1, and 4.4 × 10³ M^-1 were determined for Pd(II) and Pt(II) complexes (I-IV) at temperature of 25 °C, respectively. Also, binding constants of 1.9 × 10³ M^-1, 15.17 × 10³ M^-1, 1.9 × 10³ M^-1, and 13.1 × 10³ M^-1 were determined for Pd(II) and Pt(II) complexes (I-IV) at temperature of 37 °C, respectively. The results of CD and thermal denaturation showed that the molecular structure of HSA affected by interaction with Pt(II) and Pd(II) complexes is stable. Cytotoxicity studies represented the growth suppression effect of the Pt(II) and Pd(II) complexes toward the human colorectal carcinoma cell line. Therefore, the results suggest that the new designed Pt(II) and Pd(II) complexes are well promising candidates for use in cancer treatment, particularly for human colorectal cancer. Communicated by Ramaswamy H. Sarma.

Binding studies based on ultrafast affinity extraction and single- or two-column systems: Interactions of second- and third-generation sulfonylurea drugs with normal or glycated human serum albumin

Ultrafast affinity extraction was evaluated and used with microcolumns containing human serum albumin (HSA) to measure the global affinity constants and dissociation rate constants for several second- and third-generation sulfonylurea drugs with solution-phase normal HSA or glycated HSA. Glibenclamide, glimepiride and glipizide were used as model drugs for this work. Both single- and two-column systems were considered for the analysis of global affinities for the model drugs. These methods were optimized with respect to the flow rates, column sizes and sample residence times that were employed with each drug for ultrafast affinity extraction. Data acquired with single-column systems were further utilized to estimate the dissociation rate constants for normal HSA and glycated HSA with the given drugs. The binding constants obtained by the single- and two-column systems showed good agreement with each other and with values obtained from the literature. Use of a single-column system indicated that levels of glycation found in controlled or advanced diabetes resulted in a 18-44% decrease in the overall binding strength of the model drugs with HSA. Although the two-column system allowed work with smaller free drug fractions and clinically-relevant drug/protein concentrations, the single-column system required less protein, provided better precision, and was easier to use in binding studies.

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.

High performance affinity chromatography and related separation methods for the analysis of biological and pharmaceutical agents

The last few decades have witnessed the development of many high-performance separation methods that use biologically related binding agents. The combination of HPLC with these binding agents results in a technique known as high performance affinity chromatography (HPAC). This review will discuss the general principles of HPAC and related techniques, with an emphasis on their use for the analysis of biological compounds and pharmaceutical agents. Various types of binding agents for these methods will be considered, including antibodies, immunoglobulin-binding proteins, aptamers, enzymes, lectins, transport proteins, lipids, and carbohydrates. Formats that will be discussed for these methods range from the direct detection of an analyte to indirect detection based on chromatographic immunoassays, as well as schemes based on analyte extraction or depletion, post-column detection, and multi-column systems. The use of biological agents in HPLC for chiral separations will also be considered, along with the use of HPAC as a tool to screen or study biological interactions. Various examples will be presented to illustrate these approaches and their applications in fields such as biochemistry, clinical chemistry, and pharmaceutical research.

Chromatographic studies of drug interactions with alpha1-acid glycoprotein by ultrafast affinity extraction and peak profiling

Interactions with serum proteins such as alpha₁-acid glycoprotein (AGP) can have a significant effect on the behavior and pharmacokinetics of drugs. Ultrafast affinity extraction and peak profiling were used with AGP microcolumns to examine these processes for several model drugs (i.e., chlorpromazine, disopyramide, imipramine, lidocaine, propranolol and verapamil). The association equilibrium constants measured for these drugs with soluble AGP by ultrafast affinity extraction were in the general range of 10⁴-10⁶M^-1 at pH 7.4 and 37°C and gave good agreement with literature values. Some of these values were dependent on the relative drug and protein concentrations that were present when using a single-site binding model; these results suggested a more complex mixed-mode interaction was actually present, which was also then used to analyze the data. The apparent dissociation rate constants that were obtained by ultrafast affinity extraction when using a single-site model varied from 0.14 to 7.0s^-1 and were dependent on the relative drug and protein concentrations. Lower apparent dissociation rate constants were obtained by this approach as the relative amount of drug versus protein was decreased, with the results approaching those measured by peak profiling at low drug concentrations. This information should be useful in better understanding how these and other drugs interact with AGP in the circulation. In addition, the chromatographic approaches that were optimized and used in this report to examine these systems can be adapted for the analysis of other solute-protein interactions of biomedical interest.

Study on the interactions of sulfonylurea antidiabetic drugs with normal and glycated human serum albumin by capillary electrophoresis-frontal analysis

Diabetes is one of the most widespread diseases characterized by a deficiency in the production of insulin or its ineffectiveness. As a result, the increased concentrations of glucose in the blood lead not only to damage to many of the body's systems but also cause the nonenzymatic glycation of plasma proteins affecting their drug binding. Since the binding ability influences its pharmacokinetics and pharmacodynamics, this is a very important issue in the development of new drugs and personalized medicine. In this study, capillary electrophoresis-frontal analysis was used to evaluate the affinities between human serum albumin or its glycated form and the first generation of sulfonylurea antidiabetics, since their inadequate concentration may induce hypoglycaemia or on the contrary hyperglycaemia. The binding constants decrease in the sequence acetohexamide > tolbutamide > chlorpropamide > carbutamide both for normal and glycated human serum albumins, with glycated giving lower values. These results provide a more quantitative picture of how these drugs bind with normal and modified human serum albumin and indicate capillary electrophoresis-frontal analysis to be another tool for examining the changes arising from modifications of albumin, or any other protein, with all its benefits like short analysis time, small sample requirement, and automation.

Analysis of free drug fractions in serum by ultrafast affinity extraction and two-dimensional affinity chromatography using α1-acid glycoprotein microcolumns

In the circulatory system, many drugs are reversibly bound to serum proteins such as human serum albumin (HSA) and alpha1-acid glycoprotein (AGP), resulting in both free and protein-bound fractions for these drugs. This report examined the use of microcolumns containing immobilized AGP for the measurement of free drug fractions by ultrafast affinity extraction and a two-dimensional affinity system. Several drugs known to bind AGP were used as models to develop and evaluate this approach. Factors considered during the creation of this method included the retention of the drugs on the microcolumns, the injection flow rate, the microcolumn size, and the times at which a second AGP column was placed on-line with the microcolumn. The final system had residence times of only 110-830ms during sample passage through the AGP microcolumns and allowed free drug fractions to be determined within 10-20min when using only 3-10μL of sample per injection. This method was used to measure the free fractions of the model drugs at typical therapeutic levels in serum, giving good agreement with the results obtained by ultrafiltration. This approach was also used to estimate the binding constants for each drug with AGP in serum, even for drugs that had significant interactions with both AGP and HSA in such samples. These results indicated that AGP microcolumns could be used with ultrafast affinity extraction to measure free drug fractions in a label-free manner and to study the binding of drugs with AGP in complex samples such as serum.

High-Performance Affinity Chromatography: Applications in Drug-Protein Binding Studies and Personalized Medicine

The binding of drugs with proteins and other agents in serum is of interest in personalized medicine because this process can affect the dosage and action of drugs. The extent of this binding may also vary with a given disease state. These interactions may involve serum proteins, such as human serum albumin or α1-acid glycoprotein, or other agents, such as lipoproteins. High-performance affinity chromatography (HPAC) is a tool that has received increasing interest as a means for studying these interactions. This review discusses the general principles of HPAC and the various approaches that have been used in this technique to examine drug-protein binding and in work related to personalized medicine. These approaches include frontal analysis and zonal elution, as well as peak decay analysis, ultrafast affinity extraction, and chromatographic immunoassays. The operation of each method is described and examples of applications for these techniques are provided. The type of information that can be obtained by these methods is also discussed, as related to the analysis of drug-protein binding and the study of clinical or pharmaceutical samples.

Analysis of Hormone-Protein Binding in Solution by Ultrafast Affinity Extraction: Interactions of Testosterone with Human Serum Albumin and Sex Hormone Binding Globulin

Ultrafast affinity extraction was used to study hormone-protein interactions in solution, using testosterone and its transport proteins human serum albumin (HSA) and sex hormone binding globulin (SHBG) as models. Both single column and two-dimensional systems based on HSA microcolumns were utilized to measure the free fraction of testosterone in hormone/protein mixtures at equilibrium or that were allowed to dissociate for various lengths of time. These data were used to determine the association equilibrium constants (Ka) or global affinities (nKa') and dissociation rate constants (kd) for testosterone with soluble HSA and SHBG. This method was also used to measure simultaneously the free fraction of testosterone and its equilibrium constants with both these proteins in physiological mixtures of these agents. The kd and Ka values obtained for HSA were 2.1-2.2 s(-1) and 3.2-3.5 × 10(4) M(-1) at pH 7.4 and 37 °C. The corresponding constants for SHBG were 0.053-0.058 s(-1) and 0.7-1.2 × 10(9) M(-1). All of these results gave good agreement with literature values, indicating that this approach could provide information on a wide range of rate constants and binding strengths for hormone-protein interactions in solution and at clinically relevant concentrations. The same method could be extended to alternative hormone-protein systems or other solutes and binding agents.

Analysis of free drug fractions in human serum by ultrafast affinity extraction and two-dimensional affinity chromatography

Ultrafast affinity extraction and a two-dimensional high performance affinity chromatographic system were used to measure the free fractions for various drugs in serum and at typical therapeutic concentrations. Pooled samples of normal serum or serum from diabetic patients were utilized in this work. Several drug models (i.e., quinidine, diazepam, gliclazide, tolbutamide, and acetohexamide) were examined that represented a relatively wide range of therapeutic concentrations and affinities for human serum albumin (HSA). The two-dimensional system consisted of an HSA microcolumn for the extraction of a free drug fraction, followed by a larger HSA analytical column for the further separation and measurement of this fraction. Factors that were optimized in this method included the flow rates, column sizes, and column switching times that were employed. The final extraction times used for isolating the free drug fractions were 333-665 ms or less. The dissociation rate constants for several of the drugs with soluble HSA were measured during system optimization, giving results that agreed with reference values. In the final system, free drug fractions in the range of 0.7-9.5% were measured and gave good agreement with values that were determined by ultrafiltration. Association equilibrium constants or global affinities were also estimated by this approach for the drugs with soluble HSA. The results for the two-dimensional system were obtained in 5-10 min or less and required only 1-5 μL of serum per injection. The same approach could be adapted for work with other drugs and proteins in clinical samples or for biomedical research.

Analysis of multi-site drug-protein interactions by high-performance affinity chromatography: Binding by glimepiride to normal or glycated human serum albumin

High-performance affinity chromatography (HPAC) was used in a variety of formats to examine multi-site interactions between glimepiride, a third-generation sulfonylurea drug, and normal or in vitro glycated forms of the transport protein human serum albumin (HSA). Frontal analysis revealed that glimepiride interacts with normal HSA and glycated HSA at a group of high affinity sites (association equilibrium constant, or Ka, 9.2-11.8×10(5)M(-1) at pH 7.4 and 37°C) and a group of lower affinity regions (Ka, 5.9-16×10(3)M(-1)). Zonal elution competition studies were designed and carried out in both normal- and reversed-role formats to investigate the binding by this drug at specific sites. These experiments indicated that glimepiride was interacting at both Sudlow sites I and II. Allosteric effects were also noted with R-warfarin at Sudlow site I and with tamoxifen at the tamoxifen site on HSA. The binding at Sudlow site I had a 2.1- to 2.3-fold increase in affinity in going from normal HSA to the glycated samples of HSA. There was no significant change in the affinity for glimepiride at Sudlow site II in going from normal HSA to a moderately glycated sample of HSA, but a slight decrease in affinity was seen in going to a more highly glycated HSA sample. These results demonstrated how various HPAC-based methods can be used to profile and characterize multi-site binding by a drug such as glimepiride to a protein and its modified forms. The information obtained from this study should be useful in providing a better understanding of how drug-protein binding may be affected by glycation and of how separation and analysis methods based on HPAC can be employed to study systems with complex interactions or that involve modified proteins.