Determination of protein-drug binding constants by pressure-assisted capillary electrophoresis (PACE)/frontal analysis (FA)

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.

Toward an automated workflow for the study of plasma protein-drug interactions based on capillary electrophoresis-frontal analysis combined with in-capillary mixing of interacting partners

Capillary electrophoresis-frontal analysis (CE-FA) together with mobility shift affinity CE is the most frequently used mode of affinity CE for a study of plasma protein-drug interactions, which is a substantial part of the early stage of drug discovery. Whereas in the classic CE-FA setup the sample is prepared by off-line mixing of the interaction partners in the sample vial outside the CE instrument and after a short incubation period loaded into the capillary and analysed, in this work a new methodological approach has been developed that combines CE-FA with the mixing of interacting partners directly inside the capillary. This combination gives rise to a fully automated and versatile methodology for the characterization of these binding interactions besides a substantial reduction in the amounts of sample compounds used. The minimization of possible experimental errors due to the full involving of sophisticated CE instrument in the injection procedure, mixing and separation instead of manual manipulation is another fundamental benefit. The in-capillary mixing is based on the transverse diffusion of laminar flow profile methodology introduced by Krylov et al. using its multi-zone injection modification presented by Řemínek at al.. Actually, after the method optimization, the alternate introduction of six plugs of drug and six plugs of bovine serum protein in BGE, each injected for 3 s at a pressure of -10 mbar (-1 kPa) into the capillary filled by BGE, was found to be the best injection procedure. The method repeatability calculated as RSDs of plateau highs of bovine serum albumin and propranolol as model sample compounds were better than 3.44 %. Its applicability was finally demonstrated on the determination of apparent binding parameters of bovine serum albumin for basic drugs propranolol and lidocaine and acid drug phenylbutazone. The values obtained by a new on-line CE-FA methodology are in agreement with values estimated by classic off-line CE-FA, as well as with literature data obtained using different techniques.

Investigation of the enantioselective interaction between selected drug enantiomers and human serum albumin by mobility shift-affinity capillary electrophoresis

Mobility shift-affinity capillary electrophoresis was employed for enantioseparation and simultaneous binding constant determination. Human serum albumin was used as a chiral selector in the background electrolyte composed of 20 mM phosphate buffer, pH 7.4. The applied setup supports a high mobility shift since albumin and the drug-albumin complex hold negative net charges, while model compounds of amlodipine and verapamil are positively charged. In order to have an accurate effective mobility determination, the Haarhoff-van der Linde function was utilized. Subsequently, the association constant was determined by nonlinear regression analysis of the dependence of effective mobilities on the total protein concentration. Differences in the apparent binding status between the enantiomers lead to mobility shifts of different extends (α). This resulted in enantioresolutions of Rs = 1.05-3.63 for both drug models. R-(+)-Verapamil (K_A 1844 M^-1 ) proved to bind stronger to human serum albumin compared to S-(-)-verapamil (K_A 6.6 M^-1 ). The association constant of S-(-)-amlodipine (K_A 25 073 M^-1 ) was found to be slightly higher compared to its antipode (K_A 22 620 M^-1 ) when applying the racemic mixture. The low measurement uncertainty of this approach was demonstrated by the close agreement of the association constant of the enantiopure S-(-)-form (K_A 25 101 M^-1 ).

Study of Interactions between Antigens and Polymeric Adjuvants in Vaccines by Frontal Analysis Continuous Capillary Electrophoresis

Vaccine adjuvants are used to enhance the immune response induced by antigens that have insufficient immunostimulatory capabilities. The present work aims at developing a frontal analysis continuous capillary electrophoresis (FACCE) methodology for the study of antigen-adjuvant interactions in vaccine products. After method optimization using three cationic model proteins, namely lysozyme, cytochrome c, and ribonuclease A, FACCE was successfully implemented to quantify the free antigen and thus to determine the interaction parameters (stoichiometry and binding constant) between an anionic polymeric adjuvant (polyacrylic acid, SPA09) and a cationic vaccine antigen in development for the treatment for Staphylococcus aureus. The influence of the ionic strength of the medium on the interactions was investigated. A strong dependence of the binding parameters with the ionic strength was observed. The concentration of the polymeric adjuvant was also found to significantly modify the ionic strength of the formulation, the extent of which could be estimated and corrected.

Studies on the interaction between HSA and new halogenated metformin derivatives: influence of lipophilic groups in the binding ability

In the type II diabetes mellitus, Metformin hydrochloride is recommended as a common FAD approved drug. Synthesis of novel metformin series has been widely explored, mainly due to its biological importance and to improve their pharmacokinetic profile. Generally, human serum albumin (HSA) is the main protein used to study drug viability in vitro analysis. Thus, the present study reports the synthesis of three new halogenated metformin derivatives (MFCl, MFBr and MFCF₃) and its interaction toward HSA by multiple spectroscopic techniques (UV-Vis, circular dichroism, steady-state, time-resolved and synchronous fluorescence), combined to computational methods (molecular docking and quantum chemical calculation). The interaction between each halogenated metformin derivative and HSA is spontaneous (ΔG°<0), entropically driven (ΔS°>0), moderate (K_a and K_b ≈ 10⁴ M^-1) and occurs preferentially in the subdomain IIA (close to Trp-214 residue). Molecular docking results suggested hydrogen bonding, van der Waals and hydrophobic interactions as the main binding forces. Quantum chemical calculations suggested imino groups as the most intense electrostatic negative potentials, while the positive electrostatic potential is located at the hydrogen atoms on N,N-dimethyl and the phenyl systems which can help the hydrophobic interactions. [Formula: see text]Communicated by Ramaswamy H. Sarma.

Frontal analysis capillary electrophoresis: recent advances and future perspectives

The developments on frontal analysis capillary electrophoresis (FACE) from 2012 to 2017 to study interactions of simple and complex systems are reviewed. Most research papers focused on therapeutic drug-related studies; however, other studies include chemical sensing, drug delivery, inhibitor screening and capillary coating. New ligand–substrate systems such as template-molecularly imprinted polymer systems were reported. Comparison of FACE with other analytical techniques used to investigate binding interaction, and the determination of binding parameters using different isotherm models are also covered. In 2017, eight research papers were reported including new detection by ESI–MS. Future research direction of FACE may include high sensitivity detection and throughput screening of drugs, natural products and biomarkers for clinical diagnosis.

Pressure-assisted capillary electrophoresis frontal analysis for faster binding constant determination

Adding external pressure during the process of capillary electrophoresis usually add to the band broadening, especially if the pressure induced flow is significant. The resolution is normally negatively affected in pressure-assisted capillary electrophoresis (PACE). Frontal analysis (FA), however, can potentially benefit from using an external pressure while avoiding the drawbacks in other modes of CE. In this work, possible impact from the external pressure was simulated by COMSOL Multiphysics®. Under a typical CE-FA set-up, it was found that the detected concentrations of analyte will not be significantly affected by an external pressure less than 5 psi. Besides, the measured ligand concentration in PACE-FA was also not affected by common variables (molecular diffusion coefficient (10-8 to 10-11 m2 /s), capillary length etc). To provide an experimental proof, PACE-FA is used to study the binding interactions between hydroxypropyl β-cyclodextrin (HP-β-CD) and small ligand molecules. Taking the HP-β-CD /benzoate pair as an example, the binding constants determined by CE-FA (18.3 ± 0.8 M-1 ) and PACE-FA (16.5 ± 0.5 M-1 ) are found to be similar. Based on the experimental results, it is concluded that PACE-FA can reduce the time of binding analysis while maintaining the accuracy of the measurements.

Analysis of DNA in Phosphate Buffered Saline Using Kinetic Capillary Electrophoresis

Kinetic capillary electrophoresis (KCE) methods are useful in the study of kinetics and equilibrium properties of interactions between DNA and its binding partners (ligands). KCE experiments are typically performed in a narrow set of "conventional" low-conductivity run buffers while DNA-ligand interactions in biological systems occur in physiological fluids, characterized by high ionic strengths. The nature and ionic strength of the buffer, in which DNA-ligand interaction occurs, can significantly influence the binding. Therefore, KCE experiments meant to study such interactions would greatly benefit if they could be performed in physiological buffers, such as phosphate buffered saline (PBS). No previous KCE studies of DNA used PBS as the run buffer. Here, we test the feasibility of using PBS as a KCE run buffer for analysis of DNA and show that its usage under standard KCE conditions renders DNA undetectable. We uncover the causes of this previously unreported detrimental effect and come up with a modification of KCE which allows one to overcome it. We apply the modified KCE method to an experimental model of a platelet-derived growth factor (PDGF) protein and its DNA aptamer, which was selected in PBS, and show that the results obtained in PBS run buffer are much closer to previously reported values than those which were obtained with a conventional low-conductivity capillary electrophoresis (CE) buffer.

Photoactive assemblies of organic compounds and biomolecules: drug–protein supramolecular systems

Modification of the drug excited state properties within proteins provides information on binding and may result in a different photoreactivity.

High performance affinity chromatography (HPAC) as a high-throughput screening tool in drug discovery to study drug-plasma protein interactions

Drug-plasma protein binding is an important parameter that, together with other physicochemical properties such as lipophilicity and pK(a), greatly influences drug absorption, distribution, metabolism, and excretion (ADME). Therefore, it is important for pharmaceutical companies to develop a rapid screening assay to examine plasma protein binding during the early stages of the drug discovery process. Human serum albumin (HSA) and α(1)-acid glycoprotein (AGP) are the most important plasma proteins that are capable of binding drugs. In this work, an automated and high-throughput (<3 min/compound) strategy was developed using high performance affinity chromatography (HPAC) with commercial HSA and AGP columns to evaluate drug-plasma protein interactions for drug screening. A generic gradient was used throughout the study to separate drugs that were weakly and tightly bound to HSA and AGP. To accelerate the analysis time, the system was calibrated in a single run by pooling reference compounds without overloading the column. For both HSA and AGP studies, the developed methods were successfully transferred from HPAC-UV to HPAC-MS with single quadrupole MS detection and ammonium acetate, pH 7.0 as a volatile mobile phase. The MS detection enhanced the sensitivity, selectivity, and throughput of the method by pooling unknown compounds. For HSA analyses, the binding percentages obtained using HPAC were well correlated with the binding percentages from the literature. This method was also able to rank compounds based on their affinity for HSA. Concerning the AGP analyses, the quality of the correlation between the binding percentages obtained in HPAC and those from the literature was weaker. However, the method was able to classify compounds into weak, medium, and strong binders and rank compounds based on their affinity for AGP.

Photophysical properties of a water-soluble adjacently substituted bisnaphthalophthalocyanine

Spectral properties of a water soluble metal free tetracarboxyphenoxy bisnaphthalophthalocyanine (3) were studied in water and organic solvents. It was found that in protic solvents, complex 3 was highly aggregated. The surfactant, Triton X100, and bovine serum albumin (BSA) do not effect disaggregation while cetyl trimethylammonium chloride (CTAC) caused the molecule to disaggregate. The fluorescence quantum yields were higher in the presence of CTAC. Studying the interaction of BSA with complex 3 using fluorometry revealed that BSA is highly quenched by the latter. A 1:1 stoichiometric binding ratio between BSA and the Pc was found. Triplet quantum yields in water containing CTAC were higher than in organic solvents.

High-throughput analysis of drug dissociation from serum proteins using affinity silica monoliths

A noncompetitive peak decay method was used with 1 mm×4.6 mm id silica monoliths to measure the dissociation rate constants (kd) for various drugs with human serum albumin (HSA) and α1-acid glycoprotein (AGP). Flow rates up to 9 mL/min were used in these experiments, resulting in analysis times of only 20-30 s. Using a silica monolith containing immobilized HSA, dissociation rate constants were measured for amitriptyline, carboplatin, cisplatin, chloramphenicol, nortriptyline, quinidine, and verapamil, giving values that ranged from 0.37 to 0.78 s(-1). Similar work with an immobilized AGP silica monolith gave kd values for amitriptyline, nortriptyline, and lidocaine of 0.39-0.73 s(-1). These kd values showed good agreement with values determined for drugs with similar structures and/or affinities for HSA or AGP. It was found that a kd of up to roughly 0.80 s(-1) could be measured by this approach. This information made it possible to obtain a better understanding of the advantages and possible limitations of the noncompetitive peak decay method and in the use of affinity silica monoliths for the high-throughput analysis of drug-protein dissociation.

The N-terminus of the intrinsically disordered protein α-synuclein triggers membrane binding and helix folding

Alpha-synuclein (αS) is a 140-amino-acid protein that is involved in a number of neurodegenerative diseases. In Parkinson's disease, the protein is typically encountered in intracellular, high-molecular-weight aggregates. Although αS is abundant in the presynaptic terminals of the central nervous system, its physiological function is still unknown. There is strong evidence for the membrane affinity of the protein. One hypothesis is that lipid-induced binding and helix folding may modulate the fusion of synaptic vesicles with the presynaptic membrane and the ensuing transmitter release. Here we show that membrane recognition of the N-terminus is essential for the cooperative formation of helical domains in the protein. We used circular dichroism spectroscopy and isothermal titration calorimetry to investigate synthetic peptide fragments from different domains of the full-length αS protein. Site-specific truncation and partial cleavage of the full-length protein were employed to further characterize the structural motifs responsible for helix formation and lipid-protein interaction. Unilamellar vesicles of varying net charge and lipid compositions undergoing lateral phase separation or chain melting phase transitions in the vicinity of physiological temperatures served as model membranes. The results suggest that the membrane-induced helical folding of the first 25 residues may be driven simultaneously by electrostatic attraction and by a change in lipid ordering. Our findings highlight the significance of the αS N-terminus for folding nucleation, and provide a framework for elucidating the role of lipid-induced conformational transitions of the protein within its intracellular milieu.

Neutral coatings for the study of polycation/multicharged anion interactions by capillary electrophoresis: application to dendrigraft poly-L-lysines with negatively multicharged molecules

The study of interactions between oppositely multicharged (macro)molecules remains a challenging issue. In frontal analysis capillary electrophoresis (FACE), it is difficult to avoid the adsorption of one of the interacting partners onto the capillary wall. In this work, we demonstrate the possibility to use FACE and affinity capillary electrophoresis (ACE) on a neutrally coated capillary for the study of interactions between a polycationic dendrigraft (or linear) poly-l-lysines, on one hand, and a multicharged anionic biomolecule (adenosine monophosphate, AMP, or adenosine triphosphate, ATP), on the other hand. A systematic comparison of four different neutral coatings (hydroxypropyl cellulose, polydimethylacrylamide, polyacrylamide, polyethylene glycol) has been performed based on the repeatability of the electrophoretic migration of the dendrigraft poly-l-lysines at pH close to neutrality. Both FACE and ACE methodogies were then used to study the interactions and to get the association constants and the stoichiometry of the complex. Multisite interactions, with two classes of independent sites, were determined. The specificity of the dendritic polylysine structure compared with linear polylysine in the interaction with ATP or AMP is also emphasized.

Analysis of lidocaine interactions with serum proteins using high-performance affinity chromatography

High-performance affinity chromatography was used to study binding by the drug lidocaine to human serum albumin (HSA) and alpha(1)-acid glycoprotein (AGP). AGP had strong binding to lidocaine, with an association equilibrium constant (K(a)) of 1.1-1.7 x 10(5) M(-1) at 37 degrees C and pH 7.4. Lidocaine had weak to moderate binding to HSA, with a K(a) in the range of 10(3) to 10(4) M(-1). Competitive experiments with site selective probes showed that lidocaine was interacting with Sudlow site II of HSA and the propranolol site of AGP. These results agree with previous observations in the literature and provide a better quantitative understanding of how lidocaine binds to these serum proteins and is transported in the circulation. This study also demonstrates how HPAC can be used to examine the binding of a drug with multiple serum proteins and provide detailed information on the interaction sites and equilibrium constants that are involved in such processes.

Flow injection-capillary electrophoresis frontal analysis method for the study of the interactions of a series of drugs with human serum albumin

In this paper, a fast method for the study on the interactions of a series of drugs used in the treatment of hypertension with human serum albumin (HSA) by flow injection-capillary electrophoresis (FI-CE) was developed based on the principle of frontal analysis (FA). The binding parameters were determined by FI-CE-FA from Scatchard equation and compared with results obtained by non-CE methods and literature values. A multiple linear regression (MLR) model between the drug-protein binding constants (K) and structural descriptors of drugs was constructed. L-tryptophan (L-try) and phenylbutazone (PB) were used as displacement reagents to investigate the binding sites of a series of drugs on HSA. The binding synergism effect between drugs and the effects of many metal ions existing in human plasma on protein binding were also investigated systematically.

Studies of imipramine binding to human serum albumin by high-performance affinity chromatography

Binding by the drug imipramine to the protein human serum albumin (HSA) was studied by using high-performance affinity chromatography. The association equilibrium constants and number of binding sites for imipramine with HSA were first estimated by utilizing frontal analysis. Imipramine was found to have one major binding site on HSA with an association equilibrium constant of 1.6 x 10(5) M(-1) at pH 7.4 and 37 degrees C, as well as a second group of weaker and non-specific binding regions (8-9 mol/mol HSA) with an average association equilibrium constant of 1.5 x 10(3) M(-1). Competition studies based on zonal elution were performed to identify the location of the major binding site for imipramine on HSA. Imipramine was found to have direct competition with L-tryptophan, which indicated that imipramine was interacting with Sudlow site II, or the indole-benzodiazepine site of HSA. No competition or allosteric effects were noted between imipramine and warfarin, a probe for Sudlow site I or the warfarin-azapropazone site of HSA. The association equilibrium constant found for imipramine at its site of competition with L-tryptophan also agreed with the value that was obtained for the major binding site of imipramine in the frontal analysis studies. These results confirmed that Sudlow site II was the location of the major binding site for imipramine on HSA. These results gave good agreement with previous observations made in the literature and should provide a more detailed description of how imipramine is transported in blood and of how it may interact with other drugs in the body.

Studies of verapamil binding to human serum albumin by high-performance affinity chromatography

The binding of verapamil to the protein human serum albumin (HSA) was examined by using high-performance affinity chromatography. Many previous reports have investigated the binding of verapamil with HSA, but the exact strength and nature of this interaction (e.g. the number and location of binding sites) is still unclear. In this study, frontal analysis indicated that at least one major binding site was present for R- and S-verapamil on HSA, with estimated association equilibrium constants on the order of 10(4)M(-1) and a 1.4-fold difference in these values for the verapamil enantiomers at pH 7.4 and 37 degrees C. The presence of a second, weaker group of binding sites on HSA was also suggested by these results. Competitive binding studies using zonal elution were carried out between verapamil and various probe compounds that have known interactions with several major and minor sites on HSA. R/S-Verapamil was found to have direct competition with S-warfarin, indicating that verapamil was binding to Sudlow site I (i.e. the warfarin-azapropazone site of HSA). The average association equilibrium constant for R- and S-verapamil at this site was 1.4 (+/-0.1)x10(4)M(-1). Verapamil did not have any notable binding to Sudlow site II of HSA but did appear to have some weak allosteric interactions with l-tryptophan, a probe for this site. An allosteric interaction between verapamil and tamoxifen (a probe for the tamoxifen site) was also noted, which was consistent with the binding of verapamil at Sudlow site I. No interaction was seen between verapamil and digitoxin, a probe for the digitoxin site of HSA. These results gave good agreement with previous observations made in the literature and help provide a more detailed description of how verapamil is transported in blood and of how it may interact with other drugs in the body.

Study of interaction between drug enantiomers and human serum albumin by flow injection-capillary electrophoresis frontal analysis

Flow injection (FI)-CE coupled with frontal analysis (FA) was applied to the study of stereoselectivity binding of amlodipine (AL) to HSA. Under protein-drug binding equilibrium, the unbound concentrations of drug enantiomers were measured by plateau height. The stereoselectivity of AL binding to HSA was proved by the different free fractions of two enantiomers. In physiological phosphate solution (pH 7.4, ionic strength 0.17) when 200 microM (+/-)AL was equilibrated with 300 microM HSA, the concentration of unbound R-AL was about 1.5 times higher than that of its antipode. The binding constants of two enantiomers, KR-AL and KS-AL, were 9910-11200 and 90200-104000 M(-1), respectively. The results obtained by the method were compared with those determined by conventional equilibrium dialysis (ED)-CE and fluorescence spectra. Hydroxypropyl-beta-CD (HP-beta-CD) (10 mM) was used as a chiral selector in pH 3.7 phosphate buffer. L-tryptophan (L-try) and ketoprofen (Ket) were used as displacement reagents to investigate the binding sites of AL to HSA. A binding synergism effect between hydrochlorothiazide (QL) and AL was observed and the results suggested that QL can destroy binding equilibrium of R-AL and S-AL toward HSA and they can occupy the same binding site of HSA (site I). The reproducibility was confirmed by RSD (RSD<1.5%) of the plateau height determined by FI-CE frontal analysis (FI-CE-FA). The FI-CE-FA was a good method to study protein-drug interaction.

Use of Triplet Excited States for the Study of Drug Binding to Human and Bovine Serum Albumins

The triplet excited states of (S)- and (R)-flurbiprofen (FBP) have been used as reporters for the microenvironments experienced within the binding sites of human and bovine serum albumins. Regression analysis of triplet decay provides valuable information on the degree of protection that these excited states are afforded from attack by a second FBP molecule, oxygen, or other reagents. The multiexponential fitting of these decays can be satisfactorily correlated with the distribution of the drug among the two binding sites and its presence as the noncomplexed form in the bulk solution. This assignment has been confirmed by using (S)-ibuprofen or capric acid as selective site II replacement probes. Triplet lifetimes and site occupancy are sensitive to the type of serum albumin employed (human versus bovine). Finally, the binding behaviour of (S)- and (R)-FBP exhibits little stereoselectivity.

Bioanalytical interaction studies executed by preincubation affinity capillary electrophoresis

The versatility of CE is beneficial for the study of many types of molecular interactions, because different experimental designs can be made to suit the characteristics of a particular interaction. A very versatile starting point is the preequilibration type of affinity CE that has been used extensively for characterizing biomolecular interactions in the last 15 years. We review this field here and include a comprehensive overview of the existing preincubation ACE modes including their advantages and limitations as well as the methodological developments and applications within the bioanalytical field.

Electrophoresis of pharmaceutical proteins:Status quo

The biotechnology industry has undergone rapid growth in recent years largely due to the development and success of protein-based therapeutics for a wide range of disorders. Similar to traditional pharmaceuticals, characterization of a therapeutic protein for its physicochemical properties, process monitoring and lot release is crucial. Electrophoresis in the slab-gel format has and continues to be a mainstay of the protein laboratory; and more recently, CE has begun to make significant inroads for protein analysis in industrial settings. This review focuses on the electrophoresis of proteins with an emphasis on protein-based therapeutics in the capillary, slab-gel and to a lesser extent, the microchip format. Reported applications of electrophoresis at several stages of the biopharmaceutical industry covering the period of 2000-2005 will be discussed.