Immobilized nicotinic receptor stationary phases: going with the flow in high-throughput screening and pharmacological studies

Screening potential ligands of endothelin receptor A from Choerospondias axillaris and evaluation of their drug-like properties by affinity chromatographic methods

Tibetan medicine is traditionally prescribed as crude extracts or mixtures owing to the theoretical basis with cross fertilization from other medical systems like Ayurveda and traditional Chinese medicine. This is challenged to elucidate the action mechanism and material foundation of Tibetan medicine due to lacking a method to confirm the bioactive compounds determining the therapy. This work created a new strategy for screening and evaluating the bioactive compounds against cardiovascular ailments from Choerospondias axillaris. It involved the immobilization of endothelin receptor A (ET_AR) by a one-step covalent assay, the screening and identification of the bioactive compounds by ET_AR column combined with tandem mass spectrometry, and the evaluation of their drug-like properties by calculating the efficiency indexes using the data collected by frontal analysis and adsorption energy distribution. The immobilized ET_AR remained good stability in three weeks in terms of specificity and repeatability. Catechin, pinocembrin, and hyperoside were identified as potential ET_AR ligands from Choerospondias axillaris with two types of binding sites on the immobilized receptor. Their association constants on the high and low affinity binding sites were (2.53 ± 0.11) × 10⁵ and (9.94 ± 0.02) × 10³ M^-1 for catechin, (1.01 ± 0.12) × 10⁶ and (7.40 ± 0.03) × 10⁴ for hyperoside, and (2.05 ± 0.04) × 10⁵ and (2.47 ± 0.09)× 10⁴ M^-1 for pinocembrin, respectively. Owing to the highest association constant, hyperoside presented a surface efficiency index of 7.95, and binding efficiency index of 20.7, and the ligand-lipophilicity efficiency of 1.38. These indicated that the three compounds were the main ingredients for the therapy of Choerospondias axillaris, and had potential to become lead compounds for anti-cardiovascular drugs based on drug-ET_AR interaction. The immobilized receptor-based strategy is possible to become an alternative for screening and assessing bioactive compounds from Tibetan medicine.

Screening the bioactive compound from Coptis chinensis inflorescence by immobilized Peroxisome proliferator activated receptor gamma

Coptis chinensis inflorescence is a by-product of Coptis chinensis Franch and riches in alkaloids. We screened the bioactive compounds in the by-product through an immobilized peroxisome proliferator-activated receptor gamma. The receptor was covalently immobilized on the macroporous silica gel through amino groups to generate the affinity stationary phase and was applied for screening. Berberine, palmatine, and jatrorrhizine were identified as the retained components of the herb on the affinity column. We evaluated the binding of the three bioactive compounds with the receptor by nonlinear chromatography and molecular docking. The affinities of the compounds to the receptor were (1.42±0.10) ×10⁸ M^-1 , (4.88±0.38) ×10⁷ M^-1 , and (1.65±0.13) ×10⁷ M^-1 for berberine, palmatine, and jatrorrhizine, with dissociation rate constants of (17.70±0.03) ×10^-3 S^-1 , (5.18±0.25) ×10^-2 S^-1 , and (15.7±0.05) ×10^-2 S^-1 , respectively. Cys285, Arg288, Ile326, Leu330, and His449 in the agonist binding pocket of the receptor participated in the formation of bioactive compound-receptor conjugates. These data indicated that the immobilized receptor is a reliable alternative for screening the bioactive compounds. In addition, Coptis chinensis inflorescence has the potential to be a source for drug discovery. This article is protected by copyright. All rights reserved.

Rapidly identifying bioactive compounds from Zhisou oral liquid by immobilized receptor-based high-performance affinity chromatography

The identification of bioactive compounds in complex matrices remains a major challenge due to the lack of highly efficient and specific methods. This work developed an approach based on high-performance affinity chromatography to identify the potential antitussive compounds from Zhisou oral liquid . The main methods include the synthesis of immobilized beta2-adrenoceptor by a one-step method, the screening and identification of the potential bioactive compounds by the receptor column coupled with mass spectrometry, and the binding mechanism analysis of the compounds to the receptor by the in vivo experiment, injection amount dependent method and molecular simulation. We identified the potential bioactive compounds of Zhisou oral liquid as glycyrrhizic acid, platycodin D, tuberostemonine, and hesperidin. In vivo experiment showed that the combinational utilization of the four compounds was possible to present an equivalent antitussive effect to the formula. The docking results demonstrated that hydrogen bonds and Van der Waals forces were the main forces to drive the binding of the four compounds to beta2-adrenoceptor. We concluded that the four compounds are the effective components in Zhisou oral liquid. The proposed strategy is possible to provide an alternative for the development of highly efficient methods to pursue the bioactive compounds of complex matrices.

Identification of selective ligands targeting two GPCRs by receptor-affinity chromatography coupled with high-throughput sequencing techniques

The rapid growth of demands for drug discovery has necessitated the ongoing pursuit of new methods for specific ligands screening and identification. This work combined receptor-affinity chromatography (RAC) with high-throughput sequencing techniques to rapidly screen and identify the specific ligands. By this method, immobilized angiotensin II type I receptor (AT₁R) and endothelin receptor A (ET_AR) based on RAC were utilized for lead screening from a DNA-encoded library. The specific ligands of AT₁R (ligand A₁, A₂) and ET_AR (ligand B₁, B₂) were synthesized after decoding by high-throughput sequencing techniques. The dissociation rate constants (k_d) of ligand A₁, A₂ to AT₁R and B₁, B₂ to ET_AR were 9.65 × 10^-4, 31.1 × 10^-4 and 0.66, 1.22 s^-1 by peak profiling assay. The association constant (K_A) to the receptors of four ligands was 5.4 × 10⁶, 3.3 × 10⁶ and 1.6 × 10⁶, 2.2 × 10⁵ by injection amount dependent method. The kinetic and thermodynamic parameters of the four specific ligands are similar to those of the positive drugs. This indicates that they are promising to drug candidates. The druggability of the four ligands through pharmacokinetic investigation by HPLC-MS/MS presented desired pharmacokinetic behavior including the fast absorption, the relatively slow elimination. These results, taking together, indicated that the RAC combined with high-throughput sequencing techniques can screen and identify the specific ligands according to various proteins, thus creating a general strategy for rapid discovery of promising drug candidates.

Immobilized angiotensin II type I receptor: A powerful method of high throughput screening for antihypertensive compound identification through binding interaction analysis

The enormous growth in drug discovery paradigm has necessitated continuous exploration of new methods for drug-protein interaction analysis. To enhance the role of these methodologies in designing rational drugs, this work extended an immobilized angiotensin II type I receptor (AT1R) based affinity chromatography in antihypertensive compound identification. We fused haloalkane dehalogenase at C-terminus of AT1R and expressed the fusion receptor in E. coli. The expressed receptor was covalently immobilized onto 8.0 μm microspheres by mixing the cell lysate with 6-chlorocaproic acid-modified amino polystyrene microspheres. The immobilized AT1R was utilized for thermodynamic and kinetic interaction analysis between the receptor and four specific ligands. Following confirmation of these interactions by molecular docking, we identified puerarin and rosmarinic acid by determining their binding to the receptor. Azilsartan, candesartan, valsartan and olmesartan displayed two kinds of binding sites to AT1R by injection amount-dependent method. By molecular docking, we recognize the driving forces of the interaction as electrostatic interaction, hydrogen bonds and van der Waals force. The dissociation rate constants (kd) of azilsartan, candesartan, valsartan and olmesartan to AT1R were 0.01138 ± 0.003, 0.05142 ± 0.003, 0.07547 ± 0.004 and 0.01310 ± 0.005 min-1 by peak profiling assay. Comparing with these parameters, puerarin and rosmarinic acid presented lower affinity (KA: 0.12 × 104 and 1.5 × 104/M) and slower kinetics (kd: 0.6864 ± 0.03 and 0.3005 ± 0.01 min-1) to the receptor. These results, taking together, indicated that the immobilized AT1R has the capacity to probe antihypertensive compounds.

Development and characterization of the α3β4α5 nicotinic receptor cellular membrane affinity chromatography column and its application for on line screening of plant extracts

The α3β4α5 nAChR has been recently shown to be a useful target for smoking cessation pharmacotherapies. Herein, we report on the development and characterization of the α3β4α5 nicotinic receptor column by frontal displacement chromatography. The binding affinity of the nicotine and minor alkaloids found in tobacco smoke condensates were determined for both the α3β4 and α3β4α5 nicotinic receptors. It was demonstrated that while no subtype selectivity was observed for nicotine and nornicotine, anabasine was selective for the α3β4α5 nicotinic receptor. The non-competitive inhibitor binding site was also studied and it was demonstrated while mecamylamine was not selective between subtypes, buproprion showed subtype selectivity for the α3β4 nicotinic receptor. The application of this methodology to complex mixtures was then carried out by screening aqueous-alcoholic solutions of targeted plant extracts, including Lycopodium clavatum L. (Lycopodiaceae) and Trigonella foenum graecum L. (Fabaceae) against both the α3β4 and α3β4α5 nAChRs.

Emerging challenges in ligand discovery: new opportunities for chromatographic assay

Ligand discovery initiatives are facing interesting challenges as ever-increasing numbers of proteins are entering screening programs. As an answer to steady pressure to improve performance in drug discovery, ligand discovery can expect to play an expanded role in generating small molecules as probes to help uncover the function of novel proteins. Chromatographic assay formats can offer new entry points into standard interaction characterization (binding and rate constants) as well as powerful, scaleable methods for compound screening. This review presents recent advancements in chromatographic assay technology, with a particular focus on frontal affinity chromatography as a platform technology for interaction analysis.

The synthesis and characterization of a nuclear membrane affinity chromatography column for the study of human breast cancer resistant protein (BCRP) using nuclear membranes obtained from the LN-229 cells

BCRP expression has been reported in glioblastoma cell lines and clinical specimens and has been shown to be expressed both in purified nuclei and in the soluble cytoplasmic fraction. To date, the nuclear BCRP has not been characterized. Our laboratory has previously developed an online chromatographic approach for the study of binding interactions between ligands and protein, cellular membrane affinity chromatography. To this end, we have immobilized the nuclear membrane fragments onto an immobilized artificial membrane stationary phase (IAM), resulting in the nuclear membrane affinity chromatography (NMAC) column. Initial characterization was carried out on the radio flow detector, as well as the LC-MSD, using frontal displacement chromatography techniques. Etoposide, a substrate for BCRP, was initially tested, to determine the functional immobilization of BCRP. Frontal displacement experiments with multiple concentrations of etoposide were run and the binding affinity was determined to be 4.54 μM, which is in close agreement with literature. The BCRP was fully characterized on the NMAC column and this demonstrates that for the first time the nuclear membranes have been successfully immobilized.

Recent developments in asymmetric multicomponent reactions

Multicomponent reactions (MCRs) receive increasing attention because they address both diversity and complexity in organic synthesis. Thus, in principle diverse sets of relatively complex structures can be generated from simple starting materials in a single reaction step. The ever increasing need for optically pure compounds for pharmaceutical and agricultural applications as well as for catalysis promotes the development of asymmetric multicomponent reactions. In recent years, asymmetric multicomponent reactions have been applied to the total synthesis of various enantiopure natural products and commercial drugs, reducing the number of required reaction steps significantly. Although many developments in diastereoselective MCRs have been reported, the field of catalytic enantioselective MCRs has just started to blossom. This critical review describes developments in both diastereoselective and catalytic enantioselective multicomponent reactions since 2004. Significantly broadened scopes, new techniques, more environmentally benign methods and entirely novel MCRs reflect the increasingly inventive paths that synthetic chemist follow in this field. Until recently, enantioselective transition metal-catalyzed MCRs represented the majority of catalytic enantioselective MCRs. However, metal contamination is highly undesirable for drug synthesis. The emergence of organocatalysis greatly influences the quest for new asymmetric MCRs.

Screening of tobacco smoke condensate for nicotinic acetylcholine receptor ligands using cellular membrane affinity chromatography columns and missing peak chromatography

This manuscript reports an approach to the screening of natural product extracts for compounds which are active at membrane-bound receptors, ion channels and transporters. The technique is based upon cellular membrane affinity chromatography (CMAC) columns created through the immobilization of cellular membrane fragments on liquid chromatography stationary phases. In this study a CMAC(nAChR(+)) column was created out of membranes from a transfected cell line expressing the alpha3beta4 neuronal nicotinic acetylcholine receptor (nAChR) and the column was used to screen tobacco smoke condensates. A strategy involving parallel screening with a CMAC column created from a non-transfected form of the same cell line, CMAC(nAChR(-)) was adopted. The condensate was chromatographed on both columns, timed fractions collected and concentrated. Each fraction was analyzed on a C18 column in order to establish a chromatographic fingerprint of each fraction and a differential elution profile of each compound. Comparison of the elution profiles from the CMAC(nAChR(+)) and CMAC(nAChR(-)) columns identified patterns that could be associated with high affinity ligands and with low-affinity/non-binding compounds. Known strong ligands ((S)-nicotine, (R,S)-anatabine, N'-nitrosonornicotine), weak ligands ((R,S)-nornicotine, anabasine) as well as known non-ligands (N-methyl-gamma-oxo-3-pyridinebutanamide, (1'S,2'S)-nicotine 1'-oxide) have been identified in the complex extract. The results demonstrate that CMAC-based screens can be used in the identification of compounds within natural product extracts that bind to membrane-based targets.

Solid-phase assays for small molecule screening using sol-gel entrapped proteins

With compound libraries exceeding one million compounds, the ability to quickly and effectively screen these compounds against relevant pharmaceutical targets has become crucial. Solid-phase assays present several advantages over solution-based methods. For example, a higher degree of miniaturization can be achieved, functional- and affinity-based studies are possible, and a variety of detection methods can be used. Unfortunately, most protein immobilization methods are either too harsh or require recombinant proteins and thus are not amenable to delicate proteins such as kinases and membrane-bound receptors. Sol-gel encapsulation of proteins in an inorganic silica matrix has emerged as a novel solid-phase assay platform. In this minireview, we discuss the development of sol-gel derived protein microarrays and sol-gel based monolithic bioaffinity columns for the high-throughput screening of small molecule libraries and mixtures.

Frontal affinity chromatography-mass spectrometry

Frontal affinity chromatography (FAC) is a biophysical method for the discovery and characterization of molecular interactions in a flow-based system. Several different modes of analysis are possible by interfacing to the mass spectrometer, including robust single-compound characterizations as well as high-throughput screening of over 1,000 compounds per run. The method supports thermodynamic and kinetic characterization of interactions for a wide range of molecular species and possesses similarities to flow-based biosensors such as surface plasmon resonance. It offers sensitive detection of ligands present well below their respective dissociation constants, and can be assembled from readily available laboratory components. Direct coupling of the FAC cartridge to the mass spectrometer is useful for the interrogation of single compounds or mixtures of limited complexity. An offline fractionation schema is more appropriate for discovery-mode applications. A high-performance FAC system enabling both modes can be assembled in 2-3 h. Measurements of dissociation constants can be made with such a system in 0.5-3 h, and the system supports higher-throughput screening modes at a rate of 10,000 compounds d(-1).

Automated ligand fishing using human serum albumin-coated magnetic beads

Human serum albumin, HSA, was immobilized onto the surface of silica-based magnetic beads. The beads were used to isolate known HSA ligands from a mixture containing ligands and nonligands. The separation was accomplished manually and was also automated. The results indicate that an automated "ligand-fishing" technique can be developed using magnetic beads containing an immobilized protein.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

Immobilized P2X2 purinergic receptor stationary phase for chromatographic determination of pharmacological properties and drug screening

The purinergic receptor signaling system plays an important role in communication between cells in the nervous system and opens new opportunities for screening of potential drugs. Our objective was to explore the pharmacological properties and establish a new methodology for ligand screening for the P2X2 receptor, which has been developed by the combinatorial library approach Systematic Evolution of Ligands by Exponential enrichment (SELEX). To this end, membranes of 1321N1 cells stably transfected with rat P2X2 receptors were resuspended in 2% cholate detergent and subsequently coupled onto an immobilized artificial membrane (IAM). The IAM-cholate-P2X2 mixture was then dialyzed, centrifuged and packed into a FPLC column. Equilibrium binding to the receptor and competition between ATP and the purinergic antagonists suramin and 2'3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) were analyzed by a chromatographic assay using 32P alpha ATP as a radioligand. Our data indicate that suramin does not compete with ATP for the ligand binding site and TNP-ATP is a competitive antagonist, confirming previous studies [C.A. Trujillo, A.A. Nery, A.H. Martins, P. Majumder, F.A. Gonzalez, H. Ulrich, Biochemistry 45 (2006) 224-233]. In addition, we demonstrate that this assay can be used in in vitro selection procedures for RNA aptamers binding to P2X2 receptors. The results demonstrate that the receptor can be immobilized in a stable format and reused over an extended period of time, facilitating the exploration of ligand-receptor interactions and screening of combinatorial pools for possible ligands.

Capillary-scale frontal affinity chromatography/MALDI tandem mass spectrometry using protein-doped monolithic silica columns

Frontal affinity chromatography (FAC) interfaced with electrospray mass spectrometry (ESI-MS) has been reported as a potential method for screening of compound mixtures against immobilized target proteins. However, the interfacing of bioaffinity columns to ESI-MS requires that the eluent that passes through the protein-loaded column have a relatively low ionic strength to produce a stable spray. Such low ionic strength solvents can cause serious problems with protein stability and may also affect binding constants and lead to high nonspecific binding to the column. Herein, we report on the interfacing of bioaffinity columns to matrix-assisted laser desorption/ionization (MALDI) MS/MS as a new platform for FAC/MS studies. Capillary columns containing a monolithic silica material with entrapped dihydrofolate reductase were used for frontal affinity chromatography of small-molecule mixtures. The output from the column was combined with a second stream containing alpha-cyano-hydoxycinnamic acid in methanol and was deposited using a nebulizer-assisted electrospray method onto a conventional MALDI plate that moved relative to the column via a computer-controlled x-y stage, creating a semipermanent record of the FAC run. The use of MALDI MS/MS allowed for buffers with significantly higher ionic strength to be used for FAC studies, which reduced nonspecific binding of ionic compounds and allowed for better retention of protein activity over multiple runs. Following deposition, MALDI analysis required only a fraction of the chromatographic run time, and the deposited track could be rerun multiple times to optimize ionization parameters and allow signal averaging to improve the signal-to-noise ratio. Furthermore, high levels of potential inhibitors could be detected via MALDI with limited ion suppression effects. Both MALDI- and ESI-based analysis showed similar retention of inhibitors present in compound mixtures when using identical ionic strength conditions. The results show that FAC/MALDI-MS should provide advantages over FAC/ESI-MS for high-throughput screening of compound mixtures.

Asymmetric multicomponent reactions (AMCRs): the new frontier

Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously. This kind of addition and reaction has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects. All these advantages, together with the high level of stereoselectivity attained in some of these reactions, will force chemists in industry as in academia to adopt this new strategy of synthesis, or at least to consider it as a viable option. The positive aspects as well as the drawbacks of this strategy are discussed in this Review.

Live cell extraction and HPLC–MS analysis for predicting bioactive components of traditional Chinese medicines

A novel strategy for predicting bioactive components in traditional Chinese medicines (TCM) using live cell extraction and high performance liquid chromatography-diode array detection-mass spectrometry (HPLC-DAD-MS) analysis was proposed. The hypothesis is that when cells are incubated together with the extract of TCM, the potential bioactive components in the TCM should selectively combine with the cells, and the relative concentrations of the cell-combining components in the suspension medium should decrease, while the cell-combining components would be detectable in the extract of denatured cells. The identities of the cell-combining components could be determined by HPLC-DAD-MS analysis. Using the proposed approach, the potential bioactive components of Danggui Buxue decoction, a commonly used TCM for anaemia, and its compositions, Radix Angelica Sinensis and Radix Astragli for endothelial cells, were investigated. Six compounds in the extract of Danggui Buxue decoction were detected as the components selectively combined with endothelial cells, among them two were contributed by Radix Angelica Sinensis, and four by Radix Astragli. The identities of four of the six potential bioactive compounds were elucidated as ononoside, calycosin, 3-butylphthalide and ligustilide by HPLC-DAD-MS analysis. The results indicate that the proposed approach may be applied to predict the bioactive candidates in TCM.

Immobilized enzyme reactor chromatography: Optimization of protein retention and enzyme activity in monolithic silica stationary phases

Our group recently reported on the application of protein-doped monolithic silica columns for immobilized enzyme reactor chromatography, which allowed screening of enzyme inhibitors present in mixtures using mass spectrometry for detection. The enzyme was immobilized by entrapment within a bimodal meso/macroporous silica material prepared by a biocompatible sol-gel processing route. While such columns proved to be useful for applications such as screening of protein-ligand interactions, significant amounts of entrapped proteins leached from the columns owing to the high proportion of macropores within the materials. Herein, we describe a detailed study of factors affecting the morphology of protein-doped bioaffinity columns and demonstrate that specific pH values and concentrations of poly(ethylene glycol) can be used to prepare essentially mesoporous columns that retain over 80% of initially loaded enzyme in an active and accessible form and yet still retain sufficient porosity to allow pressure-driven flow in the low muL/min range. Using the enzyme gamma-glutamyl transpeptidase (gamma-GT), we further evaluated the catalytic constants of the enzyme entrapped in capillary columns with different silica morphologies as a function of flowrate and backpressure using the enzyme reactor assay mode. It was found that the apparent activity of the enzyme was highest in mesoporous columns that retained high levels of enzyme. In such columns, enzyme activity increased by approximately 2-fold with increases in both flowrate (from 250 to 1000 nL/min) and backpressure generated (from 500 to 2100 psi) during the chromatographic activity assay owing to increases in k(cat) and decreases in K(M), switching from diffusion controlled to reaction controlled conditions at ca. 2000 psi. These results suggest that columns with minimal macropore volumes (<5%) are advantageous for the entrapment of soluble proteins for bioaffinity and bioreactor chromatography.

Receptor–ligand binding assays: Technologies and Applications

Receptor-ligand interactions play a crucial role in biological systems and their measurement forms an important part of modern pharmaceutical development. Numerous assay formats are available that can be used to screen and quantify receptor ligands. In this review, we give an overview over both radioactive and non-radioactive assay technologies with emphasis on the latter. While radioreceptor assays are fast, easy to use and reproducible, their major disadvantage is that they are hazardous to human health, produce radioactive waste, require special laboratory conditions and are thus rather expensive on a large scale. This has led to the development of non-radioactive assays based on optical methods like fluorescence polarization, fluorescence resonance energy transfer or surface plasmon resonance. In light of their application in high-throughput screening environments, there has been an emphasis on so called "mix-and-measure" assays that do not require separation of bound from free ligand. The advent of recombinant production of receptors has contributed to the increased availability of specific assays and some aspects of the expression of recombinant receptors will be reviewed. Applications of receptor-ligand binding assays described in this review will relate to screening and the quantification of pharmaceuticals in biological matrices.

Capillary-Scale Monolithic Immunoaffinity Columns for Immunoextraction with In-Line Laser-Induced Fluorescence Detection

A bimodal meso/macroporous monolithic silica capillary column containing an entrapped antibody was prepared by a biocompatible sol-gel process and used for nanoflow immunoaffinity chromatography and immunoextraction studies. Stationary phases were prepared by combining the protein-compatible silane precursor diglycerylsilane with an aqueous solution containing 10,000 Da poly(ethylene glycol) and the antibody. An analytical method was developed that was capable of determining both the dissociation constant and binding site content for the anti-fluorescein antibody within the stationary phase. The assay showed that while the antibody residing in macropores was easily removed, approximately 20% of initially loaded antibody remained active and accessible after several washes, consistent with the antibody being entrapped within the mesopores of the sol-gel matrix. The dissociation constants for fluorescein binding to the anti-fluorescein antibody were similar in solution and in the meso/macroporous silica, indicating that the entrapped antibody retained its native conformation within such a matrix. The mixture was loaded into a 250-microm-i.d. fused-silica capillary where the polymer phase separated from the silica followed by gelation of the silica. The capillary-scale immunoaffinity columns could be operated at low back pressure using a syringe pump and were capable of performing chromatographic separations that were dependent on the presence of the antibody within the stationary phase. Such columns could also be operated using in-line laser-induced fluorescence detection. The use of the capillary-scale monolithic columns for on-column immunoextraction and preconcentration is also demonstrated.

Interaction of Noncompetitive Inhibitors with an Immobilized α3β4 Nicotinic Acetylcholine Receptor Investigated by Affinity Chromatography, Quantitative−Structure Activity Relationship Analysis, and Molecular Docking

A large number of drug substances act as noncompetitive inhibitors (NCIs) of the nicotinic acetylcholine receptor (nAChR) by blocking the ion flux through the channel. An affinity chromatography technique has been developed for investigating the interactions between NCIs and the alpha3beta4 subtype of neuronal nAChR. The data obtained from the chromatographic study were used to construct QSAR models of the NCI-nAChR binding with both electronic and steric parameters observed as important descriptors. A molecular model of the transmembrane domain of the alpha3beta4 subtype of nAChR was constructed and used to simulate the docking of a series of NCIs. A key aspect of the model was the discovery of the cleft produced by the incorporation of the bulky phenylalanine moiety into the nonpolar section of the lumen by the beta4 subunit. Quantitatively, the results of docking simulations modeled the experimental affinity data better than QSAR results. The computational approach, combined with the modeling of NCI-nAChR interaction by affinity chromatography, can be used to predict possible toxicities and adverse interactions.

Entrapment of Src Protein Tyrosine Kinase in Sugar-Modified Silica

A novel sugar-modified silica has been used to entrap for the first time a protein tyrosine kinase (PTK). Silane precursors bearing covalently attached gluconamide moieties were used in combination with the biocompatible precursor diglycerylsilane (DGS) to generate sol-gel derived silica that was able to encapsulate highly active Src PTK and preserve the activity of the enzyme over multiple uses. The relative activity of the enzyme was assayed using a LANCE based fluorescence resonance energy transfer method involving time-gated detection of fluorescence from a europium labeled antiphosphotyrosine antibody and Cy5 labeled streptavidin upon mutual binding to biotinylated phosphopeptides. Using this detection method, with the antibody and streptavidin external to the sol-gel matrix, it was possible to detect the phosphorylation of peptides with molecular weights of up to 2300 Da using the entrapped enzyme in N-(3-triethoxysilylpropyl)gluconamide (GLTES) doped glasses. Src kinase-doped glasses, derived from precursors such as tetramethyl orthosilicate, tetraethyl orthosilicate, or DGS that did not contain GLTES, provided no detectable enzyme activity. The addition of 1 mM ATP to the GLTES/DGS sol before the encapsulation of the protein increased the activity of the enzyme in the resulting gel, likely through a ligand-based stabilization mechanism. The use of such a system for determination of PTK activity and inhibition is demonstrated, setting the stage for the development of chromatographic and microarray based methods for the screening of kinase inhibitors.

Protein-Doped Monolithic Silica Columns for Capillary Liquid Chromatography Prepared by the Sol−Gel Method: Applications to Frontal Affinity Chromatography

The development of bioaffinity chromatography columns that are based on the entrapment of biomolecules within the pores of sol-gel-derived monolithic silica is reported. Monolithic nanoflow columns are formed by mixing the protein-compatible silica precursor diglycerylsilane with a buffered aqueous solution containing poly(ethylene oxide) (PEO, MW 10,000) and the protein of interest and then loading this mixture into a fused-silica capillary (150-250-microm i.d.). Spinodal decomposition of the PEO-doped sol into two distinct phases prior to the gelation of the silica results in a bimodal pore distribution that produces large macropores (>0.1 microm), to allow good flow of eluent with minimal back pressure, and mesopores (approximately 3-5-nm diameter) that retain a significant fraction of the entrapped protein. Addition of low levels of (3-aminopropyl)triethoxysilane is shown to minimize nonselective interactions of analytes with the column material, resulting in a column that is able to retain small molecules by virtue of their interaction with the entrapped biomolecules. Such columns are shown to be suitable for pressure-driven liquid chromatography and can be operated at relatively high flow rates (up to 500 microL x min(-1)) or with low back pressures (<100 psi) when used at flow rates of 5-10 microL x min(-1). The clinically relevant enzyme dihydrofolate reductase was entrapped within the bioaffinity columns and was used to screen mixtures of small molecules using frontal affinity chromatography with mass spectrometric detection. Inhibitors present in compound mixtures were retained via bioaffinity interactions, with the retention time being dependent on both the ligand concentration and the affinity of the ligand for the protein. The results suggest that such columns may find use in high-throughput screening of compound mixtures.

Technological Advances in High-Throughput Screening

High-throughput screening (HTS) is the process of testing a large number of diverse chemical structures against disease targets to identify 'hits'. Compared to traditional drug screening methods, HTS is characterized by its simplicity, rapidness, low cost, and high efficiency, taking the ligand-target interactions as the principle, as well as leading to a higher information harvest. As a multidisciplinary field, HTS involves an automated operation-platform, highly sensitive testing system, specific screening model (in vitro), an abundant components library, and a data acquisition and processing system. Various technologies, especially the novel technologies such as fluorescence, nuclear-magnetic resonance, affinity chromatography, surface plasmon resonance, and DNA microarray, are now available, and the screening of more than 100,000 samples per day is already possible. Fluorescence-based assays include the scintillation proximity assay, time-resolved energy transfer, fluorescence anisotropy, fluorescence correlation spectroscopy, and fluorescence fluctuation spectroscopy. Fluorescence-based techniques are likely to be among the most important detection approaches used for HTS due to their high sensitivity and amenability to automation, giving the industry-wide drive to simplify, miniaturize, and speed up assays. The application of NMR technology to HTS is another recent trend in drug research. One advantage afforded by NMR technology is that it can provide direct information on the affinity of the screening compounds and the binding location of protein. The structure-activity relationship acquired from NMR analysis can sharpen the library design, which will be very important in furnishing HTS with well-defined drug candidates. Affinity chromatography used for library screening will provide the information on the fundamental processes of drug action, such as absorption, distribution, excretion, and receptor activation; also the eluting curve can give directly the possibility of candidate drug. SPR can measure the quantity of a complex formed between two molecules in real-time without the need for fluorescent or radioisotopic labels. SPR is capable of characterizing unmodified biopharmaceuticals, studying the interaction of drug candidates with macromolecular targets, and identifying binding partners during ligand fishing experiments. DNA microarrays can be used in HTS be used to further investigate the expression of biological targets associated with human disease, which then opens new and exciting opportunities for drug discovery. Without doubt, the addition of new technologies will further increase the application of HTS in drug screening and its related fields.

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.