Conformational Mobility of Immobilized α3β2, α3β4, α4β2, and α4β4 Nicotinic Acetylcholine Receptors

Relationship between MRGPRX2 and pethidine hydrochloride- or fentanyl citrate-induced LAD2 cell degranulation

OBJECTIVES

Pethidine hydrochloride (PH) and fentanyl citrate (FC) are opioid receptor agonists commonly used to treat pain clinically. PH and FC have been reported to have a high potential for pseudoallergic effects, but the underlying mechanism has not been well studied. MRGPRX2 is a novel atypical opioid receptor that is mainly expressed in human mast cells and considered to mediate drug-induced pseudoallergic reactions. This study aimed to investigate the allergy effect of these two opioid receptor agonists and the possible association of MRGPRX2 with this response.

METHODS

HEK293-MRGPRX2/CMC assay, molecular docking assay, calcium mobilization assay, the test of β-hexosaminidase, histamine and cytokine release assay were performed in this article.

KEY FINDINGS

PH but not FC induced LAD2 cell activation and degranulation dose-dependently. Histamine, tumour necrosis factor (TNF)-α, interleukin (IL)-8, monocyte chemotactic protein 1 (MCP-1) and macrophage inflammatory protein (MIP-1β) levels were upregulated by PH, but not FC. The PH-induced activation of mast cell was MRGPRX2-dependent.

CONCLUSIONS

PH but not FC activated mast cells, leading to degranulation mediated via MRGPRX2 receptors, which could be greatly significant in future clinical applications of opioid receptor drugs.

Comparison of analytical techniques for the identification of bioactive compounds from natural products

Covering: 2000 to 2016Natural product extracts are a rich source of bioactive compounds. As a result, the screening of natural products for the identification of novel biologically active metabolites has been an essential part of several drug discovery programs. It is estimated that more than 70% of all drugs approved from 1981 and 2006, were either derived from or structurally similar to nature based compounds indicating the necessity for the development of a rapid method for the identification of novel compounds from plant extracts. The screening of biological matrices for the identification of novel modulators is nevertheless still challenging. In this review we discuss current techniques in phytochemical analysis and the identification of biologically active components.

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.

Interactions between histamine H1 receptor and its antagonists by using cell membrane chromatography method

Objectives

A high histamine H1 receptor (H1R) expression cell membrane chromatography (CMC) method was developed to investigate the affinity of ligands for H1R.

Methods

The affinity of ligands for H1R was evaluated by frontal analysis. Competition studies and molecular docking study were utilized to study the interactions that occurred at specific binding sites on H1R.

Key findings

The KD values measured by frontal analysis were (8.72 ± 0.21) × 10−7 M for azelastine, (9.12 ± 0.26) × 10−7 M for cyproheptadine, (9.90 ± 0.18) × 10−7 M for doxepin, (1.42 ± 0.13) × 10−6 M for astemizole, (2.25 ± 0.36) × 10−6 M for chlorpheniramine and (3.10 ± 0.27) × 10−6 M for diphenhydramine. The results had a positive correlation with those from radioligand binding assay. The ability of displacement order measured on the binding sites occupied by doxepin was doxepin (KD, (2.95 ± 0.21) × 10−8 M) > astemizole (KD, (5.03 ± 0.18) × 10−7 M) > chlorpheniramine (KD, (1.27 ± 0.16) × 10−6 M) > cyproheptadine (KD, (1.61 ± 0.27) × 10−6 M), whose order met with the scores by molecular docking study.

Conclusions

The studies showed CMC could be applied to investigate drug–receptor interactions.

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.

Naturally-expressed nicotinic acetylcholine receptor subtypes

Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.

Interaction of ibogaine with human alpha3beta4-nicotinic acetylcholine receptors in different conformational states

The interaction of ibogaine and phencyclidine (PCP) with human (h) alpha3beta4-nicotinic acetylcholine receptors (AChRs) in different conformational states was determined by functional and structural approaches including, radioligand binding assays, Ca2+ influx detections, and thermodynamic and kinetics measurements. The results established that (a) ibogaine inhibits (+/-)-epibatidine-induced Ca2+ influx in h(alpha)3beta4 AChRs with approximately 9-fold higher potency than that for PCP, (b) [3H]ibogaine binds to a single site in the h(alpha)3beta4 AChR ion channel with relatively high affinity (Kd = 0.46 +/- 0.06 microM), and ibogaine inhibits [3H]ibogaine binding to the desensitized h(alpha)3beta4 AChR with slightly higher affinity compared to the resting AChR. This is explained by a slower dissociation rate from the desensitized ion channel compared to the resting ion channel, and (c) PCP inhibits [3H]ibogaine binding to the h(alpha)3beta4 AChR, suggesting overlapping sites. The experimental results correlate with the docking simulations suggesting that ibogaine and PCP interact with a binding domain located between the serine (position 6') and valine/phenylalanine (position 13') rings. This interaction is mediated mainly by van der Waals contacts, which is in agreement with the observed enthalpic contribution determined by non-linear chromatography. However, the calculated entropic contribution also indicates local conformational changes. Collectively our data suggest that ibogaine and PCP bind to overlapping sites located between the serine and valine/phenylalanine rings, to finally block the AChR ion channel, and in the case of ibogaine, to probably maintain the AChR in the desensitized state for longer time.

Interaction of 18-methoxycoronaridine with nicotinic acetylcholine receptors in different conformational states

The interaction of 18-methoxycoronaridine (18-MC) with nicotinic acetylcholine receptors (AChRs) was compared with that for ibogaine and phencyclidine (PCP). The results established that 18-MC: (a) is more potent than ibogaine and PCP inhibiting (+/-)-epibatidine-induced AChR Ca(2+) influx. The potency of 18-MC is increased after longer pre-incubation periods, which is in agreement with the enhancement of [(3)H]cytisine binding to resting but activatable Torpedo AChRs, (b) binds to a single site in the Torpedo AChR with high affinity and inhibits [(3)H]TCP binding to desensitized AChRs in a steric fashion, suggesting the existence of overlapping sites. This is supported by our docking results indicating that 18-MC interacts with a domain located between the serine (position 6') and valine (position 13') rings, and (c) inhibits [(3)H]TCP, [(3)H]ibogaine, and [(3)H]18-MC binding to desensitized AChRs with higher affinity compared to resting AChRs. This can be partially attributed to a slower dissociation rate from the desensitized AChR compared to that from the resting AChR. The enthalpic contribution is more important than the entropic contribution when 18-MC binds to the desensitized AChR compared to that for the resting AChR, and vice versa. Ibogaine analogs inhibit the AChR by interacting with a luminal domain that is shared with PCP, and by inducing desensitization.

Tricyclic antidepressants and mecamylamine bind to different sites in the human alpha4beta2 nicotinic receptor ion channel

The interaction of tricyclic antidepressants with the human (h) alpha4beta2 nicotinic acetylcholine receptor in different conformational states was compared with that for the noncompetitive antagonist mecamylamine by using functional and structural approaches. The results established that: (a) [(3)H]imipramine binds to halpha4beta2 receptors with relatively high affinity (K(d)=0.83+/-0.08 microM), but imipramine does not differentiate between the desensitized and resting states, (b) although tricyclic antidepressants inhibit (+/-)-epibatidine-induced Ca(2+) influx in HEK293-halpha4beta2 cells with potencies that are in the same concentration range as that for (+/-)-mecamylamine, tricyclic antidepressants inhibit [(3)H]imipramine binding to halpha4beta2 receptors with affinities >100-fold higher than that for (+/-)-mecamylamine. This can be explained by our docking results where imipramine interacts with the leucine (position 9') and valine (position 13') rings by van der Waals contacts, whereas mecamylamine interacts electrostatically with the outer ring (position 20'), (c) van der Waals interactions are in agreement with the thermodynamic results, indicating that imipramine interacts with the desensitized and resting receptors by a combination of enthalpic and entropic components. However, the entropic component is more important in the desensitized state, suggesting local conformational changes. In conclusion, our data indicate that tricyclic antidepressants and mecamylamine efficiently inhibit the ion channel by interacting at different luminal sites. The high proportion of protonated mecamylamine calculated at physiological pH suggests that this drug can be attracted to the channel mouth before binding deeper within the receptor ion channel finally blocking ion flux.

Interaction of bupropion with muscle-type nicotinic acetylcholine receptors in different conformational states

To characterize the binding sites and the mechanisms of inhibition of bupropion on muscle-type nicotinic acetylcholine receptors (AChRs), structural and functional approaches were used. The results established that bupropion (a) inhibits epibatidine-induced Ca(2+) influx in embryonic muscle AChRs, (b) inhibits adult muscle AChR macroscopic currents in the resting/activatable state with approximately 100-fold higher potency compared to that in the open state, (c) increases the desensitization rate of adult muscle AChRs from the open state and impairs channel opening from the resting state, (d) inhibits binding of [(3)H]TCP and [(3)H]imipramine to the desensitized/carbamylcholine-bound Torpedo AChR with higher affinity compared to the resting/alpha-bungarotoxin-bound AChR, (e) binds to the Torpedo AChR in either state mainly by an entropy-driven process, and (f) interacts with a binding domain located between the serine (position 6') and valine (position 13') rings, by a network of van der Waals, hydrogen bond, and polar interactions. Collectively, our data indicate that bupropion first binds to the resting AChR, decreasing the probability of ion channel opening. The remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process. Bupropion interacts with a luminal binding domain shared with PCP that is located between the serine and valine rings, and this interaction is mediated mainly by an entropy-driven process.

The preparation and development of cellular membrane affinity chromatography columns

Cellular membrane affinity chromatography is a technique that is based on the immobilization of a target trans-membrane protein onto a stationary phase. The target protein is isolated by homogenization and solubilization of a source (e.g., cell line) followed by immobilization on either the immobilized artificial membrane-phosphatidyl choline (IAM-PC) stationary phase or the surface of an open tubular capillary during a dialysis step. The procedure typically takes 3-4 d for the IAM-PC stationary phase, whereas the open-tubular method takes an extra week for the preparation of the capillary. The resulting columns can then be used to characterize binding sites on the target protein through frontal chromatographic and/or nonlinear chromatographic studies using a wide variety of ligands including small molecules and polypeptides. The columns have been used in drug discovery as well as in the screening of tobacco smoke condensates.

Exploring enantiospecific ligand-protein interactions using cellular membrane affinity chromatography: chiral recognition as a dynamic process

The chiral recognition mechanisms responsible for the enantioselective binding on the alpha3beta4 nicotinic acetylcholine receptor (alpha3 beta4 nAChR) and human organic cation transporter 1 (hOCT1) have been reviewed. The results indicate that chiral recognition on the alpha3beta4 nAChR is a process involving initial tethering of dextromethorphan and levomethorphan at hydrophobic pockets within the central lumen followed by hydrogen bonding interactions favoring dextromethorphan. The second step is the defining enantioselective step. Studies with the hOCT1 indentified four binding sites within the transporter that participated in chiral recognition. Each of the enantiomers of the compounds used in the study interacted with three of these sites, while (R)-verapamil interacted with all four. Chiral recognition arose from the conformational adjustments required to produce optimum interactions. With respect to the prevailing interaction-based models, the data suggest that chiral recognition is a dynamic process and that the static point-based models should be amended to reflect this.

Characterization of a multiple ligand-gated ion channel cellular membrane affinity chromatography column and identification of endogenously expressed receptors in astrocytoma cell lines

Cellular membranes obtained from the 1321N1 and A172 astrocytoma cell lines were immobilized on a chromatographic phase to create cellular membrane affinity chromatography (CMAC) columns, CMAC(1321N1) and CMAC(A172). The columns were characterized using frontal affinity chromatography with [(3)H]-epibatidine as the marker ligand and epibatidine, nicotine, and methyllycaconitine as the displacers. The results indicated that the columns contained homomeric alpha7 nicotinic acetylcholine receptors (alpha7 nAChR) and heteromeric nicotinic acetylcholine receptors (alpha(x)beta(y) nAChRs), which was confirmed by the addition of subtype-specific inhibitors, alpha-bungarotoxin (alpha7 nAChR) and kappa-bungarotoxin (alpha(x)beta(y) nAChR) to the mobile phase. The presence of two additional ligand-gated ion channels (LGICs), gamma-aminobutyric acid (GABA(A)) and N-methyl-D-aspartic acid (NMDA), was established using frontal affinity chromatography with flunitrazepam and diazepam (GABA(A) receptor) and MK-801 and NMDA (NMDA receptor). The presence of the four LGICs was confirmed using confocal microscopy and flow cytometry. The results indicate that the CMAC(1321N1) and CMAC(A172) columns contain four independently functioning LGICs, that the columns can be used to characterize binding affinities of small molecules to each of the receptors, and that the CMAC approach can be used to probe the expression of endogenous membrane receptors.

Development of an immobilized GPR17 receptor stationary phase for binding determination using frontal affinity chromatography coupled to mass spectrometry

A liquid chromatographic stationary phase containing immobilized membranes from cells expressing the P2Y-like receptor GPR17 is described. Cellular membranes from 1321N1 cells transiently transfected with GPR17 vector [GPR17+] and from the same cell line transfected with the corresponding empty vector [GPR17(-)] were entrapped on immobilized artificial membrane (IAM) support and packed into 6.6-mm-i.d. glass columns to create GPR17(+)-IAM and GPR17(-)-IAM stationary phases. Frontal chromatography experiments on both GPR17(+)-IAM and GPR17(-)-IAM demonstrated the presence of a specific interaction with GPR17 only in the former that was maximized by increasing the membrane/IAM ratio. GPR17(+)-IAM was used in frontal affinity chromatography experiments to calculate the dissociation constants (K(d)) of three ligands-the antagonist cangrelor (formerly AR-C69931MX, a P2Y(12)/P2Y(13) antagonist), MRS2179 (a P2Y(1) receptor antagonist), and the agonist UDP-all of which have been reported to also interact with GPR17. Immobilized GPR17 retained its ability to specifically bind the three analytes, as demonstrated by the agreement of the calculated K(d) values with previously reported data. Preliminary ranking experiments suggest the application of GPR17(+)-IAM in ranking affinity studies for the selection of new potential candidates.

Initial synthesis and characterization of an alpha7 nicotinic receptor cellular membrane affinity chromatography column: effect of receptor subtype and cell type

In this study, cellular membrane fragments from SH-EP1-pCEP4-halpha7 and alpha7 HEK-293 cell lines were used to synthesize cellular membrane affinity chromatography (CMAC) columns containing functional alpha7 nicotinic acetylcholine receptors, CMAC(alpha7 nAChR) columns. The synthesis of stable columns required the addition of cholesterol to the 2% cholate solubilization/immobilization (s/i) buffer and to the mobile phase. In addition, when membranes from the SH-EP1 cell line were used, l-alpha-phosphatidylserine and l-alpha-phosphatidylethanolamine also had to be added to the s/i buffer. A CMAC(alpha4beta2 nAChR) column was prepared using membrane fragments from a SH-EP1-pCEP4-halpha4beta2 cell line, and this process required the addition of l-alpha-phosphatidylserine and l-alpha-phosphatidylethanolamine to the s/i buffer, but not cholesterol. The s/i buffers from the three columns were compared with the s/i buffer utilized in the preparation of a CMAC(alpha4beta2 nAChR) column prepared using an alpha4beta2 HEK-293 cell line, which required no additions to the 2% cholate s/i buffer. The data demonstrate that both cell type and receptor type affect the protocol required to produce a stable CMAC column and that, at the current time, the development of an optimum immobilization protocol is an empirical process. The results are also consistent with the observation that the alpha7 nAChR is localized in lipid rafts in both of these cell lines and that the cholate detergent removed cholesterol from these microdomains.

Allosteric modifiers of neuronal nicotinic acetylcholine receptors: new methods, new opportunities

Allosteric, non-competitive inhibitors (NCIs) of neuronal nicotinic acetylcholine receptors (nAChRs) have been shown to produce a wide variety of clinically relevant responses. Many of the observed effects are desired as the nAChR is the therapeutic target, while others are undesired consequences due to off-target binding at the nAChR. Thus, the determination of whether or not a lead drug candidate is an NCI should play an important role in drug discovery programs. However, the current experimental techniques used to identify NCIs are challenging, expensive, and time consuming. This review focuses on an alternative approach to the investigation of interactions between test compounds and nAChRs based upon liquid chromatographic stationary phases containing cellular fragments from cell lines expressing nAChRs. The development and validation of these phases as well as their use in drug discovery and pharmacophore modeling are discussed.

Development and characterization of immobilized human organic anion transporter-based liquid chromatographic stationary phase: hOAT1 and hOAT2

This paper reports the development of liquid chromatographic columns containing immobilized organic anion transporters (hOAT1 and hOAT2). Cellular membrane fragments from MDCK cells expressing hOAT1 and S2 cells expressing hOAT2 were immobilized on the surface of the immobilized artificial membrane (IAM) liquid chromatographic stationary phase. The resulting stationary phases were characterized by frontal affinity chromatography, using the marker ligand [3H]-adefovir for the hOAT1 and [14C]-p-aminohippurate for the hOAT2 in the presence of multiple displacers. The determined binding affinities (Kd) for eight OAT1 ligands and eight OAT2 ligands were correlated with literature values and a statistically significant correlation was obtained for both the hOAT1 and hOAT2 columns: r2=0.688 (p<0.05) and r2=0.9967 (p<0.0001), respectively. The results indicate that the OAT1 and OAT2 have been successfully immobilized with retention of their binding activity. The use of these columns to identify ligands to the respective transporters will be presented.

Enantiomeric separation using an l-RNA aptamer as chiral additive in partial-filling capillary electrophoresis

In this paper, we report the chiral resolution of arginine using an anti-arginine l-RNA aptamer chiral selector in partial-filling CE. The effects of the capillary temperature, sample load, and aptamer plug length on the enantiomeric separation were assessed. Very high chiral resolving capability was observed at low or moderate capillary temperatures (the target peak being not detected in the separation window), whereas the practical chiral resolution was achieved only at high enough temperatures (50-60 degrees C). Over this high-temperature range, the electrophoretic behavior of the target enantiomer appeared to result from a combination of binding site heterogeneity, slow desorption kinetics, and concentration overload of aptamer binding sites. From additional thermal UV melting experiments, three RNA conformations were identified for the 50-60 degrees C temperatures. It was suggested that the presence of these different RNA conformations was a plausible source of the binding site heterogeneity.

Conformational mobility of immobilized proteins

Cellular membrane fragments have been immobilized on the surface of a silica-based liquid chromatographic support and on the surface of glass capillaries to create immobilized receptor and drug transporter columns. These columns have included phases containing one subtype of the nicotinic receptor (alpha3beta2, alpha3beta4, alpha4beta2, alpha4beta4) and the P-glycoprotein transporter. A key question in the application of these columns to drug discovery and development is the ability of the immobilized receptor or transporter to undergo ligand and/or co-factor induced conformational changes. Using frontal affinity chromatographic techniques and non-linear chromatographic techniques it has been demonstrated that the immobilized nicotinic receptors undergo agonist-induced conformational shifts from the resting to desensitized states with corresponding changes in binding affinities and enantioselectivities. Ligand-induced allosteric interactions and ATP-driven conformational changes have also been demonstrated with the immobilized Pgp stationary phase. The results demonstrate that the immobilized proteins retained their ability to undergo conformational mobility and that this is an attractive alternative to allow for the full characterization of multiple protein conformations.

Non-Competitive Inhibitory Activities of Morphinan and Morphine Derivatives at the α3β4 Neuronal Nicotinic Acetylcholine Receptor Determined Using Nonlinear Chromatography and Chemometric Techniques

PURPOSE

A series of morphine and morphinan derivatives were chromatographed on a column containing immobilized cellular membranes from a cell line expressing the alpha 3 beta 4 neuronal nicotinic acetylcholine receptor (alpha 3 beta 4 nAChR).

METHODS

The results were analyzed using chemometric and molecular modeling techniques in order to predict the noncompetitive inhibitory (NCI) activity of these compounds, the molecular basis for the predicted activity and the binding sites of the inhibitors.

RESULTS

The data demonstrated that seven of seven morphinans were NCIs and bound in the central lumen of the nAChR while only 2 of 13 morphine derivatives had NCI activity and these compounds most likely bound at the quinacrine binding site on the nAChR. The predicted activities were confirmed using functional inhibition studies.

CONCLUSIONS

The results indicate that this approach can be used to rapidly assess pharmacological activity and to guide new drug design.

Development of immobilized membrane-based affinity columns for use in the online characterization of membrane bound proteins and for targeted affinity isolations

Membranes obtained from cell lines that express or do not express a target membrane bound protein have been immobilized on a silica-based liquid chromatographic support or on the surface of an activated glass capillary. The resulting chromatographic columns have been placed in liquid chromatographic systems and used to characterize the target proteins and to identify small molecules that bind to the target. Membranes containing ligand gated ion channels, G-protein coupled receptors and drug transporters have been prepared and characterized. If a marker ligand has been identified for the target protein, frontal or zonal displacement chromatographic techniques can be used to determine binding affinities (K(d) values) and non-linear chromatography can be used to assess the association (k(on)) and dissociation (k(off)) rate constants and the thermodynamics of the binding process. Membrane-based affinity columns have been created using membranes from a cell line that does not express the target protein (control) and the same cell line that expresses the target protein (experimental) after genomic transfection. The resulting columns can be placed in a parallel chromatography system and the differential retention between the control and experimental columns can be used to identify small molecules and protein that bind to the target protein. These applications will be illustrated using columns created using cellular membranes containing nicotinic acetylcholine receptors and the drug transporter P-glycoprotein.

Qualitative assessment of IC50 values of inhibitors of the neuronal nicotinic acetylcholine receptor using a single chromatographic experiment and multivariate cluster analysis

It has been widely demonstrated that affinity chromatography can be used to derive binding affinities, and that these affinities can be correlated to data obtained using standard techniques such as membrane binding, ultrafiltration and equilibrium dialysis. The purpose of this study is to evaluate the use of immobilized nicotinic acetylcholine receptor stationary phase in chromatographic experiments to assess the functional activity of series of noncompetitive inhibitors (NCIs) as reflected in their IC50 values. Chromatographically determined retention values and computer generated molecular descriptors were obtained for 29 compounds and the data were analyzed by cluster analysis. The approach qualitatively ranked the test compounds as efficient NCIs (low IC50 values) or poor NCIs (high IC50 values). The data obtained with the 29 compounds used in this study demonstrate that the experimental approach had been able to place 25 of these compounds in the correct IC(50) clusters. To our knowledge, this is the first relationship established between chromatographic retention and IC50 for membrane-bound receptors. These results suggest that the chromatographic approach may be useful in development of lead drug candidates including the determination of off-target binding.

Enantioselective binding to the human organic cation transporter-1 (hOCT1) determined using an immobilized hOCT1 liquid chromatographic stationary phase

A liquid chromatography stationary phase containing immobilized membranes obtained from a cell line that expresses the human organic cation transporter (hOCT1-IAM) has been used to study the binding of the enantiomers of propranolol, atenolol, pseudoephedrine, and alpha-methylbenzylamine to the immobilized hOCT1. Frontal displacement chromatography was used to determine the binding affinities (K(d) values), and the data demonstrate that there was an enantioselective difference in the K(d) values of the enantiomers of propranolol, atenolol, and pseudoephedrine, while alpha-methylbenzylamine did not significantly bind to the transporter. Competitive inhibition studies with the cell line used to create the chromatographic column demonstrated that, for the enantiomers of propranolol, the ratio of the chromatographically determined K(d) values [K(d (+)-(R)-propranolol)/K(d (-)-(S)-propranolol) = 2.98] reflected an enantioselective difference in the functional activity of the two enantiomers [IC(50 (+)-(R)-propranolol)/IC(50 (-)-(S)-propranolol) = 2.75]. The chromatographically determined K(d) values were used to construct an initial pharmacophore which contains a hydrogen bond donating site that appears to be responsible for the observed enantioselectivity.