Competitive MS binding assays for dopamine D2 receptors employing spiperone as a native marker

Development of a non-radioactive mass spectrometry-based binding assay at the μ-opioid receptor and its application for the determination of the binding affinities of 17 opiates/opioids as well as of the designer opioid isotonitazene and five further 2-benzylbenzimidazoles

Radioactive ligand binding assays are the most commonly applied method for the determination of binding affinities of compounds at a particular receptor. While they are highly sensitive and high-throughput capable they come with major disadvantages due to the radioactive ligands utilized. Here we present the development of a mass-spectrometry-based binding assay for the determination of binding affinities at the human μ-opioid receptor using non-labelled DAMGO ([D-Ala², N-MePhe⁴, Gly⁵-ol]-enkephalin). The runtime of the LC-MS/MS method was 5.5 min per data point and allowed for the highly sensitive detection of 38.5 fg DAMGO on column. The assay shows low non-specific binding and the equilibrium dissociation constant of DAMGO was 0.57 nM. The assay was applied to determine the K_i values of 17 opiates/opioids and the results were in good agreement with the data from radioactive receptor binding assays published in the literature. Additionally, the K_i value of six 2-benzylbenzimidazoles, including the widely abused designer opioid isotonitazene, were determined ranging from 0.654 to 72.9 nM. Consequently, the developed assay provides a suitable alternative to radioactive binding assays as it allows for a reliable and rapid determination of receptor binding affinities of e.g. newly emerging designer opioids.

G protein-coupled receptors: structure- and function-based drug discovery

As one of the most successful therapeutic target families, G protein-coupled receptors (GPCRs) have experienced a transformation from random ligand screening to knowledge-driven drug design. We are eye-witnessing tremendous progresses made recently in the understanding of their structure-function relationships that facilitated drug development at an unprecedented pace. This article intends to provide a comprehensive overview of this important field to a broader readership that shares some common interests in drug discovery.

TP10-Dopamine Conjugate as a Potential Therapeutic Agent in the Treatment of Parkinson's Disease

Parkinson's disease (PD) is a common progressive neurodegenerative disorder for which the current treatment is not fully satisfactory. One of the major drawbacks of current PD therapy is poor penetration of drugs across the blood-brain barrier (BBB). In recent years, cell-penetrating peptides (CPPs) such as Tat, SynB, or TP10 have gained great interest due to their ability to penetrate cell membranes and to deliver different cargos to their targets including the central nervous system (CNS). However, there is no data with respect to the use of CPPs as drug carriers to the brain for the treatment of PD. In the presented research, the covalent TP10-dopamine conjugate was synthesized and its pharmacological properties were characterized in terms of its ability to penetrate the BBB and anti-parkinsonian activity. The results showed that dopamine (DA) in the form of a conjugate with TP10 evidently gained access to the brain tissue, exhibited low susceptibility to O-methylation reaction by catechol- O-methyltransferase (lower than that of DA), possessed a relatively high affinity to both dopamine D₁ and D₂ receptors (in the case of D₁, a much higher than that of DA), and showed anti-parkinsonian activity (higher than that of l-DOPA) in the MPTP-induced preclinical animal model of PD. The presented results prove that the conjugation of TP10 with DA may be a good starting point for the development of a new strategy for the treatment of PD.

Label-Free, LC-MS-Based Assays to Quantitate Small-Molecule Antagonist Binding to the Mammalian BLT1 Receptor

We have developed and validated label-free, liquid chromatography-mass spectrometry (LC-MS)-based equilibrium direct and competition binding assays to quantitate small-molecule antagonist binding to recombinant human and mouse BLT1 receptors expressed in HEK 293 cell membranes. Procedurally, these binding assays involve (1) equilibration of the BLT1 receptor and probe ligand, with or without a competitor; (2) vacuum filtration through cationic glass fiber filters to separate receptor-bound from free probe ligand; and (3) LC-MS analysis in selected reaction monitoring mode for bound probe ligand quantitation. Two novel, optimized probe ligands, compounds 1 and 2, were identified by screening 20 unlabeled BLT1 antagonists for direct binding. Saturation direct binding studies confirmed the high affinity, and dissociation studies established the rapid binding kinetics of probe ligands 1 and 2. Competition binding assays were established using both probe ligands, and the affinities of structurally diverse BLT1 antagonists were measured. Both binding assay formats can be executed with high specificity and sensitivity and moderate throughput (96-well plate format) using these approaches. This highly versatile, label-free method for studying ligand binding to membrane-associated receptors should find broad application as an alternative to traditional methods using labeled ligands.

MS Binding Assays for D1 and D5 Dopamine Receptors

MS Binding Assays are a label-free alternative to radioligand binding assays. They provide basically the same capabilities as the latter, but an unlabeled reporter ligand is used instead of a radioligand. The study presented herein describes the development of MS Binding Assays that address D1 and D5 dopamine receptors. A highly sensitive, rapid and robust LC-ESI-MS/MS quantification method for the selective D1 dopamine receptor antagonist SCH23390 ((5R)-8-chloro-3-methyl-5-phenyl-1,2,4,5-tetrahydro-3-benzazepin-7-ol) was established and validated, using its 8-bromo analogue SKF83566 as an internal standard. This quantification method proved to be suitable for the characterization of SCH23390 binding to human D1 and D5 receptors. Following the concept of MS Binding Assays, saturation experiments for D1 and D5 receptors were performed, as well as competition experiments for D1 receptors. The results obtained are in good agreement with results from radioligand binding assays and therefore indicate that the established MS Binding Assays addressing D1 and D5 receptors are well-suited substitutes for radioligand binding assays, the technique that has so far dominated affinity determinations toward these targets.

Shuttle-cargo fusion molecules of transport peptides and the hD2/3 receptor antagonist fallypride: a feasible approach to preserve ligand-receptor binding?

To determine if the conjugation of a small receptor ligand to a peptidic carrier to potentially facilitate transport across the blood-brain barrier (BBB) by "molecular Trojan horse" transcytosis is feasible, we synthesized several transport peptide-fallypride fusion molecules as model systems and determined their binding affinities to the hD2 receptor. Although they were affected by conjugation, the binding affinities were found to be still in the nanomolar range (between 1.5 and 64.2 nM). In addition, homology modeling of the receptor and docking studies for the most potent compounds were performed, elucidating the binding modes of the fusion molecules and the structure elements contributing to the observed high receptor binding. Furthermore, no interaction between the hybrid compounds and P-gp, the main excretory transporter of the BBB, was found. From these results, it can be inferred that the approach to deliver small neuroreceptor ligands across the BBB by transport peptide carriers is feasible.

Receptor-based high-throughput screening and identification of estrogens in dietary supplements using bioaffinity liquid-chromatography ion mobility mass spectrometry

A high-throughput bioaffinity liquid chromatography-mass spectrometry (BioMS) approach was developed and applied for the screening and identification of recombinant human estrogen receptor α (ERα) ligands in dietary supplements. For screening, a semi-automated mass spectrometric ligand binding assay was developed applying (13)C2, (15) N-tamoxifen as non-radioactive label and fast ultra-high-performance-liquid chromatography-electrospray ionisation-triple-quadrupole-MS (UPLC-QqQ-MS), operated in the single reaction monitoring mode, as a readout system. Binding of the label to ERα-coated paramagnetic microbeads was inhibited by competing estrogens in the sample extract yielding decreased levels of the label in UPLC-QqQ-MS. The label showed high ionisation efficiency in positive electrospray ionisation (ESI) mode, so the developed BioMS approach is able to screen for estrogens in dietary supplements despite their poor ionisation efficiency in both positive and negative ESI modes. The assay was performed in a 96-well plate, and all these wells could be measured within 3 h. Estrogens in suspect extracts were identified by full-scan accurate mass and collision-cross section (CCS) values from a UPLC-ion mobility-Q-time-of-flight-MS (UPLC-IM-Q-ToF-MS) equipped with a novel atmospheric pressure ionisation source. Thanks to the novel ion source, this instrument provided picogram sensitivity for estrogens in the negative ion mode and an additional identification point (experimental CCS values) next to retention time, accurate mass and tandem mass spectrometry data. The developed combination of bioaffinity screening with UPLC-QqQ-MS and identification with UPLC-IM-Q-ToF-MS provides an extremely powerful analytical tool for early warning of ERα bioactive compounds in dietary supplements as demonstrated by analysis of selected dietary supplements in which different estrogens were identified.

Recent developments in protein-ligand affinity mass spectrometry

This review provides an overview of direct and indirect technologies to screen protein–ligand interactions with mass spectrometry. These technologies have as a key feature the selection or affinity purification of ligands in mixtures prior to detection. Specific fields of interest for these technologies are metabolic profiling of bioactive metabolites, natural extract screening, and the screening of libraries for bioactives, such as parallel synthesis libraries and small combichem libraries. The review addresses the principles of each of the methods discussed, with a focus on developments in recent years, and the applicability of the methods to lead generation and development in drug discovery.

Figure

Competitive mass spectrometry binding assay for characterization of three binding sites of tubulin

Tubulin is an attractive and established target for anticancer therapy. To date, the only method to determine the binding of inhibitor to tubulin has been competitive radioligand binding assays. We developed a non-radioactive mass spectrometry (MS) binding assay to study the tubulin binding of colchicine, vinblastine and paclitaxel and to identify which of these three binding sites that a novel inhibitor binds. The method involves a very simple step of separating the unbound ligand from macromolecules using ultrafiltration. The unbound ligand in the filtrate can be accurately determined using highly sensitive and specific liquid chromatography tandem mass spectrometry (LC-MS/MS) method using multiple reaction monitoring (MRM) mode. The assay was validated using podophyllotoxin, vincristine and docetaxel, drugs that compete to the colchicine-, vinblastine- and paclitaxel-binding sites in tubulin, respectively. This competitive binding assay allowed the reliable detection of interactions of these drugs with three binding sites on tubulin. This method was subsequently applied to determine the tubulin-binding site of 4-substituted methoxylbenzoyl-aryl-thiazoles (SMART-H), a potent antitubulin agent developed in our laboratory. The results indicated that SMART-H specifically and reversibly bound only to the colchicine-binding site, but not to vinblastine- or paclitaxel sites. This new non-radioligand binding method to determine the binding site on tubulin will function as a useful tool to study the binding sites of tubulin inhibitors.

Development of a microfluidic confocal fluorescence detection system for the hyphenation of nano-LC to on-line biochemical assays

One way to profile complex mixtures for receptor affinity is to couple liquid chromatography (LC) on-line to biochemical detection (BCD). A drawback of this hyphenated screening approach is the relatively high consumption of sample, receptor protein and (fluorescently labeled) tracer ligand. Here, we worked toward minimization of sample and reagent consumption, by coupling nano-LC on-line to a light-emitting diode (LED) based capillary confocal fluorescence detection system capable of on-line BCD with low-flow rates. In this fluorescence detection system, a capillary with an extended light path (bubble cell) was used as a detection cell in order to enhance sensitivity. The technology was applied to a fluorescent enhancement bioassay for the acetylcholine binding protein, a structural analog of the extracellular ligand-binding domain of neuronal nicotinic acetylcholine receptors. In the miniaturized setup, the sensitive and low void volume LED-induced confocal fluorescence detection system operated in flow injection analysis mode allowing the measurement of IC₅₀ values, which were comparable with those measured by a conventional plate reader bioassay. The current setup uses 50 nL as injection volume with a carrier flow rate of 400 nL/min. Finally, coupling of the detection system to gradient reversed-phase nano-LC allowed analysis of mixtures in order to identify the bioactive compounds present by injecting 10 nL of each mixture.

Drastic decrease in dopamine receptor levels in the striatum of acetylcholinesterase knock-out mouse

BACKGROUND

The acetylcholinesterase knock-out mouse lives to adulthood despite 60-fold elevated acetylcholine concentrations in the brain that are lethal to wild-type animals. Part of its mechanism of survival is a 50% decrease in muscarinic and nicotinic receptors and a 50% decrease in adrenoceptor levels.

HYPOTHESIS

The hypothesis was tested that the dopaminergic neuronal system had also adapted.

METHODS

Radioligand binding assays measured dopamine receptor level and binding affinity in the striatum. Immunohistochemistry of brain sections with specific antibodies visualized dopamine transporter. Effects on the intracellular compartment were measured as cAMP content, PI-phospholipase C activity.

RESULTS

Dopamine receptor levels were decreased 28-fold for the D(1)-like, and more than 37-fold for the D(2)-like receptors, though binding affinity was normal. Despite these huge changes in receptor levels, dopamine transporter levels were not affected. The intracellular compartment had normal levels of cAMP and PI-phospholipase C activity.

CONCLUSION

Survival of the acetylcholinesterase knock-out mouse could be linked to adaptation of many neuronal systems during development including the cholinergic, adrenergic and dopaminergic. These adaptations balance the overstimulation of cholinergic receptors caused by high acetylcholine concentrations and thus maintain homeostasis inside the cell, allowing the animal to live.

MS-Binding Assays: Kinetic, Saturation, and Competitive Experiments Based on Quantitation of Bound Marker as Exemplified by the GABA Transporter mGAT1

A new kind of binding assay is described in which the amount of a nonlabeled marker bound to the target is quantified by LC-ESI-MS-MS. This new approach was successfully implemented with nonlabeled NO 711 as marker and the GABA transporter subtype mGAT1 as target. The native marker bound to the target was liberated from the receptor protein by methanol denaturation after filtration. A reliable and sensitive LC-ESI-MS-MS method for the quantitation of NO 711 was developed, and data from mass spectrometric detection were analyzed by nonlinear regression. Kinetic MS-binding experiments yielded values for k+1 and k-1, while in saturation MS-binding experiments, Kd and Bmax values were determined. In competitive MS-binding experiments, Ki values were obtained for various test compounds covering a broad range of affinities for mGAT1. All experiments were performed in 96-well plate format with a filter plate for the separation step which improved the efficiency and throughput of the procedure. The method was validated by classical radioligand-binding experiments with the labeled marker [3H2]NO 711 in parallel. The results obtained from MS-binding experiments were found to be in good agreement with the results of the radioligand-binding assays. The new kind of MS-binding assay presented herein is further adapted to the conventional radioligand-binding assay in that the amount of bound marker is securely quantified. This promises easy implementation in accordance with conventional binding assays without the major drawbacks that are inherent in radioligand or fluorescence binding assays. Therefore, MS-binding assays are a true alternative to classical radioligand-binding assays.