Screening for new ligands of the MB327-PAM-1 binding site of the nicotinic acetylcholine receptor

Intoxications with organophosphorus compounds (OPCs) effect a severe impairment of cholinergic neurotransmission that, as a result of overstimulation may lead to desensitization of nicotinic acetylcholine receptors (nAChRs) and finally to death due to respiratory paralysis. So far, therapeutics, that are capable to address and revert desensitized neuromuscular nAChRs into their resting, i.e. functional state are still missing. Still, among a class of compounds termed bispyridinium salts, which are characterized by the presence of two pyridinium subunits, constituents have been identified, that can counteract organophosphate poisoning by resensitizing desensitized nAChRs. According to comprehensive modeling studies this effect is mediated by an allosteric binding site at the nAChR termed MB327-PAM-1 site. For MB327, the most prominent representative of the bispyridinium salts and all other analogues studied so far, the affinity for the aforementioned binding site and the intrinsic activity measured in ex vivo and in in vivo experiments are distinctly too low, to meet the criteria to be fulfilled for therapeutic use. Hence, in order to identify new compounds with higher affinities for the MB327-PAM-1 binding site, as a basic requirement for an enhanced potency, two compound libraries, the ChemDiv library with 60 constituents and the Tocriscreen Plus library with 1280 members have been screened for hit compounds addressing the MB327-PAM-1 binding site, utilizing the [2H6]MB327 MS Binding Assay recently developed by us. This led to the identification of a set of 10 chemically diverse compounds, all of which exhibit an IC50 value of ≤ 10 µM (in the [2H6]MB327 MS Binding Assay), which had been defined as selection criteria. The three most affine ligands, which besides a quinazoline scaffold share similarities with regard to the substitution pattern and the nature of the substituents, are UNC0638, UNC0642 and UNC0646. With binding affinities expressed as pKi values of 6.01 ± 0.10, 5.97 ± 0.05 and 6.23 ± 0.02, respectively, these compounds exceed the binding affinity of MB327 by more than one log unit. This renders them promising starting points for the development of drugs for the treatment of organophosphorus poisoning by addressing the MB327-PAM-1 binding site of the nAChR.

MS Binding Assays with UNC0642 as reporter ligand for the MB327 binding site of the nicotinic acetylcholine receptor

Intoxications with organophosphorus compounds (OPCs) based chemical warfare agents and insecticides may result in a detrimental overstimulation of muscarinic and nicotinic acetylcholine receptors evolving into a cholinergic crisis leading to death due to respiratory failure. In the case of the nicotinic acetylcholine receptor (nAChR), overstimulation leads to a desensitization of the receptor, which cannot be pharmacologically treated so far. Still, compounds interacting with the MB327 binding site of the nAChR like the bispyridinium salt MB327 have been found to re-establish the functional activity of the desensitized receptor. Only recently, a series of quinazoline derivatives with UNC0642 as one of the most prominent representatives has been identified to address the MB327 binding site of the nAChR, as well. In this study, UNC0642 has been utilized as a reporter ligand to establish new Binding Assays for this target. These assays follow the concept of MS Binding Assays for which by assessing the amount of bound reporter ligand by mass spectrometry no radiolabeled material is required. According to the results of the performed MS Binding Assays comprising saturation and competition experiments it can be concluded, that UNC0642 used as a reporter ligand addresses the MB327 binding site of the Torpedo-nAChR. This is further supported by the outcome of ex vivo studies carried out with poisoned rat diaphragm muscles as well as by in silico studies predicting the binding mode of UNC0646, an analog of UNC0642 with the highest binding affinity, in the recently proposed binding site of MB327 (MB327-PAM-1). With UNC0642 addressing the MB327 binding site of the Torpedo-nAChR, this and related quinazoline derivatives represent a promising starting point for the development of novel ligands of the nAChR as antidotes for the treatment of intoxications with organophosphorus compounds. Further, the new MS Binding Assays are a potent alternative to established assays and of particular value, as they do not require the use of radiolabeled material and are based on a commercially available compound as reporter ligand, UNC0642, exhibiting one of the highest binding affinities for the MB327 binding site known so far.

Structural Modification of the Natural Product Valerenic Acid Tunes RXR Homodimer Agonism

Retinoid X receptors (RXR) are ligand-sensing transcription factors with a unique role in nuclear receptor signaling as universal heterodimer partners. RXR modulation holds potential in cancer, neurodegeneration and metabolic diseases but adverse effects of RXR activation and lack of selective modulators prevent further exploration as therapeutic target. The natural product valerenic acid has been discovered as RXR agonist with unprecedented preference for RXR subtype and homodimer activation. To capture structural determinants of this activity profile and identify potential for optimization, we have studied effects of structural modification of the natural product on RXR modulation and identified an analogue with enhanced RXR homodimer agonism.

Structure-Guided Design of Nurr1 Agonists Derived from the Natural Ligand Dihydroxyindole

The neuroprotective transcription factor Nurr1 was recently found to bind the dopamine metabolite 5,6-dihydroxyindole (DHI) providing access to Nurr1 ligand design from a natural template. We screened a custom set of 14 k extended DHI analogues in silico for optimized descendants to select 24 candidates for microscale synthesis and in vitro testing. Three out of six primary hits were validated as novel Nurr1 agonists with up to sub-micromolar binding affinity, highlighting the druggability of the Nurr1 surface region lining helix 12. In vitro profiling confirmed cellular target engagement of DHI descendants and demonstrated remarkable additive effects of combined Nurr1 agonist treatment, indicating diverse binding sites mediating Nurr1 activation, which may open new avenues in Nurr1 modulation.

Cryo-EM structure of GABA transporter 1 reveals substrate recognition and transport mechanism

The inhibitory neurotransmitter γ-aminobutyric acid (GABA) is cleared from the synaptic cleft by the sodium- and chloride-coupled GABA transporter GAT1. Inhibition of GAT1 prolongs the GABAergic signaling at the synapse and is a strategy to treat certain forms of epilepsy. In this study, we present the cryo-electron microscopy structure of Rattus norvegicus GABA transporter 1 (rGAT1) at a resolution of 3.1 Å. The structure elucidation was facilitated by epitope transfer of a fragment-antigen binding (Fab) interaction site from the Drosophila dopamine transporter (dDAT) to rGAT1. The structure reveals rGAT1 in a cytosol-facing conformation, with a linear density in the primary binding site that accommodates a molecule of GABA, a displaced ion density proximal to Na site 1 and a bound chloride ion. A unique insertion in TM10 aids the formation of a compact, closed extracellular gate. Besides yielding mechanistic insights into ion and substrate recognition, our study will enable the rational design of specific antiepileptics.

Targeting the Alternative Vitamin E Metabolite Binding Site Enables Noncanonical PPARγ Modulation

The lipid-sensing transcription factor PPARγ is the target of antidiabetic thiazolidinediones (TZD). At two sites within its ligand binding domain, it also binds oxidized vitamin E metabolites and the vitamin E mimetic garcinoic acid. While the canonical interaction within the TZD binding site mediates classical PPARγ activation, the effects of the second binding on PPARγ activity remain elusive. Here, we identified an agonist mimicking dual binding of vitamin E metabolites and developed a selective ligand of the second site, unveiling potential noncanonical regulation of PPARγ activities. We found that this alternative binding event can simultaneously occur with orthosteric ligands and it exerted different effects on PPARγ-cofactor interactions compared to both orthosteric PPARγ agonists and antagonists, indicating the diverse roles of the two binding sites. Alternative site binding lacked the pro-adipogenic effect of TZD and mediated no classical PPAR signaling in differential gene expression analysis but markedly diminished FOXO signaling, suggesting potential therapeutic applications.

Development of a Potent Nurr1 Agonist Tool for In Vivo Applications

Nuclear receptor related 1 (Nurr1) is a neuroprotective transcription factor and an emerging target in neurodegenerative diseases. Despite strong evidence for a role in Parkinson's and Alzheimer's disease, pharmacological control and validation of Nurr1 are hindered by a lack of suitable ligands. We have discovered considerable Nurr1 activation by the clinically studied dihydroorotate dehydrogenase (DHODH) inhibitor vidofludimus calcium and systematically optimized this scaffold to a Nurr1 agonist with nanomolar potency, strong activation efficacy, and pronounced preference over the highly related receptors Nur77 and NOR1. The optimized compound induced Nurr1-regulated gene expression in astrocytes and exhibited favorable pharmacokinetics in rats, thus emerging as a superior chemical tool to study Nurr1 activation in vitro and in vivo.

Discovery of new, highly potent and selective inhibitors of BuChE - design, synthesis, in vitro and in vivo evaluation and crystallography studies

The symptomatic and disease-modifying effects of butyrylcholinesterase (BuChE) inhibitors provide an encouraging premise for researching effective treatments for Alzheimer's disease. Here, we examined a series of compounds with a new chemical scaffold based on 3-(cyclohexylmethyl)amino-2-hydroxypropyl, and we identified a highly selective hBuChE inhibitor (29). Based on extensive in vitro and in vivo evaluations of the compound and its enantiomers, (R)-29 was identified as a promising candidate for further development. Compound (R)-29 is a potent hBuChE inhibitor (IC₅₀ = 40 nM) with selectivity over AChE and relevant off-targets, including H₁, M₁, α_1A and β₁ receptors. The compound displays high metabolic stability on human liver microsomes (90% of the parent compound after 2 h of incubation), and its safety was confirmed through examining the cytotoxicity on the HepG2 cell line (LC₅₀ = 2.85 μM) and hERG inhibition (less than 50% at 10 μM). While (rac)-29 lacked an effect in vivo and showed limited penetration to the CNS in pharmacokinetics studies, compound (R)-29 exhibited a procognitive effect at 15 mg/kg in the passive avoidance task in scopolamine-treated mice.

Benocyclidine (BTCP) as Non-labelled Reporter Ligand for MS Binding Assays for the PCP Ion Channel Binding Site of the Desensitized Torpedo Nicotinic Acetylcholine Receptor (nAChR)

In this study we present MS Binding Assays for the PCP ion channel binding site of Torpedo californica nicotinic acetylcholine receptor (nAChR) as an alternative to radioligand binding assays. As MS Marker Benocyclidine (BTCP) was employed, found to be more affine (K_d of 84.2 nM) than the radioligands, e. g. [³ H]PCP, used so far in respective binding assays. Based on a highly sensitive and fast LC-ESI-MS/MS method for quantification of BTCP samples, BTCP MS Binding Assays for the PCP ion channel binding site of Torpedo nAChR could be established comprising saturation, kinetic and competition experiments. The affinities obtained in competitive BTCP MS Binding Assays for ligands addressing the PCP ion channel binding site of Torpedo nAChR were in excellent accord with those reported from radioligand experiments. Thus, the new BTCP MS Binding Assays represent a potent and reliable alternative to radioligand binding assays used so far for the characterization of ligand binding to the PCP ion channel binding site of the nAChR.

Simultaneous LC-ESI-MS/MS Quantification of Levosimendan and Its Metabolites for Therapeutic Drug Monitoring of Cardiac Surgery Patients

Levosimendan is used in severe chronic cardiac insufficiency, also within the peri-operative setting. Real-life pharmacokinetic data in surgical patients is lacking, making therapeutic drug monitoring (TDM) of levosimendan, its pharmacologically active metabolite OR-1896, and its intermediate OR-1855 important. A simultaneous highly sensitive quantification of levosimendan and its metabolites in small-volume samples has not yet been described. Here, levosimendan (LLOQ 0.450 nM), OR-1896, and OR-1855 (LLOQ both 1.0 nM) were successfully quantified by LC-ESI-MS/MS after liquid-liquid extraction in 300 µL of blood. A short C8 column under reversed-phase conditions enabled simultaneous and fast quantification of levosimendan in the negative and the metabolites in the positive ionization mode in a single run within 2 min. Interestingly and unexpectedly, constitutional isomers of levosimendan metabolites with identical mass transitions and similar retention times were observed in surgical patients’ samples, which we identified as the metamizole metabolites 4-aminoantipyrine and 4-acetamidoantipyrine. A longer C8 column and a modified mobile phase enabled selective quantification of all analytes in a single run within 7 min. We developed, validated, and applied highly sensitive LC-ESI-MS/MS methods for simultaneous quantification of levosimendan and its metabolites, enabling efficient TDM of cardiac surgery patients even with additional metamizole administration.

Screening for New Inhibitors of Glycine Transporter 1 and 2 by Means of MS Binding Assays

A straightforward screening of a compound library comprising 2439 substances for the identification of new inhibitors for the neurotransmitter transporters GlyT1 and GlyT2 is described. Screening and full-scale competition experiments were performed using recently developed GlyT1 and GlyT2 MS Binding Assays. That way for both targets, GlyT1 and GlyT2, ligands were identified, which exhibited affinities (pKi values) in the low micromolar to sub-micromolar range. The majority of these binders exhibit new chemical scaffolds in the class of GlyT1 and GlyT2 inhibitors, which could be of interest for the development of new ligands with improved affinities for the target proteins. Additionally, compounds with excellent fluorescent properties were found for GlyT2, which renders them promising compounds for future fluorescence-based techniques. All in all, this study demonstrates that MS Binding Assays represent a powerful technology platform also well suited for the screening of compound libraries in a highly reliable and effective manner.

Discovery of multifunctional anti-Alzheimer's agents with a unique mechanism of action including inhibition of the enzyme butyrylcholinesterase and γ-aminobutyric acid transporters

Looking for an effective anti-Alzheimer's agent is very challenging; however, a multifunctional ligand strategy may be a promising solution for the treatment of this complex disease. We herein present the design, synthesis and biological evaluation of novel hydroxyethylamine derivatives displaying unique, multiple properties that have not been previously reported. The original mechanism of action combines inhibitory activity against disease-modifying targets: β-secretase enzyme (BACE1) and amyloid β (Aβ) aggregation, along with an effect on targets associated with symptom relief - inhibition of butyrylcholinesterase (BuChE) and γ-aminobutyric acid transporters (GATs). Among the obtained molecules, compound 36 exhibited the most balanced and broad activity profile (eeAChE IC₅₀ = 2.86 μM; eqBuChE IC₅₀ = 60 nM; hBuChE IC₅₀ = 20 nM; hBACE1 IC₅₀ = 5.9 μM; inhibition of Aβ aggregation = 57.9% at 10 μM; mGAT1 IC₅₀ = 10.96 μM; and mGAT2 IC₅₀ = 19.05 μM). Moreover, we also identified 31 as the most potent mGAT4 and hGAT3 inhibitor (IC₅₀ = 5.01 μM and IC₅₀ = 2.95 μM, respectively), with high selectivity over other subtypes. Compounds 36 and 31 represent new anti-Alzheimer agents that can ameliorate cognitive decline and modify the progress of disease.

MS Binding Assays for Glycine Transporter 2 (GlyT2) Employing Org25543 as Reporter Ligand

This study describes the first binding assay for glycine transporter 2 (GlyT2) following the concept of MS Binding Assays. The selective GlyT2 inhibitor Org25543 was employed as a reporter ligand and it was quantified with a highly sensitive and rapid LC-ESI-MS/MS method. Binding of Org25543 at GlyT2 was characterized in kinetic and saturation experiments with an off-rate of 7.07×10-3  s-1 , an on-rate of 1.01×106  M-1  s-1 , and an equilibrium dissociation constant of 7.45 nM. Furthermore, the inhibitory constants of 19 GlyT ligands were determined in competition experiments. The validity of the GlyT2 affinities determined with the binding assay was examined by a comparison with published inhibitory potencies from various functional assays. With the capability for affinity determination towards GlyT2 the developed MS Binding Assays provide the first tool for affinity profiling of potential ligands and it represents a valuable new alternative to functional assays addressing GlyT2.

Combination of MS Binding Assays and affinity selection mass spectrometry for screening of structurally homogenous libraries as exemplified for a focused oxime library addressing the neuronal GABA transporter 1

This study presents an efficient screening approach based on combination of mass spectrometry (MS) based binding assays (MS Binding Assays) and affinity selection mass spectrometry (ASMS) customized for screening of structurally homogeneous libraries sharing a common mass spectrometric fragmentation pattern. After reaction of a nipecotic acid derivative possessing a hydroxylamine functionality with aldehydes, the resulting oxime library was screened accordingly toward the GABA transporter subtype 1 (GAT1), a drug target for several neurological disorders. After assessing sublibraries' activities for inhibition of reporter ligand binding, hits in active ones were directly identified. This could be achieved by recording mass transitions for the reporter ligand as well as those predicted for the library components in a single LC-MS/MS run with a triple quadrupole mass spectrometer in the multiple reaction monitoring mode. Identification of hits with a predefined affinity could be reliably accomplished by calculation of IC₅₀-values from specific binding concentrations of library constituents and reporter ligand. Application of this strategy revealed six hits, from which two of them were resynthesized for further biological evaluation. Thereby, the best one displayed a pK_i of 7.38 in MS Binding Assays and a pIC₅₀ of 6.82 in [³H]GABA uptake assays for GAT1.

Synthesis and biological evaluation of α- and β-hydroxy substituted amino acid derivatives as potential mGAT1–4 inhibitors

In this study, we report the synthesis and biological evaluation of a variety of α- and β-hydroxy substituted amino acid derivatives as potential amino acid subunits in inhibitors of GABA uptake transporters (GATs). In order to ensure that the test compounds adopt a binding pose similar to that presumed for related larger GAT inhibitors, lipophilic residues were introduced either at the amino nitrogen atom or at the alcohol function. Several of the synthesized compounds were found to exhibit similar inhibitory activity at the GAT subtypes mGAT2, mGAT3, and mGAT4, respectively, as compared with the reference N-butylnipecotic acid. Hence, these compounds might serve as starting point for future developments of more complex GAT inhibitors.

N-Substituted Nipecotic Acids as (S)-SNAP-5114 Analogues with Modified Lipophilic Domains

Potential mGAT4 inhibitors derived from the lead substance (S)-SNAP-5114 have been synthesized and characterized for their inhibitory potency. Variations from the parent compound included the substitution of one of its aromatic 4-methoxy and 4-methoxyphenyl groups, respectively, with a more polar moiety, including a carboxylic acid, alcohol, nitrile, carboxamide, sulfonamide, aldehyde or ketone function, or amino acid partial structures. Furthermore, it was investigated how the substitution of more than one of the aromatic 4-methoxy groups affects the potency and selectivity of the resulting compounds. Among the synthesized test substances (S)-1-{2-[(4-formylphenyl)bis(4-methoxyphenyl)-methoxy]ethyl}piperidine-3-carboxylic acid, that features a carbaldehyde function in place of one of the aromatic 4-methoxy moieties of (S)-SNAP-5114, was found to have a pIC50 value of 5.89±0.07, hence constituting a slightly more potent mGAT4 inhibitor than the parent substance while showing comparable subtype selectivity.

A Library Screening Strategy Combining the Concepts of MS Binding Assays and Affinity Selection Mass Spectrometry

The primary objective of early drug development is to identify hits and leads for a target of interest. To achieve this aim, rapid, and reliable screening techniques for a huge number of compounds are needed. Mass spectrometry based binding assays (MS Binding Assays) represent a well-established technique for library screening based on competitive binding experiments revealing active sublibraries due to reduced binding of a reporter ligand and following hit identification for active libraries by deconvolution in further competitive binding experiments. In the present study, we combined the concepts of MS Binding Assays and affinity selection mass spectrometry (ASMS) to improve the efficiency of the hit identification step. In that case, only a single competitive binding experiment is performed that is in the first step analyzed for reduced binding of the reporter ligand and—only if a sublibrary is active—additionally for specific binding of individual library components. Subsequently, affinities of identified hits as well as activities of reduced sublibraries (i.e., all sublibrary components without hit) are assessed in additional competitive binding experiments. We exemplified this screening concept for the identification of ligands addressing the most widespread GABA transporter subtype in the brain (GAT1) studying in the beginning a library composed of 128 and further on a library of 1,280 well-characterized GAT1 inhibitors, drug substances, and pharmacological tool compounds. Determination of sublibraries' activities was done by quantification of bound NO711 as reporter ligand and hit identification for the active ones achieved in a further LC-ESI-MS/MS run in the multiple reaction monitoring mode enabling detection of all sublibrary components followed by hit verification and investigation of reduced sublibraries in further competitive binding experiments. In this way, we could demonstrate that all GAT1 inhibitors reducing reporter ligand binding below 50% at a concentration of 1 μM are detected reliably without generation of false positive or false negative hits. As the described strategy is apart from its reliability also highly efficient, it can be assumed to become a valuable tool in early drug research, especially for membrane integrated drug targets that are often posing problems in established screening techniques.

Synthesis and Biological Evaluation of Nipecotic Acid and Guvacine Derived 1,3-Disubstituted Allenes as Inhibitors of Murine GABA Transporter mGAT1

A new class of nipecotic acid and guvacine derivatives has been synthesized and characterized for their inhibitory potency at mGAT1-4 and binding affinity for mGAT1. Compounds of the described class are defined by a four-carbon-atom allenyl spacer connecting the nitrogen atom of the nipecotic acid or guvacine head with an aromatic residue. Among the compounds investigated, the mixture of nipecotic acid derivatives rac-{(R_a )-1-[4-([1,1':2',1''-terphenyl]-2-yl)buta-2,3-dien-1-yl](3R)-piperidine-3-carboxylic acid} and rac-{(S_a )-1-[4-([1,1':2',1''-terphenyl]-2-yl)buta-2,3-dien-1-yl](3R)-piperidine-3-carboxylic acid} (21 p), possessing an o-terphenyl residue, was identified as highly selective and the most potent mGAT1 inhibitor in this study. For the (R)-nipecotic acid derived form of 21 p, the inhibitory potency in [³ H]GABA uptake assays was determined as pIC₅₀ =6.78±0.08, and the binding affinity in MS Binding Assays as pK_i =7.10±0.12. The synthesis of the designed compounds was carried out by a two-step procedure, generating the allene moiety via allenylation of terminal alkynes which allows broad variation of the terminal phenyl and biphenyl subunit.

MS-Based Screening of 5-Substituted Nipecotic Acid Derived Hydrazone Libraries as Ligands of the GABA Transporter 1

A screening of compound libraries based on nipecotic acid derivatives with lipophilic residues attached to the scarcely explored 5-position of the core structure was used for the search of new inhibitors of the γ-aminobutyric acid (GABA) transporter 1 (mGAT1). The generated compound libraries, which were based on hydrazone chemistry commonly used in dynamic combinatorial chemistry but rendered pseudostatic, were screened for their binding affinities toward mGAT1 by means of MS Binding Assays. With nipecotic acid derived hydrazone rac-16 h [rac-(3R,5S)-{5-[(E)-2-{[5-(2-phenylethynyl)thiophen-2-yl]methylidene}hydrazin-1-yl]piperidine-3-carboxylic acid}-sodium chloride (1/2)], one hit was found and evaluated displaying sub-micromolar potency (pK_i =6.62±0.04) and a noncompetitive interaction mode at mGAT1. By bearing a 5-(2-phenylethynyl)thiophen-2-yl residue attached to the 5-position of nipecotic acid via a three-atom spacer, compound rac-16 h contains a structural moiety so far unprecedented for these kinds of bioactive molecules, and complements novel 5-substituted nipecotic acid derived ligands of mGAT1 revealed in a recently published screening campaign. This new class of ligands, with an inhibition mode distinct from that of benchmark mGAT1 inhibitors, could serve as research tools for investigations of mGAT1-mediated GABA transport.

Development and validation of an LC-ESI-MS/MS method for the quantification of D-84, reboxetine and citalopram for their use in MS Binding Assays addressing the monoamine transporters hDAT, hSERT and hNET

MS Binding Assays represent a label-free alternative to radioligand binding assays. In this study, we present an LC-ESI-MS/MS method for the quantification of (R,R)-4-(2-benzhydryloxyethyl)-1-(4-fluorobenzyl)piperidin-3-ol [(R,R)-D-84, (R,R)-1], (S,S)-reboxetine [(S,S)-2], and (S)-citalopram [(S)-3] employed as highly selective nonlabeled reporter ligands in MS Binding Assays addressing the dopamine [DAT, (R,R)-D-84], norepinephrine [NET, (S,S)-reboxetine] and serotonin transporter [SERT, (S)-citalopram], respectively. The developed LC-ESI-MS/MS method uses a pentafluorphenyl stationary phase in combination with a mobile phase composed of acetonitrile and ammonium formate buffer for chromatography and a triple quadrupole mass spectrometer in the multiple reaction monitoring mode for mass spectrometric detection. Quantification is based on deuterated derivatives of all three analytes serving as internal standards. The established LC-ESI-MS/MS method enables fast, robust, selective and highly sensitive quantification of all three reporter ligands in a single chromatographic run. The method was validated according to the Center for Drug Evaluation and Research (CDER) guideline for bioanalytical method validation regarding selectivity, accuracy, precision, calibration curve and sensitivity. Finally, filtration-based MS Binding Assays were performed for all three monoamine transporters based on this LC-ESI-MS/MS quantification method as read out. The affinities determined in saturation experiments for (R,R)-D-84 toward hDAT, for (S,S)-reboxetine toward hNET, and for (S)-citalopram toward hSERT, respectively, were in good accordance with results from literature, clearly demonstrating that the established MS Binding Assays have the potential to be an efficient alternative to radioligand binding assays widely used for this purpose so far.

Simultaneous Multiple MS Binding Assays for the Dopamine, Norepinephrine, and Serotonin Transporters

In this work, we present label-free, mass-spectrometry-based binding assays (MS Binding Assays), targeting the human dopamine, norepinephrine, and serotonin transporters (hDAT, hNET, and hSERT) in simultaneous binding experiments. Using a validated LC-ESI-MS/MS method for quantification of the selective dopamine transporter inhibitor (R,R)-4-(2-benzhydryloxyethyl)-1-(4-fluorobenzyl)piperidin-3-ol ((R,R)-D-84), the selective norepinephrine transporter inhibitor (S,S)-reboxetine, and the selective serotonin reuptake inhibitor (S)-citalopram, binding affinities at the three monoamine transporters could be characterized simultaneously in a single binding experiment. The performed simultaneous saturation and competition experiments yielded results that are in good accordance with those determined in MS Binding Assays addressing the monoamine transporters individually. The results obtained from this study underscore the potential of MS Binding Assays for simultaneous affinity determination at different targets, which is difficult to accomplish with conventional radioligand binding assays.

Synthesis and Bioactivity of Novel N-Benzylic and N-Phenethylic Ephedrine Derivatives

A range of N-benzylic and N-phenethylic ephedrine derivatives were prepared in a one-pot procedure starting from the two enantiomers of ephedrine using the Potier reagent, and a polyfunctional aryl- or benzylic organozinc halide. The biological activity in indatraline MS Binding Assays addressing hDAT, hNET and hSERT was determined and discussed.

Development of MS Binding Assays targeting the binding site of MB327 at the nicotinic acetylcholine receptor

The bispyridinium compound MB327 has been shown previously to have a positive pharmacological effect against poisoning with organophosphorous compounds (OPCs). The mechanism by which it exerts its therapeutic effect seems to be directly mediated by the nicotinic acetylcholine receptor (nAChR). In the present study, the development of mass spectrometry based binding assays (MS Binding Assays) for characterization of the binding site of MB327 at the nAChR from Torpedo californica is described. MS Binding Assays follow the principle of radioligand binding assays, but do not, in contrast to the latter, require a radiolabeled reporter ligand, as the readout is in this case based on mass spectrometric detection. For [²H₆]MB327, a deuterated MB327 analogue employed as reporter ligand in the MS Binding Assays, an LC-ESI-MS/MS method was established allowing for its fast and reliable quantification in samples resulting from binding experiments. Using centrifugation for separation of non-bound [²H₆]MB327 from target-bound [²H₆]MB327 in saturation and autocompetition experiments (employing native MB327 as competitor) enabled reliable determination of specific binding. In this way, the affinities for [²H₆]MB327 (K_d=15.5±0.9μmolL^-1) and for MB327 (K_i=18.3±2.6μmolL^-1) towards the nAChR could be determined for the first time. The almost exactly matching affinities for MB327 and [²H₆]MB327 obtained in the MS Binding Assays are in agreement with potencies previously found in functional studies. In summary, our results demonstrate that the established MS Binding Assays represent a promising tool for affinity determination of test compounds towards the binding site of MB327 at the nAChR.

Simultaneous Multiple MS Binding Assays Addressing D1 and D2 Dopamine Receptors

MS Binding Assays are a label-free alternative to radioligand binding assays. They provide basically the same capabilities as the latter, but use a non-labeled reporter ligand instead of a radioligand. In contrast to radioligand binding assays, MS Binding Assays offer-owing to the selectivity of mass spectrometric detection-the opportunity to monitor the binding of different reporter ligands at different targets simultaneously. The present study shows a proof of concept for this strategy as exemplified for MS Binding Assays selectively addressing D₁ and D₂ dopamine receptors in a single binding experiment. A highly sensitive, rapid and robust LC-ESI-MS/MS quantification method capable of quantifying both SCH23390 and raclopride, selectively addressing D₁ and D₂ receptors, respectively, was established and validated for this purpose. Based thereon, simultaneous saturation and competition experiments with SCH23390 and raclopride in the presence of both D₁ and D₂ receptors were performed and analyzed by LC-MS/MS within a single chromatographic cycle. The present study thus demonstrates the feasibility of this strategy and the high versatility of MS Binding Assays that appears to surpass that common for conventional radioligand binding assays.

Synthesis of 4-substituted nipecotic acid derivatives and their evaluation as potential GABA uptake inhibitors

In this study, we disclose the design and synthesis of novel 4-susbtituted nipecotic acid derivatives as inhibitors of the GABA transporter mGAT1. Based on molecular modeling studies the compounds are assumed to adopt a binding pose similar to that of the potent mGAT1 inhibitor nipecotic acid. As substitution in 4-position should not cause an energetically unfavorable orientation of nipecotic acid as it is the case for N-substituted derivatives this is expected to lead to highly potent binders. For the synthesis of novel 4-substituted nipecotic acid derivatives a linear synthetic strategy was employed. As a key step, palladium catalyzed cross coupling reactions were used to attach the required biaryl moieties to the ω-position of the alkenyl- or alkynyl spacers of varying length in the 4-position of the nipecotic acid scaffold. The resulting amino acids were characterized with respect to their binding affinities and inhibitory potencies at mGAT1. Though the biological activities found were generally insignificant to poor, two compounds, one of which possesses a reasonable binding affinity for mGAT1, rac-57, the other a notable inhibitory potency at mGAT4, rac-84, both displaying a slight subtype selectivity for the individual transporters, could be identified.

Azidobupramine, an Antidepressant-Derived Bifunctional Neurotransmitter Transporter Ligand Allowing Covalent Labeling and Attachment of Fluorophores

The aim of this study was to design, synthesize and validate a multifunctional antidepressant probe that is modified at two distinct positions. The purpose of these modifications was to allow covalent linkage of the probe to interaction partners, and decoration of probe-target complexes with fluorescent reporter molecules. The strategy for the design of such a probe (i.e., azidobupramine) was guided by the need for the introduction of additional functional groups, conveying the required properties while keeping the additional moieties as small as possible. This should minimize the risk of changing antidepressant-like properties of the new probe azidobupramine. To control for this, we evaluated the binding parameters of azidobupramine to known target sites such as the transporters for serotonin (SERT), norepinephrine (NET), and dopamine (DAT). The binding affinities of azidobupramine to SERT, NET, and DAT were in the range of structurally related and clinically active antidepressants. Furthermore, we successfully visualized azidobupramine-SERT complexes not only in SERT-enriched protein material but also in living cells stably overexpressing SERT. To our knowledge, azidobupramine is the first structural analogue of a tricyclic antidepressant that can be covalently linked to target structures and further attached to reporter molecules while preserving antidepressant-like properties and avoiding radioactive isotopes.

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.