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.

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 pKi of 7.38 in MS Binding Assays and a pIC50 of 6.82 in [3H]GABA uptake assays for GAT1.

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 (pKi =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.

Novel Allosteric Ligands of γ-Aminobutyric Acid Transporter 1 (GAT1) by MS Based Screening of Pseudostatic Hydrazone Libraries.

This study describes the screening of dynamic combinatorial libraries based on nipecotic acid as core structure with substituents attached to the 5- instead of the common 1-position for the search of novel inhibitors of the GABA transporter GAT1. The generated pseudostatic hydrazone libraries included a total of nearly 900 compounds and were screened for their binding affinities toward GAT1 in competitive mass spectrometry (MS) based Binding Assays. Characterization of the hydrazones with the highest affinities (with cis-configured rac-16gf bearing a 5-(1-naphthyl)furan-2-yl residue and a four atom spacer being the most potent) in binding and uptake experiments revealed an allosteric interaction at GAT1, which was not reported for any other nipecotic acid derivative up to now. Therefore, the herein introduced 5-substituted nipecotic acid derivatives could serve as valuable tools for investigations of allosterically modulated GABA transport mediated by GAT1 and furthermore as starting point for a new class of GAT1 inhibitors.

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.

Application of MS Transport Assays to the Four Human γ-Aminobutyric Acid Transporters.

γ-Aminobutyric acid (GABA) transporters (GATs) are promising drug targets for various diseases associated with imbalances in GABAergic neurotransmission. For the development of new drugs or pharmacological tools addressing GATs, screening techniques to identify new inhibitors and to characterize their potency at each GAT subtype are indispensable. By now, the technique by far dominating is based on radiolabeled GABA. We recently described "MS Transport Assays" for hGAT-1 by employing ((2) H6 )GABA as the substrate. In the present study, we applied this approach to all four human GAT subtypes and determined the KM values for GAT-mediated transport of ((2) H6 )GABA at each subtype. Furthermore, a comprehensive set of GAT inhibitors reflecting the whole range of potency and subtype selectivity known so far was evaluated for their potency. The comparison of pIC50 values obtained in conventional [(3) H]GABA uptake assays with those obtained in MS Transport Assays indicated the reliability of the latter. The MS Transport Assays enable a throughput similar to that of conventional radiometric transport assays performed in a 96-well format but avoid the use of radiolabeled substrates.

Synthesis and biological evaluation of 4-hydroxy-4-(4-methoxyphenyl)-substituted proline and pyrrolidin-2-ylacetic acid derivatives as GABA uptake inhibitors.

A series of enantiomerically pure 4-hydroxy-4-(4-methoxyphenyl)-substituted proline and pyrrolidin-2-ylacetic acid derivatives have been synthesized starting from the respective N-protected 4-hydroxy derivatives via oxidation to the corresponding 4-oxo compounds, subsequent addition of organometallic reagents, final hydrolysis and deprotection. The major diastereoisomers obtained by the addition of the Grignard reagents were found to have opposite stereoconfigurations depending on whether cerium trichloride was present or absent as an additive. The final compounds were evaluated for their capability to inhibit the GABA transport proteins GAT1 and GAT3. 4-Hydroxyproline derivatives substituted with a tris(4-methoxyphenyl)methyloxyethyl residue at the nitrogen and a 4-methoxyphenyl group in 4-position showed, with the exception of the (2R,4R)-diastereomer, an improved inhibition at GAT3 compared to the derivatives missing the 4-methoxyphenyl group in 4-position. This may imply that an appropriate lipophilic group at the C-4 position of the proline moiety is beneficial for potent inhibition at GAT3.

Synthesis and biological evaluation of new derivatives of 2-substituted 4-hydroxybutanamides as GABA uptake inhibitors.

This study presents the synthesis of novel substituted 4-hydroxybutanamides and their influence on the activity of murine GABA transport proteins GAT1-GAT4. The active compounds, derivatives of N-arylalkyl-2-(4-diphenylmethylpiperazin-1-yl)-4-hydroxybutyramide, are characterized by pIC(50) values in range of 3.92-5.06 and by slight subtype-selectivity. Among them N-4-chlorobenzylamide was the most potent GAT inhibitor (mGAT3), while N-benzylamide was the most active in GAT1-binding assay (pK(i) = 4.96). The results pointed out that benzhydryl and benzylamide moieties are crucial for the activity of this class of compounds as murine GAT inhibitors.