GABAA receptor-mediated seizure liabilities: a mixed-methods screening approach

GABAA receptors, members of the pentameric ligand-gated ion channel superfamily, are widely expressed in the central nervous system and mediate a broad range of pharmaco-toxicological effects including bidirectional changes to seizure threshold. Thus, detection of GABAA receptor-mediated seizure liabilities is a big, partly unmet need in early preclinical drug development. This is in part due to the plethora of allosteric binding sites that are present on different subtypes of GABAA receptors and the critical lack of screening methods that detect interactions with any of these sites. To improve in silico screening methods, we assembled an inventory of allosteric binding sites based on structural data. Pharmacophore models representing several of the binding sites were constructed. These models from the NeuroDeRisk IL Profiler were used for in silico screening of a compiled collection of drugs with known GABAA receptor interactions to generate testable hypotheses. Amoxapine was one of the hits identified and subjected to an array of in vitro assays to examine molecular and cellular effects on neuronal excitability and in vivo locomotor pattern changes in zebrafish larvae. An additional level of analysis for our compound collection is provided by pharmacovigilance alerts using FAERS data. Inspired by the Adverse Outcome Pathway framework, we postulate several candidate pathways leading from specific binding sites to acute seizure induction. The whole workflow can be utilized for any compound collection and should inform about GABAA receptor-mediated seizure risks more comprehensively compared to standard displacement screens, as it rests chiefly on functional data.

Molecular Mingling: Multimodal Predictions of Ligand Promiscuity in Pentameric Ligand-Gated Ion Channels

Background: Human pentameric ligand-gated ion channels (pLGICs) comprise nicotinic acetylcholine receptors (nAChRs), 5-hydroxytryptamine type 3 receptors (5-HT3Rs), zinc-activated channels (ZAC), γ-aminobutyric acid type A receptors (GABAARs) and glycine receptors (GlyRs). They are recognized therapeutic targets of some of the most prescribed drugs like general anesthetics, anxiolytics, smoking cessation aids, antiemetics and many more. Currently, approximately 100 experimental structures of pLGICs with ligands bound exist in the protein data bank (PDB). These atomic-level 3D structures enable the generation of a comprehensive binding site inventory for the superfamily and the in silico prediction of binding site properties. Methods: A panel of high throughput in silico methods including pharmacophore screening, conformation analysis and descriptor calculation was applied to a selection of allosteric binding sites for which in vitro screens are lacking. Variant abundance near binding site forming regions and computational docking complement the approach. Results: The structural data reflects known and novel binding sites, some of which may be unique to individual receptors, while others are broadly conserved. The membrane spanning domain, comprising four highly conserved segments, contains ligand interaction sites for which in vitro assays suitable for high throughput screenings are critically lacking. This is also the case for structurally more variable novel sites in the extracellular domain. Our computational results suggest that the phytocannabinoid Δ9-tetrahydrocannabinol (Δ9-THC) can utilize multiple pockets which are likely to exist on most superfamily members. Conclusion: With this study, we explore the potential for polypharmacology among pLGICs. Our data suggest that ligands can display two forms of promiscuity to an extent greater than what has been realized: 1) Ligands can interact with homologous sites in many members of the superfamily, which bears toxicological relevance. 2) Multiple pockets in distinct localizations of individual receptor subtypes share common ligands, which counteracts efforts to develop selective agents. Moreover, conformational states need to be considered for in silico drug screening, as certain binding sites display considerable flexibility. In total, this work contributes to a better understanding of polypharmacology across pLGICs and provides a basis for improved structure guided in silico drug development and drug derisking.

Tricyclic antipsychotics and antidepressants can inhibit α5-containing GABAA receptors by two distinct mechanisms

Background and Purpose

Many psychotherapeutic drugs, including clozapine, display polypharmacology and act on GABA_A receptors. Patients with schizophrenia show alterations in function, structure and molecular composition of the hippocampus, and a recent study demonstrated aberrant levels of hippocampal α5 subunit‐containing GABA_A receptors. The purpose of this study is to investigate the effects of tricyclic compounds on α5 subunit‐containing receptor subtypes.

Experimental Approach

Functional studies of effects by seven antipsychotic and antidepressant medications were performed in several GABA_A receptor subtypes by two‐electrode voltage‐clamp electrophysiology using Xenopus laevis oocytes. Computational structural analysis was employed to design mutated constructs of the α5 subunit, probing a novel binding site. Radioligand displacement data complemented the functional and mutational findings.

Key Results

The antipsychotic drugs clozapine and chlorpromazine exerted functional inhibition on multiple GABA_A receptor subtypes, including those containing α5‐subunits. Based on a chlorpromazine binding site observed in a GABA‐gated bacterial homologue, we identified a novel site in α5 GABA_A receptor subunits and demonstrate differential usage of this and the orthosteric sites by these ligands.

Conclusion and Implications

Despite high molecular and functional similarities among the tested ligands, they reduce GABA currents by differential usage of allosteric and orthosteric sites. The chlorpromazine site we describe here is a new potential target for optimizing antipsychotic medications with beneficial polypharmacology. Further studies in defined subtypes are needed to substantiate mechanistic links between the therapeutic effects of clozapine and its action on certain GABA_A receptor subtypes.

Molecular basis of mood and cognitive adverse events elucidated via a combination of pharmacovigilance data mining and functional enrichment analysis

Drug-induced Mood- and Cognition-related adverse events (MCAEs) are often only detected during the clinical trial phases of drug development, or even after marketing, thus posing a major safety concern and a challenge for both pharmaceutical companies and clinicians. To fill some gaps in the understanding and elucidate potential biological mechanisms of action frequently associated with MCAEs, we present a unique workflow linking observational population data with the available knowledge at molecular, cellular, and psychopharmacology levels. It is based on statistical analysis of pharmacovigilance reports and subsequent signaling pathway analyses, followed by evidence-based expert manual curation of the outcomes. Our analysis: (a) ranked pharmaceuticals with high occurrence of such adverse events (AEs), based on disproportionality analysis of the FDA Adverse Event Reporting System (FAERS) database, and (b) identified 120 associated genes and common pathway nodes possibly underlying MCAEs. Nearly two-thirds of the identified genes were related to immune modulation, which supports the critical involvement of immune cells and their responses in the regulation of the central nervous system function. This finding also means that pharmaceuticals with a negligible central nervous system exposure may induce MCAEs through dysregulation of the peripheral immune system. Knowledge gained through this workflow unravels putative hallmark biological targets and mediators of drug-induced mood and cognitive disorders that need to be further assessed and validated in experimental models. Thereafter, they can be used to substantially improve in silico/in vitro/in vivo tools for predicting these adversities at a preclinical stage.

Allosteric GABAA Receptor Modulators-A Review on the Most Recent Heterocyclic Chemotypes and Their Synthetic Accessibility

GABAA receptor modulators are structurally almost as diverse as their target protein. A plethora of heterocyclic scaffolds has been described as modulating this extremely important receptor family. Some made it into clinical trials and, even on the market, some were dismissed. This review focuses on the synthetic accessibility and potential for library synthesis of GABAA receptor modulators containing at least one heterocyclic scaffold, which were disclosed within the last 10 years.

Variations on a scaffold - Novel GABAA receptor modulators

Allosteric ligands of GABA_A receptors exist in many different chemotypes owing to their great usefulness as therapeutics, with benzodiazepines being among the best known examples. Many allosteric binding sites have been described, among them a site at the extracellular interface between the alpha principal face and the beta complementary face (α+/β-). Pyrazoloquinolinones have been shown to bind at α+/β-binding sites of GABA_A receptors, exerting chiefly positive allosteric modulation at this location. In order to further explore molecular determinants of this type of allosteric modulation, we synthesized a library of ligands based on the PQ pharmacophore employing a ring-chain bioisosteric approach. In this study we analyzed the structure-activity-relationship (SAR) of these novel ligands based on an azo-biaryl structural motif in α1β3 GABA_A receptors, indicating interesting novel properties of the compound class.

Engineered Flumazenil Recognition Site Provides Mechanistic Insight Governing Benzodiazepine Modulation in GABAA Receptors

The anxiolytic, anticonvulsant, muscle-relaxant, and sedative-hypnotic effects of benzodiazepine site ligands are mainly elicited by allosteric modulation of GABA_A receptors via their extracellular αx+/γ2- ( x = 1, 2, 3, 5) interfaces. In addition, a low affinity binding site at the homologous α+/β- interfaces was reported for some benzodiazepine site ligands. Classical benzodiazepines and pyrazoloquinolinones have been used as molecular probes to develop structure-activity relationship models for benzodiazepine site activity. Considering all possible α+/β- and α+/γ- interfaces, such ligands potentially interact with as many as 36 interfaces, giving rise to undesired side effects. Understanding the binding modes at their binding sites will enable rational strategies to design ligands with desired selectivity profiles. Here, we compared benzodiazepine site ligand interactions in the high affinity α1+/γ2- site with the homologous α1+/β3- site using a successive mutational approach. We incorporated key amino acids known to contribute to high affinity benzodiazepine binding of the γ2- subunit into the β3- subunit, resulting in a quadruple mutant β3(4mut) with high affinity flumazenil (Ro 15-1788) binding properties. Intriguingly, some benzodiazepine site ligands displayed positive allosteric modulation in the tested recombinant α1β3(4mut) constructs while diazepam remained inactive. Consequently, we performed in silico molecular docking in the wildtype receptor and the quadruple mutant. The results led to the conclusion that different benzodiazepine site ligands seem to use distinct binding modes, rather than a common binding mode. These findings provide structural hypotheses for the future optimization of both benzodiazepine site ligands, and ligands that interact with the homologous α+/β- sites.

Expression of miR-208b and miR-499 in Greek Patients with Acute Myocardial Infarction

BACKGROUND/AIM

Certain microRNAs (miRs) present in human plasma are candidate biomarkers for cardiovascular diseases, including acute myocardial infarction (AMI). We examined the expression of two cardiac-specific miRs (miR-208b and miR-499) in a Greek pathological population.

MATERIALS AND METHODS

Plasma samples from AMI patients and healthy subjects (controls) were analyzed using TaqMan® MicroRNA assays.

RESULTS

The concentration of both miRs was significantly elevated in AMI patients compared to healthy controls. Moreover, receiver-operating characteristic (ROC) curve analysis showed that miR-208b and miR-499 displayed similar properties with the established AMI biomarker cardiac troponin T (cTnT).

CONCLUSION

We showed, for the first time, that these miRs could be used as AMI biomarkers in our population as well. Our data are in agreement with those of studies based on different population groups and further strengthen the observation that plasma levels of circulating miR-208b and miR-499 could serve as potential AMI biomarkers.

Towards functional selectivity for α6β3γ2 GABAA receptors: a series of novel pyrazoloquinolinones

BACKGROUND AND PURPOSE

The GABAA receptors are ligand-gated ion channels, which play an important role in neurotransmission. Their variety of binding sites serves as an appealing target for many clinically relevant drugs. Here, we explored the functional selectivity of modulatory effects at specific extracellular α+/β- interfaces, using a systematically varied series of pyrazoloquinolinones.

EXPERIMENTAL APPROACH

Recombinant GABAA receptors were expressed in Xenopus laevis oocytes and modulatory effects on GABA-elicited currents by the newly synthesized and reference compounds were investigated by the two-electrode voltage clamp method.

KEY RESULTS

We identified a new compound which, to the best of our knowledge, shows the highest functional selectivity for positive modulation at α6β3γ2 GABAA receptors with nearly no residual activity at the other αxβ3γ2 (x = 1-5) subtypes. This modulation was independent of affinity for α+/γ- interfaces. Furthermore, we demonstrated for the first time a compound that elicits a negative modulation at specific extracellular α+/β- interfaces.

CONCLUSION AND IMPLICATIONS

These results constitute a major step towards a potential selective positive modulation of certain α6-containing GABAA receptors, which might be useful to elicit their physiological role. Furthermore, these studies pave the way towards insights into molecular principles that drive positive versus negative allosteric modulation of specific GABAA receptor isoforms.

Expeditious synthesis of polyacetylenic water hemlock toxins and their effects on the major GABAA receptor isoform

Classical synthetic approaches to highly unsaturated polyene/yne natural products rely on iterative cross-coupling of linear fragments.

Carvedilol inhibits cADPR- and IP3-induced Ca2+ release

Spontaneous Ca²⁺ waves, also termed store-overload-induced Ca²⁺ release (SOICR), in cardiac cells can trigger ventricular arrhythmias especially in failing hearts. SOICR occurs when RyRs are activated by an increase in sarcoplasmic reticulum (SR) luminal Ca²⁺. Carvedilol is one of the most effective drugs for preventing arrhythmias in patients with heart failure. Furthermore, carvedilol analogues with minimal β-blocking activity also block SOICR showing that SOICR-inhibiting activity is distinct from that for β-block. We show here that carvedilol is a potent inhibitor of cADPR-induced Ca²⁺ release in sea urchin egg homogenate. In addition, the carvedilol analog VK-II-86 with minimal β-blocking activity also suppresses cADPR-induced Ca²⁺ release. Carvedilol appeared to be a non-competitive antagonist of cADPR and could also suppress Ca²⁺ release by caffeine. These results are consistent with cADPR releasing Ca²⁺ in sea urchin eggs by sensitizing RyRs to Ca²⁺ involving a luminal Ca²⁺ activation mechanism. In addition to action on the RyR, we also observed inhibition of inositol 1,4,5-trisphosphate (IP₃)-induced Ca²⁺ release by carvedilol suggesting a common mechanism between these evolutionarily related and conserved Ca²⁺ release channels.

Effects of ranolazine on left ventricular diastolic and systolic function in patients with chronic coronary disease and stable angina

INTRODUCTION

The present study examined the effect of ranolazine, which acts via the mechanism of selective inhibition of late INa+, on parameters of left ventricular systolic and diastolic function in patients suffering from angiographically confirmed chronic coronary artery disease, presenting with chronic stable angina.

METHODS

We studied 40 patients (age 67 ± 9 years; 30 men, 10 women) with chronic coronary artery disease who reported angina symptoms on optimal medication and who were not suitable for invasive treatment. Patients were randomized to the ranolazine group (group A, 20 patients taking oral ranolazine 500 mg bid for 3 months) and the control group (group B, 20 patients who did not receive the drug). Left ventricular systolic and diastolic function was assessed echocardiographically at baseline and after the end of the three-month treatment period. Left ventricular ejection fraction by the modified Simpson's method, E and A left ventricular filling velocities, E/A ratio, deceleration time (DT) of E, isovolumic relaxation time (IVRT), E and A waves, and the E/E ratio were measured using 2-dimensional echocardiography, Doppler and tissue Doppler imaging (TDI).

RESULTS

Group A patients demonstrated a clear improvement of their initial angina symptoms. There were no adverse effects from ranolazine requiring withdrawal from the study. There was no statistically significant change in left ventricular systolic function in either group. A statistically significant change was seen in indexes of diastolic function measured using both conventional Doppler and TDI in Group A patients compared with Group B patients after three months' ranolazine treatment period. The changes in left ventricular diastolic function indexes in Group A patients were as follows: E 0.58 ± 0.11 vs. 0.76 ± 0.12 m/s, p<0.001; A 0.71 ± 0.22 vs. 0.83 ± 0.19 m/s, p<0.001; E/A 0.81 ± 0.14 vs. 0.97 ± 0.17, p<0.005; 5.4 ± 0.7 vs. 6.8 ± 0.9 cm/s, p<0.005; 7.2 ± 0.8 vs. 8.3 ± 1.1 cm/s, p<0.005; E/ 10.7 ± 1.1 vs. 11.1 ± 0.8, p=ns; DT 251 ± 14 vs. 226 ± 17 ms, p<0.004; IVRT 95 ± 11 vs. 74 ± 9 ms, p<0.001. Systolic function did not change: EF 46.3 ± 3.4 vs. 46.7 ± 2.7%, p: ns.

CONCLUSIONS

The use of ranolazine in patients suffering from chronic coronary artery disease has a favorable impact on diastolic function parameters. Accordingly, a clinical benefit could be observed due to an improvement in patients' symptoms.