Discovery and Preclinical Evaluation of BMS-955829, a Potent Positive Allosteric Modulator of mGluR5

Pharmacological characterization of SAGE-718, a novel positive allosteric modulator of N-methyl-d-aspartate receptors

BACKGROUND AND PURPOSE

Select neuroactive steroids tune neural activity by modulating excitatory and inhibitory neurotransmission, including the endogenous cholesterol metabolite 24(S)-hydroxycholesterol (24(S)-HC), which is an N-methyl-d-aspartate (NMDA) receptor positive allosteric modulator (PAM). NMDA receptor PAMs are potentially an effective pharmacotherapeutic strategy to treat conditions associated with NMDA receptor hypofunction.

EXPERIMENTAL APPROACH

Using in vitro and in vivo electrophysiological recording experiments and behavioural approaches, we evaluated the effect of SAGE-718, a novel neuroactive steroid NMDA receptor PAM currently in clinical development for the treatment of cognitive impairment, on NMDA receptor function and endpoints that are altered by NMDA receptor hypoactivity and assessed its safety profile.

KEY RESULTS

SAGE-718 potentiated GluN1/GluN2A-D NMDA receptors with equipotency and increased NMDA receptor excitatory postsynaptic potential (EPSP) amplitude without affecting decay kinetics in striatal medium spiny neurons. SAGE-718 increased the rate of unblock of the NMDA receptor open channel blocker ketamine on GluN1/GluN2A in vitro and accelerated the rate of return on the ketamine-evoked increase in gamma frequency band power, as measured with electroencephalogram (EEG), suggesting that PAM activity is driven by increased channel open probability. SAGE-718 ameliorated deficits due to NMDA receptor hypofunction, including social deficits induced by subchronic administration of phencyclidine, and behavioural and electrophysiological deficits from cholesterol and 24(S)-HC depletion caused by 7-dehydrocholesterol reductase inhibition. Finally, SAGE-718 did not produce epileptiform activity in a seizure model or neurodegeneration following chronic dosing.

CONCLUSIONS AND IMPLICATIONS

These findings provide strong evidence that SAGE-718 is a neuroactive steroid NMDA receptor PAM with a mechanism that is well suited as a treatment for conditions associated with NMDA receptor hypofunction.

Oxazolidinones as versatile scaffolds in medicinal chemistry

Oxazolidinone is a five-member heterocyclic ring with several biological applications in medicinal chemistry. Among the three possible isomers, 2-oxazolidinone is the most investigated in drug discovery. Linezolid was pioneered as the first approved drug containing an oxazolidinone ring as the pharmacophore group. Numerous analogues have been developed since its arrival on the market in 2000. Some have succeeded in reaching the advanced stages of clinical studies. However, most oxazolidinone derivatives reported in recent decades have not reached the initial stages of drug development, despite their promising pharmacological applications in a variety of therapeutic areas, including antibacterial, antituberculosis, anticancer, anti-inflammatory, neurologic, and metabolic diseases, among other areas. Therefore, this review article aims to compile the efforts of medicinal chemists who have explored this scaffold over the past decades and highlight the potential of the class for medicinal chemistry.

Oscillatory Deficits in the Sub-Chronic PCP Rat Model for Schizophrenia Are Reversed by mGlu5 Receptor-Positive Allosteric Modulators VU0409551 and VU0360172

The cognitive deficits of schizophrenia are linked to imbalanced excitatory and inhibitory signalling in the prefrontal cortex (PFC), disrupting gamma oscillations. We previously demonstrated that two mGlu5 receptor-positive allosteric modulators (PAMs), VU0409551 and VU0360172, restore cognitive deficits in the sub-chronic PCP (scPCP) rodent model for schizophrenia via distinct changes in PFC intracellular signalling molecules. Here, we have assessed ex vivo gamma oscillatory activity in PFC slices from scPCP rats and investigated the effects of VU0409551 and VU0360172 upon oscillatory power. mGlu5 receptor, protein kinase C (PKC), and phospholipase C (PLC) inhibition were also used to examine ‘modulation bias’ in PAM activity. The amplitude and area power of gamma oscillations were significantly diminished in the scPCP model. Slice incubation with either VU0409551 or VU0360172 rescued scPCP-induced oscillatory deficits in a concentration-dependent manner. MTEP blocked the PAM-induced restoration of oscillatory power, confirming the requirement of mGlu5 receptor modulation. Whilst PLC inhibition prevented the power increase mediated by both PAMs, PKC inhibition diminished the effects of VU0360172 but not VU0409551. This aligns with previous reports that VU0409551 exhibits preferential activation of the phosphatidylinositol-3-kinase (PI3K) signalling pathway over the PKC cascade. Restoration of the excitatory/inhibitory signalling balance and gamma oscillations may therefore underlie the mGluR5 PAM-mediated correction of scPCP-induced cognitive deficits.

Applications of Isosteres of Piperazine in the Design of Biologically Active Compounds: Part 1

Piperazine and homopiperazine are well-studied heterocycles in drug design that have found gainful application as scaffolds and terminal elements and for enhancing the aqueous solubility of a molecule. The optimization of drug candidates that incorporate these heterocycles in an effort to refine potency, selectivity, and developability properties has stimulated the design and evaluation of a wide range of bioisosteres that can offer advantage. In this review, we summarize the design and application of bioisosteres of piperazine and homopiperazine that have almost exclusively been in the drug design arena. While there are ∼100 approved drugs that incorporate a piperazine ring, only a single marketed agricultural product is built on this heterocycle. As part of the review, we discuss some of the potential reasons underlying the relatively low level of importance of this heterocycle to the design of agrochemicals and highlight the potential opportunities for their use in contemporary research programs.

Enzyme- and Chemo-enzyme-Catalyzed Stereodivergent Synthesis

Multiple stereoisomers can be found when a substance contains chiral carbons in its chemical structure. To obtain the desired stereoisomers, asymmetric synthesis was proposed in the 1970s and developed rapidly at the beginning of this century. Stereodivergent synthesis, an extension of asymmetric synthesis in organic synthesis with the hope to produce all stereoisomers of chiral substances in high conversion and selectivity, enriches the variety of available products and serves as a reference suggestion for the synthesis of their derivatives and other compounds. Since biocatalysis has outstanding advantages of economy, environmental friendliness, high efficiency, and reaction at mild conditions, the biocatalytic reaction is regarded as an efficient strategy to perform stereodivergent synthesis. Thus, in this review, we summarize the stereodivergent synthesis catalyzed by enzymes or chemo-enzymes in cases where a compound contains two or three chiral carbons, i.e., at most four or eight stereoisomers are present. The types of reactions, including reduction of substituent ketones, cyclization reactions, olefin addition, and nonredox transesterification reactions, are also discussed for the understanding of the progress and application of biocatalysis in stereodivergent synthesis.

Reversal of synapse loss in Alzheimer mouse models by targeting mGluR5 to prevent synaptic tagging by C1Q

Microglia-mediated synaptic loss contributes to the development of cognitive impairments in Alzheimer's disease (AD). However, the basis for this immune-mediated attack on synapses remains to be elucidated. Treatment with the metabotropic glutamate receptor 5 (mGluR5) silent allosteric modulator (SAM), BMS-984923, prevents β-amyloid oligomer-induced aberrant synaptic signaling while preserving physiological glutamate response. Here, we show that oral BMS-984923 effectively occupies brain mGluR5 sites visualized by [¹⁸F]FPEB positron emission tomography (PET) at doses shown to be safe in rodents and nonhuman primates. In aged mouse models of AD (APPswe/PS1ΔE9 overexpressing transgenic and App^NL-G-F/hMapt double knock-in), SAM treatment fully restored synaptic density as measured by [¹⁸F]SynVesT-1 PET for SV2A and by histology, and the therapeutic benefit persisted after drug washout. Phospho-TAU accumulation in double knock-in mice was also reduced by SAM treatment. Single-nuclei transcriptomics demonstrated that SAM treatment in both models normalized expression patterns to a far greater extent in neurons than glia. Last, treatment prevented synaptic localization of the complement component C1Q and synaptic engulfment in AD mice. Thus, selective modulation of mGluR5 reversed neuronal gene expression changes to protect synapses from damage by microglial mediators in rodents.

GluN2A NMDA Receptor Enhancement Improves Brain Oscillations, Synchrony, and Cognitive Functions in Dravet Syndrome and Alzheimer's Disease Models

NMDA receptors (NMDARs) play subunit-specific roles in synaptic function and are implicated in neuropsychiatric and neurodegenerative disorders. However, the in vivo consequences and therapeutic potential of pharmacologically enhancing NMDAR function via allosteric modulation are largely un-known. We examine the in vivo effects of GNE-0723, a positive allosteric modulator of GluN2A-subunit-containing NMDARs, on brain network and cognitive functions in mouse models of Dravet syndrome (DS) and Alzheimer’s disease (AD). GNE-0723 use dependently potentiates synaptic NMDA receptor currents and reduces brain oscillation power with a predominant effect on low-frequency (12–20 Hz) oscillations. Interestingly, DS and AD mouse models display aberrant low-frequency oscillatory power that is tightly correlated with network hypersynchrony. GNE-0723 treatment reduces aberrant low-frequency oscillations and epileptiform discharges and improves cognitive functions in DS and AD mouse models. GluN2A-subunit-containing NMDAR enhancers may have therapeutic benefits in brain disorders with network hypersynchrony and cognitive impairments.

Hanson et al. examine the therapeutic effects of enhancing GluN2A-subunit-containing NMDAR function in Dravet syndrome and Alzheimer’s disease mice. GNE-0723 treatment reduces aberrant low-frequency oscillations and epileptiform discharges and improves cognitive functions in both disease models. GluN2A NMDAR enhancers may benefit brain disorders with network hypersynchrony and cognitive impairments.

Acetylene Group, Friend or Foe in Medicinal Chemistry

The use of an acetylene (ethynyl) group in medicinal chemistry coincides with the launch of the Journal of Medicinal Chemistry in 1959. Since then, the acetylene group has been broadly exploited in drug discovery and development. As a result, it has become recognized as a privileged structural feature for targeting a wide range of therapeutic target proteins, including MAO, tyrosine kinases, BACE1, steroid receptors, mGlu5 receptors, FFA1/GPR40, and HIV-1 RT. Furthermore, a terminal alkyne functionality is frequently introduced in chemical biology probes as a click handle to identify molecular targets and to assess target engagement. This Perspective is divided into three parts encompassing: (1) the physicochemical properties of the ethynyl group, (2) the advantages and disadvantages of the ethynyl group in medicinal chemistry, and (3) the impact of the ethynyl group on chemical biology approaches.

Atom-based 3D-QSAR, induced fit docking, and molecular dynamics simulations study of thieno[2,3-b]pyridines negative allosteric modulators of mGluR5

Atom-based three dimensional-quantitative structure-activity relationship (3D-QSAR) model was developed on the basis of 5-point pharmacophore hypothesis (AARRR) with two hydrogen bond acceptors (A) and three aromatic rings for the derivatives of thieno[2,3-b]pyridine, which modulates the activity to inhibit the mGluR5 receptor. Generation of a highly predictive 3D-QSAR model was performed using the alignment of predicted pharmacophore hypothesis for the training set (R² = 0.84, SD = 0.26, F = 45.8, N = 29) and test set (Q² = 0.74, RMSE = 0.235, Pearson-R = 0.94, N = 9). The best pharmacophore hypothesis AARRR was selected, and developed three dimensional-quantitative structure activity relationship (3D-QSAR) model also supported the outcome of this study by means of favorable and unfavorable electron withdrawing group and hydrophobic regions of most active compound 42d and least active compound 18b. Following, induced fit docking and binding free energy calculations reveals the reliable binding orientation of the compounds. Finally, molecular dynamics simulations for 100 ns were performed to depict the protein-ligand stability. We anticipate that the resulted outcome could be supportive to discover potent negative allosteric modulators for metabotropic glutamate receptor 5 (mGluR5).

Schizophrenia: synthetic strategies and recent advances in drug design

Schizophrenia is a complex and unpredictable mental disorder which affects several domains of cognition and behaviour. It is a heterogeneous illness characterised by positive, negative, and cognitive symptoms, often accompanied by signs of depression. In this tutorial review, we discuss recent progress in understanding the target sites and mechanisms of action of second-generation antipsychotic drugs. Progress in identifying and defining target sites has been accelerated recently by advances in neuroscience, and newly developed agents that regulate signalling by the main excitatory neurotransmitters in the brain are surveyed. Examples of novel molecules for the treatment of schizophrenia in preclinical and clinical development and their industrial sponsors are highlighted.

In Vitro Functional Characterization of GET73 as Possible Negative Allosteric Modulator of Metabotropic Glutamate Receptor 5

The present study was aimed to further characterize the pharmacological profile of N-[4-(trifluoromethyl) benzyl]-4-methoxybutyramide (GET73), a putative negative allosteric modulator (NAM) of metabotropic glutamate subtype 5 receptor (mGluR5) under development as a novel medication for the treatment of alcohol dependence. This aim has been accomplished by means of a series of in vitro functional assays. These assays include the measure of several down-stream signaling [intracellular Ca⁺⁺ levels, inositol phosphate (IP) formation and CREB phosphorylation (pCREB)] which are generally affected by mGluR5 ligands. In particular, GET73 (0.1 nM-10 μM) was explored for its ability to displace the concentration-response curve of some mGluR5 agonists/probes (glutamate, L-quisqualate, CHPG) in different native preparations. GET73 produced a rightward shift of concentration-response curves of glutamate- and CHPG-induced intracellular Ca⁺⁺ levels in primary cultures of rat cortical astrocytes. The compound also induced a rightward shift of concentration response curve of glutamate- and L-quisqualate-induced increase in IP turnover in rat hippocampus slices, along with a reduction of CHPG (10 mM)-induced increase in IP formation. Moreover, GET73 produced a rightward shift of concentration-response curve of glutamate-, CHPG- and L-quisqualate-induced pCREB levels in rat cerebral cortex neurons. Although the engagement of other targets cannot be definitively ruled out, these data support the view that GET73 acts as an mGluR5 NAM and support the significance of further investigating the possible mechanism of action of the compound.

Silent Allosteric Modulation of mGluR5 Maintains Glutamate Signaling while Rescuing Alzheimer's Mouse Phenotypes

Metabotropic glutamate receptor 5 (mGluR5) has been implicated in Alzheimer's disease (AD) pathology. We sought to understand whether mGluR5's role in AD requires glutamate signaling. We used a potent mGluR5 silent allosteric modulator (SAM, BMS-984923) to separate its well-known physiological role in glutamate signaling from a pathological role in mediating amyloid-β oligomer (Aβo) action. Binding of the SAM to mGluR5 does not change glutamate signaling but strongly reduces mGluR5 interaction with cellular prion protein (PrP^C) bound to Aβo. The SAM compound prevents Aβo-induced signal transduction in brain slices and in an AD transgenic mouse model, the APPswe/PS1ΔE9 strain. Critically, 4 weeks of SAM treatment rescues memory deficits and synaptic depletion in the APPswe/PS1ΔE9 transgenic mouse brain. Our data show that mGluR5's role in Aβo-dependent AD phenotypes is separate from its role in glutamate signaling and silent allosteric modulation of mGluR5 has promise as a disease-modifying AD intervention with a broad therapeutic window.

Pharmacological property optimization for allosteric ligands: A medicinal chemistry perspective

New strategies to potentially improve drug safety and efficacy emerge with allosteric programs. Biased allosteric modulators can be designed with high subtype selectivity and defined receptor signaling endpoints, however, selecting the most meaningful parameters for optimization can be perplexing. Historically, "potency hunting" at the expense of physicochemical and pharmacokinetic optimization has led to numerous tool compounds with excellent pharmacological properties but no path to drug development. Conversely, extensive physicochemical and pharmacokinetic screening with only post hoc bias and allosteric characterization has led to inefficacious compounds or compounds with on-target toxicities. This field is rapidly evolving with new mechanistic understanding, changes in terminology, and novel opportunities. The intent of this digest is to summarize current understanding and debates within the field. We aim to discuss, from a medicinal chemistry perspective, the parameter choices available to drive SAR.

Difluorocyclobutylacetylenes as positive allosteric modulators of mGluR5 with reduced bioactivation potential

Schizophrenia is a serious illness that affects millions of patients and has been associated with N-methyl-d-aspartate receptor (NMDAR) hypofunction. It has been demonstrated that activation of metabotropic glutamate receptor 5 (mGluR5) enhances NMDA receptor function, suggesting the potential utility of mGluR5 positive allosteric modulators (PAMs) in the treatment of schizophrenia. Herein we describe the optimization of an mGluR5 PAM by replacement of a phenyl with aliphatic heterocycles and carbocycles as a strategy to reduce bioactivation in a biaryl acetylene chemotype. Replacement with a difluorocyclobutane followed by further optimization culminated in the identification of compound 32, a low fold shift PAM with reduced bioactivation potential. Compound 32 demonstrated favorable brain uptake and robust efficacy in mouse novel object recognition (NOR) at low doses.

Development of 1H-Pyrazolo[3,4-b]pyridines as Metabotropic Glutamate Receptor 5 Positive Allosteric Modulators

The metabotropic glutamate receptor 5 (mGluR5) is an attractive target for the treatment of schizophrenia due to its role in regulating glutamatergic signaling in association with the N-methyl-d-aspartate receptor (NMDAR). We describe the synthesis of 1H-pyrazolo[3,4-b]pyridines and their utility as mGluR5 positive allosteric modulators (PAMs) without inherent agonist activity. A facile and convergent synthetic route provided access to a structurally diverse set of analogues that contain neither the aryl-acetylene-aryl nor aryl-methyleneoxy-aryl elements, the predominant structural motifs described in the literature. Binding studies suggest that members of our new chemotype do not engage the receptor at the MPEP and CPPHA mGluR5 allosteric sites. SAR studies culminated in the first non-MPEP site PAM, 1H-pyrazolo[3,4-b]pyridine 31 (BMT-145027), to improve cognition in a preclinical rodent model of learning and memory.

Overview of Critical Parameters for the Design and Execution of a High-Throughput Screen for Allosteric Ligands

Allosteric ligands modulate the activity of receptor targets by binding to sites that are distinct from the orthosteric (native agonist) binding site. Allosteric modulators have potential therapeutic advantages over orthosteric agonists and antagonists, including improved selectivity, and maintenance of the spatial and temporal fidelity of native signaling patterns. The identification of allosteric ligands presents unique challenges because of the requirement for screening in the presence of an orthosteric agonist, the small signal window that is produced by many allosteric modulators, the proclivity of allosteric modulators to exhibit activity switching within a chemotype (e.g., one compound may be a positive allosteric modulator while a close analog is a negative allosteric modulator), and probe dependence (differential interactions with different orthosteric agonists). Described in this unit are emerging strategies for the identification of allosteric ligands by high-throughput screening (HTS), including the use of multiple-add/multiple-read HTS assays and tool molecule-based screening formats. © 2016 by John Wiley & Sons, Inc.

Diverse modes of NMDA receptor positive allosteric modulation: Mechanisms and consequences

NMDA Receptors (NMDARs) play key roles in synaptic physiology and NMDAR hypofunction has been implicated in various neurological conditions. In recent years an increasing number of positive allosteric modulators (PAMs) of NMDARs have been discovered and characterized. These diverse PAM classes vary not only in their binding sites and GluN2 subunit selectivity profiles, but also in the nature of their impacts on channel function. Major differences exist in the degree of slowing of channel deactivation and shifting of apparent agonist affinity between different classes of PAMs. Here we review the diverse modes of potentiation by the currently known classes of NMDAR PAMs and discuss the potential consequences of different types of potentiation in terms of desirable and undesirable effects on brain function. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Oxazolidinone-based allosteric modulators of mGluR5: Defining molecular switches to create a pharmacological tool box

Herein we describe the structure activity relationships uncovered in the pursuit of an mGluR5 positive allosteric modulator (PAM) for the treatment of schizophrenia. It was discovered that certain modifications of an oxazolidinone-based chemotype afforded predictable changes in the pharmacological profile to give analogs with a wide range of functional activities. The discovery of potent silent allosteric modulators (SAMs) allowed interrogation of the mechanism-based liabilities associated with mGluR5 activation and drove our medicinal chemistry effort toward the discovery of low efficacy (fold shift) PAMs devoid of agonist activity. This work resulted in the identification of dipyridyl 22 (BMS-952048), a compound with a favorable free fraction, efficacy in a rodent-based cognition model, and low potential for convulsions in mouse.