Neuroactive Steroid N-Methyl-d-aspartate Receptor Positive Allosteric Modulators: Synthesis, SAR, and Pharmacological Activity

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.

Population pharmacokinetics, enzyme occupancy, and pharmacodynamic modeling of soticlestat in patients with developmental and epileptic encephalopathies

Soticlestat (TAK-935) is a first-in-class, selective inhibitor of cholesterol 24-hydroxylase (CH24H) under phase III development for the treatment of the developmental and epileptic encephalopathies (DEEs), Dravet syndrome (DS), and Lennox-Gastaut syndrome (LGS). A previous model characterized the pharmacokinetics (PKs), CH24H enzyme occupancy (EO), and pharmacodynamics (PDs) of soticlestat in healthy volunteers. The present study extended this original model for patients with DEEs and investigated sources of variability. Model-based simulations were carried out to optimize dosing strategies for use in clinical trials. Data from eight phase I and II trials of healthy volunteers or patients with DEEs receiving oral soticlestat 15-1350 mg were included, encompassing 218 individuals for population PK (PopPK) analyses and 306 individuals for PK/PD analyses. Dosing strategies were identified through model-based simulations. The final mixed-effect PopPK/EO/PD model consisted of a two-compartment PK model and an effect-site compartment in the PK/EO model; soticlestat concentrations at the effect site were linked to 24S-hydroxycholesterol plasma concentrations using a semimechanistic inhibitory indirect response model. Covariates were included to account for sources of variability. Pediatric dosing strategies were developed for four body weight bands (10 to <15, 15 to <30, 30 to <45, and 45-100 kg) to account for covariate effects by body weight. The final PopPK and PK/EO/PD models accurately described PK, EO, and PD profiles of soticlestat in healthy volunteers and patients with DEEs. Covariate analyses and model-based simulations facilitated optimization of phase III trial dosing strategies for patients with DS or LGS.

Therapeutic potential of N-methyl-D-aspartate receptor modulators in psychiatry

N-methyl-D-aspartate (NMDA) receptors mediate a slow component of excitatory synaptic transmission, are widely distributed throughout the central nervous system, and regulate synaptic plasticity. NMDA receptor modulators have long been considered as potential treatments for psychiatric disorders including depression and schizophrenia, neurodevelopmental disorders such as Rett Syndrome, and neurodegenerative conditions such as Alzheimer's disease. New interest in NMDA receptors as therapeutic targets has been spurred by the findings that certain inhibitors of NMDA receptors produce surprisingly rapid and robust antidepressant activity by a novel mechanism, the induction of changes in the brain that well outlast the presence of drug in the body. These findings are driving research into an entirely new paradigm for using NMDA receptor antagonists in a host of related conditions. At the same time positive allosteric modulators of NMDA receptors are being pursued for enhancing synaptic function in diseases that feature NMDA receptor hypofunction. While there is great promise, developing the therapeutic potential of NMDA receptor modulators must also navigate the potential significant risks posed by the use of such agents. We review here the emerging pharmacology of agents that target different NMDA receptor subtypes, offering new avenues for capturing the therapeutic potential of targeting this important receptor class.

Design and synthesis of novel GluN2A NMDAR positive allosteric modulators via scaffold hopping strategy as anti-stroke therapeutic agents

NMDA receptor subunits have differential roles in mediating excitotoxic neuronal death both in vitro and in vivo . Activation of NR2A-containing NMDA receptors promotes neuronal survival and exerts a neuroprotective action, whereas over activating GluN2B-containing receptor results in excitotoxicity, increasing neuronal apoptosis. Our previous study has identified Npam 43 as a NMDAR positive allosteric modulators. However, the cis-trans isomerization impedes the development of Npam 43 as potential neuroprotective agents. To discover more potent and selective GluN2A NMDAR positive allosteric modulators, 38 derivatives were synthesized and evaluated their neuroprotective effect on glutamate-exposed PC-12 cells. The allosteric activities of compounds were evaluated using calcium imaging approaches. Among them, compound 5c exhibit GluN1/2A selectivity over GluN1/2B and show neuroprotective activity in vitro and in vivo. This study reported a series of GluN1/2A positive allosteric modulators as neuroprotective agents, and provided a potential opportunity to discover new drugs for stroke treatment.

Novel neuroactive steroids as positive allosteric modulators of NMDA receptors: mechanism, site of action, and rescue pharmacology on GRIN variants associated with neurological conditions

N-methyl-D-aspartate receptors (NMDARs) play vital roles in normal brain functions (i.e., learning, memory, and neuronal development) and various neuropathological conditions, such as epilepsy, autism, Parkinson's disease, Alzheimer's disease, and traumatic brain injury. Endogenous neuroactive steroids such as 24(S)-hydroxycholesterol (24(S)-HC) have been shown to influence NMDAR activity, and positive allosteric modulators (PAMs) derived from 24(S)-hydroxycholesterol scaffold can also enhance NMDAR function. This study describes the structural determinants and mechanism of action for 24(S)-hydroxycholesterol and two novel synthetic analogs (SGE-550 and SGE-301) on NMDAR function. We also show that these agents can mitigate the altered function caused by a set of loss-of-function missense variants in NMDAR GluN subunit-encoding GRIN genes associated with neurological and neuropsychiatric disorders. We anticipate that the evaluation of novel neuroactive steroid NMDAR PAMs may catalyze the development of new treatment strategies for GRIN-related neuropsychiatric conditions.

X-ray structure analysis of the cholesterol 25- and 20-hydroperoxides, the elusive primary sidechain autoxidation products of cholesterol

The systematic X-ray structure analyses of the primary cholesterol sidechain autoxidation products cholesterol 25- and 20β(S)-hydroperoxide are presented and compared to cholesterol and 25-hydroxycholesterol. Intermolecular interactions in crystal structures of the molecules are revealed through Hirshfeld surface analysis and fingerprint plots. The magnitude of energy frameworks is presented by combining efficient calculations of intermolecular interaction energies with novel graphical representation.

SAGE-718: A First-in-Class N-Methyl-d-Aspartate Receptor Positive Allosteric Modulator for the Potential Treatment of Cognitive Impairment

N-Methyl-d-aspartate receptor (NMDAR) positive allosteric modulators (PAMs) have received increased interest as a powerful mechanism of action to provide relief as therapies for CNS disorders. Sage Therapeutics has previously published the discovery of endogenous neuroactive steroid 24(S)-hydroxycholesterol as an NMDAR PAM. In this article, we detail the discovery of development candidate SAGE-718 (5), a potent and high intrinsic activity NMDAR PAM with an optimized pharmacokinetic profile for oral dosing. Compound 5 has completed phase 1 single ascending dose and multiple ascending dose clinical trials and is currently undergoing phase 2 clinical trials for treatment of cognitive impairment in Huntington's disease.

Differential regulation of STP, LTP and LTD by structurally diverse NMDA receptor subunit-specific positive allosteric modulators

Different types of memory are thought to rely on different types of synaptic plasticity, many of which depend on the activation of the N-Methyl-D Aspartate (NMDA) subtype of glutamate receptors. Accordingly, there is considerable interest in the possibility of using positive allosteric modulators (PAMs) of NMDA receptors (NMDARs) as cognitive enhancers. Here we firstly review the evidence that NMDA receptor-dependent forms of synaptic plasticity: short-term potentiation (STP), long-term potentiation (LTP) and long-term depression (LTD) can be pharmacologically differentiated by using NMDAR ligands. These observations suggest that PAMs of NMDAR function, depending on their subtype selectivity, might differentially regulate STP, LTP and LTD. To test this hypothesis, we secondly performed experiments in rodent hippocampal slices with UBP714 (a GluN2A/2B preferring PAM), CIQ (a GluN2C/D selective PAM) and UBP709 (a pan-PAM that potentiates all GluN2 subunits). We report here, for the first time, that: (i) UBP714 potentiates sub-maximal LTP and reduces LTD; (ii) CIQ potentiates STP without affecting LTP; (iii) UBP709 enhances LTD and decreases LTP. We conclude that PAMs can differentially regulate distinct forms of NMDAR-dependent synaptic plasticity due to their subtype selectivity.

This article is part of the Neuropharmacology Special Issue on ‘Glutamate Receptors – NMDA receptors’.

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NMDAR-dependent STP, LTP and LTD can be dissociated pharmacologically

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GluN2A/2B PAM UBP714 potentiates LTP and reduces LTD

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GluN2C/D PAM CIQ potentiates STP without affecting LTP

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NMDAR pan-PAM UBP709 potentiates LTD and reduces LTP

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Positive allosteric modulation of NMDA receptors: mechanisms, physiological impact and therapeutic potential

NMDA receptors (NMDARs) are glutamate-gated ion channels that play key roles in synaptic transmission and plasticity. Both hyper- and hypo-activation of NMDARs are deleterious to neuronal function. In particular, NMDAR hypofunction is involved in a wide range of neurological and psychiatric conditions like schizophrenia, intellectual disability, age-dependent cognitive decline, or Alzheimer's disease. While early medicinal chemistry efforts were mostly focused on the development of NMDAR antagonists, the last 10 years have seen a boom in the development of NMDAR positive allosteric modulators (PAMs). Here we review the currently developed NMDAR PAMs, their pharmacological profiles and mechanisms of action, as well as their physiological effects in healthy animals and animal models of NMDAR hypofunction. In light of the complexity of physiological outcomes of NMDAR PAMs in vivo, we discuss the remaining challenges and questions that need to be addressed to better grasp and predict the therapeutic potential of NMDAR positive allosteric modulation.

Impact of Allosteric Modulation in Drug Discovery: Innovation in Emerging Chemical Modalities

Recent years have seen an unprecedented level of innovation in allosteric drug discovery and development, with multiple drug candidates advancing into clinical studies. From early examples of allosteric drugs like GABA_A receptor modulators (benzodiazepines) in the 1960s to more recent GPCR negative allosteric modulators of CCR5 (maraviroc) approved in 2007, the opportunities for interrogating allosteric sites in drug discovery have expanded to other target classes such as protein-protein interactions, kinases, and nuclear hormone receptors. In this Innovation Letter, the authors highlight the latest advances of allosteric drug discovery from different target classes and novel emerging chemical modalities beyond small molecules.

Opportunities for Tapping into Three-Dimensional Chemical Space through a Quaternary Carbon

A quaternary carbon bears four other carbon substituents or combination of four non-hydrogen substituents at four vertices of a tetrahedron. The spirocyclic quaternary carbon positioned at the center of a bioactive molecule offers conformational rigidity, which in turn reduces the penalty for conformational entropy. The quaternary carbon is a predominant feature of natural product structures and has been associated with more effective and selective binding to target proteins compared to planar compounds with a high sp² count. The presence of a quaternary carbon stereocenter allows the exploration of novel chemical space to obtain new molecules with enhanced three-dimensionality. These characteristics, coupled to an increasing awareness to develop sp³-rich molecules, boosted utility of quaternary carbon stereocenters in bioactive compounds. It is hoped that this Perspective will inspire the chemist to utilize quaternary carbon stereocenters to enhance potency, selectivity, and other drug-like properties.