Amanda Barth PhD

Pain is both a sensory and emotional experience, which serves an adaptive purpose by protecting the body from prolonged and repeated tissue damage. However, the presence of chronic pain can transition to a maladaptive state leading to life-long suffering and is currently a health care burden due to limited effective pharmacotherapies. This review aims to describe the transition to a chronic pain state and the central changes (supraspinal) that occur to prolong the unpleasant feelings of pain. In addition to alterations in the periphery and spinal cord, regions in the brain involved in pain perception as well as an executive functioning undergo changes in activity that reflect painful experiences from nonpainful stimuli. These pathological changes to the nociceptive system are partly mediated by expression and activity patterns of N-methyl-d-aspartate receptors (NMDARs), which process and propagate excitatory transmission in response to glutamate release in the periphery, spinal cord, and brain and have an important role in learning and memory. Importantly, NMDARs have been implicated in the cellular mechanisms responsible for the maintenance of the centralized, chronic pain state within the brain. Here we present preclinical data describing the analgesic effects of NYX-2925, a novel NDMAR modulator in a variety of neuropathic pain rodent models. These data provide compelling evidence that targeting brain regions responsible for the affective and cognitive aspects of pain may alleviate chronic pain and serve as a novel mechanism to treat chronic pain conditions, including those refractory to current analgesics.

Background: NYX-458 is a N-methyl-d-aspartate receptor (NMDAR) modulator that enhances synaptic plasticity. Dopaminergic cell loss in Parkinson's disease (PD) leads to NMDAR dysregulation in the cortico-striato-pallidal-thalmo-cortical network and altered plasticity in brain regions important to cognitive function. We hypothesize that targeting the NMDAR may be an efficacious approach to treating cognitive impairment in PD.

Objectives: NYX-458 was evaluated in 2 nonhuman primate models of PD. The first, a chronic low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-administration model, was used to assess the effects of NYX-458 on cognitive domains impacted early in PD including attention, working memory, executive function, and visuospatial learning. The second, a high-dose MPTP-administration model, was used to assess potential for NYX-458 induced change in motor symptoms.

Methods: NYX-458 was evaluated in the chronic low-dose MPTP model using the variable delayed response measure to assess attention and working memory and simple discrimination reversal to assess executive function. NYX-458 was also assessed in the high-dose MPTP model as a monotherapy and in combination with low-dose or high-dose levodopa to assess potential impact on motor symptoms.

Results: NYX-458 administration resulted in rapid and long-lasting improvement in cognitive function across the domains of attention, working memory, and executive function. Dose levels effective in improving cognitive performance had no effect on PD motor symptoms, the antiparkinsonian benefit of levodopa, or dyskinesia.

Conclusions: NYX-458 provides benefit in specific domains known to be impaired in PD in a dopamine depletion model of PD-like cognitive impairment. These data support the continued evaluation of NYX-458 as a potential therapeutic for cognitive decline in PD. © 2020 International Parkinson and Movement Disorder Society.

Previous studies have shown that oral administration of the NMDAR modulator NYX-2925 alleviates pain in several animal models of neuropathic pain and this appears to be through mPFC, but not spinal, mediated mechanisms. While much is known about the impact of neuropathic pain on NMDAR-mediated signaling in the spinal cord, limited studies have focused on the brain. In the current study, we assess signaling changes associated with NMDAR-mediated plasticity in the mPFC and the impact of NYX-2925 administration on the normalization of these signaling changes. We found a decrease in activated Src levels in the mPFC of animals with chronic constriction injury (CCI) of the sciatic nerve. While Src mediated activation of NMDARs was also decreased in CCI animals, the main NMDAR phosphorylation site of CAMKII was not affected. This is in opposition to what has been found in the spinal cord, where both Src and CAMKII activation are increased. Oral administration of NYX-2925 restored levels of activated Src and Src phosphorylation sites on GluN2A and GluN2B in the mPFC, with no effect on activated CAMKII levels. The analgesic effect of NYX-2925 appears dependent on this restoration of Src activation in the mPFC, as co-administering Src activation inhibitors prevented the NYX-2925 analgesic effect. Overall, these data suggest that NMDAR-mediated signaling plays a key role in neuropathic pain, albeit in different directions in the spinal cord vs. the mPFC. Furthermore, the analgesic effect of NYX-2925 appears to involve a restoration of NMDAR-mediated signaling in the mPFC.

Background: Modulation of glutamatergic synaptic transmission by N-methyl-D-aspartate receptors can produce rapid and sustained antidepressant effects. Rapastinel (GLYX-13), initially described as a N-methyl-D-aspartate receptor partial glycine site agonist, exhibits rapid antidepressant effect in rodents without the accompanying dissociative effects of N-methyl-D-aspartate receptor antagonists.

Methods: The relationship between rapastinel's in vitro N-methyl-D-aspartate receptor pharmacology and antidepressant efficacy was determined by brain microdialysis and subsequent pharmacological characterization of therapeutic rapastinel concentrations in N-methyl-D-aspartate receptor-specific radioligand displacement, calcium mobilization, and medial prefrontal cortex electrophysiology assays.

Results: Brain rapastinel concentrations of 30 to 100 nM were associated with its antidepressant-like efficacy and enhancement of N-methyl-D-aspartate receptor-dependent neuronal intracellular calcium mobilization. Modulation of N-methyl-D-aspartate receptors by rapastinel was independent of D-serine concentrations, and glycine site antagonists did not block rapastinel's effect. In rat medial prefrontal cortex slices, 100 nM rapastinel increased N-methyl-D-aspartate receptor-mediated excitatory postsynaptic currents and enhanced the magnitude of long-term potentiation without any effect on miniature EPSCs or paired-pulse facilitation responses, indicating postsynaptic action of rapastinel. A critical amino acid within the NR2 subunit was identified as necessary for rapastinel's modulatory effect.

Conclusion: Rapastinel brain concentrations associated with antidepressant-like activity directly enhance medial prefrontal cortex N-methyl-D-aspartate receptor activity and N-methyl-D-aspartate receptor-mediated synaptic plasticity in vitro. At therapeutic concentrations, rapastinel directly enhances N-methyl-D-aspartate receptor activity through a novel site independent of the glycine coagonist site. While both rapastinel and ketamine physically target N-methyl-D-aspartate receptors, the 2 molecules have opposing actions on N-methyl-D-aspartate receptors. Modest positive modulation of N-methyl-D-aspartate receptors by rapastinel represents a novel pharmacological approach to promote well-tolerated, rapid, and sustained improvements in mood disorders.

Background: N-methyl-D-aspartate receptors are one member of a family of ionotropic glutamate receptors that play a pivotal role in synaptic plasticity processes associated with learning and have become attractive therapeutic targets for diseases such as depression, anxiety, schizophrenia, and neuropathic pain. NYX-2925 ((2S, 3R)-3-hydroxy-2-((R)-5-isobutyryl-1-oxo-2,5-diazaspiro[3.4]octan-2-yl)butanamide) is one member of a spiro-β-lactam-based chemical platform that mimics some of the dipyrrolidine structural features of rapastinel (formerly GLYX-13: threonine-proline-proline-threonine) and is distinct from known N-methyl-D-aspartate receptor agonists or antagonists such as D-cycloserine, ketamine, MK-801, kynurenic acid, or ifenprodil.

Methods: The in vitro and in vivo pharmacological properties of NYX-2925 were examined.

Results: NYX-2925 has a low potential for "off-target" activity, as it did not exhibit any significant affinity for a large panel of neuroactive receptors, including hERG receptors. NYX-2925 increased MK-801 binding to human N-methyl-D-aspartate receptor NR2A-D subtypes expressed in HEK cells and enhanced N-methyl-D-aspartate receptor current and long-term potentiation (LTP) in rat hippocampal slices (100-500 nM). Single dose ex vivo studies showed increased metaplasticity in a hippocampal LTP paradigm and structural plasticity 24 hours after administration (1 mg/kg p.o.). Significant learning enhancement in both novel object recognition and positive emotional learning paradigms were observed (0.01-1 mg/kg p.o.), and these effects were blocked by the N-methyl-D-aspartate receptor antagonist CPP. NYX-2925 does not show any addictive or sedative/ataxic side effects and has a therapeutic index of >1000. NYX-2925 (1 mg/kg p.o.) has a cerebrospinal fluid half-life of 1.2 hours with a Cmax of 44 nM at 1 hour.

Conclusions: NYX-2925, like rapastinel, activates an NMDA receptor-mediated synaptic plasticity process and may have therapeutic potential for a variety of NMDA receptor-mediated central nervous system disorders.

Rapastinel (GLYX-13) is an N-methyl-d-aspartate receptor (NMDAR) modulator that has characteristics of a glycine site partial agonist. Rapastinel is a robust cognitive enhancer and facilitates hippocampal long-term potentiation (LTP) of synaptic transmission in slices. In human clinical trials, rapastinel has been shown to produce marked antidepressant properties that last for at least one week following a single dose. The long-lasting antidepressant effect of a single dose of rapastinel (3mg/kg IV) was assessed in rats using the Porsolt, open field and ultrasonic vocalization assays. Cognitive enhancement was examined using the Morris water maze, positive emotional learning, and contextual fear extinction tests. LTP was assessed in hippocampal slices. Dendritic spine morphology was measured in the dentate gyrus and the medial prefrontal cortex. Significant antidepressant-like or cognitive enhancing effects were observed that lasted for at least one week in each model. Rapastinel facilitated LTP 1day-2weeks but not 4weeks post-dosing. Biweekly dosing with rapastinel sustained this effect for at least 8weeks. A single dose of rapastinel increased the proportion of whole-cell NMDAR current contributed by NR2B-containing NMDARs in the hippocampus 1week post-dosing, that returned to baseline by 4weeks post-dosing. The NMDAR antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) blocked the antidepressant-like effect of rapastinel 1week post dosing. A single injection of rapastinel also increased mature spine density in both brain regions 24h post-dosing. These data demonstrate that rapastinel produces its long-lasting antidepressant effects via triggering NMDAR-dependent processes that lead to increased sensitivity to LTP that persist for up to two weeks. These data also suggest that these processes led to the alterations in dendritic spine morphologies associated with the maintenance of long-term changes in synaptic plasticity associated with learning and memory.

Rapastinel (GLYX-13) is a NMDA receptor modulator with glycine-site partial agonist properties. It is a robust cognitive enhancer and shows rapid and long-lasting antidepressant properties in both animal models and in humans. Contextual fear extinction (CFE) in rodents has been well characterized and used extensively as a model to study the neurobiological mechanisms of post-traumatic stress disorder (PTSD). Since CFE is NMDA receptor modulated and neural circuitry in the medial prefrontal cortex (MPFC) regulates both depression and PTSD, studies were undertaken to examine the effects of rapastinel for its therapeutic potential in PTSD and to use rapastinel as a tool to study its underlying glutamatergic mechanisms. A 21-day chronic mild unpredictable stress (CUS) rat model was used to model depression and PTSD. The effects of CUS alone compared to No CUS controls, and the effects of rapastinel (3 mg/kg IV) on CUS-treated animals were examined. The effect of rapastinel was first assessed using CUS-treated rats in three depression models, Porsolt, sucrose preference, and novelty-induced hypophagia tests, and found to produce a complete reversal of the depressive-like state in each model. Rapastinel was then assessed in a MPFC-dependent positive emotional learning paradigm and in CFE and again a reversal of the impairments induced by CUS treatment was observed. Both synaptic plasticity and metaplasticity, as measured by the induction of long-term potentiation in rat MPFC slice preparations, was found to be markedly impaired in CUS-treated animals. This impairment was reversed when CUS-treated rats were administered rapastinel and tested 24 h later. Transcriptomic analysis of MPFC mRNA expression in CUS-treated rats corroborated the link between rapastinel's behavioral effects and synaptic plasticity. A marked enrichment in both the LTP and LTD connectomes in rapastinel-treated CUS rats was observed compared to CUS-treated controls. The effects of rapastinel on depression models, PEL, and most importantly on CFE demonstrate the therapeutic potential of rapastinel for the treatment of PTSD. Moreover, rapastinel appears to elicit its therapeutic effects through a NMDA receptor-mediated, LTP-like, metaplasticity process in the MPFC.

Recent human clinical studies with the NMDA receptor (NMDAR) antagonist ketamine have revealed profound and long-lasting antidepressant effects with rapid onset in several clinical trials, but antidepressant effects were preceded by dissociative side effects. Here we show that GLYX-13, a novel NMDAR glycine-site functional partial agonist, produces an antidepressant-like effect in the Porsolt, novelty induced hypophagia, and learned helplessness tests in rats without exhibiting substance abuse-related, gating, and sedative side effects of ketamine in the drug discrimination, conditioned place preference, pre-pulse inhibition and open-field tests. Like ketamine, the GLYX-13-induced antidepressant-like effects required AMPA/kainate receptor activation, as evidenced by the ability of NBQX to abolish the antidepressant-like effect. Both GLYX-13 and ketamine persistently (24 h) enhanced the induction of long-term potentiation of synaptic transmission and the magnitude of NMDAR-NR2B conductance at rat Schaffer collateral-CA1 synapses in vitro. Cell surface biotinylation studies showed that both GLYX-13 and ketamine led to increases in both NR2B and GluR1 protein levels, as measured by Western analysis, whereas no changes were seen in mRNA expression (microarray and qRT-PCR). GLYX-13, unlike ketamine, produced its antidepressant-like effect when injected directly into the medial prefrontal cortex (MPFC). These results suggest that GLYX-13 produces an antidepressant-like effect without the side effects seen with ketamine at least in part by directly modulating NR2B-containing NMDARs in the MPFC. Furthermore, the enhancement of 'metaplasticity' by both GLYX-13 and ketamine may help explain the long-lasting antidepressant effects of these NMDAR modulators. GLYX-13 is currently in a Phase II clinical development program for treatment-resistant depression.

Models of hippocampal function suggest that the modulation of CA3 afferent input during theta rhythm allows for a rapid alternation between encoding and retrieval states, with each phase enhancing either extrinsic or intrinsic CA3 afferents, favoring either encoding or retrieval, respectively. Here, we show that during the initial exploration of a novel environment by rats, intrinsic CA3-CA3 synaptic inputs are attenuated on CA3 theta peaks, favoring extrinsic CA3 inputs, whereas extrinsic perforant path-CA3 synaptic inputs are attenuated on CA3 theta troughs, favoring intrinsic CA3 inputs. This modulation is absent when animals are re-exposed to the same environment 2 or 48 h later and thus habituates with familiarity, suggesting a process involved in learning. Modulation of CA3 synaptic inputs during novelty was blocked by atropine at a dose that blocks type 2 theta rhythm. Re-exposure to the same novel environment 48 h later in the absence of atropine did not result in habituation, but instead modulated CA3 synaptic responses as though the environment were novel and explored for the first time. The NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), administered in a dose that blocks long-term potentiation induction, did not alter CA3 synaptic modulation during initial exploration. However, like atropine, CPP blocked the habituation of synaptic modulation normally observed with re-exposure, as though the environment were novel and explored for the first time. Thus, as predicted theoretically, recurrent and cortical CA3 afferents are differentially modulated during phases of theta rhythm. This modulation is atropine sensitive and habituates in an NMDA receptor-dependent manner, suggesting an NMDA receptor-dependent process that, in conjunction with theta rhythm, contributes to encoding of novel information in the hippocampus.