Evaluation of the phencyclidine-like discriminative stimulus effects of novel NMDA channel blockers in rats

GABAkines - Advances in the discovery, development, and commercialization of positive allosteric modulators of GABAA receptors

Positive allosteric modulators of γ-aminobutyric acid-A (GABA_A) receptors or GABAkines have been widely used medicines for over 70 years for anxiety, epilepsy, sleep, and other disorders. Traditional GABAkines like diazepam have safety and tolerability concerns that include sedation, motor-impairment, respiratory depression, tolerance and dependence. Multiple GABAkines have entered clinical development but the issue of side-effects has not been fully solved. The compounds that are presently being developed and commercialized include several neuroactive steroids (an allopregnanolone formulation (brexanolone), an allopregnanolone prodrug (LYT-300), Sage-324, zuranolone, and ganaxolone), the α2/3-preferring GABAkine, KRM-II-81, and the α2/3/5-preferring GABAkine PF-06372865 (darigabat). The neuroactive steroids are in clinical development for post-partum depression, intractable epilepsy, tremor, status epilepticus, and genetic epilepsy disorders. Darigabat is in development for epilepsy and anxiety. The imidazodiazepine, KRM-II-81 is efficacious in animal models for the treatment of epilepsy and post-traumatic epilepsy, acute and chronic pain, as well as anxiety and depression. The efficacy of KRM-II-81 in models of pharmacoresistant epilepsy, preventing the development of seizure sensitization, and in brain tissue of intractable epileptic patients bodes well for improved therapeutics. Medicinal chemistry efforts are also ongoing to identify novel and improved GABAkines. The data document gaps in our understanding of the molecular pharmacology of GABAkines that drive differential pharmacological profiles, but emphasize advancements in the ability to successfully utilize GABA_A receptor potentiation for therapeutic gain in neurology and psychiatry.

Application and assessment of deep learning for the generation of potential NMDA receptor antagonists

Uncompetitive antagonists of the N-methyl d-aspartate receptor (NMDAR) have demonstrated therapeutic benefit in the treatment of neurological diseases such as Parkinson's and Alzheimer's, but some also cause dissociative effects that have led to the synthesis of illicit drugs. The ability to generate NMDAR antagonists in silico is therefore desirable for both new medication development and preempting and identifying new designer drugs. Recently, generative deep learning models have been applied to de novo drug design as a means to expand the amount of chemical space that can be explored for potential drug-like compounds. In this study, we assess the application of a generative model to the NMDAR to achieve two primary objectives: (i) the creation and release of a comprehensive library of experimentally validated NMDAR phencyclidine (PCP) site antagonists to assist the drug discovery community and (ii) an analysis of both the advantages conferred by applying such generative artificial intelligence models to drug design and the current limitations of the approach. We apply, and provide source code for, a variety of ligand- and structure-based assessment techniques used in standard drug discovery analyses to the deep learning-generated compounds. We present twelve candidate antagonists that are not available in existing chemical databases to provide an example of what this type of workflow can achieve, though synthesis and experimental validation of these compounds are still required.

Evaluating the abuse potential of psychedelic drugs as part of the safety pharmacology assessment for medical use in humans

Psychedelics comprise drugs come from various pharmacological classes including 5-HT_2A agonists, indirect 5-HT agonists, e.g., MDMA, NMDA antagonists and κ-opioid receptor agonists. There is resurgence in developing psychedelics to treat psychiatric disorders with high unmet clinical need. Many, but not all, psychedelics are schedule 1 controlled drugs (CDs), i.e., no approved medical use. For existing psychedelics in development, regulatory approval will require a move from schedule 1 to a CD schedule for drugs with medical use, i.e., schedules 2-5. Although abuse of the psychedelics is well documented, a systematic preclinical and clinical evaluation of the risks they pose in a medical-use setting does not exist. We describe the non-clinical tests required for a regulatory evaluation of abuse/dependence risks, i.e., drug-discrimination, intravenous self-administration and physical dependence liability. A synopsis of the existing data for the various types of psychedelics is provided and we describe our findings with psychedelic drugs in these models. FDA recently issued its guidance on abuse/dependence evaluation of drug-candidates (CDER/FDA, 2017). We critically review the guidance, discuss the impact this document will have on non-clinical abuse/dependence testing, and offer advice on how non-clinical abuse/dependence experiments can be designed to meet not only the expectations of FDA, but also other regulatory agencies. Finally, we offer views on how these non-clinical tests can be refined to provide more meaningful information to aid the assessment of the risks posed by CNS drug-candidates for abuse and physical dependence. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

NMDA Receptors Containing the GluN2D Subunit Control Neuronal Function in the Subthalamic Nucleus

The GluN2D subunit of the NMDA receptor is prominently expressed in the basal ganglia and associated brainstem nuclei, including the subthalamic nucleus (STN), globus pallidus, striatum, and substantia nigra. However, little is known about how GluN2D-containing NMDA receptors contribute to synaptic activity in these regions. Using Western blotting of STN tissue punches, we demonstrated that GluN2D is expressed in the rat STN throughout development [age postnatal day 7 (P7)-P60] and in the adult (age P120). Immunoelectron microscopy of the adult rat brain showed that GluN2D is predominantly expressed in dendrites, unmyelinated axons, and axon terminals within the STN. Using subunit-selective allosteric modulators of NMDA receptors (TCN-201, ifenprodil, CIQ, and DQP-1105), we provide evidence that receptors containing the GluN2B and GluN2D subunits mediate responses to exogenously applied NMDA and glycine, as well as synaptic NMDA receptor activation in the STN of rat brain slices. EPSCs in the STN were mediated primarily by AMPA and NMDA receptors and GluN2D-containing NMDA receptors controlled the slow deactivation time course of EPSCs in the STN. In vivo recordings from the STN of anesthetized adult rats demonstrated that the spike firing rate was increased by the GluN2C/D potentiator CIQ and decreased by the GluN2C/D antagonist DQP-1105, suggesting that NMDA receptor activity can influence STN output. These data indicate that the GluN2B and GluN2D NMDA receptor subunits contribute to synaptic activity in the STN and may represent potential therapeutic targets for modulating subthalamic neuron activity in neurological disorders such as Parkinson's disease.

N-methyl-D-aspartate receptor channel blocker-like discriminative stimulus effects of nitrous oxide gas

Nitrous oxide (N2O) gas is a widely used anesthetic adjunct in dentistry and medicine that is also commonly abused. Studies have shown that N2O alters the function of the N-methyl-d-aspartate (NMDA), GABAA, opioid, and serotonin receptors among others. However, the receptors systems underlying the abuse-related central nervous system effects of N2O are unclear. The present study explores the receptor systems responsible for producing the discriminative stimulus effects of N2O. B6SJLF1/J male mice trained to discriminate 10 minutes of exposure to 60% N2O + 40% oxygen versus 100% oxygen served as subjects. Both the high-affinity NMDA receptor channel blocker (+)-MK-801 maleate [(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate] and the low-affinity blocker memantine partially mimicked the stimulus effects of N2O. Neither the competitive NMDA antagonist, CGS-19755 (cis-4-[phosphomethyl]-piperidine-2-carboxylic acid), nor the NMDA glycine-site antagonist, L701-324 [7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(1H)-quinolinone], produced N2O-like stimulus effects. A range of GABAA agonists and positive modulators, including midazolam, pentobarbital, muscimol, and gaboxadol (4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-3-ol), all failed to produce N2O-like stimulus effects. The μ-, κ-, and δ-opioid agonists, as well as 5-hydroxytryptamine (serotonin) 1B/2C (5-HT1B/2C) and 5-HT1A agonists, also failed to produce N2O-like stimulus effects. Ethanol partially substituted for N2O. Both (+)-MK-801 and ethanol but not midazolam pretreatment also significantly enhanced the discriminative stimulus effects of N2O. Our results support the hypothesis that the discriminative stimulus effects of N2O are at least partially mediated by NMDA antagonist effects similar to those produced by channel blockers. However, as none of the drugs tested fully mimicked the stimulus effects of N2O, other mechanisms may also be involved.

GLYX-13, an NMDA receptor glycine site functional partial agonist enhances cognition and produces antidepressant effects without the psychotomimetic side effects of NMDA receptor antagonists

INTRODUCTION

The N-methyl-d-aspartate receptor-ionophore complex plays a key role in learning and memory and has efficacy in animals and humans with affective disorders. GLYX-13 is an N-methyl-d-aspartate receptor (NMDAR) glycine-site functional partial agonist and cognitive enhancer that also shows rapid antidepressant activity without psychotomimetic side effects.

AREAS COVERED

The authors review the mechanism of action of GLYX-13 that was investigated in preclinical studies and evaluated in clinical studies. Specifically, the authors review its pharmacology, pharmacokinetics, and drug safety that were demonstrated in clinical studies.

EXPERT OPINION

NMDAR full antagonists can produce rapid antidepressant effects in treatment-resistant subjects; however, they are often accompanied by psychotomimetic effects that make chronic use outside of a clinical trial inpatient setting problematic. GLYX-13 appears to exert its antidepressant effects in the frontal cortex via NMDAR-triggered synaptic plasticity. Understanding the mechanistic underpinning of GLYX-13's antidepressant action should provide both novel insights into the role of the glutamatergic system in depression and identify new targets for therapeutic development.

1-Methyl-1,2,3,4-tetrahydroisoquinoline, an endogenous amine with unexpected mechanism of action: new vistas of therapeutic application

This review outlines the effects of 1,2,3,4-tetrahydroisoquinoline (TIQ) and its derivative, 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), endogenous substances imbued with high pharmacological potential and broad spectrum of action in brain. 1MeTIQ has gained special interest as a neuroprotectant, and its ability to antagonize the behavioral syndrome produced by well-known neurotoxins (e.g., MPTP; rotenone). This review is thus focused on mechanisms of action of 1MeTIQ in behavioral, neurochemical, and molecular studies in rodents; also, effects of TIQ and 1MeTIQ on dopamine metabolism; and neuroprotective properties of TIQ and 1MeTIQ in vitro and in vivo. Finally, antiaddictive properties of 1MeTIQ will be described in cocaine self-administered rats. Findings implicate TIQ and especially its methyl derivative 1MeTIQ in unique and complex mechanisms of neuroprotection in various neurodegenerative illnesses of the central nervous system. We believe that MAO inhibition, free radicals scavenging properties, and antagonism to the glutamatergic system may play an essential role in neuroprotection. In addition, the results strongly support the view that 1MeTIQ has a considerable potential as a drug for combating substance abuse, through the attenuation of craving.

The discriminative stimulus effects of N-methyl-D-aspartate glycine-site ligands in NMDA antagonist-trained rats

RATIONALE

Many N-methyl-D-aspartate (NMDA) antagonists produce phencyclidine (PCP)-like side effects that limit their clinical utility. NMDA glycine-site antagonists may be less likely to produce these effects than other site-selective NMDA antagonists.

OBJECTIVES

The objective of the study is to compare the discriminative stimulus effects of novel NMDA glycine-site drugs to those of channel blocking and competitive NMDA antagonists.

MATERIALS AND METHODS

Drug discrimination studies were performed in separate groups of rats trained with saline vs. PCP (2 mg/kg i.p.) or the competitive antagonist NPC 17742 (4 mg/kg i.p.) using a standard two-lever operant conditioning procedure under an FR32.

RESULTS

Neither the partial glycine-site agonists aminocyclopropane carboxylic acid methyl ester and (+)-HA-966 nor the antagonists L701,324; MDL 100,458; MDL 100,748; MDL 103,371; MDL 104,472; MDL 105,519; MRZ 2/571; MRZ 2/576; and ACEA 0762 produced >50% PCP-lever selection, though all were tested over a sufficient dose range to produce response rate decreasing effects. All of the antagonists, except MDL 100,458 and MDL 100,748, were also tested for NPC 17742-like effects, producing somewhat more variable results than in PCP-trained rats. ACEA-0762 produced full substitution for NPC 17742, whereas MDL 105,519 produced no substitution. The remaining compounds engendered between 20% and 80% drug-lever selection.

CONCLUSION

These results provide evidence that NMDA glycine-site partial agonists and antagonists generally do not produce discriminative stimulus effects similar to those of representative NMDA channel blockers or competitive antagonists. This suggests that these NMDA glycine-site antagonists should be less likely to produce the undesirable behavioral side effects seen in clinical trials with many other NMDA antagonists.