Analgesic potential of PF-06372865, an α2/α3/α5 subtype-selective GABAA partial agonist, in humans

Nonsedating anxiolytics

Anxiety disorders are the most prevalent psychiatric pathology with substantial cost to society, but the existing treatments are often inadequate. This has rekindled the interest in the GABAA-receptor (GABAAR) positive allosteric modulator (PAM) compounds, which have a long history in treatment of anxiety beginning with diazepam, chlordiazepoxide, and alprazolam. While the GABAAR PAMs possess remarkable anxiolytic efficacy, they have fallen out of favor due to a host of adverse effects including sedation, motor impairment, addictive potential and tolerance development. A substantial effort was thus devoted to the design of GABAAR PAMs as anxiolytics with reduced sedative liabilities. Several non-benzodiazepine (BZD) GABAAPAMs progressed to clinical trials (bretazenil, abecarnil, alpidem, and ocinaplon) with alpidem obtaining regulatory approval as anxiolytic, but later withdrawn from market due to hepatotoxicity. Advances in molecular biology gave birth to a host of subtype selective GABAAR-PAMs which suffered from signs of sedation and motor impairment and only three compounds progressed to proof-of-concept studies (TPA-023, AZD7325 and PF-06372865). TPA-023 was terminated due to toxicity in preclinical species while AZD7325 and PF-06372865 did not achieve efficacy endpoints in patients. We highlight a new compound, KRM-II-81, that is an imidazodiazepine selective for GABAAR containing α2/3 and β3 proteins. In preclinical studies KRM-II-81 produced anxiolytic-like effects but with minimal sedation, respiratory depression, and abuse liability. Thus, KRM-II-81 is a newly discovered, non- BZD anxiolytic compound, which targets a selective population of GABAAR for improved therapeutic gain and reduced side effects.

Antinociceptive Effects of α2/α3-Subtype-Selective GABAA Receptor Positive Allosteric Modulators KRM-II-81 and NS16085 in Male Rats: Behavioral Specificity

Recent studies suggest that among the gamma-aminobutyric acid type A (GABAA)receptor subtype heterogeneity, α2/α3 subunits of GABAA receptors mediate pain processing. Therefore, α2/α3 subtype-selective GABAA receptor-positive allosteric modulators (PAMs) may be candidate analgesics. Antinociceptive effects of α2/α3 subtype-selective GABAA receptor PAMs have been reported, but the behavioral effects of these compounds have not been systematically evaluated. This study examined the behavioral effects of two α2/α3 subtype-selective GABAA receptor PAMs, KRM-II-81 and NS16085, in male rats. The antinociceptive effects of KRM-II-81 and NS16085 were examined using rat models of inflammatory (complete Freund's adjuvant) and neuropathic pain (chronic constriction injury). The effect of KRM-II-81 on affective pain was measured using the place escape/avoidance paradigm (PEAP). Rate-response of food-maintained operant responding, horizontal wire test, and the spontaneous alternation T-maze were assessed to study the side-effect profiles of KRM-II-81 and NS16085. The benzodiazepine midazolam was used as a comparator in these studies. KRM-II-81 and NS16085 attenuated mechanical allodynia but not thermal hyperalgesia in both pain states, and their effects were attenuated by the benzodiazepine receptor antagonist flumazenil. KRM-II-81 attenuated affective pain-related behavior in the PEAP test. In the operant responding procedure and horizontal wire test, only midazolam produced significant effects at the dose that produced maximal antinociception. In the T-maze assay, only midazolam significantly decreased the percentage of alternation at an antinociceptive dose. Thus, KRM-II-81 and NS16085 but not midazolam selectively produced antinociceptive effects. Collectively, these data suggest that α2/α3 subtype-selective GABAA PAMs could be a novel class of analgesics and warrant further investigation. SIGNIFICANCE STATEMENT: This study demonstrates that α2/α3 subtype-selective GABAA PAMs KRM-II-81 and NS16085 produce selective antinociceptive effects devoid of sedation, myorelaxation, and cognitive impairment in two rat models of persistent pain. This study supports the development of α2/α3 subtype-selective GABAA PAMs, rather than classical benzodiazepines, as safe and novel analgesics for pain management.

Propofol Alleviates Anxiety-Like Behaviors Associated with Pain by Inhibiting the Hyperactivity of PVNCRH Neurons via GABAA Receptor β3 Subunits

Pain, a comorbidity of anxiety disorders, causes substantial clinical, social, and economic burdens. Emerging evidence suggests that propofol, the most commonly used general anesthetic, may regulate psychological disorders; however, its role in pain-associated anxiety is not yet described. This study investigates the therapeutic potential of a single dose of propofol (100 mg kg-1) in alleviating pain-associated anxiety and examines the underlying neural mechanisms. In acute and chronic pain models, propofol decreased anxiety-like behaviors in the elevated plus maze (EPM) and open field (OF) tests. Propofol also reduced the serum levels of stress-related hormones including corticosterone, corticotropin-releasing hormone (CRH), and norepinephrine. Fiber photometry recordings indicated that the calcium signaling activity of CRH neurons in the paraventricular nucleus (PVNCRH) is reduced after propofol treatment. Interestingly, artificially activating PVNCRH neurons through chemogenetics interfered with the anxiety-reducing effects of propofol. Electrophysiological recordings indicated that propofol decreases the activity of PVNCRH neurons by increasing spontaneous inhibitory postsynaptic currents (sIPSCs). Further, reducing the levels of γ-aminobutyric acid type A receptor β3 (GABAAβ3) subunits in PVNCRH neurons diminished the anxiety-relieving effects of propofol. In conclusion, this study provides a mechanistic and preclinical rationale to treat pain-associated anxiety-like behaviors using a single dose of propofol.

Managing Chronic Neuropathic Pain: Recent Advances and New Challenges

Aim. Neuropathic pain affects 7–10% of the population, with most of the patients receiving inadequate and incomplete treatment. Owing to the high financial burden and the poor quality of life of the patients and their caretakers, there is a dire need to address the challenges in diagnosing and treating chronic neuropathic pain. Methods. This literature review was conducted to review novel treatments and related challenges through a systematic search from sources such as PubMed, Google Scholar with the combination of MESH words such as neuropathic pain, management of neuropathic pain. Articles from non-English literature, reports without human subjects, animal studies, and abstracts/posters were excluded. However, human studies and studies published in English were included. Result. This review article discusses novel treatment modalities while acknowledging the challenges medical workers face while encountering neuropathic pain. Despite the recent advances in diagnosis and treatment modalities, several challenges still exist. Hence, there is still a need to explore the various treatment modalities, emphasizing the cause and underlying pathophysiology of neuropathic pain. Conclusion. We recommend integrated multimodal treatment with the current treatment facility, including various medical disciplines. However, a personalized approach would work the best depending on the ’patient’s medical history. Therefore, this article recommends an integrated, cause-specific, cost-effective approach to address this problem of chronic neuropathic pain.

Curcumin Diethyl γ-Aminobutyrate, a Prodrug of Curcumin, for Enhanced Treatment of Inflammatory Pain

Curcumin is a naturally occurring polyphenol compound with potential analgesic effects. It has been shown to improve pain-like behaviors in numerous models of pain. Despite its potential, curcumin exhibits poor physicochemical and pharmacokinetic properties, which hinder its oral therapeutic efficacy. Curcumin diethyl γ-aminobutyrate (CUR-2GE), a carbamate prodrug of curcumin, was designed to overcome these limitations and demonstrated greater anti-neuroinflammatory effects compared to curcumin in vitro. Thus, this study evaluated the effect of CUR-2GE and its parent compound on pain-like behaviors in carrageenan- and LPS-induced mouse models. The possible side effects of CUR-2GE were also assessed by exploring its effects on motor coordination and spontaneous locomotor activity after acute and chronic treatments. The results showed that CUR-2GE improved mechanical and thermal hyperalgesia and locomotor activity to a greater extent than curcumin in carrageenan-induced mice. These results are in line with the ability of CUR-2GE to suppress peripheral inflammation in the paw tissue of carrageenan-induced mice, indicated by a significant decrease in TNF-α and IL-6 expression levels. Similarly, in LPS-induced mice, CUR-2GE improved sickness and pain-like behaviors (exploratory behaviors and long-term locomotor activity) to a greater extent than curcumin. Furthermore, CUR-2GE significantly reduced the level of proinflammatory cytokines in both the plasma and spinal cord tissue of LPS-induced mice, exhibiting significantly higher inhibition than curcumin. Moreover, the motor coordination, and locomotive behaviors of mice were not affected by both acute and chronic administration of CUR-2GE, indicating no potential CNS side effects. Thus, CUR-2GE demonstrated enhanced therapeutic efficacy in mouse models of inflammatory pain without any possible CNS side effects, suggesting its potential to be developed as an analgesic agent against inflammatory pain.

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.

Comparison of bromazepam and ibuprofen influence on tooth pulp-evoked potentials in humans

Introduction/Objective Somatosensory evoked potentials are a neurophysiological tool for testing the effects of drugs in humans and animals. The aim of this study was to estimate the way that bromazepam and ibuprofen had on tooth pulp-evoked potentials (TPEPs) after non-painful stimuli, as well as to detect possible differences in this activity. Methods Sixty young healthy subjects were included in the study. They were arranged into three groups: ibuprofen, bromazepam and placebo. To record TPEPs response, dental pulp was electrically stimulated through intact enamel with non-painful stimuli. For stimulation and registration, we used Xltek Protektor 32 system, software EPWorks, version 5.0. The experiment consisted of two testing sessions. Five recordings were performed in each session. The first test session was before, and the second was 45 minutes after administration of a single dose of the ibuprofen (400 mg), bromazepam (1.5 mg) or placebo. Results The results of the present study exhibit that both ibuprofen and bromazepam significantly increased all the latencies; ibuprofen decreased amplitudes of all the waves except the first one (p < 0.05), and bromazepam decreased amplitudes of all the waves except the first one (p < 0.05); placebo did not modified TPEPs waves (p > 0.05). Additionally, there were no significant differences in influence on TPEPs between bromazepam and ibuprofen (p > 0.05). Conclusion Our study showed that both bromazepam and ibuprofen had the same influence on TPEPs after non-painful stimuli. That indicates that anxiolytic dose of bromazepam affects neurotransmission in the same manner as non-opioid analgesics ibuprofen.

The efficacy of γ-aminobutyric acid type A receptor (GABA AR) subtype-selective positive allosteric modulators in blocking tetramethylenedisulfotetramine (TETS)-induced seizure-like behavior in larval zebrafish with minimal sedation

The chemical threat agent tetramethylenedisulfotetramine (TETS) is a γ-aminobutyric acid type A receptor (GABA _AR) antagonist that causes life threatening seizures. Currently, there is no specific antidote for TETS intoxication. TETS-induced seizures are typically treated with benzodiazepines, which function as nonselective positive allosteric modulators (PAMs) of synaptic GABA_ARs. The major target of TETS was recently identified as the GABA_AR α2β3γ2 subtype in electrophysiological studies using recombinantly expressed receptor combinations. Here, we tested whether these in vitro findings translate in vivo by comparing the efficacy of GABA_AR subunit-selective PAMs in reducing TETS-induced seizure behavior in larval zebrafish. We tested PAMs targeting α1, α2, α2/3/5, α6, ß2/3, ß1/2/3, and δ subunits and compared their efficacy to the benzodiazepine midazolam (MDZ). The data demonstrate that α2- and α6-selective PAMs (SL-651,498 and SB-205384, respectively) were effective at mitigating TETS-induced seizure-like behavior. Combinations of SB-205384 and MDZ or SL-651,498 and 2–261 (ß2/3-selective) mitigated TETS-induced seizure-like behavior at concentrations that did not elicit sedating effects in a photomotor behavioral assay, whereas MDZ alone caused sedation at the concentration required to stop seizure behavior. Isobologram analyses suggested that SB-205384 and MDZ interacted in an antagonistic fashion, while the effects of SL-651,498 and 2–261 were additive. These results further elucidate the molecular mechanism by which TETS induces seizures and provide mechanistic insight regarding specific countermeasures against this chemical convulsant.

5-O-methylcneorumchromone K Exerts Antinociceptive Effects in Mice via Interaction with GABAA Receptors

The proper pharmacological control of pain is a continuous challenge for patients and health care providers. Even the most widely used medications for pain treatment are still ineffective or unsafe for some patients, especially for those who suffer from chronic pain. Substances containing the chromone scaffold have shown a variety of biological activities, including analgesic effects. This work presents for the first time the centrally mediated antinociceptive activity of 5-O-methylcneorumchromone K (5-CK). Cold plate and tail flick tests in mice showed that the 5-CK-induced antinociception was dose-dependent, longer-lasting, and more efficacious than that induced by morphine. The 5-CK-induced antinociception was not reversed by the opioid antagonist naloxone. Topological descriptors (fingerprints) were employed to narrow the antagonist selection to further investigate 5-CK's mechanism of action. Next, based on the results of fingerprints analysis, functional antagonist assays were conducted on nociceptive tests. The effect of 5-CK was completely reversed in both cold plate and tail-flick tests by GABA_A receptor antagonist bicuculline, but not by atropine or glibenclamide. Molecular docking studies suggest that 5-CK binds to the orthosteric binding site, with a similar binding profile to that observed for bicuculline and GABA. These results evidence that 5-CK has a centrally mediated antinociceptive effect, probably involving the activation of GABAergic pathways.

Experimental GABA A Receptor Agonists and Allosteric Modulators for the Treatment of Focal Epilepsy

GABA A receptors are ubiquitous in the central nervous system and there is a huge diversity of receptor subtypes in almost all regions of the brain. However, the expression of GABA A receptor subtypes is altered in both the gray and white matter of patients with focal epilepsy. Although there is a number of anticonvulsants with marketing authorization for the treatment of focal epilepsy which act through GABA A receptors, potentiating the inhibitory effects of GABA, it is necessary to develop more potent and more specific GABAergic anticonvulsants that are effective in drug-resistant patients with focal epilepsy. There are three orthosteric and at least seven allosteric agonist binding sites at the GABA A receptor. In experimental and clinical studies, full agonists of GABA A receptors showed a tendency to cause desensitization of the receptors, tolerance, and physical dependence; therefore, partial orthosteric agonists and positive allosteric modulators of GABA A receptors were further developed. Preclinical studies demonstrated the anticonvulsant efficacy of positive allosteric modulators with selective action on GABA A receptors with α2/α3 subunits, but only a handful of them were further tested in clinical trials. The best results were obtained for clobazam (already marketed), ganaxolone (in phase III trials), CVL-865 (in phase II trials), and padsevonil (in phase III trials). Several compounds with more selective action on GABA A receptors, perhaps only in certain brain regions, have the potential to become effective drugs against specific subtypes of focal-onset epilepsy. However, their development needs time, and in the near future we can expect only one or two new GABA A agonists to obtain marketing authorization for focal epilepsy, an advance that would be of use for just a fraction of patients with drug-resistant epilepsy.

Harmaline potentiates morphine-induced antinociception via affecting the ventral hippocampal GABA-A receptors in mice

Morphine is one of the most effective medications for treatment of pain, but its side effects limit its use. Therefore, identification of new strategies that can enhance morphine-induced antinociception and/or reduce its side effects will help to develop therapeutic approaches for pain relief. Considering antinociceptive efficacy of harmaline and also highlighted the important role of GABA-A receptors in the pain perception, this research aimed to determine whether the ventral hippocampal (vHip) GABA-A receptors are involved in the possible harmaline-induced enhancement of morphine antinociception. To achieve this, vHip regions of adult male mice were bilaterally cannulated and pain sensitivity was measured in a tail-flick apparatus. Intraperitoneally administration of morphine (0, 2, 4 and 6 mg/kg) or harmaline (0, 1.25, 5 and 10 mg/kg) increased the percentage of maximal possible effect (%MPE) and induced antinociception. Interestingly, co-administration of sub-effective doses of harmaline (5 mg/kg) and morphine (2 mg/kg) induced antinociception. Intra-vHip microinjection of muscimol (0, 200 and 300 ng/mice), a GABA-A receptor agonist, enhanced the anti-nociceptive effects of harmaline (2.5 mg/kg)+morphine (2 mg/kg) combination. Microinjection of the same doses of muscimol into the vHip by itself did not alter tail-flick latency. Intra-vHip microinjection of bicuculline (100 ng/mouse), a GABA-A receptor antagonist, did not cause a significant change in MPE%. Bicuculline (60 and 100 ng/mouse, intra-vHip) was administered with the harmaline (5 mg/kg)+morphine (2 mg/kg), and inhibited the potentiating effect of harmaline on morphine response. These findings favor the notion that GABAergic mechanisms in the vHip facilitate harmaline-induced potentiation of morphine response in the tail-flick test in part through GABA-A receptors. These findings shall provide insights and strategies into the development of pain suppressing drugs.

Mice lacking spinal α2GABAA receptors: Altered GABAergic neurotransmission, diminished GABAergic antihyperalgesia, and potential compensatory mechanisms preventing a hyperalgesic phenotype

Diminished synaptic inhibition in the superficial spinal dorsal horn contributes to exaggerated pain responses that accompany peripheral inflammation and neuropathy. α2GABA_A receptors (α2GABA_AR) constitute the most abundant GABA_AR subtype at this site and are the targets of recently identified antihyperalgesic compounds. Surprisingly, hoxb8-α2^-/- mice that lack α2GABA_AR from the spinal cord and peripheral sensory neurons exhibit unaltered sensitivity to acute painful stimuli and develop normal inflammatory and neuropathic hyperalgesia. Here, we provide a comprehensive analysis of GABAergic neurotransmission, of behavioral phenotypes and of possible compensatory mechanisms in hoxb8-α2^-/- mice. Our results confirm that hoxb8-α2^-/- mice show significantly diminished GABAergic inhibitory postsynaptic currents (IPSCs) in the superficial dorsal horn but no hyperalgesic phenotype. We also confirm that the potentiation of dorsal horn GABAergic IPSCs by the α2-preferring GABA_AR modulator HZ-166 is reduced in hoxb8-α2^-/- mice and that hoxb8-α2^-/- mice are resistant to the analgesic effects of HZ-166. Tonic GABAergic currents, glycinergic IPSCs, and sensory afferent-evoked EPSCs did not show significant changes in hoxb8-α2^-/- mice rendering a compensatory up-regulation of other GABA_AR subtypes or of glycine receptors unlikely. Although expression of serotonin and of the serotonin producing enzyme tryptophan hydroxylase (TPH2) was significantly increased in the dorsal horn of hoxb8-α2^-/- mice, ablation of serotonergic terminals from the lumbar spinal cord failed to unmask a nociceptive phenotype. Our results are consistent with an important contribution of α2GABA_AR to spinal nociceptive control but their ablation early in development appears to activate yet-to-be identified compensatory mechanisms that protect hoxb8-α2^-/- mice from hyperalgesia.

GABAA receptor subtype modulators in medicinal chemistry: an updated patent review (2014-present)

Introduction: Ligands at the benzodiazepine binding site of the GABA_A receptor (GABA_AR) act by modulating the effect of GABA (γ-aminobutyric acid). The benzodiazepine drugs are conventionally categorized as positive allosteric modulators enhancing the chloride ion current GABA-induced. In literature there are also reported ligands that act as negative allosteric modulators, reducing chloride ion current, and silent allosteric modulators not influencing the chloride ion flux.Areas covered: This review covers patents published from 2014 to present on ligands for the benzodiazepine binding site of the GABA_ARs. Patents filed from different companies and research groups report many compounds that may be used in the treatment/prevention of a large variety of diseases.Expert opinion: Since the discovery of the first benzodiazepine about 60 years have passed and about 50 years since the identification of their target, GABA_A receptor. Even if benzodiazepines are the most popular anxiolytic drugs, the research in this field is still very active. From patents/application analysis arises that most of them claim methods for alleviating specific symptoms in different neurodegenerative diseases and their related memory deficits. Noteworthy is the presence of the α4- and α5-GABA_A receptor subtype ligands as new pharmacological tools for airway hyperresponsiveness, inflammation diseases, and asthma.

GABAergic modulation of secondary hyperalgesia: A randomized controlled 4-way crossover trial with the α2-subunit preferring GABA positive allosteric modulator, N-desmethyl-clobazam in healthy volunteers

The antihyperalgesic and sedative effects of the α2-subunit preferring GABA_A positive allosteric modulator (GAM), N-desmethyl-clobazam (NDMC), 20 and 60 mg, were assessed in a randomized, placebo and active-controlled (clonazepam 1,5 mg), 4-way crossover study, in healthy volunteers, using the ultraviolet B-induced experimental pain model. Single (20, 40, 60 mg) and repeated doses (20 mg over 15 days) of NDMC pharmacokinetics were evaluated. Thirty-two subjects participated in the study. Primary outcome parameter was maximal change in the area of cutaneous UVB irradiation-induced secondary hyperalgesia (ASH). ASH decreased under all treatments. Mean (SD) relative change was 79 (22)%, 83 (24)%, 77 (30)% and 92 (16)% for placebo, NDMC20, NDMC60 and clonazepam, respectively. Neither absolute change nor relative change in ASH was significantly different between NDMC60 and placebo (mean difference = 2.3 cm² [95% CI 4.0-8.5], p = .462 and 0.4% [-11.9 to 12.6], p = .952, respectively). An overall treatment effect was found on level of sedation. Compared to placebo, sedation was higher under clonazepam (mean difference = 39 mm [30-49] on a visual analogue scale, p < .001) while NDMC was free of sedative effect. NDMC pharmacokinetics after single doses showed poor absorption, but was linear. Steady-state plasma concentrations of NDMC20 were attained within 14 days, with low between-subjects variability. Mean steady-state concentration (C_S-S , SD) reached 209 (22) ng/ml. NDMC absence of sedative effect and its overall well-characterized safety coming from years of utilization as a metabolite from clobazam, raise the prospect of dose escalating trials in patients to quantify its clinical utility. SIGNIFICANCE: This article, presenting the Phase I data of the new antihyperalgesic compound, α2-subunit GABA_A positive allosteric modulator, N-desmethyl-clobazam (NDMC) is exploring the modulation of a new target in the treatment of neuropathic pain. Based on these results and on its preclinical properties NDMC would qualify as a good tool compound to seek confirmation of the clinical utility of selective GABA allosteric modulators in neuropathic pain patients.