Category: Articles

Background: Melioidosis is a relatively high-incidence, high-mortality tropical infectious disease caused by Burkholderia pseudomallei. To date, the few prevention and management strategies in practice have failed to reduce mortality from septic shock in patients with severe melioidosis. Up to 60.9% of patients also have pre-existing type 2 diabetes mellitus (T2DM), the most significant risk factor for melioidosis. An effective tertiary prevention strategy against melioidosis for these diabetic individuals would impact significantly on the burden of disease. Glibenclamide, a drug belonging to the sulfonylurea class and commonly used as an antidiabetic, may have the potential to be one such strategy.

Aim: To examine the underlying inflammatory cause of morbidity and mortality in melioidosis and to assess the potential for glibenclamide to ameliorate these causes.

Results: Bacteraemia with B. pseudomallei leads to widespread infection. Multi-organ damage and loss of function results from both direct bacterial damage and an excessive inflammatory response, the latter of which also causes potentially fatal septic shock in 21% of cases. Massive cellular infiltration of the lungs is particularly damaging. The immunomodulatory effects of T2DM further exacerbate the deleterious immune response into a dysregulated, hyperinflammatory state. On the other hand, the several anti-inflammatory effects of glibenclamide have been demonstrated to significantly reduce mortality from septic shock in melioidosis.

Conclusion: For diabetics living in regions where melioidosis is endemic, choosing glibenclamide over other sulfonylureas and metformin for antidiabetic treatment could be a promising tertiary prevention measure against melioidosis.

Introduction

Melioidosis, a tropical infectious disease, is a significant cause of death and illness in northern Australia and Southeast Asia. Its non-specific prodrome and clinical presentation makes early detection difficult.[1] Yet, timely diagnosis is critical because responsiveness to treatment declines with progression of the disease over time.[2] Morbidity and mortality remains high despite aggressive antibiotic treatment. As of yet, no vaccine is available for primary prevention and novel developments such as granulocyte colony-stimulating factor (G-CSF) and corticosteroid therapy lack evidence as good measures of tertiary prevention. Considering the significant population health threat, any effective prevention strategy would be of value. This article suggests the potential use of glibenclamide, an antidiabetic sulfonylurea, for tertiary prevention of melioidosis in diabetics. Because diabetics are approximately 13 times more likely to contract melioidosis,[3,4] this strategy may have widespread application.

One of the major and dangerous end-stage complications of melioidosis is septic shock. Especially in settings such as Northeast (NE) Thailand where melioidosis is endemic but intensive care is unavailable, morbidity and mortality from septic shock are unacceptably high. Systemically-spread B. pseudomallei stimulates massive release of cytokines into the blood, this event being instrumental in the immunopathogenesis of septic systemic vasodilation, hypotension, vascular hyperpermeability, and disseminated intravascular coagulation (DIC). Furthermore, poor glycaemic control in diabetics increases the risk and severity of these events by precipitating a compensatory, dysregulated, and hyperinflammatory response to bacterial immunogenic stimuli. Demanding particular attention is the stimulated production of cytokine IL-1β because it promotes excessive neutrophilic recruitment to the lungs and bacterial persistence inside cells. Fortunately, there is strong evidence that glibenclamide acts protectively against IL-1β-mediated damage by targeting IL-1β production directly as well as multiple upstream components in the IL-1β production pathway. The result of glibenclamide use for diabetics is a significantly reduced morbidity and mortality from melioidosis septic shock compared with diabetics not taking the drug.

Epidemiology of Melioidosis

Melioidosis is endemic in Southeast Asia, northern Australia, India, Hong Kong, southern China and Taiwan.[5,6] Isolated sporadic cases have also appeared in Central America, the Caribbean, New Caledonia, Mauritius, Africa, and the Middle East.[7] Of particular concern are the hyperendemic hotspots – NE Thailand and the Top End of the Northern Territory of Australia. A 2006 NE Thailand study that gathered results from four large hospitals reported the region experienced an estimated minimum annual incidence of 21.3 cases per 100,000 people and case fatality rate of 40.5%, melioidosis locally being the third highest infectious cause of death and second most common pathogenic cause of community-acquired bacteraemia.[8,3] From 2004 to 2009, according to data collected at Royal Darwin Hospital (RDH), the annual incidence in the Top End was 21.6 per 100,000 people and average case fatality rate plateaued at 9% having decreased from 30% since 1989. The lower fatality rate in Darwin is attributed to greater access to intensive care unit (ICU) supportive care and early sepsis management that prolongs life in the event of fatal septic shock, a complication that develops in at least 21% of cases.[9]

Type 2 diabetes mellitus (T2DM) is the largest risk factor for melioidosis. 48% and 60.9% of individuals with melioidosis were confirmed to have T2DM at RDH and in NE Thailand respectively.[10,11] The 2006 Thai study reported that the relative risk (RR) of melioidosis in adult diabetics compared with non-diabetics was 12.4.[3] In the Top End region, combining diabetes with the additional risk factor of Indigenous ethnicity raised the RR from 13.1 to 20.6. Other notable risk factors associated with a significant RR are chronic lung disease (RR 4.3), age ≥ 45 years (RR 4.0), chronic renal disease (RR 3.2), Indigenous ethnicity (RR 3.0), male sex (RR 2.4) and hazardous alcohol consumption (RR 2.1).[6]

The presence of any of these risk factors in infected individuals also dramatically increases their chance of developing septic shock (RR 4.5) and death (RR 9.2), the risk factors with highest independent association with death being age ≥ 50 years (RR 2.1) and chronic lung disease (RR 1.5). Malignancy (RR 1.9) and rheumatic heart disease and/or congestive cardiac failure (RR 1.7) are statistically insignificant risk factors for death. Clinical presentations most likely to result in death are septic shock (RR 11.2) and, out of the non-septic shock cases, the presence of neurological infection foci (RR 4.7).[9] Additionally, case fatality during the peak rainfall months of December through February is 1.6 times greater than it is during other months.[9]

Immunopathogenesis and pathophysiology

Melioidosis is caused by Gram-negative bacterium B. pseudomallei. Bacteria, found in contaminated soil, rodents, food, water, and excretions, are transmitted via inhalation, ingestion, or percutaneous inoculation – usually direct contact with open skin lesions.[12,13] Post-inoculation, B. pseudomallei can invade most human cell types. Employing Type 3 Secretion System (TTSS) clusters, bacteria enter non-phagocytic cells. If bacteria are phagocytosed, TTSS enables exit from intracellular phagocytic endosomes such that degradation is evaded. Once escaped into the cytoplasm of cells both phagocytic and non-phagocytic, bacteria replicate and self-induce polarised actin filamentation that confers motility, facilitating spread to neighbouring cells by forcing host-cell membrane protrusion and fusion.[14]

B. pseudomallei possesses highly immunogenic factors that trigger a strong host immune reaction essential to the host for early bacterial containment.[15] B. pseudomallei expresses pathogen-associated molecular patterns including lipopeptides, peptidoglycan, lipopolysaccharide, flagellin, TTSS, and DNA. They are recognised by host cell toll-like receptors (TLR) and NOD-like receptors (NLR). TLRs and NLRs are expressed by immune cells both professional – macrophages and dendritic cells – and non-professional – epithelial cells, endothelial cells, and fibroblasts.[16]

Activation of NLRC4 or NLRP3 induces binding of caspase-1, Asc (apoptosis-associated speck-like protein containing CARD) and NLRC4 or NLRP3 to form an inflammasome. The NLRC4 inflammasome in infected macrophages triggers pyroptosis, serving to limit intracellular B. pseudomallei growth and proliferation, while the NLRP3 inflammasome performs proteolytic activation of pro-IL-1β and pro-IL-18 into their mature forms for secretion.[17] IL-1β recruits neutrophils to infection site(s). However, excessive recruitment is often deleterious because neutrophils, lacking NLRC4, fail to pyroptose and instead provide a favourable intracellular environment that sustains chronic bacterial persistence. In the lungs, persistence leads to damaging pulmonary abscess formation and acute respiratory distress syndrome.[17,18] In contrast, elevated IL-18 production correlates with survival and immunoprotection because IL-18 induces IFN-γ production.[17,19] IFN-γ activates macrophages, stimulating their direct antimicrobial processes – phagolysosomal fusion and toxic reactive nitrogen species synthesis that produces nitrosative stress.[20] The cytokine also facilitates macrophage antigen processing and presentation, recruits leukocytes to infection site(s), upregulates Th1 CD4+ cell population, enhances natural killer cell function, regulates B cell anti-lipopolysaccharide antibody production and isotype switching.[21]

Figure 1. NLRC4 inflammasome leads to immunoprotective effects while NLRP3 inflammasome produces both protective and deleterious responses. Glibenclamide works by inhibiting NLRP3 inflammasome formation and its deleterious effects.

TLR activation upregulates secretion of principal inflammatory mediators including type 1 interferon, chemokines, antimicrobial proteins, and pro-inflammatory cytokines.[22] Elevated expression of TLR1, TLR2, TLR3, TLR4, TLR5, TLR8, and TLR10 has been demonstrated in patients with septic melioidosis.[16] In particular, stimulation of the TLR2-mediated signalling pathway is a principal step in recognising the immune challenge of B. pseudomallei and initiating early inflammatory processes.[23] TNF-α and IL-6, along with IL-1β, increase vascular permeability, induce acute phase protein production, and recruit leukocytes to the site of infection.[24] TNF-α and IL-6 also activate the complement and coagulation cascades that are key defense mechanisms of the innate response.[25] However, excessive inflammatory cytokine production induced by widespread bacteraemia often leads to septic shock characterised by systemic vasodilatory hypotension, vascular hyperpermeability causing major cellular and fluid leakage from the intravascular to extravascular compartments, DIC, and death in the absence of immediate treatment.[26]

Through the establishment of bacteraemia, B. pseudomallei spreads from primary foci of infection to other body tissues, usually the lungs, genitourinary tract, skin, joints, bones, liver, spleen, skeletal muscles, prostate, parotid gland, and nervous system. Multi-organ spread leading to impaired organ function and failure is the main source of morbidity and mortality in melioidosis.[27] Thus, a rapid but non-deleterious inflammatory response is critical before B. pseudomallei establishes an intracellular niche that enables its persistence and protection against eradication by a subsequent immune attack. Paradoxically, subsequent immune attacks may cause further damage to the host.

Effect of type 2 diabetic state on host response

Diabetics, due to their generally immunosuppressed state, compared with non-diabetics, are more susceptible to developing sepsis from most organisms,[28] with the most common source of systemic spread being respiratory, followed by urinary and abdominal.[29] Gram-positive bacteria have become the leading cause of sepsis since the 1980s, and sepsis due to causes other than B. pseudomallei actually affects diabetics more commonly than melioidosis.[29,30] However, it is still of significance that diabetics are particularly vulnerable to pathogens such as B. pseudomallei and Mycobacterium tuberculosis because immune defence against these intracellular bacteria is highly macrophage-mediated – a function that is critically impaired in poorly-controlled T2DM.[31]

T2DM, especially if poorly controlled, causes marked immunomodulation that produces B. pseudomallei-induced immune responses that are damaging and yet insufficient to offer host protection. Morris et al. created an ex vivo whole-blood assay using human peripheral blood to compare inflammatory responses to B. pseudomallei between diabetics and non-diabetics. Immune insufficiency was worst in poorly controlled diabetics, the assay detecting: elevated serum IL-10 (an anti-inflammatory cytokine); reduced CD11b on polymorphonuclear leukocytes (PMNs) leading to decreased PMN function with reduced activation, adhesion, transmigration, and migratory capacity towards IL-8; reduced endotoxaemia-induced upregulation of ICAM-1; and defects in phagocytic detection and response to the bacteria.[32] In vivo and in vitro models using streptozotocin (STZ)-induced, leptin deficiency, leptin receptor deficiency, and diet-induced diabetic mice found similar immune impairments.[31,33-35] Transcriptional analysis of STZ-diabetic mice responses over the first 42 hours of B. pseudomallei exposure attributed delayed defence and splenic dysfunction (permitting uncontrolled bacterial replication intracellularly causing susceptibility to sepsis) to diminished TLR2 recognition of the bacterium.[34]

Incompetence of the early inflammatory response to contain an initial infection precipitates a compensatory, dysregulated, hyperinflammatory response to the spreading infection. The intensity of this ensuing reaction is exacerbated by chronic low-grade inflammation and increased oxidative stress, both characteristic of T2DM. Morris et al. found poorly controlled diabetics to have the most elevated levels of pro-inflammatory markers ESR, CRP, TNF-α, IL-1β, IL-6, IL-8, IL-12p70, MCP-1, and MPO (which indicates increased oxidative burst activity in PMNs). At focal infection sites, there was extensive infiltration with PMNs and greater risk of tissue damage, as well as generalised endothelial dysfunction and vascular inflammation.[32]

Hyperglycaemia is a major link between T2DM, immune derangement, and susceptibility to sepsis. The supra-physiological hyperosmolarity of hyperglycaemic blood directly potentiates TLR4-mediated cytokine production, as well as dampening phagocytic responses and granulocyte oxidative burst.[36] Activation of the RAGE pathway by advanced glycation end-products formed secondary to hyperglycaemia perpetuates inflammation and worsens survival in septic mice.[37] Furthermore, in the event of sepsis, poor glycaemic control allowing for periods of hyperglycaemia in both diabetics and non-diabetics correlates with longer hospital stay, greater morbidity, and possibly decreased survival.[38] In addition to immunomodulatory effects, hyperglycaemia exacerbates hypotension in septic shock by promoting glycosuric diuresis and cardiac hyperresponsiveness to cholinergic stimulation and the baroreflex.[39] Elevated risk of myocardial infarction, stroke, and venous thromboembolism is associated with the prothrombotic effect of acute hyperglycaemia that is further potentiated by hyperinsulinaemia or an inflammatory stress state.[40]

Prevention and management

No human vaccine is available.[41] CDC prevention guidelines recommend avoiding contact with soil and stagnant water in endemic regions.[42] High-risk individuals should stay indoors during heavy wind and rain in endemic regions due to possible bacterial aerosolisation that greatly facilitates transmission.[43]

Treatment involves two weeks of intravenous antibiotics – ceftazidime, meropenem, or imipenem, in combination with trimethoprim-sulfamethoxazole – followed by 3 months of oral eradication therapy to prevent recrudescence.[44] However, even with early antibiotic administration, case fatality from septicaemia remains at 37% in Thailand.[45] The development of septic shock is an indication for ICU supportive care usually involving invasive monitoring, fluid resuscitation, renal replacement therapy, and vasopressor, inotropic, and mechanical ventilatory support.[46,47]

Adjunctive use of recombinant G-CSF holds no clinical utility for improving sepsis from melioidosis and other aetiologies. Both animal and human trials – including prospective, multicentre, randomised, double-blind, and placebo-controlled studies – confirm this.[47-49] In fact, G-CSF may cause increased hepatic dysfunction and higher peak troponin I levels.[50] Other immunomodulatory adjuncts such as corticosteroids do not improve survival.[46] In light of the paucity of effective evidence-based prevention strategies and adjunct therapies for underserved populations, we turn to a novel potential for tertiary prevention.

Glibenclamide and its protective effect

Glibenclamide is a sulfonylurea used in the management of diabetes. It antagonises ATP-dependent potassium (KATP) channels in pancreatic β-cells, stimulating insulin release. The salient risk associated with glibenclamide is accidentally developing hypoglycaemia, an event of varying seriousness in patients who are not critically ill but a great predictor of mortality in acute sepsis. Furthermore, recent studies comparing glibenclamide with metformin and other sulfonylureas elucidated its significant adverse effects in addition to severe hypoglycaemia, such as increased risk of malignancy and mortality from cardiovascular disease due to interference with ischaemic preconditioning.[51,52] Despite glibenclamide still being a popular choice, a trend in medical opinion has arisen against its use in favour of alternative sulfonylureas.[53,54]

However, for T2DM individuals living in regions endemic with melioidosis, choosing glibenclamide over other antidiabetic drugs has an additional benefit that clinicians must consider. In a 5-year prospective cohort study from 2002 to 2006, Koh et al. followed 1160 adult patients with culture-confirmed melioidosis for 28 days during and after their admission to a major NE Thailand hospital. 71.3% of diabetic individuals taking glibenclamide survived from melioidosis after 28 days, while only 50.7% of diabetics not taking glibenclamide and 47.0% of non-diabetics survived (survival of patients discharged from hospital within 28 days were assumed to have survived). Using a logistics regression model, glibenclamide treatment reduced case fatality with an adjusted odds ratio (AOR) of 0.34 when compared to diabetics not taking glibenclamide and an AOR of 0.47 compared to non-diabetics. Incidences of hypotension (AOR 0.48) and respiratory failure (AOR 0.50) were also reduced in these patients.[55] Therefore, this protective effect must be factored into the drug’s risk-benefit comparison with metformin and with other sulfonylureas. If it informs clinicians’ drug choices for glycaemic control in management of T2DM, glibenclamide can contribute to tertiary prevention of melioidosis.

Although preventing hyperglycaemia in sepsis improves patient outcomes, glibenclamide does not rely on its glucose-lowering effect.[56] Using a mouse model, Koh et al. attributed the mechanism of glibenclamide’s protective effect to two main anti-inflammatory pathways. Firstly, glibenclamide reduces IL-1β secretion, attenuating IL-8 production and neutrophilic and monocytic influx into the lungs (without completely ablating the neutrophilic response).[55,57] In vitro evidence suggests that glibenclamide does this by partially blocking NLRP3 inflammasome formation and/or by directly inhibiting the secretion of mature IL-1β (Figure 1).[57-59] Glibenclamide may also attenuate transcription and translation of caspase-1, NLRP3, and IL-1β genes.[57] Secondly, by reducing systemic vasodilation and maintaining normal peripheral vascular resistance, glibenclamide restores systemic mean arterial pressure in the event of septic shock. KATP channels in vascular smooth muscle preferentially open during sepsis.[60] When they do, they hyperpolarise the cell, inhibiting Ca2+ influx (via voltage-dependent Ca2+ channels) and causing muscular relaxation. Glibenclamide opposes this effect by blocking the KATP channels. However, the only evidence for this is found in animal models.[61,62]

Conclusion

Melioidosis remains a significant public health concern due to its high incidence, mortality and case fatality rates particularly in the Top End of Northern Territory and NE Thailand. T2DM is the largest risk factor for melioidosis. The antidiabetic drug, glibenclamide, reduces morbidity and mortality from melioidosis-induced septic shock and so plays a potential role in tertiary prevention of melioidosis. In the debate over glibenclamide use, consideration of the drug’s protective effect for diabetics living in melioidosis-endemic regions, particularly those that are resource-limited, is critical.

Acknowledgements

None

Conflict of Interests

None declared

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Amnesic effects of benzodiazepines are in part the result of the activity of α₅-subunit containing GABA_A receptors (GABRA5). Negative modulators at this receptor could improve cognition. In order to explore this beneficial effect, this article reviews the evidence on the effects of GABRA₅ negative modulators and searches potential uses for such drugs. A literature search found a number of GABRA5 negative modulators. These drugs generally improve hippocampal-dependant learning via an increase in longterm potentiation (LTP) in the hippocampus. Passive avoidance learning was also improved. In addition, the compounds examined demonstrated minimal side effects partly due to lack of binding to different alpha subunit-containing GABA_A types. Due to its beneficial properties, there is potential for such a drug in treating Alzheimer’s, alcohol-related amnesia and Down syndrome. Despite the myriad animal studies that utilised GABRA5 negative modulators, only three human studies were found. Due to its cognitive enhancing properties and minimal side effects, further human trials should be conducted in order to ascertain the potential of such drugs in treating cognitive deficits.

Introduction
γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain and is responsible for regulating neuronal excitability. There are at least three different receptors that it targets — GABA_A, GABA_B and GABA_C. [1] The GABA_A receptor is the main target for the popular class of drugs, the benzodiazepines. This receptor is an ionotropic membrane receptor, which facilitates the movement of chloride into cells. In neurons, this increases the threshold needed to excite them.[2] Benzodiazepines are positive modulators at this receptor and exert their effects by binding to the interface between the γ₂ and α subunits on the GABA_A receptor. [3] Since they are modulators and not agonists, they do not work in the absence of GABA. [4] They only bind to the receptors containing the α₅, α₃, α₂, and α₁ subunits. [5] Each of these subunits mediates different effects. In knockout mice, the α₁ subunit has shown hypnotic/sedative effects, whilst the α₃, α₂ and α₅ subunits have anti-anxiolytic effects and these have been exploited for therapeutic use.[6]

Despite their uses in treating various conditions, traditional benzodiazepines have numerous side effects. The main side effects associated with therapeutic use are amnesia, confusion, impaired coordination and dizziness. There may be tolerance due to rapid escalation in the dose needed to provide the required effect. There are also long-term issues with dependence. In acute overdose the most life-threatening effect is respiratory depression, especially when combined with alcohol. [7]

Interestingly, it is thought that positive modulators at the α₅ subunit-containing GABA_A receptors (GABRA5) produce the anterograde amnesia associated with benzodiazepine use. [7] Most of these receptors are found in the hippocampus (a brain region associated with memory) and provide tonic inhibition in this region. [8] The exploitation of this receptor has led to the increasing use of the infamous ‘date rape’ drug flunitrazepam, which is a positive modulator at the GABRA5 in addition to its other functions. Because of its amnestic effects, victims are unable to recall events following intoxication and this provides a major challenge for prosecutors. [7] Despite these negative properties, by using a GABRA5 negative modulator the opposite effect might The α₅ subunit-containing GABA_A receptor: a target for the treatment of cognitive defects be achieved and cognition improved. The use of such a drug could potentially improve the quality of life for those living with cognitive defects and could also counteract drug-induced amnesia (for example, alcoholic ‘blackout’). However, this must be balanced with the inverse activity of non-selective negative modulators which could produce
convulsant or anxiogenic effects. [4]

Based on the premise that a GABRA5 negative modulator could improve cognition, the aim of this literature review was to review the evidence on (1) the effects of GABRA5 negative modulators on cognition; and (2) investigate the potential of GABRA5 negative modulators in managing conditions involving cognitive defects.

Effects of GABRA5 selective negative modulators

A number of GABRA5 negative modulator compounds were examined. Many of the studies used animals as subjects. Studies in animals provide a solid starting platform for understanding the various physiological changes that a drug induces. [9] Of particular importance is the avoidance of potential side effects as a result of non-selective actions at other GABA_A receptors. Side effects could include an increase in anxiety, aggressiveness, motor impairment, inability to sleep and proconvulsant effects. [7] Ultimately, the knowledge gained from animal experimentation can be used to conduct safe and effective clinical trials.

Dawson et al. [10] examined a compound named α51A (3-(5-Methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-yl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine), which has selective negative modulator effects at GABRA5. The authors found that α51A reversed the inhibiting effects of GABRA5 in the hippocampus in rats and mice. This resulted in an increase in performance in a memory test named the “delayed matching-to-position version of the Morris water maze”, which is a hippocampus-dependant cognitive test. [11] In addition, ‘long-term potentiation’ (LTP), which is thought to underlie the synaptic changes that take place during memory formation, was found to be enhanced in the hippocampus. [11,12] Benzodiazepine (agonist) effects and non-selective GABA_A negative modulator effects were also examined. The authors found no anxiogenic, convulsant, withdrawal or motor-impairing effects from the drug. [10]

Only certain components of memory have shown to be improved by GABRA5 negative modulators. Collinson et al. [13] extrapolated on the results obtained by Dawson et al. [10] and looked at the effect of a modified version of α51A, α51A-II. They separated memory into three components — encoding, consolidation (conversion into long-term memory) and recall. Results were obtained by measuring performance in the delayed matching-to-position (DMTP) version of the Morris water maze in rats. The authors found that the compound improved encoding and recall but not consolidation in this hippocampaldependant memory test. [13]

The effect on cognition by another selective GABRA5 negative modulator was examined by Ballard et al. [14] This was done by the use of an imidazo-triazolo-benzodiazepine compound named RO4938581. The effect of this compound on cognition was examined in rats. [15] This compound demonstrated similar effects to those found by Dawson et al. [10] in that they found no convulsant or anxiogenic effects. In addition, similar to Dawson et al. [10], there was an increase in hippocampal LTP. The authors also found that working memory was enhanced since RO493881 reversed scopolamine-induced working memory impairment. [14] This was shown by an increase in performance in the DMTP task, which is used to assess spatial working memory. [14,15] It also reversed diazepam-induced spatial impairment. This was demonstrated by an increase in performance in the Morris water maze task. [14]

‘Moderate’ GABRA5 negative modulators improve passive avoidance learning but generally have no effect on active learning. [16] This was shown by an experiment conducted by Savic et al. [16] in which they examined effects of PWZ-029 (a ‘moderate’ GABRA5 negative modulator) on passive and active learning avoidance in rats. The result was obtained through various shuttle-box based behavioural experiments. This experiment proved that even at ‘moderate’ efficacy a GABRA5 negative modulator can induce memory formation. The compound also had no effect on muscle tension and anxiety (nonselective side effects). [16] Although promising, this study was limited by the fact that the compound only had ‘moderate’ negative modulator activity at GABRA5 so using a more efficient compound may display different effects on avoidance learning. Despite this limitation, this shows that the use of a ‘moderate’ GABRA5 negative modulator would be beneficial in the treatment of disease due to its limited side effects and its memory-enhancing properties. [16]

Application in management

The compounds examined in this review show that selective GABRA5 negative modulators have nootropic effects without any serious side effects, which are seen in non-selective negative modulators at the alpha subunit of the GABA_A receptor. [17] Thus, there is strong potential for the use of GABRA5 negative modulators in healthcare settings. One major limitation is that most of the data obtained for this review was from animals. Further human trials need to be conducted to ascertain the potential of this drug. Drawing on the literature, possible future uses for a GABRA5 negative modulator are detailed below.

GABRA5 negative modulators could be used to treat Alzheimer’s disease since GABRA5 is preserved in Alzheimer’s disease patients.
[18] Alzheimer’s disease is commonly characterised by the gradual worsening of ability to remember new information. [19] Administration of a GABRA5 negative modulator could help with the ‘encoding’ and ‘recall’ of this information. [8] It could also be used to treat mild cognitive impairment (MCI), which is a risk factor for later developing the disease. [20] Administration of such a drug to patients may provide relief to older caregivers, who often show signs of sleep detriment. [21]

A review by Attack [22] in 2010 found two human trials on the GABRA5 negative modulator α5Ia and one trial on MRK-016. Since then no human trials were found and this could be an area of future research. The first study found that a potential application of GABRA5 negative modulators is the treatment of alcohol-induced amnesia. Nutt et al. [23] found that pre-treatment reduces alcohol’s amnestic effects in humans. This was measured by word list learning which is linked to hippocampal processing. Alcohol-induced amnesia has been shown to predict future alcohol-related injury. [24] Therefore, the use of a GABRA5 negative modulator may help in reducing this risk. In addition, it may also reduce alcohol-related stress since it has been found that amnestic episodes related to alcohol have resulted in moderate psychological stress. [25]

Unfortunately, GABRA5 does not improve age-related cognitive defects. In fact it has been found that α5IA significantly impairs cognition in the elderly despite having positive effects on the young. Attack [22] found that young subjects (mean age 22 years) performed much better than older subjects (mean age 72 years) on the paired associates learning test, which is sensitive to age-related cognitive decline. Therefore, this trial showed no potential in reversing age-related cognitive decline. This demonstrates that careful consideration based on age should be taken in to account when using this drug.

MRK-016 (3-tert-butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)-pyrazolo[1,5-d]-[1,2,4]triazine) is another negative modulator and showed greater LTP in rat hippocampal slices than α51A. It also enhanced performance in the DMTP and Morris water maze tasks, which are used to test spatial memory. In humans, it was well tolerated in young adults with a maximum tolerated dose of 5 mg with 75% occupancy. In elderly subjects, however, it was poorly tolerated even at 10% of the maximum dosage in young adult males. Therefore, this particular drug has been precluded for development. [26]

Recent trials of GABRA5 agonists, in particular L-655,708 and MRK-016, have focused on restoring post-anaesthetic cognitive deficits. Lecker et al. [27] found that L-655,708 and MRK-016 reduced the potentiation of GABRA5 post-inhalation of isoflurane and sevoflurane. A further study by Zureck et al. [28] found that short-term memory assessed by the novel object recognition task was fully reversed by L-655,708 after isoflurane anaesthesia. This demonstrates the potential use of L-655,708 in reducing post-anaesthetic amnesia. However, further studies which include those performed on humans are needed to validate the potential of MRK-016 and other GABRA5 negative modulators in reducing post-anaesthetic amnesia.

The use of GABRA5 negative modulators could help with treating cognitive deficits related to Down syndrome. A recent review by Martínez-Cué et al. [29] investigated this specific application. It identified two studies that examined the effects of a GABRA5 inverse modulator on a Down syndrome mouse model (Ts65Dn). Braudeau et al. [30] found that acute treatment with the GABRA negative modulator α5IA improved learning deficits in the Morris water maze task. The second study also showed that chronic administration of a similar drug, RO4938581 has also been shown to have memory-promoting effects in the Morris water maze task on Ts65Dn mice. [31] In a practical sense, administration of such a drug could improve performance in learning a wide range of functional skills in those living with Down syndrome. For example, in children this may include learning how to use the toilet and administering self-care. [32]

Conclusion

The literature supporting the use of a GABRA5 negative modulator in the treatment of cognitive deficits is promising. GABRA5 negative modulators exert their actions by enhancing hippocampal dependant memory formation. There are minimal side effects as no withdrawal symptoms, convulsant, anxiogenic or motor-impairing effects were found. There is great potential for the use of GABRA5 negative modulators as they have been shown to reduce alcohol-related amnesia and may have potential in the treatment of Alzheimer’s disease. They could also treat cognitive deficits in Down syndrome patients, increasing the speed at which they learn functional skills. Due to these favourable findings, there is an increased need for human clinical trials in order to validate the potential for this important receptor target.

Acknowledgements

A/Prof Zoltan Sarnyai, for reviewing this article when I submitted it as an assignment.

Conflict of interest

None declared.

Correspondence

A Bhandari: abhishta.bhandari@my.jcu.edu.au

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