Argon: A Noble, But Not Inert, Treatment For Brain Trauma?

Traumatic brain injury (TBI) is an acquired neurological condition resulting from external mechanical forces and is a leading cause of morbidity and mortality worldwide.1 The global incidence of TBI is estimated to be 60 million cases annually.2 Although improvements in the clinical management of TBI have improved outcomes and reduced mortality, current treatments are largely supportive focusing on maintaining what are considered to be appropriate physiological levels of, for example, tissue oxygenation and ICP.1,3 

Click to cistanche tcm for neuroprotection effect

Advances in the understanding of TBI pathophysiology have revealed a complex network of interacting cellular responses and biochemical pathways underlying the potentially preventable secondary injury that occurs after the mechanical insult.4,5 

However, clinically proven treatments for TBI specifically targeting neuronal injury and secondary injury pathways are currently lacking. Despite the apparent molecular complexity of the injurious processes underlying secondary injury, effective treatment could involve the use of a simple monatomic gas. Argon is a member of the series of noble gases that have low chemical reactivity and comprise group 18 of the periodic table, but they are neither inert nor biologically inactive. 

Argon, together with its larger neighbor krypton, has anesthetic properties at elevated pressure, while xenon is an anesthetic under normobaric conditions.6 Argon has previously been evaluated in preclinical models of ischaemic stroke with both positive7,8 and negative9 findings. Both argon and xenon reduce injury development after TBI in vitro, 10e13, and xenon have been shown to reduce secondary injury, improve long-term locomotor function, prevent late-onset cognitive impairment and neuronal loss, and improve long-term survival after experimental TBI in mice.14,15 

Thus far there have been no reports of the efficacy of argon in an animal model of TBI. The study by Moro and colleagues16 in the current issue of the British Journal of Anaesthesia represents a significant advance in this respect, with an evaluation of argon as a potential treatment for TBI in mice. Moro and colleagues16 used the highly reproducible controlled cortical impact model of blunt contusional TBI to investigate the efficacy of argon in mice after a severe injury. 

Spontaneously breathing male mice were treated with argon 70%:oxygen 30% for 24 h starting 10 min after brain injury. The authors used two measures of sensorimotor function: simple neuro assessment of asymmetric impairment (SNAP) and neuro score. 

Argon treatment improved both the SNAP score and the neuro score at 24 h and 7 days after injury, but not at 2, 3, or 6 weeks after injury. Motor impairment was reduced in the argon treatment group, assessed by counting the number of missteps as mice walk along a narrow beam 2 days after injury, but impairment was not assessed at later time points. 

Argon treatment also reduced memory impairment at 4 weeks after TBI, measured using the Barnes maze paradigm that assesses hippocampus-dependent spatial learning and memory by quantifying the time taken to find a hidden escape box after learning the maze during a series of training trials. These behavioral outcomes are clinically relevant as deficits in locomotor function and learning and memory are frequent clinical sequelae of TBI. 

However, the long-term persistence of the treatment effects is as yet unclear. Another translationally relevant aspect of the study is the utilization of MRI to quantify vasogenic edema and white matter integrity. Diffusion-weighted imaging at 3 days after injury revealed a significant reduction in edema in the argon-treated group. 

White matter damage at 5 weeks after TBI, assessed using diffusion tensor imaging, was reduced in the ipsilateral fimbria that connects the hippocampus with subcortical brain regions and in the corpus callosum in the argon-treated group. Although edema volume is not routinely measured clinically, diffusion tensor imaging is used as a surrogate outcome measure for white matter injury in clinical trials.17 

At the cellular level Moro and colleagues16 show that argon treatment reduced the number of proinflammatory microglia in the ipsilateral cortex, hippocampus, corpus callosum, and fimbria. The strengths of the study are the use of the well-characterized controlled cortical impact model, randomization and blinding, relatively large group sizes for animal studies (n¼20 in each group for behavioral tests up to 1 week after injury), clinically relevant behavior outcomes, use of translationally relevant imaging modalities, and inclusion of supplementary data. 

Nevertheless, there are several caveats before firm conclusions on the efficacy of argon for TBI can be drawn. From a clinical translation perspective, perhaps the greatest limitation is the short time window of 10 min between injury and the start of argon treatment. The preclinical literature on acquired brain injury is littered with putative treatments that have failed to translate to human treatments.18 

There are many possible explanations for these failures, but a significant factor has been the failure to determine whether there is a clinically relevant time window for treatments. In some preclinical studies, treatments have been given before or during the injury thus potentially modulating the magnitude of initial insult in the treatment group. 

This is not the case here where treatment was given 10 min after trauma. Although there are specific circumstances where treatment could be given within minutes of injury by first responders, there are likely to be limited opportunities for such early interventions. A more realistic clinically relevant time window would extend to a few hours (or at least the first hour) after injury, by which time a patient would have reached the hospital. In the dynamic and rapidly evolving pathophysiology of TBI, treatments aimed at reducing or preventing secondary injury are likely to need to be started within hours. 

As Moro and colleagues16 acknowledge, further preclinical studies with argon are required to establish the time window within which efficacy is maintained. Another aspect is that the authors used a relatively high concentration of 70% argon. Although this makes sense for early preclinical studies on efficacy, in clinical TBI, oxygen concentrations over 30% may well be required. It would therefore be essential to determine whether lower concentrations of argon (e.g. 50%) are equally effective, as noted by the authors. The study should be commended for looking at outcomes up to 6 weeks after injury. 

This is a relatively long time for animal studies, and given the short lifespan of rodents, is equivalent to several months or a few years in humans. Given the clear association of TBI with increased risk of premature mortality and dementia in humans,19,20 another area for further investigation would be to look at even longer-term outcomes. 

The author’s laboratory has been one of relatively few, along with our own, to study TBI in rodents up to 1 yr or more after injury,15,21,22 and an investigation of argon’s long-term efficacy would be very helpful. An intriguing aspect of the study is that argon treatment improved hippocampus-dependent memory function in the Barnes maze test, but there was no difference in neuronal density in the ipsilateral hippocampus in the argon group, although there was a reduction in inflammatory microglia. The lack of a neuronal correlate of the functional outcome may seem strange, and microglia are known to modulate synaptic connectivity, however, it should be noted that the histopathology was at a different time point (7 days) than the Barnes maze test (4 weeks) and in a different cohort of animals. 

The authors focussed their attention on analyzing the ipsilateral hemisphere and regions proximal to the severe impact site; it would be interesting to know whether there is neuronal loss, and possibly preservation with argon treatment, in the contralateral hemisphere. This information is of clinical relevance, given that in TBI patients there is the well-known coup-contrecoup phenomenon and that injury is often observed distant from the site of the primary injury. Despite these caveats, the work of Moro and colleagues16 represents an important first step in the evaluation of the noble gas argon as a potential treatment for TBI. 

There are no clinically proven, specifically targeted neuroprotective treatments for TBI, and noble gases such as argon and xenon are attractive candidates. Attention has focussed on xenon, which is already approved for use as a general anesthetic and has undergone a successful early clinical trial for ischaemic brain injury.17 Xenon is effective in preventing short- and very long-term deficits after experimental TBI in animals.14,15 

However, the current work on argon for TBI, and other studies on argon for ischaemic brain injury,7e9,11,23 suggest that this noble gas may also have promise as a treatment for acquired brain injuries. Argon is less expensive than xenon, and if it is equally as effective as xenon, this could favor its use in the absence of closed rebreathing circuits. As argon and xenon appear to act via different mechanisms,13,23 there is the possibility that combinations of these two noble gases could have a synergistic effect. 

However, caution should be exercised before human trials of argon, because many treatments that have shown promise in brain injury in animals have failed to translate to humans.18 One of the features of TBI that makes human trials challenging is that TBI is a very heterogeneous condition both in the severity of the injury and in the age of patients. 

To avoid the failures of the past, rigorous preclinical data packages should be assembled including determining the therapeutic time window, concentration-response, efficacy in different injury severities (in both female and male animals of different ages), efficacy in other species, and a clear mechanism of action. 

This interesting first study by Moro and colleagues16 is an important first step that should prompt additional research into argon as a potential novel treatment for TBI.

What is the mechanism of the Cistanche neuroprotection effect?

Cistanche is a traditional Chinese medicinal herb that has been found to have neuroprotective effects. Its mechanism of action for neuroprotection is believed to involve several different pathways. 

One pathway involves the modulation of oxidative stress in neurons. Cistanche contains compounds called phenylethanoid glycosides, which have been shown to scavenge free radicals and reduce oxidative stress in cells. This mechanism can help to protect neurons from damage caused by oxidative stress.

Another pathway involves the regulation of inflammation in the brain. Chronic inflammation has been linked to several neurodegenerative diseases, and Cistanche has been shown to reduce the expression of pro-inflammatory cytokines in cells. This can help to prevent or reduce inflammation in the brain, which may help to protect neurons and prevent neurodegeneration.

Finally, Cistanche has been shown to promote neurogenesis or the growth of new neurons in the brain. This effect may help to improve cognitive function and prevent or slow the progression of neurodegenerative diseases.

References 

1. Maas AIR, Menon DK, Adelson PD, et al. Traumatic brain injury: integrated approaches to improve prevention, clinical care, and research. Lancet Neurol 2017; 16: 987e1048 2. Feigin VL, Theadom A, Barker-Collo S, et al. Incidence of traumatic brain injury in New Zealand: a population-based study. Lancet Neurol 2013; 12: 53e64

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12. Campos-Pires R, Koziakova M, Yonis A, et al. Xenon protects against blast-induced traumatic brain injury in an in vitro model. J Neurotrauma 2018; 35: 1037e44 

13. Harris K, Armstrong SP, Campos-Pires R, Kiru L, Franks NP, Dickinson R. Neuroprotection against traumatic brain injury by xenon, but not argon, is mediated by inhibition at the N-methyl-D-aspartate receptor glycine site. Anesthesiology 2013; 119: 1137e48 

14. Campos-Pires R, Armstrong SP, Sebastiani A, et al. Xenon improves neurologic outcomes and reduces secondary injury following trauma in an in vivo model of traumatic brain injury. Crit Care Med 2015; 43: 149e58 

15. Campos-Pires R, Hirnet T, Valeo F, et al. Xenon improves long-term cognitive function, reduces neuronal loss and chronic neuroinflammation, and improves survival after traumatic brain injury in mice. Br J Anaesth 2019; 123: 60e73 

16. Moro F, Fossi F, Magliocca A, et al. Efficacy of inhaled argon administered acutely after traumatic brain injury in mice. Br J Anaesth 2020. https://doi.org/10.1016/ j.bja.2020.08.027 

17. Laitio R, Hynninen M, Arola O, et al. Effect of inhaled xenon on cerebral white matter damage in comatose survivors of out-of-hospital cardiac arrest: a randomized clinical trial. JAMA 2016; 315: 1120e8 

18. Warner DS, James ML, Laskowitz DT, Wijdicks EF. Translational research in acute central nervous system injury: lessons learned and the future. JAMA Neurol 2014; 71: 1311e8 

19. McMillan TM, Teasdale GM, Weir CJ, Stewart E. Death after head injury: the 13-year outcome of a case-control study. J Neurol Neurosurg Psychiatry 2011; 82: 931e5 

20. Smith DH, Johnson VE, Stewart W. Chronic neuropathologies of single and repetitive TBI: substrates of dementia? Nat Rev Neurol 2013; 9: 211e21 

21. Pischiutta F, Micotti E, Hay JR, et al. Single severe traumatic brain injury produces progressive pathology with ongoing contralateral white matter damage one year after injury. Exp Neurol 2018; 300: 167e78 

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23. Koziakova M, Harris K, Edge CJ, Franks NP, White IL, Dickinson R. Noble gas neuroprotection: xenon and argon protect against hypoxic-ischaemic injury in rat hippocampus in vitro via distinct mechanisms. Br J Anaesth 2019; 123: 601e9

Christopher J. Edge1,2 and Robert Dickinson3,4,* 

1 Department of Life Sciences, Imperial College London, London, UK, 

2 Department of Anaesthetics, Royal Berkshire Hospital NHS Foundation Trust, Reading, UK, 

3 Anaesthetics, Pain Medicine and Intensive Care Section, Department of Surgery and Cancer, London, UK and 

4 Royal British Legion Centre for Blast Injury Studies, Department of Bioengineering, Imperial College London, London, UK