Treatment with inhaled Argon: a systematic review of pre-clinical and clinical studies with meta-analysis on neuroprotective effect

Abstract

Background: Argon (Ar) has been proposed as a potential therapeutic agent in multiple clinical conditions, specifically in organ protection. However, conflicting data on pre-clinical models, together with a great variability in Ar administration protocols and outcome assessments, have been reported. The aim of this study was to review evidence on treatment with Ar, with an extensive investigation on its neuroprotective effect, and to summarise all tested administration protocols.

Methods: Using the PubMed database, all existing pre-clinical and clinical studies on the treatment with Ar were systematically reviewed (registration: https://doi.org/10.17605/OSF.IO/7983D). Study titles and abstracts were screened, extracting data from relevant studies post full-text review. Exclusion criteria included absence of full text and non-English language. Furthermore, meta-analysis was also performed to assess Ar potential as neuroprotectant agent in different clinical conditions: cardiac arrest, traumatic brain injury, ischemic stroke, perinatal hypoxic-ischemic encephalopathy, subarachnoid haemorrhage. Standardised mean differences for neurological, cognitive and locomotor, histological, and physiological measures were evaluated, through appropriate tests, clinical, and laboratory variables. In vivo studies were evaluated for risk of bias using the Systematic Review Center for Laboratory Animal Experimentation tool, while in vitro studies underwent assessment with a tool developed by the Office of Health Assessment and Translation.

Findings: The systematic review detected 60 experimental studies (16 in vitro, 7 ex vivo, 31 in vivo, 6 with both in vitro and in vivo) investigating the role of Ar. Only one clinical study was found. Data from six in vitro and nineteen in vivo studies were included in the meta-analyses. In pre-clinical models, Ar administration resulted in improved neurological, cognitive and locomotor, and histological outcomes without any change in physiological parameters (i.e., absence of adverse events).

Interpretation: This systematic review and meta-analysis based on experimental studies supports the neuroprotective effect of Ar, thus providing a rationale for potential translation of Ar treatment in humans. Despite adherence to established guidelines and methodologies, limitations in data availability prevented further analyses to investigate potential sources of heterogeneity due to study design.

Funding: This study was funded in part by Italian Ministry of Health-Current researchIRCCS and by Ministero della Salute Italiano, Ricerca Finalizzata, project no. RF 2019-12371416.

Keywords: Argon; Meta-analysis; Neuroprotection; Noble gas; Organ protection.

Fig. 1

Framework of findings.

Fig. 2

PRISMA flow diagram for literature search and study selection process of the systematic review.

Fig. 3

Analysing the evolution of scientific objectives and published findings on the effects of Ar administration in the pre-clinical studies (in vitro, ex vivo, in vivo). Data are referred to 60 pre-clinical studies (16 in vitro, 7 ex vivo, 31 in vivo, 6 with both in vitro and in vivo). Panel a. Distribution of primary aims regarding Ar administration effects: supportive, opposing, and neutral findings. “Supportive” means that the authors of the study found that Ar administration led to an improved condition in all the evaluated outcomes compared to the control group; “opposing” means that the authors of the study found that Ar administration did not find any benefit associated with Ar administration in any of the analysed outcomes; “neutral” means that the authors of the study found Ar benefits in some of the analysed outcomes and no benefits in the remaining. Of note, all the studies were positive regarding the safety of Ar administration. Panel b. Number of studies on Ar effect according publication year and investigated primary outcome on the effects of Ar administration.

Fig. 4

Treatment protocols of all clinical conditions derived from ischemia reperfusion injury (CA, ischemic stroke, retinal IRI, perinatal HIE, myocardial infarction, SAH). Gas mixture. The pie chart represents the blend composition, in violet the percentage of Ar, in white the percentage of oxygen, in yellow the percentage of nitrogen. Onset timing. Defining temporal parameters: “Early” within one and a half hours pre- or post-injury; “Late” after one and a half hours pre- or post-injury. Duration (time) of administration. The bar chart consists of three segments: the first represents durations of 1 h or less, the second represents durations between 1.5 h and 12 h (inclusive), and the third represents durations between 12 h and 24 h (inclusive). Outcome. The coloured square denotes the study outcome. “Supportive” (green) means that the authors of the study found that Ar administration led to an improved condition in all the evaluated outcomes compared to the control group; “opposing” (red) means that the authors of the study found that Ar administration did not find any benefit associated with Ar administration in any of the analysed outcomes; “neutral” (yellow) means that the authors of the study found Ar benefits in some of the analysed outcomes and no benefits in the remaining. Of note, all the studies were positive regarding the safety of Ar administration.

Fig. 5

Treatment protocols of all clinical conditions not derived from an IRI. Gas mixture. The pie chart represents the blend composition, in violet the percentage of Ar, in white the percentage of oxygen, in yellow the percentage of nitrogen. Onset timing. Defining temporal parameters: “Early” within one and a half hours pre- or post-injury; “Late” after one and a half hours pre- or post-injury. Duration (time) of administration. The bar chart consists of three segments: the first represents durations of 1 h or less, the second represents durations between 1.5 h and 12 h (inclusive), and the third represents durations between 12 h and 24 h (inclusive). Outcome. The coloured square denotes the study outcome. “Supportive” (green) means that the authors of the study found that Ar administration led to an improved condition in all the evaluated outcomes compared to the control group; “opposing” (red) means that the authors of the study found that Ar administration did not find any benefit associated with Ar administration in any of the analysed outcomes; “neutral” (yellow) means that the authors of the study found Ar benefits in some of the analysed outcomes and no benefits in the remaining. Of note, all the studies were positive regarding the safety of Ar administration.

Fig. 6

Forest plot of Ar administration effect on cell protection in vitro studies. Random effects models were used to estimate the pooled estimates for cell protection both overall and by subgroups injury models (OGD and traumatic brain injury). Data are expressed as SMD and 95% CI. Size of each square represents the study weight in the analysis. The diamond represents the pooled effect from the included studies (the width of the diamond represents the 95% CI for the overall effect) both overall and by subgroups injury models. CIs crossing zero (vertical line) indicate inconclusive results regarding the support for or against Ar.

Fig. 7

Forest plots of Ar administration effects on functional domain in pre-clinical studies (in vivo). Random effects models were used to estimate the pooled estimates for neurological outcome (panel a). Three-level nested random effects model was used to estimate the pooled estimates for cognitive and locomotor outcome (panel b). Data are expressed as SMD and 95% CI. Size of each square represents the study weight in the analysis. The diamond represents the overall pooled effect from the included studies (the width of the diamond represents the 95% CI for the overall effect). CIs crossing zero (vertical line) indicate inconclusive results regarding the support for or against Ar.

Fig. 8

Forest plots of Ar administration effects on respiratory exchange outcome in pre-clinical studies (in vivo). Random effects models were used to estimate the pooled estimates for gas exchange parameters, such as arterial partial pressure of oxygen (panel a) and arterial partial pressure of carbon dioxide (panel b). Data are expressed as SMD and 95% CI. Size of each square represents the study weight in the analysis. The diamond represents the overall pooled effect from the included studies (the width of the diamond represents the 95% CI for the overall effect). CIs crossing zero (vertical line) indicate inconclusive results regarding the support for or against Ar.

Fig. 9

Forest plots of Ar administration effects on histological outcomes in pre-clinical studies (in vivo). Three-level nested random effects model was used to estimate the pooled estimates for neurodegeneration (panel a) and inflammation (panel b) outcome. Data are expressed as SMD and 95% CI. Size of each square represents the study weight in the analysis. The diamond represents the overall pooled effect from the included studies (the width of the diamond represents the 95% CI for the overall effect). CIs crossing zero (vertical line) indicate inconclusive results regarding the support for or against Ar.