Dynamic TSPO-PET for assessing early effects of cerebral hypoxia and resuscitation in new born pigs
Introduction
Intrapartum events or birth asphyxia represents an important global cause of mortality in children and is associated with high morbidity, mainly from long-lasting neurological deficits and cognitive impairment [[1], [2], [3], [4], [5]]. During the past years, neonatal care has developed and improved considerably, but the prevalence of hypoxic ischemic encephalopathy (HIE) in the Western countries has not changed accordingly [6]. Resuscitation after birth asphyxia is often needed and ventilator support is one of the initial steps in the reanimation. International guidelines now recommend to start resuscitation with air in term or near term new born infants as high-level oxygen has proven harmful to the brain and other organs with potential negative effect on survival [[7], [8], [9], [10], [11], [12]]. The provision of severity of hypoxic ischemic injury (HI) and prediction of future handicap is important to establish and decide for early interventions. Therapeutic options are still limited where avoiding high-level oxygen seems to be an efficient tool to reduce HIE [13,14] but for the time being, hypothermia is the only established treatment [3,[15], [16], [17]]. Still there is a need for other neuroprotective therapies, and several are presently being investigated [18,19].
Clinical grading and characteristics are not reliable to predict prognosis and other tools to select individuals who might benefit from active treatment is warranted.
Neuroimaging techniques are used to investigate cerebral HI, where magnetic resonance imaging (MRI) and ultrasonography (US) are considered the most important for diagnosis and prognosis [[20], [21], [22], [23]]. Positron emission tomography (PET) imaging technologies are not commonly used. However, by the use of different PET radioligands, biological and pathological processes such as neuroinflammation, can be assessed by qualitative and quantitative means.
Tissue damage after hypoxia-ischemia follows upon an interruption of cerebral blood flow and oxygen delivery to the brain and induces a cascade of deleterious biochemical events at different molecular levels. An important factor is the early inflammatory response elicited by activation of microglia in the brain [24]. Activated microglial cells express the 18 kDa translocator protein, TSPO (formerly known as the peripheral benzodiazepine receptor, PBR) that has shown to be a sensitive marker to visualize and measure glial cell activation in various neuro-inflammatory disorders. Activated microglia/macrophages upregulate the expression of TSPO, which can be depicted in vivo by selective PET TSPO radioligands, of which [11C]PK11195 has been most studied [25]. New generation TSPO radioligands with higher performance, providing improved signal to-noise ratio (SNR) have renewed the interest in TSPO as a neuroinflammatory biomarker. Flutriciclamide, [18F]GE180 is a recently developed third-generation TSPO radioligand. Until now, there are a few publications showing promising results with increased specific binding with [18F]GE180 compared to [11C]PK11195 in rat studies after injection of Lipopolysacharide (LPS), after cerebral ischemia and in a model of stroke in rats [[26], [27], [28], [29]]. Recently, Fan et al. and Feeney et al. published human PET studies using [18F]GE180 in healthy subjects [30,31].
Studies on detecting tissue at risk of permanent injury early after perinatal hypoxia with metabolic and inflammatory stress, in new born animals and human babies are currently lacking as well as the influence of high-level oxygen versus room air resuscitation modes on microglial activation.
The main aim of our study was to explore if [18F]GE180 could detect immediate TSPO activation in the brain after experimental global hypoxia, using a clinical positron emission tomography-computed tomography (PET-CT) scanner on a new born piglet model. Secondary aims were to find the optimal time-point after [18F]GE180 injection to detect microglial activation in this preclinical model and evaluate the impact of oxygen concentration in the resuscitation air.
Section snippets
Methods
This study included 18 new born Noroc (LYxLD) pigs. The inclusion criteria being an age of 12–36 h, B-haemoglobin values >5 g/100 mL, and good general condition. The piglets were included within a study period of 11 weeks.
Cohort characterization during the experiment
The summary of the cohort characteristics before hypoxia, end hypoxia, end resuscitation, 4 h after end resuscitation, 24 h after end resuscitation and at the end of the experiment is given in Table 1. There were no statistical differences between the groups at start hypoxia. At end-hypoxia the two hypoxia-exposed groups showed equal changes, but at 30 min resuscitation, the FiO2 1.0 group had significant lower BE and pH indicating a slower recovery after hypoxia-ischemia than the piglets in
Discussion
In this study, we have performed dynamic [18F]GE180 PET aiming to explore if early inflammatory response after severe hypoxia in the brain and liver of new born pigs could be detected.
Even though different time points after hypoxia and also multiple time points after [18F]GE180 injection were analyzed, we found that global hypoxia did not cause any changes in [18F]GE180 uptake in the brain and no increase in activity in the liver in our piglet model.
Previous studies looking at TSPO activation
Conclusion
In this feasibility study using an established piglet model, global hypoxia did not cause any significant changes in cerebral microglial activation as measured by [18F]GE180 and consequently no significant influence of hyperoxic resuscitation in the early post hypoxic period and up to 32 h after hypoxia, thus reaching beyond the period of secondary energy failure. [18F]GE180 PET seems not to be the preferable technique for detecting an inflammatory response after perinatal hypoxia as simulated
Funding
The study was funded by grants from Vestfold Hospital Trust, and from Div. of Radiology and Nuclear Medicine (grant number 36701) and Dept. of Paediatric Research, Oslo University Hospital. GE Healthcare provided the [18F]GE180.
Conflict of interest
The authors declare that they have no conflict of interest.
Ethics approval
The experimental protocol was approved by the Norwegian Council for Animal Research (Approval Number 6623, August 2014). All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
Acknowledgements
We thank the bioengineers and radiographers at the PET center, Ullevaal, Oslo University Hospital, for providing the technical assistance during scanning. We would also like to thank GE Healthcare Norway, for providing the tracer [18F]GE180.
We are grateful to Senior Medical Photographer Øystein H. Horgmo, at the Medical Photography Section, Institute of Clinical Medicine and the library staff at the University of Oslo Medical Library for the contribution to Figure 1.
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