Elsevier

Nuclear Medicine and Biology

Volume 66, November 2018, Pages 49-57
Nuclear Medicine and Biology

Dynamic TSPO-PET for assessing early effects of cerebral hypoxia and resuscitation in new born pigs

https://doi.org/10.1016/j.nucmedbio.2018.08.004Get rights and content

Abstract

Introduction

Inflammation associated with microglial activation may be an early prognostic indicator of perinatal hypoxic ischemic injury, where translocator protein (TSPO) is a known inflammatory biomarker. This piglet study used dynamic TSPO-PET with [18F]GE180 to evaluate if microglial activation after global perinatal hypoxic injury could be detected.

Methods

New born anesthetized pigs (n = 14) underwent hypoxia with fraction of inspired oxygen (FiO2)0.08 until base excess −20 mmol/L and/or a mean arterial blood pressure decrease to 20 mm Hg, followed by resuscitation with FiO2 0.21 or 1.0. Three piglets served as controls and one had intracranial injection of lipopolysaccharide (LPS). Whole body [18F]GE180 Positron emission tomography-computed tomography (PET-CT) was performed repeatedly up to 32 h after hypoxia and resuscitation. Volumes of interest were traced in the basal ganglia, cerebellum and liver using MRI as anatomic correlation. Standardized uptake values (SUVs) were measured at baseline and four time-points, quantifying microglial activity over time. Statistical analysis used Mann Whitney- and Wilcoxon rank test with significance value set to p < 0.05.

Results

At baseline (n = 5), mean SUVs ±1 standard deviation were 0.43 ± 0.10 and 1.71 ± 0.62 in brain and liver respectively without significant increase after hypoxia at the four time-points (n = 5–13/time point). Succeeding LPS injection, SUV increased 80% from baseline values.

Conclusions

Cerebral inflammatory response caused by severe asphyxia was not possible to detect with [18F]GE180 PET CT the first 32 h after hypoxia and only sparse hepatic uptake was revealed.

Advances in knowledge

Early microglial activation as indicator of perinatal hypoxic ischemic injury was not detectable by TSPO-PET with [18F]GE180.

Implications for patient care

TSPO-PET with [18F]GE180 might not be suitable for early detection of perinatal hypoxic ischemic brain injury.

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.

References (51)

  • S. Saikali et al.

    A three-dimensional digital segmented and deformable brain atlas of the domestic pig

    J Neurosci Methods

    (2010)
  • D. O'Shea et al.

    Exploration of the structure-activity relationship of a novel tetracyclic class of TSPO ligands-potential novel positron emitting tomography imaging agents

    Bioorg Med Chem Lett

    (2013)
  • H. Wadsworth et al.

    [18F]GE-180: a novel fluorine-18 labelled PET tracer for imaging translocator protein 18 kDa (TSPO)

    Bioorg Med Chem Lett

    (2012)
  • M.C. Petit-Taboue et al.

    Brain kinetics and specific binding of [11C]PK 11195 to omega 3 sites in baboons: positron emission tomography study

    Eur J Pharmacol

    (1991)
  • B. Begni et al.

    Neuroligand binding endophenotypes in blood cells distinguish two subsets of borderline personality disorder patients

    Neurosci Lett

    (2009)
  • S. Kannan et al.

    Applications of positron emission tomography in the newborn nursery

    Semin Perinatol

    (2010)
  • J.S. Wyatt et al.

    Determinants of outcomes after head cooling for neonatal encephalopathy

    Pediatrics

    (2007)
  • A.C. Lee et al.

    Intrapartum-related neonatal encephalopathy incidence and impairment at regional and global levels for 2010 with trends from 1990

    Pediatr Res

    (2013)
  • M. Vento et al.

    Room-air resuscitation causes less damage to heart and kidney than 100% oxygen

    Am J Respir Crit Care Med

    (2005)
  • O.D. Saugstad et al.

    Resuscitation of newborn infants with 21% or 100% oxygen: an updated systematic review and meta-analysis

    Neonatology

    (2008)
  • R. Solberg et al.

    Resuscitation of hypoxic newborn piglets with oxygen induces a dose-dependent increase in markers of oxidation

    Pediatr Res

    (2007)
  • B.H. Munkeby et al.

    Resuscitation of hypoxic piglets with 100% o2 increases pulmonary metalloproteinases and IL-8

    Pediatr Res

    (2005)
  • J.M. Perlman et al.

    Part 7: neonatal resuscitation: 2015 international consensus on cardiopulmonary resuscitation and emergency cardiovascular care science with treatment recommendations

    Circulation

    (2015)
  • D.V. Azzopardi et al.

    Moderate hypothermia to treat perinatal asphyxial encephalopathy

    N Engl J Med

    (2009)
  • D. Azzopardi et al.

    Effects of hypothermia for perinatal asphyxia on childhood outcomes

    N Engl J Med

    (2014)
  • Cited by (5)

    • TSPO imaging in animal models of brain diseases

      2021, European Journal of Nuclear Medicine and Molecular Imaging
    View full text