Defining the optimal temperature for newborn hypothermic neuroprotection and effect of sodium/proton exchanger blockade

Brain damage due to a transient shortage of oxygen and blood supply during birth leads to death or severe permanent disability in around 1-2 per 1000 infants born in the UK, and is a major cause of childhood disability around the world. Until recently no treatment given after birth has been shown to improve the outcome, but evidence is now accumulating that mild cooling may be an effective protective treatment. In addition, new research suggests that the brain alkalosis that occurs immediately after HI is damaging to the brain and that mild acidosis and prevention of the alkaline overshoot reduces brain damage. Amiloride, a drug used to treat water imbalance in patients, maintains mild brain acidosis and prevents the alkaline overshoot. We propose to study 2 questions:

(1) What is the optimal temperature for neuroprotection and is it the same for moderate and severe HI insults? We will compare delayed whole body cooling to either 33oC or 35oC from 2-26 hours after resuscitation.

(2) Does amiloride administered immediately after HI reduce the extent of brain injury and is this effect additive when hypothermia is also applied?

A well-characterised experimental animal model of brain injury will be used and the changes in brain energy levels and number of cells dying at 48 hours will be compared. These results will guide the clinical application of cooling and one other neuroprotective therapy in newborn infants, ensuring that treatment is applied in the most effective manner and tailored to the individual needs of each patient. Translating the results from this study to the clinical setting is likely to improve neurodevelopmental outcome in a large number of infants worldwide.

Neonatal encephalopathy (NE) secondary to intrapartum hypoxia-ischaemia (HI) is an important cause of neurologic injury at birth, occurring in 1-2 per 1000 term live births. Recently results from 3 clinical trials of hypothermia suggest that brain injury may be ameliorated in some infants with NE. Fundamental questions remain, however, about the optimal cooling temperature and whether this differs in different patterns of brain injury. Furthermore, as optimal neuroprotection is likely to require a combination of agents affecting different aspects of the neurotoxic cascade, it is important to assess the potential benefit of adding other neuroprotective agents to cooling.

Questions: (1) What is the optimal temperature for newborn neuroprotection and is this the same for both moderate and severe HI brain injuries? Based on work leading to this proposal, I wish to test the hypothesis that delayed systemic cooling to 35oC provides better neuroprotection in deep grey matter (particularly affected in severe HI) than 33oC whereas delayed systemic cooling to 33oC provides better neuroprotection in cortical grey matter (particularly affected in moderate HI) than 35oC.

(2) Is sodium/proton exchanger (NHE) blockade using amiloride immediately after HI neuroprotective and is this effect synergistic with delayed systemic hypothermia?

Methods: A validated neonatal piglet model of HI and secondary energy failure, with continuous acquisition of quantitative magnetic resonance (MR) imaging and proton (1H) and phosphorus-31 (31P) spectroscopic imaging data at 7T over 48-60 hours. 90 piglets randomised to: (i) normothermia (n=15); (ii) whole body cooling (T 35oC) (n=15); (iii) whole body cooling (T 33oC) (n=15); (iv) amiloride analogue and normothermia (n=15); (v) amiloride analogue and whole body cooling (T 35oC); (vi) amiloride analogue and whole body cooling (T 33oC) (n=15). Stratified randomisation will create a balance of moderate and severe HI insults. Cooling will extend from 2-26 hours following resuscitation in groups ii, iii, v, and vi. The effects of cooling, amiloride and cooling plus amiloride will be compared with no intervention by comparing (i) brain energetics over 48 hours; and (ii) brain regional histopathology.

Expected outcome: This study will guide appropriate implementation of hypothermic neuroprotection in infants with NE, allowing intervention to be tailored to the specfic requirements of each infant. This study will provide data on the possible neuroprotective effect of amiloride analogues immediately after HI. Amiloride is already used clinically; if proven to be beneficial following transient HI it may be possible to translate this intervention rapidly into the clinical arena.