Acute crush injury to the spinal cord is immediately followed by secondary tissue damage, linked to massive release of ATP and activation of high-affinity P2X7 receptors. Researchers at the University of Rochester Medical Center have previously shown that intraspinal injection of the P2X7 antagonist, adenosine 5′-triphosphate-2′,3′-dialdehyde (OxATP), improves outcomes of spinal injury in rats, but intraspinal injection – together with cardiovascular toxicity – makes this treatment unattractive for human trauma victims. In a new study, the team have shown that outcomes in rats can also be improved by systemic treatment with a different P2X7 antagonist, the Coomassie dye, Brilliant Blue G.
Writing in the journal PNAS, they show that iv administration of Brilliant Blue G (10 or 50 mg/kg) 15 minutes after injury, and for three consecutive days, protected spinal cord neurons from purinergic excitotoxicity and also reduced local inflammatory responses, resulting in reduced spinal cord anatomic damage and improved motor recovery. After 6 weeks, treated animals recovered sufficiently to walk with a limp whereas untreated animals did not walk again. Although it seems unlikely that Brilliant Blue G would efficiently cross the intact blood brain barrier, the dye was found to accumulate in the lesions in the injured animals.
There is currently no effective treatment to prevent secondary damage in patients with acute spinal cord injury and the team hope that their work will lead to safe practical treatments that could be administered soon after injury to improve outcomes for spinal injury victims.
Brilliant Blue G is a noncompetitive inhibitor of rat and human P2X7 receptors with IC50 values of 10 and 200 nM respectively.
The team chose to use Brilliant Blue G for their experiments because they saw structural and functional similarities with a food additive, FD&C blue dye No 1 (E133), used in a variety of processed foods and generally considered to be safe. A number of groups have now designed selective P2X7 antagonists – some of which have entered the clinic – and it would be interesting to see the effect of these newer compounds in the rat spinal injury model. Because of the differing affinities of antagonists for rat and human receptors, care will be needed in the choice of appropriate molecules for study, and in extrapolation of results from rodents to humans.