A substance taken from the saliva of vampire bats can be used for up to three times as long as the currently available stroke treatment with less risk for brain damage, a new study says.
The study was done in mice, and it's not known how effective the treatment will be in humans. Moreover, some experts believe the emphasis on a longer time window obscures the necessity of treating stroke victims as quickly as possible."Even if this drug is better than the current one, you still need to get people treated very quickly. This does not mean that you now can wait longer," says Dr. Keith Siller, medical director of New York University's Comprehensive Stroke Care and Research Center. "You have a limited time window to restore blood to the brain and that window doesn't change, no matter what you give."
An ischemic stroke happens when one or more blood clots block blood supply to the brain.
Right now, about the only immediate hope for a person suffering this type of stroke is recombinant tissue plasminogen activator, or t-PA. Although t-PA is the only FDA-approved treatment for ischemic stroke, it carries an increased risk of cerebral hemorrhage (bleeding in the brain) and it may cause brain cell death, say the study authors.
The report appears in tomorrow's issue of Stroke.
"One of the problems with thrombolytic [clot-busting] agents is they're fairly nonspecific. They have effects throughout the circulation whether there's a clot there or not," says Dr. David Sherman, a professor of medicine and neurology at the University of Texas Health Science Center at San Antonio who has investigated the blood-clotting properties of the venom of the Malaysian pit viper. "So the focus has been on getting more and more specific and refined thrombolytic agents because what you would like in the best of all possible worlds is something that only affects that clot and doesn't injure the vessels."
Scientists believe they may have a candidate in the new compound extracted from vampire bat saliva, called Desmodus rotundus salivary plasminogen activator (DSPA), or desmoteplase.
DSPA takes advantage of vampire feeding principles and tries to make them work in humans -- without leaving puncture marks in the patient's neck.
When vampire bats bite into their victims, they release a clot-dissolving substance that keeps the blood flowing long enough to suck a full meal. Without the clot-buster, the victim's blood would dry up, leaving the bat hungry and, presumably, frustrated.
In this study, the investigators injected either DSPA or t-PA into the brains of mice, then watched to see how the brain cells survived.
Like t-PA, DSPA activates an enzyme called plasminogen, which destroys fibrin, the stuff of which blood clots are made. DSPA, however, did not have the brain damage associated with t-PA. "DSPA did not activate the receptors associated with brain cell death, whereas t-PA did," reports study author Dr. Robert L. Medcalf, a senior research fellow at Monash University Department of Medicine at Box Hill Hospital in Victoria, Australia.
This is because DSPA appears to become active only in the presence of fibrin, the "building blocks" of a clot. In fact, DSPA's clot-busting properties increase about 13,000-fold in the presence of fibrin, whereas t-PA's only increase by a factor of 72. "DSPA is almost inactive in the absence of fibrin and therefore much more fibrin-specific than t-PA," Medcalf says.
This is important because blood clots and, therefore, fibrin are not actually located in the brain, so DSPA has no effect in this vulnerable region.
"Theoretically, it seems like DSPA is much more specific in terms of its potential beneficial effects in restoring circulation and it doesn't have the downside of the injury to the brain," Sherman explains.
According to the authors, DSPA can be administered up to nine hours after the onset of a stroke. T-PA can only be given for up to three hours.
Granted, this study looked only at mice, but DSPA is currently being tested (by some of the same investigators) in human stroke patients in Europe, Asia and Australia.
"It's a nice idea but what it still comes down to is you have to get the people in as soon as possible," Siller says. "The overwhelming key factor is treating as soon as possible and the conclusion should not be that using this drug allows you to do this after three hours. It may be more powerful or safer to use but the time window is still the same."
What To Do
For more on ischemic stroke, visit the National Stroke Association.