A new clot-buster: vampire bat saliva
Every minute counts when someone suffers a stroke. Yet because symptoms are often subtle, most victims arrive at the hospital too late to use medication to dissolve the blood clots that cause most of the brain attacks. As a consequence, thousands of victims suffer severe brain damage or even die.
Bat saliva could hold the key to saving them.
An experimental drug based on a vampire bat protein has shown promise in clearing away clots up to several hours after a stroke. Such a drug could not only save more lives, it could also reduce the incidence of permanent disability.
“Because of the prolonged time window, this could be a significant advance that could potentially benefit many more patients,” says Dr. Mark Alberts, a neurologist and director of the stroke program at Northwestern Memorial Hospital in Chicago.
About 88% of strokes occur when a blood clot or a narrowing of blood vessels prevents blood from getting to the brain. Deprived of needed oxygen and nutrients, brain cells die off quickly.
Drugs that break up these clots can halt this process and preserve brain function. However, the only approved clot-busting medication, TPA (tissue plasminogen activator), has serious drawbacks.
TPA must be used within three hours after the onset of symptoms (such as weakness, vision changes and slurred speech) but most stroke victims delay getting treatment, and fewer than 4% get to the ER within that time. It must also be dispensed through an intravenous drip, and can take up to an hour to dissolve larger clots. And because TPA causes cerebral hemorrhages in 6% of patients, it can’t be given to people prone to bleeding.
In contrast, the new drug, Desmoteplase, lasts longer, is easier to administer and doesn’t seem to activate certain proteins in the brain that boost bleeding risks. It’s a genetically engineered version of a protein in the saliva of the vampire bat. (Bats can take up to 30 minutes to feed on their prey, so they secrete a powerful chemical that thins the blood and prevents the formation of blood clots. This ensures a steady flow of blood from the puncture wound.)
The injectable medication can be used up to nine hours after the onset of symptoms, and studies indicate the hemorrhage rate is as low as 2%. “With a nine-hour therapeutic window, we could treat perhaps 20% to 25% of all stroke victims,” says Dr. Anthony Furlan, a stroke expert at the Cleveland Clinic in Ohio and the lead investigator on a recent clinical trial of Desmoteplase.
The results of the study were encouraging. It involved 38 stroke victims who didn’t get to the hospital within the three-hour time limit for TPA.
They were screened with special MRI tests to determine if the brain cells could still be salvaged.
“Certain MRI techniques allow us to tell within an hour of the stroke what part of the brain is dead or if it is still treatable,” Furlan says.
Eligible volunteers received a high or low dose of Desmoteplase, or a placebo. Blood flow was restored in more than half of patients receiving the drug’s highest dose, compared to 38% in the placebo group.
Three months after treatment, as many as 60% of patients who got Desmoteplase were able to live independently, compared to 23% who had a dummy shot.
A larger trial that will eventually encompass 170 patients was launched recently. If all goes well, Desmoteplase could be added to the therapeutic arsenal within five years. “TPA is not a home run,” says Dr. Stuart A. Lipton, scientific director of the Burnham Center for Neuroscience and Aging in La Jolla. Desmoteplase may be better, he says, and cause fewer side effects.
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Limiting the damage
Other drugs also could prevent some of the brain damage and disability caused by stroke.
The experimental medication Cerovive, which could be on the market by 2007, protects the brain by mopping up the toxic molecules, known as free radicals, that are produced when brain cells are starved of vital nutrients. Although Cerovive won’t save brain cells that die from lack of oxygen near a clot, it can limit the brain damage that occurs after a stroke when these highly reactive chemicals are released.
Existing medications, such as Lipitor and Zocor, which are currently used to control cholesterol, and the Alzheimer’s drug memantine may also help.
“We need a good cocktail of drugs to treat stroke,” says Dr. Stuart Lipton.
