Experimental Drug Derived From Snake Venom Shows Promise In Reversing Effects Of Stroke

Scientists announced the results of a 500-person trial examining the effects of an experimental stroke drug called ANCROD here today at the 24th American Heart Association International Conference on Stroke and Cerebral Circulation.

Researchers found that 42 percent of stroke victims treated with ANCROD within three hours of stroke onset had recovered the physical and mental abilities that they had prior to the stroke.

In comparison, 34 percent of those receiving an inert substance, or placebo, regained their previous faculties. However, stroke patients who received ANCROD were also at a higher risk of bleeding in the brain (5 percent of patients) when compared to those who received the placebo (2 percent).

ANCROD is not yet approved by the Food and Drug Administration for stroke treatment. Tissue plasminogen activator (TPA) is the only FDA-approved acute stroke treatment.

ANCROD is derived from the venom of a pit viper snake. Researchers discovered that in people who were bitten by the snake, blood failed to clot. Based on that observation, the venom was utilized as an anticoagulant, helping blood flow more freely through vessels.

"Like clot-dissolving drugs TPA and urokinase, the goal was for ANCROD to be administered within three hours of stroke onset," says the study's lead author, David Sherman, M.D. "Also, none of these three drugs are for use in patients who have hemorrhagic strokes, caused by a burst or leaking blood vessel in the brain."

There are sharp contrasts between the treatments, however, says Sherman.

TPA is administered in a single-dose, hour-long injection and results can be seen soon thereafter. ANCROD is given intravenously through a catheter over a 3-to-5 day span in the hospital. While TPA dissolves the clots causing the most common type of stroke, ANCROD lowers the level of fibrinogen in the bloodstream.

Fibrinogen is a clotting factor that is produced naturally by the body. It can help form clots that can in turn block blood flow to the brain, resulting in a stroke. ANCROD improves the bloods viscosity, a measurement of how well it flows through blood vessels.

"We found that patients who had their fibrinogen levels lowered promptly, within 6 hours, and maintained in the target range after treatment had the best response to the treatment," says Sherman, professor and chief, division of neurology, University of Texas Health Science Center, San Antonio.

The initial amount of ANCROD administered is determined by the patients blood fibrinogen level and body weight.

The additional amounts of ANCROD administered is adjusted based on the amount of fibrinogen in the blood, which is regularly monitored for 3-to-5 days during hospitalization, according to Sherman. By keeping the fibrinogen levels between 40-70 milligrams per deciliter of blood, Sherman says it's possible to reverse the effects of the initial stroke, minimize the bleeding risk and protect against a recurrent stroke during therapy.

"What I envision if ANCROD is approved is that the physician would have the option of using either TPA or ANCROD, based on what would be best for that particular patient," says Sherman. "Overall, TPA has been shown to be a little bit more effective than ANCROD, but it also carries a little bit more danger with the possibility of bleeding in the brain. It's also easier to use -- one shot, one hour and youre done."

"With ANCROD, you're treating someone for 3 to 5 days and making sure their fibrinogen levels dont get too low or too high. There's more of a user-friendliness with ANCROD in the sense that youre able to monitor the patient more closely as you move forward with the treatment."

In the study, patients were treated between 3-to-5 days and then followed up at 3 months post-stroke; their progress was measured by various tests to determine neurological damage and ability to function in a normal manner.

"We think that its probably going to turn out to be safer than TPA in terms of risk of bleeding," says Sherman. "With the new therapy, the effects of the drug are sustained for five days instead of an hour or two, without increasing the bleeding risk over time."

The major risk in using clot-busting drugs is bleeding, or hemorrhage, in the brain.

Sherman says that ANCROD should not be considered a clot-busting therapy like TPA or urokinase, even though it does have similar properties.

"It's given through an IV infusion, so in that sense its similar to TPA," says Sherman. "TPA is usually an hour-long infusion while ANCROD is continuous over three days so you dont have the dramatic changes in coagulation that you can get with other types of clot-dissolving therapy."