Waiting six hours to give stroke victims an agent derived from snake venom is too late, European researchers have found.

The drug ancrod is purified from venom of the Malaysian pit viper. It is thought to act on the blood's ability to clot, and a previous trial suggested it helps ischemic stroke victims if given within a three-hour window.

Ischemic stroke — which accounts for about 80 per cent of cases — is caused by interruption of blood supply to the brain.

In the European Stroke Treatment with Ancrod Trial or ESTAT, neurologist Dr. Michael Hennerici of the University of Heidelberg in Mannheim, Germany, and his colleagues randomly assigned 1,222 patients to receive ancrod or a placebo within six hours.

Neurological recovery was worse and there were more hemorrhages in the ancrod group than in the placebo group, the team reports in the Nov. 25 issue of the medical journal The Lancet.

After three months, mortality was also higher in the ancrod group, but there was no significant difference at 12 months.

"On the basis of our findings, ancrod should not be recommended for use in acute ischemic stroke beyond three hours," the researchers concluded.

Brain imaging may identify candidates

Most patients who were included after three hours of the onset of stroke did not receive brain imaging tests.

Using other vascular and brain-tissue imaging techniques might help in identifying patients who gain greater benefit and are at reduced risk of taking the therapy after three hours, the researchers said.

"Although the study was unsuccessful, it delivers an important message: that time from onset of symptoms to treatment matters, and in ESTAT it was too long," Markku Kaste of the University of Helsinki wrote in an accompanying commentary.

"The study also highlights how important it is to publish unsuccessful trials. Such trials often include valuable pieces of information, which can guide future studies. Furthermore, it is unethical not to publish the results of all well-designed and executed trials just because they fail to reach their targets."

Early treatment of stroke patients "requires continuous education, practice in training of the whole chain in stroke care," Kaste said.

In Canada and Europe, ancrod has been used to treat vascular disorders such as deep-vein thrombosis.

Stroke is the third-largest cause of death in the United States and Canada after coronary heart disease and all forms of cancer.