Clot buster attached to red blood cells avoids complications.

When a person is rushed to an emergency room with a heart attack or stroke, doctors often prescribe immediate infusions of tissue-type plasminogen activator (tPA). The drug can dissolve clots blocking blood flow to the heart or brain. But because tPA indiscriminately attacks clots throughout the body, it can damage older clots that had repaired blood vessels. In those eases, it can cause internal bleeding.

Researchers now report that attaching tPA to red blood cells in mice and rats reduces this problem by concentrating the drug's clot-busting efforts on newly formed, troublesome dots.

"After we couple tPA to red blood cells, the drug lasts longer and becomes much safer," says Vladimir Muzykantov, of the University of Pennsylvania in Philadelphia. In people, the drug might reduce the amount of tPA that diffuses from blood vessels into the brains of stroke victims, where it can cause neurological complications, Muzykantov says. His group's study appears in the August Nature Biotechnology.

The drug might also prove helpful to people recovering from surgery. Such patients need to retain blood coagulation capabilities for healing, yet because they're immobile, they're prone to forming harmful clots.

It remains unclear how long the tPA-red blood cell combination would stay in circulation. The animal experiments lasted only a few hours, but they suggest that the combination outlasts unattached tPA, Muzykantov says.--N.S.