By Ransdell Pierson and Bill Berkrot
Dallas (Reuters) - An experimental antidote to the widely used blood clot preventer Pradaxa worked immediately and completely in an early-stage trial among healthy volunteers, raising hopes that the drug's blood-thinning effects can be reversed in emergency situations.
"These are absolutely exciting findings," said Dr. Stephan Glund, a research executive of privately held Boehringer Ingelheim Pharmaceuticals, the German drugmaker that developed Pradaxa and is testing the antidote.
Pradaxa, also known as dabigatran, was approved in 2010 to prevent strokes in patients with atrial fibrillation, an irregular heartbeat that affects more than 2.5 million American adults and which raises the risk of stroke fivefold. It works by blocking thrombin, a blood enzyme involved with clotting.
Pradaxa was the first in a new crop of blood clot and stroke-prevention drugs meant to replace warfarin, a half-century-old pill that requires frequent blood monitoring, carries serious bleeding risks and has dietary and lifestyle restrictions.
Pradaxa and two newer agents that work differently from the Boehringer drug - Xarelto from Johnson & Johnson and Bayer AG, and Eliquis from Bristol-Myers Squibb Co and Pfizer Inc - are all at least as effective as warfarin in preventing stroke and are easier to manage.
But unlike warfarin, none of them have antidotes to stop or prevent dangerous bleeding, such as after a car accident or a fall, or before emergency surgery. The absence of antidotes has deterred some doctors from prescribing the newer drugs.
But researchers on Monday said data from a Phase 1 study of Pradaxa involving almost 150 healthy volunteers suggest a safe and effective antidote to Pradaxa may be within reach in coming years.
The antidote is a fragment of a monoclonal antibody, a type of protein made in living cells, meant to mop up Pradaxa circulating in the bloodstream.
Volunteers received the antidote through 5-minute intravenous infusions after taking Pradaxa for four days.
"At the end of the infusion we observed an immediate and complete reversal" of Pradaxa in terms of the drug's anti-coagulation effect, Boehringer's Glund said. "It binds (Pradaxa) and prevents it from having any further action."
Moreover, he said no serious side effects were seen with the antidote. "It has a very positive safety profile."
The data were presented at the annual scientific sessions of the American Heart Association in Dallas.
Glund said more and bigger trials will be needed to prove the antidote's safety and effectiveness. He declined to speculate when the product might come to market.
Many doctors attending the five-day Dallas meeting said any approved antidotes would bolster confidence in the use of Pradaxa, Xarelto, Eliquis and future similar stroke-prevention drugs.
"It's always easier to prescribe a therapy if you feel you have a bailout strategy," said Dr. Jonathan Piccini, a cardiologist with Duke University Medical Center who was not involved in the Pradaxa study.
Eliquis and Xarelto work by blocking a clotting protein called Factor Xa. Portola Pharmaceuticals Inc is attempting to develop effective antidotes to Eliquis and Xarelto.
Portola in May said its experimental antidote, called PRT4445, caused a rapid and sustained reversal of the anticoagulant effects of Eliquis. The mid-stage trial also involved healthy volunteers.
(Reporting by Ransdell Pierson and Bill Berkrot; Editing by John Wallace)