FDA approves new type of blood thinner for hip, knee surgery patients

Monday, December 10th 2001, 12:00 am

By: News On 6

WASHINGTON (AP) _ The government approved a new type of blood thinner Monday that generated doctor excitement when studies suggested it cut the risk of blood clots after hip and knee surgery far more than standard therapy.

But in approving Arixtra, the Food and Drug Administration warned that it may not really be superior to standard therapy because of a quirk in how those studies were performed.

Plus, it cannot be given to some patients _ those with serious kidney failure or who weigh less than 110 pounds _ because it could cause dangerous bleeding.

Blood clots in the deep veins of the legs are a common and severe threat after hip replacement, knee replacement or a hip fracture. The clots can kill if they break off and travel through the bloodstream to the lungs.

To prevent this condition, called deep-vein thrombosis, those patients always are given blood thinners after surgery, typically heparin or newer ``low molecular-weight heparins'' that sell under various brand names, including Lovenox.

Arixtra is the first synthetic option. It was made by culling one portion of the heparin molecule, a piece that inhibits activated Factor X, a key component in causing blood clots.

In manufacturer-sponsored clinical trials of more than 4,000 patients, those given Arixtra were 50 percent less likely to suffer a clot than those given Lovenox.

``I've never, in 40 years of clinical trial experience, seen such a big benefit, particularly against an active and very effective drug,'' said Dr. Alexander G.G. Turpie of McMaster University in Hamilton, Ontario, one of the lead researchers.

But here's the quirk: In those studies, patients were injected with Arixtra six hours after surgery, while the competing Lovenox was not given for 18 hours, said FDA's Dr. Victor Raczkowski. That is the way Lovenox is generally used today, he said _ but had both drugs been given at the earlier time, they might have worked equally well.

That means the FDA won't call Arixtra superior to today's top treatment.

Like all anticoagulants, Arixtra can cause serious bleeding. But in the studies, the patients most at risk for this side effect were those with seriously impaired kidney function or who weigh less than 110 pounds. The FDA ordered that Arixtra not be used in those patients.

Risk also seemed to increase with age. Add these three risk factors together, and it turns out some of the very frail elderly women who are most at risk of hip fractures shouldn't be given Arixtra, Raczkowski acknowledged.

The drug takes hours longer to be cleared by the body than competing anticoagulants, and it only comes in a pre-filled syringe with one dose _ so doctors cannot simply give smaller or sicker people a lower dose, he said.

Arixtra, known chemically as fondaparinux, will be available starting in late January, said manufacturer Organon Sanofi-Synthelabo. The company refused to disclose the price.