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Wed Mar 19, 2008 1:45pm EDT

By Lisa Richwine

WASHINGTON (Reuters) - Health regulators said on Wednesday they have identified a contaminant found in batches of Baxter International Inc's blood-thinner heparin that were linked to serious reactions and deaths.

Food and Drug Administration officials said they are trying to determine whether the chemical -- over-sulfated chondroitin sulfate -- was purposely or inadvertently added during manufacturing in China.

"We don't know whether the contaminant was introduced intentionally or by accident," said Dr. Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research.

Chondroitin sulfate is widely sold as a dietary supplement to treat joint pain. The over-sulfated version is not known to occur naturally and therefore likely was chemically modified, Woodcock told reporters on a conference call.

It is still unclear whether the contaminant is the cause of up to 19 deaths and hundreds of serious breathing problems and other reactions reported to the FDA, she said.

The substance mimics the blood-thinning actions of heparin but is not FDA-approved for use in medicines, Woodcock said. In some batches, the contaminant accounted for between 2 percent and 50 percent of the samples, she said.

FDA officials said chemically manipulating chondroitin sulfate, which is widely available from animal sources, would be cheaper than getting raw heparin from pig intestines.

Last year, FDA officials found that pet food ingredients imported from China were tainted, killing more than 200 cats and dogs and causing widespread alarm. The Chinese manufacturer involved was later indicted in the U.S. for adding the chemical melamine in an attempt to bolster the value of its ingredients.

The pet food case drew attention to China's role in the U.S. food and drug supply and ignited criticism over the FDA's lack of oversight -- and little cooperation from its Chinese counterparts.

This time around, FDA officials said they are getting needed help from China.

"It's quite a different scenario than we had a year ago," FDA's Deputy Commissioner for International Affairs Murray Lumpkin, told reporters from his trip China.

FDA's Woodcock said it remains unclear whether the heparin contamination was intentional. Many scientists are studying over-sulfated chondroitin sulfate, which could have accidentally made its way into heparin products, she said.

The FDA announced on March 5 it had found a "heparin-like" contaminant in some of the drug's active ingredient supplied to Baxter by a plant in Changzhou, China, owned by Wisconsin-based Scientific Protein Labs. Investigators had been working since then to determine its precise identity.

Heparin is used in kidney dialysis, plus heart and other surgeries to prevent blood clots.

A Baxter spokeswoman could not immediately be reached for comment. The company recalled most of its U.S. supplies of heparin last month.

The FDA has received more than 785 reports of heparin reactions, including 19 deaths in patients treated with some brand of heparin. Baxter has said only four fatalities might be related to its heparin and there is not enough information to say the drug was the cause.

The agency has told border inspectors to stop all shipments of imported heparin intended for the U.S. market so they can be tested for the contaminant.

Germany also has ordered a recall of heparin made by Rotexmedica after reports of allergic reactions there.

(Reporting by Lisa Richwine, Susan Heavey, and Debra Sherman; editing by Tim Dobbyn and Gerald E. McCormick)