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FDA green lights Felimazole to treat feline hyperthyroidism

Methimazole used off-label by veterinarians since 1970s

June 11, 2009
By Jennifer Fiala

The U.S. Food and Drug Administration (FDA) announced yesterday that it approved Dechra Veterinary Product's Felimazole (methimazole), the first drug approved for treatment of hyperthyroidism in cats, yet some veterinarians are questioning the move's relevance. 

Practitioners have been using the human version of methimazole — an antithyroid drug and works by blocking the creation of thyroid hormones — off label for more than 30 years to treat feline hyperthyroidism, one of the most common diseases in middle-age and senior cats. So at least some veterinarians suspect that Felimazole's move through the regulatory process might be an attempt to dress an old drug differently.

"This is regular methimazole ... not a slow-release product or anything different than we are already using," says Margie Scherk, DVM, DABVP (Feline), a Veterinary Information Network consultant and editor of the Journal of Feline Medicine and Surgery. "The only reason this has been licensed, as best I can tell, is because in the UK, if there is a licensed drug, vets have to prescribe that rather than a generic or off label."

That rule does not apply to Canadian or U.S. veterinarians on a national level, although Dr. Bill Folger, owner of Memorial Cat Hospital in Houston, explains that Texas law requires veterinarians to use licensed drugs when there's a choice to do so. "Now some follow it, and some ignore it," he says. "It's very questionable whether (the administrative rule) is enforceable because half the stuff we use is off label."

Kansas-based Dechra Veterinary Products is the U.S. subsidiary of Dechra Pharmaceuticals PLC, a UK-listed company. While Dechra officials could not be reached by press time, Mike Eldred, president of the company's U.S. operations, explains in a news release that the approval of Felimazole coupled with last year's FDA approval of Vetoryl "further strengthens our position in the endocrinology market segment." 

Why the company chose to manufacture and seek regulatory approval for a drug already in use by DVMs could not be confirmed by anyone at Dechra before press time.

Still, "the approval of Felimazole offers cat owners, in consultation with their veterinarians, an effective medical treatment for this serious disease," says Dr. Bernadette Dunham, director of FDA's Center for Veterinary Medicine in a news release.

According to FDA, the safety and efficacy of Felimazole were evaluated in field studies conducted in the United Kingdom and the United States. Success was measured by a decrease in the production of thyroid hormones or T4 levels and improvement in clinical signs, such as weight gain, a return to normal eating habits and the cessation of vomiting.

A total 166 hyperthyroid cats were treated with Felimazole. By day 42, 77 percent of the cats in the UK study had normal T4 levels and 81 percent had reduced disease severity. In the U.S. study, 61 percent of the cats were considered treatment successes by day 42, with a significant decrease in T4 concentration and clinical signs.

The most common side effects associated with Felimazole include a change in food consumption (increase or decrease), lethargy, vomiting and diarrhea. Less common side effects include anemia and low platelet count in some cats, FDA says.

The coated tablets are available in 2.5 mg. and 5 mg. Company officials say that the drug will hit the U.S. market within the next three months.

In the meantime, Scherk and others hope FDA will take a look at Intervet's carbimazole, a common oral treatment for feline hyperthyroidism not yet approved in North America. Carbimazole is a prodrug that is converted to methimazole after metabolism. 

Sold as Vidalta in the UK and licensed as a once-a-day, the controlled-release drug would truly be new to the market, Scherk adds. Phone calls to Intervet, part of Schering Plough Animal Health, inquiring about the drug and whether or not FDA is reviewing it were not returned before press time. The regulatory agency does not release such information. 

For more on Felimazole, see FDA's freedom of information summary.

VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email

Information and opinions expressed in letters to the editor are those of the author and are independent of the VIN News Service. Letters may be edited for style. We do not verify their content for accuracy.


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