The U.S. Food and Drug Administration (FDA) approved two new drugs this week that are the first of their kind for treating specific conditions in animals. On June 3, the FDA approved the first cancer drug therapy for dogs, and on June 9, the agency gave the green light to a hyperthyroidism drug for cats.

The feline hyperthyroidism drug, Felimazole (methimazole), is manufactured by U.K.-based drug maker Dechra, and is the first drug approved by the FDA for treating this condition in cats. Methimazole is an antithyroid drug that blocks the creation of thyroid hormones.

According to an FDA news release, Felimazole was tested in field studies in the United States and the United Kingdom. It is administered orally in initial doses of 2.5 mg per animal every 12 hours.

“Success was measured by a decrease in the production of thyroid hormones or T4 levels and improvement in clinical signs, i.e., weight gain, no longer vomiting, return to normal eating habits, etc.,” the FDA said. “A total of 166 hyperthyroid cats were treated with Felimazole. By day 42, 77% of cats in the U.K. study had normal T4 levels and 81% had reduced disease severity. In the U.S. study, by day 42, 61% of cats were considered treatment successes with a significant decrease in T4 concentration and improved clinical signs.”

Rebecka Hess, DVM, DACVIM, associate professor of internal medicine at the University of Pennsylvania’s School of Veterinary Medicine has experience with methimazole in cats.

“I use methimazole frequently, prior to [radioactive iodine] I-131 therapy, to assess renal function once T4 levels have normalized,” said Hess, who is also president of the society for comparative endocrinology. “I also use it in cats with renal insufficiency in which I-131 is not indicated.”

She said the FDA approval was a step forward, and is a better choice than using extra-label human drugs to treat the condition.

“It is best to use labeled drugs that are marketed specifically for veterinary use by companies which serve the veterinary field,” she said. “The use of these products is also monitored carefully by the company for any adverse side effects and this is an excellent way of collecting large amounts of data about the drug and remaining well informed about it.”

The dog cancer drug, manufactured by Pfizer Animal Health, is sold under the name Palladia (toceranib phosphate). It is designed to treat cutaneous mast cell tumors (MCTs) in dogs. The drug is approved to treat the tumors with or without regional lymph node involvement, according to the FDA.

The approval is significant because it is the first animal-specific cancer drug to be OK’d by the FDA.

"Prior to this approval, veterinarians had to rely on human oncology drugs, without knowledge of how safe or effective they would be for dogs,” said Bernadette Dunham, DVM, PhD, director of FDAs Center for Veterinary Medicine in a news release..”[The] approval offers dog owners, in consultation with their veterinarian, an option for treatment of their dogs cancer."

Palladia is a tyrosine kinase inhibitor that kills tumor cells and cuts off the blood supply to the tumor.

In a double-blind, centrally randomized, multicenter group sequential clinical trial sponsored by Pfizer, dogs received either Palladia (3.25 mg/kg) orally every other day, or placebo tablets for six weeks. After the six week blinded phase, eligible dogs were given open-label Palladia. The dogs used in the study were more than a year old, weighed more than 5 kg, and had pathologically confirmed recurrent MCT and at least one MCT of at least 20 mm in its longest diameter.

“There was a statistically significant improvement in the primary end point (objective response) for Palladia treatment compared with placebo treatment,” the study says. “The objective response rate in Palladia-treated dogs was 37.2% (32/86; 7 complete response, 25 partial response) compared with 7.9% (5/63; 5 partial response) for placebo-treated dogs.”

Veterinary oncologist Gregory K. Ogilvie, DVM, DACVIM (Internal Medicine, Oncology), DECVIM-CA (Oncology) said the approval was welcome news.

“That ushers in a whole new era of molecular therapeutics,” said Ogilvie, who is director of the Angel Care Cancer Center at California Veterinary Specialists and president of the Special Care Foundation for Companion Animals. “It’s a good product not only for the direct treatment of mast tumors but augmenting other therapies. There is a great deal of evidence to suggest this drug has many other applications that involve mast cells.”

Rick Goulart, director of public relations for Pfizer Animal Health, said Palladia will be available for sale in early 2010. Goulart said the company is beginning to work on other animal cancer drugs as well.

“Pfizer is committing resources to Pet Oncology, a new therapeutic area in Pfizers Veterinary Medicine Research & Development labs,” Goulart said. “Although we have a number of active programs in our new Pet Oncology pipeline, it is premature to discuss when those cancer therapies may be available.”