FRIDAY, Sept. 17 (HealthDay News) -- A targeted drug called pazopanib could prove effective against difficult-to-treat cases of thyroid cancer cases, researchers say.

Most thyroid cancers can be treated with surgery or radioiodine, but about 5 percent of patients will develop an aggressive, life-threatening form of the disease.

Pazopanib (Votrient), already approved by the U.S. Food and Drug Administration for treating advanced kidney cancer, works by inhibiting growth of blood vessels essential for tumor growth and survival.

"Thyroid cancers, when they grow, they need to form a lot of blood vessels," explained researcher Dr. Julian Molina, an assistant professor of oncology at the Mayo Clinic and co-author of the study. "For blood vessels to grow, the growth factor VEGF is required, and the drug targets this protein," he said.

Not only does the drug block production of new blood vessels, it also interferes with the tumor cells' ability to continue growing, Molina said.

Pazopanib, a pill that's taken daily, doesn't have the severe side effects of standard chemotherapy, Molina said. But it is expensive. According to the U.S. Department of Health and Human Services, the average wholesale price, which is used to set drug reimbursements, is $6,595 a month.

For this phase 2 study, published in the Sept. 17 online edition of The Lancet Oncology, Molina and colleagues tested the efficacy and safety of pazopanib in 37 patients with advanced, rapidly progressing thyroid cancer.

About half of the patients --18 -- had a partial response to the drug, meaning their tumor shrank 30 percent or more, Molina said. None had a complete response, in which the tumor disappears completely, he added.

Twelve patients are still responding to the drug about a year later. "Patients take the drug until the drug is no longer effective," Molina said, noting these patients still take pazopanib every day.

Side effects were commonplace but typically mild. The most usual were diarrhea, hypertension, and an increase in a liver enzyme called aminotransferase. However, 15 patients had to have their dose lowered because of side effects, the researchers note.

The U.S. National Cancer Institute funded the study.

Molina noted that larger phase 3 trials will start soon, and he hopes the drug, made by GlaxoSmithKline, will be approved for treating advanced thyroid cancer.

"One of the goals in cancer care is to make cancer a chronic disease," Molina said. "If we can make cancer something like that, where your pills keep your disease controlled, that's a good goal," he said.

One expert said that any effective therapy would be welcome.

"There are not a lot of treatment options available for people with advanced thyroid cancer," said Dr. Leonard Lichtenfeld, deputy chief medical officer at the American Cancer Society.

While the study results will need to be replicated and tested against other treatment options before pazopanib can be approved as a therapy for thyroid cancer, he agreed that the drug shows promise.

"Targeted therapies are providing some effective treatment options for patients with diseases that traditionally have been difficult to treat," he said. "I find that very helpful and very exciting."

More information

For more information on thyroid cancer, visit the American Cancer Society.