A newly developed drug shows promise in treating the most aggressive form of thyroid cancer, according to a new study.

About 5 percent of thyroid cancer patients have a very aggressive form of the cancer which spreads to other organs, such as the lungs, and can be fatal. Because these tumors cannot usually be treated effectively with surgery or radioactive iodine, patients often have no choice but to be treated with chemotherapy, which is not very effective.

In the new study, 37 patients with aggressive thyroid cancer took a drug called pazopanib every day. In nearly half of the patients, the size of their tumor decreased by at least 30 percent, said study author Dr. Julian Molina, an oncologist at the Mayo Clinic in Rochester, Minn.

On average, patients were able to take pazopanib for about a year before their cancer started growing again.

"For a disease for which we did not have any effective treatment, in which the treatments were vey toxic, we were able to find that this drug pazopanib works in 50 percent of patients, allowing the tumor to shrink and the patient to have a good quality of life," Molina said.

This is the best response reported yet for aggressive thyroid cancer, according to the researchers. Another drug that is also now in clinical trials, called axitinib, is effective in about 31 percent of patients, Molina said.

About 37,000 patients a year are diagnosed with thyroid cancer, making it more common than ovarian, esophageal and gastric cancer. Cases of thyroid cancer have doubled over the last decade, which researchers suspect is due to an increase in detection. For about 90 to 95 percent of patients, thyroid cancer is treatable and not fatal, Molina said.

Both pazopanib and axitinib work by blocking the formation of blood vessels in tumor cells, starving the tumor of oxygen and nutrients it needs to survive and grow. These drugs have side effects, including hypertension , diarrhea and inflammation of the liver. In the study, 16 patients had to have their dose lowered because of the side effects they experienced.

Future research needs to be done in larger populations in order to confirm the findings, Molina said. And he stressed pazopanib is not a cure, but rather, a way to slow the growth of aggressive thyroid cancer. These drugs might make the cancer more of a chronic disease, such as diabetes or hypertension, in which patients take a pill everyday to manage their condition.

The study was a phase II clinical trial, and a phase III trial needs to be conducted before the drug's manufacturer, GlaxoSmithKline, Plc., can seek approval from the U.S. Food and Drug Administration to use pazopanib as a treatment for this cancer.

Pazopanib is currently approved for treatment of kidney cancer. This means that doctors could theoretically prescribe pazopanib for thyroid cancer, but since the drug is not FDA-approved for this use, the treatment might not be covered by insurance.

The study was funded by the National Cancer Institute, and was published online today (Sept. 17) in the journal The Lancet Oncology.

The researchers reported no conflict of interest. GlaxoSmithKline did not fund the study and did not have access to the data.