CHICAGO (Reuters) - In a small study, GlaxoSmithKline’s cancer drug Votrient helped shrink tumors in nearly half of patients with an advanced form of thyroid cancer, U.S. and international researchers said on Friday.

For many, the effect of the kidney cancer pill, known generically as pazopanib, lasted for more than a year, said Dr. Keith Bible of the Mayo Clinic in Rochester, Minnesota, whose findings were published in Lancet Oncology.

“These responses that occur in half of all patients are very durable. The majority, in fact two-thirds, last greater than one year,” Bible said in a video interview on clinic’s website.

The study was done in 37 patients with the most aggressive form of differentiated thyroid cancer. Many patients with these types of cancers can be cured with surgery and radioiodine. But 5 percent of these patients develop a rapidly progressing life-threatening disease that has few treatment options.

“At the time we began this trial about two years ago, there were no effective therapies beyond radioactive iodine. We selected patients who were resistant to radioactive iodine specifically for this trial,” Bible said.

Of these patients, 18, or 49 percent, had a long-lasting response to pazopanib.

Bible said normally, they would have expected the cancer to progress within six months. Instead, it took about a year on average before the cancers started growing again.

“These are not just short-term results or benefits to the patient. These are results which really last over a significant period of time.”

The team said the response rate is the highest yet in such aggressive cases of differentiated thyroid cancer.

The findings suggest the drug has “a high level of clinical activity in advanced rapid progressive differentiated thyroid cancers,” Bible said.

But the drug caused side effects severe enough that the dose had to be lowered in 16 patients. Two patients experienced significant bleeding, and two died from pre-existing disease while taking part in the study.

For that reason, the team does not recommend the drug for use in people with a slow-growing form of the cancer. And they said it was not possible to tell whether the drug will ultimately keep people alive longer.

That will take a much larger, randomized clinical trial, which is already being organized.

Votrient targets the vascular endothelial growth factor, or VEGF receptor, to try to limit the growth of new blood vessels that can feed tumors.

As a treatment for kidney cancer, the drug is expected to have global sales of $284 million by 2014. The study was funded by the National Cancer Institute.

Thyroid cancer affects women more often than men and usually occurs in people between the ages of 25 and 65 years.