WASHINGTON (AP) — The Food and Drug Administration on Thursday approved a new drug to treat a rare form of thyroid cancer after it has spread to other parts of the body.

The agency cleared Cometriq from Exelixis Inc. for patients with advanced medullary thyroid cancer, which attacks thyroid cells that maintain calcium levels in the blood.

The disease is among the rarest forms of thyroid cancer, affecting an estimated 2,250 U.S. patients per year, according to the National Cancer Institute.

Exelixis' capsule-based drug is the second medicine for the condition approved in the last two years. In April 2011 the FDA approved Caprelsa from AstraZeneca PLC for the same indication.

"Patients with this rare and difficult to treat disease had limited therapeutic treatment options," before the two drugs were approved, said Dr. Richard Pazdur, FDA's director for oncology products, in a statement.

The agency approved Cometriq based on study results showing that patients on the drug lived an average of 11.2 months without tumor growth, compared with four months for patients receiving a placebo.

The drug carries a boxed warning that "severe and fatal bleeding and holes in the colon occurred in some patients."

Exelixis is based in South San Francisco, Calif.