* FDA approves pancreatic cancer therapy

* Treatment to carry black box warning

* Stock halted (Adds background, analyst's comment)

By Samantha Kareen Nair

Oct 22 (Reuters) - The U.S. Food and Drug Administration approved Merrimack Pharmaceutical Inc's pancreatic cancer therapy Onivyde, but with a serious warning, marking the company's first ever drug to win regulatory clearance.

The drug in combination with chemotherapy treatments 5-fluorouracil and leucovorin treats metastatic pancreatic cancer in patients, who have failed to respond to chemotherapy drug gemcitabine.

Onivyde, however, carries a black box warning about severe risks of diarrhea and low white blood cell count, the agency said, adding that it was not approved for use as a single agent. (http://1.usa.gov/1kvFKVt)

Pancreatic cancer, which is known as the "deadliest" kind of cancer, is seldom detected in its early stages and is responsible for 7 percent of cancer deaths in the United States, according to the American Cancer Society.

Guggenheim Securities LLC's analyst Tony Butler said he expected the drug to be priced between $5,000-$10,000 per course and rake in peak annual sales of about $1.5 billion in the United States and Europe.

Butler said Onivyde, which is currently under European review, is likely to be launched in the fourth quarter.

Baxter International had licensed the rights to market the drug outside the United States in September last year for an upfront fee of $100 million.

The drug is also being tested in early-stage studies for breast cancer and pediatric sarcoma, a type of bone or connective tissue cancer in children.

Merrimack shares were halted prior to the announcement. The company's shares closed at $10.01 on the Nasdaq on Wednesday, having fallen about 11 percent this year.

(Reporting By Samantha Kareen Nair in Bengaluru; Editing by Don Sebastian and Anil D'Silva)