By Diane Bartz and Ransdell Pierson

WASHINGTON, Feb 7 (Reuters) - The patent on Merck & Co's cholesterol fighters Zetia and Vytorin, two of the drugmaker's biggest products, is valid, a U.S. appeals court ruled on Thursday.

Vytorin combines Zocor, a member of the statin family developed by Merck, with a newer Merck cholesterol treatment called Zetia. Sales of Vytorin are $1.75 billion while sales of Zetia used by itself are another $2.6 billion a year.

The U.S. Court of Appeals for the Federal Circuit did not discuss the ruling at length and simply affirmed a decision by a lower court in New Jersey.

The U.S. District Court for the District of New Jersey had ruled in April that the patent on Merck's Zetia and on Vytorin was valid and issued an injunction blocking approval of a generic version by Mylan Inc until the patent expires.

The patent expires in April 2017, Merck said.

"We invest heavily in the R&D that is needed to discover innovative medicines like Vytorin and Zetia, and we will vigorously defend our intellectual property rights," Merck spokesman Ron Rogers said in an statement.

Mylan did not immediately respond to requests for comment.

Zocor, which is available as a generic, cuts the liver's production of "bad" LDL cholesterol while Zetia blocks absorption of LDL in the intestines. Vytorin, which combines both in a pill, was approved by U.S. regulators in 2004.

Although Vytorin is a big seller, demand for it has been hurt by uncertainty whether the combination of drugs lowers risk of heart attack and stroke any more than using Zocor alone.

A long-term study, involving thousands of patients, is now under way to answer that question.

The case is Merck Sharp & Dohme Corp. V. Mylan Pharmaceuticals. The number at the U.S. Court of Appeals for the Federal Circuit is 2012-1434. The number in the U.S. District Court for the District of New Jersey is 10-3085.

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