UPDATE 1-U.S. reexamines Merck's Singulair patent

* Patent Office review seen taking perhaps years

* Reexamination granted after online group challenge (Adds details on patent reexamination, patent challenge)

By Ransdell Pierson

NEW YORK, May 28 (Reuters) - Merck & Co Inc (MRK.N) said on Thursday the U.S. Patent Office is reexamining the patent on its biggest product, $4.5 billion-a-year asthma treatment Singulair.

The U.S. Patent and Trademark Office (PTO) ordered the reexamination on May 20, according to PTO records. It did so after a request by a new online community called Article One Partners LLC that recruits scientists worldwide to look for evidence patents have been improperly issued.

"The PTO has made the decision to reexamine the patent and we can't speculate how long it will take," said Merck spokesman Ron Rogers.

Rogers said Merck remains confident its U.S. patent is valid and enforceable. Should the patent be invalidated, generic drugmakers could begin selling cheaper versions of Singulair before the patent lapses in 2012.

But Rogers noted patent reexaminations can take a number of years.

"According to the PTO website, the average time it takes the PTO to decide ex parte reexaminations is roughly 24 months," Rogers said. "The PTO's decision can be appealed to the Court of Appeals for the Federal Circuit before it becomes final."