Momenta Pharmaceuticals Inc. surged 76 percent in midday trading Friday on the U.S. Food and Drug Administration’s approval of its generic version of a blockbuster Sanofi-Aventis drug to treat blood clots.

The approval is a major milestone for the Cambridge-based company, as it shifts from a research and development firm to a commercial biopharmaceutical company.

The drug was developed under a collaboration agreement between Momenta and Germany-based Sandoz, and is an alternative to Lovenox, an injectable drug to prevent and sometimes treat blood clots in the leg. It’s also used in combination with aspirin to prevent complications from angina (chest pain) and heart attacks. United States sales of Lovenox were $2.7 billion in 2009.

Craig Wheeler, Momenta’s president and CEO, said in a prepared statement: “This is the first product based on Momenta’s technology platform to be approved, and demonstrates our ability to characterize and develop a complex mixture drug like Lovenox.”

Other applications for other generic versions of the drug have been filed.

Momenta’s (Nasdaq: MNTA) shares were trading at $21.21 in midday trading, up from $11.93 at Thursday’s previous close.

In 2009, Momenta announced its initial public offering, seeking to raise $41 million.