A divided federal appeals court overturned an order in favor of Momenta Pharmaceuticals that had blocked Watson Pharmaceuticals from selling a generic version of the blood-thinning drug Lovenox until a patent infringement trial is held.

Friday's 2-1 decision by the U.S. Federal Circuit Court of Appeals in Washington allows Watson and its partner Amphastar Pharmaceuticals to market for now their own generic version of the drug, which is intended to prevent blood clots.

Momenta, which holds a patent related to the drug, and Novartis's Sandoz unit won U.S. Food and Drug Administration approval in July 2010 to sell a generic version of Lovenox, resulting in about $1 billion of annual revenue.

In September 2011, the FDA gave approval for Amphastar and Watson to sell their own generic version of the drug, whose chemical name is enoxaparin. Two days later, Momenta and Sandoz sued.

France's Sanofi sells the branded version of Lovenox.

Momenta and its lead lawyer did not immediately respond to requests for comment On Friday.

U.S. District Judge Nathaniel Gorton in Boston in October 2011 temporarily blocked Amphastar and Watson from bringing their version to market.

He cited the strength of Momenta's infringement claim and the risk that the Cambridge, Massachusetts-based biotechnology company could suffer "long-term irreparable harm" from a rival version.

But Circuit Judge Kimberly Moore wrote for the appeals court that "Momenta's admission that Amphastar's testing is carried out to satisfy the FDA's requirements makes it unlikely that Momenta will succeed on the merits of its infringement claim."

Federal Circuit Chief Judge Randall Rader dissented, saying Amphastar "stepped in and took Momenta's patented invention without permission," and suggested the majority's reasoning would allow it "to trespass for years to come."

The case is Momenta Pharmaceuticals et al v. Amphastar Pharmaceuticals et al, U.S. Federal Circuit Court of Appeals, Nos. 2012-1062, 2012-1103, 2012-1104.