Amgen Inc. announced Tuesday that a federal district court judge determined a new drug from competitor Roche violates one of Amgen's patents.

The Thousand Oaks-based biotech giant has been battling Roche on its planned introduction of Mircera, a drug that would compete directly with Amgen's top-selling anemia drug Aranesp and Johnson & Johnson's Procrit.

Roche says Mircera is a monthly dose anemia drug, meaning it would require fewer patient injections.

Roche planned to have the drug approved for sale by the end of this year.

Amgen sued Roche over its introduction of Mircera in November 2005, arguing that the drug violated several patents held by Amgen.

On Tuesday, the U.S. Federal District Court in Boston ruled that Roche's drug infringed upon Amgen's "pharmaceutical composition" patent, meaning that the components to produce the Roche drug are covered by Amgen's patent.

Several other patent issues that remain in dispute will be argued during a trial set to open Tuesday.

"We're pleased with today's outcome and look forward to moving ahead with trial next week," Amgen spokesman David Polk said in an e-mail.

Amgen continues to argue that Mircera infringes on other patents it holds relating to the type of drug and how it is produced.

Roche issued a statement Tuesday afternoon: "Roche remains confident in our position that all of the Amgen patents are invalid and not infringed. Today's ruling in Boston is only one step along the way and the trial has yet to begin. While we disagree with the Judge on the matter of infringement, the ruling does not determine the ultimate validity of any Amgen patents. Roche looks forward to presenting its case in Court."

Roche has moved forward with trying to get its drug approved while fighting Amgen in court.

In May, the U.S. Food and Drug Administration determined that it would make a decision on Mircera after its Sept. 11 committee meeting to discuss the safety of anemia drugs for patients with kidney disease.

Anemia drugs have come under fire after some studies raised safety questions at certain doses or used in certain patient groups. The FDA put warning labels on anemia drugs earlier this year, and Medicare has since changed how it pays for those drugs.

The fallout of those moves have hurt Amgen, which relies heavily on its two anemia drugs, particularly Aranesp. The drug made up almost 30 percent of Amgen's total sales revenue in 2006.

Since the beginning of this year, Aranesp sales have fallen, driving Amgen to orchestrate the first restructuring in its history. That includes cutting 12 to 14 percent of its work force worldwide.

Company officials talked about the Mircera threat in the last conference call with investors and how it could hurt Amgen's anemia drug sales even more if the drug comes on the market.

CEO Kevin Sharer said, at the time, that the company felt confident that Mircera would be found to violate Amgen's patents.

The company's shares (Nasdaq: AMGN) fell 91 cents to $49.01 Tuesday, before the company made the announcement.

Also this week, the court determined against some of Roche's defense in the trial.