Amgen Inc. will face new competition after Novartis AG secured the first-ever FDA approval to sell a “biosimilar” drug – provided it can overcome a legal challenge by the Thousand Oaks company.

The Food and Drug Administration, following a January recommendation of an agency advisory panel, said that the Swiss drug company may sell a copycat version of Neupogen, one of Amgen's best-selling drugs with revenue last year of about $1.16 billion.

Neupogen stimulates production of white blood cells in bone marrow and is for patients undergoing radiation or chemotherapy, which depletes white cell counts.

Biosimilars are medicines that copy proprietary biotech drugs when patent protection expires. Because small differences in the environment or manufacturing processes may alter the final product, the drugs are called similar – but not identical – to the original drug. Neupogen’s patent expired last year.

On Feb. 6, Amgen attorneys filed paperwork seeking a preliminary injunction to stop Novartis from selling the biosimilar, dubbed Zarxio for marketing purposes, arguing the company hadn’t complied with the time elements specified in the law allowing biosimilars. Novartis attorneys later filed documents claiming Amgen’s lack of cooperation caused delays and the timing issues.

A U.S. District Court hearing is set for March 13. Novartiz has agreed to not launch the drug until after a decision or April 10, whichever comes first. Novartiz has not released pricing information for Zarxio.

Also, the FDA is still working on a pharmaceutical naming system for biosimiliars that do not use the original commercial trademarked name for the drug. Zarxio will use a variation on the drug’s scientific name, filgrastim.

In a statement, Amgen said that Neupogen continues to play an important role for patients.

“The approval of the first U.S. biosimilar with an interim distinguishable nonproprietary name is an important first step in the creation of a biosimilar program,” the statement said. “Biosimilars will succeed in the U.S. marketplace because consumers – patients and doctors – have confidence in their quality. FDA should take the necessary steps to ensure a permanent distinguishable nonproprietary name, which will foster confidence and thus success of the U.S. biosimilar marketplace.”

Shares closed down $4.72 or 3 percent to $154.88 on the Nasdaq.