Top-selling anti-inflammatory drug Enbrel from Amgen, Inc. (NASDAQ:AMGN) is facing steep competition from biosimilars that are hitting the market at a very rapid speed. And in its efforts to defend this competition, Amgen has sued Novartis’ AG (ADR) (NYSE:NVS) Sandoz unit with claims of infringement of Enbrel’s patents. The company considered the infringement by the merits of that Sandoz had sought the FDA’s approval for its biosimilar of the med.

To that effect, Amgen wants the U.S. District Court to offer an injuction towards the prevention of Sandoz from selling its biosimilar in the event it obtains the FDA’s approval. Sandoz’s application for its Enbrel biosimilar was accepted in October by the FDA which led the company to getting into higher heights and close to gaining full approval. Amgen feels like Sandoz is taking credit on the fruits and earning on its hard earned efforts in the development of Enbrel.

Amgen argues that it has taken two decades to develop this drug which is known for treatment of an assortment of autoimmune diseases the likes of psoriasis and ankylosing spondylitis. On its suit, Amgen stated that Sandoz had only carried out clinical trials of its biosimilar for psoriasis patients yet it is seeking for the approval of the med.

There is such a big line up of Amgen’s drug having registered in more than $5 billion in Enbrel sales last year in the U.S. And it is now looking up to new patent it worn in 2011. Apparently, the FDA was just about to accept Amgen’s application for ABP 501for its biosimilar of AbbVie Inc (NYSE:ABBV) but it could not go through because the U.S. Patent and Trademark Office declined the company’s attempt.

Truth be told, the suit is a throwback for Sandoz which was already up in speed with its Enbrel biosimilar even though it has been leading the pack of biosimilar development. Bernstein analyst Ronny Gal says that biosimilars are coming in though on a slow bit. Nevertheless, Sandoz is optimistic to forge ahead despite the challenges.