Enbrel, which is used to treat rheumatoid arthritis and psoriasis, was one of several biotechnology drugs that were expected to face competition in the next few years from copycat versions, eventually saving the health care system billions of dollars a year.

The 2010 health care law established a way for such biologic drugs, which can cost tens of thousands of dollars a year, to face competition from near generic versions, which are often called biosimilars. A new law was needed because biologic drugs, which are made in living cells, were not covered by the 1984 law governing most pharmaceutical competition.

The main patent on Enbrel was to expire in October of next year. But the new patent could stave off such biosimilar competition until Nov. 22, 2028. By that time, Enbrel will have been on the market 30 years, far longer than the 20 years of protection expected in patent law.

Enbrel had sales of $3.5 billion in the United States and Canada in 2010, accounting for nearly one-quarter of Amgen’s revenue. The drug costs more than $20,000 a year. Pfizer sells Enbrel abroad.

Merck announced in June that it planned to develop a biosimilar version of Enbrel, in a partnership with Hanwha Chemical of South Korea.

“Enbrel is widely considered to be one of the most important biosimilar molecules,” a Merck executive said in a statement at that time. Merck had no comment Tuesday on Amgen’s patent.

The application for the new patent was filed in 1995. But it took until Tuesday to get through the Patent Office because it was reworked and at one point rejected, forcing Amgen to appeal.

Patents now run 20 years from the date of application, to avoid situations like this where an invention gets extended protection because of delays or maneuvers at the patent office. But since this patent was filed before the law changed, it is governed by the old rules and lasts for 17 years from the date of issuance.

Amgen benefited from a similar situation with its anemia drug Epogen, which is still protected by patents even though it has been on the market since 1989.

The new patent on Enbrel, No. 8,063,182, is owned by Roche but was licensed to Amgen, which took it through the Patent Office.

Amgen executives said earlier this year that they did not anticipate biosimilar competition to Enbrel in the next five years anyway, in part because of other patents covering the use or formulations of Enbrel. But such patents tend to be weaker than one that covers the basic composition of the drug, as Amgen says the new patent does.

Still, it is possible that some biosimilar manufacturers will try to challenge the patent or work around it. And Enbrel might face competition from generic versions of other arthritis drugs, including Abbott Laboratories’ Humira, and from new oral drugs that might reach the market in the next few years.

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