In June, Merck teamed up with South Korea’s Hanwha Chemical to bring to market its biosimilar of Amgen’s arthritis blockbuster Enbrel, which was set to lose patent protection in 2012. But Amgen may have gotten an early Christmas gift from the U.S. Patent and Trademark office.

Just before Thanksgiving, Amgen announced the USPTO issued a new patent on Enbrel, extending patent protection for another 16 years past its 2012 expiration date.

Amgen’s new patent could deal a big blow to Merck and other developers of biosimilars targeting Enbrel. Merck obtained global marketing rights outside of South Korea and Turkey from Hanwha for an undisclosed upfront payment. While Merck can continue global development, the new patent will impact entry into the U.S. market, which accounts for almost half of Enbrel’s global sales of $6.8 billion. The drug was first approved in 1998 to treat rheumatoid arthritis and its indications were later expanded to include the treatment of psoriasis and other conditions.

The new patent, U.S Patent 8,063,182, is owned by Roche, but it is exclusively licensed to Amgen as a result of its 2002 acquisition of Immunex. In 2004, Amgen made Roche a one-time payment and obtained an exclusive license to the pending patent, now granted. It describes and claims the fusion protein that is etanercept, a fully human TNF receptor inhibitor and Enbrel’s established name.

Drugmakers, including Teva Pharmaceuticals, Merck, Hospira, Celltrion, and Hanwha to name a few, have been developing technology to produce biosimilars in expectation of several patent expiring in the next few years. It is a market that is expected to exceed $2 billion annually by 2015, according to IMS Health. These drugs are more difficult to produce than generics because they must be grown in cell cultures rather than by made through chemical processes.

An analysis of Amgen’s newly issued Enbrel patent by Geoffrey Porges of Sanford C. Bernstein found that it could have greater value for Amgen than previously thought because its existing marketing partnership with Pfizer on Enbrel in the United States is set to expire.

“The application for this patent was first filed in 1990 and has been rejected, modified, resubmitted, opposed, and updated during the course of its prosecution,” writes Porges. “Conversations with the three involved parties (Roche, Amgen, Pfizer) suggest that the sunsetting of Amgen’s profit sharing obligation on Enbrel to Pfizer in the 2013-2015 period are unaffected by this patent, and Roche is not entitled to any royalties on Enbrel now or in the future.”

Porges estimates that the “extended patent protection for Enbrel could be worth as much as $6 per share in NPV to Amgen, assuming the patent held for its full term, Enbrel survived other branded competitors, and Amgen’s operating margins on Enbrel improve as we expect.”

Even without competition from biosimilars, Enbrel does face several challengers in the rheumatoid arthritis treatment space, including Abbott’s Humira, J&J’s Simponi, and Roche’s Rituxan. Pfizer also has great hopes for its new treatment tofacitinib, which is a pill versus an injection.