The U.S. Supreme Court declined to hear Novartis AG’s appeal of a lower court’s decision on Monday, allowing Amgen Inc. to keep its patent protection for the rheumatoid arthritis drug Enbrel until 2029.

Enbrel, also known as etanercept, is Thousand Oaks-based Amgen's top-selling drug, accounting for nearly $5 billion of the company's $24.2 billion in product sales in 2020. The medication is used to treat adults with moderate to severe active rheumatoid arthritis and was launched in 1998 by Immunex Corp., which Amgen acquired in 2002.

In the initial 2016 lawsuit, Novartis subsidiary Sandoz Inc. sought to market a generic version of the drug. Immunex sued for patent infringement and Sandoz accused Immunex of wrongfully extending the life of its monopoly on the drug through a 2004 deal that took over a rival company’s patent applications on similar research and amended them to cover Enbrel, according to reporting by Reuters. The successful applications now protect Enbrel from competition until 2029.

Last year, the Washington-based U.S. Court of Appeals for the Federal Circuit rejected Sandoz's argument, saying that Immunex did not fully take over the patents, which were therefore still valid through 2029. The Supreme Court affirmed that ruling Monday by declining to hear Sandoz’s challenge.

Shares of Amgen Inc (AMGN) closed up Monday by $1.27, or a fraction of a percent, to $252.63 on the Nasdaq.