CAMBRIDGE, Mass. (AP) — Idenix Pharmaceuticals Inc. shares sank Monday after the U.S. Patent and Trademark Office determined a competitor was the first to request a patent for a hepatitis therapy.

The biopharmaceutical company said an appeals board decided that Idenix’s patent application for compounds used to treat hepatitis C was filed after Gilead Pharmasset LLC’s application. The U.S. Patent and Trademark Office appeals board will next determine which company invented the product first.

Idenix said that it does not believe that the patent application is relevant to any of the compounds it is currently developing, including two that are expected to begin midstage studies within the next few months, or another compound for which it soon plans to file a new drug application.

The news sent shares of Cambridge, Mass.-based Idenix down more than 15 percent to $3.89 by early afternoon, its lowest level in months. The company’s stock is approaching the bottom of its 52-week trading range of $3.35 to $11.30.

Hepatitis C is a virus that can lead to life-threatening liver damage and is the main cause of liver transplants in the U.S. The disease is spread through the blood, which can happen through sharing intravenous drug needles or having sex with an infected person. There are around 3 million Americans with the disease, which can go undetected for many years until the liver is severely damaged.

Shares of Gilead Sciences Inc. fell 65 cents to $44.86.