Gilead Sciences Inc. on Tuesday sued to stop a rival drugmaker from bringing a generic version of Sovaldi to market, as the company continues its campaign to protect the hepatitis C treatment amid rapidly declining sales.

The 26-count complaint, filed in the U.S. District Court for the District of Delaware, accused Israeli-based Teva Pharmaceutical Industries Ltd. and its U.S. subsidiary of infringing 17 patents for the drug, which is used to treat adult and teens suffering from chronic hepatitis C infection.

The filing came in response to Teva's application earlier this year with the U.S. Food and Drug Administration to develop a generic using the same active ingredient and dosage levels as Sovaldi. In it, Gilead asked for an injunction barring Teva and its affiliates from making or selling its planned generic until the last of its patents expire in 2029.

Teva told Gilead in a letter last month that it believes its patents for the drug are invalid and unenforceable, according to the complaint. A Teva spokeswoman declined to comment on Wednesday.

The suit comes on the heels of a separate filing earlier this month accusing another generic drugmaker, Natco Pharma Ltd., of infringing six patents with its own planned generic.

“If we are unsuccessful in these lawsuits, some or all of our original claims in the patents may be narrowed or invalidated and the patent protection for these products could be substantially shortened,” Gilead said in an annual filing with the U.S. Securities and Exchange Commission.

“The sale of generic versions of these products earlier than their patent expiration would have a significant negative effect on our revenues and results of operations.”

Sovaldi, the brand name for sofosbuvir, has been credited with saving the lives of millions of hepatitis C patients, and has led to billions of dollars in revenue for Gilead, which is based in Foster City, California.

However, Gilead said that increased competition and lower market patient starts have combined to take a big bite out the company's bottom line in recent years.

In 2017, the company reported just $964 million in worldwide sales for the drug, compared to $4 billion the year before and nearly $5.3 billion in 2015, according to its 10-K form. U.S. sales accounted for just $130 million of that total last year, down from $1.9 billion in 2016.

In both the Teva and Natco cases, Gilead has enlisted the services of attorneys from Fish & Richardson, the same firm that helped the company wipe out a massive $2.5 billion infringement verdict earlier this year. An online docket-tracking service listed Robert M. Oakes and Gregory R. Booker as representing Gilead.

Attorneys for Teva were not yet listed.

Gilead is also seeking damages and an order declaring the case exceptional under U.S. patent law. A call to Gilead's press office was not immediately returned on Wednesday.

The case is captioned Gilead v. Teva.