Drugmaker AbbVie Inc. filed a lawsuit Thursday against Thousand Oaks biotech Amgen Inc. claiming patent infringement of its top-selling drug Humira, which brought in $14 billion in revenue in 2015.

Amgen is developing a biosimilar of the popular arthritis medication, and its version is currently in late-stage development.

Chicago-based AbbVie alleges that Amgen’s drug infringes on at least 10 of AbbVie’s patents and said it reserved the right to assert as many as 51 other patents.

The suit was filed in a federal court in Delaware.

Biosimilars are highly comparable to their original bioengineered counterparts but are not exact replicas, because they are made in living cells.

Amgen shares closed up 99 cents, or less than 1 percent, to $173.66 on the Nasdaq.