11:08 AM EDT, 09/12/2025 (MT Newswires) -- (Updates with Ionis' response in the fourth paragraph and the latest stock movement in the fifth paragraph.)

Arrowhead Pharmaceuticals ( ARWR ) said Thursday it filed a complaint for declaratory judgment against Ionis Pharmaceuticals ( IONS ) to defend its potential familial chylomicronemia treatment plozasiran against Ionis' patent infringement allegations.

Arrowhead said plozasiran, which is under review by the US Food and Drug Administration, does not infringe an Ionis patent. Ionis' claims are baseless and its patent is invalid, Arrowhead added.

Arrowhead said it is not claiming any monetary relief in this litigation.

Ionis is "not seeking to block" the roll out of this new therapy to patients, an Ionis spokesperson told MT Newswires in an email Thursday. "We are confident in our IP around FCS and will vigorously defend our patent position," the spokesperson added.

Shares of Ionis edged 0.8% lower in recent Friday trading, while Arrowhead's stock was 4% higher.

Price: 30.78, Change: +1.06, Percent Change: +3.55