Ionis Pharmaceuticals Sues Arrowhead Pharmaceuticals Over Patent Infringement.

Ionis Pharmaceuticals, Inc. has filed a complaint against Arrowhead Pharmaceuticals, Inc. in the United States District Court for the Central District of California. The complaint alleges that Arrowhead breached its contract with Ionis and failed to make payments for royalties owed for the sale of Arrowhead's drug, ARO-APOC3. Ionis is seeking damages and injunctive relief.

Ionis Pharmaceuticals, Inc. has taken legal action against Arrowhead Pharmaceuticals, Inc. in the United States District Court for the Central District of California, alleging patent infringement and breach of contract. The lawsuit, filed on September 11, 2025, seeks damages and injunctive relief.

According to the complaint, Ionis alleges that Arrowhead copied its patented messenger RNA (mRNA) technology to develop its drug for familial chylomicronemia syndrome (FCS), a genetic disorder that prevents the body from breaking down fats. The lawsuit also claims that Arrowhead applied for FDA approval of its competing drug, plozasiran, in January, in "blatant disregard for Ionis’s patent rights."

In response, Arrowhead filed a separate lawsuit in Delaware federal court on Wednesday, seeking to invalidate Ionis' patent. Arrowhead's complaint states that its drug was "based entirely on work developed internally at Arrowhead, which Ionis was not involved with and provided no contribution to whatsoever."

The U.S. Food and Drug Administration approved Ionis' Tryngolza in December, the company's first wholly owned drug and the United States' first approved treatment for FCS. Tryngolza significantly reduces patients' triglyceride levels, with analysts estimating that sales could peak near $2.5 billion annually. Arrowhead's plozasiran, on the other hand, is a first-in-class investigational RNA interference (RNAi) therapeutic designed to reduce production of apolipoprotein C-III (APOC3), a key regulator of triglyceride metabolism.

The cases are Ionis Pharmaceuticals Inc v. Arrowhead Pharmaceuticals Inc, U.S. District Court for the Central District of California, No. 2:25-cv-08609 and Arrowhead Pharmaceuticals Inc v. Ionis Pharmaceuticals Inc, U.S. District Court for the District of Delaware, No. 1:25-cv-01130.

Ionis shares jumped by 34% earlier this month after Tryngolza showed promise in clinical trials. Arrowhead, however, has been an innovator in RNAi therapeutics for decades, with multiple issued US patents covering its plozasiran for the treatment of FCS.

This legal dispute highlights the intense competition in the biotechnology sector, particularly in the development of mRNA-based therapies. The outcome of these lawsuits could significantly impact the availability of new treatments for patients with FCS and other genetic disorders.