Arrowhead still defending Ionis patent suit over plozasiran after Delaware case was dismissed

Arrowhead Pharmaceuticals, Inc. (Nasdaq: ARWR) is defending a patent infringement suit brought by Ionis Pharmaceuticals, Inc. over its siRNA drug plozasiran, which is approved as REDEMPLO, according to its 10-Q filed August 4, 2026. Ionis filed the complaint on September 11, 2025 in the U.S. District Court for the Central District of California. It alleges infringement of U.S. Patent No. 9,593,333, which the filings call the '333 patent, and seeks damages. The 10-Q gives no case number.

Arrowhead is the only defendant the filings name. No officer or director is described as a defendant. Arrowhead says it disputes the allegations of wrongdoing and intends to vigorously defend itself.

The case

The complaint alleges that Arrowhead's planned commercialization of investigational plozasiran infringes the '333 patent. The 10-Q does not describe what the patent claims, how much Ionis seeks, or whether Ionis asked for an injunction. These are Ionis's allegations. Arrowhead's own filings do not concede any of them.

Arrowhead moved first. On September 10, 2025 it filed a complaint for declaratory judgment against Ionis in the U.S. District Court for the District of Delaware, asking the court to declare the '333 patent invalid and not infringed by the planned commercialization of plozasiran. Arrowhead reported that case in an 8-K filed September 11, 2025. It then learned of the Ionis suit in California and disclosed that in an 8-K filed September 12, 2025.

Defendants

In the California case Arrowhead Pharmaceuticals, Inc. is the defendant and Ionis is the plaintiff. The filings name no individual defendants, underwriters or other entities. In the Delaware case the positions were reversed: Arrowhead was the plaintiff and Ionis the defendant.

Where it stands

Arrowhead's 10-Q for the quarter ended March 31, 2026, filed May 7, 2026, says that on December 23, 2025 the Delaware court granted Ionis's motion to dismiss the declaratory judgment action. That filing does not say why. The latest 10-Q no longer repeats the Delaware dismissal. It describes only the California suit and says there were no other material developments in the legal proceedings disclosed in the company's annual report.

Neither 10-Q reports a ruling, a trial date or a settlement in the California case. The company says it recorded no contingent liabilities as of June 30, 2026. In the same filing it says litigation "can be expensive and disruptive to normal business operations" and that the results of complex proceedings cannot be predicted with any certainty.

Why it matters to shareholders

Arrowhead's annual report for the fiscal year ended September 30, 2025 describes the suit in its risk factors. It says the Ionis claim "may impact our planned commercialization of REDEMPLO." It also says that if the claim or any future claim by another party succeeds, Arrowhead may be required to pay substantial damages, be forced to abandon any affected drug product or candidate, or seek a license from the patent holder.

The same risk factor says the claim, whether or not it succeeds, may cause Arrowhead to incur significant expenses and divert the attention of management and key personnel.

The 10-Q says the FDA approved REDEMPLO on November 18, 2025 as an adjunct to diet to reduce triglycerides for adults with familial chylomicronemia syndrome, and that it is also approved for the same indication by the European Commission, the Chinese National Medical Products Administration, Health Canada and the Australian Therapeutic Goods Administration. The 10-Q describes the claim as aimed at planned commercialization and does not say whether it covers marketed product.

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