aibioArcturus sues AbbVie and Capstan over trade secrets, and Capstan counterclaims
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Arcturus filed in the Southern District of California on September 23, 2025. Capstan Therapeutics answered with counterclaims against the company and two employees, and no trial date is set.
Key facts
- Company
- Arcturus Therapeutics Holdings Inc. (NASDAQ: ARCT)
- News type
- Legal
- Primary source
- Arcturus Therapeutics Holdings 10-Q for the quarter ended June 30, 2026 (SEC EDGAR)
Arcturus Therapeutics Holdings Inc. (Nasdaq: ARCT) is the plaintiff in a trade secret suit against AbbVie Inc., Capstan Therapeutics, Inc. and other defendants, and it is now also a counterclaim defendant. The 10-Q filed August 6, 2026 says Arcturus filed the suit on September 23, 2025 in the U.S. District Court for the Southern District of California. It asserts trade secret misappropriation and breach of contract. The 10-Q gives no case number.
The "other defendants" are not named in the 10-Q, the 10-K or the October 22, 2025 8-K. Capstan has counterclaimed against Arcturus and two of its employees.
The case
The 10-Q states the claims only in general terms and does not describe the alleged trade secrets or the contracts. It states no damages figure. Arcturus filed an Amended Complaint on April 13, 2026 against the same defendants, asserting the same claims.
The defendants answered on May 11, 2026. Capstan Therapeutics asserted counterclaims for trade secret misappropriation against Arcturus and two of its employees, and for breach of contract against the two employees. The 10-Q does not name the employees. Arcturus and the two employees answered the counterclaims on June 12, 2026.
Defendants
In Arcturus's suit the named defendants are AbbVie Inc. and Capstan Therapeutics, Inc., along with others the filings do not identify. On the counterclaims, the defendants are Arcturus and two of its employees. No officer or director of Arcturus is named in the 10-Q as a party.
Where it stands
The defendants filed a motion to dismiss in December 2025, and Arcturus opposed it in January 2026. On March 30, 2026, the court granted the motion in part and denied it in part, and gave Arcturus leave to amend on the points where it granted the motion. The court has not set a trial date. The 10-Q gives no estimate of loss from the counterclaims.
The October 22, 2025 8-K gave December 1, 2025 as the defendants' deadline to respond to the complaint, and the 10-K filed March 3, 2026 said the court had not set a case schedule.
Related matters
The same section reports the end of an arbitration with CSL Seqirus. Arcturus began an International Chamber of Commerce arbitration on May 30, 2025 to seek payment of a milestone under its collaboration agreement with Seqirus, Inc., based on the European Commission's grant of marketing authorization for a presentation of KOSTAIVE. CSL Seqirus answered on July 25, 2025 that it had no current obligation to pay the milestone. The March 31, 2026 10-Q had said a hearing was scheduled for the third quarter of 2026.
On August 3, 2026, the parties signed a Termination and Settlement Agreement and exchanged mutual releases. Arcturus dismissed the arbitration on August 5, 2026, and the tribunal acknowledged the dismissal that day. Under the agreement Arcturus received a one-time cash payment of $12.0 million and was released from a liability and R&D credit repayment with an aggregate value of approximately $16.0 million.
Why it matters to shareholders
The 10-Q says Arcturus is not otherwise a party to any material legal proceedings. The 10-K's risk factors say Arcturus may not prevail on its claims and that the defendants may assert counterclaims against it. They add that even a win in whole or in part could bring substantial expense and divert management's attention. The termination with CSL Seqirus also returned strategic control of the vaccine portfolio, including KOSTAIVE, to Arcturus.