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UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT
PURSUANT TO SECTION 13 OR 15(D) OF
THE SECURITIES EXCHANGE ACT OF 1934
September 11, 2025
Date of Report
(Date of earliest event reported)

Arrowhead Pharmaceuticals, Inc.
(Exact name of registrant as specified in its charter)

Delaware |  | 001-38042 |  | 46-0408024
(State or other jurisdiction of incorporation) |  | (Commission File Number) |  | (IRS Employer Identification No.)

177 E. Colorado Blvd, Suite 700, Pasadena, CA 91105
(Address of principal executive offices, including Zip Code)
(626) 304-3400
(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐ | Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)
☐ | Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)
☐ | Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))
☐ | Pre-commencement communications pursuant to Rule 13e-4 (c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:
Title of each class |  | Trading Symbol(s) |  | Name of each exchange on which registered
Common Stock, par value $0.001 per share |  | ARWR |  | The Nasdaq Global Select Market

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).
Emerging growth company ☐
If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 8.01 Other Events
Arrowhead Pharmaceuticals, Inc. (the “Company”) was notified that on September 11, 2025, Ionis Pharmaceuticals, Inc. (“Ionis”) filed a Complaint for Patent Infringement against the Company in the United States District Court for the Central District of California alleging patent infringement of United States Patent No. 9,593,333 (“the ’333 patent”). As the Company previously reported, on September 10, 2025, the Company filed a Complaint for Declaratory Judgment in the United States District Court for the District of Delaware against Ionis to declare that the ’333 patent is invalid and not infringed by the Company’s planned commercialization of investigational plozasiran. A copy of Ionis’s complaint is attached hereto as Exhibit 99.1 (the “Complaint”).
Forward-Looking Statements
This Current Report on Form 8-K contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995. Any statements contained in this report except for historical information may be deemed to be forward-looking statements. Without limiting the generality of the foregoing, words such as “may,” “will,” “expect,” “believe,” “anticipate,” “hope,” “intend,” “plan,” “project,” “could,” “estimate,” “continue,” “target,” “forecast” or “continue” or the negative of these words or other variations thereof or comparable terminology are intended to identify such forward-looking statements. These forward-looking statements include, but are not limited to, statements about our planned commercialization of investigational plozasiran and beliefs and expectations regarding the Company’s claims and lawsuits against Ionis and the validity and enforceability of the ‘333 Patent. These statements are based upon our current expectations and speak only as of the date hereof. Our actual results or outcomes may differ materially and adversely from those expressed in any forward-looking statements as a result of numerous factors and uncertainties, including the risks and uncertainties described in our most recent Annual Report on Form 10-K, subsequent Quarterly Reports on Form 10-Q and other documents filed with the Securities and Exchange Commission from time to time. We assume no obligation to update or revise forward-looking statements to reflect new events or circumstances.
Item 9.01. Financial Statements and Exhibits.
(d) Exhibits

Exhibit No. |  | Description
99.1 |  | Complaint
104 |  | Cover Page Interactive Data File (the cover page tags are embedded within the Inline XBRL document).

SIGNATURES
Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.
Date: September 12, 2025

ARROWHEAD PHARMACEUTICALS, INC.
By: |  | /s/ Daniel Apel
|  | Daniel Apel
|  | Chief Financial Officer