Annexon Inc — Current Report (Form 8-K)

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

Date of Report (Date of earliest event reported): November 12, 2025

ANNEXON, INC.

(Exact name of registrant as specified in its charter)

1400 Sierra Point Parkway, Bldg C, Suite 200

Brisbane, California 94005

(Address of principal executive offices) (Zip Code)

Registrant’s telephone number, including area code: (650) 822-5500

Not Applicable

(Former name or former address, if changed since last report)

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:

Securities registered pursuant to Section 12(b) of the Act:

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. ☐

On November 12, 2025, Annexon, Inc. (the “Company’) entered into an underwriting agreement (the “Underwriting Agreement”) with Goldman Sachs & Co. LLC, TD Securities (USA) LLC and Wells Fargo Securities, LLC, as representatives of the several underwriters listed therein (collectively, the “Underwriters”), to issue and sell 25,096,153 shares (the “Shares”) of the Company’s common stock, par value $0.001 per share (the “Common Stock”) at a price to the public of $2.60 per share and pre-funded warrants (the “Pre-Funded Warrants”) to purchase 3,750,000 shares of Common Stock at a price to the public of $2.599 per Pre-Funded Warrant, which represents the per share public offering price for the Shares less the $0.001 exercise price of each Pre-Funded Warrant (the “Offering”). The Offering was made pursuant to an effective registration statement on Form S-3 (File No. 333-278246) and related prospectus and prospectus supplement, in each case filed with the Securities and Exchange Commission (the “SEC”). All of the Shares and the Pre-Funded Warrants were sold by the Company. Pursuant to the Underwriting Agreement, the Company has also granted the Underwriters an option exercisable for 30 days to purchase up to an additional 4,326,922 shares of Common Stock at the public offering price, which was exercised in full by the Underwriters on November 13, 2025. The net proceeds from the Offering were approximately $80.4 million after deducting underwriting discounts and commissions and estimated offering expenses. The Offering closed on November 14, 2025.

The Pre-Funded Warrants are exercisable at any time after the date of issuance. A holder of Pre-Funded Warrants may not exercise the warrant if the holder, together with its affiliates, would beneficially own more than 9.99% of the number of shares of Common Stock outstanding immediately after giving effect to such exercise. A holder of Pre-Funded Warrants may increase or decrease this percentage to a percentage not in excess of 19.99% by providing at least 61 days’ prior notice to the Company.

The Underwriting Agreement contains customary representations, warranties, covenants and agreements by the Company, customary conditions to closing, indemnification obligations of the Company and the Underwriters, including for liabilities under the Securities Act of 1933, as amended (the “Securities Act”), other obligations of the parties and termination provisions. The representations, warranties and covenants contained in the Underwriting Agreement were made only for purposes of such agreement and as of specific dates, were solely for the benefit of the parties to such agreement, and may be subject to limitations agreed upon by the contracting parties.

A copy of the Underwriting Agreement and the form of Pre-Funded Warrant are filed as Exhibits 1.1 and 4.1, respectively, and are incorporated herein by reference. The foregoing descriptions of the Underwriting Agreement and the Pre-Funded Warrants do not purport to be complete and are qualified in their entirety by reference to such exhibits. A copy of the opinion of Cooley LLP relating to the legality of the securities issued and sold in the Offering is filed herewith as Exhibit 5.1.

On November 12, 2025, the Company issued a press release announcing the pricing of the Offering, a copy of which is filed as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

On November 14, 2025, the Company issued a press release announcing the closing of the Offering, a copy of which is filed as Exhibit 99.2 to this Current Report on Form 8-K and is incorporated herein by reference.

Caution Concerning Forward Looking Statements

This Current Report on Form 8-K may contain forward-looking statements made in reliance upon the safe harbor provisions of Section 27A of the Securities Act and Section 21E of the Securities Exchange Act of 1934, as amended. Forward-looking statements include all statements that do not relate solely to historical or current facts, and can be identified by the use of words such as “may,” “will,” “expect,” “project,” “estimate,” “anticipate,” “plan,” “believe,” “potential,” “should,” “continue” or the negative versions of those words or other comparable words. These forward-looking statements include statements about the Offering, such as the expected net proceeds and anticipated closing date. These forward-looking statements are based on information currently available to the Company and its current plans or expectations, and are subject to a number of uncertainties and risks that could significantly affect current plans. Actual results and performance could differ materially from those projected in the forward-looking statements as a result of many factors, including the uncertainties related to market conditions and the completion of the public offering on the anticipated terms or at all. The Company’s forward-looking statements also involve assumptions that, if they prove incorrect, would cause its results to differ materially from those expressed or implied by such forward-looking statements. These and other risks concerning the Company’s business are described in additional

detail in the Company’s Annual Report on Form 10-K for the year ended December 31, 2024, Quarterly Reports on Form 10-Q for the quarters ended March 31, 2025, June 30, 2025 and September 30, 2025 and in the Company’s other SEC filings. The Company is under no obligation to (and expressly disclaims any such obligation to) update or alter its forward-looking statements, whether as a result of new information, future events or otherwise.

(d) Exhibits

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 hereunto duly authorized.