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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

Date of Report (Date of earliest event reported): September 8, 2025

DIANTHUS THERAPEUTICS, INC.

(Exact name of Registrant as Specified in Its Charter)

Registrant’s Telephone Number, Including Area Code: 929 999-4055

(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 September 8, 2025, Dianthus Therapeutics, Inc. (the “Company”) issued a press release announcing positive top-line data from the Phase 2 MaGic trial evaluating the safety and efficacy of claseprubart (DNTH103) in adults with acetylcholine receptor antibody positive (AChR+) generalized Myasthenia Gravis (gMG). The Company will host a conference call and webcast today, Monday, September 8, 2025, at 8:00 am, Eastern Time, to discuss the data results.

A copy of the press release is furnished as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated by reference herein. The exhibit furnished under Item 7.01 of this Current Report on Form 8-K shall not be deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any filing under the Exchange Act or the Securities Act of 1933, as amended, regardless of any general incorporation language in such filing.

On September 8, 2025, the Company made publicly available a data presentation announcing positive top-line data from the Phase 2 MaGic trial evaluating the safety and efficacy of claseprubart (DNTH103) in adults with acetylcholine receptor antibody positive (AChR+) generalized Myasthenia Gravis (gMG) (the “Presentation”) on the investor relations section of its website. The Presentation is filed as Exhibit 99.2 and is incorporated by reference into this Item 8.01.

(d) Exhibits. The following exhibit is being furnished herewith:

EXHIBIT INDEX

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.