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SCHEDULE 13D/A 0001193125-23-239624 0001802528 XXXXXXXX LIVE 4 Common Stock, $0.001 par value 11/21/2025 false 0001690585 252828108 DIANTHUS THERAPEUTICS, INC. 7 Times Square 43rd Floor New York NY 10036 Ms. Erin O'Connor (267) 262-5300 Fairmount Funds Management LLC 200 Barr Harbor Drive, Suite 400 West Conshohocken PA 19428 0001802528 N Fairmount Funds Management LLC a AF N DE 0.00 2139635.00 0.00 2139635.00 2139635.00 N 4.9 IA The securities include (i) 2,137,191 shares of common stock, $0.001 par value ("Common Stock"), and (ii) pre-funded warrants to purchase up to 2,444 shares of Common Stock (the "Pre-Funded Warrants"), the exercise of which is subject to a beneficial ownership limitation of 4.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 42,878,549 shares of Common Stock outstanding, consisting of 42,876,015 shares of Common Stock outstanding as of November 3, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, and the 2,444 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation. 0001769651 N Fairmount Healthcare Fund II L.P. a AF N DE 0.00 2139635.00 0.00 2139635.00 2139635.00 N 4.9 PN The securities include (i) 2,137,191 shares of Common Stock, and (ii) Pre-Funded Warrants to purchase up to 2,444 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 4.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 42,878,549 shares of Common Stock outstanding, consisting of 42,876,015 shares of Common Stock outstanding as of November 3, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, and the 2,444 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation. 0001663607 N Peter Evan Harwin a AF N X1 0.00 2139635.00 0.00 2139635.00 2139635.00 N 4.9 IN The securities include (i) 2,137,191 shares of Common Stock, and (ii) Pre-Funded Warrants to purchase up to 2,444 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 4.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 42,878,549 shares of Common Stock, consisting of 42,876,015 shares of Common Stock outstanding as of November 3, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, and the 2,444 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation. 0001830177 N Tomas Kiselak a AF N 2B 0.00 2139635.00 0.00 2139635.00 2139635.00 N 4.9 IN The securities include (i) 2,137,191 shares of Common Stock, and (ii) Pre-Funded Warrants to purchase up to 2,444 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 4.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Row 13 is based on 42,878,549 shares of Common Stock outstanding, consisting of 42,876,015 shares of Common Stock outstanding as of November 3, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, and the 2,444 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation. Common Stock, $0.001 par value DIANTHUS THERAPEUTICS, INC. 7 Times Square 43rd Floor New York NY 10036 This Amendment No. 4 amends and supplements the statement on Schedule 13D originally filed with the Securities and Exchange Commission on September 21, 2023, Amendment No. 1 filed on January 24, 2024, Amendment No. 2 filed on April 1, 2024, and Amendment No. 3 filed on September 15, 2025 (as amended, the "Statement" or the "Schedule 13D") by the Reporting Persons with respect to the common stock, $0.001 par value ("Common Stock"), of Dianthus Therapeutics, Inc. (the "Company"). Unless otherwise defined herein, capitalized terms used in this Amendment No. 4 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged. The address of the principal executive offices of the Company is 7 Times Square, 43rd Floor, New York, NY 10036. This Schedule 13D is being filed jointly by (1) Fairmount Funds Management LLC, a Delaware limited liability company and Securities and Exchange Commission registered investment adviser under the Investment Advisers Act of 1940 ("Fairmount"); (2) Fairmount Healthcare Fund II L.P., a Delaware limited partnership ("Fund II"); (3) Peter Harwin; and (4) Tomas Kiselak (Mr. Harwin, Mr. Kiselak, Fairmount, and Fund II are collectively referred to herein as the "Reporting Persons"). The joint filing agreement of the Reporting Persons is attached as Exhibit 99.1 to this Schedule 13D. Fairmount serves as investment manager for Fund II and may be deemed a beneficial owner for purposes of Section 13(d) of the Securities Exchange Act of 1934 (the "Act") of any securities of the Company held by Fund II. Fund II has delegated to Fairmount the sole power to vote and the sole power to dispose of all securities held in Fund II's portfolio, including the shares of Common Stock reported herein. Because Fund II has divested itself of voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days' notice, Fund II disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act and therefore disclaims any obligation to report ownership of the reported securities under Section 13(d) of the Act. As managers of Fairmount, Mr. Harwin, and Mr. Kiselak may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Company beneficially owned by Fairmount. Fairmount, Mr. Harwin and Mr. Kiselak disclaim beneficial ownership of the securities reported in this Schedule 13D other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13D shall not be deemed an admission that any of Fairmount, Mr. Harwin or Mr. Kiselak is the beneficial owner of such securities for any other purpose. The principal business address of each of the Reporting Persons is 200 Barr Harbor Drive, Suite 400, West Conshohocken, PA 19428. The principal business of Fairmount is to provide discretionary investment management services to qualified investors through Fund II, a private pooled investment vehicle. The principal occupation of Mr. Harwin and Mr. Kiselak is investment management. During the last five years, none of the Reporting Persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors). During the last five years, none of the Reporting Persons has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceedings was or is subject to a judgment, decree, or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violations with respect at such laws. Fairmount is a Delaware limited liability company. Fund II is a Delaware limited partnership. Mr. Harwin is a citizen of the United States of America. Mr. Kiselak is a citizen of Slovakia. Item 3 of the Statement is hereby amended and supplemented as follows: In aggregate, the Reporting Persons have voting and dispositive power over 2,139,635 shares of Common Stock of the Company, which is comprised of (a) 2,137,191 shares of Common Stock, and (b) Pre-Funded Warrants to purchase up to 2,444 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 4.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. Item 4 of the Statement is hereby amended and supplemented as follows: On November 24, 2025, Fund II provided notice to the Company to decrease the Maximum Percentage of the Pre-Funded Warrants from 9.99% to 4.99%, such that the Reporting Persons may only exercise the Pre-Funded Warrants to the extent that after giving effect to such exercise the holders thereof and their affiliates and any persons who are members of a Section 13(d) group with the holders would beneficially own in the aggregate no more than 4.99% of the outstanding Common Stock of the Company. By notice to the Company, the Reporting Persons may increase or decrease the Maximum Percentage to any other percentage not in excess of 19.99%; provided that any such increase will not be effective until the 61st day after such notice is delivered to the Company. The decrease of the Maximum Percentage became effective immediately. The Reporting Persons do not have any present plans or proposals that relate to or would result in any of the actions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D, although, the Reporting Persons, at any time and from time to time, may review, reconsider and change their position and/or change their purpose and/or develop such plans and may seek to influence management or the board of directors of the Company with respect to the business and affairs of the Company and may from time to time consider pursuing or proposing such matters with advisors, the Company or other persons. The information in rows 11 and 13 of each of the cover pages of this Amendment No. 4 is incorporated by reference herein. The percentages used in this Schedule 13D are calculated based upon 42,878,459 shares of Common Stock outstanding, consisting of 42,876,015 shares of Common Stock outstanding as of November 3, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, and 2,444 shares of Common Stock underlying the Pre-Funded Warrants owned by the Reporting Persons, subject to the beneficial ownership limitation. The information in rows 7 through 10 of each of the cover pages of this Amendment No. 4 is incorporated by reference herein. The Reporting Persons' securities consist of (a) 2,137,191 shares of Common Stock, and (b) Pre-Funded Warrants to purchase up to 2,444 shares of Common Stock, the exercise of which is subject to a beneficial ownership limitation of 4.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon exercise of the Pre-Funded Warrants in excess of the beneficial ownership limitation. On November 21, 2025, Fund II sold in a block trade a total of 1,170,000 shares of Common Stock at a price of $39.50 per share. Fairmount is the investment manager to Fund II and has voting and dispositive power over shares of Common Stock held on behalf of Fund II. As of the date of this filing, the Reporting Persons no longer beneficially own 5% or more of the total outstanding shares of Common Stock. Consequently, this Amendment No. 4 is the final amendment to the Schedule 13D and constitutes an "exit filing" for the Reporting Persons. Exhibit 99.1 Joint Filing Agreement Fairmount Funds Management LLC /s/ Peter Harwin Peter Harwin, Managing Member 11/25/2025 /s/ Tomas Kiselak Tomas Kiselak, Managing Member 11/25/2025 Fairmount Healthcare Fund II L.P. /s/ Peter Harwin Peter Harwin, Managing Member 11/25/2025 /s/ Tomas Kiselak Tomas Kiselak, Managing Member 11/25/2025 Peter Evan Harwin /s/ Peter Harwin Peter Harwin 11/25/2025 Tomas Kiselak /s/ Tomas Kiselak Tomas Kiselak 11/25/2025