DIANTHUS THERAPEUTICS, INC.
4.90%
2,139,635
1690585
252828108
Nov 20, 2025
Nov 25, 2025, 09:00 PM
Reporting Persons (4)
This is a joint filing. The reported shares may overlap between reporting persons and should not be summed.
| Name | Type | % of Class | Aggregate | Sole Voting | Shared Voting |
|---|---|---|---|---|---|
| Fairmount Funds Management LLC | Investment Adviser | 4.90% | 2,139,635 | 0 | 2,139,635 |
| Fairmount Healthcare Fund II L.P. | Partnership | 4.90% | 2,139,635 | 0 | 2,139,635 |
| Peter Evan Harwin | Individual | 4.90% | 2,139,635 | 0 | 2,139,635 |
| Tomas Kiselak | Individual | 4.90% | 2,139,635 | 0 | 2,139,635 |
Disclosure Items (6)
Common Stock, $0.001 par value
DIANTHUS THERAPEUTICS, INC.
7 Times Square, 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 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.
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