# EDGAR Filing Document

**Accession Number:** 0001802528
**File Stem:** 0001104659-25-099401
**Filing Date:** 2025-10
**Character Count:** 21533
**Document Hash:** 967b70f8393fdd06dce4d6f427742773
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-25-099401.hdr.sgml**: 20251014

**ACCESSION NUMBER**: 0001104659-25-099401

**CONFORMED SUBMISSION TYPE**: SCHEDULE 13D/A

**PUBLIC DOCUMENT COUNT**: 2

**FILED AS OF DATE**: 20251014

**DATE AS OF CHANGE**: 20251014

**SUBJECT COMPANY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Apogee Therapeutics, Inc.
- **CENTRAL INDEX KEY:** 0001974640
- **STANDARD INDUSTRIAL CLASSIFICATION:** BIOLOGICAL PRODUCTS (NO DIAGNOSTIC SUBSTANCES) [2836]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 934958665
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D/A
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 005-94122
- **FILM NUMBER:** 251393046

**BUSINESS ADDRESS:**
- **STREET 1:** 221 CRESCENT ST.
- **STREET 2:** BUILDING 17, SUITE 102B
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02453
- **BUSINESS PHONE:** 650-394-5230

**MAIL ADDRESS:**
- **STREET 1:** 221 CRESCENT ST.
- **STREET 2:** BUILDING 17, SUITE 102B
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02453

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Apogee Therapeutics, LLC
- **DATE OF NAME CHANGE:** 20230420
**FILED BY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Fairmount Funds Management LLC
- **CENTRAL INDEX KEY:** 0001802528

**ORGANIZATION NAME:**
- **EIN:** 364841215
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D/A

**BUSINESS ADDRESS:**
- **STREET 1:** 200 BARR HARBOR DRIVE
- **STREET 2:** SUITE 400
- **CITY:** WEST CONSHOHOCKEN
- **STATE:** PA
- **ZIP:** 19428
- **BUSINESS PHONE:** 267-262-5300

**MAIL ADDRESS:**
- **STREET 1:** 200 BARR HARBOR DRIVE
- **STREET 2:** SUITE 400
- **CITY:** WEST CONSHOHOCKEN
- **STATE:** PA
- **ZIP:** 19428

## Exhibit 99.1

**EXHIBIT 99.1**

**JOINT FILING AGREEMENT**

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, the undersigned agree to the joint filing on behalf of each of them of a Statement on Schedule 13D (including any and all amendments thereto) with respect to the Common Stock, $0.00001 par value per share, of Apogee Therapeutics, Inc. and further agree that this Joint Filing Agreement shall be included as an exhibit to such joint filing.

The undersigned further agree that each party hereto is responsible for the timely filing of such Statement on Schedule 13D and any amendments thereto, and for the accuracy and completeness of the information concerning such party contained therein; provided, however, that no party is responsible for the accuracy or completeness of the information concerning any other party, unless such party knows or has reason to believe that such information is inaccurate.

This Joint Filing Agreement may be signed in counterparts with the same effect as if the signature on each counterpart were upon the same instrument.

Date: October 14, 2025

---

| | | |
|:---|:---|:---|
| **FAIRMOUNT FUNDS MANAGEMENT LLC** | **FAIRMOUNT FUNDS MANAGEMENT LLC** | **FAIRMOUNT FUNDS MANAGEMENT LLC** |
| By: | /s/ Peter Harwin | /s/ Tomas Kiselak |
|  | Peter Harwin | Tomas Kiselak |
|  | Managing Member | Managing Member |
| **FAIRMOUNT HEALTHCARE FUND II L.P.** | **FAIRMOUNT HEALTHCARE FUND II L.P.** | **FAIRMOUNT HEALTHCARE FUND II L.P.** |
| By: | /s/ Peter Harwin | /s/ Tomas Kiselak |
|  | Peter Harwin | Tomas Kiselak |
|  | Managing Member | Managing Member |

---

---

| |
|:---|
| /s/ Peter Harwin |
| Peter Harwin |
| /s/ Tomas Kiselak |
| Tomas Kiselak |

---

### UNITED STATES SECURITIES AND EXCHANGE COMMISSION
**Washington, D.C. 20549**

## SCHEDULE 13D

### Under the Securities Exchange Act of 1934

**(Amendment No. 3)**

**Apogee Therapeutics, Inc.**

*(Name of Issuer)*

**Common Stock, $0.00001 par value**

*(Title of Class of Securities)*

**03770N101**

*(CUSIP Number)*

**Ms. Erin O'Connor**<br>Fairmount Funds Management LLC<br>200 Barr Harbor Drive, Suite 400<br>West Conshohocken PA 19428<br>(267) 262-5300

*(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)*

**10/10/2025**

*(Date of Event Which Requires Filing of this Statement)*

| **CUSIP No.** | **03770N101** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Fairmount Funds Management LLC** | Name of reporting person<br>**Fairmount Funds Management LLC** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**AF** | Source of funds (See Instructions)<br>**AF** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**DELAWARE** | Citizenship or place of organization<br>**DELAWARE** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**5678239.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**5678239.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**5678239.00** | Aggregate amount beneficially owned by each reporting person<br>**5678239.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**9.81%** | Percent of class represented by amount in Row (11)<br>**9.81%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**IA** | Type of Reporting Person (See Instructions)<br>**IA** | |

---

**Comment for Reporting Person:** The securities include (i) 2,048,647 shares of common stock, $0.00001 par value per share (the "Common Stock") and (ii) 3,629,592  shares of Common Stock issuable upon conversion of 3,629,592 shares of Non-Voting Common Stock, par value $0.00001 per share (the "Non-Voting Common Stock"), the conversion of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock, both directly held by Fairmount Healthcare Fund II L.P., a Delaware limited partnership ("Fund II"). The securities exclude shares of Common Stock issuable upon conversion of shares of Non-Voting Common Stock held by Fund II in excess of the beneficial ownership limitation.

Row 13 is based on 57,863,578 shares of Common Stock outstanding as of October 10, 2025, consisting of (i) 54,149,570 shares of Common Stock outstanding as of October 10, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 8, 2025, (ii) 84,416 shares underlying vested stock options owned by the Reporting Persons and (iii) 3,629,592 shares of Common Stock underlying the 3,629,592 shares of Non-Voting Common Stock owned by Fund II, applying the beneficial ownership limitation.

| **CUSIP No.** | **03770N101** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Fairmount Healthcare Fund II L.P.** | Name of reporting person<br>**Fairmount Healthcare Fund II L.P.** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**AF** | Source of funds (See Instructions)<br>**AF** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**DELAWARE** | Citizenship or place of organization<br>**DELAWARE** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**5678239.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**0.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**5678239.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**5678239.00** | Aggregate amount beneficially owned by each reporting person<br>**5678239.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**9.81%** | Percent of class represented by amount in Row (11)<br>**9.81%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**PN** | Type of Reporting Person (See Instructions)<br>**PN** | |

---

**Comment for Reporting Person:** The securities include (i) 2,048,647 shares of Common Stock and (ii) 3,629,592 shares of Common Stock issuable upon conversion of 3,629,592 shares of Non-Voting Common Stock, the conversion of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock, both directly held by Fund II. The securities exclude shares of Common Stock issuable upon conversion of shares of Non-Voting Common Stock held by Fund II in excess of the beneficial ownership limitation.

Row 13 is based on 57,863,578 shares of Common Stock outstanding as of October 10, 2025, consisting of (i) 54,149,570 shares of Common Stock outstanding as of October 10, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 8, 2025, (ii) 84,416 shares underlying vested stock options owned by the Reporting Persons and (iii) 3,629,592 shares of Common Stock underlying the 3,629,592 shares of Non-Voting Common Stock owned by Fund II, applying the beneficial ownership limitation.

| **CUSIP No.** | **03770N101** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Peter Evan Harwin** | Name of reporting person<br>**Peter Evan Harwin** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**AF** | Source of funds (See Instructions)<br>**AF** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**X1** | Citizenship or place of organization<br>**X1** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**93374.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**5683507.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**93374.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**5683507.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**5776881.00** | Aggregate amount beneficially owned by each reporting person<br>**5776881.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**9.99%** | Percent of class represented by amount in Row (11)<br>**9.99%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**IN** | Type of Reporting Person (See Instructions)<br>**IN** | |

---

**Comment for Reporting Person:** The securities include (a) 51,166 shares of Common Stock held directly by Mr. Harwin, (b) 42,208 shares of Common Stock underlying vested options held by Mr. Harwin* and (c) Fund II's direct holdings of (i) 2,048,647 shares of Common Stock and (ii) 3,634,860 shares of Common Stock issuable upon conversion of 3,634,860 shares of Non-Voting Common Stock, the conversion of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon conversion of shares of Non-Voting Common Stock in excess of the beneficial ownership limitation.

Row 13 is based on 57,826,638 shares of Common Stock outstanding as of October 10, 2025, consisting of (i) 54,149,570 shares of Common Stock outstanding as of October 10, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 8, 2025, (ii) 42,208 shares of Common Stock underlying vested options held by Mr. Harwin and (iii) 3,634,860 shares of Common Stock underlying the 3,634,860 shares of Non-Voting Common Stock owned by Fund II, subject to the beneficial ownership limitation.

* Under Mr. Harwin's arrangement with Fairmount, Mr. Harwin holds the options for one or more investment vehicles managed by Fairmount (each, a "Fairmount Fund"). Mr. Harwin is obligated to turn over to Fairmount any net cash or stock received from the option for the benefit of such Fairmount Fund. Mr. Harwin therefore disclaims beneficial ownership of the option and underlying common stock.

| **CUSIP No.** | **03770N101** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Tomas Kiselak** | Name of reporting person<br>**Tomas Kiselak** | |
| 2 | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | Check the appropriate box if a member of a Group (See Instructions)<br>[x] (a)<br>[ ] (b) | |
| 3 | SEC use only | SEC use only | |
| 4 | Source of funds (See Instructions)<br>**AF** | Source of funds (See Instructions)<br>**AF** | |
| 5 | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)<br>[ ] | |
| 6 | Citizenship or place of organization<br>**2B** | Citizenship or place of organization<br>**2B** | |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 7 | Sole Voting Power<br>**93374.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**5683507.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**93374.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**5683507.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**5776881.00** | Aggregate amount beneficially owned by each reporting person<br>**5776881.00** | |
| 12 | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)<br>[ ] | |
| 13 | Percent of class represented by amount in Row (11)<br>**9.99%** | Percent of class represented by amount in Row (11)<br>**9.99%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**IN** | Type of Reporting Person (See Instructions)<br>**IN** | |

---

**Comment for Reporting Person:** The securities include (a) 51,166 shares of Common Stock held directly by Mr. Kiselak, (b) 42,208 shares of Common Stock underlying vested options held by Mr. Kiselak* and (c) Fund II's direct holdings of (i) 2,048,647 shares of Common Stock and (ii) 3,634,860 shares of Common Stock issuable upon conversion of 3,634,860 shares of Non-Voting Common Stock, the conversion of which is subject to a beneficial ownership limitation of 9.99% of the outstanding Common Stock. The securities exclude shares of Common Stock issuable upon conversion of shares of Non-Voting Common Stock in excess of the beneficial ownership limitation.

Row 13 is based on 57,826,638 shares of Common Stock outstanding as of October 10, 2025, consisting of (i) 54,149,570 shares of Common Stock outstanding as of October 10, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 8, 2025, (ii) 42,208 shares of Common Stock underlying vested options held by Mr. Kiselak and (iii) 3,634,860 shares of Common Stock underlying the 3,634,860 shares of Non-Voting Common Stock owned by Fund II, subject to the beneficial ownership limitation.

* Under Mr. Kiselak's arrangement with Fairmount, Mr. Kiselak holds the options for one or more investment vehicles managed by Fairmount (each, a "Fairmount Fund"). Mr. Kiselak is obligated to turn over to Fairmount any net cash or stock received from the option for the benefit of such Fairmount Fund. Mr. Kiselak therefore disclaims beneficial ownership of the option and underlying common stock.

**Item 1. Security and Issuer**

**(a) Title of Class of Securities:**
Common Stock, $0.00001 par value

**(b) Name of Issuer:**
Apogee Therapeutics, Inc.

**(c) Address of Issuer's Principal Executive Offices:**
221 Crescent Street, Building 17, Suite 102b, Waltham, MA, 02453

This Amendment No. 3 amends and supplements the statement on Schedule 13D originally filed withthe Securities and Exchange Commission (the "SEC") on July 21,2023, Amendment No. 1 filed on January 31, 2024 and Amendment No. 2 filed on April 1, 2024 (as amended, the "Statement" or the "Schedule 13D") by the Reporting Persons with respect to the Common Stock of the Company. Unless otherwise defined herein, capitalized terms used in this Amendment No. 3 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged. This Amendment No. 3 is being filed solely to report the Reporting Persons' entry into a lock-agreement with the Company in connection with the Company's underwritten public offering; the number of shares of Common Stock and Non-Voting Common Stock beneficially owned by the Reporting Persons has not changed since Amendment No. 2 to the Statement, filed on April 1, 2024.

**Item 5. Interest in Securities of the Issuer**

**(a)**
The information in rows 11 and 13 of each of the cover pages (including the explanatory note in the "Comments") of this Amendment No. 1 is incorporated by reference herein.

**(b)**
The information in rows 7 through 10 of each of the cover pages of this Amendment No. 3 is incorporated by reference herein.

**(c)**
None

**Item 6. Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer.**

Lock-Up Agreement

In connection with the Company's underwritten public offering of common stock and pre-funded warrants that closed on October 10, 2025, the Fund II entered into a customary lock-up letter agreement (the "Lock-Up Agreement") with Jefferies LLC, as representative of the several underwriters party to an underwriting agreement with the Company. Pursuant to the Lock-Up Agreement, Fund II agreed to refrain from selling shares of the Company's securities without the consent of Jefferies LLC, and other customary lock-up conditions, for a period of 60 days from the date of the Company's final prospectus supplement related to the offering.

The Reporting Persons did not purchase any securities of the Company or otherwise participate in the offering.

The foregoing description of the Lock-Up Agreement does not purport to be complete and is qualified in its entirety by reference to the form of the Lock-Up Agreement, which is filed as Exhibit 99.2 to this Schedule 13D and incorporated herein by reference.

### SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Reporting Person:** Fairmount Funds Management LLC

**Signature:** /s/ Peter Harwin

**Name/Title:** Peter Harwin, Managing Member

**Date:** 10/14/2025

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Reporting Person:** Fairmount Healthcare Fund II L.P.

**Signature:** /s/ Peter Harwin

**Name/Title:** Peter Harwin, Managing Member

**Date:** 10/14/2025

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Reporting Person:** Peter Evan Harwin

**Signature:** /s/ Peter Harwin

**Name/Title:** Peter Harwin

**Date:** 10/14/2025

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

**Reporting Person:** Tomas Kiselak

**Signature:** /s/ Tomas Kiselak

**Name/Title:** Tomas Kiselak

**Date:** 10/14/2025