# EDGAR Filing Document

**Accession Number:** 0001636282
**File Stem:** 0001104659-25-100449
**Filing Date:** 2025-10
**Character Count:** 22881
**Document Hash:** f574f26fe5a0e8bdb67cd1060ea960fc
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-25-100449.hdr.sgml**: 20251017

**ACCESSION NUMBER**: 0001104659-25-100449

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

**PUBLIC DOCUMENT COUNT**: 2

**FILED AS OF DATE**: 20251017

**DATE AS OF CHANGE**: 20251017

**SUBJECT COMPANY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Spyre Therapeutics, Inc.
- **CENTRAL INDEX KEY:** 0001636282
- **STANDARD INDUSTRIAL CLASSIFICATION:** PHARMACEUTICAL PREPARATIONS [2834]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 464312787
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D/A
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 005-89456
- **FILM NUMBER:** 251401432

**BUSINESS ADDRESS:**
- **STREET 1:** 221 CRESCENT STREET
- **STREET 2:** BUILDING 23, SUITE 105
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02453
- **BUSINESS PHONE:** 6176515940

**MAIL ADDRESS:**
- **STREET 1:** 221 CRESCENT STREET
- **STREET 2:** BUILDING 23, SUITE 105
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02453

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Aeglea BioTherapeutics, Inc.
- **DATE OF NAME CHANGE:** 20150311
**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, $0.0001 par value per share, of Spyre 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 17, 2025

---

| | | |
|:---|:---|:---|
| **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. 5)**

**SPYRE THERAPEUTICS, INC.**

*(Name of Issuer)*

**Common Stock, $0.0001 par value**

*(Title of Class of Securities)*

**00773J202**

*(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)*

**10/15/2025**

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

| **CUSIP No.** | **00773J202** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**7272341.00** |
| 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>**7272341.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**7272341.00** | Aggregate amount beneficially owned by each reporting person<br>**7272341.00** | |
| 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>**8.99%** | Percent of class represented by amount in Row (11)<br>**8.99%** | |
| 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) 4,018,101 shares of common stock, $0.0001 par value per share (the "Common Stock") and (ii) 3,254,240 shares of Common Stock issuable upon conversion of 81,356 shares of Series A Preferred Stock, par value $0.0001 per share (the "Series A Preferred Stock"), held directly by Fairmount Healthcare Fund II L.P. ("Fund II"). The securities exclude shares of Common Stock issuable upon conversion of shares of Series A Preferred Stock and Series B Preferred Stock, par value $0.0001 per share (the "Series B Preferred Stock") held directly by Fund II in excess of the beneficial ownership limitation of 9.99%.

Row 13 is based on 80,925,407 shares of Common Stock outstanding as of October 15, 2025, consisting of (i) 77,467,521 shares of Common Stock outstanding as of October 15, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 14, 2025, (ii) 203,646 shares underlying options that are currently exercisable or will be exercisable within 60 days of the date of this filing by the Reporting Persons and (iii) 3,254,240 shares of Common Stock underlying the 81,356 shares of Series A Preferred Stock owned by Fund II, applying the beneficial ownership limitation.

| **CUSIP No.** | **00773J202** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**7272341.00** |
| 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>**7272341.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**7272341.00** | Aggregate amount beneficially owned by each reporting person<br>**7272341.00** | |
| 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>**8.99%** | Percent of class represented by amount in Row (11)<br>**8.99%** | |
| 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) 4,018,101 shares of Common Stock and (ii) 3,254,240 shares of Common Stock issuable upon conversion of 81,356 shares of Series A Preferred Stock held directly by Fund II. The securities exclude shares of Common Stock issuable upon conversion of shares of Series A Preferred Stock and Series B Preferred Stock held directly by Fund II in excess of the beneficial ownership limitation of 9.99%.

Row 13 is based on 80,925,407 shares of Common Stock outstanding as of October 15, 2025, consisting of (i) 77,467,521 shares of Common Stock outstanding as of October 15, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 14, 2025, (ii) 203,646 shares underlying options that are currently exercisable or will be exercisable within 60 days of the date of this filing by the Reporting Persons and (iii) 3,254,240 shares of Common Stock underlying the 81,356 shares of Series A Preferred Stock owned by Fund II, applying the beneficial ownership limitation.

| **CUSIP No.** | **00773J202** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**507861.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**7599021.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**507861.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**7599021.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**8106882.00** | Aggregate amount beneficially owned by each reporting person<br>**8106882.00** | |
| 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) 406,038 shares of Common Stock held directly by Mr. Harwin, (b) 101,823 shares of Common Stock issuable upon the exercise of options that are currently exercisable or will be exercisable within 60 days of the date of this filing held directly by Mr. Harwin*, and (c) Fund II's direct holdings of (i) 4,018,101 shares of Common Stock and (ii) 3,580,920 shares of Common Stock issuable upon conversion of 89,523 shares of Series A Preferred Stock held directly by Fund II. The securities exclude shares of Common Stock issuable upon conversion of shares of Series A Preferred Stock and Series B Preferred Stock held directly by Fund II in excess of the beneficial ownership limitation of 9.99%.

Row 13 is based on 81,150,264 shares of Common Stock outstanding as of October 15, 2025, consisting of (i) 77,467,521 shares of Common Stock outstanding as of October 15, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 14, 2025, (ii) 101,823 shares of Common Stock underlying options that are currently exercisable or will be exercisable within 60 days of the date of this filing by Mr. Harwin and (iii) 3,580,920 shares of Common Stock underlying the 89,523 shares of Series A Preferred Stock owned by Fund II, applying 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.** | **00773J202** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**507861.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**7599021.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**507861.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**7599021.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**8106882.00** | Aggregate amount beneficially owned by each reporting person<br>**8106882.00** | |
| 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) 406,038 shares of Common Stock held directly by Mr. Kiselak, (b) 101,823 shares of Common Stock issuable upon the exercise of options that are currently exercisable or will be exercisable within 60 days of the date of this filing held directly by Mr. Kiselak*, and (c) Fund II's direct holdings of (i) 4,018,101 shares of Common Stock and (ii) 3,580,920 shares of Common Stock issuable upon conversion of 89,523 shares of Series A Preferred Stock held directly by Fund II. The securities exclude shares of Common Stock issuable upon conversion of shares of Series A Preferred Stock and Series B Preferred Stock held directly by Fund II in excess of the beneficial ownership limitation of 9.99%.

Row 13 is based on 81,150,264 shares of Common Stock outstanding as of October 15, 2025, consisting of (i) 77,467,521 shares of Common Stock outstanding as of October 15, 2025, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated October 14, 2025, (ii) 101,823 shares of Common Stock underlying options that are currently exercisable or will be exercisable within 60 days of the date of this filing by Mr. Kiselak and (iii) 3,580,920 shares of Common Stock underlying the 89,523 shares of Series A Preferred Stock owned by Fund II, applying 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, $0.0001 par value

**(b) Name of Issuer:**
SPYRE THERAPEUTICS, INC.

**(c) Address of Issuer's Principal Executive Offices:**
221 Crescent Street, Building 23, Suite 105, Waltham, MA, 02453

This Amendment No. 5 amends and supplements the statement on Schedule 13D originally filed withthe Securities and Exchange Commission (the "SEC") on June 30, 2023, Amendment No. 1 filed on November 29, 2023, Amendment No. 2 filed on December 11, 2023, Amendment No. 3 filed on December 29, 2023, and Amendment No. 4 filed on April 25, 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. 5 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. 5 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, Series A Preferred Stock, and Series B Preferred Stock beneficially owned by the Reporting Persons has not changed since Amendment No. 4 to the Statement, filed on April 25, 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. 5 is incorporated by reference herein.

Together, the Reporting Persons' securities include (i) 4,830,177 shares of Common Stock, (ii) 203,646 shares of Common Stock issuable upon the exercise of options held by the Reporting Persons that are currently exercisable or will be exercisable within 60 days of the date of this filing and, (iii) 3,028,000 shares of Common Stock issuable upon conversion of 75,700 shares of Series A Preferred Stock subject to the 9.99% beneficial ownership limitation giving effect all of the securities owned by the Reporting Persons. The securities exclude shares of Common Stock issuable upon conversion of 270,345 shares of Series A Preferred Stock and 16,667 shares of Series B Preferred Stock held directly by Fund II in excess of the beneficial ownership limitation of 9.99%.

**(b)**
The information in rows 7 through 10 of each of the cover pages of this Amendment No. 5 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 that closed on October 15, 2025, the Fund II entered into a customary lock-up letter agreement (the "Lock-Up Agreement") with Jefferies LLC, TD Securities (USA) LLC, Leerink Partners LLC and Stifel, Nicolaus & Company Incorporated as the representatives (the "Representatives") of the underwriters 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 the Representatives, 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:** 10/17/2025

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:** 10/17/2025

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:** 10/17/2025

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:** 10/17/2025