# EDGAR Filing Document

**Accession Number:** 0001636282
**File Stem:** 0001104659-26-045578
**Filing Date:** 2026-4
**Character Count:** 21138
**Document Hash:** 19ad5c72a83111e95aa4d5ebe14dfb21
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-26-045578.hdr.sgml**: 20260420

**ACCESSION NUMBER**: 0001104659-26-045578

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

**PUBLIC DOCUMENT COUNT**: 1

**FILED AS OF DATE**: 20260420

**DATE AS OF CHANGE**: 20260420

**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:** 26876284

**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

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

## SCHEDULE 13D

### Under the Securities Exchange Act of 1934

**(Amendment No. 6)**

**SPYRE THERAPEUTICS, INC.**

*(Name of Issuer)*

**Common Stock, $0.0001 par value**

*(Title of Class of Securities)*

**—**

*(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)*

**04/16/2026**

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

| **CUSIP No.** | **—** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**8184941.00** |
| 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>**8184941.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**8184941.00** | Aggregate amount beneficially owned by each reporting person<br>**8184941.00** | |
| 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>**9.09%** | Percent of class represented by amount in Row (11)<br>**9.09%** | |
| 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) 4,166,840 shares of Common Stock issuable upon conversion of 104,171 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 90,060,297 shares of Common Stock outstanding as of April 16, 2026, consisting of (i) 85,664,811 shares of Common Stock outstanding as of April 16, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated April 14, 2026, (ii) 228,646 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) 4,166,840 shares of Common Stock underlying the 104,171 shares of Series A Preferred Stock owned by Fund II, applying the beneficial ownership limitation.

| **CUSIP No.** | **—** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**8184941.00** |
| 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>**8184941.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**8184941.00** | Aggregate amount beneficially owned by each reporting person<br>**8184941.00** | |
| 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>**9.09%** | Percent of class represented by amount in Row (11)<br>**9.09%** | |
| 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) 4,166,840 shares of Common Stock issuable upon conversion of 104,171 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 90,060,297 shares of Common Stock outstanding as of April 16, 2026, consisting of (i) 85,664,811 shares of Common Stock outstanding as of April 16, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated April 14, 2026, (ii) 228,646 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) 4,166,840 shares of Common Stock underlying the 104,171 shares of Series A Preferred Stock owned by Fund II, applying the beneficial ownership limitation.

| **CUSIP No.** | **—** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Peter Harwin** | Name of reporting person<br>**Peter 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>**520361.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**8496341.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**520361.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**8496341.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**9016702.00** | Aggregate amount beneficially owned by each reporting person<br>**9016702.00** | |
| 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) 114,323 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) 4,478,240 shares of Common Stock issuable upon conversion of 111,956 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 90,257,374 shares of Common Stock outstanding as of April 16, 2026, consisting of (i) 85,664,811 shares of Common Stock outstanding as of April 16, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated April 14, 2026, (ii) 114,323 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) 4,478,240 shares of Common Stock underlying the 111,956 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.** | **—** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**520361.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 8 | Shared Voting Power<br>**8496341.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 9 | Sole Dispositive Power<br>**520361.00** |
| Number of Shares<br>Beneficially Owned by<br>Each Reporting Person With: | 10 | Shared Dispositive Power<br>**8496341.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**9016702.00** | Aggregate amount beneficially owned by each reporting person<br>**9016702.00** | |
| 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) 114,323 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) 4,478,240 shares of Common Stock issuable upon conversion of 111,956 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 90,257,374 shares of Common Stock outstanding as of April 16, 2026, consisting of (i) 85,664,811 shares of Common Stock outstanding as of April 16, 2026, as reported in the Company's final prospectus supplement filed pursuant to Rule 424(b)(5) dated April 14, 2026, (ii) 114,323 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) 4,478,240 shares of Common Stock underlying the 111,956 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. 6 amends and supplements the statement on Schedule 13D originally filed with the 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, Amendment No. 4 filed on April 25, 2024, and Amendment No. 5 filed on October 17, 2025 (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. 6 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. 6 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. 5 to the Statement, filed on October 17, 2025.

**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. 6 is incorporated by reference herein.

Together, the Reporting Persons' securities include (i) 4,830,177 shares of Common Stock, (ii) 228,646 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) 3,912,800 shares of Common Stock issuable upon conversion of 97,820 shares of Series A Preferred Stock subject to the 9.99% beneficial ownership limitation applied to all of the securities owned by the Reporting Persons. The securities exclude shares of Common Stock issuable upon conversion of 248,225 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. 6 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 April 16, 2026,  the Reporting Persons entered into a customary lock-up letter agreement (the "Lock-Up Agreement") with Jefferies LLC, Goldman Sachs & Co. LLC, Evercore Group L.L.C. and Guggenheim Securities, LLC as the representatives (the "Representatives") of the underwriters to an underwriting agreement with the Company. Pursuant to the Lock-Up Agreement, the Reporting Persons 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:** 04/20/2026

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:** 04/20/2026

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 Harwin

**Signature:** /s/ Peter Harwin

**Name/Title:** Peter Harwin

**Date:** 04/20/2026

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:** 04/20/2026