# EDGAR Filing Document

**Accession Number:** 0001802528
**File Stem:** 0001104659-25-113219
**Filing Date:** 2025-11
**Character Count:** 13851
**Document Hash:** cc8e784b2a0e8c7345714d5ac86a095f
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-25-113219.hdr.sgml**: 20251117

**ACCESSION NUMBER**: 0001104659-25-113219

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

**PUBLIC DOCUMENT COUNT**: 2

**FILED AS OF DATE**: 20251117

**DATE AS OF CHANGE**: 20251117

**SUBJECT COMPANY**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Cogent Biosciences, Inc.
- **CENTRAL INDEX KEY:** 0001622229
- **STANDARD INDUSTRIAL CLASSIFICATION:** PHARMACEUTICAL PREPARATIONS [2834]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 465308248
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** SCHEDULE 13D/A
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 005-90431
- **FILM NUMBER:** 251491508

**BUSINESS ADDRESS:**
- **STREET 1:** 275 WYMAN STREET
- **STREET 2:** 3RD FLOOR
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02451
- **BUSINESS PHONE:** 617-945-5576

**MAIL ADDRESS:**
- **STREET 1:** 275 WYMAN STREET
- **STREET 2:** 3RD FLOOR
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02451

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Unum Therapeutics Inc.
- **DATE OF NAME CHANGE:** 20180402

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Unum Therapeutics, Inc.
- **DATE OF NAME CHANGE:** 20141014
**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, $0.001 par value per share, of Cogent Biosciences, 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: November 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 GP LLC** | **FAIRMOUNT HEALTHCARE FUND II GP LLC** |  |
| By: | /s/ Peter Harwin | /s/ Tomas Kiselak |
|  | Peter Harwin | Tomas Kiselak |
|  | Managing Member | Managing Member |

---

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

## SCHEDULE 13D

### Under the Securities Exchange Act of 1934

**(Amendment No. 8)**

**Cogent Biosciences, Inc.**

*(Name of Issuer)*

**Common Stock, $0.001 par value**

*(Title of Class of Securities)*

**19240Q201**

*(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)*

**11/13/2025**

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

| **CUSIP No.** | **19240Q201** |

---

| | | | |
|:--|:--|:--|:--|
| 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>**15877418.00** |
| 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>**15877418.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**15877418.00** | Aggregate amount beneficially owned by each reporting person<br>**15877418.00** | |
| 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>**9.9%** | Percent of class represented by amount in Row (11)<br>**9.9%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**IA** | Type of Reporting Person (See Instructions)<br>**IA** | |

---

**Comment for Reporting Person:** The securities include (a) 9,003,418 shares of common stock, $0.001 par value ("Common Stock"), and (b) 6,874,000 shares of Common Stock issuable upon conversion of approximately 27,496 shares of Series A Convertible Preferred Stock, par value $0.001 per share (the "Series A Preferred Stock") directly held 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 held directly by Fund II in excess of the beneficial ownership limitation of 9.9%.

Row 13 is based on 160,379,562 shares of Common Stock outstanding as of November 13, 2025, consisting of (i) 143,376,529 shares of Common Stock outstanding as of November 5, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) 11,129,033 shares of Common Stock sold in the offering, as reported in the Company's Current Report on Form 8-K filed on November 13,2025, and (iii) 6,874,000 shares of Common Stock underlying the 27,496 shares of Series A Preferred Stock owned by Fund II, subject to the beneficial ownership limitation.

| **CUSIP No.** | **19240Q201** |

---

| | | | |
|:--|:--|:--|:--|
| 1 | Name of reporting person<br>**Fairmount Healthcare Fund II GP LLC** | Name of reporting person<br>**Fairmount Healthcare Fund II GP 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>**15877418.00** |
| 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>**15877418.00** |
| 11 | Aggregate amount beneficially owned by each reporting person<br>**15877418.00** | Aggregate amount beneficially owned by each reporting person<br>**15877418.00** | |
| 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>**9.9%** | Percent of class represented by amount in Row (11)<br>**9.9%** | |
| 14 | Type of Reporting Person (See Instructions)<br>**OO** | Type of Reporting Person (See Instructions)<br>**OO** | |

---

**Comment for Reporting Person:** The securities include (a) 9,003,418 shares of common stock, $0.001 par value ("Common Stock"), and (b) 6,874,000 shares of Common Stock issuable upon conversion of approximately 27,496 shares of Series A Convertible Preferred Stock, par value $0.001 per share (the "Series A Preferred Stock") directly held 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 held directly by Fund II in excess of the beneficial ownership limitation of 9.9%.

Row 13 is based on 160,379,562 shares of Common Stock outstanding as of November 13, 2025, consisting of (i) 143,376,529 shares of Common Stock outstanding as of November 5, 2025, as reported in the Company's most recent Quarterly Report on Form 10-Q, (ii) 11,129,033 shares of Common Stock sold in the offering, as reported in the Company's Current Report on Form 8-K filed on November 13,2025, and (iii) 6,874,000 shares of Common Stock underlying the 27,496 shares of Series A Preferred Stock owned by Fund II, subject to the beneficial ownership limitation.

**Item 1. Security and Issuer**

**(a) Title of Class of Securities:**
Common Stock, $0.001 par value

**(b) Name of Issuer:**
Cogent Biosciences, Inc.

**(c) Address of Issuer's Principal Executive Offices:**
275 Wyman Street, 3rd Floor, Waltham, MA, 02451

This Amendment No. 8 amends and supplements the statement on Schedule 13D originally filed with the Securities and Exchange Commission on July 6, 2020, Amendment No. 1 filed on June 21, 2022, Amendment No. 2 filed on June 13, 2023, Amendment No. 3 filed on February 16, 2024, Amendment No. 4 filed on March 27, 2024, Amendment No. 5 filed on April 1, 2024, Amendment No. 6 filed on June 13, 2024, and Amendment No. 7 filed on July 14, 2025 (as amended, the "Statement" or the "Schedule 13D") by the Reporting Persons with respect to the Common Stock of Cogent Biosciences, Inc. (the "Company"). Unless otherwise defined herein, capitalized terms used in this Amendment No. 8 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged.

**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. 8 is incorporated by reference herein.

**(b)**
The information in rows 7 through 10 of each of the cover pages of this Amendment No. 8 is incorporated by reference herein.

**(c)**
—

**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 November 13, 2025 and the Company's underwritten public offering of convertible senior notes that is expected to close on November 18, 2025, Fairmount Funds Management entered into a customary lock-up letter agreement (the "Lock-Up Agreement") with J.P. Morgan Securities LLC, Jefferies LLC, Leerink Partners LLC and Guggenheim Securities, LLC as representatives (the "Representatives") of the several underwriters to an underwriting agreement with the Company. Pursuant to the Lock-Up Agreement, Fairmount Funds Management 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 45 days after the date of the Company's final prospectus supplement related to the offerings.

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

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:** 11/17/2025

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 GP LLC

**Signature:** /s/ Peter Harwin

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

**Date:** 11/17/2025