UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13D/A

(Rule 13d-101)

Information to be Included in Statements Filed Pursuant

to § 240.13d-1(a) and Amendments Thereto Filed

Pursuant to § 240.13d-2(a)

Under the Securities Exchange Act of 1934

(Amendment No. 6)

Solid Biosciences Inc.

(Name of Issuer)

Common Stock, $0.001

par value per share

(Title of Class of Securities)

83422E 105

(CUSIP Number)

Bain Capital Life Sciences Investors, LLC

200 Clarendon Street

Boston, MA 02116

617-516-2000

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

December 2, 2022

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ☐

Note. Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934, as amended (“Act”), or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

This Amendment No. 6 to Schedule 13D relates to the Common Stock of Solid Biosciences Inc. and amends the initial statement on Schedule 13D filed by BCLS SB Investco, LP on February 1, 2018, as amended by Amendment No. 1 filed on July 30, 2019, Amendment No. 2 filed on December 11, 2020, Amendment No. 3 filed on March 25, 2021, Amendment No. 4 filed on August 18, 2022 and Amendment No. 5 filed on October 3, 2022 (the “Initial Statement” and, as further amended by this Amendment No. 6, the “Schedule 13D”). Capitalized terms used but not defined herein shall have the meanings ascribed to them in the Initial Statement.

On October 27, 2022, the Issuer effected a one-for-15 reverse stock split of its issued and outstanding shares of Common Stock (the “Reverse Stock Split”). As a result of the Reverse Stock Split, every 15 shares of Common Stock issued and outstanding were converted into one share of Common Stock. All information in this Schedule 13D gives effect to the Reverse Stock Split.

Item 2. Identity and Background

Item 2 of the Initial Statement is hereby amended and restated as follows:

(a) This Schedule 13D is being filed jointly by BCLS SB Investco, LP, a Delaware limited partnership (“BCLS SB”), Bain Capital Life Sciences Fund II, L.P., a Cayman Islands exempted limited partnership (“BCLS Fund II”), BCLS II Investco, LP, a Delaware limited partnership (“BCLS II Investco”), and BCIP Life Sciences Associates, LP (“BCIPLS” and, together with BCLS SB, BCLS Fund II and BCLS II Investco, the “Reporting Persons”).

Bain Capital Life Sciences Investors, LLC, a Delaware limited liability company (“BCLSI”), is the general partner of Bain Capital Life Sciences Partners, LP, a Cayman Islands exempted limited partnership (“BCLSP”), which is the general partner of BCLS SB.

BCLSI is the manager of Bain Capital Life Sciences Investors II, LLC, a Cayman Islands limited liability company (“BCLSI II”), which is the general partner of BCLS Fund II.

BCLS II Investco (GP), LLC, a Delaware limited liability company (“BCLS II Investco GP”), whose managing member is BCLS Fund II, is the general partner of BCLS II Investco.

Boylston Coinvestors, LLC, a Delaware limited liability company (“Boylston” and, together with the Reporting Persons, BCLSI, BCLSP, BCLSI II and BCLS II Investco GP, the “Bain Capital Life Sciences Entities”), is the general partner of BCIPLS. BCLSI governs the investment strategy and decision-making process with respect to investments held by BCIPLS.

As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Reporting Persons.

The Reporting Persons have entered into a Joint Filing Agreement, dated December 5, 2022, pursuant to which the Reporting Persons have agreed to file this Schedule 13D jointly in accordance with the provisions of Rule 13d-1(k)(1) promulgated under the Act.

(b) The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, MA 02116.

(c) Each of the Bain Capital Life Sciences Entities is principally engaged in the business of investment in securities.

(d) During the last five years, none of the Bain Capital Life Sciences Entities has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(e) During the last five years, none of the Bain Capital Life Sciences Entities has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violation of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(f) BCLS Fund II, BCLSP and BCLSI II are organized under the laws of the Cayman Islands. BCLS SB, BCLS II Investco, BCIPLS, BCLSI, BCLS II Investco GP and Boylston are organized under the laws of the State of Delaware.

Item 3. Source and Amount of Funds

Item 3 of the Initial Statement is hereby amended and supplemented to add the following:

AavantiBio Merger Closing

On December 2, 2022, the Issuer consummated the Merger. Pursuant to the terms of the Merger Agreement, BCLS Fund II, BCLS II Investco and BCIPLS received 267,257, 138,835, and 32,550 shares of Common Stock, respectively, in consideration for the shares of AavantiBio preferred stock held by BCLS Fund II, BCLS II Investco and BCIPLS prior to the Merger.

Private Placement Closing

The Private Placement closed on December 2, 2022. Pursuant to the terms of the Securities Purchase Agreement, substantially concurrently with the Merger, BCLS II Investco purchased 2,163,120 shares of Common Stock from the Issuer at a price per share of $7.05, for total consideration of $15,249,996. BCLS II Investco used its own working capital to purchase such shares of Common Stock.

Item 4. Purpose of Transaction

Item 4 of the Initial Statement is hereby amended and supplemented to add the following:

Following the closing of the Merger and the Private Placement, Dr. Adam Koppel, a Managing Director of BCLSI, joined the Issuer’s board of directors on December 2, 2022.

Item 5. Interest in Securities of the Issuer

Item 5 of the Initial Statement is hereby amended and restated as follows:

The information set forth in Items 2 and 3 and on the cover pages of this Schedule 13D is incorporated by reference in its entirety into this Item 5.

(a) – (c) As of the date hereof, (i) BCLS SB holds 528,661 shares of Common Stock, representing approximately 2.7% of the outstanding shares of Common Stock, (ii) BCLS Fund II holds 267,257 shares of Common Stock, representing approximately 1.4% of the outstanding shares of Common Stock, (iii) BCLS II Investco holds 2,301,955 shares of Common Stock, representing approximately 11.8% of the outstanding shares of Common Stock, and (iv) BCIPLS holds 32,550 shares of Common Stock, representing approximately 0.2% of the outstanding shares of Common Stock.

As a result of the foregoing and the relationships described in Item 2(a) of this Schedule 13D, the Reporting Persons may be deemed to beneficially own in the aggregate 3,130,423 shares of Common Stock, representing approximately 16.0% of the outstanding shares of Common Stock.

The percentage of the outstanding shares of Common Stock beneficially owned by the Reporting Persons is based on 19,527,440 shares of Common Stock outstanding, which consists of (i) 7,534,892 shares of Common Stock outstanding as of November 4, 2022, as disclosed by the Issuer in its Quarterly Report on Form 10-Q for the quarter ended September 30, 2022, filed with the Securities and Exchange Commission on November 10, 2022, (ii) an aggregate of 1,354,258 shares of Common Stock issued to AavantiBio shareholders in the Merger, as disclosed by the Issuer in its Current Report on Form 8-K filed with the Securities and Exchange Commission on December 5, 2022, and (iii) an aggregate of 10,638,290 shares of Common Stock issued in the Private Placement, as disclosed by the Issuer in its Current Report on Form 8-K filed with the Securities and Exchange Commission on December 5, 2022.

(d) Except as otherwise described in this Item 5, no one other than the Reporting Persons has the right to receive, or the power to direct the receipt of, dividends from, or the proceeds from the sale of, any of the shares of Common Stock beneficially owned by the Reporting Persons as described in this Item 5.

(e) Not applicable.

Item 7. Material to be Filed as Exhibits

Item 7 of the Initial Statement is hereby amended and supplemented to add the following:

Exhibit M     Joint Filing Agreement

SIGNATURES

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

Exhibit M

AGREEMENT REGARDING THE JOINT FILING OF

SCHEDULE 13D

The undersigned being duly authorized thereunto, hereby execute this agreement as an exhibit to this Schedule 13D to evidence the agreement of the below-named parties in accordance with the rules promulgated pursuant to the Securities Exchange Act of 1934, as amended, to file this Schedule 13D (including amendments thereto) jointly on behalf of each such party.