Tectonic Therapeutic (TECX) SC 13DTectonic Therapeutic / MCGUIRE TERRANCE ownership change

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. )

TECTONIC THERAPEUTIC, INC.

(Name of Issuer)

Common Stock, $0.0001 par value

(Title of Class of Securities)

878972108

(CUSIP Number)

Lauren Crockett

Polaris Partners

One Marina Park Drive, 8th Floor

Boston, MA 02210

(781) 290-0770

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

June 20, 2024

(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 ☐.

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 (“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).

(Continued on following pages)

Item 1. Security and Issuer.

This joint statement on Schedule 13D (this “Statement”) is filed with respect to the common stock, par value $0.0001 per share (“Common Stock”), of Tectonic Therapeutic, Inc., a Delaware corporation (the “Issuer”). The address of the principal executive offices of the Issuer is 490 Arsenal Way, Suite 210, Watertown, MA 02472. Information given in response to each item shall be deemed incorporated by reference in all other items, as applicable.

Item 2. Identity and Background.

(a) This Statement is being filed by Polaris Partners IX, L.P. (“PP IX”), Polaris Partners GP IX, L.L.C. (“PP GP IX”), Polaris Founders Capital Fund I, L.P. (“PFC I”), Polaris Founders Capital Management Co. I, L.L.C. (“PFCM I”), Polaris Founders Capital Fund II, L.P. (“PFC II”) and Polaris Founders Capital Management Co. II, L.L.C. (“PFCM II”, and together with PP IX, PP GP IX, PFC I, PFCM I and PFC II, the “Reporting Entities”) and Terrance McGuire (“McGuire”), a member of the Issuer’s board of directors (the “Board”) and Jonathan Flint (“Flint”, and together with McGuire, the “Reporting Individuals”). The Reporting Entities and the Reporting Individuals are collectively referred to as the “Reporting Persons”. The agreement among the Reporting Persons to file jointly in accordance with the provisions of Rule 13d-1(k)(1) under the Act is attached to this Statement as Exhibit 1. Other than those securities reported herein as being held directly by such Reporting Person, each Reporting Person disclaims beneficial ownership of all securities reported in this Statement except to the extent of such Reporting Person’s pecuniary interest therein.

(b) The address of the business offices of each of the Reporting Persons is One Marina Park Drive, 8^th Floor, Boston, MA 02210.

(c) PP IX, PFC I and PFC II are each a venture capital investment entity. PP GP IX, PFCM I and PFCM II are the general partners of the venture capital investment entities. McGuire is a member of the Board, a managing member of PFCM I and PFCM II and an interest holder of PP GP IX. Flint is a managing member of PFCM I and PFCM II. Each of David Barrett, Brian Chee, Amir Nashat and Amy Schulman are the managing members of PP GP IX (the “PP GP IX Managing Members”).

(d) During the past five years, none of the Reporting Persons have been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(e) None of the Reporting Persons are, nor during the last five years have 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 violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(f) Each of PP GP IX, PFCM I and PFCM II are limited liability companies organized under the laws of the State of Delaware. Each of PP IX, PFC I and PFC II are limited partnerships organized under the laws of the State of Delaware. Each McGuire and Flint are citizens of the United States of America.

Item 3. Source and Amount of Funds or Other Consideration.

All of the securities reported herein as beneficially owned by the Reporting Persons were acquired in the Business Combination (as defined below). Avrobio, Inc., a Delaware corporation (“Avrobio”), and Tectonic Therapeutic, Inc., a Delaware corporation (“Tectonic”), entered into an Agreement and Plan of Merger and Reorganization (the “Business Combination Agreement”) on January 30, 2024, pursuant to which, among other matters, Alpine Merger Subsidiary, Inc., a direct, wholly owned subsidiary of Avrobio (“Merger Sub”), will merge with and into Tectonic, with Tectonic surviving as a wholly owned subsidiary of Avrobio (such transaction, the “Business Combination”). After the completion of the Business Combination, Avrobio will change its corporate name to “Tectonic Therapeutic, Inc.” with the surviving entity becoming the Issuer for purposes of this Statement.

At the effective time of the Business Combination (the “Closing”), each share of Tectonic common stock, par value $0.0001 per share (“Tectonic Common Stock”), after giving effect to the conversion of each share of Tectonic preferred stock, par value $0.0001 per share, into Tectonic Common Stock as well as the issuance of the private financing shares (as defined below) and of Company SAFEs (as defined below) will be converted into the right to receive a number of shares of Avrobio common stock (the “Conversion Shares”), par value $0.0001 per share, equal to the exchange ratio.

In connection with the Business Combination, certain investors, including each of PP IX and PFC II agreed to purchase shares of Tectonic Common Stock at a purchase price of $12.39908 per share subject to and immediately prior to the Closing, pursuant to the terms of a subscription agreement entered into by such investors and Tectonic, and certain investors, including PP IX and PFC II consummated certain additional purchases of Tectonic Common Stock pursuant to the conversion of certain simple agreements for future equity entered into by such investors and Tectonic (the “Company SAFEs”). The above summary is qualified by reference to such description and the full text of the Business Combination Agreement, which is filed as Exhibit 2 to this Statement and is incorporated herein by reference.

Following the Closing, the Reporting Persons hold an aggregate of 1,157,976 shares of the Issuer’s Common Stock.

All securities of the Issuer covered by this Statement were originally acquired by each of PP IX, PFC I, and PFC II using investment funds provided to each of PP IX, PFC I, and PFC II by their respective limited and general partner investors. Unless noted above, no part of the purchase price was borrowed by any Reporting Person for the purpose of acquiring any securities discussed in this Item 3.

Item 4. Purpose of Transaction.

The information set forth in Item 3 of this Statement is incorporated herein by reference. The Reporting Persons hold the securities of the Issuer for general investment purposes. The Reporting Persons may, from time to time, depending on prevailing market, economic and other conditions, acquire additional shares of Common Stock or other securities of the Issuer, dispose of any such securities, or engage in discussions with the Issuer concerning such acquisitions or dispositions or further investments in the Issuer. The Reporting Persons intend to review their investment in the Issuer on a continuing basis and, depending upon the price and availability of shares of Common Stock or other securities of the Issuer, subsequent developments affecting the Issuer, the Issuer’s business and prospects, other investment and business opportunities available to the Reporting Persons, general stock market and economic conditions, tax considerations and other factors considered relevant, may decide at any time to increase or to decrease the size of their investment in the Issuer in the open market, in privately negotiated transactions, pursuant to 10b5-1 trading plans or otherwise.

McGuire is a member of the Issuer’s board of directors. In addition, McGuire, in his capacity as a director, may be entitled to receive cash compensation and equity compensation, including stock option or other equity awards, pursuant to the Issuer’s 2024 Equity Incentive Plan (the “Option Plan”), which became effective upon the Closing.

Except as set forth above, the Reporting Persons have no present plans or intentions which would result in or relate to any of the transactions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D.

Item 5. Interest in Securities of the Issuer.

(a) and (b) See Items 7-11 and 13 of the cover pages of this Statement for each Reporting Person and Item 2 above.

(c) Except as reported in this Statement, none of the Reporting Persons has effected any transactions in the Issuer’s securities within the past 60 days.

(d) Under certain circumstances set forth in the respective limited partnership agreements of each of PP IX, PFC I, and PFC II (the “Funds”), the respective limited and general partners of the Funds may be deemed to have the right to receive dividends from, or the proceeds from, the sale of shares of the Issuer owned by such entity of which they are a partner.

(e) Not applicable.

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

The information set forth in Items 3 and 4 of this Statement is incorporated herein by reference.

The Conversion Shares held by the Funds are subject to certain lock-up restrictions, which prohibit, subject to certain limited exceptions, the sale, transfer or conveyance of any of the Conversion Shares held by the Funds for 180 days following the closing of the Business Combination. The above summary is qualified by reference to such description and the full text of the Lock-Up Agreement, which is filed as Exhibit 3 to this Statement and is incorporated herein by reference.

McGuire, in his capacity as a director, may be entitled to receive cash compensation and equity compensation, including stock option or other equity awards, pursuant to the Option Plan, and the Issuer’s non-employee director compensation policy (the “Compensation Policy”). The terms and provisions of the Option Plan and the Compensation Policy are described in the Option Plan filed as Exhibit 10.6 to the Issuer’s Current Report on Form 8-K as filed with the Commission on June 20, 2024 (File No. 001-38537). The full text of the Option Plan is filed as Exhibit 4 to this Statement and is incorporated herein by reference.

The Issuer has entered into indemnification agreements with its directors and executive officers. These agreements provide that the Issuer will indemnify each of its directors and executive officers to the fullest extent permitted by Delaware law. The Issuer will advance expenses, including attorneys’ fees (but excluding judgments, fines and settlement amounts), to each indemnified director or executive officer in connection with any proceeding in which indemnification is available and the Issuer will indemnify its directors and officers for any action or proceeding arising out of that person’s services as a director or officer brought on behalf of the Issuer or in furtherance of the Issuer’s rights. Additionally, certain of the Issuer’s directors may have certain rights to indemnification, advancement of expenses or insurance provided by their affiliates or other third parties, which indemnification relates to and might apply to the same proceedings arising out of such director’s services as a director referenced herein. Nonetheless, the Issuer has agreed in the indemnification agreements that the Issuer’s obligations to those same directors are primary and any obligation of such affiliates or other third parties to advance expenses or to provide indemnification for the expenses or liabilities incurred by those directors are secondary.

Item 7. Material to be Filed as Exhibits.

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.

Dated: June 25, 2024

[* This Statement was executed pursuant to a Power of Attorney. Note that copies of the applicable Powers of Attorney are already on file with the appropriate agencies.]