X0202 SCHEDULE 13D 0001346824 XXXXXXXX LIVE Common Stock, par value $0.001 per share 10/30/2026 false 0002044950 058937103 Bambusa Therapeutics, Inc.

22 Boston Wharf Road, Floor 7 Boston MA 02210

Peter Kolchinsky 617.778.2500 RA Capital Management, L.P. 200 Berkeley Street, 18th Floor Boston MA 02116 0001346824 N RA Capital Management, L.P. AF N DE 0.00 7429758.00 0.00 7429758.00 7429758.00 N 14.6 IA PN 0001384859 N Peter Kolchinsky AF N X1 0.00 7429758.00 0.00 7429758.00 7429758.00 N 14.6 HC IN 0001619841 N Rajeev Shah AF N X1 0.00 7429758.00 0.00 7429758.00 7429758.00 N 14.6 HC IN 0001315082 N RA Capital Healthcare Fund, L.P. WC N DE 0.00 5933787.00 0.00 5933787.00 5933787.00 N 11.6 PN Common Stock, par value $0.001 per share Bambusa Therapeutics, Inc. 22 Boston Wharf Road, Floor 7 Boston MA 02210 This Schedule 13D is being filed on behalf of RA Capital Management, L.P. ("RA Capital"), Peter Kolchinsky, Rajeev Shah and RA Capital Healthcare Fund, L.P. (the "Fund"). RA Capital, Dr. Kolchinsky, Mr. Shah and the Fund are collectively referred to herein as the "Reporting Persons." The agreement among the Reporting Persons to file this Schedule 13D jointly in accordance with Rule 13d-1(k) of the Securities Exchange Act of 1934, as amended, (the "Act") is filed as Exhibit 99.1. The Reporting Persons' ownership of the Issuer's securities includes (i) 5,933,787 shares of common stock held directly by the Fund; and (ii) 1,495,971 shares of common stock held directly by RA Capital Nexus Fund III, L.P. (the "Nexus Fund III"). RA Capital Healthcare Fund GP, LLC is the general partner of the Fund and RA Capital Nexus Fund III GP, LLC is the general partner of the Nexus Fund III. The general partner of RA Capital is RA Capital Management GP, LLC, of which Dr. Kolchinsky and Mr. Shah are the controlling persons. RA Capital serves as investment adviser for each of the Fund and the Nexus Fund III and may be deemed a beneficial owner, for purposes of Section 13(d) of the Act, of any securities of the Issuer held by the Fund or the Nexus Fund III. Each of the Fund and the Nexus Fund III has delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in its portfolio, including the shares of the Issuer's common stock reported herein. Because each of the Fund and the Nexus Fund III has divested itself of voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days' notice, each of the Fund and the Nexus Fund III disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act and therefore disclaims any obligation to report ownership of the reported securities under Section 13(d) of the Act. As managers of RA Capital, Dr. Kolchinsky and Mr. Shah may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of the securities reported in this Schedule 13D other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13D shall not be deemed an admission that either RA Capital, Dr. Kolchinsky, or Mr. Shah is the beneficial owner of such securities for any other purpose. The address of the principal business office of each of the Reporting Persons is 200 Berkeley Street, 18th Floor, Boston, MA 02116. The Fund is a private investment vehicle. RA Capital provides investment management services to the Fund and the Nexus Fund III. The principal occupation of each of Dr. Kolchinsky and Mr. Shah is investment management. During the last five years, none of the Reporting Persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors). During the last five years, none of the Reporting Persons 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 of 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. See Item 6 of the cover pages. Prior to the Issuer's initial public offering (the "IPO"), (i) the Fund purchased 2,844,910 shares of Series A-1 preferred stock of the Issuer and 299,948 shares of Series A-2 preferred stock of the Issuer; and (ii) the Nexus Fund III purchased 1,278,148 shares of Series A-1 preferred stock of the Issuer and 52,932 shares of Series A-2 preferred stock of the Issuer. The foregoing share numbers give effect to a 1.54485-for-1 forward stock split of the Issuer's outstanding capital stock effected on October 23, 2026. The aggregate purchase price of the foregoing securities was $27,777,749. In connection with the closing of the IPO on November 2, 2026, the Series A-1 preferred stock and Series A-2 preferred stock automatically converted into shares of common stock on a 1-to-1 basis. The Fund and the Nexus Fund III also purchased 2,335,109 and 164,891 shares of common stock, respectively, from the underwriters of the IPO at the IPO price of $16.00 per share, for an aggregate purchase price of $40,000,000. On October 30, 2026, the Fund purchased the following shares of the Issuer's common stock in open market transactions: - 189,909 shares at prices ranging from $13.61 to $14.58 per share (a weighted-average price of $14.23 per share); and - 263,911 shares at prices ranging from $14.65 to $15.00 per share (a weighted-average price of $14.92 per share). All purchases of the securities described herein were for cash and were funded by working capital of the Fund and the Nexus Fund III, as applicable. The Reporting Persons acquired the common stock reported herein for investment purposes and not with an intent, purpose or effect of changing control of the Issuer. The Reporting Persons may, from time to time, acquire additional equity securities or debt securities of the Issuer, which debt securities may be convertible or non-convertible, secured or unsecured, and could involve the monetization of potential revenue streams deriving from development-stage or commercial programs of the Issuer, or dispose of Issuer securities they beneficially own, on the open market or in private transactions or otherwise (including by means of 10b5-1 programs), consistent with their investment purposes and in amounts, on such terms and at such times as to be determined by the Reporting Persons based upon a number of factors, including, without limitation, their ongoing assessment of the Issuer's business prospects, the availability of Issuer securities at prices that would make the purchase or sale of such securities desirable, prevailing market conditions, the availability of other investment opportunities, and/or other considerations. In addition, consistent with their investment purpose, the Reporting Persons may engage in communications with persons associated with the Issuer, including stockholders of the Issuer, officers of the Issuer, members of the board of directors of the Issuer, and/or or other third parties, to discuss matters regarding the Issuer, including but not limited to its operations, strategic direction, governance or capitalization, and potential business combinations or dispositions involving the Issuer or certain of its businesses. Dr. Derek DiRocco, a Partner of RA Capital, currently serves as a director of the Issuer and therefore will engage in regular discussions with the Issuer's board of directors and management as part of his duties as a director. Neither Dr. DiRocco (other than in his capacity as a director) nor the Reporting Persons have any present plans or proposals that relate to, or could result in, any of the matters referred to in paragraphs (a) through (j), inclusive, of the instructions to Item 4 of Schedule 13D. Depending on various factors including, without limitation, the Issuer's financial position, results and strategic direction, actions taken by the Issuer's management and Board, other investment opportunities available to RA Capital, the price levels of the Issuer securities, conditions in the securities markets and general economic and industry conditions, the Reporting Persons may change their purpose and formulate and implement plans or proposals with respect to the Issuer at any time and from time to time. Any such action may be made by the Reporting Persons alone or in conjunction with other stockholders, potential acquirers, financing sources and/or other third parties and could include one or more purposes, plans or proposals that relate to or would result in actions required to be reported herein in accordance with Item 4 of Schedule 13D. Rows 11 and 13 of each Reporting Person's cover page to this Schedule 13D set forth the aggregate number of shares of common stock and percentage of outstanding common stock beneficially owned by such Reporting Person and are incorporated by reference. The percentage set forth in each row 13 is based upon 50,942,430 shares of common stock outstanding as of November 2, 2026, as reported in the Issuer's final prospectus dated October 29, 2026 filed with the Securities and Exchange Commission (the "SEC") on October 30, 2026. Rows 7 through 10 of each Reporting Person's cover page to this Schedule 13D set forth the number of shares of common stock as to which such Reporting Person has the sole or shared power to vote or direct the vote and sole or shared power to dispose or to direct the disposition and are incorporated by reference. Except as set forth herein, none of the Reporting Persons has effected any transactions with respect to the securities of the Issuer during the past sixty days. No person (other than the Reporting Persons) is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the shares of common stock subject to this 13D. Not applicable. Lock-up Agreements The holders of substantially all of the Issuer's pre-IPO capital stock, including the Fund and the Nexus Fund III, and each of the Issuer's directors, entered into lock-up agreements (the "Lock-Up Agreements") with the representatives of the underwriters of the Issuer's IPO. Pursuant to the Lock-Up Agreements, neither the Fund nor the Nexus Fund III is permitted, with limited exceptions, for a period of 180 days from October 29, 2026, to dispose of or hedge any of the Issuer's securities, except with the prior written consent of BofA Securities, Inc. and Evercore Group L.L.C. The description of the Lock-Up Agreements contained in this Item 6 is qualified in its entirety by reference to the full text of the form of Lock-up Agreement filed as Exhibit 99.2 to this Schedule 13D and incorporated by reference herein. Amended and Restated Investors' Rights Agreement Certain investors of the Issuer, including the Fund and the Nexus Fund III, entered into the amended and restated investors' right agreement dated February 12, 2025 (the "Investors' Rights Agreement") with the Issuer, pursuant to which the Issuer agreed to register for resale certain shares of common stock and other equity securities of the Issuer that are held by the investors party thereto (the "Registrable Securities"). Pursuant to the Investors' Rights Agreement, at any time beginning 180 days after October 29, 2026, the holders of at least a majority of the Registrable Securities then outstanding can request that the Issuer file a registration statement on Form S-1 to register the offer and sale of their shares. The Issuer is only obligated to effect one such registration. The request for registration must cover at least 40% of the Registrable Securities then outstanding. These demand registration rights are subject to specified conditions and limitations, including the right of the underwriters to limit the number of shares included in any such registration under certain circumstances. If the Issuer determines that it would be materially detrimental to itself to effect such a demand registration, then the Issuer has the right to defer such registration, not more than once in any 12-month period (provided the Issuer has not already deferred a request for registration on Form S-3, within that period), for a period of not more than 120 days. In addition, the holders of the Registrable Securities then outstanding may request the Issuer to register the offer and sale of their shares of the Issuer's common stock on a registration statement on Form S-3 so long as the request covers securities the anticipated aggregate public offering price of which, net of certain selling expenses, is at least $5 million. The Issuer is obligated to effect up to two such registrations within any 12-month period. These Form S-3 registration rights are subject to specified conditions and limitations, including the right of the underwriters to limit the number of shares included in any such registration under certain circumstances. If the Issuer determines that it would be materially detrimental to itself to effect such a registration, then the Issuer has the right to defer such registration, not more than once in any 12-month period (provided the Issuer has not already deferred a request for registration on Form S-1 within that period), for a period of not more than 120 days. The investors are also entitled to customary "piggyback" registration rights, subject to certain requirements and conditions. The Issuer will be responsible for the registration expenses (other than underwriting discounts, selling commissions, and stock transfer taxes) of the holders of the shares to be offered and sold pursuant to the registrations described above, including the reasonable fees and disbursements, not to exceed $30,000 per registration, of one counsel chosen by the holders of the shares included in such registrations. The registration rights granted under the Investors' Rights Agreement will terminate upon the earliest of (1) as to a given holder of registration rights, when such holder of registration rights, together with its affiliates (as defined under SEC Rule 144) holds less than 1% of our outstanding securities and can sell all of such holder's registrable securities without limitation in a three-month period pursuant to Rule 144 promulgated under the Securities Act, (2) immediately prior to the closing of certain liquidation events, and (3) November 2, 2031. The description of the Investors' Rights Agreement contained in this Item 6 is qualified in its entirety by reference to the full text of the Amended and Restated Investors' Rights Agreement filed as Exhibit 99.3 to this Schedule 13D and incorporated by reference herein. Equity Awards Dr. Derek DiRocco, a Partner of RA Capital, is a member of the Board of Directors of the Issuer. From time to time, Dr. DiRocco may receive stock options or other awards of equity-based compensation pursuant to the Issuer's compensation arrangements for non-employee directors. Pursuant to an agreement with RA Capital, Dr. DiRocco is obligated to transfer any securities issued under any such stock options or other awards, or the economic benefit thereof, to RA Capital. Exhibit 99.1 Joint Filing Agreement Exhibit 99.2 Form of Lock-up Agreement (incorporated by reference to Exhibit A to the Form of Underwriting Agreement filed as Exhibit 1.1 to the Issuer's Registration Statement on Form S-1, as amended (Registration No. 333-299405), filed with the SEC on October 26, 2026). Exhibit 99.3 Amended and Restated Investors' Rights Agreement (incorporated by reference to Exhibit 4.1 to the Issuer's Registration Statement on Form S-1 (Registration No. 333-299405), filed with the SEC on October 29, 2026). RA Capital Management, L.P. /s/ Peter Kolchinsky 11/06/2026 Peter Kolchinsky /s/ Peter Kolchinsky 11/06/2026 Rajeev Shah /s/ Rajeev Shah 11/06/2026 RA Capital Healthcare Fund, L.P. /s/ Peter Kolchinsky 11/06/2026