Dyne Therapeutics, Inc. Debt Disclosure

7. Long-Term Debt

On June 27, 2025 (the “Closing Date”), the Company entered into the loan agreement with Hercules (as subsequently amended, the “Loan Agreement”), in its capacity as administrative agent and collateral agent (the “Agent”) and as a lender, and certain other financial institutions that from time to time become parties to the Loan Agreement as lenders (collectively, the “Lenders”). On December 8, 2025, the Company entered into the First Amendment to the Loan Agreement with Hercules. The Loan Agreement, as amended by the First Amendment, provides for term loans in an aggregate principal amount of up to $275.0 million under multiple tranches (the “Term Loans”), available as follows: (i) an initial term loan tranche funded on the Closing Date in aggregate principal amount of $100.0 million (the “Initial Tranche”); (ii) subject to the achievement of specified clinical, regulatory and commercial milestones, three additional term loan tranches totaling up to $125.0 million; and (iii) subject to approval by the Lenders’ investment committee, in their discretion, a final term loan tranche of up to $50.0 million.

All unpaid principal and accrued and unpaid interest with respect to the Term Loans is due and payable in full on July 1, 2030 (the “Maturity Date”). The outstanding principal balance of the Term Loans bears

interest at a floating interest rate per annum equal to the Wall Street Journal prime rate, subject to a floor of 7.50%, plus 2.45%. Accrued interest on the outstanding Term Loans is payable monthly. The Company may make payments of interest only until July 1, 2029. The interest only period may be extended until the Maturity Date upon the achievement of specified clinical, regulatory and commercial milestones. At the end of the interest only period, the Company is required to begin repayment of the outstanding principal of the Term Loans in equal monthly installments (or, in a single installment, if the interest-only period has been extended to the Maturity Date).

As collateral for the obligations under the Loan Agreement, the Company has granted to the Agent, for the benefit of the Lenders, a first-priority security interest in substantially all of its property, inclusive of intellectual property, subject to customary permitted liens and other exceptions set forth in the Loan Agreement.

The Loan Agreement contains customary representations and warranties, events of default and affirmative and negative covenants, including a minimum cash covenant (the “Minimum Cash Covenant”) requiring the Company to maintain specified levels of cash in accounts subject to a control agreement in favor of the Agent (“Qualified Cash”) during the period commencing on January 1, 2027. The Minimum Cash Covenant will initially be set at 60% of the then outstanding principal balance of the Term Loans, is subject to adjustment and will not be tested at any time when the Company’s market capitalization is greater than $1.65 billion. The Company is also required to maintain minimum net product revenue from the sale of z-rostudirsen and z-basivarsen starting nine months after U.S. Food and Drug Administration approval of z-rostudirsen or z-basivarsen (the “Minimum Revenue Covenant”) if the outstanding principal balance of the Term Loans exceeds $100.0 million. The Minimum Revenue Covenant will not be tested for any month to the extent that for each day during such month either (i) Qualified Cash is at least 100% of the Company’s outstanding obligations under the Loan Agreement or (ii) the Company’s market capitalization is greater than $1.65 billion and Qualified Cash is at least 50% of the Company’s outstanding obligations under the Loan Agreement. Certain negative covenants under the Loan Agreement limit the ability of the Company, among other things, to incur future debt, grant liens, make investments, make acquisitions, distribute dividends and sell assets, subject in each case to certain exceptions.

Upon the occurrence of an event of default, including the failure by the Company to comply with the covenants under the Loan Agreement or the occurrence of a material adverse effect on the business, operations, properties, assets or financial condition of the Company, in each case subject to certain exceptions, and subject to any specified cure periods, all amounts owed by the Company under the Loan Agreement may be declared immediately due and payable by the Agent, and the Agent may foreclose on collateral.

The Loan Agreement requires the Company to pay closing fees, prepayment penalties and an end-of-term charge equal to 5.5% of the amount of Term Loans borrowed, which amount is due at the earlier of prepayment or the Maturity Date. A prepayment penalty applies to any prepayment of the Term Loans prior to the Maturity Date equal to (i) 2.0% of the principal amount prepaid if the prepayment occurs on or prior to the first anniversary of the Closing Date, (ii) 1.5% of the principal amount prepaid if the prepayment occurs after the first anniversary and on or prior to the second anniversary of the Closing Date, and (iii) 0.75% of the principal amount prepaid if the prepayment occurs after the second anniversary through the day before the Maturity Date.

In connection with the execution of the First Amendment to the Loan Agreement, the Company borrowed a second term loan tranche in an aggregate principal amount of $50.0 million. During the year ended December 31, 2025, the average interest rate for the loan was 9.95%.

Unamortized debt discount and issuance costs were recorded as a reduction of the carrying amount on the Term Loans and are amortized as interest expense using the effective-interest method. The Company recorded total debt discount and debt issuance costs of $1.6 million as of December 31, 2025. The Company is accruing the end-of-term charge on its consolidated balance sheet. During the year ended December 31, 2025, the effective interest rate for the loan was 11.5%.

In addition, debt issuance costs of $0.5 million were recorded in restricted cash and other assets as of December 31, 2025.

As of December 31, 2025, the carrying value of the Term Loans approximates its fair value.

The table of future principal payments excludes the end-of-term charge of 5.5% of the principal amount of the Term Loans borrowed, which is due upon the maturity of the Term Loans.