Dyne Therapeutics Reports Second Quarter 2023 Financial Results and Business Highlights - On Track to Report Initial Data from ACHIEVE Clinical Trial of DYNE-101 in DM1 and DELIVER Clinical Trial of DYNE-251 in DMD Durin

Dyne Therapeutics Reports Second Quarter 2023 Financial Results and Business Highlights

- On Track to Report Initial Data from ACHIEVE Clinical Trial of DYNE-101 in DM1 and DELIVER Clinical Trial of DYNE-251 in DMD During the Second Half of 2023 -

WALTHAM, Mass., August 3, 2023 Dyne Therapeutics, Inc. (Nasdaq: DYN), a clinical-stage muscle disease company focused on advancing innovative life-transforming therapeutics for people living with genetically driven diseases, today reported financial results for the second quarter of 2023 and business highlights.

This is an exciting time for the entire Dyne team as we continue to enroll and dose patients in our ACHIEVE and DELIVER trials and are on track to report our first clinical data from both during the second half of this year, including evaluating important biomarkers of splicing in DM1 and dystrophin in DMD, said Joshua Brumm, president and chief executive officer of Dyne. In addition to progressing our clinical programs, we were also pleased to present preclinical data at ASGCT in May demonstrating the FORCE platform achieved TfR1-mediated delivery to the CNS, building on previous work showing delivery to skeletal, smooth and cardiac muscle in multiple well-validated preclinical models. CNS symptoms contribute significantly to the burden of neuromuscular disease, and we look forward to further exploring this application of the FORCE platform. Our commitment to advancing the treatment and care of individuals living with rare muscle diseases continues to drive our efforts with a sense of urgency.

-Enrollment continues in ACHIEVE, a Phase 1/2 global clinical trial evaluating DYNE-101 in adult patients with myotonic dystrophy type 1 (DM1). ACHIEVE, which is designed to be a registrational trial, consists of a 24-week multiple ascending dose (MAD), randomized, placebo-controlled period, a 24-week open-label extension and a 96-week long-term extension. The primary endpoints are safety and tolerability, with secondary endpoints of pharmacokinetics and pharmacodynamics, including change from baseline in splicing, as well as measures of muscle strength and function.

-Enrollment continues in DELIVER, a Phase 1/2 global clinical trial evaluating DYNE-251 in males with Duchenne muscular dystrophy (DMD) who have mutations amenable to exon 51 skipping. DELIVER, which is designed to be a registrational trial, consists of a 24-week MAD, randomized, placebo-controlled period, a 24-week open-label extension and a 96-week long-term extension. The primary endpoints are safety, tolerability and change from baseline in dystrophin levels as measured by Western blot. Secondary endpoints include measures of muscle function, exon skipping and pharmacokinetics.

-New preclinical data were featured in an oral presentation at the American Society of Gene & Cell Therapy (ASGCT) 26th Annual Meeting in May 2023 demonstrating the FORCE platform achieved delivery to the central nervous system (CNS) in non-human primates and robust pharmacological effects in the brain in a model of DM1.

-The European Medicines Agency (EMA) granted orphan drug designation for DYNE-101 in DM1 in May 2023.

-Dyne anticipates reporting initial data in the second half of 2023 from:

othe MAD placebo-controlled portion of the ACHIEVE trial of DYNE-101 in DM1 on safety, tolerability and splicing; and

othe MAD placebo-controlled portion of the DELIVER trial of DYNE-251 in DMD on safety, tolerability and dystrophin.

Second Quarter 2023 Financial Results

Cash position: Cash, cash equivalents and marketable securities were $207.7 million as of June 30, 2023, which is anticipated to fund operations through 2024.

Research and development (R&D) expenses: R&D expenses were $59.1 million for the quarter ended June 30, 2023, compared to $46.7 million for the quarter ended June 30, 2022.

General and administrative (G&A) expenses: G&A expenses were $7.6 million for the quarter ended June 30, 2023, compared to $6.1 million for the quarter ended June 30, 2022.

Net loss: Net loss for the quarter ended June 30, 2023 was $64.9 million, or $1.08 per basic and diluted share. This compares with a net loss of $52.3 million, or $1.01 per basic and diluted share, for the quarter ended June 30, 2022.

About Dyne Therapeutics

Forward-Looking Statements

This press release contains forward-looking statements that involve substantial risks and uncertainties. All statements, other than statements of historical facts, contained in this press release, including statements regarding Dyne's strategy, future operations, prospects and plans, objectives of management, the potential of the FORCE platform, the anticipated timelines for reporting data from the DYNE-251 and DYNE-101 clinical trials, the trial design of the DYNE-251 and DYNE-101 clinical trials, and the sufficiency of Dyne's existing cash resources for the period anticipated, constitute forward-looking statements within the meaning of The Private Securities

Litigation Reform Act of 1995. The words anticipate, believe, continue, could, estimate, expect, intend, may, might, objective, ongoing, plan, predict, project, potential, should, or would, or the negative of these terms, or other comparable terminology are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. Dyne may not actually achieve the plans, intentions or expectations disclosed in these forward-looking statements, and you should not place undue reliance on these forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in these forward-looking statements as a result of various important factors, including: uncertainties inherent in the identification and development of product candidates, including the initiation and completion of preclinical studies and clinical trials; uncertainties as to the availability and timing of results from preclinical studies and clinical trials; the timing of and Dyne's ability to initiate and enroll patients in clinical trials; whether results from preclinical studies will be predictive of the results of later preclinical studies and clinical trials; whether Dyne's cash resources will be sufficient to fund the Company's foreseeable and unforeseeable operating expenses and capital expenditure requirements; as well as the risks and uncertainties identified in Dyne's filings with the Securities and Exchange Commission (SEC), including the Company's most recent Form 10-Q and in subsequent filings Dyne may make with the SEC. In addition, the forward-looking statements included in this press release represent Dyne's views as of the date of this press release. Dyne anticipates that subsequent events and developments will cause its views to change. However, while Dyne may elect to update these forward-looking statements at some point in the future, it specifically disclaims any obligation to do so. These forward-looking statements should not be relied upon as representing Dyne's views as of any date subsequent to the date of this press release.