Letter to the Duchenne Community: Organizational Update to Advance Development of Duchenne Gene Therapy Pipeline Programs

Dear Duchenne Community,

In a press release issued this morning, we shared an update to our company's strategy and reiterated our commitment to advancing both of our gene therapy programs for Duchenne: SGT-001 and SGT-003. We wanted to provide you with a bit more context and what this means for the future of these ongoing programs.

SGT-001

Last month, we provided an update on patients dosed with SGT-001 two years after treatment. These data show sustained benefit two years after treatment compared with natural history trajectories across functional, pulmonary, and patient reported outcomes.

In the update provided this morning, we announced that we have decided to conclude enrollment in the IGNITE DMD Phase I/II clinical trial in order to FDA and engagement and planning for future clinical activities. Per study protocol, all patients dosed will continue with their scheduled follow-up visits 5 years post-infusion. As part of the clinical activities for IGNITE DMD, we will be developing a primary analysis of data from all nine patients who were treated in IGNITE DMD through the primary end point at one year. We plan to make information related to the full study, as well as individual patient data, available to all study participants.

We will also be making the shift to a commercially scaled transient transfection-based manufacturing process for SGT-001. Following a robust manufacturing analysis, we believe that by transitioning to a transient-based process, we will be able to produce a high-quality drug product and see improvements to manufacturability and clinical supply capabilities.

This was a natural opportunity for the company to complete enrollment in the study. We will be moving aggressively through the transition process and have already contacted the FDA to determine how we can rapidly return to the clinic with SGT-001. We expect to resume dosing with SGT-001 in the first half of 2023.

We would like to take this opportunity to express our immense gratitude for the patients who have participated in IGNITE DMD, and for all patients and families who choose to participate in clinical trials. Without your bravery and willingness to participate in clinical trials, we would not be able to advance this important science in our pursuit to develop meaningful therapies for Duchenne.

SGT-003

We also shared new data that support SGT-003, a novel, next generation capsid candidate that we believe may have meaningful advantages for the delivery of muscle-related gene therapies. New data from a non-human primate study using a reporter transgene in our novel capsid demonstrated increased muscle tropism, decreased liver biodistribution and improved efficiency compared with AAV9. These results are consistent with earlier in vitro and in vivo studies in mouse models, which suggested improved muscle tropism with our novel capsid as well as improved expression of Solid's microdystrophin compared with AAV9. In the SGT-003 program for Duchenne, we combine this novel, muscle tropic capsid with our differentiated microdystrophin. We are excited about these compelling new data and remain on track for an anticipated early 2023 IND submission.

As a result of our decision to align Solid behind these two promising gene therapy programs, we had to make some tough choices to re-focus our resources. As part of the organizational changes, we will reduce our work force by approximately thirty five percent and curtail activities on our other research and development programs unrelated to SGT-001, SGT-003 and next generation capsids. We are grateful for the efforts and contributions of every employee who has worked tirelessly to advance our mission.

Thank you for your continued support of Solid. As always, our commitment to the Duchenne community is unwavering. We will continue to work every day with resolve and urgency towards the unified goal of developing meaningful treatments and improving the lives of patients with Duchenne.

Upcoming Community Presentations

• CureDuchenne FUTURES National Conference: May 27-29, 2022
• Parent Project Muscular Dystrophy Annual Conference: June 23-26, 2022

As always, we appreciate your continued support. #TogetherWeAreSolid

Sincerely,

Your Solid Biosciences Team