1300 Rancho Conejo Blvd.
Thousand Oaks, CA 91320
To the STXBP1 Community:
We wanted to provide an update following our January 7, 2026 letter.
As we shared previously, the autopsy report for the patient treated with CAP-002 who tragically passed away in September identified cerebral edema (brain swelling) as the cause of death. Importantly, the report did not identify the underlying reason for this occurrence. We recognize how difficult this remains for the family and for this community, and we remain mindful of their loss.
Since that time, we have been conducting additional laboratory research to better understand what happened. This work is focused on how the delivery system used in CAP-002, which is designed to carry the therapy into the brain, interacts with the body. Included in this work is studying further how it engages with a protein called ADAM15, which helps the therapy cross into the brain.
Our goal is to understand whether there were unexpected differences in how this process occurred in the patient compared to what we have seen in prior studies, and whether those differences may have contributed to this outcome. We are also working to understand whether what we observed is specific to the capsid that carries the therapy, the receptor it binds to, the therapy itself or other factors. This analysis is complex and will take more time to understand.
Since late September, the SYNRGY trial for STXBP-1 related DEE has been on clinical hold. Based on what we know today, we have made the decision to close out the trial. This was not a decision we took lightly. Our priority is to more fully understand what happened and to ensure that any future work is guided by those learnings.
We are deeply grateful to the patients, families, and advocates who have supported this work and who continue to engage with us during this difficult time. We remain committed to advancing research for this community and will continue to provide updates as we have more information to share.
Sincerely,
The entire Capsida Biotherapeutics team