News|Articles|September 11, 2025
First Patient Treated in Capsida Biotherapeutics’ Trial for Encephalopathy Gene Therapy CAP-002 Passes Away
Author(s)Noah Stansfield
Capsida has made the move to pause the trial voluntarily.
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The first patient treated in the phase 1/2a SYNRGY clinical trial (NCT06983158), which is evaluating CAP-002, an investigational gene therapy intended to treat developmental and epileptic encephalopathy caused by syntaxin-binding protein 1 (STXBP1-DEE) mutations, has died following dosing.1
Capsida has made the move to pause the trial voluntarily and informed the FDA of the death. The company is currently working to figure out the root cause of the patient’s death and has stated its intention to give a full report to the FDA in accord with regulatory protocol. The company will evaluate its next steps for the program following the aforementioned steps.
“We are committed to keeping the community informed about what happened and our plans for the program as soon as we have information to share,” the Capsida Biotherapeutics team wrote in an open letter addressed to the STXBP1 community.1 “We understand that a letter will not ease the pain you are all feeling, but we want you to know we are mourning together with the patient’s family and all of you. We are committed to timely and transparent communication with the STXBP1 community.”
The investigational new drug (IND) application for SYNRGY was cleared by the FDA in May 2025.2 CAP-002 utilizes a capsid that has been engineered by the company with the intent of enabling brain-wide neuronal expression, while cutting down on tropism for the cells of the liver seen in many adeno-associated virus (AAV) capsids used for gene therapy products.3 According to Capsida, wild-type AAVs are not capable of neuronal transduction at levels widespread enough to constitute a disease-modification.2
“STXBP1-related disorders present devastating challenges in communication, development, motor function, and seizures,” Charlene Son Rigby, the STXBP1 Foundation’s president and cofounder, said in a statement at the time of the IND clearance.2 “We are in dire need of targeted therapies that can improve the lives and functioning of our children and families.”
Transduction at levels greater than 70% of neurons across critical brain areas was observed for CAP-002 in preclinical research in nonhuman primates (NHPs). Alongside this, the gene therapy showed the ability to detarget liver tissue and dorsal root ganglia. With regard to safety, no adverse histopathology was observed in the NHP Good Laboratory Practice (GLP) toxicology study for the program. Capsida characterized the safety profile of CAP-002 as well-tolerated. Notably, CAP-002 previously received orphan drug designation from the FDA in October 2024 and fast track designation from the FDA in May 2025.3,4
“This is the first potentially disease-modifying treatment for STXBP1-DEE, and we are excited to be part of the SYNRGY clinical trial,” Ingo Helbig, MD, a pediatric neurologist and director of clinical research at the Center for Epilepsy and Neurodevelopmental Disorders (ENDD) at Children’s Hospital of Philadelphia, and a paid consultant to Capsida, added to the statement.2
REFERENCES
1. A letter to the STXBP1 community. Press release. Capsida Biotherapeutics. September 11, 2025. Accessed September 11, 2025. https://capsida.com/a-letter-to-the-stxbp1-community/
2. Capsida receives FDA IND clearance for its first-in-class, IV-administered gene therapy for STXBP1 developmental and epileptic encephalopathy. News release. Capsida Biotherapeutics. May 12, 2025. Accessed September 11, 2025. https://capsida.com/capsida-receives-fda-ind-clearance-for-its-first-in-class-iv-administered-gene-therapy-for-stxbp1-developmental-and-epileptic-encephalopathy/
3. FDA grants orphan drug designation to Capsida Biotherapeutics for potential treatment of STXBP1 developmental and epileptic encephalopathy. News release. Capsida Biotherapeutics. October 30, 2024. Accessed September 11, 2025. https://capsida.com/fda-grants-orphan-drug-designation-to-capsida-biotherapeutics-for-potential-treatment-of-stxbp1-developmental-and-epileptic-encephalopathy/
4. Capsida receives FDA fast track designation for its potential first-in-class iv-administered gene therapy for STXBP1 developmental and epileptic encephalopathy. News release. Capsida Biotherapeutics. May 29, 2025. Accessed September 11, 2025. https://capsida.com/capsida-receives-fda-fast-track-designation-for-its-potential-first-in-class-iv-administered-gene-therapy-for-stxbp1-developmental-and-epileptic-encephalopathy/
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