Capsida Biotherapeutics has suspended a clinical trial of its experimental gene therapy after the first child to receive the treatment died, sending shockwaves through the rare disease and gene therapy communities. According to BioPharma Dive, the trial, which had just begun enrolling children with a severe neurodevelopmental disorder tied to mutations in the STXBP1 gene, is now under urgent review as Capsida and regulators seek answers.
The company publicly disclosed the incident on September 10, 2025, in a letter addressed to the patient community. While specifics about the child’s death have not been released, Capsida has notified the Food and Drug Administration and pledged to provide a comprehensive report as the circumstances are investigated. “We understand this devastating news will raise questions and uncertainty, and we are working with urgency to gather information and find answers,” Capsida stated.
The therapy, called CAP-002, uses a specially engineered viral vector to deliver healthy copies of the STXBP1 gene directly to the brain. This approach was necessary because commonly used viral vectors in gene therapy do not effectively reach enough brain neurons. The goal was to help children with STXBP1-related disorders, who experience early-life seizures, developmental delays, low muscle tone, and difficulties with movement and speech. Tragically, these children are also at risk for sudden death due to epilepsy.
Capsida’s trial received FDA approval in May, with plans to enroll around a dozen children at sites including Weill Cornell Medicine in New York and the Children’s Hospital of Philadelphia. The community had pinned hopes on CAP-002, which aims to restore a critical protein lacking in affected children, potentially offering benefits beyond currently available anti-seizure drugs that only control some symptoms.
This fatality comes at a time of increased scrutiny of gene therapy after deaths in other recent clinical trials for muscular dystrophy and rare neurological diseases. Capsida is now working to determine whether its treatment was directly responsible or if other factors played a role. Both the company and the STXBP1 Foundation, a patient advocacy group, have promised to keep the community informed as the required safety review progresses.
“On behalf of everyone at Capsida Biotherapeutics, we extend our heartfelt sympathies to the patient’s family and loved ones,” said CEO Peter Anastasiou, emphasizing Capsida’s commitment to transparency and collaboration with all involved parties.
Capsida is also advancing gene therapy programs for Parkinson’s disease and Friedreich’s ataxia, though it is unclear if any patients have yet been dosed in those studies. The outcome of the CAP-002 trial review will likely have ripple effects throughout the field, highlighting both the promise and the inherent risks of pioneering new treatments for some of the world’s rarest and most challenging diseases.
