Capsida Pauses Gene Therapy Trial Following Patient Incident

September 11, 2025 – Capsida Biotherapeutics has suspended a recently initiated clinical trial for its experimental AAV-based gene therapy, CAP-002, after the first participant, a child, experienced a significant adverse event following treatment. The investigational therapy was being tested for STXBP1 developmental and epileptic encephalopathy, a rare neurodevelopmental disorder.

Capsida disclosed the event in a letter to the patient community, expressing its “deepest condolences” and stating it is working with urgency to determine the “root cause.” The company has informed the Food and Drug Administration (FDA) and will provide regulators with a full report. It remains unclear whether the treatment was directly to blame for the event. The news comes just four months after the FDA granted a green light to begin the trial, which had opened recruitment at Weill Cornell Medicine in New York and the Children’s Hospital of Philadelphia in Pennsylvania.

Capsida’s therapy utilizes a specially engineered AAV-based virus to deliver a functional copy of the STXBP1 gene, aiming to make up for deficient protein levels that are essential for nerve communication in the central nervous system. The trial was designed to enroll approximately 12 children with the disorder.