Patient Death Halts Capsida's Gene Therapy Trial for Rare Epilepsy Disorder

Capsida Biotherapeutics has suspended a clinical trial of its experimental gene therapy CAP-002 following the death of the first patient treated in the study. The therapy, designed to address STXBP1-related epileptic encephalopathy disorders, was in early-stage testing when the incident occurred.

Trial Suspension and FDA Notification

Capsida voluntarily paused the study after the patient's death and has informed the U.S. Food and Drug Administration (FDA) of the development. The company is currently investigating the root cause of the death and plans to provide a full report to regulators.

"We understand this devastating news will raise questions and uncertainty, and we are working with urgency to gather information and find answers," Capsida stated in a letter to the patient community. The biotech company has yet to disclose the exact cause of death and is still in the process of gathering information.

STXBP1-Related Disorders and CAP-002

STXBP1-related disorders are rare neurodevelopmental conditions characterized by early-onset seizures, developmental delays, and intellectual disabilities. Patients with these disorders also face a heightened risk of sudden death.

CAP-002, Capsida's investigational gene therapy, is administered intravenously and aims to provide a stable supply of the STXBP1 protein throughout the brain. The therapy utilizes a specially engineered virus to reach brain neurons, as naturally occurring viruses typically used in gene therapies do not adequately penetrate this area.

The FDA had previously granted CAP-002 fast track and orphan drug designations, highlighting the urgent need for treatments in this rare disease space. Capsida's trial, which began in May 2025, aimed to enroll approximately 12 children with STXBP1-related disorders at Weill Cornell Medicine in New York and the Children's Hospital of Philadelphia.

Industry-Wide Concerns and Recent Patient Deaths

This incident adds to growing concerns in the gene therapy field, following several patient deaths reported in clinical trials over the past year. Notable cases include:

1. Two deaths linked to Sarepta Therapeutics' Duchenne muscular dystrophy treatment, Elevidys
2. A third death associated with Sarepta's investigational product for limb-girdle muscular dystrophy
3. Allogene Therapeutics' discontinuation of ALLO-647 after a patient death in a Phase II CAR-T therapy study
4. A mortality in CytomX's early-stage study for an investigational antibody-drug conjugate in colorectal cancer
5. Three deaths reported by Agios Pharmaceuticals in patients treated with its approved anemia drug, Pyrukynd

These incidents underscore the challenges and risks associated with developing advanced therapies for complex and rare disorders.