Capsida reports child death in gene therapy trial

Capsida Biotherapeutics reported that the first patient to participate in its phase 1/2a clinical trial of a gene therapy for a rare, severe genetic disorder has passed away.

In a letter posted on its website, California-based Capsida said it has voluntarily paused the SYNRGY study for CAP-002, while determining the root cause of the patient’s passing. CAP-002 is the company’s investigational IV-administered gene therapy for the treatment of STXBP1 developmental and epileptic encephalopathy (STXBP1-DEE), and is the first IV-delivered, AAV blood brain barrier-crossing genetic medicine program to enter a human clinical trial.

CAP-002 received FDA orphan drug designation in October 2024 and IND clearance and fast track designation in May 2025.

STXBP1 is caused by changes in the STXBP1 gene, leading to a form of developmental and epileptic encephalopathy. Mutations in the STXBP1 gene are associated with early-onset seizures, severe developmental delay and intellectual disability, motor abnormalities, and a risk of sudden unexpected death in epilepsy

Capsida says it has alerted the FDA and will be providing the agency with a full report in compliance with regulations. The company will then assess next steps with respect to the STXBP1 program.