Capsida Biotherapeutics said it has closed its SYNRGY Phase I/II gene therapy trial in STXBP1-related developmental and epileptic encephalopathy after a child death following infusion of CAP-002. The company previously disclosed the death and said it would pause trial activity pending additional investigation, but now it says it has decided to close out the study. Capsida attributed the fatal outcome to cerebral edema, or brain swelling, and said it still has not confirmed the underlying driver of the event. The company also disclosed a partial mechanism of action for CAP-002, stating the AAV gene therapy engages ADAM15, a protein involved in cellular adhesion and cytokine processing. The announcement lands amid renewed scrutiny of AAV gene therapies following other recent fatal events, highlighting continued safety uncertainties for first-in-human and early-stage programs.