Neurogene Halts High-Dose Arm of Rett Syndrome Gene Therapy Trial Following Serious Adverse Event
Key Insights
Neurogene has suspended enrollment in the high-dose arm of its NGN-401 gene therapy trial for Rett syndrome (search) after a patient developed life-threatening systemic hyperinflammatory syndrome (search), a known risk with AAV vector therapies.
Despite early efficacy signals showing "meaningful gains of skills and developmental milestones" in low-dose patients, the company's shares plummeted nearly 44% following the safety concerns announcement.
The trial will continue with the low-dose cohort, though with delayed enrollment timeline, as the company works to balance promising efficacy data against serious safety considerations for this devastating neurological disorder.
Neurogene announced today the suspension of enrollment in the high-dose arm of its NGN-401 gene therapy trial for Rett syndrome (search) after a patient developed a life-threatening immune response. The decision comes amid a turbulent period for the company, whose shares have fallen sharply despite initially promising efficacy data.
The patient, who received the higher dose of NGN-401, developed systemic hyperinflammatory syndrome (search), a rare but serious adverse event associated with adeno-associated virus (AAV) vectors used in gene therapies. According to the company, the patient is currently in "critical condition," and the situation continues to evolve.
"We are deeply saddened for the family," said Rachel McMinn, Neurogene's Chief Executive Officer, in a statement. "While no words could possibly provide comfort to her family, we ask the Rett syndrome (search) community to join us in sending heartfelt thoughts to her family, friends, and the dedicated clinicians who are caring for her."
Understanding Rett Syndrome and NGN-401
Rett syndrome (search) is a devastating neurological disorder primarily affecting females, caused by mutations in the MECP2 (search) gene on the X chromosome. The condition leads to severe impairments in brain function, resulting in behavioral problems, loss of speech and motor skills, seizures, spinal curvature, and sleep disturbances. Approximately 15,000 girls and women in the US and 350,000 worldwide live with this condition, which currently has no approved treatments.
NGN-401 is designed to deliver a functional copy of the MECP2 (search) gene to patients through a one-time intra-cerebroventricular infusion. The therapy aims to address the underlying genetic cause of Rett syndrome (search), potentially offering hope where traditional treatments have failed.
Promising Efficacy Amid Safety Concerns
Prior to this setback, Neurogene had reported encouraging results from the low-dose cohort of the trial. The first four patients treated with the low dose all achieved a "much improved" rating on the Clinical Global Impression Scale of Improvement (CGI-I), representing a clinically meaningful two-point improvement from baseline.
Additionally, all four patients showed improvements on the Rett Syndrome (search) Behaviour Questionnaire (RSBQ), with gains ranging from 28% to 52% compared to baseline scores. These results suggested potential clinical benefit from the therapy.
However, the company emphasized that no other treatment-related serious adverse events have occurred in the trial, including among the five participants who received the low-dose therapy and two other participants who received the higher dose without incident.
Market Response and Future Directions
The market reaction to these developments has been severe. Neurogene's shares fell 44% after the initial revelation of the serious adverse event and dropped an additional 32% in pre-market trading following the announcement of the trial modification.
The company has confirmed that enrollment in the low-dose cohort, which had been expected to complete by the end of the year, will now be delayed due to necessary protocol changes. Neurogene had also previously initiated an adolescent/adult cohort enrolling three participants aged 16 and above at the high dose, though the status of this arm is now unclear given the safety concerns.
Balancing Risk and Potential
This case highlights the delicate balance in gene therapy development between promising efficacy signals and serious safety considerations. The AAV vectors used to deliver gene therapies like NGN-401 carry known risks, including potential liver damage and inflammation. Additionally, excessive production of MECP2 (search) itself carries risks of toxicity, including epileptic seizures.
For Neurogene, this setback comes at a particularly challenging time, as the company recently ended development of NGN-101, a gene therapy for Batten disease (search), after the FDA declined to approve a streamlined regulatory pathway for the program.
Despite these challenges, the company remains committed to advancing potential treatments for Rett syndrome (search), a condition with significant unmet medical need. The continued development of the low-dose arm may still offer hope for patients and families affected by this devastating disorder, though with a more cautious approach to dosing and safety monitoring.
The company plans to report additional data from the trial in the second half of 2025, with details on the design of a potential registrational trial expected in the first half of next year.