As a consequence of the adverse event, Neurogene's stock is projected to decline by 7.12% on May 19, 2025, reflecting a staggering total drop of 74.84% since the initial report.

Despite these challenges, Neurogene is continuing its Phase I/II trial using a lower dose of NGN-401, which has not resulted in any HLH cases thus far.

Neurogene has introduced a new monitoring system for its gene therapy following the tragic death of a Rett syndrome patient during a clinical trial.

This new protocol was announced at the ASGCT Annual Meeting and aims to detect the hyperinflammatory syndrome HLH, which has been associated with high doses of AAV gene therapy.

The monitoring system will focus on daily tracking of elevated ferritin levels, fever, and falling blood counts, collectively known as the 'three Fs', during the week after dosing, as these indicators are present in over 90% of HLH patients.

CEO Dr. Rachel McMinn highlighted the importance of sharing findings on HLH to enhance monitoring and treatment practices within the gene therapy community.

The adverse event that prompted these changes was initially reported in November 2024, with the patient's death confirmed shortly thereafter, marking the third case linked to high-dose NGN-401 administration.

Neurogene's experience is not isolated; other companies, including Sarepta and Pfizer, have also encountered fatal adverse events linked to their gene therapies, leading to pauses or terminations of their clinical trials.

For HLH treatment, high-dose corticosteroids are typically used, with Amgen's Kineret (anakinra) available as a second-line option.