Neurogene (NGNE, Financials) shares fell 35% in Tuesday's pre-market trading following the release of interim results from a phase 1/2 trial for NGN-401, its gene therapy candidate targeting Rett syndrome.

The study included pediatric patients given low and high dosages of NGN-401. All four of the low-dose group members exhibited improvement, scoring 2 on the Clinical Global Impression Scale of Improvement, thereby indicating significant change from baseline. The business also reports that these individuals showed a 28% to 52% improvement on the Rett Syndrome Behavior Questionnaire.

Nonetheless, consistent with recognized dangers of AAV (adeno-associated virus) gene treatments, one of the two patients in the high-dose group had a major adverse event connected to the therapy, as reported in a corporate statement.

In order to evaluate the effectiveness of NGN-401 across a larger population, Neurogene also started a new cohort within the study including teenage and adult participants aged 16 and above. Three people are likely to be part of the high-dose group in this new cohort.

In a separate update, Neurogene said its only other clinical-stage asset, NGN-101 gene therapy candidate for CLN5 Batten disease, would not be further developed.