UCB Announces Positive Phase 3 Study Results for Fenfluramine in Treating CDKL5 Deficiency Disorder

UCB Unveils Encouraging Results from GEMZ Phase 3 Study on Fenfluramine

UCB, the global biopharmaceutical company, has presented persuasive efficacy and safety data from its GEMZ phase 3 study of adjunctive fenfluramine (FFA) for children and adults affected by CDKL5 deficiency disorder (CDD) during the American Epilepsy Society (AES) meeting held from December 5 to 9, 2025, in Atlanta. The findings reveal a significant breakthrough in managing this ultra-rare condition characterized by treatment-resistant seizures.

The study met its primary endpoint, affirming fenfluramine's ability to significantly reduce countable motor seizure frequency (CMSF) compared with a placebo. Specifically, patients receiving fenfluramine showed an impressive median reduction of 47.6% in CMSF, dramatically higher than the mere 2.8% decrease observed in the placebo group. This translated into a median difference of -52.7% between the treatment and placebo groups during the 14-week period of study.

In addition to the primary endpoint, the trial achieved two of its three secondary endpoints. The study indicated that 45.2% of patients treated with fenfluramine attained at least a 50% reduction in CMSF, whereas only 4.5% of those on placebo experienced similar relief. Moreover, 38.1% of fenfluramine patients were regarded as