Acadia Daybu receives final approval as first EU treatment for Rett syndrome
First EU Approval for Rett Syndrome Treatment
The European Union Executive Committee approved Acadia Pharmaceuticals' Daybu (trofinetide) on August 20, 2026, as a treatment for the neurobehavioral symptoms of Rett syndrome. The approval applies to adults and children aged five years and older across the 27 EU member states, as well as Iceland, Liechtenstein, and Norway. The EMA's Committee for Medicinal Products for Human Use (CHMP) adopted a positive opinion on June 25 following a re-examination process, making Daybu the first and currently the only approved treatment for Rett syndrome in Europe. However, the timing of actual revenue generation will depend on the speed of price and reimbursement negotiations across individual countries.
Drug and Clinical Basis
Daybu is an oral solution containing 200 mg/ml of trofinetide, a synthetic analog of the tripeptide glycine-proline-glutamic acid (GPE) derived from insulin-like growth factor-1 (IGF-1). The Phase 3 LAVENDER study, which formed the basis for approval, evaluated 187 patients aged 5 to 20 years over 12 weeks and demonstrated statistically significant improvement compared to placebo in the co-primary endpoints of the Rett Syndrome Behavior Questionnaire (RSBQ) and Clinical Global Impression-Improvement (CGI-I). While EMA acknowledged the small effect size, it concluded that the overall evidence package supports clinical relevance. Key adverse reactions such as diarrhea, vomiting, and weight loss are critical management variables that may influence prescription persistence and real-world adoption.
Regulatory History and Commercial Value
The U.S. FDA approved Daybue for the treatment of Rett syndrome in patients aged two years and older on March 10, 2023, without an advisory committee meeting. Daybue is already in the Marketed phase in the U.S., with Q2 2026 net sales reaching USD 125 million and annual company guidance set at USD 480–510 million, reflecting the real size of the rare disease market. The EU approval serves as a catalyst to expand the existing U.S.-centric revenue base into a multi-national rare disease franchise. With no previously approved treatments in Europe, initial competition will be shaped by diagnostic rates, access to specialist centers, and reimbursement conditions.
Competitive Landscape and Rights Structure
The current standard of care combines anticonvulsants with symptom management for sleep, breathing, and gastrointestinal issues, along with physical and speech therapies. Daybu is the first approved drug to directly address the condition, currently without direct competition. However, Taysha Gene Therapies (TSHA)'s MECP2 gene therapy TSHA-102, in Phase 1/2 and pivotal REVEAL study, and Neurogene (NGNE)'s MECP2 gene therapy NGN-401, in Phase 1/2 and registration-targeting Embolden study, are expected to intensify mid- to long-term competition. Acadia Pharmaceuticals (ACAD) secured global exclusive rights to trofinetide from Neuren Pharmaceuticals (NEU.AX), paying an upfront USD 100 million. The agreement includes potential milestone payments of up to USD 426.3 million for non-North American trofinetide milestones, a USD 35 million milestone for the first commercial sale in Europe, and tiered royalties in the mid-teens to low-twenties range on European sales.
This approval opens Acadia Pharmaceuticals (ACAD)'s first major commercial region outside the U.S., establishing a foundation to add European sales to the USD 480–510 million 2026 revenue guidance for DAYBUE in the U.S. From a research perspective, the RSBQ and CGI-I improvements in the Phase 3 LAVENDER study have been reconfirmed by regulators, but the impact of adverse events such as diarrhea, vomiting, and weight loss, along with EMA's assessment of the small effect size, will determine long-term adherence rates. For the industry, the European Rett syndrome market, previously without approved therapies, is now emerging, with Phase 1/2 and registration-stage TSHA-102 and NGN-401 signaling future competition between symptomatic treatments and one-time MECP2 gene therapies. Short-term value will hinge on reimbursement adoption across countries and the USD 35 million milestone for the first European commercial sale, while mid- to long-term value will depend on gene therapy clinical results in 2027 and Daybu's market resilience.
Source: EMA (ema)