The Food and Drug Administration on Friday approved Daybue, or trofinetide, for adults and children age 2 and older with Rett syndrome, creating the first FDA-approved treatment for a rare neurodevelopmental disorder that until now has been managed through supportive and symptom-directed care.

The FDA’s approval notice marks a major clinical milestone for families affected by Rett syndrome, a condition most often caused by mutations in the MECP2 gene and characterized by developmental regression, loss of purposeful hand use and communication, abnormal movements and a range of motor, respiratory and autonomic problems.

Approval rests on a 187-patient Phase 3 trial

Acadia Pharmaceuticals said the approval is supported by LAVENDER, a randomized, double-blind, placebo-controlled Phase 3 trial involving 187 girls and young women ages 5 to 20. The company’s March 10 approval announcement said trofinetide produced statistically significant differences on both co-primary measures: the caregiver-rated Rett Syndrome Behaviour Questionnaire and the clinician-rated Clinical Global Impression-Improvement scale.

The trial was designed to capture changes across the diverse symptoms of Rett rather than focus on one isolated manifestation. Acadia’s earlier top-line results reported statistically significant improvements on both co-primary endpoints and on a key measure of communication. The findings were important because Rett syndrome affects multiple domains—communication, behavior, motor function, breathing, sleep and gastrointestinal function—and no drug had previously demonstrated sufficient evidence to win FDA approval for the disorder itself.

Daybue is an oral formulation of trofinetide, a synthetic analog related to a naturally occurring peptide fragment. Its exact mechanism in Rett syndrome remains uncertain. The development program has focused on whether the drug can improve core clinical features rather than treating only complications such as seizures, constipation or sleep disturbance.

A treatment for a disorder with no prior approved therapy

Rett syndrome typically emerges after a period of apparently normal early development. Children may then lose language and purposeful hand skills and develop repetitive hand movements, gait abnormalities and other neurologic features. The condition occurs predominantly in girls, although males can also be affected. Acadia estimates that 6,000 to 9,000 people in the United States have Rett syndrome, with a smaller diagnosed population.

The International Rett Syndrome Foundation called the approval a landmark in a March 11 statement. The organization has supported trofinetide research and trial recruitment over many years and emphasized that the approval is the culmination of a long development program involving patients, caregivers, investigators and advocacy groups.

That history matters because rare-disease development faces structural obstacles: small patient populations, heterogeneous symptoms and difficulty selecting outcome measures that can detect meaningful change. Rett syndrome also places substantial demands on caregivers, making caregiver-observed measures an important part of assessing whether a therapy changes daily function.

Safety and tolerability will matter in routine care

Clinical benefit must be weighed against adverse effects. Diarrhea and vomiting were among the most common problems reported in the development program, and the prescribing information includes guidance for managing gastrointestinal effects and hydration. For patients who already face feeding, gastrointestinal or nutritional challenges, tolerability may determine whether the treatment can be maintained.

Acadia submitted the new-drug application in July 2022. Its submission announcement said the filing covered adults and pediatric patients age 2 and older and relied principally on the LAVENDER efficacy and safety data, supplemented by additional clinical experience.

Two months later, the FDA accepted the application and granted priority review. Acadia’s September regulatory update set a March 12, 2023 action date and said the agency did not plan an advisory-committee meeting. Friday’s approval arrives two days ahead of that deadline.

Access becomes the next test

Approval does not mean immediate availability. Acadia says Daybue is expected to become commercially available in the United States by the end of April. The company is establishing patient-support services to help families and clinicians navigate insurance coverage and distribution.

That transition from trial to practice will answer questions the controlled study could not. Clinicians will need to determine which patients are most likely to benefit, how gastrointestinal adverse effects can be managed and how changes on trial scales translate into communication, mobility and daily function that matter to families.

The approval may also reshape Rett research. Until now, drug development has occurred in a field with no regulatory precedent for an approved disease-specific therapy. Daybue gives researchers a validated development pathway, established clinical endpoints and a treatment benchmark against which future therapies can be compared.

For families, however, the significance is more immediate. Rett syndrome remains a severe lifelong condition, and Daybue is not described as a cure or a genetic correction. The FDA decision instead establishes that a drug can produce measurable improvement in core symptoms sufficient to meet the agency’s benefit-risk standard. After decades with no approved treatment, that is a consequential change in the clinical landscape.