Rett Syndrome Drug Faces EU Regulatory Hurdle as Acadia Prepares Response Strategy

Acadia Pharmaceuticals is bracing for a setback in its European expansion plans, with the company anticipated to receive a negative opinion from the European Medicines Agency’s (EMA) Committee for Medicinal Products for Human Use (CHMP) regarding trofinetide, its treatment candidate for Rett syndrome. Following an oral explanation of the committee’s position, Acadia has been notified of a negative trend vote on the marketing application. The company now plans to pursue a re-examination request once the official CHMP opinion is finalized this month. This regulatory challenge represents a significant obstacle to bringing Rett syndrome treatment options to European patients in the near term.

Rett syndrome remains one of the most serious rare neurodevelopmental disorders affecting infants and young children, impacting approximately one in 10,000 to 15,000 female births globally. The condition typically manifests between six and 18 months of age after a period of apparently normal early development. Children with Rett syndrome experience progressive loss of acquired motor, cognitive, and communication skills, along with the emergence of characteristic repetitive hand movements. Though some degree of stabilization may occur over time, patients frequently face ongoing motor deterioration throughout their lives, creating significant clinical and caregiving challenges for families.

Current Market Status and FDA Success

Despite the EU setback, Acadia has already achieved a major regulatory milestone domestically. The FDA granted approval for trofinetide under the brand name Daybue in 2023, becoming the first and only FDA-approved medication specifically designed to treat Rett syndrome in both adult and pediatric patients aged two years and older. This approval marked a breakthrough in addressing a previously untreated rare disease, offering hope to the thousands of American families affected by the condition.

The early commercial performance of Daybue in the United States demonstrates encouraging market reception. During the first nine months of 2025, Daybue generated $281.8 million in sales, reflecting a 12% year-over-year increase driven by expansion into additional patient populations. This growth trajectory indicates that healthcare providers and caregivers increasingly recognize the therapeutic value of Rett syndrome treatment options. To further strengthen its market position, Acadia recently received FDA clearance for Daybue Stix, a new dye- and preservative-free powder formulation that offers an alternative delivery method. The company plans to introduce Daybue Stix with a limited launch in the first quarter of 2026, expanding to broader market availability in the second quarter while maintaining the original oral solution formulation alongside it.

Acadia’s Broader Product Pipeline Supports Long-Term Vision

The company’s growth trajectory extends well beyond its Rett syndrome franchise. Nuplazid, Acadia’s flagship product addressing hallucinations and delusions associated with Parkinson’s disease psychosis, continues to drive substantial revenue. In the first nine months of 2025, Nuplazid recorded $505.7 million in sales, representing a 13% increase from the previous year, primarily fueled by volume expansion as more patients receive prescriptions.

Financial projections reveal management’s confidence in the company’s commercial potential. Acadia anticipates combined net sales for Nuplazid and Daybue reaching approximately $1.7 billion by 2028, with Nuplazid expected to contribute around $1 billion and Daybue contributing approximately $700 million. Nuplazid’s strong competitive position is further reinforced by patent protection extending through 2038 in the United States, providing over a decade of exclusivity that insulates the product from generic competition and enables sustained revenue generation.

Market Context and Competitive Landscape

Within the broader biotech sector, Acadia’s current Zacks Rank stands at #3 (Hold), reflecting a moderate outlook. Other companies within the biotech space have garnered stronger investor sentiment. Alkermes, Castle Biosciences, and Immunocore each maintain a Zacks Rank #1 (Strong Buy) designation. Over recent 60-day periods, Alkermes’ 2026 earnings per share estimates improved from $1.54 to $1.91, with shares appreciating 30.6% over six months. Castle Biosciences has demonstrated particularly strong momentum, with shares surging 157.8% over the past six months alongside improved profitability projections for 2026. Immunocore’s shares rose 1.6% over the same period as loss-per-share estimates narrowed.

The contrasting performance across these biotech companies underscores the sector’s diversity and the individual impact of regulatory and commercial developments on valuation multiples. For Acadia specifically, the EU regulatory challenge for Rett syndrome treatment represents a near-term headwind that may weigh on investor sentiment. However, the strong U.S. market traction for Daybue and the continued growth momentum in Nuplazid sales suggest the company maintains pathways to revenue expansion despite the European setback.

The Rett syndrome treatment landscape remains nascent and underserved, with the condition affecting tens of thousands of families worldwide. Beyond the regulatory disappointment in Europe, Acadia’s position as the sole FDA-approved treatment provider grants the company significant first-mover advantages in the American market. The company’s product diversification and patent-protected revenue streams provide financial resilience to weather regulatory challenges in international markets while maintaining momentum in key therapeutic areas. As Acadia pursues its re-examination strategy with the EMA and expands Daybue’s market penetration domestically, the company’s long-term prospects for becoming a meaningful player in rare disease treatments remain intact.