Eisai's TREM2-targeting Alzheimer's drug candidate encountered two significant regulatory setbacks this week when the Food and Drug Administration declined to grant priority review and an independent data safety monitoring board recommended pausing new patient enrollment pending a full safety analysis.
The FDA's decision to deny priority review means the application will be assessed under a standard twelve-month review timeline rather than the expedited six-month process Eisai had sought. Priority review is typically reserved for drugs that address serious conditions and represent a potential substantial improvement over existing therapies. The agency cited "unresolved safety questions" in its decision letter without specifying them in detail.
The data safety monitoring board, which reviews blinded trial data at pre-specified intervals and operates independently of the drug's developer, recommended the enrollment pause following its most recent interim review. Eisai stated that adverse events observed in the trial were "within expected parameters," but acknowledged that the pause was in effect. An enrollment pause is distinct from a full trial halt — patients already enrolled may continue receiving the drug while the board conducts its review.
A neurologist not involved in the trial who reviewed the publicly available safety data said the combination of a priority review denial and a safety board pause was unusual and reflected genuine regulatory concern. "The FDA citing safety questions in a priority review denial is not boilerplate," she said. "That is a signal that the agency has identified something specific it needs answered before the standard review can proceed normally."
The safety concern that prompted the board's action is understood to relate to a rate of amyloid-related imaging abnormalities — brain swelling events known as ARIA — that was higher than anticipated at a subset of trial sites. ARIA is a known risk associated with anti-amyloid drugs and can range from asymptomatic to severe.
Eisai said it would work with the FDA and the safety board to address the outstanding questions and expected to provide a public update within 90 days. The company said the drug's primary efficacy endpoint — a 35 percent reduction in cognitive decline over 18 months — remained unaffected by the safety review.
Shares in Eisai fell 27.8 percent on the day of the announcements. Patient advocacy organisations said they were watching the safety review closely and urged the company to be transparent about the data the board had reviewed.