Biogen defends anti-tau drug after stock slides on Alzheimer's readout

Biogen said yesterday that detailed data from the phase II CELIA study of its experimental tau-targeting Alzheimer's drug diranersen confirmed the trial did not meet its primary endpoint of demonstrating a dose-response relationship on the Clinical Dementia Rating-Sum of Boxes score at week 76. The company presented the findings at the Alzheimer's Association International Conference. Shares fell 8.2% following the presentation. Biogen had disclosed in May 2026 that the study failed its primary endpoint. The data showed the lowest dose of 60 mg every six months produced the strongest clinical results, slowing CDR-SB decline by 26%. Higher doses of 115 mg given every six months and every three months showed 14% and 9% slowing, respectively. The company stated that higher doses were not associated with greater slowing of decline. The drug also showed reductions in cerebrospinal fluid tau and brain tau pathology. Investors appeared concerned about plans to advance diranersen into phase III development given the lack of dose-dependent benefit. Biogen has not disclosed a timeline for initiating late-stage development. The drug is being developed with Ionis Pharmaceuticals.