Questions linger over Biogen's tau drug for Alzheimer's, as low dose reports better results

Recent results from Biogen’s Phase 2 CELIA trial of the tau-targeting Alzheimer’s drug diranersen show that the lowest tested dose (60 mg every six months) produced the strongest clinical and biomarker signals, even though the trial missed its primary dose-response endpoint on the CDR-SB scale. Across multiple cognitive and functional tests, the 60 mg dose slowed decline by roughly 23–50% versus placebo, with particularly notable effects on CDR-SB, ADAS-Cog13, MMSE, iADRS, and ADCOMS. The drug also produced robust reductions in tau pathology in the brain and cerebrospinal fluid across all doses, representing the first evidence that a tau-directed therapy can both lower tau and slow cognitive decline in early Alzheimer’s disease. However, the counterintuitive finding—that higher doses did not yield greater benefit—has raised questions about dose selection, trial design, and how to design a definitive Phase 3 program. Despite these uncertainties, Biogen plans to advance diranersen into late-stage development, highlighting the 60 mg dose as its preferred regimen based on overall efficacy and tolerability.