Biogen’s diranersen (BIIB-080) generated a renewed case for phase 3 escalation after phase 2 Celia data presented at AAIC showed cognitive slowdown roughly on par with approved anti-amyloid benchmarks, despite a formal phase 2 miss on dose-response. Lower-dose arms demonstrated slowing across multiple clinical scores, while biomarker analyses showed substantial tau reductions. Multiple reports emphasized the trial’s practical signal: cognitive decline decelerated across endpoints and tau reductions were observed on clinical and imaging measures. Biogen is therefore moving forward with phase 3, fueling debate about how phase 2 surrogate biology translates into patient-centered outcomes. The development keeps tau as an active regulatory target and highlights how evidence thresholds may hinge on endpoint selection and effect magnitude within dementia trial designs.