Biogen’s diranersen (BIIB-080) drew renewed attention at the Alzheimer’s Association International Conference after updated reporting on the Phase 2 Celia trial. Researchers described proof-of-concept signals that tau-lowering slowed cognitive and functional decline, with Biogen moving the program into Phase 3 despite a formal phase II dose-response miss. Reporting cited comparative slowing rates versus placebo across multiple cognitive and clinical scoring measures, with improvements strongest in lower-dose cohorts. Biomarker data also emphasized substantial tau reductions assessed through PET imaging and cerebrospinal measures, supporting the target engagement story. The takeaway for the field is that tau-directed approaches continue to compete against amyloid-centric strategies, but trial design and endpoint sensitivity remain pivotal for interpreting clinical meaning. The stock reaction and ongoing debate reflect the tension between tau lowering magnitude and whether the effects translate into clinically durable outcomes. For trial planners, Celia reinforces the need for endpoints that can capture modest but meaningful change—and the importance of interpreting signals when primary endpoint architecture differs from how clinicians and investors judge efficacy.