The latest results presented at the Alzheimer's Association International Conference show that patients treated with diranersen experienced a 26% reduction in cognitive deterioration. While this figure aligns with the performance of Leqembi, the market had anticipated more robust efficacy from the tau-targeting candidate. Despite missing its primary clinical objective during trials in May, the drug demonstrated a 65% reduction of the tau protein in spinal fluid. Biogen executives confirmed intentions to advance the program into late-stage testing, banking on the biological impact of the drug despite the lackluster clinical outcomes. The sell-off marks the steepest decline for the company since 2020, occurring only one day after shares climbed 5% following FDA approval for a self-administered version of Leqembi. As pharmaceutical firms pivot toward tau-focused research to find more potent interventions, the pressure on Biogen to deliver a breakthrough beyond current modest treatment standards remains high.