Biogen presented data from the Phase 2 CELIA study at the Alzheimer’s Association International Conference (AAIC) 2026 demonstrating that diranersen (BIIB080), an investigational antisense oligonucleotide therapy, achieved robust reductions in tau pathology and slowed clinical decline in individuals with early Alzheimer’s disease. While the study failed to meet its primary endpoint of demonstrating dose response-related effects on change in Clinical Dementia Rating-Sum of Boxes (CDR-SB), the data showed mean reductions of 50–65% in cerebrospinal fluid total tau and decreases across all evaluated brain regions on tau PET imaging. These findings provide the first evidence from a randomized trial that a tau-targeting drug can produce both a robust biomarker effect and a signal of clinical benefit, signaling momentum toward addressing the full pathobiology of Alzheimer's.

Diranersen demonstrated efficacy across all studied doses at 18 months, with consistent results across multiple prespecified clinical endpoints. The 60 mg dose administered intrathecally every six months showed the strongest response, slowing clinical decline by 42% on ADAS-Cog13 and 50% on MMSE. Compared with placebo, this dose also demonstrated a 26% slowing on CDR-SB. The majority of these endpoint differences achieved nominal statistical significance. However, the central unanswered question from the study is the discordance between the biomarker and clinical findings; higher doses led to greater tau reduction, while the lowest dose showed the strongest clinical signal.

Clinical and Biomarker Results

The study evaluated three dose regimens over an 18-month placebo-controlled treatment period. While the 60 mg dose showed the most pronounced effect, other regimens also demonstrated slowing of clinical decline compared with placebo. The study did not meet its primary endpoint of dose response on CDR-SB at 18 months, as higher doses were not associated with greater slowing of decline.

Diranersen is the first tau-directed therapy to demonstrate reductions in both CSF total tau and brain tau pathology, as measured by PET, across all studied doses in a Phase 2 study. The therapy targets MAPT mRNA to reduce the production of all tau isoforms, lowering both intracellular and extracellular tau protein.

Safety and Tolerability

Diranersen was generally well tolerated during the placebo-controlled period. Most adverse events were mild or moderate in severity, non-serious, and did not result in treatment discontinuation. The most frequent adverse events were procedural pain, post-lumbar puncture syndrome, and confusional state. Amyloid-related imaging abnormalities (ARIA) were not observed, consistent with the therapy's tau-targeting mechanism of action. More than 90% of participants who completed the placebo-controlled period elected to continue into the extension study.

The CELIA study enrolled 416 participants representative of early Alzheimer’s disease. Participants had a mean age of 68 years, 51% were female, and 60% were classified as having mild cognitive impairment. ApoE4 carriers represented approximately 69% of participants. Biogen plans to advance diranersen into confirmatory Phase 3 development as it continues to analyze the full data to understand the divergence between biomarker and clinical effects.

Truist Securities analyst Srikripa Devarakonda has upgraded Biogen Inc from Hold to Buy, raising the price target to $235 from $190. The rating change reflects a more positive outlook on the stock's performance potential. Biogen shares closed at $199.15 on Friday.

Rating and Target Changes

The upgrade moves Biogen out of a neutral stance, signaling confidence in future gains. The new price target of $235 represents a significant increase from the previous $190 level.