Biogen’s stock has dropped after it released mixed data on its tau-targeting investigational antisense oligonucleotide (ASO), compounding a previous announcement of the trial failing to meet its primary endpoint.
Data from the randomised, double blind, Phase II CELIA study (NCT05399888) was presented at the Alzheimer’s Association International Conference (AAIC) 2026, taking place in London between12-15 July.
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Compared to placebo, diranersen 60mg – the lowest dose tested – was the most beneficial after 18 months of treatment. In this arm, patients demonstrated slowing of clinical decline by 0.54 points, or 26%, on clinical dementia rating sum of boxes (CDR-SB). Biogen also measured efficacy on various cognitive scale endpoints, all of which demonstrated patient improvement.
The majority of these endpoint differences achieved nominal statistical significance compared with placebo. Compared to placebo, the six-month 115mg cohort saw a slowing of clinical decline by 0.28, while the three-monthly 115mg cohort reported an 0.18 point slowing in decline on the CDR-SB. This was the equivalent of a 14% and 9% reduction in clinical decline.
When viewed alongside the other cognitive tests, the 115mg cohorts did not perform as well as patients who took the 60mg dose of the drug.
While these were not statistically significant, Biogen said they were clinically beneficial and that CDR-SB favoured diranersen across all studied doses.
CELIA was designed to investigate whether higher doses of diranersen could provide greater clinical benefit on the CDR-SB after 18 months. As this was not observed, the study failed to meet its primary endpoint, as previously announced in May 2026. Biogen did not release any data at that time.
This unexpected data led to a drop in Biogen’s stock of 8.17% on 14 July, from a 13 July close of $209.03 to a 14 July close of $191.95. Biogen is listed on the Nasdaq with a market cap of $28.3bn.
Diranersen demonstrated target engagement and robust reductions in cerebrospinal fluid (CSF) total tau across all studied doses, with mean reductions of 50–65% from baseline. In the tau PET imaging sub-study, decreases from baseline were seen across all evaluated brain regions for all diranersen doses.
Diranersen was generally well tolerated. During the placebo-controlled period, most patients who experienced adverse events (AEs) had events that were mild or moderate in severity, non-serious, and did not result in treatment discontinuation or study withdrawal.
Cross-trial comparisons favour amyloid-targeting drugs
Analysts did see benefit in the data, but questions remain when comparing the drug to approved, amyloid-targeting therapies.
Phillipa Salter, senior neurology analyst at GlobalData, said: “The results seen in the CELIA trial are a positive step forward in demonstrating that targeting tau is a meaningful way to provide clinical benefit for patients with Alzheimer’s disease. However, questions remain given that the clinical results were not dose-dependent, with the lowest dose resulting in the highest clinical effect, something that Biogen will need to address going forward, especially when considering its dosing for a Phase III trial. Additionally, whilst it’s exciting that diranersen demonstrated clinical efficacy across multiple endpoints, an indirect comparison shows similar results to the anti-amyloid therapies, which have been viewed as modest in terms of meaningful clinical benefit.
“Following the launch of the anti-amyloid therapies, tau has been considered as the next step in Alzheimer’s management, and if diranersen can back up its positive results in a Phase III study, interest will quickly focus on whether a combination of amyloid and tau therapies can provide more benefit than either as monotherapies alone.”
GlobalData is the parent company of Clinical Trials Arena.
William Blair analysts agreed that the anti-amyloid drugs do appear to show more benefit, while acknowledging the caveats in cross-trial comparisons. They added, however, that the data does appear to be stronger than UCB’s bepranemab TOGETHER dataset, supporting diranersen’s mechanism of action in breaking down tau.
According to GlobalData analysis, the Alzheimer’s disease market is expected to grow at a high compound annual growth rate (CAGR) of 21.8% from $2.4bn in 2023 to $17bn by 2033 across the eight major markets (8MM: the US, France, Germany, Italy, Spain, the UK, Japan, and China).
Disease-modifying therapies (DMTs), like amyloid-targeting drugs, are expected to dominate the global Alzheimer’s disease market, contributing 69.2% of the market by 2033, with drugs targeting amyloid beta (Aβ) making up the majority of this. GlobalData forecasts that Biogen and Eisai’s Leqembi (lecanemab) and Eli Lilly’s Kisunla (donanemab) could generate global sales of approximately $2.9bn and $2.3bn, respectively, by 2033. However, these drugs have had significant reimbursement challenges, which may impact peak sales values.
