Biogen Diranersen Phase 2 Shows Mixed Results

Biogen (BIIB) presented detailed Phase 2 CELIA data for diranersen on July 14, 2026, showing robust tau biomarker reductions and consistent, though modest, slowing of clinical decline. The study missed its prespecified dose-response primary endpoint, but the company said it will advance the program into Phase 3.

CELIA Phase 2 Outcomes and Clinical Signals

The CELIA study was a global, randomized, double-blind, placebo-controlled, dose-ranging Phase 2 trial in individuals with early Alzheimer’s disease that ran through Week 76 (18 months). It did not meet its primary endpoint, which required a dose-response pattern for change from baseline on the Clinical Dementia Rating–Sum of Boxes (CDR-SB) at Week 76.

Biogen reported consistent slowing of clinical decline versus placebo across multiple prespecified secondary endpoints at 18 months. For the two 115 mg intrathecal dosing regimens, the 115 mg every six months arm slowed decline by 0.28 points on CDR-SB (14%) and the 115 mg every three months arm slowed by 0.18 points (9%). The company also reported slowing on other cognitive scales: ADAS-Cog13 by 32% and 29%, MMSE by 34% and 38%, modified iADRS by 29% and 18%, and ADCOMS by 21% and 7% for those regimens. Functional domains within CDR-SB favored diranersen across doses.

Secondary reporting at the Alzheimer’s Association International Conference noted that the lowest-dose regimen—60 mg every six months—produced the strongest clinical signal, showing about 26% slowing on at least one measure over 18 months and benefit on five of six assessment tools. This effect was described as roughly comparable to those reported for amyloid-directed therapies.

On a dedicated daily-function measure, ADCS-ADL-MCI, Biogen reported no separation from placebo at 18 months and said longer-term follow-up is ongoing to evaluate functional impact.

Safety in CELIA was generally tolerable. Most adverse events during the placebo-controlled period were mild or moderate and non-serious, not prompting treatment discontinuation or study withdrawal. The most frequent events were procedural pain, post-lumbar-puncture syndrome, and confusional state. Serious adverse events were more frequent at the highest dose, consistent with earlier clinical experience.

Tau Biomarkers and Mechanism of Action

Diranersen is an investigational antisense oligonucleotide targeting tau, the microtubule-associated protein involved in neurofibrillary tangles in Alzheimer’s disease. Discovered by Ionis Pharmaceuticals, Biogen holds an exclusive license and leads development.

In CELIA, diranersen showed clear target engagement, reducing cerebrospinal fluid total tau by roughly 50–65% from baseline across dose groups. It also produced robust reductions in brain tau measured by tau PET imaging across studied doses, a combination not previously reported in a single mid-stage tau study.

These biomarker effects position diranersen as a differentiated tau-directed approach that could complement Biogen’s amyloid program.

Phase 3 Plans and Regulatory Status

Biogen said in a GlobeNewswire press release, “Based on the growing and consistent body of evidence from the Phase 1b and Phase 2 studies, Biogen plans to advance diranersen into confirmatory Phase 3 development.” Ionis confirmed this in a Business Wire release.

Diranersen holds FDA Fast Track designation granted in 2025. Biogen frames the program as a tau-directed complement to its amyloid portfolio. Separately, the FDA extended the PDUFA action date for LEQEMBI IQLIK, a once-weekly subcutaneous formulation of lecanemab, to August 24, 2026, providing regulatory context for the company’s Alzheimer’s strategy.

Advancing to confirmatory trials despite missing the prespecified dose-response on CDR-SB, combined with stronger clinical signals at lower, less frequent dosing and more serious adverse events at higher doses, creates a complex evidentiary package. This will influence dose selection, safety monitoring, and regulatory discussions as Biogen prepares for registrational development.