Oligonucleotide drug slows Alzheimer’s decline across every dose tested in Phase 2 and reframes the field’s central therapeutic target.
The Alzheimer’s Association International Conference has spent the last several years dominated by amyloid. This year, tau got its moment. Ionis Pharmaceuticals and partner Biogen presented full Phase 2 CELIA data for diranersen, an antisense oligonucleotide designed to reduce tau protein at its genetic source, and the results gave the field its clearest evidence yet that going after tau directly can move the needle clinically, not just biologically [1].
Across all three dose regimens tested in the 416-participant study, diranersen slowed clinical decline relative to placebo on the Clinical Dementia Rating–Sum of Boxes (CDR-SB), the trial’s key measure of cognitive and functional status. The 60 mg dose, given intrathecally every six months, produced the strongest response: a 26% slowing on CDR-SB, alongside a 42% effect on ADAS-Cog13 and 50% on MMSE at 18 months.
Notably, the primary endpoint – testing whether higher doses produced proportionally greater benefit – was not met; more drug didn’t mean more effect, an unusual but not disqualifying outcome for a dose-ranging study.
The biomarker story was, if anything, more striking. Diranersen produced mean reductions of 50–65% in cerebrospinal fluid total tau across every dose group, and PET imaging showed decreases in tau pathology across all evaluated brain regions – making it, per Ionis, the first tau-directed therapy to demonstrate that combination of effects in a Phase 2 study.
Diranersen works upstream of where most current Alzheimer’s biology has focused. Rather than clearing amyloid plaques or mopping up misfolded tau protein directly, it targets MAPT messenger RNA, cutting off tau production before the protein is even made – intracellular and extracellular forms alike. It’s a manufacturing shutdown rather than a demolition crew, and that distinction may carry real clinical consequences.
Amyloid-targeting antibodies have made real progress against Alzheimer’s, but they’ve done so while carrying a persistent asterisk: amyloid-related imaging abnormalities, or ARIA, a class of brain swelling and microbleeds that requires routine MRI monitoring and has, in some cases, proven fatal. It’s a risk baked into the amyloid-clearing mechanism itself.
Because diranersen doesn’t touch amyloid biology at all, Ionis and Biogen say ARIA isn’t expected – and the CELIA results were consistent with that expectation. The adverse events that did show up were largely procedural: injection-related pain, post-lumbar puncture syndrome, and transient confusional states that mostly resolved within a week. More than 90% of participants who completed the placebo-controlled period chose to continue into the long-term extension, a reasonable proxy for how tolerable the regimen felt in practice.
Diranersen isn’t replacing amyloid-targeted care – the study enrolled only patients who hadn’t previously received anti-amyloid therapy, leaving open how it might perform as an add-on or sequential treatment. But with Biogen now moving toward confirmatory Phase 3 development, CELIA gives tau-lowering therapy its strongest clinical proof of concept to date, and hands the field a genuine second front in the fight against a disease that has, until recently, felt like a one-target story.
