Biogen’s diranersen shows mid-stage Alzheimer’s gains

Biogen presented new mid-stage trial results for its experimental Alzheimer’s therapy diranersen at the Alzheimer’s Association International Conference in London on July 14, 2026. The antisense oligonucleotide is designed to reduce production of tau, a protein linked to neurodegeneration, and was tested in people with mild cognitive impairment or mild Alzheimer’s disease.

The trial enrolled roughly 400 participants who were randomized to receive different doses of diranersen or placebo. Dosing regimens included options such as administration every six months, allowing investigators to compare multiple exposure levels against a control group.

Across the study, diranersen treatment lowered tau protein levels in recipients’ brains, indicating that the drug was engaging its intended biological target. This biomarker effect was accompanied by clinical signals, with patients on diranersen generally experiencing slower cognitive decline than those on placebo.

On one cognitive test, the lowest diranersen dose administered every six months translated into a 26% reduction in cognitive decline versus placebo. Overall, five of the six cognitive assessments used in the study showed slower deterioration among participants who received the drug compared with those who received placebo.

Despite these positive findings, the dose–response pattern was not straightforward. The lowest dose produced the clearest clinical signal, so the trial did not meet its original goal of demonstrating greater benefit at higher doses.

Investigators reported that diranersen’s side effects in the trial included injection‑site pain, which is common with injectable therapies. Another observed adverse effect was a transient state of confusion that could appear a few days after dosing and last about a week.

Importantly for a therapy targeting the brain, the study did not find evidence of brain inflammation among trial participants. This absence of observed inflammation may be relevant for long‑term development, given safety concerns seen with some other neurological treatments.

On the basis of the mid-stage data, Biogen is planning a larger, pivotal Phase 3 clinical trial of diranersen. The goal of that study will be to determine whether the biomarker changes and early clinical signals observed so far translate into a reproducible and clinically meaningful benefit for people with early Alzheimer’s disease.