Biogen's experimental Alzheimer's drug diranersen failed to meet its primary goal in a Phase 2 trial, yet the company is moving it into late-stage testing anyway. The decision reflects a changing landscape in dementia research, where scientists are increasingly willing to pursue therapies that show cognitive benefits even when they don't hit their original targets. At the Alzheimer's Association International Conference (AAIC) in London, researchers presented data showing that while the drug didn't demonstrate a dose-dependent reduction in brain tau as planned, it still slowed cognitive decline and reduced toxic tau tangles in people with early Alzheimer's disease.

What Happened in the Diranersen Trial?

Biogen enrolled 416 people with early Alzheimer's in the CELIA trial and randomly assigned them to receive either a placebo or one of three doses of diranersen, delivered via intraspinal injection. The drug works by silencing the genetic signal that tells brain cells to produce tau proteins, a hallmark of Alzheimer's disease. The primary endpoint was designed to test whether higher doses would produce stronger cognitive benefits than lower doses, a pattern typical of effective treatments.

Instead, something unexpected happened. The lowest 60-milligram dose produced the strongest results, slowing cognitive decline on the Clinical Dementia Rating Sum of Boxes (CDR-SB) scale by 26 percent compared to placebo after 18 months. Higher doses showed less benefit, raising questions about why the dose-response pattern didn't follow the expected trajectory.

"Typically, when a treatment is effective, both its benefits and its side effects tend to increase together with the dose. Seeing the largest apparent benefit in the lowest-dose group raises some concern that the finding could reflect cognitive placebo effects," said Sarah Ackley, an epidemiologist at Brown University.

Sarah Ackley, Epidemiologist at Brown University

Despite this concern, when researchers grouped all participants who received the drug and compared them collectively against placebo, the treatment showed a slowing of decline across five of six cognitive measures. The drug also reduced biomarkers of tau in cerebrospinal fluid and on brain imaging scans, marking the first Phase 2 evidence that a drug can remove toxic tau tangles in people with Alzheimer's disease.

Why Are Researchers Optimistic About a Failed Endpoint?

In traditional drug development, a Phase 2 trial that misses its primary endpoint often signals the end of the road. But experts say the secondary findings in the diranersen trial are compelling enough to justify moving forward. Jessica Langbaum, senior director of Alzheimer's prevention and research at Banner Health, noted that while it's rare for drugs that fail their primary outcome to succeed in Phase 3, the data here is promising enough for a larger trial.

The reasoning centers on what the drug actually accomplished. Diranersen demonstrated that gene-silencing therapies can target tau, a protein that tracks more closely with clinical severity than amyloid-beta, the other major pathological hallmark of Alzheimer's disease. This opens a new therapeutic avenue beyond the amyloid-targeting drugs Leqembi and Kisunla, which are currently the only FDA-approved disease-modifying treatments for early Alzheimer's.

Side effects were mostly mild to moderate and related to the spinal injection itself, including injection-site pain and confusion. No serious adverse events were reported that would disqualify the drug from further testing.

How Does Diranersen Compare to Other Experimental Alzheimer's Drugs?

At the same AAIC conference, two other experimental Alzheimer's therapies also presented mixed results. AriBio's AR1001, a repurposed erectile dysfunction pill, failed to show a statistically significant effect on cognitive function in its Phase 2 trial, though the company is proceeding with a larger Phase 3 study of 1,535 participants with confirmed Alzheimer's biomarkers. Johnson & Johnson discontinued development of posdinemab, an anti-tau antibody, after it failed to slow cognitive decline or halt tau spread, though imaging revealed some unexpected reduction in tau tangle accumulation in dense brain regions.

The contrast highlights why diranersen's results stand out. Unlike posdinemab, which showed no cognitive benefit, diranersen demonstrated measurable slowing of decline. Unlike AR1001, which showed no significant effect in its initial trial, diranersen showed consistent tau reduction across multiple measures.

What Comes Next for Tau-Targeting Therapies?

Biogen is now designing a definitive Phase 3 trial to confirm whether diranersen's secondary findings translate into meaningful clinical benefit. The company's decision to advance the drug despite the failed primary endpoint signals confidence in the underlying biology and the potential of gene-silencing approaches for Alzheimer's disease.

Experts believe the field is moving toward therapies that address multiple pathological processes simultaneously. Current approved treatments target amyloid-beta, but emerging research suggests that combining amyloid-targeting drugs with tau-directed therapies, brain-shuttle delivery systems, and lifestyle interventions may offer better outcomes than single-target approaches.

Steps to Understanding Your Alzheimer's Risk and Treatment Options

• Blood Biomarker Testing: Two FDA-cleared blood tests can help identify Alzheimer's pathology before symptoms appear. The Lumipulse pTau217/beta-amyloid 1-42 ratio is intended for specialized-care evaluation, while Elecsys pTau181 is primarily used as a rule-out test before specialty referral. Neither should be used for population screening without a full clinical assessment.
• Clinical Evaluation: A modern Alzheimer's assessment includes detailed history, cognitive testing, neurological examination, brain imaging (MRI or CT), and laboratory work to rule out reversible causes like thyroid disease or vitamin B12 deficiency. Biomarker results must be interpreted together with clinical findings, not in isolation.
• Treatment Decisions: If Alzheimer's pathology is confirmed, FDA-approved options include Leqembi and Kisunla, both amyloid-targeting monoclonal antibodies. Leqembi now has an FDA-approved once-weekly subcutaneous starting regimen as an alternative to intravenous infusion, approved on July 13, 2026. Both drugs require careful MRI-based safety monitoring and are indicated for mild cognitive impairment or mild dementia due to Alzheimer's disease.

The emergence of new delivery technologies and multi-target approaches represents a significant shift in how researchers think about Alzheimer's treatment. Langbaum emphasized that molecular brain shuttles, which help drugs cross the blood-brain barrier more efficiently, could be "a real game changer for the field" by moving beyond antibodies that require intravenous infusion. One such brain shuttle drug was already approved for treating Hunter syndrome, a rare childhood dementia, and Roche's brain shuttle anti-amyloid drug trontinemab is currently in Phase 3 trials.

Langbaum

For patients and families navigating Alzheimer's care, the takeaway is that the field is rapidly expanding beyond the first generation of disease-slowing drugs. While diranersen's path to approval remains uncertain, its advancement signals that researchers are willing to pursue promising leads even when initial targets aren't met, and that multiple therapeutic strategies are being developed in parallel. The next few years will likely bring clarity on which approaches work best and for whom.