In a brain scan, a swirl of red marked the damage inside one person's mind—then something changed. The red faded after they received an experimental drug called diranersen, made by the company Biogen. That image, shared at a conference in London this week, offered a small but striking glimpse of hope in the fight against Alzheimer's disease.
Alzheimer's affects more than 7 million Americans and tens of millions of people worldwide. Today's treatments target a protein called amyloid, which forms sticky plaques in the brain years before memory problems start. Two drugs, lecanemab and donanemab, work this way and can modestly slow cognitive decline. But they don't work for everyone, and they can cause serious side effects like brain swelling.
Now, researchers are testing a different approach. Instead of clearing amyloid, diranersen goes after a second protein called tau. Scientists believe that as amyloid builds up over decades, it triggers tau to form twisted tangles inside brain cells—these tangles are what actually kills neurons and destroys memory. Tau has been a tough target; many past drugs aimed at it have failed.
The new study followed about 400 people with early-stage Alzheimer's or mild memory loss. Participants received injections of diranersen into the fluid around their spinal cord—a straighter path to the brain than today's amyloid drugs, which require infusions through the bloodstream. The drug works not by attacking existing tau clumps, but by telling the brain to produce less tau in the first place. "If you lower tau production, you are lowering the amount of abnormal tau that needs to be cleared," explained Dr. Cath Mummery of University College London, who led the study. "You are enabling the normal clearance mechanism to have more capacity."
The results were unexpected. The lowest dose, given twice a year, showed the strongest benefit—a 26% reduction in cognitive decline, roughly similar to what doctors had seen with amyloid drugs in earlier studies. Five out of six different brain tests showed that people taking diranersen declined more slowly than those receiving placebo shots. Side effects included some injection-site pain and temporary confusion lasting about a week, but notably no signs of the brain inflammation that can affect patients on amyloid-targeting treatments.
Researchers cautioned this is early work. The study did not meet its original goal of showing that higher doses worked better—exactly the opposite happened. Biogen is now planning a larger study to confirm whether diranersen truly helps patients.
Still, experts not involved in the study called the findings encouraging. "This is really quite promising if it were to hold up," said Jessica Langbaum of the Banner Alzheimer's Institute in Phoenix. Dr. Reisa Sperling of Mass General Brigham added that the research "will reinvigorate interest and investment in lots of tau mechanisms, and the field needs that."
Diranersen is one of several new approaches being explored, including a possible tau vaccine and other drugs that might cross into the brain more easily. It's a reminder that science moves forward in small steps—and sometimes those steps lead somewhere unexpected.