Most experimental drugs targeting tau, a protein linked to Alzheimer's disease, have failed in clinical trials because scientists may have been testing them the wrong way. Researchers at AbbVie and other institutions presented evidence at a major neuroscience conference that the standard laboratory models used to develop these medications don't accurately reflect how tau spreads in the human brain. The discovery is reshaping how scientists design the next generation of Alzheimer's treatments .

Why Are Tau Antibody Drugs Failing?

Over the past few years, several promising tau-targeting antibodies have disappointed in human trials. Posdinemab, semorinemab, gosuranemab, and tilavonemab all showed early promise in laboratory tests but failed to slow cognitive decline in people with Alzheimer's disease. Even bepranemab, which performed better than some competitors, missed its primary goal of improving thinking and memory in Phase 2 testing .

The problem, according to researchers, lies in how these drugs were evaluated before reaching patients. Most antibodies were tested using two methods: laboratory cell cultures that measure tau "seeding," or mice genetically engineered to produce abnormal tau protein. However, these mice produce a form of tau that is rarely seen in people with Alzheimer's disease. This mismatch between the test system and real human biology may explain why drugs that looked promising in the lab failed in actual patients .

What's the Difference Between Seeding and Propagation?

Scientists at AbbVie identified a critical distinction that most previous research overlooked: the difference between tau "seeding" and tau "propagation." Seeding refers to the initial clumping of tau protein in a test tube or cell culture. Propagation, by contrast, is the actual spread of tau from one neuron to another through the brain's neural networks .

To test this theory, AbbVie researchers developed a new animal model that better mimics how tau spreads in human brains. They injected tau seeds from Alzheimer's disease brain tissue into the olfactory bulb, a region at the base of the brain connected to the sense of smell. Over weeks, they tracked how tau pathology spread to connected brain regions, measuring changes at one synapse away and two synapses away from the injection site. This approach captures true propagation, not just initial seeding .

The results were striking. Two weeks after injection, tau pathology was confined to the olfactory bulb. By four weeks, it had spread to the piriform cortex. At six weeks, pathology appeared in the entorhinal cortex, thalamus, and hippocampus. By eight weeks, tau had reached all four downstream regions in nearly all of the mice tested. This gradual, synapse-by-synapse spread mirrors what happens in human Alzheimer's disease much more closely than previous mouse models .

How Does the New Antibody Perform?

Using this improved propagation model, AbbVie tested an antibody called Ab-835, which binds to a specific phosphorylated site on tau protein in the proline-rich region. The antibody was selected from an unbiased screen of hundreds of candidates generated from tau proteins isolated directly from Alzheimer's disease brain tissue, with or without post-translational modifications .

When given one day after tau injection, Ab-835 reduced pathology in the olfactory bulb and slowed propagation to downstream regions. Even when treatment was delayed until day 3, after seeding in the olfactory bulb had progressed unchecked, the antibody still reduced propagation to the piriform cortex, entorhinal cortex, hippocampus, and medial thalamus by approximately 50 percent .

Over an eight-month period, Ab-835 continued to work. The antibody reduced tau pathology in the medial thalamus by 87 percent and in the hippocampus by 54 percent compared to untreated mice. In contrast, three N-terminal antibodies that had failed in human trials showed no ability to slow propagation at all, even at high doses. Bepranemab, which had shown some promise in earlier human studies, required a dose ten times higher than Ab-835 to achieve similar results .

How to Evaluate Tau Treatments More Effectively

• Use True Propagation Models: Test antibodies in animal models that measure tau spread between connected neurons over weeks and months, rather than relying solely on cell culture seeding assays that measure tau clumping in a test tube.
• Use Human-Derived Tau: Develop antibodies against tau proteins isolated directly from Alzheimer's disease brain tissue, including post-translationally modified forms, rather than relying on genetically engineered mice that produce abnormal tau rarely seen in patients.
• Measure Long-Term Effects: Evaluate antibody performance over extended periods, such as eight months or longer, to capture how tau spreads gradually through the brain in a way that better reflects the slow progression of human disease.

Xavier Langlois, a researcher at AbbVie, explained the significance of this shift in approach. "This shows that without blocking seeding in the olfactory bulb, we could prevent propagation to primary and secondary synapses," he stated. "This better captures how tau spreads in the human brain" .

"This is true tau propagation and is the most physiologically relevant model of pathology progression," said Xavier Langlois, AbbVie researcher, in reference to the new testing approach.

Xavier Langlois, Researcher, AbbVie

What About Other Tau Antibodies in Development?

Beyond AbbVie's work, researchers at discoveric bio, a Swiss startup, presented findings on a different type of antibody called a biparatropic antibody. This antibody is designed to bind two distinct sites on tau protein simultaneously, allowing it to recognize a broader range of tau forms, including truncated or chemically modified versions .

In laboratory seeding assays, the discoveric bio antibody proved more potent at stopping tau aggregation than both bepranemab and BMS-986446, another experimental antibody that targets the microtubule-binding region of tau. This dual-binding approach may overcome one of the challenges that has plagued single-epitope antibodies: tau's ability to evade immune attack through chemical modification .

What Happens Next?

AbbVie has optimized Ab-835 for use in the human brain and renamed it ABBV-277. A Phase 1 safety trial in healthy adults began in August 2025 and is expected to continue through May 2026. This early-stage study will determine whether the antibody is safe and well-tolerated before moving to trials in people with Alzheimer's disease .

The shift toward propagation-based testing represents a fundamental rethinking of how tau-targeting drugs should be developed. If ABBV-277 and other antibodies designed using this new framework prove effective in human trials, it could mark a turning point for Alzheimer's treatment. For the millions of people living with Alzheimer's disease and their families, this change in scientific approach offers renewed hope that better tau-targeting therapies may finally be on the horizon.