Lessons from the Lilly-AC Immune Expansion: Is the "Extracellular Tau Antibody" Era Over?

Tau Pathology: A Key Breakthrough in Alzheimer’s Treatment

In the field of Alzheimer’s disease (AD), β-amyloid (Aβ) has long been the central focus. However, Aβ-targeted therapies such as Lecanemab and Donanemab can only slow disease progression to a limited extent. They are unable to fully stop the underlying neurodegenerative changes, which highlights the limitations of targeting Aβ alone. As a result, research efforts are shifting away from a single Aβ target toward broader disease mechanisms, with particular attention on Tau protein ^[4].

In the pathological process of Alzheimer’s disease, β-amyloid and Tau protein are not independent causes. Together, they form critical links in the chain of disease development. β-amyloid buildup is generally seen as an early trigger. The pathological plaques it forms in brain tissue can trigger a cascade of neuroinflammation and cell damage. Tau protein, on the other hand, becomes abnormally phosphorylated and aggregates, which disrupts the internal structure of neurons and leads to the formation of neurofibrillary tangles. These changes are more closely linked to the decline in cognitive function.

This understanding is driving a gradual shift in research focus toward Tau as a target that lies closer to the final common pathway of the disease. It signals that Alzheimer’s treatment is moving toward more diverse approaches. As an important alternative strategy, Tau pathology requires accurate animal models as the foundation for related research.

Figure 1. Evolution of Alzheimer's disease treatment: From Aβ to tau ^[4].

Progress and Core Strategies in Tau Targeting Drug Development

Current research on therapies targeting Tau protein includes diverse approaches such as gene-silencing tools like antisense oligonucleotides and siRNA, vaccines, antibodies, and specialized molecules like PROTACs and enzyme inhibitors. Among these, the ASO drug BIIB080 has shown significant progress by reducing both soluble and aggregated tau in early trials, earning it a fast-track status for further testing ^[5]. In contrast, the antibody bepranemab recently achieved a breakthrough by slowing cognitive decline in specific patient groups during Phase 2 trials, even though it did not meet all its initial goals ^[6]. Meanwhile, earlier N-terminal antibodies faced major setbacks, failing to stop the disease because they targeted the wrong part of the tau protein ^[4].

These developments highlight that successful tau-targeted therapy depends on three key factors: precisely targeting the most relevant pathological forms of the protein, selecting the right patients using advanced brain imaging, and ensuring sufficient potency to deliver meaningful clinical benefit. A prime example of this strategic approach is the small-molecule Tau inhibitor developed through the partnership between Eli Lilly and AC Immune. This drug candidate was specifically engineered for high blood-brain barrier permeability, allowing it to address Tau pathology both inside and outside of cells. In preclinical evaluations, AC Immune’s small molecule demonstrated robust efficacy, significantly reducing Tau aggregates and providing clear neuroprotective benefits in Tg4510 mouse models ^[7]. These promising results were a decisive factor in Eli Lilly’s decision to deepen its investment in the collaboration. Throughout the drug development process, mouse models like the Tg4510 remain indispensable tools, as they simulate the spread of Tau pathology across the brain and serve as a vital bridge between mechanistic research and human clinical trials.

Figure 2. Classification of tau-targeting therapies in clinical trials^[8].

Cyagen Tau Humanized Models: Precisely Replicating Human AD Pathology

To meet the growing demand for Tau-targeted drug discovery and development, Cyagen has developed a series of Tau humanized mouse models.

Among these, the huTau-P301L mouse strain (Product ID: C001835) utilizes gene-editing technology to completely replace the mouse Mapt gene with the human MAPT gene. It also introduces the same core mutation (P301L) found in Tg4510 mice. This ensures that the human Tau protein achieves physiological expression and splicing within the mouse body, more closely mimicking human pathophysiology.

When analyzing key pathological processes, this humanized mouse offers significant advantages over traditional CDS (Coding Sequence) humanized models, providing an irreplaceable research platform.

Representative validation data is shown below. For full results, refer to the product datasheets in cyagen website.

Behavioral Testing: Novel Object Recognition Test ( 6-month-old & 9-month-old)

In the Novel Object Recognition (NOR) test, huTau-P301L and huTau-P301S mice showed no significant preference for the novel object over the familiar one, indicating a clear impairment in episodic memory function. In contrast, wild-type (WT) and huTau mice exhibited normal preferential behavior.

Figure 3. The object preference (percentage of exploring the specific object/total exploration time of both objects) for mixed-sex WT, huTau, huTau-P301L, and huTau-P301S mice in novel object recognition test. Data were analyzed using the t-test; ns indicates no significant differences, p < 0.05, **p < 0.001.

Validation of Pathological Changes for htau Strains( 9-month-old)

Human Tau protein (HT7) expression was successfully detected in the hippocampal tissues of huTau, huTau-P301L, and huTau-P301S mice. Notably, the huTau-P301L and huTau-P301S mice also exhibited pathological features of neuronal structural disorganization, which were not observed in the huTau or Wild-Type (WT) mice.

*HT7 is a monoclonal antibody that binds to an epitope present on all isoforms of human tau, enabling the visualization of overall tau expression.

Figure 4. Visualization of DAPI (blue), NeuN (red), and HT7 (green) staining in the hippocampus of C57BL/6J wild-type (WT), huTau, huTau-P301L ,and huTau-P301S models.

Significant accumulation of phosphorylated Tau protein (AT8) was detected in the hippocampal tissues of huTau-P301L and huTau-P301S mice, accompanied by neuronal structural disorganization. In contrast, neither the WT mice nor the huTau mice displayed these pathological changes.

*AT8 selectively recognizes tau phosphorylated at Ser202 and Thr205, serving as a marker for pathological tau hyperphosphorylation commonly observed in Alzheimer's disease and related tauopathies.

Figure 5. Visualization of DAPI (blue), NeuN (red), and AT8 (yellow) staining in the hippocampus of C57BL/6J WT, huTau, huTau-P301L, and huTau-P301S models.

Small interfering RNA (siRNA) drugs can significantly reduce the expression of human MAPT mRNA in huTau mice.

huTau humanized mice have been widely used to validate the efficacy of siRNA drugs targeting MAPT.

Figure 6. huTau mice used for preclinical efficacy evaluation of various human Tau-targeting siRNAs (Data provided by Cyagen clients).

Conclusion

In summary, the turning point in the Tau landscape in 2026 highlights a strategic shift in Alzheimer's Disease (AD) research and development. The "wait-and-see" attitude triggered by Johnson & Johnson’s termination of cooperation stands in stark contrast to Eli Lilly’s $12.5 million increased investment in AC Immune. This not only validates the value of the Tau target but also indicates that small-molecule therapies—capable of high blood-brain barrier (BBB) penetration and precision targeting of intracellular Tau pathological conformations—have become the core breakthrough in AD R&D.

Eli Lilly’s increased commitment reflects its long-term optimism for the Tau sector, while Cyagen’s Tau humanized mouse models provide the critical instrumental support needed for these breakthroughs. Looking ahead, as the collaboration between Eli Lilly and AC Immune progresses and IND applications are initiated, Tau-related therapies are expected to enter a new clinical phase. Precise preclinical models will continue to empower R&D, driving AD treatment toward a leap from delaying disease progression to blocking it entirely, bringing new hope to patients.