Amyloid beta builds up as plaques in the brain decades before symptoms of Alzheimer’s disease (AD) appear. This slow accumulation begins with the misfolding of individual amyloid beta molecules, which causes them to stick together and form oligomers—small clusters of proteins. Oligomers adhere to one another and form pre-fibrils. Oligomers and pre-fibrils are soluble and can move freely throughout the brain. Scientists refer to these two stages as nonfibrillar amyloid aggregates.
Eventually, pre-fibrils cluster together into amyloid fibrils. These larger structures become insoluble masses that get stuck in the brain. Over time, these fibrils clump together and form the plaques we recognize as part of AD.
Researchers are not sure which of these amyloid forms should be targeted to prevent or slow the progression of the disease. There are currently two FDA-approved AD drugs that target amyloid in the brain: Leqembi and Kisunla. Both drugs target plaques, and Kisunla also targets fibrils. While these drugs are an important step forward, neither entirely stops the disease from progressing. This begs the question of whether a different form of amyloid should be targeted.
However, the challenge is that oligomers and pre-fibrils (nonfibrillar amyloid aggregates) are invisible to current detection methods. This means we can’t yet see them with brain scans or measure them reliably in living patients. Developing tools to detect amyloid at these early stages would allow scientists to study its effects in the lab and could lead to earlier diagnosis and more precise therapeutic strategies—potentially catching the disease when interventions might be most effective.
Existing amyloid PET tracers—imaging tools that use radioactive markers to light up amyloid in brain scans—were transformative for detecting AD before severe symptoms appear. These tracers were put to good use for enrolling early-stage AD patients in clinical trials. However, they only detect mature amyloid fibrils and plaques. This means they miss the earlier oligomers and pre-fibrils that may be causing damage long before plaques appear. Dr. Gandy and his collaborators want to develop a PET tracer to detect these earlier stages. They have already designed and produced a strong candidate, named Tracer 1, which interacts with these nonfibrillar amyloid aggregates. Their preliminary studies also show that Tracer 1 is safe in mice, crosses the blood-brain barrier, and is detectable in the brain via PET scans. In this proposal, they are further developing and validating this PET tracer in mice and human brain tissue. If they are successful, the next step will be to test it directly in humans.
Drs. Gandy and Guérin assembled an interdisciplinary team of investigators who proposed four aims. In the first aim, they are confirming the specificity of Tracer 1 for nonfibrillar amyloid aggregates by performing PET brain imaging in an amyloid mouse model (Dutch mice) that makes ample amounts of amyloid oligomers and pre-fibrils but does not form typical fibrils or plaques. Mice across multiple ages will be injected with Tracer 1 for PET imaging, and their brain tissue will be collected afterward. This aim will also determine how sensitive Tracer 1 is in detecting nonfibrillar aggregates. In the second aim, they are determining the specificity of Tracer 1 binding to oligomers and pre-fibrils. In the third aim, they are testing if Tracer 1 binds to nonfibrillar amyloid aggregates in human brain autopsy samples. Prior work shows that APOE4 carriers have significant nonfibrillar amyloid accumulation before cognitive decline, so they will first perform experiments in samples from APOE4 carriers before adding non-APOE4 cases. Finally, in the fourth aim, they are determining how Tracer 1 gets across the blood-brain barrier and if its delivery causes any loss of integrity of this barrier. These experiments will also be performed in mice.
Of note, this project requires interaction between laboratories in three different countries. The Lubell lab in Montreal and the Rahimipour lab in Tel Aviv are chemically preparing and analyzing the various chemical tracers used in the studies, while the Guérin lab in Sherbrooke, Quebec, and the Gandy, Ehrlich, and Teunissen labs in New York are administering the tracers to the mice and carrying out the PET scans and analyses. The human tissue is from the Mount Sinai Brain Bank and the James J Peters VA Medical Center in New York.
Much of the first funding period was dedicated to scaling up production of Tracer 1, with Dr. Lubell synthesizing the base molecule, and Dr. Guérin and Dr. Teunissen adding key elements to assemble the functional tracer. Significant work was done to identify the most efficient and effective method for producing the tracer, a critical step toward long-term scalability. They also performed pilot experiments in the Dutch mice, identifying where the tracer binds on the subcellular level. They also confirmed the tracer binds to oligomers and pre-fibrils. This is critical for understanding the exact type of amyloid that can be detected using this tracer. The second year of funding will focus primarily on testing the tracer in post-mortem human brain tissue and on determining its capacity to cross the blood-brain barrier.
