Scientists studying the non-Alzheimer’s disease tauopathies have struggled to find tracers that will bind the particular forms of tau that predominate in those diseases. At Tau2020, held February 12–13 in Washington, D.C., Gil Rabinovici, University of California, San Francisco, reviewed in vivo and autoradiography data of human tissue.
- In primary tauopathies, pathology on autopsy doesn’t always match PET.
- New tracers may bind 4R tau with high affinity.
- CryoEM structures could finally lead to α-synuclein tracers.
It showed that flortaucipir bound far more weakly to the straight and twisted filaments of the 4R tau found in progressive supranuclear palsy (PSP) and corticobasal disease (CBD), or to the 3R versions found in Pick’s disease, than it does to the 3R/4R paired helical fragments found in AD (Aug 2016 conference news; Sep 2018 news). While flortaucipir does bind to regions that accumulate tangles in primary tauopathies, such as the globus pallidus in PSP, the signal is obscure because off-target binding in healthy controls also occurs in those same regions. In toto, flortaucipir is not useful in these diseases, Rabinovici said.
New autopsy data from 19 people who had flortaucipir PET scans during life supported this. The cases ranged from 34 to 76 years old, with an average of 30 months between the scan and autopsy. Eight were diagnosed with AD, four with PSP, two each had CBD or frontotemporal dementia (FTLD) caused by a tau mutation, and one each had had FTLD-FUS, FTLD-TDP43, or argyrophilic grain disease. David Soleimani-Meigooni in Rabinovici’s lab found excellent PET-to-tau immunohistochemistry correlations among the eight AD patients, who were all at Braak stage VI.
Autopsy Versus Scan. Comparing tangle immunohistochemistry after death (bottom rows) with flortaucipir scans during life (top rows) shows tight correlation in AD and modest to poor correlation in certain primary tauopathies, such as PSP and CBD, as well as other FTLD and AGD. [Courtesy of Gil Rabinovici.]
As predicted from in vivo imaging, autopsy correlations were modest to poor for the primary tauopathies. While their scans had shown more flortaucipir retention in the brain than in normal controls, it was still much less than in AD, and the binding did not always correlate with tangle pathology. For example, non tau-mutation FTLD cases had bound tracer in frontal white matter and basal ganglia, but no tau showed up there on autopsy. The data corroborate that flortaucipir’s specificity and sensitivity for detecting tau in primary tauopathies is low, said Rabinovici.
The same seems true for a case of chronic traumatic encephalopathy (CTE), a sporadic tauopathy. Working with Rabinovici, William Mantyh at UCSF compared autopsy data from a former professional football player in the U.S., who had had a flortaucipir PET scan 52 months before he died (Mantyh et al., 2020). Postmortem pathology indicated stage 4 CTE with limbic argyrophilic grain disease, limbic-predominant TDP43 encephalopathy, and Braak stage III neurofibrillary tangles. Strong flortaucipir binding mapped to regions of high tau pathology in his left fusiform and inferior temporal gyri and in frontal white matter, and yet, other regions that lit up strongly in the scan, such as the basal ganglia, thalamus, motor cortex, and calcarine cortex, had low tau burden on autopsy.
Overall, the correlation between tau PET and pathology at the regional level was weak. “Flortaucipir does not seem to be a great tracer for CTE, at least based on this case,” said Rabinovici.
What about other tau tracers, then? Radiography data suggest that MK6240, RO948, and JNJ067 also are not specific and sensitive enough in primary tauopathies. On the other hand, PBB3 and Life Molecular Imaging’s (formerly Piramal) PI-2620 show promise, said Rabinovici. He reported recent in vivo data showing that 48 people with PSP bound much more PI-6260 in the globus pallidus, subthalamic nucleus, substantia nigra, and dentate nucleus than did 10 healthy controls.
The PBB3 derivative PM-PBB3 is being developed by Taiwan-based Aprinoia Therapeutics under the name APN-1607. Richard Margolin from Aprinoia claimed it is the only tracer to demonstrate age-associated increase in signal in a 4R tau mouse model.
About 265 people have been scanned with APN-1607 so far, including 36 subjects in a Phase 1 study that enrolled 12 healthy controls, 12 AD, three FTD, three PSP, three CBS, and three people with vascular cognitive impairment. In D.C., Margolin said that uptake in AD patients matches the known pathological distribution of tau. In PSP, the tracer bound much more strongly in the globus pallidus, subcortical white matter, and midbrain.
Binding correlated with the PSP Rating Scale, a first for a PSP tracer, said Margolin. Controls retain some APN-1607 in the meninges and choroid plexus, which is a problem with other PET tracers, as well. APN-1607 does not bind to monoamine oxidase or melanin (see also Part 3 of this series).
Tsuneya Ikezu at Boston University considers APN-1607 a tracer to watch, because it binds to all forms of tau. Rabinovici was more cautious. He noted that APN-1607’s dissociation constant (Kd), which reflects binding strength, for AD and PSP tangles is a bit weak at 4 and 6.4 nM, respectively. By comparison, the Kds for PI-2620 and MK6240 are 2.1 and 0.73 for AD tangles. Rabinovici also believes the off-target binding might still be an issue. APN-1607 is unstable under ambient light, where it isomerizes, making it more difficult to use. Rabinovici thinks better tau tracers are still needed.
One new candidate for 4R tau might be CBD-2115, which is being developed by Samuel Svensson and colleagues at CBD Solutions in Sweden. Rabinovici showed that this compound bound more than twice as strongly to frontal gyrus extracts from CBD patients than did MK6240 or flortaucipir, but he said no scans of people have been done yet with this compound.
Rabinovici is collaborating with Chet Mathis at University of Pittsburgh Medical Center, to develop new 4R tau ligands. They will exploit the different cryoEM structures of tau fibrils identified by Michel Goedert, Sjors Scheres and colleagues at the MRC in Cambridge, England. These investigators have solved structures of four different forms of tau fibril isolated from people with AD, Pick’s disease, CTE and, most recently, CBD (see Feb 2020 news).
Under a new U19 “center without walls” NINDS grant, Mathis, together with Robert Mach and colleagues at the University of Pennsylvania, Philadelphia, will run in silico modelling experiments to screen thousands of compounds in search for some that bind specifically to the unique groves formed by each of these fibrils, then whittle down the hits to candidates for clinical testing. “The other approach will be to take existing tau tracers, model how they bind to different tau forms, and see if we can tweak them to improve their binding properties,” said Rabinovici.
In parallel, Mach and E. James Petersson, also at UPenn, will use a similar strategy to screen for compounds that might make good PET ligands for α-synuclein. Mach collaborates with John Karanicolas, Fox Chase Cancer Center, who has developed in silico methods to find small molecules that bind to pockets on protein surfaces. Starting with the three-dimensional structure of those pockets, the Karanicolas method virtually “adds” 1 Ångstrom diameter spheres until the pocket is filled. He then uses the configuration of these spheres as an exemplar to find compounds that assume a similar shape (Johnson and Karanicolas, 2016).
Mach, Petersson, and colleagues have used this exemplar method to screen compound libraries for small molecules that fit sites suggested by solid-state NMR structures of α-synuclein determined by Chad Rienstra and colleagues at the University of Illinois, Urbana-Champaign (Tuttle et al., 2016).
Screening was extremely fast. Mach told Alzforum that within a few weeks they had whittled 3.5 million compounds down to a handful of candidates, ultimately identifying a compound that has 7nM affinity for α-synuclein and fivefold selectivity over binding to Aβ. “This is not great, but it’s a lot better than what we had before,” he said. Mach thinks he can improve that further using cryoEM structures of two different synuclein fibrils isolated from patients who had multiple-system atrophy. Goedert showed these structures at Tau20202 (see Part 2 of this series). “We are now using these structures to build new exemplars for further screening,” said Mach.—Tom Fagan
- Improving Tau PET: In Search of Sharper Signals
- Flortaucipir Imaging Distinguishes Alzheimer’s From Other Disorders
- Tau PET: The Field Expands Rapidly
- CryoEM of CBD Tau Suggests Another Unique Protofibril
- Behold the First Human α-Synuclein CryoEM Fibril Structure
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