Amyloid β—check. Tau—check. Microglia—? Researchers have long sought a tracer suitable for imaging the activation of these immune cells in the living brain. After some ho-hum attempts, scientists once again hope that a new compound finally might fit the bill. At the Society for Neuroscience annual meeting, held November 15-19 in Washington, D.C., researchers reported for the first time that they picked up the tracer in the brains of live Alzheimer's mouse models using micro PET scans. Transgenic AD mice took up more GE180 in their brain than did control animals. Mice treated with an experimental AD drug took up less of the tracer over time, suggesting that GE180 could possibly gauge responses to therapies that reduce the activation of microglia. Researchers at the meetings seemed impressed by the findings, but wanted to know more about the ligand’s receptor.
GE180, like many glial tracers before it, binds a mitochondrial translocator protein, TSPO for short. Many tissues of the body make large amounts of TSPO, but a healthy brain produces very little. Activated microglia, on the other hand, crank out TSPO, making it a potential marker of brain inflammation. Fortuitously, the protein has an affinity for certain chemicals. Several such carbon-11 compounds, including the prototypical ligand PK11195, have been investigated as microglial PET tracers, but as Cynthia Lemere of Brigham and Women's Hospital, Boston, explained in Washington, GE180 makes a more versatile tracer. It sports fluorine-18, which has a longer half-life than C-11 and need not be synthesized on site.
Earlier this year, researchers reported that GE180 has a better signal-to-noise ratio than PK11195 when picking up glial activation in a mouse model of ischemic stroke (see Boutin et al., 2014). Researchers in Finland tested the ligand in a rat model of acute inflammation, and their paper gave the GE compound an edge over PK1195 in assays of the compounds’ binding potential (see Dickens et al., 2014).
How does GE180 fare in AD models? Bin Liu, a senior postdoc in Lemere's lab, tracked its uptake in the brains of APP/PS1 transgenic mice and controls. Four-month-old animals took up little. By 26 months, wild-type mice showed a noticeable uptick in tracer binding in the brain, suggesting an age-related increase in glial activation. Old APP/PS1 animals took up even more of the tracer, in keeping with pathology driving glial inflammation.
To pinpoint where GE180 binds, Liu co-registered PET images to MRI scans of the mouse brain. Both the cortex and hippocampus in the old APP/PS1 mice took up more tracer than did controls. Importantly, Liu correlated uptake seen by PET with tissue autoradiography done on 1 millimeter slabs of the brain. In thinner, fixed cryo-sections, GE180 binding correlated with increased plaque deposition, said Lemere, and the TSPO-positive cells tested positive for the microglial marker CD68.
During question time, scientists at the meeting wanted to know more about TSPO. Dave Morgan, University of South Florida in Tampa, implored colleagues to develop good antibodies to the protein for immunohistochemical work. "We've been labelling microglia for years but have never had a good marker for TSPO," Morgan said. Lemere replied that AbCam now has a good monoclonal antibody to the mitochondrial protein.
Others wondered how TSPO is regulated, and if its expression returns to baseline levels once inflammation resolves. In her talk, Michelle James, Stanford University, Palo Alto, California, showed just that in another mouse model of AD.
James used micro PET to track GE180 uptake in mice expressing human APP with the London and Swedish mutations. To see if the compound can trace both the rise and fall of glial inflammation, James treated the animals with an experimental drug being developed by Frank Longo at Stanford and Stephen Massa at the University of California, San Francisco. LM11A-31 binds to the p75 neurotrophin receptor and PharmatrophiX, a company founded by Longo, recently completed a Phase 1 trial (see Nov 2013 conference story). Longo told Alzforum that a Phase 2a trial in AD patients is being planned.
James measured GE180 uptake in 5.5- to 7-month-old APP and wild-type mice to get a baseline scan. Nadia Belichenko at Longo's lab then administered LM11A-31 daily for three months before scanning the mice again and sacrificing them for autoradiography of brain tissue.
The APP mice took up much more of the tracer than the wild-type controls at baseline, indicating that neuroinflammation accompanies plaque pathology in these animals. Three months later, GE180 uptake in the cortex and hippocampus of animals treated with LM11A-31 had returned to wild-type levels, while untreated mice took up as much of the tracer as before. Analysis of the brain tissue showed lower levels of the glial activation markers Iba1 and TSPO in treated animals than controls.
James said LM11A-31 attenuates hyperphosphorylation and misfolding of tau, and prevents degeneration of neurites and synaptic spines. The compound seems to reduce inflammation as well, though it is unclear if this is a direct effect on microglia. Interestingly, James said she saw high GE180 uptake in the choroid plexus, where it also returns to baseline with treatment. The choroid plexus helps generate the cerebrospinal fluid and may serve as a conduit for removal of infiltrating immune cells.
William Trigg, a project leader at GE, told Alzforum that GE180 (which also goes by GEH120714 and flutriciclamide) is being tested in the clinic. General Electric has sponsored a Phase 1 trial for relapsing/remitting multiple sclerosis patients and healthy volunteers in Canada. Researchers at Imperial College in London have just kicked off investigator-sponsored trials of AD, as well as traumatic brain injury and multiple sclerosis. "We should see first data from those trials in 2015," said Trigg. The AD trial will enroll 30 people with AD, mild cognitive impairment, or normal cognition. Participants will also be scanned for brain amyloid.
Some people with a genetic variant in the TSPO receptor will be ineligible for GE180 imaging because their mitochondrial transporter binds poorly to the ligand. Trigg said that about 10 percent of Caucasian and African populations, and a smaller percentage in Asian populations, are homozygous for this variant, rendering them unsuitable candidates for the tracer. GE has implemented a blood test for binding affinity, and a genotype assay is available as well. Binding heterogeneity in the population could be a concern; however, Lemere said she does not expect the low-binding issue to hamper development of the tracer. Trigg encouraged researchers who want to plan studies using the compound to contact GE. "We have fairly open access," he claimed.—Tom Fagan
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- Liu GJ, Middleton RJ, Hatty CR, Kam WW, Chan R, Pham T, Harrison-Brown M, Dodson E, Veale K, Banati RB. The 18 kDa Translocator Protein, Microglia and Neuroinflammation. Brain Pathol. 2014 Nov;24(6):631-53. PubMed.