Researchers would like better ways to image inflammation in the brain. Currently, some use PET tracers targeting the mitochondrial transporter TSPO to measure active microglia, but these ligands are plagued by poor specificity and high variability. In the January 7 Proceedings of the National Academy of Sciences of the United States, online, researchers led by Andrew Horti and Martin Pomper at the Johns Hopkins University School of Medicine in Baltimore present a new tracer candidate that binds colony stimulating factor 1 receptor (CSF1R). In the brain, CSF1R is found only on microglia and infiltrating macrophages. In animal models, the ligand, CPPC, had bound poorly in healthy brain, but this doubled under inflammatory conditions. Likewise, binding in postmortem AD brain was about twice that of healthy tissue. Tracer uptake could be completely blocked by pretreating with other CSF1R ligands, demonstrating high specificity. “This is the first tool to look specifically at the microglial component of neuroinflammation in vivo,” Pomper told Alzforum. He said the next step will be to test CPPC in clinical trials.
- Researchers debut a new PET tracer that binds specifically to activated microglia.
- In animal models, uptake in inflamed brain was twice that of healthy brain.
- Ditto in postmortem AD brain; human trials are being planned.
Other researchers said the tracer shows potential. “This is a welcome and potentially important advance,” Hugh Perry at the University of Southampton, U.K., wrote to Alzforum. Andreas Jacobs at Westfälische Wilhelms-Universität, Münster, Germany, agreed, “The new radiotracer should be of highest interest for researchers working in the field of molecular imaging of microglia in various neurological disease conditions.”
For decades, the standard microglial tracer has been PK11195, which binds TSPO, but with poor specificity and a weak signal-to-noise ratio. This has led to an effort to develop better TSPO ligands (Mar 2013 conference news; Dec 2014 conference news). Meanwhile, other researchers contend that TSPO is not an ideal target, as it can also be present in activated astrocytes and endothelial cells. In addition, a polymorphism in the human TSPO gene leads to low ligand binding in some people (Apr 2012 news; Feb 2015 news).
New Way to Image Neuroinflammation? A new PET tracer, CPPC, binds to three postmortem AD brains but not to control (top row); uptake is completely blocked by unlabeled tracer (bottom). [Courtesy of Horti et al., PNAS.]
To find a more specific tracer, Horti, Pomper, and colleagues turned to CSF1R, which is upregulated during inflammation and in AD mouse models (Walker et al., 2017; Murphy et al., 2000). They started with a CSF1R inhibitor, 5-cyano-N-(4-(4-methylpiperazin-1-yl)-2-(piperidin-1-yl)phenyl)furan-2-carboxamide, developed by Johnson & Johnson as an anti-inflammatory agent (Illig et al., 2008). They radiolabeled CPPC with carbon 11 and tested it in wild-type mice, where it had good brain uptake. When they induced neuroinflammation by injecting lipopolysaccharide into one hemisphere, binding went up by 50 percent. Signal rose even higher in a baboon, where uptake more than doubled after LPS injection. Pretreating the animal with cold CPPC abolished binding, demonstrating specificity (see image below).
Other disease models also showed strong CPPC uptake. In a mouse expressing human APP with the Swedish and Indiana mutations, [11C]CPPC uptake was up 31 percent in frontal cortex compared with wild-type mice (Melnikova et al., 2013). By contrast, in an experimental autoimmune encephalomyelitis mouse, tracer uptake doubled in the brainstem, where most demyelination occurs. In postmortem human brains, AD tissue had about twice as much signal as healthy brain. Again, binding could be blocked by unlabeled CPPC, as well as by three other CSF1R inhibitors (see image at top). Pomper noted that the signal was at least as strong as that seen with TSPO, even though the tracer binds a more select cell set.
Other researchers wanted to see more data on the tracer’s binding affinity and sensitivity, as well as detailed cellular-level staining proving that it binds specifically to microglia. Agneta Nordberg at the Karolinska Institute in Solna, Sweden, wondered whether the tracer would detect all activated microglial subtypes, including those present early in Alzheimer’s disease (Edison et al., 2018; Jun 2017 news; Dec 2018 news). “It is somewhat difficult to judge its clinical value for AD patients,” Nordberg wrote (full comment below).
Pomper plans to investigate that next. Toxicology studies have demonstrated the tracer is safe for human use. Pomper will scan people with various inflammatory brain disorders such as AD, multiple sclerosis, and traumatic brain injury to see how it performs. Meanwhile, Horti is developing an analog that can be labeled with fluorine 18. With a longer half-life, this version could be commercialized and see more widespread use in the clinic. Eventually, CPPC could be used not just to image neuroinflammation, but to measure target engagement of anti-inflammatory drugs that inhibit CSF1R, Pomper suggested. “We could measure drug occupancy in individual patients and use that to tailor therapies to each person,” he told Alzforum.—Madolyn Bowman Rogers
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