Translational Evaluation of PET Radiotracers

National Institute Of Mental Health

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Abstract

Nearly two decades ago, researchers discovered how to manipulate neuronal activity in vivo by engineering receptors to bind with specific small molecules, or ligands. This powerful new chemogenetic technology was dubbed Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). In most DREADDs experiments, a specific neuronal population is virally transfected with a receptor that has been genetically modified to respond to a specific designer drug that, in turn, is designed to pass up endogenous receptors and react with DREADD receptors only. Once introduced, DREADDs can be used to excite or inhibit target neuronal populations. It is also possible to quantify the density and distribution of DREADDs in vivo using PET. Earlier work had found that the ligand 11Cdeschloroclozapine (11CDCZ) could penetrate the brain but that the prototypical DREADD agonist Clozapine N-oxide (CNO) could not, making 11CDCZ superior for translational neuroscience applications that use DREADDs. Our laboratory sought to precisely measure those features that made 11CDCZ a superior agonist for DREADD neuroscience applications. To ensure the precision of our measurements, we quantified distinct PET signals from (1) transfected receptors, (2) endogenous receptors and targets, and (3) non-displaceable binding in other brain regions. 11CDCZ and 11CCLZ PET scans were taken two to 24 months after injecting a genetically modified muscarinic type-4 human receptor (hM4Di) into the right amygdala of a male rhesus macaque. The amount of hM4Di binding was derived by subtracting the parent radioligands baseline signal normalized to the concentration in arterial plasma (n = 3 scans/radioligand) from the concentration after receptor blockade (n = 3 scans/radioligand). As expected, both radioligands, which had comparable DREADD selectivity, showed greater uptake in the DREADD-transfected region; displaceable uptake was not uniformly distributed, perhaps representing off-target binding to endogenous receptor(s). After correction, 11CDCZ signal was 19% of that for 11CCLZ, and background uptake was 10% of that for 11CCLZ. Despite stronger 11CCLZ binding, the signal-to-background ratio for 11CDCZ was almost two-fold greater than for 11CCLZ. More significant was the fact that all reference tissue models for both radioligands underestimated the signal-to-background noise ratio in the transfected region by 40-50 percent. Together, the results indicate that the greater signal-to-background ratio of 11CDCZ was due to its lower background uptake, providing valuable information about why 11CDCZ works better than other radioligands for imaging the hM4Di DREADD.