[¹¹C]Deschloroclozapine is an improved PET radioligand for quantifying a human muscarinic DREADD expressed in monkey brain

Introduction: A previous study found that [¹¹C]deschloroclozapine ([¹¹C]DCZ) is a superior radiotracer to [¹¹C]clozapine ([¹¹C]CLZ) for imaging DREADDS¹. Building on this work, the present study used PET to quantitatively and separately measure the signal from transfected receptors, endogenous receptors/targets, and non-displaceable binding in other brain regions in order to assess the qualities that contribute to this superiority. Methods: A genetically modified muscarinic type 4 human receptor (hM₄Di) was injected into the right amygdala of an 11-year-old male rhesus macaque. PET scans with [¹¹C]DCZ and [¹¹C]CLZ were conducted two to 24 months later. Uptake was quantified relative to the concentration of parent radioligand in arterial plasma at baseline (n=3 scans/radioligand) and after receptor blockade (n=3 scans/radioligand). For [¹¹C]CLZ, the three blocking studies and i.v. doses were: a) clozapine-N-oxide (CNO) (10 mg/kg), b) CLZ (0.1 mg/kg), and c) clotrimazole (CTM; 1 mg/kg) plus CNO (10 mg/kg). For [¹¹C]DCZ, the three blocking studies and i.v. doses were: a) CNO (10mg/kg), b) low-dose DCZ (0.1 mg/kg), and c) high-dose DCZ (1 mg/kg). To assess the radiotracers’ transfer between plasma and brain quantitatively, eight different VOI-based methods were performed: the one- and two-tissue compartment models, Ichise’s multilinear reference tissue models (MRTM, MRTM0, and MRTM2), simplified reference models (SRTM and SRTM2), and Logan’s reference tissue model (Logan-REF). The cerebellum was used as the reference region.

Results: Both radioligands had greater uptake in the transfected region and displaceable uptake in other brain regions. Displaceable uptake was not uniformly distributed and might represent off-target binding to endogenous receptor(s). After correction, the [¹¹C]DCZ signal was 19% of that for [¹¹C]CLZ, and the background uptake of [¹¹C]DCZ was 10% of that for [¹¹C]CLZ. Despite stronger binding of [¹¹C]CLZ, the signal-to-background ratio for [¹¹C]DCZ was almost two-fold greater than for [¹¹C]CLZ. Both radioligands had comparable DREADD selectivity. All reference tissue models underestimated signal-to-background ratio in the transfected region by 40%-50% for both radioligands.Conclusion: Both [¹¹C]DCZ and [¹¹C]DCZ had high-affinity displaceable binding to DREADDs relative to endogenous receptor(s), However, the signal-to-background ratio of [¹¹C]DCZ was greater than that of [¹¹C]CLZ primarily because of lower nonspecific binding of [¹¹C]DCZ. Reference tissue models underestimated the signal-to-background ratio compared to two tissue compartmental model. Nevertheless, the use of a pseudo-reference region should provide useful information on the localization and comparative quantitation of the transfected receptor. Our results confirm that [¹¹C]DCZ is far superior to [¹¹C]CLZ to image the hM4Di DREADD for behavioral and translational experiments.References: 1. Nagai Y, Miyakawa N, Takuwa H, et al. Deschloroclozapine, a potent and selective chemogenetic actuator enables rapid neuronal and behavioral modulations in mice and monkeys. Nature Neuroscience volume 23, pages1157-1167(2020)

• Download figure
• Open in new tab
• Download powerpoint