The importance of small polar radiometabolites in molecular neuroimaging: A PET study with [ 11 C]Cimbi-36 labeled in two positions

Abstract

[ 11 C]Cimbi-36, a 5-HT 2A receptor agonist PET radioligand, contains three methoxy groups amenable to [ 11 C]-labeling. In pigs, [ 11 C]Cimbi-36 yields a polar (M1) and a less polar (M2) radiometabolite fraction, while changing the labeling to [ 11 C]Cimbi-36_5 yields only the M1 fraction. We investigate whether changing the labeling position of [ 11 C]Cimbi-36 eliminates M2 in humans, and if this changes the signal-to-background ratio. Six healthy volunteers each underwent two dynamic PET scans; after injection of [ 11 C]Cimbi-36, both the M1 and M2 fraction appeared in plasma, whereas only the M1 appeared after [ 11 C]Cimbi-36_5 injection. [ 11 C]Cimbi-36_5 generated higher uptake than [ 11 C]Cimbi-36 in both neocortex and cerebellum. With the simplified reference tissue model mean neocortical non-displaceable binding potential for [ 11 C]Cimbi-36 was 1.38 ± 0.07, whereas for [ 11 C]Cimbi-36_5, it was 1.18 ± 0.14. This significant difference can be explained by higher non-displaceable binding caused by demethylation products in the M1 fraction such as [ 11 C]formaldehyde and/or [ 11 C]carbon dioxide/bicarbonate. Although often considered without any impact on binding measures, we show that small polar radiometabolites can substantially decrease the signal-to-background ratio of PET radioligands for neuroimaging. Further, we find that [ 11 C]Cimbi-36 has a better signal-to-background ratio than [ 11 C]Cimbi-36_5, and thus will be more sensitive to changes in 5-HT 2A receptor levels in the brain.