Quantification of PET studies with the very high-affinity dopamine D2/D3 receptor ligand [11C]FLB 457: re-evaluation of the validity of using a cerebellar reference region.
The very high-affinity position emission tomography (PET) radioligand [(11)C]FLB 457 was developed in order to study extrastriatal tissues, where the density of dopamine D(2)/D(3) receptors is one to two orders of magnitude lower than in the striatum. The present study investigated the validity of using the cerebellum as a reference region. Ten healthy volunteers underwent a 90-min dynamic PET study after the bolus injection of [(11)C]FLB 457. The total volume of distribution (VD(t)) was estimated for the thalamus, hippocampus, frontal cortex, and cerebellum using a two-tissue compartmental model with a metabolite-corrected arterial plasma input function. VD(t) was sensitive to co-injected stable FLB 457 in all regions, including the cerebellum. Ex vivo saturation studies were also conducted in 17 rats where the dose of stable ligand was varied over five orders of magnitude. Specific binding was estimated to account for more than half of the rat cerebellar uptake of [(11)C]FLB 457, questioning the latter as an estimate of nonspecific binding in human PET studies. To check whether the cerebellum is a reference region, the binding potential (BP) was calculated either from the VD(t) ratio or using the simplified reference tissue model (SRTM). A non-negligible density of D(2)/D(3) receptors in the cerebellum was shown to lead to underestimation of BP as well as erroneous estimation of differential occupancies. Binging potential estimates from the SRTM were found to be sensitive to changes in cerebral blood flow, providing further evidence for caution in the use of the cerebellum as a reference region in measures of [(11)C]FLB 457 binding.
Asselin
Marie-Claude
MC
Hammersmith Imanet, Hammersmith Hospital, London, UK. marie-claude.asselin@manchester.ac.uk
Montgomery
Andrew J
AJ
Grasby
Paul M
PM
Hume
Susan P
SP
D000328:Adult; D000465:Algorithms; D000818:Animals; D019540:Area Under Curve; D002250:Carbon Radioisotopes; D002531:Cerebellum / Q000000981:diagnostic imaging* / Q000187:drug effects / Q000378:metabolism*; D003198:Computer Simulation; D003627:Data Interpretation, Statistical; D018492:Dopamine Antagonists; D006801:Humans; D008024:Ligands; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D049268:Positron-Emission Tomography; D011759:Pyrrolidines; D019275:Radiopharmaceuticals; D051381:Rats; D017207:Rats, Sprague-Dawley; D017448:Receptors, Dopamine D2 / Q000187:drug effects*; D050637:Receptors, Dopamine D3 / Q000187:drug effects*; D015203:Reproducibility of Results; D012457:Salicylamides
D002250:Carbon Radioisotopes; D018492:Dopamine Antagonists; D008024:Ligands; D011759:Pyrrolidines; D019275:Radiopharmaceuticals; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; D012457:Salicylamides; C065065:FLB 457