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11C-SCH442416 was reported in preclinical studies with rodents and primates to be the first nonxanthine radioligand suitable for the in vivo imaging of adenosine A2A receptors with positron emission tomography (PET). The aim of the present work was to investigate the suitability of 11C-SCH442416 for the in vivo quantification of A2A receptors in human brain.Methods: Five male healthy subjects were scanned with 364 MBq bolus injections of 11C-SCH442416. 90 minutes of dynamic PET emission data were acquired, and arterial blood samples were taken throughout the scan to generate an arterial plasma input function. Using the individual MR images, regions-of-interest (ROIs) were defined for cerebellum, caudate, putamen and thalamus. Spectral analysis was used to determine the frequency components of the 11C-SCH442416 tissue response for regional and voxel time-activity curves (TACs).Results: 11C-SCH442416 was rapidly metabolised in blood, the fraction of unmetabolised parent tracer in plasma being 41% at 15 minutes and 15% at 95 minutes, lower than that reported in rats and macaca nemestrina. No lipophilic radiolabelled metabolites were found in human plasma.Rapid uptake of 11C-SCH442416 was observed in all brain regions, reaching a maximum at about 3 minutes. When spectral analysis was applied to regional brain time activity curves (TACs), relatively rapid reversible region dependent and slower irreversible, region independent but subject specific components were identified. These components were further separated into irreversible nonspecific binding, reversible nonspecific binding, reversible specific binding and a blood component. Binding potentials of the nondisplaceable binding BPND were calculated using cerebellar volume of distribution as an estimate of the reversible nondisplaceable binding across the entire brain. Mean binding potentials BPND were: 2.5 (putamen), 1.6 (caudate) and 0.5 (thalamus).Conclusion: Our study demonstrates that A2A receptor binding can be quantified in striatal regions of the human brain with 11C-SCH442416 PET. Despite the complex tracer kinetics and its low specific binding, reliable binding potentials could be estimated with spectral analysis.

Quantitative in vivo Imaging of Adenosine A2A Receptors in the Human Brain Using 11C-SCH442416 PET: A Pilot Study