Preclinical Evaluation and Quantification of 18F-Fluoroethyl and 18F-Fluoropropyl Analogs of SCH442416 as Radioligands for PET Imaging of the Adenosine A2A Receptor in Rat Brain

摘要

id="p-2">The cerebral adenosine A_2A receptor is an attractive therapeutic target for neuropsychiatric disorders. 18F-fluoroethyl and 18F-fluoropropyl analogs of 18F-labeled pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH442416) (18F-FESCH and 18F-FPSCH, respectively) were developed as A_2A receptor-specific PET ligands. Our aim was to determine an appropriate compartmental model for tracer kinetics, evaluate a reference tissue approach, and select the most suitable PET ligand. >Methods: A 90-min dynamic PET scan with arterial blood sampling and metabolite analysis was acquired for 22 healthy male Wistar rats starting at the time of 18F-FESCH (n = 12) and 18F-FPSCH (n = 10) injection. For each tracer, half the animals were vehicle-treated whereas the other half were pretreated with the A_2A receptor-selective antagonist KW-6002, inducing full blocking. Regional tissue total volume of distribution (V_T) was estimated by 1- and 2-tissue-compartment modeling (1TCM and 2TCM, respectively) and Logan graphical analysis. Midbrain, cerebellum, and hippocampus were evaluated as the reference region by comparing baseline V_T with V_T under full blocking conditions and comparing striatal nondisplaceable binding potential (BP_ND) using a simplified reference tissue model (SRTM) with distribution volume ratio minus 1 (DVR a?’ 1) for 60- and 90-min scans. >Results: On the basis of the Akaike information criterion, 1TCM and 2TCM were the most appropriate models for 18F-FPSCH (baseline striatal V_T, 3.7 ?± 1.1) and 18F-FESCH (baseline striatal V_T, 5.0 ?± 2.0), respectively. Baseline striatal V_T did not significantly differ between tracers. After pretreatment, striatal V_T was reduced significantly, with no significant decrease in hippocampus, midbrain, or cerebellum V_T. Baseline striatal SRTM BP_ND did not differ significantly from DVR a?’ 1 except for 18F-FPSCH when using a 60-min scan and midbrain as the reference region, whereas Bland-Altman analysis found a smaller bias for 18F-FESCH and a 60-min scan. After pretreatment, striatal SRTM BP_ND did not significantly differ from zero except for 18F-FPSCH when using hippocampus as the reference region. Striatal SRTM BP_ND using midbrain or cerebellum as the reference region was significantly lower for 18F-FPSCH (range, 1.41-2.62) than for 18F-FESCH (range, 1.64-3.36). >Conclusion: Dynamic PET imaging under baseline and blocking conditions determined 18F-FESCH to be the most suitable PET ligand for quantifying A_2A receptor expression in the rat brain. Accurate quantification is achieved by a 60-min dynamic PET scan and the use of either cerebellum or midbrain as the reference region.