Development of 5-HT1A receptor radioligands to determine receptor density and changes in endogenous 5-HT

(18)F]FCWAY and (18)F]FPWAY, analogues of the high affinity 5-HT(1A) receptor (5-HT(1A)R) antagonist WAY100635, were evaluated in rodents as potential radiopharmaceuticals for determining 5-HT(1A)R density and changes in receptor occupancy due to changes in endogenous serotonin (5-HT) levels. The in vivo hippocampus specific binding ratio (hippocampus(uptake)/cerebellum(uptake))-1] of (18)F]FPWAY was decreased to 32% of the ratio of (18)F]FCWAY, indicating that (18)F]FPWAY has lower affinity than (18)F]FCWAY. The 5-HT(1A)R selectivity of (18)F]FPWAY was confirmed using ex vivo autoradiography studies with 5-HT(1A)R knockout, heterozygous, and wildtype mice.Pre- or post-treatment of awake rodents in tissue dissection studies with paroxetine had no effect on hippocampal binding of (18)F]FCWAY or (18)F]FPWAY compared to controls, indicating neither tracer was sensitive to changes in endogenous 5-HT. In mouse ex vivo autoradiography studies in which awake mice were treated with fenfluramine following the (18)F]FPWAY, a significant decrease was not observed in the hippocampus specific binding ratios. In rat dissection studies with fenfluramine administered following (18)F]FPWAY or (18)F]FBWAY ((18)F]-MPPF) in awake or urethane-anesthetized rats, no significant differences in the specific binding ratios of the hippocampus were observed compared to their respective controls. (18)F]FPWAY and (18)F]FBWAY uptakes in all brain regions were increased variably in the anesthetized group (with the greatest increase in the hippocampus) vs. the awake group, but were decreased in the fenfluramine-treated anesthetized group vs. the anesthetized group. These data are best explained by changes in blood flow caused by urethane and fenfluramine, which varies from region to region in the brain.