Validation of [11C]ORM-13070 as a PET tracer for alpha2c-adrenoceptors in the human brain.

ABSTRACT This study explored the use of the α_2C-adrenoceptor PET tracer [¹¹C]ORM-13070 to monitor α_2C-AR occupancy in the human brain. The subtype-nonselective α₂-AR antagonist atipamezole was administered to eight healthy volunteer subjects to determine its efficacy and potency ( E_max and EC₅₀) at inhibiting tracer uptake. We also explored whether the tracer could reveal changes in the synaptic concentrations of endogenous noradrenaline in the brain, in response to several pharmacological and sensory challenge conditions. We assessed occupancy from the bound-to-free ratio measured during 5-30 min post injection. Based on extrapolation of one-site binding, the maximal extent of inhibition of striatal [¹¹C]ORM-13070 uptake ( E_max) achievable by atipamezole was 78% (95% CI 69-87%) in the caudate nucleus and 65% (53-77%) in the putamen. The EC₅₀ estimates of atipamezole (1.6 and 2.5 ng/ml, respectively) were in agreement with the drug's affinity to α_2C-ARs. These findings represent clear support for the use of [¹¹C]ORM-13070 for monitoring drug occupancy of α_2C-ARs in the living human brain. Three of the employed noradrenaline challenges were associated with small, approximately 10-16% average reductions in tracer uptake in the dorsal striatum (atomoxetine, ketamine, and the cold pressor test; P < 0.05 for all), but insulin-induced hypoglycemia did not affect tracer uptake. The tracer is suitable for studying central nervous system receptor occupancy by α_2C-AR ligands in human subjects. [¹¹C]ORM-13070 also holds potential as a tool for in vivo monitoring of synaptic concentrations of noradrenaline, but this remains to be further evaluated in future studies. Synapse 69:172-181, 2015. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]