Validation Of [C-11]Orm-13070 As A Pet Tracer For Alpha(2c)-Adrenoceptors In The Human Brain

This study explored the use of the (2C)-adrenoceptor PET tracer [C-11]ORM-13070 to monitor (2C)-AR occupancy in the human brain. The subtype-nonselective (2)-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 [C-11]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 (2C)-ARs. These findings represent clear support for the use of [C-11]ORM-13070 for monitoring drug occupancy of (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 (2C)-AR ligands in human subjects. [C-11]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. (c) 2014 Wiley Periodicals, Inc.