Synthesis and characterization of a novel series of agonist compounds as potential radiopharmaceuticals for imaging dopamine D_2/3 receptors in their high-affinity state

Imaging of dopamine D_2/3 receptors (D_2/3R) can shed light on the nature of several neuropsychiatric disorders in which dysregulation of D_2/3R signaling is involved. Agonist D_2/3 tracers for PET/SPECT imaging are considered to be superior to antagonists because they are more sensitive to dopamine concentrations and may selectively label the high-affinity receptor state. Carbon-11-labeled D_2/3R agonists have been developed, but these short-lived tracers can be used only in centers with a cyclotron. Here, we report the development of a series of novel D₂R agonist compounds based on the 2-aminomethylchromane (AMC) scaffold that provides ample opportunities for the introduction of longer-lived [ ¹⁸F] or [¹²³I]. Binding experiments showed that several AMC compounds have a high affinity and selectivity for D_2/3R and act as agonists. Two fluorine-containing compounds were [¹⁸F]-labeled, and both displayed specific binding to striatal D_2/3R in rat brain slices in vitro. These findings encourage further in vivo evaluations.