Convenient synthesis of 18F-radiolabeled R-(-)-N-n-propyl-2-(3-fluoropropanoxy-11-hydroxynoraporphine

Aporphines are attractive candidates for imaging D2 receptor function because, as agonists rather than antagonists, they are selective for the receptor in the high affinity state. In contrast, D2 antagonists do not distinguish between the high and low affinity states, and in vitro data suggests that this distinction may be important in studying diseases characterized by D2 dysregulation, such as schizophrenia and Parkinson's disease. Accordingly, MCL-536 (R-(-)-N-n-propyl-2-(3-[(18)F]fluoropropanoxy-11-hydroxynoraporphine) was selected for labeling with (18)F based on in vitro data obtained for the non-radioactive ((19)F) compound. Fluorine-18-labeled MCL-536 was synthesized in 70% radiochemical yield, >99% radiochemical purity, and specific activity of 167 GBq/µmol (4.5 Ci/µmol) using p-toluenesulfonyl (tosyl) both as a novel protecting group for the phenol and a leaving group for the radiofluorination.