Synthesis of carbon-11 labelled SCH 39166, a new selective dopamine D-1 receptor ligand, and preliminary PET investigations

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SCH 39166 [(—)- trans-6,7,7 a,8,9,13b-hexahydro-3-chloro-2-hydroxy- N-methyl-5H-benzo(d)naphtho-(2,1- b)azepine] is a new more selective dopamine D-1 receptor antagonist than the widely used SCH 23390. [ 11C]SCH 39166 was prepared by N-methylation of the desmethyl compound SCH 40853 [(—)- trans-6,7,7 a,8,9,13b-hexahydro-3-chloro-2-hydroxy-5H-benzo(d)naphtho-(2,1-b)azepine] with [ 11C]methyl iodide. Reaction in acetone with subsequent straight-phase semi-preparative HPLC resulted in 20–30% radiochemical yield (from EOB and decay-corrected) with a total synthesis time of 35–40 min and a radiochemical purity >99%. The specific activity obtained at EOS was about 1500 Ci/mmol (55 GBq/μmol). [ 11C]SCH 39166 was injected into a Cynomolgus monkey. PET-analysis demonstrated accumulation in the striatum, a region known to have a high density of dopamine D-1 receptors. In a displacement experiment, radioactivity in the striatum was markedly reduced after injection of 6 mg unlabelled SCH 23390, thus demonstrating the specificity and reversibility of [ 11C]SCH 39166 binding to dopamine D-1 receptors.