Synthesis and biodistribution of [5- super(131)I]iodotropapride: A potential D sub(2) dopamine receptor imaging agent

[5 super(131)I]Iodotropapride is a benzamidic compound which displays high affinity and selectivity for dopaminergic receptors. It was prepared from the corresponding brominated compound by a nucleophilic substitution with [ super(131)I]iodine (t sub(1/2) = 8.02 days, E sub( gamma ) = 364 keV) based on the use of Cu(I) as catalyst and high specific activity of [ super(131)I]NaI. After i.v. injection in rats the tracer crosses the blood-brain barrier (0.42 plus or minus 0.06% of injected dose in the total brain) and demonstrates a high affinity binding to the striatum. The striatum-to-cerebellum ratio increases with time and reaches values of 9 and 22 at 30 and 120 min after injection, respectively. This specific uptake in the striatum is saturable and can be blocked by pretreatment with different D sub(2) antagonists. When labeled with super(123)I (t sub(1/2) = 13 h, E sub( gamma ) = 159 keV), the corresponding [ super(123)I]iodotropapride may be useful for the investigation of the D sub(2) dopamine receptors in humans with single photon emission computer tomography (SPECT).