Synthesis and biodistribution of [5-131I]iodotropapride: a potential D2 dopamine receptor imaging agent

Abstract [5- 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 [ 131 I]iodine ( t 1 2 = 8.02 days, E γ = 364 keV) based on the use of Cu(I) as catalyst and high specific activity of [ 131 I]NaI. After i.v. injection in rats the tracer crosses the blood-brain barrier (0.42 ± 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 2 antagonists. When labeled with 123 I ( t 1 2 = 13 h, E γ = 159 keV), the corresponding [ 123 I]iodotropapride may be useful for the investigation of the D 2 dopamine receptors in humans with single photon emission computer tomography (SPECT).