Synthesis of iodine-123 labelled analogues of imidazenil and ethyl-imidazenil for studying benzodiazepine receptors using SPECT

The [123I]iodinated analogues of the benzodiazepine receptor partial agonist imidazenil and N-ethyl imidazenil have been synthesised for the study of the central benzodiazepine receptor using SPECT. [123I]Iodomidazenil and [123I]N-ethyliodoimidazenil were prepared by nucleophilic bromine-iodine exchange in acetic acid at 150o. The products were purified by semi-preparative reverse-phase HPLC with average radiochemical yields of 80% in a total synthesis time of 80 minutes. The specific activity was determined to be greater than 2500 Ci/mmol. The radiochemical and chemical purity assessed by radio-TLC and HPLC were found to be 98%. Alternatively, iododestannylation reactions via the trimethyltin precursors with Na[123I] in the presence of Chloramine-T or peracetic acid resulted in yields of only 20-25% with the bulk of activity being lost as volatile methyl [123I]iodide. (author)