Radioiodinated 2'-iododiazepam: a potential imaging agent for SPECT investigations of benzodiazepine receptors.

2'-Iododiazepam (2'-IDZ) is the diazepam analogue iodinated at the 2'-position of C-5 phenyl ring which was synthesized and evaluated as a potential radiopharmaceutical for investigating brain benzodiazepine receptors by SPECT. The 125I-2'-iododiazepam was synthesized by halogen exchange reaction and purified by HPLC. In vitro competitive binding studies with 3H-diazepam, using rat cortical synaptosomal membranes, showed that the affinity of 2'-IDZ for benzodiazepam receptors was higher than that in diazepam and flumazenil (RO15-1788). Biodistribution studies in mice showed that the brain uptake of 2'-iododiazepam was rapid and profound, and in the brain higher accumulation was found in the cortex than in other regions. Furthermore, the cortical uptake was displaced by benzodiazepinergic compounds. In vivo uptake was assessed by autoradiographic studies. Thus, 2'-iododiazepam bound to benzodiazepine receptors in vivo and therefore holds great potential for in vivo benzodiazepine receptor studies.