Initial human studies with single-photon emission tomography using iodine-123 labelled 3-(5-cyclopropyl-1,2,4-oxadiazo-3-yl)-7-iodo-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]-benzodiazepine (NNC 13-8241)

The iodine-123 labelled ligand 3-(5-cyclopropyl-1,2,4-oxadiazo-3-yl)-7-iodo-5,6-dihydro-5-methyl-6oxo-4H-imidazo1,5-a]1,4]-benzodiazepine (¹²³I]NNC 13-8241) was evaluated as a probe for in vivo imaging of benzodiazepine receptor sites in the human brain. Four healthy volunteers were imaged with a high-resolution single-photon emission tomography (SPET) scanner. The metabolism of ¹²³I]NNC 13-8241 in plasma was slow. The total brain uptake was about 1.5-fold higher than that of ¹²³I]iomazenil. The specific binding in the cortical areas was high and less intense in the thalamus. The most intense uptake was seen in the occipital cortex. The peak cortical uptake of ¹²³I]NNC 13-8241 was observed 6–10 h after the injection of tracer. The radiation burden to the patient was moderate, being 2.5·10^–2 mSv/MBq (effective dose equivalent). A slow metabolism together with favourable kinetics indicates that ¹²³I]NNC 13-8241 is a specific and promising SPET ligand for imaging benzodiazepine receptor sites in the living human brain.