Development of I-123-labelled NNC 13-8241 as a radioligand for SPECT visualization of benzodiazepine receptor binding

[I-125]- and [(123)]NNC 13-8241 were prepared from the trimethyltin precursor and radioactive iodide using the chloramine T method. The total radiochemical yields of [I-125]- and [I-123]NNC 13-8241 were 60-70% and 40-50% respectively, with radiochemical purity higher than 98%. In binding studies with [I-125]NNC 13-8241 in rats in vitro and in vivo a high uptake of radioactivity was demonstrated in brain regions known to have a high density of benzodiazepine (BZ) receptors such as the occipital and frontal cortex, SPECT examination with [I-123]NNC 13-8241 in a Cynomolgus monkey demonstrated a high uptake of radioactivity in the occipital and frontal cortex. After displacement with flumazenil radioactivity in these brain regions was reduced to the level of a central region including the pens. Four hours after injection about 80% of the radioactivity in monkey plasma represented unchanged radioligand. This low degree of metabolism indicates that NNC 13-8241 is metabolically more stable than the radioligands hitherto developed for imaging of BZ-receptors in the primate brain.