Human biodistribution and dosimetry of the SPECT benzodiazepine receptor radioligand iodine-123-iomazenil

SPECT imaging of the brain with [{sup 123}I]iomazenil has shown avid uptake of the radioligand in a distribution consistent with benzodiazepine receptor binding. The purposes of this study were to measure the whole-body distribution of activity following i.v. adminstration of [{sup 123}I]iomazenil and to evaluate the resulting organ radiation burdens. Serial total body scans were obtained in healthy volunteers after thyroid blockade and demonstrated avid brain uptake of radioligand. Abdominal imaging showed significant activity retention within the urinary and gastrointestinal tracts consistent with excretion via these routes. Absorbed dose to the urinary bladder was calculated to be 0.19 mGy/MBq, to the lower large intestine 0.079 mGy/MBq, to the upper large intestine 0.066 mGy/MBq, and to the thyroid 0.063 mGy/MBq. Thyroid uptake may in part have represented binding to benzodiazepine receptors, since radioligand binding to tissue homogenates prepared from human thyroid showed the presence of benzodiazepine binding sites. 20 refs., 4 figs., 2 tabs.