HUMAN BIODISTRIBUTION AND DOSIMETRY OF THE SPECT BENZODIAZEPINE RECEPTOR RADIOLIGAND IODINE-123-IOMAZENIL

SPECT imaging of the brain with [(123)l]iomazenil has shown avid uptak e 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. administration of [I-123 ]iomazenil and to evaluate the resulting organ radiation burdens. Meth ods: Serial total body scans were obtained in healthy volunteers after thyroid blockade and demonstrated avid brain uptake of radioligand. R esults: 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 up per large intestine 0.066 mGy/MBq, and to the thyroid 0.063 mGy/MBq. C onclusion: Thyroid uptake may in part have represented binding to benz odiazepine receptors, since radioligand binding to tissue homogenates prepared from human thyroid showed the presence of benzodiazepine bind ing sites.