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

SPECT imaging of the brain with [¹²³I]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. administration of [¹²³I]iomazenil and to evaluate the resulting organ radiation burdens. Methods: Serial total body scans were obtained in healthy volunteers after thyroid blockade and demonstrated avid brain uptake of radioligand. Results: 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. Conclusion: 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.