Biodistribution and dosimetry of (iodine-123)-iodomethyl-N,N-diethyltamoxifen, an (anti)oestrogen receptor radioligand

This study reports on the distribution and radiation dosimetry of iodine-12 3 labelled trans-Z-iodomethyl-N,N-diethyltamoxifen (123-ITX), a promising r adioligand for prediction of the therapeutic efficacy of unlabelled tamoxif en in human breast carcinoma. Whole-body scans were performed up to 24 h af ter intravenous injection of 123-ITX (mean: 146 MBq, range: 142-148 MBq) in five female volunteers, four with and one without thyroid blockade. Blood samples were taken at various times up to 24 h after injection. Urine was a lso collected up to 24 h after injection, allowing calculation of renal cle arance and interpretation of whole-body clearance. Time-activity curves wer e generated for the thyroid, heart, brain, breasts, liver and gallbladder b y fitting the organ-specific geometric mean counts, obtained from regions o f interest. The MIRD formulation was applied to calculate the absorbed radi ation doses for various organs, The images showed rapid hepatobiliary excre tion, resulting in good imaging conditions for the thoracic region, whereas imaging of the abdominal region was impeded by extensive bowel activity. T he breast to nonspecific uptake ratio increased over time. 123-ITX was clea red by both the kidneys and the gastrointestinal tract. At 50 h p.i. the me an excretion in the urine was 89.4% (SD 5.7%). If the thyroid was not block ed, it was one of the critical organs. The highest absorbed doses were rece ived by the excretory organs, i.e. the urinary bladder wall, the lower and upper large intestine, and the gallbladder wall. The average effective dose of 123-ITX was estimated to be 0.0084 mSv/MBq. The amount of 123-ITX requi red for adequate imaging of tumoral uptake results in an acceptable effecti ve dose to the patient.