Iodine-123 salmon calcitonin, an imaging agent for calcitonin receptors: synthesis, biodistribution, metabolism and dosimetry in humans

Calcitonin is used to reduce high serum calcium levels in patients with malignancy, and as therapy for osteoporosis and Paget's disease. Receptors for the peptide have been identified in some human cancer cells including those of lung, breast, bone, prostate, and medullary carcinoma of the thyroid, suggesting that an imaging agent for the receptors might be useful in nuclear oncology. A modified chloramine-T method was used to label a pharmaceutical form of salmon calcitonin (SCT) with iodine-123. Labelling can be performed within 5 min including purification, resulting in >95% radiochemical purity and 70% yield. Digestion analysis shows labelling with two iodine atoms on the tyrosine residue. A Chinese hamster ovary cell-based assay showed that the receptor binding and activation were not impaired by the labelling. Biodistribution in mice was similar to that of commercially available mono-iodinatd I-125-labelled SCT, kidney being the principal target organ. Evaluation in three patients previously diagnosed as having Pager's disease (injected with 37 MBq [I-123]diiodotyrosyl(22)-SCT, containing less than 4 IU hormone, imaged dynamically up to 0.5 h and at intervals up to 24 h) shows early uptake in liver, kidney and sites of known Paget's disease but not in normal bone, and later uptake in thyroid and stomach. Blood clearance was fitted to a biexponential with half-lives of 3.4-7.4 min and 3-34 h. Radiation dosimetry was estimated using MIRDOSE 3. The highest doses (mean mGy/MBq) were to thyroid (6.8x10(-1)) and kidney (6.0x10(-2)), with a whole-body dose 3.0x10(-2). High performance liquid chromatography analysis revealed that urinary radioactivity was mostly in the form of iodide and diiodotyrosine within minutes of injection, indicating rapid in vivo breakdown. In summary, [I-123]diiodotyrosyl(22)-SCT binds to calcitonin receptors and can image sites of Paget's disease but its imaging potential is not optimal because of rapid breakdown and clearance from target tissues, and an alternative radiolabelling approach is required.