Renal excretion of iodine-131 labelledmeta-iodobenzylguanidine and metabolites after therapeutic doses in patients suffering from different neural crest-derived tumours

Iodine-131 labelledmeta-iodobenzylguanidine ([¹³¹I]MIBG) is used for diagnostic scintigraphy and radionuclide therapy of neural crest-derived tumours. After administration of therapeutic doses of [¹³¹I]MIBG (3.1–7.5 GBq) to 17 patients (n=32 courses), aged 2–73 years, 56%±10%, 73%±11%, 80%±10% and 83%±10% of the dose was cumulatively excreted as total radioactivity in urine att=24 h, 48 h, 72 h and 96 h, respectively. Except for two adult patients, who showed excretion of 14%–18% of [¹³¹I]meta-iodohippuric acid ([¹³¹I]MIHA), the cumulatively excreted radioactivity consisted of >85% [¹³¹I]MIBG, with 6% of the dose excreted as free [¹³¹I]iodide, 4% as [¹³¹I]MINA and 2.5% as an unknown iodine-131 labelled metabolite. Cumulative renal excretion rates of total radioactivity and of [¹³¹I]MIBG appeared to be higher in neuroblastoma and phaeochromocytoma patients than in carcinoid patients. Based on the excretion of small amounts of [¹³¹I]meta-iodobenzoic acid in two patients, a possible metabolic pathway for [¹³¹I]MIBG is suggested. The degree of metabolism was not related to the extent of liver uptake of radioactivity.