Radioiodinated meta-iodobenzylguanidine (MIBG), an analogue of norepinephrine, has been used in management of neuroendocrine tumors. Recent studies reveal that distribution of radioiodinated MIBG in animals depends on the specific activity of this radiopharmaceutical. In order to clarify the effect of specific activity on organ uptake of radioiodinated MIBG. the kinetics of no-carrier-added (n.c.a.) [I-123]MIBG (greater than or equal to 7.4 TBq/mu mol) were compared with those of commercial (com.) [I-123]MIBG (similar to 74 MBq/mu mol) in 3 healthy volunteers by serial imaging and blood sampling. The organ uptake of radioiodinated MIBG did not remarkably differ between the two specific activities. Due to rapid degradation a more pronounced accumulation of radioactivity was present in plasma alter n.c.a. than after com. [I-123]MIBG resulting in a higher background and thyroid activity. In addition due to a prolonged residence time of the radioactivity, the radiation exposure to organs was in general slightly higher with n.c.a. [I-123]MIBG as compared to com. [I-123]MIBG. This finding highlights the higher in vivo deiodination of n.c.a. [I-123]MIBG than of com. [I-123]MIBG in humans. In the treatment of children suffering from neuroblastoma, therefore, degradation of n.c.a. [I-123]MIBG may decrease the concentration of radioiodinated MIBG available for binding at tumor sites and result in higher radiation exposure of non-tumor tissue.

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