EFFECT OF SPECIFIC ACTIVITY ON ORGAN UPTAKE OF IODINE-123-META-IODOBENZYLGUANIDINE IN HUMANS

Radioiodinated meta-iodobenzylguanidine (MIBG), an analogue of norepin ephrine, has been used in management of neuroendocrine tumors. Recent studies reveal that distribution of radioiodinated MIBG in animals dep ends on the specific activity of this radiopharmaceutical. In order to clarify the effect of specific activity on organ uptake of radioiodin ated MIBG. the kinetics of no-carrier-added (n.c.a.) [I-123]MIBG (grea ter than or equal to 7.4 TBq/mu mol) were compared with those of comme rcial (com.) [I-123]MIBG (similar to 74 MBq/mu mol) in 3 healthy volun teers by serial imaging and blood sampling. The organ uptake of radioi odinated MIBG did not remarkably differ between the two specific activ ities. Due to rapid degradation a more pronounced accumulation of radi oactivity was present in plasma alter n.c.a. than after com. [I-123]MI BG 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]M IBG as compared to com. [I-123]MIBG. This finding highlights the highe r 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 concentr ation of radioiodinated MIBG available for binding at tumor sites and result in higher radiation exposure of non-tumor tissue.