Effect of specific activity on cardiac uptake of iodine-123-MIBG

Radioiodinated meta-iodobenzylguanidine (MIBG), an analog of norepinephrine, has been used to assess myocardial sympathetic innervation. Recent in vivo studies predict enhanced cardiac uptake of this radiopharmaceutical with high specific activity. Methods: To clarify the effect of specific activity on cardiac uptake of radioiodinated MIBG, the distribution and kinetics of no-carrier-added [₁₂₃I]MIBG (≤TBq/mol) were compared with those of commercial [¹²³I]MIBG (~74 M/Bq/μmol) in three healthy volunteers by serial imaging and blood sampling. Results: Higher specific activity result in higher uptake of radioiodinated MIBG in all volunteers in the heart (p < 0.05) and liver (p < 0.05) but not in the lung (p = 0.26). Due to rapid deiodination, a more pronounced accumulation of radioactivity was present in plasma after no-carrier-added MIBG than commercial [¹²³I]MIBG, resulting in higher background and thyroid activity after administration of the former. Calculated heart-to-liver (p = 0.96) and heart-to-lung (p = 0.42) count ratios in all volunteers revealed no significant improvement in cardiac imaging with nocarrier-added [¹²³I]MIBG compared to commercial [¹²³I]MIBG. Conclusion: This study highlights the appreciably higher in vivo deiodination of no-carrier-added [¹²³I]MIBG compared to commercial preparation of [¹²³I]MIBG in humans. Cardiac images acquired with no-carrier-added [¹²³I]MIBG do not seem to be superior to those obtained with commercial MIBG.