No-carrier-added synthesis of meta-[131I]iodobenzylguanidine

No-carrier-added meta-[131I]iodobenzylguanidine ([131I]MIBG) was prepared starting with two different metallated precursors. Attempted preparation of 3-(tri-n-butylstannyl)benzylguanidine was not successful. An alternate two-step strategy using 3-(tri-n-butylstannyl)benzylamine could be used to prepare radio-iodinated [131I]MIBG in an overall radiochemical yield of 30-33%. Synthesis of [131I]MIBG via the radioiododesilylation of 3-trimethylsilylbenzylguanidine was also investigated. Yields were dependent on temperature, precursor concentration, solvent and nature of the oxidant. Radiochemical yields of 90% were obtained in 5 min at room temperature using either N-chlorosuccinimide or hydrogen peroxide in trifluoroacetic acid as oxidants. The percentage of specific binding in vitro of no-carrier-added MIBG to SK-N-SH neuroblastoma cells remained constant over a 2 log activity range, while the binding of MIBG prepared by isotopic exchange dropped by a factor of seven. In normal mice, heart and adrenal uptake of no-carrier-added [131I]MIBG was found to be higher than that of [131I]MIBG prepared by isotopic exchange.