Evaluation of radioiodinated histamine as isotope carrier in vivo

The iodo derivative of histamine labelled with 125I has been used for many years to prepare tracers used in RIA systems. The aim of this study was to evaluate radioiodinated histamine as a potential isotope carrier for in vivo applications. The biological behaviour of radioiodinated histamine has been investigated in rodents. The observed absence of any specific iodohistamine uptake by a critical organ or tissue promises a very quick distribution of the iodohistamine in soft tissues, and a rapid rate of whole-body clearance via the urinary tract (e.g. over 50% of the injected dose (ID) during the first hour after administration). In spite of moderately low in vitro stability of iodohistamine in serum, biodistribution studies in rodents have not shown any significant release of iodine from the parent molecule in the whole animal. Low uptake was observed in the thyroid (e.g. 0.22 and 0.11% ID at 1 and 2 h after administration to rats), and not more than 3% of injected activity was detected in the stomach in all of the biodistribution experiments. Moreover, our results refute any possibility of competition between histamine and iodohistamine for receptor binding sites, and suggest that radioactive mono-iodohistamine may be used successfully to develop some new radiolabelled bioactive molecules with potential application in vivo.