Iodine-123 labelling of atrial natriuretic peptide and its analogues: initial results

Receptors for atrial natriuretic peptide (ANP) are known to be present in many organs, e.g. brain, liver and kidney. The aim of this study was to optimize the labelling method for ANP analogues with iodine-123 and to describe their pharmakokinetics. ANP 99-126, atriopeptin III, Tyr-atriopeptin II and urodilatin were labelled by the Iodogen method. Of the various labelling conditions tested, the Iodogen procedure with 10 micrograms Iodogen, 10 micrograms peptides in 100 microliters phosphate buffer (pH 7.2) and a reaction time of 60 min proved to be optimal. Separation was performed using a Sep-pak reversed-phase extraction cartridge. In all cases quality control by thin-layer chromatography resulted in a radiochemical purity higher than 98%. After intravenous administration of 15 MBq/kg kinetic profiles of various organs in rabbits were investigated by whole-body scintigraphy with a gamma camera. Organ kinetics showed accumulation and excretion via the kidney. No bowel accumulation was observed. Interestingly, increasing tracer uptake in the head region was registered with three of the four tested tracers. Liver accumulation decreased rapidly during the first 40 min. No tracer degradation was registered over the whole time period, as shown by the low thyroid accumulation with a maximum of only 1%. Thus, labelling of ANF and its analogues is possible. These compounds seem to be of interest for further experiments and perhaps clinical studies because of their potential for brain receptor imaging.