Comparison of the labelling characteristics of mercaptoacetyltriglycine (MAG3) with different S-protective groups

A number of different thiol protective groups have been synthesized and attached to mercaptoacetyltriglycine (MAG3) ligand. The newly made MAG3 analogues were labelled with 99mTc by direct labelling under alkaline condition and by stannous tartrate exchange labelling method. In the latter method, the amount of the ligand, reaction temperature and pH varied and their effects on the labelling efficiencies were studied. Radiochemical purities of 51% to 70%, 58% to 75% and 46% to 81% respectively, were obtained by radio-HPLC analysis for the studied MAG3 precursors when, 0.1 mg, 0.4 mg and 1.6 mg of the ligand was used and labelling was performed at both low temperature (70°C) and pH (pH 3). All the studied ligands were efficiently labelled with 99mTc (up to 99%) when heated for 10 min at pH 9 and 100°C. The labelling efficiency obtained by the direct labelling method for MAG3 analogues varied from 32% to 94% and was in all cases lower than after the exchange labelling at pH 9 and 100°C. It was observed that the radiochemical purities can be improved significantly by heating the “direct labelling mixture” at elevated temperature. © 1997 John Wiley & Sons, Ltd.