COMPARISON OF THE LABELING CHARACTERISTICS OF MERCAPTOACETYLTRIGLYCINE (MAG3) WITH DIFFERENT S-PROTECTIVE GROUPS

A number of different thiol protective groups have been synthesized an d attached to mercaptoacetyltriglycine (MAG3) ligand. The newly made M AG3 analogues were labelled with Tc-99m by direct labelling under alka line 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% resp ectively, were obtained by radio-HPLC analysis for the studied MAG3 pr ecursors when, 0.1 mg, 0.4 mg and 1.6 mg of the ligand was used and la belling was performed at both low temperature (70 degrees C) and pH (p H 3). All the studied ligands were efficiently labelled with Tc-99m (u p to 99%) when heated for 10 min at pH 9 and 100 degrees C. The labell ing efficiency obtained by the direct labelling method for MAG3 analog ues varied from 32% to 94% and was in all cases lower than after the e xchange labelling at pH 9 and at 100 degrees C. It was observed that t he radiochemical purities can be improved significantly by heating the ''direct labelling mixture'' at elevated temperature.