The influence of temperature and alkaline pH on the labeling of free and conjugated MAG(3) with technetium-99m

Benzoyl-protected mercaptoacetyltriglycine (S-benzoyl MAG(3)) is radiolabeled by tartrate transchelation at elevated temperatures or basic pH. The object of this investigation was to establish whether the same Tc-99m labeled species are formed when S-acetyl (S-acetyl MAG(3))-conjugated compounds are radiolabeled by tartrate transchelation at ambient temperature and neutral pH in contrast to labeling at 95 degrees C and pH 11. S-acetyl MAG(3) was conjugated to biocytin and to the amine-derivitized oligomers DNA and PNA. Along with free S-acetyl MAG(3), these were radiolabeled under the different conditions. Although labeling efficiencies were always lower when labeled at ambient temperature and neutral pH relative to labeling at 95 degrees C or pH 11 (free S-acetyl MAG(3) could not be labeled at all), size exclusion and reverse phase HPLC showed no difference with labeling conditions in the radiochemical profiles for labeled DNA and biocytin. In the case of DNA, a cysteine challenge also failed to demonstrate a difference. However, in the case of PNA, some important differences were observed in the size exclusion HPLC radiochromatograms. In addition, PNA labeled at ambient temperature and neutral pH was less stable to transchelation to cysteine. In conclusion, S-acetyl MAG(3) conjugated compounds may be radiolabeled at ambient temperature and neutral pH. In most cases, the radiochemical species produced appear to be identical to these formed when labeling is accomplished at 95 degrees C or pH 11. (C) 1997 Elsevier Science Ltd.