99mTc labeling of proteins: initial evaluation of a novel diaminedithiol bifunctional chelating agent

The initial evaluation of a novel thiolactone bifunctional chelating agent, 1, for facile introduction of the diaminedithiol (DADT) ligand system onto proteins for subsequent labeling with 99mTc under mild conditions is presented. For human serum albumin and a monoclonal anti-colon carcinoma antibody (B72.3), time-dependent coupling was achieved at pH 7-9 at room temperature. The coupled proteins could be labeled in a direct fashion (Sn2+/99mTcO4-) and by exchange labeling (with preformed 99mTc-glucoheptonate) in good to excellent yields at pH 7. In contrast, the unmodified proteins could be labeled only by direct methods with maximum yields of 30%. The labeled, coupled proteins were purified by high performance liquid chromatography and found to be stable in vitro over a 20-h period of study with no evidence for the loss of label. This exceptional stability is due to the chelation of 99mTc by the DADT ligand since 66% (3.5 h) and 47% (20 h) of the label were lost from directly labeled unmodified human serum albumin and the unmodified antibody, respectively. Biodistribution studies in normal mice and rabbits confirmed the stability of the 99mTc-DADT-labeled proteins compared to the 99mTc-labeled unmodified proteins in vivo. Thus, the novel thiolactone 1 is a useful bifunctional chelating agent for introducing the DADT ligand system onto a variety of proteins for subsequent incorporation of technetium under mild conditions such that the resultant labeled products are stable in vitro and in vivo.