Comparative studies of 111In-labeled monoclonal antibody using spacer-containing and non-spacer bifunctional chelates. 1

Bifunctional chelates containing chemical spacer, succinimido-EGS-DTPA (EGS-DTPA, diester spacer) and 1-[4-(10-maleimidopropyl)amidobenzyl]ethylenediamine-N,N,N',N'-tetraacetic acid (C10-Bz-EDTA, hydrocarbon spacer) were synthesized and compared in vitro with non-spacer chelates, cyclic DTPA dianhydride (cDTPAA) and isothiocyanatobenzyl-EDTA (SCN-Bz-EDTA). The optimum condition for conjugation with chelates, labelling efficiency, immunoreactivity and stability in the serum were investigated with respect to four chelates. The labeling efficiency for EGS-DTPA-A7 and DTPA-A7 was 90% and over 95%, respectively, when reacted by the chelate to antibody ratios of 10:1 for EGS-DTPA and 2:1 for cDTPAA. The labeling efficiency for SCN-Bz-EDTA and C10-Bz-EDTA was 70% and 80%, respectively, when both of the chelates were conjugated with the antibody in a 1:1 ratio. EGS-DTPA-A7 and DTPA-A7 were unstable when incubated in the human serum at 37degC and showed the transchelation to the transferrin fraction of the 111In activity. Furthermore, the radioactivity from 111In-EGS-DTPA-A7 was also found to be associated with the In-DTPA fraction, indicating that the diester spacer was cleaved in the serum. SCN-Bz-EDTA-A7 and C10-Bz-EDTA-A7 were very stable: there was no evidence of transchelation or any activity was dissociated from the conjugated antibody when incubated in the human serum for 168 hrs. Using C10-Bz-EDTA, immunoconjugate with sufficient specific activity was obtained and high immunoreactivity was maintained. It was considerable that the C10-Bz-EDTA immunoconjugate would produce preferable biodistribution than EGS-DTPA immunoconjugate do. (author)