99mTc Complexes with activated ester functions; ligands comprising a 3,4-diamino-benzoate backbone

Preformed 99mTc chelates with an activated ester function are useful for the gentle labeling of proteins (precomplexation route). In this context, new heterobifunctional ligands derived from 2,3,5,6-tetrafluorophenyl (TFP) 3,4-diamino-benzoates (OC1, OC3, OC4) were synthesized. Their corresponding 99mTc-complexation and protein-conjugation characteristics were elucidated and compared with the results previously reported using 2,3,5,6-tetrafluorophenyl N-(S-benzoylthioacetyl)glycylglycyl-p-aminobenzoate (OC2). The reaction temperatures and the reaction time markedly influenced complexation yields. Compared with OC2, the 99mTc-chelate formation with OC1 and OC4 was more effective, showing radiochemical yields of 60% and 70% within 20 min, respectively. Owing to steric hindrance, the complexation of OC3, however, did not exceed 10%. No-carrier-added 99mTc complexes were conjugated at pH 10 with the anti-EGF-receptor monoclonal antibody MAb425, resulting in labeling yields of 14% for 99mTc-OC1 and 7% for 99mTc-OC4 after incubating for 20 min at 30°C. Increasing the temperature to 40°C improved these results by 14% and 3%, respectively. As compared with 99mTc-OC2, which provides the chelating substituent at the 4-phenyl position only, the application of 3,4-phenyl substituents proved less appropriate for protein conjugation. However, the 3,4-diaminobenzoate backbone may be attractive for an alternative design of novel 99mTc N2S2 or N3S complexes, because they show excellent complexation characteristics.