Radiolabeling of fab and f(ab')2 antibody fragments with 99mTc(I) tricarbonyl core using a new bifunctional tridentate ligand.

Misri, Ripen; Saatchi, Katayoun; Häfeli, Urs O

The objective of this study was to design and evaluate a new histidine-modified tridentate chelator for labeling antimesothelin fab and f(ab')2 antibody fragments with the Tc(I) tricarbonyl ([Tc(CO)3]) core. N-(ortho-phenol)-histidine chelator was synthesized by modifying the single amino acid L-histidine, a natural amino acid, with an additional phenol group to obtain the bifunctional tridentate ligand after reductive amination. Bioconjugation was based on the carbodiimide activation of the carboxylate of chelator and on further reaction with the amine groups present on the antibody fragments. Radiolabeling was accomplished by replacing the three aqua ligands of the complex precursor [Tc(CO)3(H2O)3] with the tridentate chelator. The antibody fragments radiolabeled with [Tc(CO)3] core were tested for stability by the cysteine challenge test. The immunoreactivity and binding affinity of the radiolabeled fragments were studied using in-vitro cell-binding assays. Radiochemical yields achieved for [Tc(CO)3] core labeling of fab and f(ab')2 were 91.6±9.1% and 80.7±8.5%, respectively. Stability studies of radiolabeled antibody fragments showed that the Tc label was stable to transchelation by cysteine. Both Tc-fab and Tc-f(ab')2 retained their reactivity and affinity to the mesothelin antigen. From our studies, it can be concluded that the newly synthesized N-(ortho-phenol)-histidine chelator is a promising candidate for [Tc(CO)3] labeling of biomolecules and for developing other novel Tc radiopharmaceuticals.