Maleimidocysteineamido-DOTA Derivatives:  New Reagents for Radiometal Chelate Conjugation to Antibody Sulfhydryl Groups Undergo pH-Dependent Cleavage Reactions

We have synthesized two bifunctional derivatives of the macrocyclic chelating agent 1,4,7,10-tetraazacyclododecane-N,N',N",N"-tetraacetic acid (DOTA) equipped with maleimide groups for conjugation to reduced disulfide bonds of monoclonal antibodies. Using water-soluble carbodiimide chemistry, DOTA was coupled to L-cysteine to incorporate both a "pendant-type" carboxyl group for metal coordination and an orthogonal thiol group for protein attachment. The homobifunctional reagent 1,6-bis(maleimido)hexane was then used to introduce the maleimide functionality via a sulfide linkage to the macrocycle, and alternatively, the sulfide group was converted to a sulfone side chain. Both maleimide derivatives were conjugated to the anticarcinoembryonic antigen chimeric monoclonal antibody cT84.66 after light reduction of the mAb with dithiothreitol. In this manner, antibody conjugates were prepared which afforded near-quantitative labeling with the radiometals 111In(III) and 90Y(III) as well as quantitative immunoreactivity. Radioimmunoconjugates prepared with the sulfide and sulfone compounds exhibited relatively rapid linker-dependent radiometal loss when incubated in human serum and aqueous solutions at physiological temperature and pH. The unconjugated maleimidocysteineamido-DOTA derivatives and their Y(III) complexes were incubated in aqueous solution at 37 degrees C, and the resulting decomposition products were analyzed by HPLC and mass spectrometry. These studies revealed that the two bifunctional chelating agents underwent linker-specific cleavage reactions which were considerably faster at pH 7.4 than at pH 5.4. The chemically labile linker systems are expected to release chelated radiometal from mAb conjugates in a pH-dependent manner. This property may impart favorable tumor uptake and normal tissue clearance on radioimmunoconjugates prepared with these reagents, on the basis of the observation that many solid tumors are significantly more acidic than normal tissues.