Discriminated release of a hippurate-like radiometal chelate in nontarget tissues for target-selective radioactivity localization using pH-dependent dissociation of reduced antibody.

To achieve high and selective target radioactivity localization by monoclonal antibodies (Mabs) labeled with metallic radionuclides, the discriminated release of a hippurate-like radiometal chelate in nontarget tissues was performed using chemically modified Mabs.The disulfide bonds of a Mab against osteogenic sarcoma (OST7, IgG1) were reduced and 67Ga chelate of succinyldeferoxamine (SDF) was conjugated proximal to the Mab molecule via an ester bond with exposed thiol groups (67Ga-DFO-MESS-redOST7), which would impair esterase access to the ester bond of 67Ga-DFO-MESS-redOST7 due to the steric interference induced by bulky antibody molecule, stabilizing the ester bond in plasma and on the target cell's surface. Gallium-67-SDF was also conjugated to OST7 via an ester bond with 2-iminothiolane to render the ester bond in a position distal from the OST7 molecule (67Ga-DFO-MESS-IT-OST7).Although SDS-PAGE analyses of 67Ga-DFO-MESS-redOST7 showed a partial cleavage of its disulfide bonds, size-exclusion HPLC and cell binding assays indicated that the IgG structure and immunoreactivity of this conjugate were preserved in a neutral buffer and plasma of the systemic circulation.The present radiochemical design of an antibody utilizing pH-dependent dissociation would constitute a promising approach in establishing selective target radioactivity localization by Mabs.