Protein labelling via deoxyribonucleic acid hybridization

A novel method of radiolabelling antibodies and other proteins is described. A small single-stranded DNA was covalently conjugated to an antibody and labelled by hybridization following the addition of the complementary single-stranded DNA labelled with technetium-99m (⁹⁹Tc^m) or indium-111 (¹¹¹In). Antibody labelling efficiencies were 100% in about 1 h at room temperature with specific activities of up to 30 μCi μg^-1 of IgG for ⁹⁹Tc^m. Both diester and thioate DNAs were used. Both the diester- and thioate-labelled antibodies showed complete label stability in 37°C saline. After 24 h in 37°C serum, however, about 40% of the label in the case of the diester antibody was on low molecular weight species - probably labelled catabolites from nuclease degradation of the phosphodiester DNA. In contrast, the ⁹⁹Tc^m label on the thioate antibody was immediately and quantitatively bound to serum proteins - probably due to nonspecific binding through the sulphur groups. Biodistribution studies in normal mice reflect these in vitro observations: ⁹⁹Tc^m on the diester antibody was rapidly cleared through the kidneys, probably as low molecular weight catabolite, while on the thioate antibody, the ⁹⁹Tc^m label was predominately deposited in the liver. In conclusion, by modifying with a single-stranded DNA, proteins may be readily labelled with a variety of radionuclides by DNA hybridization. The properties of the radiolabel are strongly influenced by the nature of the DNA.