Conjugation of monoclonal antibodies to a synthetic peptide substrate for protein kinase: a method for labelling antibodies with 32P.

In recent years, radiolabelled monoclonal antibodies have been evaluated for their use in the diagnosis and treatment of neoplastic disease. One isotope which has not been assessed for antibody targeting is 32P, even though it has many favourable radiobiological characteristics and has been used clinically for the treatment of certain neoplastic disorders such as polycythaemia rubra vera. The main drawback so far in using 32P has been the absence of a general method for phosphorylating antibodies. We have now developed a novel process for the phosphorylation of immunoglobulins which is rapid, efficient and allows high specific activities to be achieved (greater than 10 muCi micrograms-1). The technique involves the chemical conjugation of Kemptide, a synthetic heptapeptide substrate for kinases, to immunoglobulins. The antibody-Kemptide conjugate can then be phosphorylated using protein kinases and [32P]-gamma-ATP. The procedure does not compromise the binding activity of the antibody. The 32P-labelled monoclonal antibodies were stable in human, mouse and rat plasmas in vitro, although they cleared from the bloodstream of mice with a beta-phase half life of 2 days which is approximately two times faster than that of native antibody. The application of this phosphorylation technique should allow the therapeutic potential of targeted 32P to be assessed.