Construction of RibonucleaseAntibody Conjugates for Selective Cytotoxicity

Immunotoxins based on human and humanized ribonuclease may have potential for cancer therapy while exhibiting less toxic side effects and stimulating less of an immune response in humans than immunotoxins based on plant and bacterial toxins (1 ). Both recombinant RNase fusion proteins (2 –4 see also Chapter 6 , this volume) and chemical RNase conjugates have been made and characterized. The cytotoxic potential of targeted ribonuclease was first demonstrated with bovine RNase conjugated to transferrin or an antibody directed against the human transferrin receptor (5 ). Antibody RNase conjugates have also been shown to have potent anti-tumor activity against human glioma cells in athymic mice (6 ) and to enhance the activity of vincristine in mdr1 multidrug-resistant colon cancer cells in vitro and in vivo (7 ). Recently, RNase chemically conjugated to an antibody against CD22 was found to specifically kill Daudi lymphoma cells in cell culture at picomolar concentrations (IC₅₀ , 10-50 pM) and to exhibit potent antitumor activity in SCID mice with disseminated Daudi lymphoma (unpublished data). Methods for linking RNase to specific cell binding ligands are described.