Effect of chemical linkage upon the stability and cytotoxic activity of A chain immunotoxins

Preparation of cytotoxic antibody-toxin conjugates

Conjugates of antibodies with plant toxins, such as ricin and abrin, are potent cytotoxic agents that selectively eliminate target cells from mixed cell cultures in vitro, and have great promise as antitumor agents in cancer therapy (1). Ricin and abrin are protein toxins consisting of two different polypeptide subunits, the A and B chains, which are of similar size (between 30 and 34 kDa) and are joined by a single disulfide bond. The A chain is a ribosome-inactivating protein (RIP) that inactivates eukaryotic ribosomes by a specific irreversible covalent modification of the ribosomal RNA (2). The B chain binds to cell surface galactose-containing oligosaccharide residues. Following receptor-mediated endocytosis of toxin bound to the cell surface, the A chain gains access to the cytosol and destroys the ability of the cell to make protein (3).

Selective cytotoxic activity of immunotoxins composed of a monoclonal anti-Thy 1.1 antibody and the ribosome-inactivating proteins bryodin and momordin

The ribosome-inactivating proteins, bryodin, from Bryonia dioica, and momordin, from Momordica charantia, were coupled by a disulphide bond to a monoclonal anti-Thy 1.1 antibody (OX7). Both immunotoxins were specifically cytotoxic to the Thy 1.1-expressing mouse lymphoma cell line AKR-A in vitro. The OX7-bryodin immunotoxins were the more powerfully toxic and reduced protein synthesis in AKR-A cells by 50% at a concentration of 1-4 x 10(-11) M as compared with 1 x 10(-9) M for the OX7-momordin immunotoxins. Neither of the immunotoxins was toxic to mouse lymphoma EL4 cells, which lack the Thy 1.1 antigen, at concentrations up to 3 x 10(-8) M. Further, bryodin and momordin immunotoxins made from an antibody (R10) of irrelevant specificity were without effect on AKR-A cells.