Toxin-conjugated monoclonal antibodies

Fully human antibodies can be constructed from phage libraries. In addition to the diversity of engineered antibodies, other molecules can be attached to the antibody, such as enzymes, toxins, viruses, radionuclides, and biosensors for targeting, imaging, or diagnosing. These are commonly referred to as conjugated" monoclonal antibodies. 

Conjugated monoclonal antibodies can be used as carriers of toxic therapy, such as radionuclides, (e.g., yttrium-90 ibritumomab tiuxetan and iodine-131 tositumomab), cytotoxic drugs, or cell toxins to specific cell targets. They are also being employed to create tumor vaccines by stimulating a host antibody reaction causing the production of anti-idiotype antibodies. 

The conjugation of monoclonal antibodies (MoAbs) to radioisotopes, chemotherapeutic agents, and protein toxins has also been given consideration (65). Large amounts of human MoAbs can be produced by biotechnological means. 

Lambert, J.M., Senter, P.D., Yau-Young, A., Blattler, W.A., and Goldmacher, V.S. (1985) Purified immuno-toxins that are reactive with human lymphoid cells Monoclonal antibodies conjugated to the ribosomeinactivating proteins gelonin and the pokeweed antiviral proteins./. Biol. Chem. 260, 12035-12041. 

The antibody preparations could be administered unaltered or (more commonly) after their conjugation to radioisotopes or toxins. Binding of unaltered monoclonal antibodies to a tumour surface alone should facilitate increased destruction of tumour cells (Figure 13.4). This approach, however, has yielded disappointing results, as the monoclonal antibody preparations used to date have been murine in origin. The Fc region of such mouse antibodies is a very poor activator of human immune function. Technical advances, allowing the production of human/humanized monoclonals (see later) may render this therapeutic approach more attractive in the future. 

Anti-tumour monoclonal antibodies can also be used to deliver toxins to tumour sites. Toxins conjugated to therapeutic antibodies include ricin, pokeweed toxin, Pseudomonas toxin and... 

Clinically, monoclonal antibodies are also proposed as drug delivery vehicles in certain tumors where specific tumor-associated antigens are expressed. In this context, investigators have found that by conjugating toxins such as the A chain polypeptide of the plant protein ricin or the bacterial toxin from Corynebacterium diphtheriae to monoclonal antibodies specific for certain tumor type, as few as one or two molecules of antibody-toxin conjugate can destroy a tumor cell in vitro. Some success has also been obtained in clinical trials with monoclonal antibody-toxin conjugates. 

Preclinical studies should address the potential toxicity due to inappropriate release of the conjugated toxin. Preclinical toxicology of monoclonal antibodies may not require extensive animal studies but should be examined for cross-reactivity with antigenic epitopes present on normal cells in vitro and for the presence of human or rodent vimses. Early clinical trial should involve biodistribution studies with radiolabelled material. 

D. M. Neville Jr., Monoclonal Antibody Mediated Drug Delivery and Antibody Toxin Conjugates , in Directed Drug Delivery-A Multidisciplinary Approach , Eds. R. T. Borchardt, A. J. Repta, V. J. Stella, Humana Press, Clifton, N. J., 1985, p. 211-230. 

Table 10.5. The major toxins which have been conjugated to monoclonal antibody preparations for clinical use as anti-cancer agents. Refer to text for details...

The method described has been used to prepare antibody-toxin conjugates containing mouse and rat monoclonal antibodies of various IgG subtypes and polyclonal antibody from several animal species. Modifications of this method have also been used to prepare conjugates with antibodies of other classes. 

Fig. 1. Gel permeation chromatography of an antibody-toxin conjugate reaction mixture. The reaction mixture obtained following the conjugation of a mouse monoclonal antibody (50 mg) and [l25I]-labeled abrin A chain was chromatographed on a column of Sephacryl S-200 (SF), dimensions 80 cm x 2.6 cm (id). Fractions eluting from the column were monitored spectrophotometrically at 280 nm (—) to measure total protein, and by gamma counting (—) to measure the A chain in its free or conjugated form. The hatched area indicates a typical pooled conjugate preparation.

Conjugates of monoclonal antibodies and protein toxins are undergoing extensive research for their usefulness in the treatment of cancer. Toxins of many different types... 

---