(Second of two parts)
Antibodies are excellent targeting molecules. As an important part of the immune system, they are able to bind strongly to foreign antigens introduced into the body. Since the introduction of the hybridoma technology by Köhler and Milstein in 1975 (Nature 256:495-496), large amounts of antibodies of one kind could be generated against virtually any foreign substance, whether it’s a peptide, nucleic acid, carbohydrate or some other molecule. Because of the high specificity and high binding affinity of antibodies, they are used in a wide variety of applications: as detectors of specific molecules for diagnostic purposes, as imaging tools while conjugated to colorimetric or fluorimetric probes, or as therapeutic agents while conjugated to cytotoxic drugs (see below), etc.
With Jencks’ suggestion, antibodies can also function like enzymes and catalyze chemical reactions. These molecules are called catalytic antibodies which could be generated for all sorts of chemical reactions by the use of well-designed transition-state analogs. The first studies on catalytic antibodies, now aptly called “abzymes,” were reported in 1986 by two groups in back-to-back publications in Science (Tramontano et al. 234:1566-1570; Pollack et al. 234:1570-1573). Quite a number of abzymes have been generated since then, many with high catalytic efficiency. (For examples, search http://www.ncbi.nlm.nih.gov/sites/pubmed using “catalytic antibody” as search phrase.)
Clearly an important application of abzyme technology would be to catalyze reactions that are not normally found in nature, e.g., substrates that are synthetic compounds of human design, and for which no natural enzymes exist. However, the production of abzymes of a desired specificity is dependent on the design and creation of stable transition-state analogs. Since this is not always possible, an alternative method being used involves the introduction of catalytic sites in the antibody through chemical synthesis/modification or through genetic engineering techniques.
Catalytic antibody technology has great potential in various medical applications, owing to its efficacy in enzyme catalysis coupled with the high specificity of antibodies. One example is the proposal by Landry and colleagues (Science 1993; 259:1899-1901) to use abzymes in the clearance of cocaine from the bloodstream through the breakdown of bonds in the cocaine molecule until non-toxic products are produced. Additionally, other toxic substances and even chemical warfare agents can be eliminated using similar techniques.
There are now a number of antibodies that are being used against cancer and other diseases, with some antibodies conjugated to cytotoxic drugs. Antibodies, with specificity for molecules on cancer cells, are able to direct the cytotoxicity to only those cells and not to other (normal) cells. Catalytic antibodies can play a major role in these targeted anti-cancer therapeutics.
In a strategy proposed by Bagshawe (British Journal of Cancer 1987; 56:531-532) called “antibody-directed enzyme prodrug therapy” or ADEPT, an antibody conjugated to an enzyme is first administered, the antibody being specific for a molecule found on the surface of the cancer cells. The antibodies thereby accumulate on the surface of the target cells. A prodrug (a drug that needs to be converted into its active form) is added which is then activated by the enzyme so that the drug can now direct its effect on the cancer cells. This approach reduces the highly cytotoxic effects of most chemotherapeutic drugs, since the administered prodrug can be designed to be less or non-toxic. Here also, the drug is delivered directly to the cancer cells.
In cases where the enzyme elicits an immune reaction and is neutralized by the immune system of the patient, the enzyme in the antibody-enzyme conjugate could be replaced by a catalytic antibody. This new technique, also proposed by Bagshawe (British Journal of Cancer 1989; 60: 275-281), is called “antibody-directed abzyme prodrug therapy” or ADAPT. This strategy is also being developed to target HIV-infected cells. (The immunogenicity of antibodies obtained from nonhuman sources, like mice, is routinely minimized by “humanization” techniques, or the antibodies could be generated using systems that generate “fully human” molecules. Also since only the Fab (antigen binding fragment or the “catalytic site” fragment) part of the antibody structure is needed in ADEPT or ADAPT, the Fc part that an antibody normally uses for its effector function (i.e., to bind receptors in effector cells, and to neutralize antigens), is removed.
The catalytic antibody technology gives us an opportunity to utilize antibodies other than their usual binding applications. There are many useful chemical reactions for which no natural enzymes exist. By harnessing our immune system, it should be possible, theoretically at least, to generate such enzymes.
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Jo Erika T. Narciso is an MS Molecular Biology and Biotechnology student at the National Institute of Molecular Biology and Biotechnology and a research associate in the Marine Natural Products Laboratory, Marine Science Institute, University of the Philippines Diliman. She may be contacted at jetnarciso@gmail.com.
Eduardo A. Padlan is a corresponding member of the NAST and an adjunct professor in the UP Marine Science Institute. He can be reached at eduardo.padlan@gmail.com.