Chapter 25. Antibodies as Drug Carriers and Toxicity Reversal Agents

Drug delivery systems--2. Site-specific drug delivery utilizing monoclonal antibodies

Monoclonal antibodies (MAbs) are purified antibodies produced by a single clone of cells. They are engineered to recognize and bind to a single specific antigen. Accordingly, when administered, MAbs home in on a particular circulating protein or on cells that bear the correct antigenic signature on their surfaces. It is the specificity of MAbs that has made them valuable tools for health professions. Following the discovery of Kohler and Milstein regarding the method of somatic cell hybridization, a number of investigators have successfully adopted this technique to obtain T-lymphocyte hybrid cell lines by fusion of activated T (thymus derived) lymphocytes with a T lymphoma cell line leading to an immortalization of a specific differentiated function. The hybrids thus obtained were subsequently shown to produce homogeneous effector molecules with a wide variety of immune functions such as enhancement or suppression of antibody responses, generation of helper T cells, suppressor T cells and cytotoxic T cells. Study of these regulatory molecules has been further shown to provide a greater insight into the genetic, biochemical and molecular mechanisms responsible for cellular development, and the interaction and triggering of various cell types. The successful application of hybridoma technology has now resulted into several advances in the understanding the mechanism and treatment of diseases, especially cancer and development of vaccines, promotion of organ transplantation and therapy against parasites as well. Since monoclonal antibodies could be made in unlimited supply, they have been used in genetic studies such as mRNA and gene isolation, chromosomal isolation of specific genes, immunoglobulin structure, detection of new or rare immunoglobulin gene products, structural studies of enzymes and other proteins and structural and population studies of protein polymorphisms. In some instances, the monoclonal antibodies have been found to replace conventional antisera for studies of chromosome structure and function, gene mapping, embryogenesis, characterization and biosynthesis of developmental and differentiation antigens. These antigens are those that are specific for various cell types and tissues, species specific antigen, antigens involved in chemotaxis, immunogenetics and clinical genetics including genetically inherited disorders, chromosome aberrations and transplantation antigens. Besides these monoclonal antibodies, their complexes have recently been investigated as exquisitely sensitive probes to be guided to target cells or organs. They have been used to deliver cytotoxic drugs to malignant cells or enzymes to specific cell types.(ABSTRACT TRUNCATED AT 400 WORDS)

Targeting potential of antibody conjugates

Although the use of conjugates of enzymes has been considered, their use has not been very actively pursued. Much more interest has focused on the possibilities offered by the use of toxins, their subunits or of ribosome inhibitors. Conjugates of holotoxin which were very active and specific in vitro have been prepared. High in vivo activity and some specificity together with reduced whole body toxicity has been described. When A chain subunits or ribosome inhibiting proteins are used, the results are more mixed. Some very active and specific conjugates are known but others have relatively low activity. The reasons for this may be associated with the particular antigen to which the antibody component is directed, the nature of the A chain or inhibitor and the type and physiological state of the target cell. Application to man seems likely in the first instance to involve the removal of undesirable cells from bone marrow transplants.