Antibody-Directed Enzyme-Prodrug Therapy

Over a century ago, Paul Ehrlich proposed the magic bullet theory of targeting therapeutic agents to specific tissues in order to increase their efficacy and reduce systemic toxicity. Because of their selectivity and the rapid development of hybridoma technology in the 1980s, monoclonal antibodies either alone or linked to a wide range of therapeutic agents such as radionuclides, drugs, toxins, enzymes, and growth factors have been used for diagnosis and therapy of diseases, particularly cancer. However, inherent problems with the monoclonal antibodies themselves, coupled with the heterogeneous structure and pathophysiology of solid tumours, reduce effective delivery following systemic administration. The more recent development of phage technology and antibody engineering is now addressing these problems by allowing the creation of antibodies with required characteristics for specific purposes. Antibody-directed enzyme-prodrug therapy (ADEPT) is a complex two-phase system in which an enzyme is targeted to the tumor site by an antibody, where it selectively activates a relatively nontoxic prodrug to a potent cytotoxic agent. This system has progressed from the monoclonal antibody approach to that of genetically engineered fusion proteins, in order to provide optimized molecules for clinical trials. This chapter will start by giving a brief overview of the field, concentrating on the developments in our own department using anti-CEA antibodies and the bacterial enzyme carboxypeptidase G2. This will be followed by a description of the major materials and methods involved in fusion protein production.