Antibody Conjugates Promise to Deliver

Antibodies represent the single most exciting drug class currently under clinical development. However, they are also effective agents for site specific delivery of other therapeutic agents within the body. With optimised synthesis and formulation, this approach frequently results in the generation of therapeutically useful levels of drug which have been unattainable at the site of disease through more conventional pharmaceutical delivery approaches. This short review looks at antibody conjugates that are currently in the clinic, and the main strategies being examined for the development of new therapeutic antibody conjugates.

WHY USE ANTIBODY DRUG CONJUGATES?

The application of antibody-based products in the pharmaceutical industry is well established, with over 20 antibody-based products currently marketed for the treatment of conditions including cancer, viral and inflammatory diseases. Sales from this class of therapeutic reached $2 billion in 2000 and are expected to grow to over $25 billion by 2010. It is with little surprise, therefore, that they represent the fastest growing class of drug currently in clinical development.

These biopharmaceuticals elicit their therapeutic effect by docking to a specific binding site or epitope, normally on a targeted protein at the site of disease, leading to a direct or indirect biological response. The selective ability of the antibody to localise to the targeted protein is key to success; large R&D investments across the private and public biotechnology sectors have been dedicated to the identification of suitable unique targets or biomarkers in various disease models. Historically, biomarkers have been examined for the ability of the antibody binding to elicit a desired therapeutic response. For example, trastuzumab (Herceptin) is used in the treatment of early breast cancer where it binds to the cell surface protein HER2/neu, inhibiting proliferation pathways that transduce through this signalling receptor, thus attenuating the growth of the tumour. Alternatively, bevacizumab (Avastin) inhibits the formation of new blood vessels (angiogenesis) in colorectal tumours by sequestering and removing the major proangiogenic factor vascular endothelial growth factor (VEGF) from the tumour micro-environment.

In addition to the active docking of the antibody to its cognate target, it is thought that the antibodies can also elicit their effect on diseased cells by virtue of their constant fragment (Fc)region. There are two possible mechanisms: antibodydependent cellular cytotoxicity (ADCC) and complementdependent cytotoxicity (CDC). In ADCC, the antibody binds to the surface of the diseased cell and the presence and surface density of their Fc regions leads to recruitment of immune effector cells (1). It is thought that trastuzumab is able to increase its effects against HER2/neu breast cancer cells by evoking this response. Alternatively, CDC refers to the recruitment and activation of the complement system by the antibody (2), and is thought to play a role in the therapeutic effect of the anti-CD20 antibody rituximab, used in the treatment of Non-Hodgkin’s lymphoma.