The advent of recombinant DNA technology has brought about several new protein based pharmaceutical products treating diseases ranging from hormone deficiency to myocardial infarction to several forms of cancer. With the completion of the sequencing of the human genome, undoubtedly many more protein drugs will be discovered and developed. Currently 90% of the more than 200 proteins being tested clinically are monoclonal antibodies. Consequently, in the next five years, the vast majority of protein pharmaceutical products reaching the patients will be monoclonal antibodies, and this class of protein will provide important drugs in the fight against diseases for the foreseeable future. In order to understand the potential of monoclonal antibodies as a drugs and in order to appreciate the many scientific and technological advances which enable the realization of this pursuit today, let’s take a few steps back in the history of the discovery of the humoral immune system and the many attempts to use this unique biological defense mechanism to combat illness in the past.

The discovery of the existence of antibody is generally credited to Von Berhing and Kitasato in 1890. They showed that immunity to diphteria and tetanus is due to the presence of a factor in the patients’ serum which they called antibody. This factor can neutralize the toxin secreted by these organisms. This discovery was put to use immediately. In 1891 a child dying of diphteria was given immune serum and made a dramatic recovery. Von Berhing received the 1901 Nobel price for his serum therapy approach. Until today, immuno-globulins fractionated from human serum are still used to provide passive immunization to immune deficient patients. Horse anti-venom sera are also produced and used widely as antidote for venomous snake bites.

Paul Ehrlich first described the "magic bullet concept" for antibody in 1900 speculating on the unique specificity between antibody and antigen. This principle is proven to be true today. The antibodies raised against an antigen bind specifically to this antigen. However, these antibodies usually consist of a mixture of antibodies which recognize different parts (or epitopes) on the antigen. This mixture of antibody raised against an antigen is termed polyclonal antibody. This mixture of different antibodies is actually produced by a family of B lymphocytes, each lymphocyte recognizes one epitope on the antigen. Thus, it is committed to produce one specific antibody against that epitope. Isolation and propagation of a single B lymphocyte would generate a cell line producing a monoclonal antibody i.e. an antibody which binds to a sepcific epitope on the antigen based on the clonal theory advanced by Burnett.

Passive immunization with gamma globulin isolated from human blood found limited use because of its cost and because bacterial infection can be managed effectively with antibiotics. Serum from animal raised against toxin and venom are only used in cases of emergency. Anaphylaxis reactions associated with these sera are not infrequent. The discovery of monoclonal antibody and the development of the hybridoma technology opened an important therapeutic avenue: raising antibody against human tissues and cells for targeted drug delivery. Thus, throughout the late 70’s and the 80’s, extensive basic and clinical researches on targeted drug delivery were conducted. The concept was straight forward. Mice were immunized against specific human cancer cells; antibody producing cells were isolated from the mice spleen, fused with myeloma cells to generate an immortal cell line producing murine antibody against the targeted cells. Toxins were then chemically linked to these antibodies. Upon injection into the systemic circulation, it was speculated that the antibodies would bind to the target cells delivering the toxic payload killing the cancer cells while sparing other non targeted tissues. However, the work did not deliver any pharmaceutical product despite almost twenty years of research and development by thousand of researchers in academia as well in the pharmaceutical industry. Three main obstacles remained to be overcome. First, antibody-antigen binding is a passive process, the antibody has to find the cell before it can bind to the cell. Thus, the targeted delivery was not as effective as once thought especially if the target mass is small and not well perfused. Second, toxin linked antibody broke down in the systemic circulation releasing highly toxic substances prematurely which killed cells indiscriminately resulting in severe toxicity. Third and most challenging, the patient invariably produced antibody to the mouse antibody being administered (human antibody to mouse antibody reaction or HAMA reaction). This reaction prevented repeated dosing of the drug and thus effectively limited its use. Therapy using unarmed mouse antibody also failed because the mouse effector function does not activate human antibody dependent cell cytoxicity. We will return to this topic in the later part of this review.

Analogous to the use anti-venom serum to neutralize snake venom, antibodies can be raised against a soluble protein, in this case a human protein with specific physiological function, to remove it from the systemic circulation, thereby preventing unwanted pharmacological effect. When the Mab is injected systemically, it binds to the protein antigen to form complexes which are then removed by macrophages and other scavenger cells of the reticulo-endothelial system. Remicade™ is a mouse/human chimeric antibody raised against human TNF-?, a cytokine implicated in many inflammatory responses and auto-immune diseases. The product is indicated for the treatment of Crohn’s disease, a chronic inflammation of the colon and recently, it was approved for the treament of rheumatoid arthritis. At least two Mabs have been raised against human vascular endothelial growth factor (VEGF) and tested clinically against varius forms of cancer. VEGF is believed to be the key growth factor which cancer cells produce to establish the vascular network essential for tumor growth. Thus. the removal of VEGF by anti-VEGF antibody may prevent the growth or even lead to the shrinkage of solid tumors by cutting off their blood supply.

Zenapax™ (Daclizumab) is a humanized Mab raised against the p55 alpha subunit or CD 55 of the human interleukin-2 (IL-2) receptor expressed on activated but not on resting lymphocytes . It binds specifically to CD-55 and functions as an IL-2 antagonist preventing the IL-2 mediated activation of lymphocytes. This is a critical pathway in the cellular immune response in allograft rejection. Zenapax™ is indicated for the prophylaxis of acute organ rejection in patients receiving renal transplant.

Rituxan™ (Rituximab) is a chimeric mouse/ human Mab. It is targeted against CD20 antigen, a protein expressed on the surface of normal and cancerous B- lymphocytes. However, CD 20 is not found on hematopoietic stem cells, pro B-cells and cell of other normal tissues. When bound to the target cells, Rituxan™ induces B-cell lysis via complement dependent cytotoxicity (CDC) and antibody dependent cell mediated cytotoxicity (ADCC) mediated by the effector function of the Fc fragment. Rituxan™ is indicated for patients with relapsed or refractory CD20 positive non Hodgkin’s lymphoma. Because of its specificity, Rituxan™ is relatively non-toxic compared to currently used chemo-therapy agents. Since it does not affect the pro B-cells and the stem cells. The patient B-cell population is re-established quickly following treatment.

Another strategy is illustrated in the mechanism of action of Herceptin™ (Trastuzumab). Here the Mab is raised against the cell surface receptor HER-2 which is over expressed on primary breast cancer cells. HER-2 is one of the four members of the human epidermal growth factor receptors. Herceptin™ has been shown to inhibit the proliferation of human tumor cells which over express HER-2 by preventing HER-2 dimerization. It was also shown to mediate ADCC directed preferentially to Her-2 overexpressed tumor cells.