Humanized MAbs

A number of chimerized, humanized, and one human mAb have now been approved for therapeutic use in humans in the treatment of autoimmunity, malignancy, infection and cardiovascular disease (Table 1). Some of the currently licensed mAb will be discussed here. A much larger number of mAb are currently being evaluated in Phase I, II and III trials. In general, chimeric, humanized and human mAb are very well tolerated with few side effects. Chimeric or humanized mAb still have the potential to evoke host immune response to the variable domains or CDRs of the antibody so-called HACA (human anti-chimeric antibody) or HAHA (human anti-human antibody) responses, although these responses are uncommon. Short-lived and occasionally severe infusion-related acute hypersensitivity reactions such as fever, skin itching, shivering, respiratory compromise and low blood pressure sometimes occur-. Such effects may... 

Secreted neutralizing human mab 2F5 Membrane-anchored anti-gp41 scFv ... 

Zenapax (Daclizumab, humanized Mab directed against the a chain of the IL-2 receptor)... 

Humaspect (Votumumab, human Mab directed against cytokeratin tumour-associated antigen) Organon Teknika Detection of carcinoma of the colon or rectum... 

Fuii Human Antibody Full human antibodies are the current engineered antibodies. Several techniques are used to construct these antibodies. One method is to fuse human B cells to myeloma cells. These hybridomas will produce fully human MAbs. Another method is to genetically alter mice in the laboratory to contain human antibody producing genes. In response to antigens, antibodies resembling the human antibodies are produced. 

Synagis (Palivizumab, humanized Mab directed against an epitope on the surface of respiratory syncytial virus)... 

Ten years ago monoclonal antibodies (MAb) were of considerable interest but faded after clinical and commercial failures. Further clinical and laboratory testing revealed that only humanized MAb were likely to be of any benefit in a human patient. From a practical standpoint this research developed, and humanized MAb were launched onto the market, with success in the treatment of several diseases including rheumatoid arthritis. These developments justify a chapter on the subject in this book. 

The use of specific and nonspecific antisera in human medicine is well established and dates back to 1891 when Emil von Behring developed the first diphtheria antitoxin, but their use carries associated risks such as fluid overload and transmission of disease. The potential for MABs as therapeutic agents was quickly recognized and the first MAB was approved for therapeutic use in 1986 (Ortho Biotech s OKT3, a mouse MAB to CD3, for the reversal of transplant rejection). It became clear early on that the presence of, or appearance of, human antimouse antibodies (HAMA) in the patient, which neutralized subsequent treatments, often limited the efficacy of mouse MABs. More recently, chimeric, deimmunized, or fully human MABs have been developed. 

Apart from the spleen, other lymphoid tissues, such as tonsils and the mesenteric or popliteal lymph nodes, can be used as a source of lymphocytes. In the preparation of MABs of human or veterinary origins it is often not possible to obtain lymphoid tissue, and there have been many reports of the successful use of lymphocytes separated from peripheral blood. In some cases, for ethical or practical reasons, it is not possible to immunize the lymphocyte donor, as when human MABs are required, or acutely toxic antigens are used. Also, antigen is not always available in sufficient quantities to perform a successful immunization in vivo. In these circumstances, it may be possible to perform the boosting stage or, indeed, the entire immunization procedure on the lymphocytes in vitro. 

To circumvent some of the limitations of direct immunization, phage display technology has been applied to the preparation of fully human MABs. Gene libraries of cDNA from nonimmune or immunized donor lymphocytes are expressed in bacteriophages. The bacteriophages display functional antibody fragments and can... 

The low concentration of MAbs in conventionally produced culture supernatants entails the need for use of large volumes of supernatant and large amounts of precipitant to yield useful quantities of antibody. If ascitic fluid cannot be made (as with human MAbs see Note 2) then the precipitation step can often be more easily managed if the culture supernatant is reduced in volume. This is readily carried out by using ultrafiltration membranes that remove water and small molecular weight solutes from the supernatant. 

Conventional rodent model systems have proven problematic as they do not reliable model the pharmacokinetics of humanized mAb and Fc-fusion proteins. In contrast to the failure of mouse mAbs to be protected by human FcRn, humanized mAbs have an abnormally high affinity for mouse and rat FcRn, resulting in an artificially prolonged serum persistence (13, 16). This fact has greatly diminished the preclinical utility of standard mice for therapeutic mAb development and testing. The alternative cynamolo-gous monkey model has proven to be reliable, but it is hampered by considerable expense and ethical concerns that limit its routine use. 

Further progress was achieved by the humanization of murine antibodies. In humanized mAbs, the murine fraction of 5-10% consists only of the murine CDRs and, if necessary, some exclusive parts of the framework region. The human immune system can also produce antibodies against these humanized antibodies, so-called human anti-human antibodies (HAHAs). 

More recent developments using cDNA libraries of B lymphocytes and phage display technology have made it possible to produce complete human mAbs. The... 

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