Monoclonal antibody therapy is a form of immunotherapy that uses monoclonal antibodies (mAb) to bind monospecifically to certain cells or proteins. This may then stimulate the patient's immune system to attack those cells. Alternatively, in radioimmunotherapy a radioactive dose localizes on a target cell line, delivering lethal chemical doses. More recently antibodies have been used to bind to molecules involved in T-cell regulation to remove inhibitory pathways that block T-cell responses, known as immune checkpoint therapy.
It is possible to create a mAb specific to almost any extracellular/ cell surface target. Research and development is underway to create antibodies for diseases (such as rheumatoid arthritis, multiple sclerosis, Alzheimer's disease, Ebola and different types of cancers).
Antibody structure and function
Immunoglobulin G (IgG) antibodies are large heterodimeric molecules, approximately 150 kDa and are composed of two kinds of polypeptide chain, called the heavy (~50kDa) and the light chain (~25kDa). The two types of light chains are kappa (?) and lambda (?). By cleavage with enzyme papain, the Fab (fragment-antigen binding) part can be separated from the Fc (fragment constant) part of the molecule. The Fab fragments contain the variable domains, which consist of three antibody hypervariable amino acid domains responsible for the antibody specificity embedded into constant regions. The four known IgG subclasses are involved in antibody-dependent cellular cytotoxicity.
The immune system responds to the environmental factors it encounters on the basis of discrimination between "self" and "non-self". Tumor cells are generally not specifically targeted by the immune system, since tumor cells are the patient's own cells. Tumor cells, however are highly abnormal, and many display unusual antigens.
Some such antigens are inappropriate for the cell type or its environment. Some normally present only during the organisms' development (e.g. fetal antigens). Some are rare or absent in healthy cells, and are responsible for activating cellular signal transduction pathways that cause unregulated tumor growth. Examples include ErbB2, a constitutively active cell surface receptor that is produced at abnormally high levels on the surface of approximately 30% of breast cancer tumor cells. Such breast cancer is known as HER2-positive breast cancer.
Antibodies are a key component of the adaptive immune response, playing a central role in both in the recognition of foreign antigens and the stimulation of an immune response to them. The advent of monoclonal antibody technology has made it possible to raise antibodies against specific antigens presented on the surfaces of tumors.
History
Immunotherapy developed in the 1970s following the discovery of the structure of antibodies and the development of hybridoma technology, which provided the first reliable source of monoclonal antibodies. These advances allowed for the specific targeting of tumors both in vitro and in vivo. Initial research on malignant neoplasms found mAb therapy of limited and generally short-lived success with blood malignancies. Treatment also had to be tailored to each individual patient, which was impracticable in routine clinical settings.
Four major antibody types were developed: murine, chimeric, humanised and human. Antibodies of each type are distinguished by suffixes on their name.
Murine
Initial therapeutic antibodies were murine analogues (suffix -omab). These antibodies have: a short half-life in vivo (due to immune complex formation), limited penetration into tumour sites and inadequately recruit host effector functions. Chimeric and humanized antibodies have generally replaced them in therapeutic antibody applications. Understanding of proteomics has proven essential in identifying novel tumour targets.
Initially, murine antibodies were obtained by hybridoma technology, for which Kohler and Milstein received a Nobel prize. However the dissimilarity between murine and human immune systems led to the clinical failure of these antibodies, except in some specific circumstances. Major problems associated with murine antibodies included reduced stimulation of cytotoxicity and the formation complexes after repeated administration, which resulted in mild allergic reactions and sometimes anaphylactic shock. Hybridoma technology has been replaced by recombinant DNA technology, transgenic mice and phage display.
Chimeric and humanized
To reduce murine antibody immunogenicity (attacks by the immune system against the antibody), murine molecules were engineered to remove immunogenic content and to increase immunologic efficiency. This was initially achieved by the production of chimeric (suffix -ximab) and humanized antibodies (suffix -zumab). Chimeric antibodies are composed of murine variable regions fused onto human constant regions. Taking human gene sequences from the kappa light chain and the IgG1 heavy chain results in antibodies that are approximately 65% human. This reduces immunogenicity, and thus increases serum half-life.
Humanised antibodies are produced by grafting murine hypervariable regions on amino acid domains into human antibodies. This results in a molecule of approximately 95% human origin. Humanised antibodies bind antigen much more weakly than the parent murine monoclonal antibody, with reported decreases in affinity of up to several hundredfold. Increases in antibody-antigen binding strength have been achieved by introducing mutations into the complementarity determining regions (CDR), using techniques such as chain-shuffling, randomization of complementarity-determining regions and antibodies with mutations within the variable regions induced by error-prone PCR, E. coli mutator strains and site-specific mutagenesis.
Human monoclonal antibodies
Human monoclonal antibodies (suffix -umab) are produced using transgenic mice or phage display libraries by transferring human immunoglobulin genes into the murine genome and vaccinating the transgenic mouse against the desired antigen, leading to the production of appropriate monoclonal antibodies. Murine antibodies in vitro are thereby transformed into fully human antibodies.
The heavy and light chains of human IgG proteins are expressed in structural polymorphic (allotypic) forms. Human IgG allotype is one of the many factors that can contribute to immunogenicity.
Targeted conditions
Cancer
Anti-cancer monoclonal antibodies can be targeted against malignant cells by several mechanisms.
Radioummunotherapy
Radioimmunotherapy (RIT) involves the use of radioactively-conjugated murine antibodies against cellular antigens. Most research involves their application to lymphomas, as these are highly radio-sensitive malignancies. To limit radiation exposure, murine antibodies were chosen, as their high immunogenicity promotes rapid tumor clearance. Tositumomab is an example used for non-Hodgkins lymphoma.
Antibody-directed enzyme prodrug therapy
Antibody-directed enzyme prodrug therapy (ADEPT) involves the application of cancer-associated monoclonal antibodies that are linked to a drug-activating enzyme. Systemic administration of a non-toxic agent results in the antibody's conversion to a toxic drug, resulting in a cytotoxic effect that can be targeted at malignant cells. The clinical success of ADEPT treatments is limited.
Immunoliposome therapy
Immunoliposomes are antibody-conjugated liposomes. Liposomes can carry drugs or therapeutic nucleotides and when conjugated with monoclonal antibodies, may be directed against malignant cells. Immunoliposomes have been successfully used in vivo to convey tumour-suppressing genes into tumours, using an antibody fragment against the human transferrin receptor. Tissue-specific gene delivery using immunoliposomes has been achieved in brain and breast cancer tissue.
Checkpoint therapy
Checkpoint therapy uses antibodies and other techniques to circumvent the defenses that tumors use to suppress the immune system. Each defense is known as a checkpoint. Compound therapies combine antibodies to suppress multiple defensive layers. Known checkpoints include CTLA-4 targeted by ipilimumab, PD-1 targeted by nivolumab and pembrolizumab and the tumor microenvironment.
The tumor microenvironment (TME) features prevents the recruitment of T cells to the tumor. Ways include chemokine CCL2 nitration, which traps T cells in the stroma. Tumor vasculature helps tumors preferentially recruit other immune cells over T cells, in part through endothelial cell (EC)-specific expression of FasL, ETBR, and B7H3. Myelomonocytic and tumor cells can up-regulate expression of PD-L1, partly driven by hypoxic conditions and cytokine production, such as IFN?. Aberrant metabolite production in the TME, such as the pathway regulation by IDO, can affect T cell functions directly and indirectly via cells such as Treg cells. CD8 cells can be suppressed by B cells regulation of TAM phenotypes. Cancer-associated fibroblasts (CAFs) have multiple TME functions, in part through extracellular matrix (ECM)-mediated T cell trapping and CXCL12-regulated T cell exclusion.
Autoimmune diseases
Monoclonal antibodies used for autoimmune diseases include infliximab and adalimumab, which are effective in rheumatoid arthritis, Crohn's disease and ulcerative Colitis by their ability to bind to and inhibit TNF-?. Basiliximab and daclizumab inhibit IL-2 on activated T cells and thereby help preventing acute rejection of kidney transplants. Omalizumab inhibits human immunoglobulin E (IgE) and is useful in moderate-to-severe allergic asthma.
FDA approved therapeutic antibodies
The first FDA-approved therapeutic monoclonal antibody was a murine IgG2a CD3 specific transplant rejection drug, OKT3 (also called muromonab), in 1986. This drug found use in solid organ transplant recipients who became steroid resistant. Hundreds of therapies are undergoing clinical trials. Most are concerned with immunological and oncological targets.
Recently, the bispecific antibodies, a novel class of therapeutic antibodies, have yielded promising results in clinical trials. In April 2009, the bispecific antibody catumaxomab was approved in the European Union.
Economics
Since 2000, the therapeutic market for monoclonal antibodies has grown exponentially. The current "big 5" therapeutic antibodies on the market are bevacizumab, trastuzumab (both oncology), adalimumab, infliximab (both autoimmune and inflammatory disorders, 'AIID') and rituximab (oncology and AIID) accounted for 80% of revenues in 2006. In 2007, eight of the 20 best-selling biotechnology drugs in the U.S. are therapeutic monoclonal antibodies. This rapid growth in demand for monoclonal antibody production has been well accommodated by the industrialization of mAb manufacturing.
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