What are Monoclonal antibodies?
What are antibodies?
Antibodies are proteins made by cells of the immune system. These cells are called B cells, and each B cell makes a different antibody. The human body has about 10 billion (that is 10,000,000,000) different B cells each capable of making a different antibody. When the body is infected, there are a truly astronomical number of antibodies that could be made.
Antibodies are present in the blood and in every tissue. They are Y-shaped molecules having two arms with “hands” and one part being the “body”. Both hands of a single antibody molecule are identical. The hands are binding sites which attach to a very small part of an infecting foreign bacterium or virus. The body of the antibody molecule has several functions that lead to the killing and elimination of the targeted invader. One function is to activate a cascade of enzymes (called “complement”) that punch holes in viruses. Another function is to coat bacteria and enable them to be either killed by “natural killer” (NK) cells or eaten by cells (phagocytes) that engulf and destroy the bacteria and viruses.
When an infection occurs, only those B cells that make an antibody that can attach to the foreign germ are activated, in order to produce vast amounts of the antibodies (known as “specific” antibodies) that can bind to the bacteria or viruses. These are released into the circulation where they seek out the infectious agents. This targeted action has led antibodies to be called “magic bullets”.
Antibodies have two properties. Because antibodies bind to slightly different parts of a bacterium or virus they have “specificity”. Even antibodies that bind the same target may bind more or less strongly. This is called “affinity”. Those antibodies that have the highest specificity and the greatest affinity are the best at eliminating foreign invaders.
What are monoclonal antibodies?
When individual B cells are activated to make a specific antibody, they divide to create many, many identical daughter cells making the same antibody. This family of B cells derived from a single parent cell is called a “clone”.
Usually, when the body is infected, a lot of different B cell clones are activated to make many different antibodies with specificity and affinity for the invading organism. This is called a “polyclonal” response. Of course, the polyclonal antibodies have a range of specificity and affinity, some being better than others.
In the laboratory it is possible to separate the different B cell clones from a polyclonal response into individual clones. These individual B cell clones can then be grown in test tubes and the antibodies they make can be tested for specificity and affinity. Such antibodies, produced by individual clones, are called “mono (single) clonal antibodies”.
Monoclonal antibodies can be used to treat many different conditions. Because the very best specific, high affinity monoclonal antibodies can be made in great quantities and can be monitored for quality, they have found uses in many areas of medicine.
How they work
How do monoclonal antibodies work?
Antibodies can work in two major ways:
Masking or blocking
When antibodies bind, they cover up the site where they bind. In an infection with viruses, for example, the antibodies may cover a part of the virus that enables it to infect the cells of the body. This prevents the infection spreading and is said to “neutralize” the virus.
In medicine, this blocking is used to change the way the immune system is activated. Monoclonal antibodies can bind to molecules on immune cell surfaces to prevent the cells from interacting. Alternatively, monoclonal antibodies can block receptors on the surface of cells for growth factors that might prompt immune overactivity or tumor growth.
The killing of cells is known as “cytotoxicity”. In theory, at least, if a molecule is present on tumor cells and NOT on normal cells of the body, monoclonal antibodies might be used to target tumors. Killing is brought about by activating the cascade of lethal “complement” enzymes, by arming NK cells or by increasing phagocytosis (see above). Alternatively, antibodies can induce what is called “programmed cell death” (apoptosis), which is how most normal cells in the body die and are replaced.
Recently, monoclonal antibodies specific for the COVID virus have been tested in patients, apparently with some success. These anti-COVID antibodies either neutralize or kill the COVID viruses.
Why use monoclonal antibodies for treatment?
The advent of monoclonal antibodies with any binding site we chose has opened the door to novel treatments in an ever-growing range of diseases.
The generic names of monoclonal antibodies (Mab, for short) are all called ‘something’mab. For example, the generic name of Rituxan is rituximab.
- Generic name: Rituximab
- Conditions in which rituximab is used: Rituximab is used in blood cancers and in autoimmune diseases.
- Blood cancers. Rituximab is used in B cell cancers, namely leukemias and lymphomas. These include non-Hodgkin’s lymphoma (NHL) and chronic lymphocytic leukemia (CLL). It is used with other drugs such as flurabine and cyclophosphamide in CD20-positive CLL, and hyaluronidase (an enzyme to increase tissue penetration) in follicular lymphoma, diffuse large B cell lymphoma and CLL.
- Autoimmune diseases. Rituximab is effective in Rheumatoid Arthritis, where treatments with other monoclonal antibodies (anti-TNFα) has failed. It may be used with an immunosuppressive drug, methotrexate, in severe rheumatoid arthritis.
Vasculitis (inflammation of blood vessels) such as granulomatosis with poly angiitis and microscopic polyangiitis, and a destructive skin condition called pemphigus vulgaris are other indications for treatment with rituximab in the USA.
Other autoimmune conditions where rituximab has been used include multiple sclerosis (MS), systemic lupus erythematosus (SLE), chronic inflammatory demyelinating polyneuropathy and autoimmune anemias. Rituximab treatment has also been tried in many oyher autoimmune conditions: autoimmune hemolytic anemia, pure red cell aplasia, thrombotic and idiopathic thrombocytopenia purpura, Evans syndrome (destruction of red cells and platelets), type-1 diabetes mellitus, Sjögrens syndrome, anti-NMDA receptor encephalitis and neuromyelitis optica, Graves’ opthalmopathy, and autoimmune pancreatitis.
- How it works: Rituximab binds to a molecule, CD20, which is present on the surface of almost all B cells. Treatment with rituximab eliminates mostly immature B cells, including cancerous B cells.
Once the antibody has bound to CD20, the body portion of rituximab causes cell killing (cytotoxicity) i) by Natural Killer cells and ii) by activating the lethal complement enzymes, and iii) the induction of apoptosis (programmed cell death).
Cancerous B cells are “high expressors” of CD20, meaning that they have a lot of CD20 on their surface. High expressors are efficiently targeted by rituximab. Most normal B cells are low expressors of CD20 and are less vulnerable to rituximab, giving a window for safe treatment.
- Possible side effects of rituximab: Unfortunately, no powerful treatment is without some possible adverse events. In no particular order, for rituximab these include transfusion reactions, cardiac arrest, cytokine release syndrome (like a severe dose of the ‘flu), tumor lysis syndrome (causing acute kidney injury), infections (Hepatitis B reactivation, JC virus reactivation (causing progressive multifocal leukoencephalopathy or PML), immune toxicity (depletion of too many B cells in lymphoma patients), lung damage and bowel obstruction and perforation.
- Generic name: Trastuzumab
- Conditions in which trastuzumab is used: The major use of trastuzumab is in the treatment of metastatic breast cancer. It may be used as a sole agent, or in combination with other agents such as chemotherapy, hormone blockers or lapatinib (an inhibitor of tumor cell activation).
Trastuzumab has a low toxicity and so can be used at high doses. However, up to 70% of patients become resistant and do not respond to treatment. Some chemotherapy agents have been shown to overcome resistance to trastuzumab, offering new ways to use this monoclonal antibody for effective treatment.
- How it works: This monoclonal antibody targets a molecule on the surface of cells called HER2. It is a receptor (R) that binds human epidermal (HE) growth factor. It is one of a family of molecules: HER1, HER2, HER3 and HER4. For growth factor receptors on the cell surface to function, they usually have to bind to another molecule. This process is called “dimerization”. HER2 can dimerize with another molecule of HER2, forming HER2/HER2 dimers. Trastuzumab binds to HER2, preventing the formation of HER2/HER2 dimers.
The amount of HER2 receptor on early breast cancer cells, and stomach cancer cells, is significantly increased. This allows the cancer cells to receive a greater level of stimulation by epidermal growth factor (EGF), promoting rapid growth of tumor cells.
Trastuzumab binds to the HER2 molecule, causing the internalization and removal of the molecule from the surface of tumor cells. This prevents EGF from causing uncontrolled tumor expansion.
- Possible side effects of trastuzumab: Common side effects are flu-like symptoms (with chills, fever and mild pain), nausea and diarrhea. A rare but serious side effect is cardiac toxicity, including cardiac dysfunction and congestive heart failure. This occurs because trastuzumab interferes with the action of another growth factor (NRG1) which is essential for cardiomyocytes (heart muscle cells) and, therefore, for heart function. Fortunately, this side effect is reversible with cessation of treatment. Therefore, patients given trastuzumab undergo regular screening.
- Generic name: Pertuzumab
- Conditions in which pertuzumab is used: Pertuzumab is used to treat patients with HER2-positive metastatic breast cancer, usually in combination with the chemotherapeutic agent docetaxel. The same combination has been given as “neoadjuvant” therapy in early breast cancer, to reduce the size of the tumor before surgery or radiation to increase the chances of effective removal.
- How it works: Pertuzmab, like trastuzumab (above), binds to the HER2 receptor but works in a subtlely different way. In this case, pertuzumab binds to the HER2 molecule and prevents it forming HER2/HER3 dimers which are the most potent dimers for promoting tumor growth. Blocking HER2/HER3 dimerization dramatically reduces the action of EGF on cell growth.
- Possible side effects of pertuzumab: Common side effects are diarrhea, hair loss and loss of neutrophils (white blood cells). Some of this may be caused by docetaxel rather than pertuzumab. After reduction of docetaxel the most common side effects were diarrhea, upper respiratory tract infection, rash, headache, weakness and fatigue, itchiness, joint pain, nausea and cough. Allergic or hypersensitivity reactions have also been noted. Cardiac toxicity does not appear to be a major issue with pertuzumab, although it has not been studied in predisposing heart conditions.
- Generic name: Daratumumab
- Conditions in which daratumumab is used: Daratumumab is an approved treatment for multiple myeloma. Daratumumab is given in combination with anti-cancer drugs such as lenalidomide or bortezomib and dexamethasone. The combination of daratumumab with bortezomib, melphalan and predisone has been used in newly diagnosed multiple myeloma.
- How it works: Daratumumab binds to a molecule called CD38, which is present on the surface of multiple myeloma cells in high amounts. The target cells having the greatest amount of CD38 on their surface are most susceptible to daratumumab. Daratuzumab kills multiple myeloma cells by activating complement enzymes, NK cells and antibody enhanced phagocytosis, and may also increase the immune response against the tumor.
- Possible side effects of daratumumab: The major side effects are increases in the patients’ susceptibility to bacterial and viral infections. Daratumumab frequently causes reactivation of dormant cytomegalovirus (CMV) in previously infected people. Inflammation and pain are common at the site of injection of daratumumab.