Could MAbs be the 'magic bullet'?

Sarah Houlton reviews the growing use of monoclonal antibodies in the fight against cancer and other diseases.

The perfect drug is a 'magic bullet' - totally specific, with good activity and pharmacokinetics, and no side-effects. In the real world, however, most drugs have side-effects, and many of these adverse events result from a lack of selectivity. While the drugs may have the desired killing, blocking or activating activity, they frequently damage healthy tissue too.One way of targeting drugs more precisely is to use a monoclonal antibody, either exploiting its intrinsic activity or delivering another agent to the active site. A number of such antibodies are now on the market and more are in development.

The market for monoclonal antibodies (MAbs) in Europe has been estimated at around US$1.5bn, and Frost & Sullivan (F&S) expects the European antibody market to grow at a compound annual growth rate of 34.1% to $11.4bn by 2011. There are currently 10 monoclonal antibody therapeutics on the market in Europe (see table 1). Although chimeric MAbs are dominant, humanised and fully human products are likely to increase in importance in future.

It is likely that oncology, auto-immune and inflammatory disorders (AIID) will continue to drive the market. F&S projects that oncology will retain the lead, with sales of $6.5bn in Europe in 2011, followed by AIID indications at $4.5bn.

Similar growth is likely in the US. F&S says market revenues reached $8.7bn in the past year and projects that sales will exceed $16bn in 2012.

The immune system's B cells produce antibodies as part of its defence strategy. These highly selective proteins bind specifically to other molecules - antigens - and then signal to other components within the immune system to destroy the target it is bound to. However, all too often the immune system is overwhelmed and cannot make the antibodies sufficiently quickly to remove the infected or injured cells, resulting in disease. Although each B cell makes only one type of antibody, other B cells make different antibodies that bind to different parts of the antigen. The resulting mixture of anti- bodies is termed polyclonal antibodies.

specific targets

Therapeutic antibodies, like natural ones, interact with antigens and trigger a biological reaction. However, unlike the body's polyclonal antibodies, a therapeutic antibody is targeted at a single antigen, either in an infectious agent or on a cell within the body, and interferes with a cellular process.

Making specific monoclonal antibodies for use as medicines is a little more complex than the body's method for making polyclonal antibodies. First, an animal - usually a mouse - is given an antigen challenge, and B cells are removed from its spleen. These B cells are fused with myeloma tumour cells, termed hybridomas, that are unable to make their own antibodies and are able to grow indefinitely in a culture. They multiply rapidly and make large amounts of antibodies. By diluting and growing them, a number of different colonies that each make just one antibody can be created. The hybridomas can also be grown in mice by injecting them into the peritoneal cavity, where they produce tumours that contain an antibody-rich fluid.

The big drawback is that these antibodies are mouse-specific, and are thus likely to be rejected by the human immune system. Several ways of avoiding this problem have been developed. The DNA that encodes the mouse antibody's binding protein can be merged with DNA that produces human antibodies, and this chimeric DNA can then be expressed in mammalian cell cultures, giving antibodies that are part-mouse, part-human. An alternative is to genetically engineer the mice so that they produce antibodies that resemble human ones.

The first monoclonal antibody to be given FDA approval was Ortho Biotech's muronomab-CD3 (Orthoclone OKT3), licensed in 1986 and used to treat rejection of transplanted organs. It is a murine monoclonal antibody to the CD3 antigen of human T cells and a biochemically purified immuno-globulin. The antibody is thought to reverse transplant rejection by blocking the function of all T-cells that play a major role in acute rejection.

Two other marketed antibodies are designed to treat transplant rejection: Roche's Zenapax (daclizumab) and Simulect (basiliximab) from Novartis. Daclizumab inhibits the interleukin-2 mediated activation of lymphocytes, a critical pathway in the cellular immune response that causes rejection. Basiliximab is targeted against interleukin 2 (specifically IL-2α) and also inhibits the activation of lymphocytes.

The earliest MAbs were so specific that the target population was, by definition, limited. However, there are now antibodies in clinical development that can treat more than one condition, or different forms of the same disease.

One example is infliximab (Remicade) from Centocor, which blocks TNF-α. First licensed in 1998 to treat Crohn's disease, it has since been approved for rheumatoid arthritis, psoriatic arthritis and ankylosing spondylitis, all of which are inflammatory conditions mediated by TNF-α. Another antibody active against TNF-α is Abbott's Humira (adalimumab), which again blocks the activity of the protein that signals the release of further substances causing arthritic conditions.

ReoPro (abciximab) from Centocor is used to prevent clotting. It binds to the glycoprotein IIb/IIIa receptor on human platelets, inhibiting platelet aggregation; to a further receptor found on platelets; to vessel wall endothelial cells and smooth muscle cells. This prevents the binding of fibrinogen, von Willebrand's factor and other adhesive molecules, blocking access of larger molecules to the receptor.

Thus far just one antibody designed for use in an infectious disease has been licensed. Synagis (palivizumab) from Medimmune is indicated for the prevention of serious lower respiratory tract infection caused by respiratory syncytial virus in paediatric patients at risk, particularly premature babies and children with heart conditions. The antibody is an immunoglobulin directed at an epitope on the A antigenic site of the F protein of the virus, and has a neutralising and fusion inhibitory action.

There are several ways in which antibodies can be used to treat cancer, the most obvious being to create a monoclonal antibody that binds to the tumour specific antigen and induces an immunological response within the tumour cell. Another strategy is to use the MAb to deliver a cancer destroying agent specifically to the tumour cell. This could be a cytotoxic agent, a cytokine or a radioisotope.

Some tumour-associated antigens are made by both normal and tumour cells, but there are also tumour-specific antigens that are only ever produced by tumours. The latter are usually the result of some form of tumour specific mutation, and so present an ideal target for drug therapy. Tumour antigen specific drugs have great potential as anticancer agents with minimal effects on normal cells.

A number of the antibodies already on the market are designed to combat one or more forms of cancer, almost all of them directed against some type of non-solid tumour. Genentech's Rituxan (rituximab), for example, is used to treat B cell lymphomas, where it binds to the CD20 molecule. This antigen is found on most B cells - both cancerous and healthy - but after treatment the healthy cells begin to grow again from precursors without the CD20 molecule and so evade destruction.

Campath (alemtuzumab) from Millennium is designed to treat chronic lymphocytic leukaemia by binding to the CD52 antigen on white blood cells. It is also undergoing trials as a potential treatment for multiple sclerosis.

The treatment of some aggressive forms of breast cancer has been revolutionised by Genentech's introduction of Herceptin (trastuzumab). The antibody binds to HER2, which is a growth factor receptor found on a number of tumour cells, notably some breast cancers and lymphomas. It is unusual in that it is active against solid tumours.

Also from Genentech, Avastin (bevacizumab) binds to and blocks vascular endothelial growth factor (VEGF), which plays an essential role in the growth of the new blood vessels the tumour needs to survive and grow. Licensed in the US for metastatic colorectal cancer, in combination with standard chemotherapy drugs, bevacizumab was the first drug that prevents angiogenesis to be approved.

One of the most talked about drugs in recent years - Erbitux (cetuximab) - is used to treat colorectal cancers, despite much controversy over whether or not it produced sufficient clinical benefit in trials. It targets the epidermal growth factor receptor that is involved in the regulation of cell growth, and is thought to prevent cancer cells from growing by preventing epidermal growth factor from binding to the cells, thus stopping them from stimulating the cells to grow. It is ideally dosed along with the drug irinotecan.

Another way in which MAbs can be used to treat cancer is by delivering radioisotopes in a specific way to the tumour. An example already on the market is Biogen Idec's Zevalin (ibritumomab tiuxetan), which is designed to treat non-Hodgkin's lymphoma. Zevalin combines a MAb directed at the CD20 antigen with the radioisotope yttrium-90. CD20 antigens are found on malignant B lymphocytes in patients with the disease, as well as normal mature B lymphocytes. Zevalin delivers a therapeutic dose of the radioisotope to the B cells, and the β emission from the radioimmunotherapy induces cellular damage through the formation of free radicals in the target and neighbouring cells.

limited half-life

However, yttrium-90 is less useful for solid tumours as it can take up to 48 hours for the radioisotope to penetrate into the tumour, meaning that, with a half life of 64 hours, it is likely to have decayed before it reaches its target.

A second radioisotope - MAb combination, GlaxoSmithKline's Bexxar (tositumomab) - is also designed for the treatment of non-Hodgkin's lymphoma. Here, the antibody is attached to iodine-131, which emits both beta and gamma radiation, with the beta radiation attacking the cancerous cells, and the gamma rays enabling imaging to be carried out. This allows the distribution and clearance of radiation from the body to be evaluated.

Monoclonal antibodies can also be used to target chemotherapy drugs. One of the biggest drawbacks of chemotherapy is the collateral damage that it causes to healthy tissues. A MAb could be used as a magic bullet to direct the cytotoxic drug to the tumour cells instead of healthy ones.

One example is Mylotarg (gemtuzumab ozogamacin), marketed in the US by Wyeth. This targets the CD33 molecule, an antigen that is present on the surface of cancerous cells in acute myelogenous leukaemia, but not on the normal stem cells that are involved in the formation of healthy bone marrow. It delivers the complex oligosaccharide cytotoxic agent calicheamicin, which makes double stranded breaks in DNA. It is highly toxic, so directing it in this way reduces damage to healthy cells.

adverse events

However, as with all drugs, there is always the risk that widespread use in patients after licensing will throw up side-effects that cast doubt on a product's future. This is just what has happened to Tysabri (natalizumab), a monoclonal antibody developed by Elan and Biogen Idec to treat multiple sclerosis. Brain lesions in MS patients are thought to result from autoimmune responses involving activated lymphocytes and monocytes. The glycoprotein α-4 integrin is expressed on the surface of these cells and is critical in their adhesion to the vascular endothelium and migration into the parenchyma. Natalizumab Biogen is a humanised monoclonal antibody

natalizumab that acts as an α-4 integrin antagonist. It has been shown to reduce the development in brain lesions in experimental models, but was voluntarily withdrawn in February this year following reports of two serious adverse events in patients who had been given Tysabri in combination with Avonex (interferon β-1a). They had developed progressive multifocal leukoencephalopathy (PML), a rare (and frequently fatal) demyelinating disease. Ongoing trials in MS, Crohn's disease and rheumatoid arthritis have also been suspended, but since then a patient on the Crohn's trial originally diagnosed as having malignant astrocytoma has been reassessed as suffering from PML.

The problems with Tysabri throw the potential drawbacks into sharp relief. 'The MAb action mechanism is also dependent on biological milieus and is degraded by body systems and not metabolised in a manner akin to smaller chemical drugs,' says F&S team leader Patrick Rajan. 'This subsequently makes their long term safety and efficacy evaluation difficult and risky.' However, the potential benefits are enormous, and research into monoclonal antibodies continues apace.