Denise Campagnolo, MD, MS, Director, Clinical MS Research, Barrow Neurological Institute, Phoenix, Arizona

Introduction

Monoclonal antibodies (MABs) are a new class of therapy now used to combat multiple sclerosis (MS). There are several advantages of MABs compared to the other classes of drugs. At the heart of this advantage is the fact that MABs are designed to recognize specific unique antigens. Antigens are usually proteins that provoke an immune response. In MS, MABs provide a way to target very selective molecules that might prevent at least some if not all of the autoimmune attack on myelin.

What Are MABs and How Are They Made?

Antibodies, in general, are proteins that are found in blood that are usually used by our immune systems to both identify and neutralize foreign invaders, such as bacteria and viruses. They have a unique structure that has a “business end” responsible for binding to and recognizing the antigen in the bacterium or virus infected cell. Antibodies are produced by cells called B-cells and plasma cells (you will hear more about these later). Monoclonal antibodies in the context of treating MS have an entirely different role. MABs are produced by injecting a mouse with the target antigen of interest, which in the case of MS, is one of the proteins that signal the immune system to attack myelin. In its body, the mouse makes an antibody response against this specific target protein. The B-cells and plasma cells are taken from the mouse’s spleen and combined with a tumor cell to create a “hybrid cell line” that lives on and on (immortalized) and continues to produce the very unique and specific (monoclonal) antibody that recognizes the antigen of interest. Once the hybridized immortalized B-cell that produces a MAB of interest is identified, it can be made to multiply its numbers, and thus produce the mouse monoclonal antibody in quantity. It is then purified for use as a therapy.

As you might guess, mouse MABs are not very useful as therapies in people because they are foreign to the human body and will produce their own immune (allergic) reaction if recurrently injected into a person. To avoid this problem, a new chimeric antibody is made by attaching just the portion of the mouse antibody that recognizes the antigen (‘the business end’), onto a human antibody base unit. This chimeric antibody is subsequently produced and purified and tends to produce less of an immune response. You can take it a step further by taking only the very smallest part of the mouse antibody––only the smallest part that recognizes the antigen––and fuse that to almost the whole human antibody backbone. The result is a “humanized” MAB which incites the least immune reaction, or allergy, in the person it is injected into.

You might be wondering how MABs are named. The naming scheme for assigning generic names (International Nonproprietary Names) to monoclonal antibodies was developed by the World Health Organization and the United States Adopted Names Council. In general, suffixes are used to identify a class of medicines; thus all monoclonal antibodies end with the suffix -mab. Different infixes (the syllables in the middle of the word), however, are used depending on the structure and function of the drug. The first syllable is a unique prefix developed by the pharmaceutical company. The next syllable describes the target that the antibody was developed to treat, for example “tu” is for tumor antibodies, “li” is for immune cell antibodies and so forth. The third infix is the source of the antibody, if it is mouse antibody “mo” is used, if it is humanized as described above, “zu” is used, and if it is chimeric “xi” is written. As we mentioned above, the last syllable is always the suffix “mab”, standing for monoclonal antibody.

Currently Approved MABs for Treatment of MS

Currently, there is only one FDA–approved MAB for the treatment of MS, natalizumab (Tysabri®). Natalizumab was developed by taking antibodies from a mouse that had been injected with human immune cells. These antibodies were “humanized” in the process described above, thus the “zu” in the name. Two primary cells in the immune system of mice and humans are lymphocytes and monocytes. These cells are involved in the autoimmune reaction that leads to demyelinating lesions in MS. These cells cross the blood-brain barrier, or transfer from the blood stream into the brain, only with the help of a molecule called VLA-4 or alpha 4 Beta 1 integrin. Usually lymphocytes and monocytes roll along the outside walls of blood vessels. When lymphocytes are activated, however, the VLA-4 expressed on their surface can bind to VCAM-1 (one of the integrin molecules) expressed on the wall of blood vessels. The binding of VLA-4 to VCAM-1 allows the cells to stick to the blood vessel wall, and to migrate across the vessel to enter the brain. There they mount an attack against myelin. Natalizumab is a MAB that recognizes and blocks the VLA-4 molecule, thus preventing immune cells from crossing the blood-brain barrier into the brain and spinal cord. Thus, MS attacks are slowed or halted.

Unfortunately, there is a risk of a serious brain infection associated with natalizumab use. This infection, called progressive multifocal leukoencephalopathy (PML), is an opportunistic brain infection that usually results in death or severe disability. Although there were initially three cases of PML identified, no further cases have been reported with natalizumab after its re-approval by the FDA in the summer of 2006. The distribution of natalizumab is still conducted under a restricted distribution program called the Tysabri Outreach: Unified Commitment to Health or TOUCH program, so that the risk for PML can be more precisely estimated. To be treated with natalizumab patients must be enrolled in the TOUCH program and the clinical sites that infuse the drug also have to be enrolled and TOUCH certified.

At the time of this writing, a new complication associated with use of Tysabri has been identified. That is development of liver injury, including markedly elevated blood levels of liver enzymes and high levels of bilirubin. Bilirubin is made when red blood cells break down. Too much bilirubin can cause yellowing of skin called jaundice. Patients on Tysabri should have their liver function evaluated with blood tests according to their doctor’s recommendations.

MABs Under Investigation for Use in MS

Rituximab

To understand how rituximab might work in MS, we must first consider the role of Bcells in MS. As we said earlier, lymphocytes are one of the important immune cells that orchestrate the damage in MS. There are several types of lymphocytes, among them the T-cells and the B-cells. Traditionally MS has been thought of as a disease mediated by T- cells. The importance of the role of B-cells in the pathogenesis of MS is now better understood. In other autoimmune diseases like Rheumatoid arthritis, Sjogren syndrome, Crohn’s disease, and Hashimoto thyroiditis, there is formation of clusters of lymphocytes (called lymphoid follicles) in the damaged tissues. When these lymphoid follicles are formed away from their usual location, they are referred to as ectopic. These lymphoid follicles are sites where B-cells, as well asmature B-cells called plasma cells, encounter antigen, and interact with other cells of the immune system making antibodies. It has long been recognized that antibody production is part of MS, resulting in elevated antibodies (IgG) and oligoclonal bands in the cerebrospinal fluid of those with the disease. What was unknown was whether this IgG production was just a by-product of the MS process or at the heart of it. Lymphoid follicles suggest that B-cells play an important role in the disease.

In a study by Serafini in 2004, it was shown that there were B-cell follicles in brains from persons with MS. This study was performed on post mortem brains from persons with different subsets of MS–Primary Progressive MS (PPMS), Secondary Progressive MS (SPMS), and Relapsing-remitting MS (RRMS). The study further showed the presence of ectopic B-cell follicles in the CNS, primarily under the outer covering of the brain, in two third of SPMS brains. Other important cells like plasma cells and T-lymphocytes, were also found in the ectopic B-cell follicles. Using special staining techniques and markers, it was demonstrated that the B-lymphocytes stimulated by antigen also multiply and are amplified in the follicles. This initial finding supported the notion that B-cell follicles were involved in the MS disease process. Another study by this same group of investigators in 2007, determined the association of the distribution of ectopic B-cell follicles with the clinical course of MS. The study showed that patients with SPMS who had B-cell follicles had a more severe disease course. An association was found between the location of the ectopic B-cell follicles, the location of the inflammatory process, and brain injury. These studies suggest that B-cell lymphoid follicles within the CNS could be an important target for therapeutic intervention. Depleting B-cells in rheumatoid arthritis using the MAB rituximab drastically improves symptoms in many patients. Its use in MS is now under study.

Rituximab (Rituxan®) is a MAB that targets and selectively reduces the numbers of B-cells in the body, without targeting stem cells or existing plasma cells. Rituximab first received FDA approval in 1997 for the treatment of B-cell non-Hodgkin’s lymphoma. Since it originally targeted lymphoma tumors, we get the “tu” in its name. In February 2006, rituximab received FDA approval to also be used for the treatment of other lymphomas and moderately-to-severely active rheumatoid arthritis.

Rituximab is being studied in both RRMS, and in PPMS, for which there is currently no FDA-approved therapy. Rituximab is also being studied in other autoimmune diseases. In a study published in The New England Journal of Medicine in February 2008, Dr. Hauser and colleagues report the results from a Phase II study of rituximab in RRMS. This study of 104 patients showed a statistically significant reduction in the total number of gadoliniumenhancing MS lesions observed on MRI scans of the brain at weeks 12, 16, 20, and 24 in the Rituxan-treated group compared to placebo (Hauser et al., 2008). This finding in this small Phase II trial supports the idea that B-cell targeted therapy may play an important role in the treatment of MS.

All drugs have risks and benefits. The risks associated with rituximab relate to infections, like gastroenteritis, bronchitis, upper respiratory tract infections, urinary tract infections, and sinusitis. Also common with rituximab were “first infusion-related” reactions, the majority of which were mild to moderate and were generally reversible by stopping the infusion and giving medicines like antihistamines and steroids. These can be given before infusion to prevent these problems. Rare severe infusion reactions are listed in Table 1. The pharmaceutical companies continue to monitor the long-term safety of rituximab treatment. Other serious or potentially lifethreatening adverse reactions that have been reported following rituximab therapy include Hepatitis B reactivation with fulminant hepatitis, hypersensitivity reactions, andc ardiac arrhythmias. In addition there were two cases PML in patients with systemic lupus erythematosus who received rituximab therapy.

Alemtuzumab

Alemtuzumab is a humanized (thus the ‘zu’ in its name) MAB that targets the CD52 antigen present on all types of lymphocytes as well as monocytes and macrophages. Alemtuzumab causes rapid and long-lasting lymphocyte reduction and is approved for the treatment of B-cell chronic lymphocytic leukemia (thus the ‘tu’ in its name) in many countries under the name of CAMPATH or MabCAMPATH. It is under study in the United States for use in MS.

Alemtuzumab can also bring about adverse reactions while infusing the drug, similar to those mentioned in Table 1 for rituximab. Additionally, an unexpected adverse event occurred in 27% of the SPMS group who developed Graves’ disease which is an autoimmune hyperthyroidism. The development of Graves’ disease was thought to occur because of the changed structure of the immune system and loss of self-tolerance. The high rate of Graves’ disease was not seen in a Phase II study. Another adverse effect has been noted in an ongoing Phase II, randomized trial comparing high-dose and low-dose alemtuzumab. This is a disease called idiopathic thrombocytopenic purpura (ITP) in 3 patients with 1 fatality. ITP causes bleeding; it can be controlled with steroids.

Daclizumab

Daclizumab is a MAB directed against interleukin 2 (IL2) receptor. IL2 is a chemical produced by T-cells that plays a role in their activation and multiplication. The “li” in daclizumab refers to immune system as its target and “zu” refers to the humanized form in which it is used. Daclizumab is used in combination with other agents to prevent kidney transplant rejection. Two studies were done in patients with MS to see the efficacy of daclizumab compared to placebo in those treated in combination with interferon. Although, daclizumab showed some beneficial effects, both studies were limited by small numbers of study subjects. There were some adverse effects noted like urinary tract and respiratory tract infections, transient liver abnormalities, and altered sensation. The long-term safety of daclizumab has not been assessed. Currently, a Phase II, one-year open label study is ongoing.

Conclusion

In summary, MABs have demonstrated themselves to be potent treatment options for MS. Their ability to target very specific molecules derails the autoimmune process by influencing a specific target cell (B-cell, lymphocytes in general) or receptor (VLA-4, or IL2). They have the ability to reduce inflammation and thus reduce relapses effectively. They have potentially serious side effects that should be taken into consideration. They all must be used with appropriate caution and in selected patient populations. Particularly when compared to chemotherapies and steroids, however, they represent a significant advance in therapies for autoimmune diseases including MS.

References