What is Tysabri and why was the drug withdrawn from the market

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Natalizumab (Tysabri) is a monoclonal antibody to an adhesion molecule (VLA-4) that was approved November 2004 on the basis of a marked (66%) reduction of MS exacerbations, a 91% reduction in MRI gadolinium-enhancing brain lesions, and a good safety and tolerance profile. This reduced exacerbation rate was sustained through the entire 2 years of the study. In addition, these effects were accompanied by an impressive 42% reduction in the risk of sustained progression of disability for the 2 years of the study. The reduction in the risk of attacks was achieved by selectively blocking one, and only one, Velcro-like molecule on lymphocytes preventing them from attaching to the inside of brain and spinal cord (cerebral) blood vessels. Extrapolating from animal studies, this action effectively prevented the cells from crossing the blood-brain barrier. The dramatically superior effectiveness of Tysabri in preventing attacks of MS, which was also seen after 2 years of therapy, confirmed the importance of blood lymphocytes and macrophages in their role in attacks of MS, and also proved that this step (attachment via a specific adhesion molecule VLA-4) is a central factor in attacks of MS.

The apparent synergism of Tysabri and Avonex in suppressing immune function sufficiently to allow PML to occur in two MS patients on combined therapy, but not in any patient receiving Tysabri monotherapy, is a demonstration of the potency of blocking the adhesion molecule (VLA-4). The outcome of combining Tysabri and Avonex is also a demonstration of the potential hazards of combining other apparently safe effective therapies with a more potent therapy. As a consequence of the findings, Biogen-Idec and Elan Pharmaceuticals have withdrawn Tysabri from the market. All of the facts surrounding the appearance of PML in Tysabri-treated patients have been presented to an independent panel. The panel's recommendations and FDA action will be the basis of the use of the drug in the future. It is safe to say that any use will not include combination therapy with interferon-beta of any type.

92. What are the side effects of the drugs that are used to prevent attacks? Why should I take drugs that have side effects?How do the side effects compare?

The approved drugs reduce the risk of exacerbations as well as the risk of disability and were FDA approved because of their safety as well as their effectiveness. However, most recipients of the drugs available for MS treatment (Betaseron, Avonex, Rebif, and Copax-one) do experience side effects. Flu-like symptoms occur in the majority of patients early in interferonbeta therapy regardless of which one is chosen. Generally, they are more prominent for higher dose interferon-beta (Betaseron and Rebif) and less for low dose interferon-beta (Avonex) and for Copaxone. The higher dose interferons, however, have a more rapid onset of benefit, as judged from MRI studies and the pivotal drug study results.

Local reactions to injections under the skin (subcutaneous injections) are less frequent for Copaxone, about half as common as compared with injections of interferon-beta (Betaseron and Rebif). Localized redness of the skin decreases over time but almost always persists to some degree in the majority. Most patients readily accept it as a nuisance. A few people will develop little dents in the skin in areas where the drug has been injected, similar to areas where insulin has been administered in diabetics. Others will develop hard nodules under the skin at the injection site. Some patients who are given Avonex or Rebif experience some stinging sensation with injections, which may be due to the acidity of the solution.

Copaxone, despite daily injections, is associated with fewer side effects than the interferon-beta products, but the full benefit is delayed, as it is with Avonex. Copaxone induces a local reaction in about 40% of the patients; half complain of some pain at the injection site. This again does not appear to be a major problem. A few patients develop prominent skin rashes. Approximately one quarter of patients experience a transient feeling of anxiety or shortness of breath, which is ordinarily a minor issue, but occasionally, it is more severe and longer lasting.

In summary, it can be fairly stated that the side effects from Betaseron and Rebif are initially more prominent but tend to subside sooner than in patients treated with Avonex. Generally, the side effects from Copax-

one are less severe. For all of the products, there are considerable differences from one individual to another. The reason why certain patients tolerate one drug better than others remains unknown.

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