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Natalizumab is a humanized monoclonal antibody against the cell adhesion molecule α4-integrin. It is used in the treatment of multiple sclerosis and Crohn's disease. It is marketed as Tysabri, and was previously named Antegren.

The drug is believed [sic] to work by reducing the ability of inflammatory immune cells to attach to and pass through the cell layers lining the intestines and blood–brain barrier.

Natalizumab was approved in 2004 by the U.S. Food and Drug Administration (FDA) after years of testing. It was subsequently withdrawn from the market by its manufacturer after it was linked with three cases of the rare neurological condition progressive multifocal leukoencephalopathy (PML) when administered in combination with interferon beta-1a, another immunosuppressive drug often used in the treatment of multiple sclerosis.

After a review of safety information and no further deaths, the drug was returned to the US market in 2006 under a special prescription program.

As of June 2009, ten cases of PML were known. However, twenty-four cases of PML had been reported since its reintroduction by October 2009, showing a sharp rise in the number of fatalities and prompting a review of the chemical for human use by the European Medicines Agency.

 By January 2010, 31 cases of PML were attributed to natalizumab.

 The FDA did not withdraw the drug from the market

In the European Union, it has been approved for human use only for the treatment of multiple sclerosis and only then as a monotherapy because the initial cases of PML, and later “the fatalities, were said by the manufacturers to be linked to the use of previous medicines by the patients”.

Biogen Idec announced the initiation of the first clinical trial of natalizumab as a potential cancer treatment as of September 5, 2008

Time on Tysabri when people have Death :

Time on Tysabri when people have Hallucination  :