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Natalizumab (Tysabri) was approved by the U.S. FDA to treat relapsing forms of multiple sclerosis (MS) in November 2004. On February 28, 2005, the manufacturer (Biogen Idec) suspended all use of the drug after discovering progressive multifocal leukoencephalopathy (PML; an opportunistic brain infection caused by JC virus) in two people who received natalizumab in clinical trials.
Natalizumab, a monoclonal antibody directed against the α4 subunit of α4β1 and α4β7 integrins, prevents immune cells from docking with the VCAM receptor on the vascular endothelium and penetrating the CNS. The FDA had expedited approval of the drug on the basis of highly positive 1-year efficacy data from two Phase III placebo-controlled clinical trials. In both the…