One monoclonal antibody, natalizumab (Tysabri ), was shown in clinical trials to significantly reduce the frequency of attacks in people with relapsing forms of MS and was approved for marketing by the U.S. Food and Drug Administration (FDA) in 2004.  However, in 2005 the drug's manufacturer voluntarily suspended marketing of the drug after several reports of significant adverse events.  In 2006, the FDA again approved sale of the drug for MS but under strict treatment guidelines involving infusion centers where patients can be monitored by specially trained physicians.