Elan said the number of people using Tysabri has risen 54% over the past year to 39,300, slightly more than half of them in the United States, most of the rest in the European Union. Elan's share of first-quarter Tysabri income totaled $158 million, up 48%. It said every 10,000 users of Tysabri represents $100 million in profits for Elan. It cited data published last month in the Lancet medical journal that said 37% of MS patients using Tysabri had remained free from disease progression for two years, compared with 7% of those taking a placebo. Research has shown Tysabri works better than long-established treatments but it has faced regulatory and marketing hurdles because of its link to a rare, usually fatal brain-swelling disease called PML. Elan and Biogen initially pulled Tysabri from the U.S. market in February 2005 when it reported three cases of PML in Tysabri users, two of whom died. American and EU regulators cleared Tysabri for sale in mid-2006 but under much stricter conditions, citing its PML risk. Biogen said last week that regular scans of Tysabri users had detected a sixth confirmed case of PML since the drug's reintroduction in 2006. Elan and Biogen had already informed investors of five of those cases from August 2008 to February 2009. Both companies have declined to specify whether any of the recent PML victims have died. Biogen specified that the sixth PML victim had been taking Tysabri for 31 months, longer than the previous five.