Biogen Idec ( BIIB) and Elan ( ELN) have started enrolling patients in a safety study of Tysabri, the multiple sclerosis drug that was pulled from the market last year after being linked to a rare brain disease. The companies, seeking to return Tysabri to the market, said Wednesday that they have dosed the first patients in a safety extension study program. Biogen and Elan had previously pulled all clinical trials of Tysabri after the drug was linked to two cases of progressive multifocal leukoencephalopathy, a sometimes fatal brain disease. The companies conducted a safety review on more than 3,000 patients, and no additional cases of the disease were found. The Food and Drug Administration is mulling whether to allow the drug back on the market based on its efficacy, which a number of patients attested to in an emotional public hearing at a meeting of an FDA advisory committee earlier this month. According to some patients, after only one dose of Tysabri, they saw significant improvements in movement and ability to think. Multiple sclerosis is a degenerative disease of the central nervous system that causes loss of motor skills and cognition. Following the hearing, the committee recommended that the drug be allowed to return to the market. The FDA is not required to follow advisory committee suggestions, but it often does. Last week, the FDA said it wanted another 90 days to review Tysabri's return to the market. The agency is expected to decide on the matter by June 28. Biogen shares dipped 10 cents in late trading after rising $1.36 to $47.22 during the regular session. Elan shares advanced 10 cents after hours, after trading up 49 cents to $14.59 during the day.