Drug developers Biogen Idec Inc. and Elan Corp. said Thursday data from a late-stage study of Tysabri showed it was effective in reducing cognitive degeneration in multiple sclerosis patients.

The drug was recalled three months after approval in November 2004 because of links to a fatal brain disease called progressive multifocal leukoencephalopathy. Patients died in two of three cases found.

Biogen,a biotech company headquartered in Cambridge, Mass. (Nasdaq: BIIB) and Dublin-based Elan (NYSE: ELN) worked with regulators on a program called Touch Prescribing. Patients are now prescribed the medication as part of that program and patients are monitored for the brain inflammation.

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In the latest study, which was presented to a European commission and included 942 patients worldwide, treatment with Tysabri reduced the risk of sustained cognitive worsening by 43 percent when compared to a placebo.

The drug has previously shown a 68 percent reduction in the relapse rate of patients and a 42 percent reduction in the relative risk of disability progression.

Areas affected by cognitive dysfunction include memory, ability to process information and learning. The drug reduced the risk of cognitive dysfunction by 43 percent, the study showed.

Elan shares closed down .39% to 15.48 in trading on the Nasdaq.