On Monday, Biogen IDEC (Nasdaq: BIIB) and Elan (NYSE: ELN) received an additional FDA approval for their multiple sclerosis drug Tysabri. The new approval allows Tysabri to be used as a treatment for Crohn's disease.

Biogen and Elan have had a long and winding path in getting Tysabri approved to treat Crohn's. After Tysabri was pulled from the market in early 2005, a Crohn's disease patient was the third and last-known person to be infected with the rare brain disease that held up the drug's reintroduction onto the market to treat multiple sclerosis in 2006. More testing showed that the risk of acquiring this rare brain disease was probably limited to patients who use Tysabri in combination with other drugs that are immunosuppressants.

Last year, European drug regulators rejected Biogen's proposed Crohn's disease application for Tysabri, but Biogen and Elan were more successful with the FDA. After first receiving a three-month extension to the FDA's review of the drug in October, they finally got full approval this week.

Getting Tysabri approved for Crohn's disease isn't going to significantly change its revenue potential. It will provide some additional sales, but the Tysabri labeling for this indication is relatively strict, as it's only approved for use after patients have failed other Crohn's disease treatments like Abbott Laboratories' (NYSE: ABT) Humira and Johnson and Johnson's (NYSE: JNJ) Remicade.

Since the end of the third quarter last year, Biogen has seen a 24% increase in the number of patients on Tysabri to 21,000 patients worldwide. Biogen and Elan have forecasted that 100,000 patients will be on Tysabri therapy by the end of 2010. Biogen has stated that Crohn's disease patients will represent only a "modest" slice of this 100,000 guidance, so we probably won't see much change in the trajectory of the Tysabri patient uptake.

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