It wasn't quite as exciting as the 870% returns Acorda Therapeutics (NASDAQ:ACOR) has delivered since its lows in September 2006, but I'm sure investors in the drug developer weren't complaining about the 31% pop they got yesterday.

The excitement, which is spilling over into today, stemmed from the company's multiple sclerosis (MS) treatment Fampridine-SR. The drug, developed using Elan's (NYSE:ELN) extended-release technology, doesn't lower the severity or frequency of attacks, as drugs from Biogen Idec (NASDAQ:BIIB), Pfizer (NYSE:PFE), and Teva Pharmaceutical (NASDAQ:TEVA) try to do. Instead, it treats one of the most potentially debilitating symptoms of MS -- a decrease in the ability to walk.

And it treats that problem quite well. Fampridine-SR passed its primary endpoint with flying colors in helping MS patients walk at a more rapid pace. Nearly 43% of the patients taking the drug were able to walk faster while taking the drug than the fastest they were able to manage before taking it. That's much better than the 9.3% of subjects taking a placebo who seemed to improve.

Acorda had already worked out a Special Protocol Assessment (SPA) with the FDA, so the agency should be OK with the efficacy portion of Acorda's marketing application. The drug has some side effects that were more common in subjects taking Fampridine-SR than in those taking a placebo, but they were relatively minor. For instance, the drug caused insomnia in 10% of the subjects. There had been some worry that the drug caused seizures, but that concern seems to have been laid to rest -- there were no seizures seen in subjects taking Fampridine-SR in this study.

Acorda is shooting for filing a marketing application with the FDA in the first quarter of next year. It plans to ask the agency for a priority review, and it should get one, since there aren't any drugs approved specifically to treat MS patients' walking difficulties. Priority status would put the drug on a six-month review schedule, and it could be on the market by the end of next year.

With a market cap pushing $1 billion and gross sales of just $39 million for its one marketed drug, Zanaflex, last year, Acorda knows its investors are putting a lot of hope in Fampridine-SR. The data looks good enough to gain an FDA stamp of approval, so now the only question is whether the drug will meet investors' high expectations.

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