Yesterday, Genentech (NYSE:DNA) and partner Biogen Idec (NASDAQ:BIIB) announced positive interim results from a small phase 2 trial testing the company's blockbuster drug, Rituxan, for the treatment of multiple sclerosis.

While light on the details, the companies did say that the drug showed a statistically significant reduction in the number of lesions compared with a placebo at 24 weeks. For such a small trial, consisting of only 104 patients, on the surface this is fairly impressive.

Rituxan, already approved to treat non-Hodgkin's lymphoma and rheumatoid arthritis, is in testing for a whole range of disorders besides multiple sclerosis. Last year, the drug was the best-selling oncology therapeutic in the United States, and sales are still increasing briskly, despite the drug's nearly 10 years on the market.

Rituxan U.S. Sales*

Y-O-Y change

Q2 06

526

17%

Q1 06

477

8%

Q4 05

484

13%

Q3 05

456

16%

Q2 05

450

15%
*In millions.

As you can see from the rapidly growing sales, whether or not Rituxan gets approved for additional indications, the drug will still be a blockbuster for Genentech and Biogen Idec.

Drugs to treat MS operate in a crowded market. Besides the six already-marketed MS drugs, there are quite a few new ones in development, including candidates from Rule Breakers pick PDL Biopharma (NASDAQ:PDLI) and Novartis (NYSE:NVS).

MS is a chronic disease that affects more than 350,000 people in the U.S. alone, with 10,000 new patients diagnosed annually. Rituxan's unique mechanism of action compared with other MS drugs already on the market would be a great marketing point if the drug can get approved to treat the disease.

In reality, though, Rituxan faces a difficult path to approval, littered with clinical trial failures by other companies -- most recently NeurocrineBiosciences (NASDAQ:NBIX) and its altered peptide ligand technology in March. Any potential approval is at least three to four years (and several large clinical trials) away, and the FDA has been quite conservative lately in reviewing drugs to treat chronic indications.

Getting Rituxan eventually approved to treat MS would bring meaningful amounts of revenue to Biogen and Genentech. However, conclusive results on the drug's effectiveness in this indication will take years to come to fruition.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article and welcomes your feedback. The Motley Fool has a disclosure policy .