Earlier in the week, Genzyme (NASDAQ:GENZ) released the top-line results of a three-year phase 2 study on alemtuzumab, a drug candidate for treating relapsing-remitting multiple sclerosis. This drug would be entering a significant market, but where it might fit in with its competitors Isn't clear.

Alemtuzumab, trade name Campath, has been approved in the U.S. since 2001 to treat a form of leukemia. In the phase 2 study, alemtuzumab was successful in reducing the risk of MS relapse after three years of follow-up, and in lowering the potential for disability progression compared with Merck Serono's already approved Rebif.

When earlier results of the study were released last year, I figured the drug would be dead in development because of its side effects. But that's apparently not the case; no new, serious side effects have been reported in the past year.

Based on the strong efficacy of alemtuzumab, Genzyme is now running two phase 3 studies on it. One will test it as a salvage therapy for patients failing current MS standards of care, and another will test it as a front-line therapy in the disease versus Rebif.

One of the interesting aspects of alemtuzumab is its convenient dosing schedule. Already approved MS treatments need to be given up to three times a week, but alemtuzumab has to be administered only a couple of days a year. That's even better than the once-a-month dose required of Elan's (NYSE:ELN) Tysabri.

In 2005, sales of the top four relapsing-remitting MS treatments totaled more than $5 billion. By 2011, there could be a host of new treatments for the disease on the market, including Novartis' (NYSE:NVS) oral pill FTY720, PDL BioPharma (NASDAQ:PDLI), and Biogen Idec's (NASDAQ:BIIB) daclizumab.

If they all get approved, they will each probably be assigned some treatment niche and thus expand the market, just as the four blockbuster MS treatments to date -- Avonex, Betaseron, Rebif, and Copaxone -- have done.

The million-dollar question is where these new drug candidates, including alemtuzumab, will find their spot in the treatment regimen.

PDL is a Rule Breakers newsletter selection. Take a look at all of the service's recommendations with a free 30-day trial.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Biogen is a Stock Advisor selection. The Fool has a straight-A disclosure policy.