Investors in biotech Alexion Pharmaceuticals (NASDAQ:ALXN) had been having a rough go of it before Thursday morning. Seemingly, no sooner did I name Alexion as a promising biotech stock to watch than the company reported disappointing results in a high-profile coronary bypass clinical trial.

That trial all but ended hopes that the company and its partner, Procter & Gamble (NYSE:PG), would see approval of the drug in question, pexelizumab, for coronary bypass, post-MI angioplasty, or anything else, for that matter. And as you might imagine, investors and traders did not look kindly upon the announcement.

But a swift stroke of good news came into the picture again on Thursday. The company announced results from the phase 3 TRIUMPH study -- a study of the use of eculizumab in the treatment of paroxysmal nocturnal hemoglobinuria (PNH) -- and the results were very encouraging indeed.

PNH is a rare genetic disease that essentially allows the body to attack its own red blood cells. That, in turn, leads to severe hemolysis, anemia, and a host of other co-morbidities ranging from pain and fatigue to sexual dysfunction. It also greatly increases the risk of forming deadly blood clots and half of those diagnosed with PNH are expected to die within 10 years or so of initial diagnosis. What's worse, there's no real treatment options other than fighting the symptoms through means like regular blood transfusions.

Looking at the results, the median transfusion rate in the treated group was reduced to zero, and about half of the patients achieved hemoglobin stabilization. By comparison, the median result in the placebo group had patients getting transfusions of 10 units per patient and none of them achieved stabilization. It's also worth noting that the drug scored well on quality-of-life assessments.

With this data in hand, the company is expecting to file for approval in the second half of this year and intends to call the drug Soliris (far, far easier to spell than eculizumab). At present, the company plans to market this drug themselves (they hold full rights), but I would be stunned if Genzyme (NASDAQ:GENZ) didn't at least try to get these folks on the phone. While the number of people suffering from PNH is very low (around 9,000 in North America and Europe), the price per year is likely to be extremely high ($100,000-$200,000), marking it as one of those "super orphan drugs" that Genzyme has done so well with in the past.

Hopefully this is just the beginning of the story for Alexion. Not only does Soliris have other potential uses (in transplants, for severe asthma, and maybe for some cancer patients), but success with PNH could help recharge and restock the pipeline. For you long-patient Alexion investors, enjoy the day -- this is what makes biotech investing fun and exciting.

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Fool contributor Stephen Simpson has no financial interest in any stocks mentioned (that means he's neither long nor short the shares).