Image source: Alexion Pharmaceuticals.

Investors in rare disease-focused Alexion Pharmaceuticals (NASDAQ:ALXN) were treated with some good news late on Friday afternoon as the company officially announced that the FDA has cleared its new drug, Strensiq, for sale. And yet, the stock has been practically flat since the news was released.

For background, Strensiq is an enzyme replacement therapy used to treat Hypophosphatasia, or HPP, an ultra-rare, life-threatening genetic disease. Patients with HPP do not properly produce a key enzyme that helps maintain proper calcium and phosphate levels in their bodies. Without the enzyme, patients with HPP cannot adequately regulate their calcium and phosphate levels, which causes a wide range of unwanted effects including bone deformity, muscle weakness, and seizures. Over time, the disease can even lead to organ failure and death. 

In clinical trails, Strensiq was shown to be an effective therapy that improved numerous clinical outcomes in patients with HPP, and its approval finally offers patients a real treatment option.

Strensiq is expected to become available to patients starting on October 27, 2015.

Free prize inside!
In addition to the approval announcement, Alexion also appears to have gotten its hands on a much-coveted Rare Pediatric Disease Priority Review Voucher, which gives Alexion the ability to at some point in the future speed up the FDA approval process of a rare pediatric disease drug. Given that the company's current pipeline includes eight product candidates in clinical trails for 11 different indications, and another 30 compounds in preclinical work, that voucher could certainly come in handy down the road.

The company could also sell the voucher on the open market for a hefty price tag; another voucher recently traded hands for $350 million.

Another acquisition pays off
This approval once again shows off Alexion's acquisition chops, as it originally got its hands on Strensiq through a $1.08 billion buyout of Enobia Pharma in late 2011. At the time, asfotase alfa, which is now called Strensiq, was only in Phase 2 clinical development, so Alexion was taking a bit of gamble when it acquired Enobia.

It looks like that decision could pay off in a big way, as even though HPP is an exceedingly rare disease that affects fewer than 20 people per one million, Alexion has been reported to charge around $293,000 per year per patient in Japan, so its likely to have a similar price tag in Europe and the U.S. Given that, even a small addressable patient population sales can allow for big-time sales.

Analysts currently believe Strensiq could reach between $500 million to $1 billion in peak sales. Seeing the success of this acquisition should certainly help investors feel better about the company's decision to purchase Synageva BioPharma earlier this year, which set it back a cool $8.4 billion.

Investors shrug
Still, despite releasing this good news late Friday afternoon, Alexion's shares are actually trending lower today, showing that the market was expecting this approval to sail through the FDA approval process without any problems. That's certainly understandable; since Strensiq is also approved in the European Union, Japan, and Canada, investors had no reason to assume there would be any regulatory trouble stateside. With shares currently trading hands for around 58 times trailing earnings, it's clear the market is already pricing in a strong amount of growth for the company.

In addition, analysts appear to have been expecting Strensiq to sell at a price tag of more than $400,000 per year, so its possible the drug's peak sales estimates may actually be a bit higher than reality.

Still, a win is a win, and this investor thinks Alexion shares are an attractive investment right now. Alexion has been a phenomenal stock to own for years, and I for one think biotech investors should give it a look, even at today's triple-digit price tag.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.