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What: Shares of Alexion Pharmaceuticals, Inc. (ALXN), a biopharmaceutical company focused on devastating and ultra-rare disorders, fell 23.5% in June, according to data from S&P Global Market Intelligence. Disappointing data suggests that the company's lead product, Soliris, won't be expanding its list of approved indications to refractory generalized myasthenia gravis (gMG).

So what: As one of the world's most expensive drugs, with a list price north of $500,000 per year, Alexion has positioned the expansion of Soliris' addressable patient population as a key component of its growth strategy. 

First approved in 2007, Alexion's Soliris is available for treatment of just two ultra-rare diseases, and it reached sales of $2.59 billion last year. Analysts predicted that the drug would produce about $5.5 billion by 2022, but a label expansion to the gMG population comprised a significant chunk of Soliris' peak estimates. 

When the company announced that the drug failed to reach its main goal of improved scores compared with a placebo on patient-reported tests of disease progression after 26 weeks of treatment, fears set in that the company's growth might not be achievable.

Now what: Alexion's management was quick to note success meeting secondary goals, especially a physician-administered assessment of gMG disease severity. With a lack of available therapies available for the refractory gMG population, both the medical community and Alexion would like to take another shot at an expansion to this population. 

Investors will want to keep their eyes open for news that the FDA might be willing to allow another trial with a different main goal, or other protocol tweaks.

Should the regulatory door slam completely on Soliris' expansion to the gMG indication, there are additional avenues for the drug's continued growth. The drug is in phase 3 trials for treatment of another ultra-rare disease that affects vision, and for delayed kidney transplant complications. 

Beyond Soliris, Alexion's Strensiq, approved last year as the first and only treatment for lethal, ultra-rare disease hypophosphatasia, is beginning to gain traction. It contributed $33.2 million to first-quarter sales, or nearly 5% of the company's top line for the period.

Alexion's $8.4 billion acquisition last year of Synageva BioPharma, and its lysosomal-acid lipase deficiency therapy Kanuma, is just beginning to contribute to the company's top line. The drug didn't earn approval until late last year, and the company recorded only $2.5 million in Kanuma sales during the first quarter.

Kanuma is expected to top the $1 billion mark, which would increase the company's total sales about 38% over last year's $2.6 billion. Alexion's recent phase 3 disappointment hasn't changed this drug's outlook. However, the stock's price-to-sales ratio of about 10.2 is lower than it's been since the company became profitable about seven years ago.

Maybe it's time to pick up shares of this promising rare-disease drugmaker while they're relatively cheap.