Shares of Alexion Pharmaceuticals (ALXN) are down 9% in after-hours trade after the company announced that it had failed a phase 3 trial. The phase 3 trial, known as the REGAIN study, was testing the company's drug Soliris in patients with refractory generalized myasthenia gravis (refractory gMG.)

The rare disease, refractory gMG,is characterized by muscle weakness throughout the body causing a multitude of symptoms such as slurred speech, shortness of breath, extreme weakness, and many others.

The primary endpoint was measuring the change in baseline over 26 weeks  in patients' activities of daily living profile -- MG-ADL. It did not reach statistical significance aimed for. While this miss hurts the company in the short-term, the long-term story remains intact. Soliris is already approved by the FDA to treat two other rare diseases. This means that this drop creates an excellent buying opportunity.