Alexion Pharmaceuticals Inc. said Tuesday that regulators have designated its experimental drug asfotase alfa as a breakthrough therapy for a rare metabolic disease that can cause progressive damage to vital organs.

The Food and Drug Administration gives breakthrough therapy status to speed the development of drugs that treat serious or life-threatening illnesses if the new medication could be a major improvement over existing treatments.

The disease at issue is hypophosphatasia, which is caused by an enzyme deficiency. It can cause skeletal deformity, severe muscle weakness, and progressive damage to vital organs. Asfotase alfa was deemed a breakthrough therapy for hypophosphatasia in patients whose symptoms appear before they turn 18. Alexion said it may also be able to obtain breakthrough therapy status for the drug in the treatment of adult-onset patients.

The drug is designed to normalize the patient's metabolic process and prevent or reverse the complications of the disease.

Shares of Alexion rose $1.78, or 1.8 percent, to $100.95 in midday trading.

Alexion acquired asfotase alfa when it bought Enobia Pharma in early 2012. The deal cost $610 million upfront, and the total could rise to $1.08 billion based on regulatory and sales milestones.

Alexion is based in Cheshire, Conn. Its only approved drug is Soliris. Soliris is used to treat paroxysmal nocturnal hemoglobinuria, which causes a breakdown of red blood cells and leads to anemia, and atypical hemolytic uremic syndrome, which often leads to kidney failure and death. Alexion reported $1.13 billion in Soliris revenue in 2012 and expects $1.51 billion to $1.52 billion in revenue in 2013.