Shares of the Waltham, Mass. were up 18.9% to $52.60 at last check.
Apellis said that the U.S. Food & Drug Administration had approved Empaveli, which the company said was the first and only targeted C3 therapy for treatment of adults with paroxysmal nocturnal hemoglobinuria, or PNH. The C3 gene provides instructions for making a protein that plays a key role in a part of the body's immune response.
PNH is a rare, chronic, life-threatening blood disorder caused by an acquired mutation, Apellis said, which leads to uncontrolled complement activation and the destruction of red blood cells through intravascular and extravascular hemolysis.
The approval is based on the results of a head-to-head Phase 3 Pegasus study, where Empaveli demonstrated "superiority to the medication Soliris for the change from baseline in hemoglobin level at Week 16," the company said.
Hemoglobin is a protein that carries oxygen via blood.
Alexion shares were up nearly 1% to $174.71 at last check.
Empaveli also did better compared to Soliris on the endpoint of transfusion avoidance, Apellis said.
The company said 85% of Empaveli-treated patients were transfusion-free over 16 weeks, compared with 15% for those treated with the medication Soliris.
"This approval represents a major scientific advancement as Empaveli ushers in the first new class of complement medicine in almost 15 years," Cedric Francois, co-founder and CEO, said in a statement.
The most common adverse reactions to the treatment included injection site reactions, diarrhea and abdominal pain.
The European Medicines Agency is currently reviewing the marketing authorization application for Empaveli, with the potential for a European Commission decision in the second half of the year.
Apellis said last year that its treatment had outperformed Soliris in a comparison test.
Alexion said in a statement that Apellis' Pegasus "data showed an increase in hemoglobin in a narrow patient population."
The company said its Ultomiris treatment "has been established as the new standard of care for patients with PNH, with efficacy that was demonstrated in a diverse, real-world PNH patient population across the two largest PNH trials completed to date."
Alexion agreed to the $39 billion takeover last year.