(Corrects headline and paragraphs 1, 2 to show only one of the two drugs approved by European regulators was acquired in the Synageva deal)

By Bill Berkrot

NEW YORK, Sept 1 (Reuters) - Ten weeks after closing its $8.4 billion purchase of Synageva BioPharma Corp, Alexion Pharmaceuticals Inc on Tuesday announced European approval for the key drug acquired through the deal, an enzyme-replacement therapy for an ultra-rare, life threatening disease.

European health regulators approved Kanuma for patients with lysosomal acid lipase deficiency (LAL-D). They also approved Strensiq for patients with pediatric-onset hypophosphatasia (HPP), a drug that Alexion had previously acquired. The treatments are awaiting U.S. approval decisions after receiving breakthrough therapy designations from the Food and Drug Administration.

"This is sort of a momentous year for us," Alexion Chief Executive David Hallal said in an interview. "We're moving from a one-drug company to a three-drug company, eight, nine years after the launch of our first drug."

LAL-D is a progressive metabolic disease that often leads to liver failure, multi-organ damage and premature death. HPP is a progressive metabolic disease that can lead to deformity of bones, fractures and other skeletal abnormalities, as well as profound muscle weakness, seizures, pain, and respiratory failure, causing death in infants and children.

Both are ultra-rare diseases defined as occurring in fewer than 20 patients per one million in the general population.

Hallal said he expects Kanuma to eventually generate more than $1 billion in annual sales. He declined to provide a sales forecast for Strensiq.

He sees a slow sales uptake for the drugs as raising awareness of the diseases and identifying patients is one of the biggest challenges facing the company.

"It takes time to reach the patients and the physicians," Hallal said. "When we launch, it's usually one patient at a time."

The price Alexion paid for Synageva, which was more than twice its market value, raised eyebrows and drew criticism on Wall Street. But it transforms Alexion into a multi-product company with a much larger developmental pipeline.

Alexion said the Synageva deal will add to profits beginning in 2018.

Alexion currently sells Soliris for two extremely rare diseases at a cost of around $500,000 a year. It had sales of $2.23 billion in 2014.

It is in late stage testing of Soliris for three other rare conditions, and expects early stage data on another enzyme replacement therapy later this year.

On top of that, "we added another 12 preclinical programs from Synageva to our pipeline," Hallal said.

"In aggregate we felt really good with the premium(we paid for Synageva)," he said. "We have an opportunity now to accelerate and diversify our revenue." (Reporting by Bill Berkrot and Caroline Humer; Editing by Andrew Hay and Frances Kerry)