Vertex Pharmaceuticals Inc.'s experimental cystic-fibrosis drug Orkambi, which treats the most common mutation of the disease, received U.S. regulatory approval on Thursday.

The drug is a twice-daily oral tablet for patients 12 years and older that Vertex said would be available within days. It expands the company's reach in the treatment of cystic fibrosis, a progressive lung disease that often kills patients in their 20s.

J.P. Morgan Chase estimated the drug's global sales could reach $1.6 billion next year and $4.2 billion in 2020. A decision on Vertex's European application is expected in the fourth quarter.

Boston-based Vertex already sells Kalydeco, which treats a different genetic type of cystic fibrosis and has an annual price of $311,000 per patient in the U.S.

Vertex hasn't said what it will charge for Orkambi, which could treat as many as 8,500 patients in the U.S., or roughly 28% of the 30,000 people in the U.S. with the disease.

Janney Capital Markets, in a research note Wednesday, argued that Orkambi's price is likely to be less than Kalydeco's because it is a less effective therapy that serves a larger patient population.

Vertex representatives declined an immediate comment.

In clinical studies, the drug showed lung function improved by 2.6 percentage points to three percentage points compared with that of patients receiving a placebo.

Vertex's shares were halted on the news. Before the halt, the stock had risen 3.7% Thursday to $130.90.

As of March 31, the most recent figures available, Vertex had logged $14.1 million in inventory costs ahead of the drug's expected approval. The company, founded in 1989, had accumulated losses of $4.9 billion as of March 31.

Write to Maria Armental at maria.armental@wsj.com and Joseph Walker at joseph.walker@wsj.com

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