Vertex Pharmaceuticals Inc. said its second-quarter revenue rose 20% on strong sales of Kalydeco that led the biotechnology company to raise its 2015 sales guidance for the cystic-fibrosis drug.

Shares rose 1.5% to $127.50 in recent after-hours trading as revenue beat expectations and the company's quarterly loss, excluding certain items, wasn't as wide as analysts had feared.

The Massachusetts-based biotechnology company said revenue increased to $166.1 million in the latest quarter from $138.4 million a year earlier. Analysts polled by Thomson Reuters expected revenue of $150 million.

Sales of Kalydeco, the company's main product, climbed 37% to $154.9 million in the latest quarter, mostly because additional people were treated with the drug. Analysts expected Kalydeco sales to reach $140 million, according to FactSet.

For the year, Vertex raised its Kalydeco sales guidance to $575 million to $590 million, from its previous estimate for $560 million to $580 million.

Early this month, Vertex gained U.S. Food and Drug Administration approval for Orkambi—which could treat as many as 8,500 cystic fibrosis patients in the U.S.—but carries an annual wholesale cost of $259,000 per patient.

Orkambi is aimed at treating patients with a genetic mutation that is the leading cause of the disease. The new drug combines Vertex's Kalydeco, which treats a different genetic type of cystic fibrosis, with another Vertex compound.

Orkambi also is the latest drug to carry a potentially budget-busting price. Others are treatments for cancer and hepatitis C. Kalydeco's high price has put pressure on government insurance programs, such as in Arkansas.

Over all, Vertex reported a loss of $188.8 million, or 78 cents a share, compared with a year-earlier loss of $159.4 million, or 68 cents a share, a year earlier. Excluding stock-based compensation and other items, the per-share loss was 54 cents, compared with a year-earlier per-share loss of 61 cents.

Analysts polled expected a per-share loss of 58 cents.

Write to Tess Stynes at tess.stynes@wsj.com

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