ZURICH (Reuters) - Novartis has won U.S. Food and Drug Administration approval for Kisqali to treat postmenopausal women who have a difficult-to-treat form of breast cancer, challenging U.S. rival Pfizer’s Ibrance.

A sign marks a building on Novartis' campus in Cambridge, Massachusetts, U.S., February 28, 2017. REUTERS/Brian Snyder

The Swiss company has priced Kisqali to be cheaper than Ibrance, which analysts said could help it win business, though Kisqali’s approval includes additional patient monitoring requirements that could work against the new drug.

Novartis considers Kisqali to be one of its stable of 13 new medicines with blockbuster potential for annual revenue in the billions of dollars. Analysts see global annual sales reaching $1.5 billion by 2022, Thomson Reuters data shows.

British biotech company Astex Pharmaceuticals, now part of Otsuka, will receive undisclosed payments from Novartis, having worked out the structure of the key protein targeted by Kisqali.

Approval for Kisqali, previously known as LEE011 and to be administered in combination with letrozole, is for first-line treatment of hormone receptor positive, human epidermal growth factor receptor-2 negative (HR+/HER2-) advanced or metastatic breast cancer, Novartis said late on Monday.

The quick U.S. approval provides Novartis with the jump-off point to challenge Pfizer’s Ibrance in the initial treatment of patients.

Ibrance hit about $2.1 billion in sales in 2016.

Novartis estimates that it has priced Kisqali at an 18-20 percent discount to Ibrance while providing for flexible dosing.

Leerink analyst Seamus Fernandez said in a note that the pricing could help to drive take-up for prescriptions but that cost factors could be outweighed by patient monitoring requirements that are more rigorous than those for Ibrance.

Kisqali comes with warnings for so-called QT prolongation, which can signal heart problems and liver issues.

“Physicians will likely find the monitoring requirements associated with Kisqali to be burdensome relative to Ibrance,” Fernandez wrote.

Novartis said that no head-to-head studies of Kisqali and Ibrance had been done, making efficacy and safety comparisons impossible. But the company said that most health issues that arose during Kisqali’s trials were mild to moderate and generally managed through interrupting or reducing the dose.

Kisqali plus letrozole reduced the risk of disease progression or death by 44 percent over letrozole alone, according to results of a study released last year.

The Novartis drug is also under review in Europe.