Entresto, introduced in 2015, got off to a slow start but brought in $507 million in revenue last year, just surpassing Novartis's target of half a billion dollars and beating some analysts' predictions.

Paul Hudson, head of the Swiss drugmaker's pharmaceuticals unit, told Reuters in an interview that Entresto has the potential for more than $3 billion annual sales in its current usage, for reduced ejection fraction heart failure in patients whose heart muscles do not contract effectively.

That would make it a blockbuster drug, defined as one that generates sales of at least $1 billion a year.

Novartis is now studying whether Entresto could be used to treat another form of heart failure, preserved ejection fraction, with a trial due to be completed next year seen as pivotal to expanding the medicine's use -- and revenue potential.

"We need to get great data in preserved ejection fraction, but if we do, then I can definitely see a path to between $4 billion and $5 billion combined," Hudson said at Novartis' headquarters in Basel.

Entresto will begin adding to Novartis's profit margins only in January 2019, he said, after the company was forced to massively build up its sales and marketing force to address the drug's sluggish U.S. start.

Hudson did not give a deadline for when Entresto would reach peak sales.

(Reporting by John Miller, editing by John Revill and Susan Fenton)