Novartis AG said a late-stage study showed its drug hope Afinitor slowed tumor growth in advanced breast cancer, helping its chances of becoming a new blockbuster treatment.

The Swiss drugmaker said its study of Afinitor in combination with Pfizer Inc's exemestane in post-menopausal women with advanced breast cancer had been halted early after positive interim results.

"Based on these study results, this combination has the potential to extend the time until chemotherapy is needed for these patients," Herve Hoppenot, president of the Novartis oncology unit, said in a statement on Tuesday.

Analysts at banking group ZKB said the Phase III study, results of which had not been expected until 2012, said Afinitor for breast cancer could yield an additional $1.5 billion in sales.

"With this indication ... Afinitor enters into a very big market for the first time and should acquire blockbuster characteristics quickly," they said in a note.

Around 1.38 million women across the world were diagnosed with breast cancer in 2008, according to Cancer Research UK.

Novartis said it would present results from the study at an upcoming medical conference and would make worldwide regulatory submissions by the end of 2011.

"Reaching the primary endpoint ahead of time makes us hopeful that the treatment effect will be clinically significant," said analyst Andrew Weiss at Vontobel, adding breast cancer was the most important possible application for the Afinitor franchise.

"We expect Novartis to file by end 2011 and therefore (to obtain) a marketing approval during 2012."

Afinitor is already approved for treating kidney cancer and is expected to rake in sales of $1.3 billion in 2015, according to a Thomson Reuters forecast.

In May, the U.S. Food and Drug Administration approved Afinitor for a rare type of pancreatic cancer that has few treatment options, despite concerns about serious side effects.

Novartis still trails Roche Holding AG, the world's largest maker of cancer drugs, in the oncology franchise, although some of the shine has come off its cross-town rival's portfolio after its No.1 drug Avastin hit a number of setbacks.