* Data supports Kisqali use for women before menopause

* Novartis drug now dwarfed by Pfizer's Ibrance

* Class of CDK4/6 cancer drugs getting crowded (Adds details, company and analyst comment)

By John Miller

ZURICH, Nov 8 (Reuters) - New data from Novartis's breast cancer drug Kisqali underscored its effectiveness in pre-menopausal women, the Swiss drugmaker said, amid efforts to muscle in on turf dominated by rival Pfizer's Ibrance.

A late-stage trial showed Kisqali, in concert with hormonal therapies, halted the advance of hormone-receptor positive, human epidermal growth factor receptor-2 negative advanced breast cancer in pre-menopausal women for longer than in women getting hormonal therapy alone, Novartis said on Wednesday.

Sales of Ibrance, whose 2015 approval gave it a head start over Kisqali, doubled to $643 million last year. That dwarfs Kisqali's third-quarter 2017 revenue of $26 million after it won U.S. and European approvals this year against breast cancer in women after menopause.

Novartis, which now aims to expand Kisqali's use in pre-menopausal patients, sees the drug as an eventual $1 billion-per-year seller and a linchpin of its plan to return to revenue growth starting in 2018.

"There remains a significant unmet treatment need in younger women diagnosed with pre-menopausal advanced breast cancer, as the disease tends to be more aggressive with a poorer prognosis" than in women after menopause, said Samit Hirawat, head of global drug development at Novartis Oncology.

Efforts to broaden Kisqali's use come as the crowd of so-called CDK4/6 inhibitors designed to block certain enzymes blamed for spurring tumour growth is expanding. Eli Lilly's Verzenio won U.S. approval in September.

While Novartis will wait until a December cancer symposium in San Antonio to give specifics of the latest Kisqali study, analysts said initial data bode well for the medicine's getting a leg-up against Ibrance in pre-menopausal women.

Moreover, the absence of new safety issues -- Kisqali already has warnings for so-called QT prolongation, which can signal heart and liver problems -- will also be reassuring.

"The indication clearly helps differentiate this CDK 4/6 inhibitor from its competitor Ibrance, which so far benefited from a 2-years lead... and a milder tolerability profile," said Baader Helvea analyst Bruno Bulic.

He estimates Kisqali will eventually reach peak annual sales of $5 billion. (Editing by Michael Shields)