The U.S. Food and Drug Administration on Tuesday approved Pfizer Inc's Ibrance, a potential new standard of care for advanced breast cancer, in a regulatory decision that came more than two months earlier than expected.

Wall Street has considered the drug, whose chemical name is palbociclib, to be one of the most promising medicines in Pfizer's development pipeline. It was approved for previously untreated postnopausal women whose cancer cells have receptors to the female hormone estrogen and who do not have mutations in the HER2 gene that can contribute to uncontrolled growth of breast cells.

Such patients represent the largest proportion of breast cancer cases and are typically treated with the chemotherapy tamoxifen or letrozole, a drug used to prevent production of estrogen.

Cowen and Co analyst Steve Scala has estimated Ibrance could generate annual sales of $3 billion by 2020.

It works through a new mechanism of action, by blocking two enzymes involved in cell division, CDK4 and CDK6. In one clinical trial, the average patient taking Ibrance in combination with the standard treatment letrozole went 20.2 months without a worsening of symptoms - twice the length of time of those taking letrozole alone.

"This approval represents the first treatment advance for this group of women in more than 10 years," said Mace Rothenberg, the head of oncology for Pfizer.

UCLA which helped test the drug for Pfizer, in a statement, said Ibrance produced "groundbreaking results" in studies conducted at the university and has potential to become a mainstay treatment.

Ibrance will give a big boost to Pfizer as it attempts to bolster its medicine cabinet with new cancer drugs, including ones that work by harnessing the immune system to recognize and kill tumor cells.

The largest U.S. drugmaker badly needs big-selling new drugs to boost its earnings, following patent expirations on many of its biggest brands and a relatively thin supply of promising drugs in late-stage trials.