(Reuters) - The U.S. Food and Drug Administration on Tuesday said it had approved Pfizer Inc's experimental breast cancer drug Ibrance, 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.

Cowen and Co analyst Steve Scala has estimated the drug, whose chemical name is palbociclib, could generate annual sales of $3 billion by 2020.

It works through a new mechanism of action, by blocking two enzymes involved in cell divison, 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.

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

(Reporting by Ransdell Pierson; Editing by David Gregorio, Bernard Orr)