By Ransdell Pierson (Reuters) - Health regulators have granted a "breakthrough therapy" designation to an experimental Pfizer Inc treatment for breast cancer, lifting the company's shares and putting a spotlight on its growing cancer-drug portfolio. Analysts from JPMorgan and Leerink Swann forecast the oral medicine, called palbociclib, could generate annual sales of $5 billion or more if it is approved for use against breast cancer as well as other types of cancer. Pfizer, whose stock rose 2.6 percent, said the U.S. ...