Pfizer Inc. /zigman2/quotes/202877789/composite PFE -0.52% shares fell 7% in after-hours trading Friday after the company reported that a study of a potential breast-cancer treatment was halted because it was unlikely to achieve its goal. Pfizer said that a phase 3 study of palbociclib, also known as Ibrance, which was ongoing in 400 centers spread across 21 countries, would not reach its intended change in survival rates. "This result is not what we hoped for, but we are steadfast in our commitment to advancing the science and care for people living with breast cancer," Chief Executive Albert Bourla said in the announcement . After closing a penny higher at $38.19, shares dove lower than $35.50 in extended trading following the announcement. Pfizer stock has fallen 2.5% so far this year, as the S&P 500 index /zigman2/quotes/210599714/realtime SPX -1.12% has declined 6.2%.