TRENTON, N.J. (AP) — GlaxoSmithKline PLC is seeking approval in the U.S. and Europe to sell an existing breast-cancer drug as a first-line treatment. The British pharmaceutical company’s Tykerb was approved here two years ago as part of a treatment for most women with advanced breast cancer that wasn’t stopped after chemotherapy and other drugs. Tykerb is designed for women whose breast cancer is hormone sensitive, meaning it is fueled by the estrogen their body makes. GlaxoSmithKline today applied to regulators to market Tykerb as an initial treatment for hormone-sensitive breast cancer that has spread. If approved, it will be used along with another drug that presents hormones from making cancer cells multiply. Tykerb works by a different mechanism.