WASHINGTON -- The Food and Drug Administration has approved a first-of-a-kind breast cancer medication that targets tumor cells and spares healthy ones.The drug Kadcyla, from Roche, combines the established drug Herceptin with a powerful chemotherapy drug and a third chemical linking the medicines together. The chemical keeps the cocktail intact until it binds to a cancer cell, delivering a potent dose of anti-tumor poison.Cancer researchers say the drug is an important step forward because it delivers more medication while reducing the unpleasant side effects of chemotherapy."This antibody goes seeking out the tumor cells, gets internalized and then explodes them from within. So it's very kind and gentle on the patients -- there's no hair loss, no nausea, no vomiting," said Dr. Melody Cobleigh of Rush University Medical Center. "It's a revolutionary way of treating cancer."Cobleigh helped conduct the key studies of the drug at the Chicago facility.The FDA approved the new treatment for about 20 percent of breast cancer patients with a form of the disease that is typically more aggressive and less responsive to hormone therapy. These patients have tumors that overproduce a protein known as HER-2. Breast cancer is the second-most deadly form of cancer in U.S. women and is expected to kill more than 39,000 Americans this year, according to the National Cancer Institute.The approval will help Roche's Genentech unit build on the blockbuster success of Herceptin, which has long dominated the breast cancer marketplace. The drug had sales of roughly $6 billion last year.Genentech said Friday that Kadcyla will cost $9,800 per month, compared with $4,500 per month for regular Herceptin.