WASHINGTON (February 22, 2013)—The U.S. Food and Drug Administration has approved a new form of the best-selling breast cancer drug Kadcyla, which targets tumor cells while sparing healthy ones.

The drug from Roche combines the established drug Herceptin with a powerful chemotherapy and a third chemical linking the medicines together.

The chemical keeps the drugs intact until they bind to a cancer cell, where the double-shot of medication is released.

The FDA approved the drug for about 20 percent of breast cancer patients who have a particular form of the disease involving tumors that overproduce the protein HER-2.

The approval will help Roche build on the success of Herceptin, which has long dominated the breast cancer marketplace.

The drug had sales of $6 billion last year.