The U.S. Food and Drug Administration has approved a first-of-a-kind breast cancer medication that targets tumor cells while sparing 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 double-shot of anti-tumor poison.

Cancer researchers say the drug may offer a clear advantage over older drugs because it delivers more medication with fewer side effects.

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.

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 to $4,500 per month for regular Herceptin. The company estimates a full course of Kadcyla, about nine months of medicine, will cost $94,000.

FDA scientists said they approved the drug based on company studies showing Kadcyla delayed the progression of breast cancer by several months. Researchers reported last year that patients treated with the drug lived 9.6 months before death or the spread of their disease, compared with a little more than six months for patients treated with two other standard drugs, Tykerb and Xeloda.

Overall, patients taking Kadcyla lived about 2.6 years, compared with 2 years for patients taking the other drugs.