The Food and Drug Administration approved a new "smart bomb" drug on Friday that can help women with one of the most hard-to-cure types of breast cancer.

The new drug added several months of life to women with a type of breast cancer called HER2-positive breast cancer, whose tumors had spread despite treatment. While it wasn’t a cure, it did add some healthy months of life to patients whose outlook was otherwise hopeless.

The drug is called Kadcyla, and it works in an unusual way. It combines an older drug, Herceptin, with a highly toxic type of chemotherapy called DM1. The Herceptin hones in on the tumor cells, which absorb the package and are then destroyed by the DM1, which is too strong to deliver like standard chemotherapy. It’s a member of a new class of drugs called antibody-drug conjugates or ADCs.

“Kadcyla delivers the drug to the cancer site to shrink the tumor, slow disease progression and prolong survival," Dr. Richard Pazdur, director of the FDA's office of hematology and oncology products, said in a statement.

In a trial of 991 women with advanced HER2 breast cancer, those who got Kadcyla lived on average 5.8 months longer than those getting more standard chemotherapy, researchers reported last year in the New England Journal of Medicine. It meant about 2 ½ years of life after diagnosis, compared to two years for those on standard therapy.

“Only a few studies in metastatic breast cancer have shown an improvement in overall survival. It’s tough to do,” Dr. Sunil Verma of the Sunnybrook Odette Cancer Centre in Toronto, who led the study, said in a statement on the National Cancer Institute’s website.

Genentech, which makes the drug, is now focusing on the ADC formula. The hope is it can cause fewer side-effects than ordinary chemo, which can affect healthy tissue. Herceptin is a synthetic immune system protein or monoclonal antibody called trastuzumab.

“We currently have more than 25 antibody-drug conjugates in our pipeline and hope this promising approach will help us deliver more medicines to fight other cancers in the future,” Dr. Hal Barron, the company’s chief medical officer, said in a statement.

It's not cheap. A nearly 10-month course of therapy costs $94,000, Genentech says.

Genentech promised the FDA it would help patients pay for it. “People who do not have health insurance, or who have reached the lifetime limit set by their insurance company, might qualify to receive Kadcyla free of charge,” the company said in a statement. Herceptin alone costs more than $4,000 a month.

The drug is not without side-effects. It can damage the heart, liver and lungs and pregnant women can’t take it.

Breast cancer is the biggest cancer killer of women, after lung cancer. It’s diagnosed in about 235,000 U.S. men and women every year and kills 40,000, according to the American Cancer Society.

About 20 percent of cases are known as HER2-positive breast cancer. That means the tumor cells make extra amounts of a protein called human epidermal growth factor receptor 2. It makes for a very aggressive type of cancer and it’s more likely to come back after treatment than other breast cancers.

Women newly diagnosed with HER2 breast cancer should still be treated first with Herceptin alone for a year, the National Cancer Institute says. But doctors may test the new drug in some volunteers to see if it works better.

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