The Food and Drug Administration on Friday approved a new type of drug that combines the widely used breast cancer medicine Herceptin with a powerful toxin to more effectively kill cancer cells while potentially reducing side effects.

The drug, which will be called Kadcyla but was known as T-DM1 during its development, extended the median survival of women with advanced breast cancer by nearly half a year in a clinical trial.

Genentech, which developed the drug, said it would cost about $9,800 a month, or $94,000 for a typical course of treatment. That is about twice the price of Herceptin itself, which is also made by Genentech, but it is similar to the price of some other new cancer drugs.

Kadcyla, which the company said could be available within days, is one of the first successful examples of a new class of drug that link toxins to proteins known as monoclonal antibodies. The antibodies latch onto tumors and deliver the toxic payload. Because the toxin is not activated until it reaches the tumor, healthy cells are spared and some side effects are avoided.

Such medicines, known as antibody-drug conjugates, are a hot area for cancer drug developers, with around two dozen such drugs in clinical trials. Another antibody-drug conjugate, Adcetris, developed by Seattle Genetics, was approved in 2011 as a treatment for two rare types of lymphoma.

The linker and toxin used in Kadcyla were developed by ImmunoGen, based in Waltham, Mass., which will receive royalties on sales of the drug.

The main clinical trial leading to approval of Kadcyla involved 991 patients with metastatic breast cancer that was worsening despite treatment with Herceptin and a taxane chemotherapy drug, such as paclitaxel. Half the women were given infusions of Kadcyla and the other half took two pills now commonly used for such patients: Tykerb, also known as lapatinib, and Xeloda, also known as capecitabine. The patients getting Kadcyla lived a median of 30.9 months, compared with 25.1 months for those getting the two pills. The median time before the disease worsened, a measure known as progression-free survival, was 9.6 months for those getting Kadcyla, compared with 6.4 months for those getting the other drugs.

Herceptin, also known as trastuzumab, binds to a protein on the surface of breast cancer cells called HER2.

Since Kadcyla, known generically as ado-trastuzumab emtansine, incorporates Herceptin, it too is approved only for the roughly 20 percent of breast cancer cases with an overabundance of HER2.

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