This photo provided by Seattle Genetics Inc., shows the breast cancer drug Tukysa, developed by Seattle Genetics. U.S. regulators on Thursday, April 14, 2020, approved the new drug for an aggressive type of breast cancer that’s spread in the body, including into the brain, where it’s especially tough to treat. (Seattle Genetics via AP)

This photo provided by Seattle Genetics Inc., shows the breast cancer drug Tukysa, developed by Seattle Genetics. U.S. regulators on Thursday, April 14, 2020, approved the new drug for an aggressive type of breast cancer that’s spread in the body, including into the brain, where it’s especially tough to treat. (Seattle Genetics via AP)

U.S. regulators on Friday approved a new drug for an aggressive type of breast cancer that’s spread in the body — including into the brain, where it’s especially tough to treat.

The Food and Drug Administration said Tukysa, a twice-daily pill developed by Seattle Genetics, is for people with what’s known as HER2-positive breast cancer that has spread and resisted multiple other medicines. This type of cancer is driven by an overactive gene that makes too much of the HER2 protein, which promotes cancer growth.

Each year, about 50,000 people in the U.S. are diagnosed with HER2-positive breast cancer. It’s usually curable, but when it spreads it kills most patients. In up to half of them, the cancer reaches the brain, giving that group such a poor prognosis that they have rarely been included in tests of new drugs.

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Tukysa, pronounced too-KYE’-sah and also known as tucatinib, works by attacking cancer cells from inside and outside to block production of the HER2 protein.

“This is basically taking the ammunition out of that weapon,” said Dr. Eric Winer, a Dana-Farber Cancer Institute researcher who helped lead the key patient study and consults for Seattle Genetics and other drugmakers.

In that company-funded study, half the 612 participants got Tukysa along with standard cancer drugs Herceptin and Xeloda. The other half got the two standard drugs and a dummy pill.

In the group getting Tukysa, 45% survived at least two years, compared with 27% in the placebo group. Among the participants whose cancer spread to the brain, 25% were alive after a year versus none in the placebo group.

“It’s a significant advance,” said Winer.

Common side effects included diarrhea, fatigue, vomiting and liver function changes.

Tukysa has a list price of $18,500 per month, or roughly $111,000 for an average course of treatment, without insurance. Most patients don’t pay that, and Seattle Genetics plans to offer financial aid.

Tukysa should be available almost immediately.

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Follow Linda A. Johnson at https://twitter.com/LindaJ_onPharma

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.