In the long and often dispiriting quest to cure cancer, the 1998 approval of the drug Herceptin was a tremendously hopeful moment. This drug for breast cancer was the first to use a tumor-specific protein as a homing beacon to find and kill cancer cells. And it worked. Herceptin has benefited nearly three million people since that time, dramatically increasing the 10-year survival rate—and the cancer-free rate—for what was once one of the worst medical diagnoses. “Honestly, it was sort of earth-shattering,” says oncologist Sara M. Tolaney of the Dana-Farber Cancer Institute in Boston.
But the drug has a major limitation. Herceptin’s beacon is a protein called HER2, and it works best for people whose tumors are spurred to grow by the HER2 signal—yet that’s only about one fifth of breast cancer patients. For the other 80 percent of the approximately 250,000 people diagnosed with the disease every year in the U.S., Herceptin offers no benefits.
The hunt for better treatments led researchers to reimagine targeted therapies. By 2022 they had developed one that linked Herceptin to another cancer-killing drug. This therapy, for the first time, could damage tumors that had vanishingly low levels of HER2. The drug, named Enhertu, extended the lives of people with breast cancer by several months, sometimes longer. And it did so with fewer severe side effects than standard chemotherapies. The U.S. Food and Drug Administration approved its use in that year.
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The news got even better in 2023. Researchers reported that Enhertu appeared to work even on tumors with seemingly no HER2 at all. (It’s possible the cancers did have the protein but at very low levels that escaped standard detection methods.)…
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