The test involved nearly 1,000 women with advanced breast cancer. According to researchers, the treatment extended by several months the time the women lived without their cancer getting worse.
The treatment also appears to improve survival. However, experts say it will take more time to know for sure. After two years, 65 percent of women who received it were still alive versus 47 percent of those in a comparison group given two standard cancer drugs.
The new treatment builds on Herceptin, the first gene-targeted therapy for breast cancer. It is used for about 20 percent of patients whose tumors overproduce a certain protein. Researchers combined Herceptin with a chemotherapy so toxic that it can't be given by itself, plus a chemical to keep the two linked until they reach a cancer cell where the poison can be released to kill it.
The double weapon, called T-DM1, has been dubbed the "smart bomb," although it's actually not all that smart - Herceptin isn't a homing device, just a substance that binds to breast cancer cells once it encounters them.
"Think of this like a heat-seeking missile with a nuclear warhead. So it travels around the body, does no damage until it meets the cancer, and then it deploys a massively toxic dose that wipes out the cancer," said Dr. Jennifer Ashton on "Good Morning America."
Bridget Spencer, 28, was diagnosed with stage 4 breast cancer just a week after graduating from college. Despite surgery, radiation and drugs, the cancer spread to her liver, lung and bones. She turned to a clinical trial and received T-DM1.
In just three months, Spencer's tumors started shrinking, and it's given her more time to be a normal 28-year-old woman. She is still trying to get her cancer under control, but she is hopeful that this kind of targeted therapy will one day mean a cure.
"Every day, I get up and go to work and dream about celebrating my 30th birthday, and one day living in a world that we're free of breast cancer," she said.
T-DM1 caused fewer side effects than the other drugs did. Some women on T-DM1 had signs of liver damage and low levels of factors that help blood clot, but most did not have the usual problems of chemotherapy.
"People don't lose their hair, they don't throw up. They don't need nausea medicines, they don't need transfusions," said the study's leader, Dr. Kimberly Blackwell of Duke University.
Genentech, the study's sponsor, says the price of T-DM1 has not been determined. Herceptin costs more than $4,000 a month plus whatever doctors charge to infuse it. Herceptin's U.S. patent doesn't expire until 2019.
Officials hope the benefit becomes more clear with time. In fact, so many women on the new treatment are still alive that researchers cannot yet determine average survival for the group.
Though the findings are encouraging, the drug is still experimental and it's not available yet. The drug's backers hope it can reach the market within a year.
The Associated Press contributed to this report.