Breast cancer drug trial results in ‘unheard-of’ survival rates

The patients had metastatic breast cancer that had been progressing despite rounds of harsh chemotherapy. But a treatment with a drug that targeted cancer cells with laserlike precision was stunningly successful, slowing tumor growth and extending life to an extent rarely seen with advanced cancers.

The new study, presented at the annual meeting of the American Society of Clinical Oncology and published Sunday in the New England Journal of Medicine, would change how medicine was practiced, cancer specialists said.

“This is a new standard of care,” said Dr. Eric Winer, a breast cancer specialist, director of the Yale Cancer Center and head of the ASCO. Winer was not involved with the study. He added that “it affects a huge number of patients.”

The trial focused on a particular mutant protein, HER2, which is a common villain in breast and other cancers. Drugs that block HER2 have been stunningly effective in treating breast cancers that are almost entirely populated with the protein, turning HER2-positive breast cancers from those with some of the worst prognoses into ones where patients fare very well.

But HER2-positive cases constitute only about 15% to 20% of breast cancer patients, said Dr. Halle Moore, director of breast medical oncology at the Cleveland Clinic. Patients with only a few HER2 cells — a condition known as HER2-low — were not helped by those drugs.

The clinical trial, sponsored by the pharmaceutical companies Daiichi Sankyo and AstraZeneca and led by Dr. Shanu Modi of Memorial Sloan Kettering Cancer Center, involved 557 patients with metastatic breast cancer who were HER2-low. Two-thirds took the experimental drug, trastuzumab deruxtecan, sold as Enhertu; the rest underwent standard chemotherapy.

In patients who took trastuzumab deruxtecan, tumors stopped growing for about 10 months, as compared with five months for those with standard chemotherapy. The patients with the experimental drug survived for 23.9 months, as compared with 16.8 months for those who received standard chemotherapy.

“It is unheard-of for chemotherapy trials in metastatic breast cancer to improve survival in patients by six months,” said Moore, who enrolled some patients in the study. Usually, she says, success in a clinical trial is an extra few weeks of life or no survival benefit at all but an improved quality of life.