Drug shows revolutionary results in extending life for tough-to-treat breast cancer

New data released Saturday at the 2021 European Society of Medical Oncology conference is garnering excitement for those seeking to battle some of the most challenging forms of breast cancer.

Researchers found using the antibody conjugate drug ENHERTU to treat currently incurable HER-2-positive breast cancer was significantly more successful at reducing tumor size and keeping the patient alive longer without progression of disease than the current standard of care TDM1.

ENHERTU was shown to be twice as effective as TDM1, the standard of care medication, in causing complete control of the disease in this trial. Both drugs are delivered intravenously, and both are antibody-drug conjugate medications, but ENHERTU is engineered differently, which likely explains the improved results in some people.

"I am really excited about the ENHERTU data, which is showing a better survival rate than TDM1," said Dr. Jame Abraham, chairman of the Department of Hematology and Oncology at Cleveland Clinic and who wasn't involved in this study. "The side effects are minimal to none for a majority of the patients. TDM1 is a good adjuvant drug, but we are always striving to do better. ENHERTU is offering that better option."

"The outcome of this trial is very clinically meaningful for HER-2-positive breast cancer patients," Dr. Sara Tolaney, Chief, Division of Breast Oncology, Dana-Farber Cancer Institute, who was part of the research team, told ABC News. She predicted that "this potent antibody drug ... will dramatically change the treatment for HER-2 positive breast cancer."

Breast cancer is the deadliest cancer for women worldwide, and about 1 in 5 people with breast cancer are positive for the HER-2 gene. The presence of the HER-2 protein makes breast cancer more aggressive than other types. About 1 in 3 women diagnosed with early breast cancer will have the cancer eventually spread to other organs, and about 72% of those patients will die within five years.

ENHERTU was approved by the Food and Drug Administration in 2019 for inoperable or metastatic (spreading to other organs) HER-2-positive breast cancer. It was approved for use if the patient didn't respond to the standard treatment and at least two combination options were tried.

"To use ENHERTU as an earlier line of treatment, the FDA required a head to-head comparison trial," Dr. Sunil Verma, vice president and global clinical head of breast cancer research at AstraZeneca, told ABC News. So the group initiated a trial in 2017, with the results announced publicly on Saturday.

This resulted in the phase 3 trial, which included approximately 500 patients over the age of 18 in 15 countries across Asia, Europe, North America, Oceania and South America. Half were treated with ENHERTU, half with TDM1. This trial by AstraZeneca and Daiichi Sankyo is the first global head-to-head randomized clinical trial for HER-2-positive breast cancer.

The trial compared the progression-free survival, or PFS, of ENHERTU to that of TDM1. PFS indicates disease control (tumor sizes) and looks at how long a patient lives without the cancer growing or spreading. At the 12-month mark fewer than 25% of the patients treated with ENHERTU showed disease progression or death, whereas 65% of patients treated with TDM1 experienced disease progression or death.

The safety profile of ENHERTU was consistent with that of previous trials and no new safety concerns were identified.

Verma said about 80% of patients on ENHERTU saw tumor shrinkage, and 16% of the patients' tumors could no longer be seen using imaging technology.

"With the remarkable results of this study, ENHERTU might become the new standard of care treatment for patients with HER-2-positive metastatic breast cancer following standard chemotherapy," Verma added.

This promising new treatment is called a second-line treatment as it's intended to only be used upon progression on standard first-line treatment with chemotherapy and Herceptin. But the results suggest that perhaps it could be used earlier in place of chemotherapy.

AstraZeneca and Daiichi Sankyo are already researching if patients with other cancers -- among them stomach and lung cancer -- may see similarly promising results with ENHERTU, said Verma, adding: "We are hopeful that we can see some success when this medicine can latch onto other HER-2-positive cancers."

Editor's note: This article was updated to make clear the results of the data reported were for ENHERTU alone and not as a combination.

Dr. Madhu Vennikandam, M.D., a board-certified internal medicine physician and a gastroenterology fellow at Michigan State University with Sparrow Hospital, East Lansing, is a contributor to the ABC News Medical Unit.