Women with metastatic HER2-positive breast cancer resistant to standard therapies can experience halted growth of their tumors with a new drug combination, according to early data from a clinical trial led by a researcher from the Dana-Faber Cancer Institute.

For the study, principal investigator Ian Krop, MD, PhD, addressed a population of women with metastatic HER2-positive breast cancer whose disease had progressed despite treatment with all of the FDA-approved drugs for the disease, including trastuzumab, lapatinib, and several chemotherapy agents.

The investigational agent combines a cell-killing drug, DM1, and the monoclonal antibody trastuzumab to make up the T-DM1 compound. This selectively binds to the HER2 growth signal receptor, a receptor which is highly overexpressed in HER2-positive breast tumors. According to the press release announcing the findings, approximately 20% of breast cancers are HER2-positive.

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“The antibody binds to the HER2 protein on tumor cells and delivers the drug (DM1) selectively to them—but not to normal cells,” Dr Krop explained. “This allows us to deliver high doses of the chemotherapy directly to tumor cells. And at the same time, the antibody continues to block the HER2 growth signals.”

Results indicated that the antibody-drug combination shrank tumors by 30% or more in 40% of the women with confirmed HER2-positive cancers. Additionally, another 13% had stable disease for at least 6 months, for a total clinical benefit rate of approximately 53%.

T-DM1 is also being tested in larger clinical trails to compare its effectiveness with that of other combined therapies.

The results of the study were presented at the CTRC-AACR San Antonio Breast Cancer Symposium.