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CHICAGO — Breast cancer patients with low levels of HER2 expression, previously considered untreatable with HER2-targeted therapies, benefited from the anti-HER2 antibody drug conjugate trastuzumab deruxtecan. The therapy doubled progression-free survival versus chemotherapy in patients with hormone receptor positive (HR+) and low levels of HER2.

“Overall, these results establish HER2 low metastatic breast cancer as a targetable population of breast cancer with trastuzumab deruxtecan as a new standard of care in this setting, yasmin alibhai brown ” said Shanu Modi, MD, during a press conference held in Chicago at the annual meeting of the American Society of Clinical Oncology, where she presented the results.

“I think the results of this trial clearly will be practice changing,” said ASCO spokesperson and breast cancer expert Jane Lowe Meisel, MD, during the press conference. “I think what this trial does is really extend the benefits of this agent to a whole new group of patients that traditionally is really quite difficult to treat. I think this will offer a wonderful new option for patients and also will really fundamentally change the way we think about HER2 status and how we classify this in our metastatic patients,” Meisel added.

The conjugate includes the anti-HER2 antibody trastuzumab and the topoisomerase I inhibitor deruxtecan, which interferes with DNA replication. Trastuzumab has demonstrated efficacy in patients with high levels of HER2 expression, and trastuzumab-deruxtecan received FDA approval in May 2022 for the treatment of HER2-positive breast cancer in patients who had previously received an anti-HER2 regimen.

However, anti-HER2 agents had not been shown to benefit HER2-low patients, defined as immunohistochemistry (IHC) 1+ or 2+. About 60% of breast cancer patients traditionally thought of HER2 negative could be classified as HER2 low, according to Modi, who is a medical oncologist at Memorial Sloan Kettering Cancer Center, New York.

Asked why she thought trastuzumab deruxtecan succeeded where other anti-HER2 therapies failed in this population, Modi highlighted the nature of the drug conjugate, including a high drug payload and the use of a topoisomerase inhibitor, which is rarely employed against breast cancer.

Once released from the antibody, the drug retains its ability to cross cell membranes and enter the tumor microenvironment. That ‘knock on’ effect might allow it to reach neighboring cells that don’t express HER2. “We know HER2 expression is very heterogeneous. I think that’s why, for the first time, we’re seeing activity for a targeted agent,” Modi said.

The DESTINY-Breast04 study included 557 patients in Asia, Europe, and North America with HR-negative or HR-positive, HER2-low, unresectable, and/or metastatic breast cancer. Patients were randomized to trastuzumab deruxtecan or physician’s choice of several standard chemotherapy drugs. After a median follow-up of 18.4 months, compared with the chemotherapy group, patients in the trastuzumab deruxtecan arm had a 49% reduction in risk of progression and a 36% reduction in mortality. The group also had longer progression-free survival (10.1 months vs. 5.4 months) and overall survival (23.9 months vs. 17.5 months).

Although adverse events were similar between the two groups (52.6% in trastuzumab deruxtecan, 67.4% in chemotherapy), lung toxicity occurred in 12% of the group, and there were 3 fatalities as a result (0.8%). Interstitial lung disease/pneumonitis has been linked to trastuzumab treatment in the past, with one meta-analysis finding a frequency of 2.4% and fatality rate of 0.2%.

Additional studies are in progress to determine the minimum threshold of HER2 expression needed to gain a benefit from trastuzumab deruxtecan treatment.

The study was funded by Daiichi Sankyo, and AstraZeneca. Modi has advised, consulted for, or received honoraria from Daiichi Sankyo, and AstraZeneca. Meisel has advised or consulted for Medscape and AstraZeneca.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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