Recent therapeutic advances in HER2-positive metastatic breast cancer (MBC) have begun to reshape the treatment landscape for patients. Since late 2019, the U.S. Food and Drug Administration (FDA) has approved a handful of novel agents for HER2-positive MBC — most notably, the antibody-drug conjugate (ADC) trastuzumab deruxtecan in December 2019 and the tyrosine kinase inhibitors (TKIs) tucatinib and neratinib in 2020. According to the National Cancer Institute›s Surveillance, Epidemiology, and End Results (SEER) program, the 5-year survival rate for patients with advanced disease was already on the rise between 2004 and 2018, and
“I’ve been involved in the HER2 space for a long time and have watched the field evolve,” said Adam Brufsky, MD, PhD, associate chief in the division of hematology/oncology and co-director of the Comprehensive Breast Cancer Center at the University of Pittsburgh School of Medicine. “The fact that we’re now talking about fourth- and fifth-line therapies for HER2-positive MBC represents a major advance in the management of these patients.”
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Oncologists are still building on this progress, focusing on designing more targeted therapies as well as studying different combinations of available agents. The main goal of treatment, experts say, is to prolong patients’ systemic response and prevent recurrences, especially in the brain. This news organization spoke to Dr. Brufksy and others about promising agents and therapeutic strategies on the horizon to treat HER2-positive MBC.
Inside emerging ADCs
Because many patients develop resistance to trastuzumab emtansine (T-DM1) — the first FDA-approved ADC in breast cancer — researchers have focused on developing the next generation of ADCs with more potent payloads, different linkers, and distinct mechanisms of action, according to Sayeh Lavasani, MD, MS, a medical oncologist at City of Hope, a comprehensive cancer center in Los Angeles County.
The second-generation ADC trastuzumab deruxtecan showed “really dramatic” results in HER2-positive MBC, demonstrating progression-free survival of 16 months, remarked Kevin Kalinsky, MD, acting associate professor in the department of hematology and medical oncology at Emory University School of Medicine in Atlanta and director of the Glenn Family Breast Center at the Winship Cancer Institute of Emory University. “These outcomes further changed how we treat patients with metastatic disease and prompted considerable excitement over the potential to develop novel ADCs to treat HER2-positive MBC.”
Most recently, two investigational ADCs — trastuzumab duocarmazine (SYD985) and ARX788 — have stood out. The FDA granted fast-track designations to trastuzumab duocarmazine in January 2018 and ARX788 in January 2021. Trastuzumab duocarmazine, the furthest along the pipeline, has shown promising results so far. In June 2021, Netherlands-based biopharmaceutical company Byondis reported preliminary phase 3 data from the TULIP trial. The open-label, randomized phase 3 study enrolled 436 patients with HER2-positive locally advanced or metastatic disease that had progressed on previous anti-HER2 regimens. The company shared early results that trastuzumab duocarmazine achieved its progression-free survival primary endpoint, marking a significant improvement over physician’s choice of chemotherapy, and promised more detailed results to come later this year.
Although only in early-phase trials, ARX788 has also shown robust anti-HER2 activity as well as low toxicity in HER2-positive tumors, according to recent data. The findings from two phase 1 studies, presented at the June 2021 virtual American Society for Clinical Oncology meeting (abstract 1038), revealed an overall response rate of 74% in the breast cancer cohort, but the investigators acknowledged it was too early to report median progression-free survival outcomes. Preclinical data also showed activity in HER2-low and T-DM1–resistant tumors.
Despite the encouraging initial findings, Dr. Kalinsky remains cautiously optimistic about long-term outcomes for both ADCs. “These data are hot off the press, but it’s too soon to know how these two ADCs and others in the pipeline will measure up to approved therapies,” he commented. As experts learn more about the efficacy of these novel ADCs, Dr. Brufsky would also like to better understand resistance mechanisms and how to integrate these agents into current treatment strategies. “The cellular biology of HER2-positive MBC is complicated, and many factors in these tumor cells affect where ADCs are released, how resistance develops, and whether or not resistance to one ADC applies to others,” Dr. Brufsky remarked. “As we gather more data, we’ll understand resistance mechanisms better and begin to figure out where to go with treatment sequencing.”