The Evolving HER2+ Metastatic Breast Cancer Landscape: Novel Agents and Promising Combination Therapies

Victoria Stern, MA


December 09, 2021

Recent therapeutic advances in HER2-positive metastatic breast cancer (MBC) have begun to reshape the treatment landscape for patients. Since late 2019, the US 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 the introduction of these new therapeutic options has continued to improve patients' survival odds.

"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."

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. Medscape spoke to 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 ASCO 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, 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, 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," Brufsky remarked. "As we gather more data, we'll understand resistance mechanisms better and begin to figure out where to go with treatment sequencing."

TKIs and Beyond

In addition to ADCs, TKIs continue to make their mark in the targeted HER2 therapeutic space. The approvals of tucatinib and neratinib last year represented an important advance in treating HER2-positive MBC, particularly for patients with brain metastases. The HER2CLIMB trial, for instance, found that tucatinib combined with trastuzumab and capecitabine had a 4.5-month overall survival advantage compared with placebo (21.9 vs 17.4) and a median progression-free survival advantage of 5.4 months in patients with active brain metastases (9.5 vs 4.1) and 8.3 months in patients with stable metastases (13.9 vs 5.6).

Given this progress, experts are looking to add new TKIs to the armamentarium. In particular, pyrotinib — already approved in China for treating HER2-positive MBC — has demonstrated significantly longer progression-free survival compared with a standard TKI, lapatinib. The phase 3 PHOEBE trial results, published in The Lancet in early 2021, found a median progression-free survival of 12.5 months in patients randomly assigned to receive pyrotinib plus capecitabine compared with 6.8 months in those receiving lapatinib plus capecitabine. The investigators also reported "manageable toxicity"; diarrhea was the most common grade 3 adverse event, occurring in 31% of the pyrotinib group vs 8% of the lapatinib group, and overall serious adverse events occurred in 10% of patients receiving pyrotinib vs 8% of those receiving lapatinib.

More recent data on pyrotinib come from the phase 2 PERMEATE trial, which focused on the safety and efficacy of the agent in patients with advanced disease and brain metastases. The investigators, who presented their findings at the 2021 virtual ASCO meeting (abstract 1037), reported that radiation therapy–naive patients receiving pyrotinib plus capecitabine had an overall response rate of 74.6% in the central nervous system. Patients experiencing progression after whole-brain or stereotactic radiation therapy, however, had a comparatively lower overall response rate of 42.1%.

Similarly, median progression-free survival was much higher in the radiation therapy–naive patients (12.1 vs 5.6 months in the radiation therapy cohort). Similar to the PHOEBE trial, the most common grade 3 adverse event was diarrhea (23.1%), followed by decreased neutrophil and white blood cell counts (12.8% for both), anemia (9%), and hand-foot syndrome (7.7%). The main question for Kalinsky is how well pyrotinib will ultimately stack up to tucatinib and neratinib. "Pyrotinib — like neratinib — was shown to be superior to lapatinib plus capecitabine , but its role may be limited by its gastrointestinal toxicity," he said. In addition to research focused on expanding the selection of novel ADCs and TKIs, researchers are also exploring new combinations of approved treatments and whether these combinations can be used earlier in treatment sequencing.

Take the CompassHER2 trials. The ongoing phase 3 trial in patients with high-risk HER2-positive breast cancer and residual disease will explore whether tucatinib plus T-DM1 compared with T-DM1 alone improves overall survival and recurrence-free survival and prevents brain metastases. Another possibility currently under investigation is pairing tucatinib and trastuzumab deruxtecan, instead of T-DM1. "Overall, it's exciting that we are increasing the number of therapeutic options and combinations," commented Debu Tripathy, MD, professor and chairman in the Department of Breast Medical Oncology at The University of Texas MD Anderson Cancer Center in Houston. "Having more choices allows us to tailor therapies to manage resistance and prolong patients' responses."

Curbing brain metastasis, according to Brufksy, is particularly important, and experts need to explore the extent to which ADCs can penetrate the blood-brain barrier. Already, a subgroup analysis of the DESTINY-Breast01 trial found that trastuzumab deruxtecan appeared to be active in patients with brain metastases. Investigators reported an overall response rate of 58.3% and a median progression-free survival of 18.1 months — results in line with those in the general study cohort — but the study population did not include patients with untreated or progressive brain metastases. A phase 2 study currently under way will examine whether patients with HER2-positive and HER2-low breast cancer who have untreated or progressive brain metastases respond to trastuzumab deruxtecan as well. Ultimately, Brufksy hopes the recent successes with preventing brain metastases in pediatric acute lymphoblastic leukemia (ALL) foreshadow what's to come in HER2-positive MBC.

"When we figured out how to treat brain metastases prophylactically in childhood ALL, we saw a huge improvement in the cure rate, which is ultimately my vision for HER2-positive disease," Brufsky remarked. "Are there cures for HER2-positive MBC on the horizon? We don't know yet, but the field has really exploded in recent years."

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