Treating triple-negative breast cancer (TNBC), one of the more lethal breast cancer subtypes, remains a challenge. By definition, TNBC lacks the three telltale molecular signatures known to spur tumor growth: estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). A growing amount of literature shows that these frequently aggressive tumors harbor a rich array of molecular characteristics but no clear oncogenic driver.
“TNBC is incredibly heterogeneous, which makes it challenging to treat,” said Rita Nanda, MD, director of the breast oncology program and associate professor of medicine at the University of Chicago. “We have subsets of TNBC that don’t respond to currently available therapies and, as of yet, have no identifiable therapeutic targets.”
Overall, about 40% of patients with TNBC show a pathologic complete response after first-line neoadjuvant chemotherapy – typically anthracycline and taxane-based agents. But for 50% of patients, chemotherapy leaves behind substantial residual cancer tissue. These patients subsequently face a 40%-80% risk for recurrence and progression to advanced disease.
When triple-negative disease metastasizes, survival rates plummet. The most recent data from the National Cancer Institute, which tracked patients by stage of diagnosis between 2010 and 2016, showed steep declines in 5-year survival as TNBC progressed from local (91.2%) to regional (65%) to advanced-stage disease (11.5%).
Experts have started to make headway identifying and targeting different molecular features of advanced TNBC. These approaches often focus on three key areas: targeting cell surface proteins or oncogenes, stimulating an anticancer immune response, or inhibiting an overactive signaling pathway.
“For a patient with metastatic breast cancer, finding a molecular target or an oncogenic driver is essential,” said Kelly McCann, MD, PhD, a hematologist/oncologist in the department of medicine at the University of California, Los Angeles. “Because TNBC encompasses many different molecular subsets of breast cancer, the development of effective new therapeutics is going to depend on subdividing TNBC into categories with more clear targets.”
A targeted strategy
The Food and Drug Administration’s approval of sacituzumab govitecan, the first antibody-drug conjugate to treat metastatic TNBC, marked an important addition to the TNBC drug armamentarium. “Sacituzumab govitecan is one of the most exciting drugs available for the treatment of metastatic disease,” Dr. Nanda said.
Sacituzumab govitecan, approved as third-line therapy for metastatic TNBC, works by targeting the cell surface protein TROP2, expressed in about 88% of TNBC tumors but rarely in healthy cells.
In the phase 1/2 ASCENT trial, the median progression-free survival was 5.5 months and overall survival was 13.0 months in 108 patients with metastatic TNBC who had received at least two therapies prior to sacituzumab govitecan.
A subsequent phase 3 trial showed progression-free survival of 5.6 months with sacituzumab govitecan and 1.7 months with physician’s choice of chemotherapy. The median overall survival was 12.1 months and 6.7 months, respectively.
But, according to the analysis, TROP2 expression did not necessarily predict who would benefit from sacituzumab govitecan. A biomarker study revealed that although patients with moderate to high TROP2 expression exhibited the strongest treatment response, those with low TROP2 expression also survived longer when given sacituzumab govitecan, compared with chemotherapy alone.
In other words, “patients did better on sacituzumab govitecan regardless of TROP2 expression, which suggests we do not have a good biomarker for identifying who will benefit,” Dr. Nanda said.
Two other investigational antibody-drug conjugates, trastuzumab deruxtecan and ladiratuzumab vedotin, show promise in the metastatic space as well. For instance, the recent phase 2 trial evaluating trastuzumab deruxtecan in patients with HER2-positive breast cancer reported treatment response in 44% of patients with HER2-low tumors.
Given that about 36.6% of TNBC tumors exhibit low levels of HER2 expression, “trastuzumab deruxtecan represents potential in treating HER2-low TNBC,” said Yuan Yuan, MD, PhD, medical oncologist at City of Hope, a comprehensive cancer center in Los Angeles County.
Early results from a phase 1b study showed that trastuzumab deruxtecan produced a response rate of 37% in patients with HER2-low breast cancer.
Investigators are now recruiting for an open-label phase 3 trial to determine whether trastuzumab deruxtecan extends survival in patients with HER2-low metastatic breast cancers.