Identifying biomarkers in metastatic breast cancer (MBC) has become an integral part of choosing treatments and understanding disease progression. The American Society of Clinical Oncology Clinical Practice Guideline, published in 2015, recommends an initial biopsy to confirm estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER2) status as well as repeat biopsies to watch for receptor status changes over time.
“Decisions concerning the initiation of systemic therapy or selection of systemic therapy for metastatic breast cancer should be guided by ER, PR, and HER2 status in conjunction with clinical evaluation, judgment, and the patient’s goals for care,” according to the guideline authors.This news organization reached out to Kelly McCann, MD, PhD, a hematologist and oncologist in the department of medicine at the David Geffen School of Medicine, University of California, Los Angeles, to explore the role biomarker testing plays in managing MBC.
Question: How important is biomarker testing in guiding MBC treatments? Is there a standard or recommended process?
Dr. McCann: Biomarker testing is essential to breast cancer treatment and the development of targeted therapies. Oncologists typically identify a tumor’s canonical biomarkers — ER, PR, and HER2 — using immunohistochemistry or fluorescence in situ hybridization (FISH) testing and then try to match the tumor biology to drugs that target that subtype.
For tumors that lack canonical biomarkers — for example, triple-negative breast cancer (TNBC) — I send the tumor tissue for next-generation sequencing at the time of metastatic diagnosis to identify a wider range of potential targets or oncogenic drivers, such as somatic or germline mutations in homologous recombination repair genes ( BRCA1, BRCA2, and PALB2 ) or mutations in the PI3K/AKT/mTOR pathway.
In our attempts to define tumor biology and design a treatment strategy, two additional issues quickly arise. First, tumors are heterogeneous from the start. Second, tumors evolve.
Let’s start with how we define or subtype a tumor. Would you walk us through this process?
Defining a breast tumor can be tricky because these cancers often don’t fit neatly into predefined categories. Let’s take the estrogen receptor. In clinical trials, we need to define the cutoff for what constitutes ER-positive MBC or TNBC. Some trials define ER-positive as 1% or greater, others define it as 10% or greater.
But is a PR- and HER2-negative tumor with 1% or even 5% ER expression really ER-positive in the biological or prognostic sense? Probably not. A tumor with less than 10% ER expression, for instance, will actually behave like a triple-negative tumor. Instead of choosing a regimen targeting the ER-positive cells, I’ll lean more toward cytotoxic chemotherapy, the standard treatment for TNBC.
Tumors may have multiple drivers as well. What are some aberrations in addition to the main subtypes?
Tumors also often harbor more than one targetable driver. For instance, PIK3CA gene mutations are present in about 40% of hormone receptor–positive, HER2-negative tumors. Activating mutations in ESR1 develop in anywhere from 10% to 50% of MBCs as a resistance mechanism to estrogen deprivation therapy, conferring estrogen independence to the cells. Activating mutations in ERBB2, which essentially turns HER2 into an active receptor, are found in 2%-4% of breast cancers, including ER-positive, HER2-mutant breast cancers, and are enriched in lobular breast cancers, which are typically ER positive, HER2 negative.