NEW YORK – Researchers uncovered a distinct subgroup of HER2-negative breast cancer that has poor survival outcomes on standard treatments, suggesting the need for a better understanding of HER2-driven disease through more precise testing and treatment selection.
The study, published in ESMO Open earlier this month, was undertaken due to recent data that upended the traditional understanding and measurement of HER2 expression in breast cancer using immunohistochemistry (IHC). As AstraZeneca and Daiichi Sankyo's HER2 antibody-drug conjugate Enhertu (trastuzumab deruxtecan) continues to show activity in patients with lower and lower levels of HER2 expression in earlier-stage metastatic tumors, researchers want to better characterize the differences between true HER2-negative disease and cancers with some HER2 expression that can still trigger a treatment response.
Investigators, led by Rinath Jeselsohn, an assistant professor of medicine in the breast oncology center at the Dana-Farber Cancer Institute, used ERBB2 mRNA expression data as a more quantitative measurement of HER2 expression in this study to capture any differences in HER2-negative tumors and better select patients for treatment with Enhertu.
With Enhertu recently receiving US approval for treating HER2-ultralow breast cancer, which includes patients with IHC 0 scores that have some staining, and AstraZeneca and Daiichi Sankyo now eyeing a further indication for Enhertu in HER2-negative breast cancers, the researchers hoped to identify additional biomarkers to guide treatment with the drug, Jeselsohn said.
"There are clear definitions for what's HER2-positive, and the guidelines for HER2 positivity have been really well established," Jeselsohn said. "But for estrogen receptor (ER)-positive, HER2-negative disease as well as triple-negative disease, this is still a work in progress. We still don't have the best biomarkers to really determine who does benefit or doesn't benefit from trastuzumab deruxtecan in ER-positive, HER2-negative and triple-negative breast cancer."
The researchers analyzed genomic data from HER2-negative breast cancer patients included in several databases, including the Molecular Taxonomy of BC International Consortium (METABRIC)'s database, the Cancer Genome Atlas (TCGA), and a cohort of real-world patients from Dana-Farber.
The researchers first explored ERBB2 mRNA protein expression in the ER-positive HER2-negative breast cancer samples from METABRIC and TCGA datasets. Both datasets included whole-exome sequencing and transcriptomic data of primary treatment-naive breast cancers. METABRIC was used as the discovery cohort with 1,298 HER2-negative breast cancer samples, and TCGA was the validation cohort with 524 samples. The researchers grouped ERBB2 expression into three categories: minimal, moderate, and enhanced.
They found that all HER2-negative breast cancer samples, using the traditional clinical HER2-negative definition of an IHC score of 0, 1+, or 2+ with a negative in situ hybridization (ISH) result, in the METABRIC and TCGA cohorts had some degree of ERBB2 expression based on the mRNA reads.
In the TCGA cohort, the level of ERBB2 expression correlated with IHC score, with IHC 2+ samples having the highest levels of expression and IHC 0 samples having the lowest. However, they noted that the HER2 IHC scoring and ERBB2 mRNA subgroups were not entirely concordant. For example, a third of the IHC 1+ tumors had enhanced ERBB2 mRNA expression.
The researchers also explored the concordance between IHC scores and ERBB2 mRNA levels based on quantitative reverse transcription-PCR in the real-world Dana-Farber cohort of 971 patients. They found that each IHC score included a range of ERBB2 mRNA expression, suggesting these testing methods correlate but are not fully concordant.
In the IHC 0 group, for example, nearly half of the samples fell into the ERBB2 mRNA moderate and enhanced expression categories. The IHC 2+ group, frequently considered HER2-positive, more than half the samples were categorized as having ERBB2 mRNA minimal or moderate expression.
"[HER2 IHC testing] is really a field that still needs work to really better define who is considered a candidate for trastuzumab deruxtecan," Jeselsohn said. "We were interested in looking at a more quantitative measure using mRNA levels of HER2 that is not dependent on pathology review. The idea of this study was to take something that's quantitative and see if using that we can classify HER2-negative breast cancers and if we see differences between these subgroups."
The researchers also wanted to understand the genomic characteristics of ERBB2 mRNA subgroups and found differences in the prevalence of several genetic mutations between them. PIK3CA mutations were more prevalent in the ERBB2 mRNA moderate and enhanced groups compared with the ERBB2 mRNA minimal subgroup, while TP53 mutations, which are associated with poor prognosis and treatment resistance, were more prevalent in the ERBB2 mRNA minimal group compared to the moderate and enhanced groups.
Homing in on subgroups based on IHC score subgroups, researchers found limited differences and no significant differences in the prevalence of PIK3CA and TP53 mutations. Based on these findings, they concluded that ERBB2 mRNA levels categorize primary hormone receptor-positive breast cancers into biologically distinct subgroups.
The researchers also found that the ERBB2 mRNA minimal group had a higher prevalence of ERBB2 loss of heterozygosity compared to the other groups. Between 35 percent and 52 percent of samples in the minimal group across the datasets exhibited ERBB2 loss compared to 9 percent to 13 percent in ERBB2 mRNA moderate and 4 percent to 5 percent in ERBB2 mRNA enhanced groups.
When they looked further into ERBB2 loss of heterozygosity on chromosome 17, the chromosome that codes for many proteins including HER2, the researchers found that samples with a 17q12 deletion and ERBB2 loss of heterozygosity had a distinct biological profile. These tumors with 17q12 deletions and ERBB2 loss had a higher prevalence of TP53, PTEN, CDKN2A, RB1, and MAP2K4 heterozygous loss and were enriched in the expression of genes related to cell proliferation and immune response.
In the Dana-Farber real-world cohort, the researchers found the same differences among HER2-negative breast cancer samples with 17q12 deletion and ERBB2 loss. Within the outcomes data from this cohort, they found the 17q12 deletion with ERBB2 loss tumors had worse overall survival outcomes compared to the ERBB2 copy neutral/gain cancers, 33.8 months versus 47.8 months median survival, respectively. However, the survival outcomes were not different when stratified by HER2 IHC 0 compared to HER2-low breast cancers.
The researchers also sought to validate this finding in a separate group of more than 800 HER2-negative breast cancer patients from MSK's MetTropism cohort. They found similar trends of worse median overall survival in patients with a 17q12 deletion with ERBB2 heterozygous loss, 16.4 months, compared with patients who were copy number neutral, 30.8 months. For both the Dana-Farber and MSK cohorts, researchers controlled for TP53 mutations, since they are associated with poorer outcomes, and found that the trend toward worse survival outcomes still held for those with 17q12 deletions.
Jeselsohn added that the 17q12 deletion group made up about 30 percent of patients with low levels of ERBB2 mRNA. This group has other genomic features that could be targeted by different therapies, including loss of RB1, which is "a key gene that's important for [determining] CDK4/6 inhibitor sensitivity," she said.
"We need to better study this patient population and identify the better treatments for [them] because, based on their genomics, they could be resistant to multiple types of drugs, and based on the survival data, they also have poor survival," she continued.
A limitation of this study was that the researchers did not have specific outcomes data by ERBB2 subgroups for patients treated with Enhertu, Jeselsohn noted, adding that her team hopes to conduct that analysis in the future. She highlighted that in the DAISY trial, which evaluated Enhertu in breast cancer patients with a range of HER2 expression levels, researchers identified a small number of patients with ERBB2 heterozygous deletions who derived less benefit from treatment.
Overall, in the DAISY trial, the cohort of HER2 non-expressing breast cancer patients, as measured by IHC, did not benefit as much from Enhertu, with a response rate of 29.7 percent, compared to 70.6 percent and 37.5 percent in the HER2-overexpressing and -low groups, respectively.
This data, combined with the data from the ESMO Open study, suggest that a more quantitative test is needed to identify best responders to Enhertu, Jeselsohn said. She's not certain, however, that measurement of ERBB2 mRNA levels like in the ESMO Open study is the best method for patient selection.
"We need a better quantitative measure that's more standardized," she said. "Right now, the HER2 [IHC] testing was really designed to identify patients who have HER2-positive disease, it wasn't developed to identify patients who respond to trastuzumab deruxtecan or other [antibody-drug conjugates] that target HER2."
In general, Jeselsohn is of the view that protein assessment is better than mRNA because that's what these drugs are designed to target. "But it's clear we need better tools," she said.