Much attention has been focused recently on the idea that breast cancer with a low expression of HER2 can be treated with HER2-targeted agents. Not surprisingly, manufacturers of these drugs have pounced on this idea, as it opens up a whole new patient population: previously these drugs were only for tumors with a high HER2 expression.
There is a large potential market at stake: HER2-low (also referred to as ERBB2-low), as defined by a score of 0 to 3+ on immunohistochemistry (IHC), is seen in approximately 50%-60% of all breast cancers
However,
A new large-scale analysis concludes that it is not. The authors argued that it actually it represents a series of biological differences from HER2-negative disease that do not have a strong bearing on outcomes.
The analysis was published online in JAMA Oncology.
For the study, researchers from the University of Chicago examined data on more than 1.1 million breast cancer patients recorded as HER2-negative in the U.S. National Cancer Database, and re-classified almost two thirds as HER2-low on further analysis.
They found that HER2-low status was associated with higher estrogen receptor (ER) expression, as well as a lower rate of pathologic complete response, compared with HER2-negative disease. It was also linked to an improvement in overall survival on multivariate analysis of up to 9% in advance stage triple negative tumors.
“However, the clinical significance of these differences is questionable,” the researchers commented.
HER2-low status alone “should not influence neoadjuvant treatment decisions with currently approved regimens,” they added.
These results “do not support classification of HER2-low breast cancer as a distinct clinical subtype,” the team concluded.
Not necessarily, according to Giuseppe Curigliano, MD, PhD, director of the new drugs and early drug development for innovative therapies division at the European Institute of Oncology, Milan. He argued the opposite case, that HER2-low is a separate clinical entity, in a recent debate on the topic held at the 2022 San Antonio Breast Cancer Symposium.
In a comment, Dr. Curigliano said that a “major strength” of the current study is its large patient cohort, which reflects the majority of cancer diagnoses in the United States, but that it nevertheless has “important limitations that should be considered when interpreting the results.”
The inclusion of only overall survival in the dataset limits the ability to make associations between HER2-low status and cancer-specific prognosis, as “survival may lag years behind recurrence.”
The lack of centralized assessment of IHC results is also an issue, as “some of the results may be associated with regional variation in practice of classifying cases as HER2 0 versus HER2 1+.”
“In my opinion, this is a great limitation,” he said, in being able to conclude that there is “no prognostic difference between ERBB2-low and -negative patients.”
He also noted that, from a molecular point of view, “the key determinant of the gene expression profile is the expression of hormone receptors, [and] if we perform a correction for hormone receptor expression, only marginal differences in gene expression are found” between HER2-low and HER2-negative tumors.
“Similarly, large genomic studies have identified no specific and consistent difference in genomic profiles,” Dr. Curigliano said, and so, HER2-low disease, “as currently defined, should not be considered a distinct molecular entity, but rather a heterogeneous group of tumors, with biology primarily driven by hormone receptor expression.”
