My Treatment Approach: Targeting HER2 in Metastatic Colorectal Cancer - Episode 3
Experts describe which patients should be screened for HER2 amplification in colorectal cancer and how best to detect it.
Andrea Cercek, MD: Moving on to detecting HER2 [human epidermal growth factor receptor 2]. There’s still a lot for us to learn about with HER2 in colorectal cancer, and I’m so glad you’re here to help guide us through this in terms of how we can best detect HER2 amplification in colorectal cancer. Which tests should we use? Which patients should we screen?
Jaclyn Hechtman, MD: The tests that we use are dependent on a few things when we break them out. How quickly do we need the results? Oftentimes for colon cancer, we do have a good amount of time, so we have the sectioned specimen with decent tissue. When time is not of the essence, next-generation sequencing is available. That usually takes 2 to 3 weeks for these broader panels, and they can identify whether there’s an amplification of HER2. But if you have something like a lymph node or something that has a lot of other types of cells that aren’t cancer and your purity is low, the signal of how many HER2 copies you have will be drowned out by all these other cells. Or if we have a small clone of HER2amplification or a relatively small clone—and a lot of tumor cells that don’t have that—you might not be able to pick up the HER2 amplification when we’re doing next-generation sequencing because all the DNA is assessed together.
When we’re looking for IHC [immunohistochemistry], the difference is that each cell is individually visualized, so you can determine if this is clonal vs subclonal. Meaning that if the majority of tumor cells have HER2 amplification or if it’s a population of them, we don’t run into the problem of drowning out the HER2amplification signal with IHC. It’s also faster and cheaper, but it doesn’t tell you whether there’s a concomitant downstream mutation in KRAS, RAS, or BRAF, which are important when you’re deciding which therapy you’re going to use. [It doesn’t tell you] whether the patient is eligible for a HER2 inhibitor and which 1.
FISH [fluorescence in situ hybridization] is similar to IHC in that the tumor cells are directly visualized. But instead of reviewing it with a regular microscope, we use a fluorescence microscope. There are a few more steps to this exam, and rather than protein overexpression, which we see with IHC, when we do this refracts FISH testing for those equivocal cases, and you’re counting the number of signals of HER2. It’s normalized against another portion of the same chromosome CEP17. And you derive an average of how many HER2 copies based on an area of the tumor that you think has the highest HER2 population.
Andrea Cercek, MD: That’s really helpful. As a medical oncologist, what we know about HER2 in metastatic colorectal cancer is that it’s more likely to occur in left-sided tumors and the rectum, but it’s not exclusive to that location. I wouldn’t not do HER2 testing in a right-sided tumor; it would just be more likely to be amplified in left-sided tumors. It’s about 3% to 5% of all patients, so it’s not a small number considering how common colorectal cancer is.
As you mentioned, and it’s really important, it’s not mutually exclusive from RAS and BRAF mutations. We do the IHC, and it helps us, but we still need to know the RAS status and the BRAF status because that changes the therapies we can offer. In many ways, it changes our approach to HER2 targeting. But there are also data showing that patients who have HER2-amplified tumors that are RAS wild-type, left-sided, still don’t benefit from anti-EGFR therapies. It should give us a little pause if we know that a tumor is left-sided but HER2 amplified to use anti-EGFR therapies. It gets a bit complicated, but it’s an important marker to pay attention to at this point in terms of different therapies that we can offer.
Jaclyn Hechtman, MD: Andrea, at what point in the patients’ journey do you usually obtain HER2testing in these colon cancer specimens?
Andrea Cercek, MD: That’s a great question. Historically, I try to get next-generation sequencing on all my patients with metastatic colorectal cancer when I meet them. Ideally, they’re newly diagnosed before starting first-line therapies, so I will usually know the HER2status a few weeks after meeting them. At the moment, our therapies that target HER2 are approved after the first progression on the first line. However, as is usually the case, we’ll be moving treatment into earlier lines of therapy—the earlier, the better. But at some point when the patient is on first-line chemotherapy, we should get that information.
Transcript edited for clarity.