Your BTC patient’s HER2 IHC result landed you in the gray zone — Now what?
Key Takeaways
Industry Buzz
“All patients with biliary tract cancer should have molecular profiling, which would include HER2 IHC to assess for HER2 expression and next generation sequencing.” — James J. Harding, MD
“If the results of the IHC are equivocal, which is defined as 2+, then the next step should be FISH testing. A positive FISH test [means] positive HER2 status." — Anwaar Saeed, MD
"CISH is actually considered a suboptimal assay, but it’s cheaper for some labs." — Aatur D. Singhi, MD, PhD
"A patient with 2+ IHC would not be anticipated to benefit from HER2-directed therapies." — Douglas Rubinson, MD, PhD
Find more of your peers' perspectives and insights below.
HER2 (human epidermal growth factor receptor 2) overexpression and amplification, long recognized as an important biomarker in breast and gastric cancers, has emerged as a relevant therapeutic target in BTCs as well.
Significant alterations in HER2 are evidenced in a substantial minority of patients with biliary tract cancer—especially in extrahepatic cholangiocarcinoma and gallbladder cancer, where HER2 positivity rates have been reported as high as 10-15% in some studies.[]
According to James J. Harding, MD, gastrointestinal medical oncologist and early drug development specialist at Memorial Sloan Kettering Cancer Center, “All patients with biliary tract cancer should have molecular profiling, which would include HER2 IHC [immunohistochemistry] to assess for HER2 expression and next generation sequencing.”
However, results may not always be clear-cut. One conundrum that confounds management is the presence of an equivocal initial IHC test result based on staining patterns. What should we do in these cases?
Let’s walk through the answer—and why it matters.
Testing refresher
The most common method to determine HER2 status in formalin-fixed paraffin-embedded tissue are IHC, FISH, and CISH.
IHC, which measures the number of HER2 receptors located on the cell surface and thus detects receptor overexpression and
Fluorescence in situ hybridization (FISH) or chromogenic in situ hybridization (CISH), both of which detect gene amplification by counting the number of copies of the ERBB2 gene in tumor cells.
Originally, these analysis were applied to breast and gastric cancers but have since expanded to biliary tract cancers.[]
Accurate testing is increasingly important as targeted agents are evaluated in trials and clinical settings for HER2-positive BTCs. Several anti-HER2 therapies, such as trastuzumab, pertuzumab, trastuzumab-deruxtecan, zanidatamab-hrii, and neratinib, have been investigated.[]
Most recently, in November 2024, zanidatamab-hrii was granted accelerated FDA approval for adults with previously treated, unresectable or metastatic HER2-positive (IHC 3+) BTC. It is the first and only option specifically indicated for HER2-positive BTC.
While other therapies like fam-trastuzumab deruxtecan-nxki are also used, they are more broadly approved for HER2-positive solid tumors.
What is an equivocal IHC result?
HER2 IHC is scored based on the intensity and robust nature of cell membrane staining. Results may read:
0 or 1+: Negative
2+: Equivocal
3+: Positive
An IHC score of 2+ indicates uncertain HER2 expression, with additional testing necessary to determine whether HER2 gene amplification is present. Compared with 3+, which suggests strong overexpression indicative of HER2-positive, 2+ falls in a gray zone, which is neither positive nor negative.[]
HER2 IHC showing positive expression (3+) in a cholangiocarcinoma. Slides credit Dr. Aatur Singhi, University of Pittsburgh Department of Pathology.
Next steps
“If the results of the IHC are equivocal, which is defined as 2+, then the next step should be FISH testing. A positive FISH test means positive HER2 status. This happens in about 10% to 15% of cases,” said Anwaar Saeed, MD, an associate professor of Medicine and the Chief of the Gastrointestinal (GI) Medical Oncology Program at the University of Pittsburgh, University of Pittsburgh Medical Center (UPMC), and UPMC Hillman Cancer Center.
ERBB2 FISH testing with the red probe demonstrating amplification of the ERBB2 gene in the same cholangiocarcinoma. The green probe is the control probe for chromosome 17. Slides credit Dr. Aatur Singhi, University of Pittsburgh Department of Pathology.
This approach is based on ASCO guidelines, noted Aatur D. Singhi, MD, PhD, a surgical pathologist with subspecialty training in gastrointestinal, liver, and pancreatobiliary pathology, and associate professor of pathology at UPMC.
“ISH can be followed by FISH for the actual gene ERBB2, or CISH. CISH is actually considered a suboptimal assay, but it’s cheaper for some labs. Here at UPMC, the gold standard is considered FISH, so we utilize FISH,” he noted.
Treatment implications
Data from the HERIZON-BTC-01 trial demonstrated that for treatment-refractory patients with HER2 3+ expression, treatment with zanidatamab is associated with meaningful benefit.[]
“Unfortunately, the benefit seemed to be limited to patients with 3+ expression by immunohistochemistry regardless of amplification status of ERBB2. Thus, a patient with 2+ IHC would not be anticipated to benefit from HER2-directed therapies,” said Douglas Rubinson, MD, PhD, gastrointestinal oncologist at the Dana-Farber Cancer Institute.
What percentage of your BTC patients show HER2 3+ expression and could benefit from treatment with zanidatamab?
In cases of equivocal results—as with all other BTC—combining treatment with immunotherapy or chemotherapy could prove useful, stated Dr. Saeed.
"This next generation of trials will lead to even better results for this subgroup. This is the future."
— Anwaar Saeed, MD
Dr. Singhi also explained that there’s still hope for treatment with an equivocal result that persists after FISH testing. “There are other options—other testing and stains that can be done to identify additional targets.”
Disclosure: Dr. Harding works with Jazz Pharmaceuticals, the maker of zanidatamab (Ziihera), and AstraZeneca, the maker of trastuzumab-deruxtecan (Enhertu).