Nuances of HER2 Expression Underscore Need for More Precise Testing in Breast Cancer

David Rimm, MD, PhD

Accurate biomarker testing has become a vital step in the diagnosis and treatment decision-making across various tumor types. Within the breast cancer space, immunohistochemistry (IHC) testing has served as a staining process to detect the presence of HER2 or hormone receptors on the surface of cells from biopsied breast cancer tissue.¹

However, since the introduction of IHC testing to identify patients with HER2-positive breast cancer, HER2 expression has become more nuanced. Previously, patients with an IHC score of 3+ were considered HER2 positive; those with an IHC score of 0 or 1+ were deemed to have HER2-negative disease; and patients with an IHC score of 2+ were considered borderline, depending on in situ hybridization (ISH) status. Now, an IHC score of 1+ or 2+/ISH– is considered HER2-low.

In the real-world setting, properly classifying a patient’s specific subtype is crucial for making optimal treatment decisions, and pathologists said that accurate testing beyond determining HER2-positive and -negative disease remains a challenge.

“We’ve [completed] studies that have shown that pathologists can’t [always] accurately distinguish HER2 0 from 1+. A score of 2+/ISH– and 1+ is considered HER2-low, and 0 is considered HER2-negative, but [it can be difficult to distinguish] 0 from 1+,” David Rimm, MD, PhD, explained in an interview with OncLive®. “Being asked to distinguish 0 from 1+ was already [a challenge]. Now, they’re asking us to distinguish a 0.5+ from a 0. [This] certainly is not well thought out, because we already couldn’t tell 0 from 1. It is baffling to me that they would move in this direction.”

Rimm serves as the Anthony N. Brady Professor of Pathology, professor of medicine, Medical Oncology, the director of the Yale Cancer Center Tissue Microarray Facility, director of the Yale Pathology Tissue Services, and director of the Physician Scientist Training Program, Pathology Research, at Yale School of Medicine in New Haven, Connecticut.

In an editorial published in Archives of Pathology and Laboratory Medicine in January 2023, Rimm and colleagues Sanja Dacic, MD, PhD, of the Yale School of Medicine, and Stuart J. Schnitt, MD, of the Dana-Farber Cancer Institute in Boston, Massachusetts, wrote, "We should not be forced to issue diagnoses that lead to $100,000 therapies that are no more reproducible than the flip of a coin.”²

Rimm and other pathologists continue to stress the clinical and financial consequences of inaccurate IHC results on health care systems, and, more importantly, on patients, due to these challenges presented in IHC regulation and testing.

In a letter to the editor published in the Journal of Histotechnology in November 2023, members of the National Society for Histotechnology (NSH) endorsed a proposal to reform IHC regulations.³

“Since its inception as a diagnostic tool in anatomic pathology, IHC has been referred to as a ‘stain.’ This notion is an offshoot of traditional histochemical techniques that supplemented morphological interpretation and is antiquated terminology,” NSH members wrote. “Given the complexity of performing clinical IHC testing, the number of tests performed annually, and the importance of the results to patient care, NSH firmly believes that it is time to take the steps necessary to improve the quality of clinical IHC practice.”

A Quality Control Crisis of Inconsistent IHC Reporting

IHC is currently regulated as a stain rather than a diagnostic assay, which may lead to a lack of standard assay quality assurance requirements for the test, contributing to inconsistent results.⁴

In another 2023 editorial published in Archives of Pathology and Laboratory Medicine, Barbarajean Magnani, MD, PhD, of Tufts University School of Medicine in Boston, Massachusetts, and Clive R. Taylor, MD, Dphil, of the Keck School of Medicine at the University of Southern California in Los Angeles, explained that IHC should be regulated as an assay rather than a stain.

“Any accrediting organizations should view [IHC] not as a stain, but as an immunoassay, because we’re using it differently than we did 40 years ago,” Magnani told OncLive.

Within the breast cancer space, false-negative or -positive IHC results for HER2 expression now have direct clinical implications, largely in part due to the integration of fam-trastuzumab deruxtecan-nxki (T-DXd; Enhertu) into the treatment paradigm. Originally granted accelerated approval by the FDA in 2019 and regular approval in 2022 for the treatment of select patients with pretreated unresectable or metastatic HER2-positive breast cancer, the HER2-directed antibody-drug conjugate also received FDA approval for the treatment of patients with unresectable or metastatic HER2-low breast cancer in 2022.^5,6

The approval of T-DXd for patients with HER2-low breast cancer was based on data from the phase 3 DESTINY-Breast04 trial (NCT03734029), where HER2-low expression was defined as IHC 1+ or IHC 2+/ISH–.

T-DXd was then evaluated in the phase 3 DESTINY-Breast06 trial (NCT04494425), which included patients with HER2-low breast cancer or HER2-ultralow disease, defined as IHC 0 with membrane staining.⁷

At the 2024 ASCO Annual Meeting, lead study author Giuseppe Curigliano, MD, PhD, of the University of Milan and European Institute of Oncology, shared results from the primary analysis of DESTINY-Breast06, which showed that T-DXd generated a statistically significant and clinically meaningful improvement in progression-free survival (PFS) vs chemotherapy in pretreated patients with hormone receptor–positive, HER2-low metastatic breast cancer. A comparable benefit was observed in a subgroup of patients with HER2-ultralow disease.

Patients with HER2-low disease who were given T-DXd (n = 359) experienced a median PFS of 13.2 months per blinded independent central review assessment vs 8.1 months for those given investigator’s choice of chemotherapy (n = 354; HR, 0.62; 95% CI, 0.51-0.74; P <.0001). In the HER2-ultralow population, the median PFS was 13.2 months for T-DXd (n = 76) vs 8.3 months for chemotherapy (n = 76; HR, 0.78; 95% CI, 0.50-1.21).

During a discussion following the presentation of the DESTINY-Breast06 data at the ASCO Annual Meeting, Ian Krop, MD, PhD, of Yale Cancer Center, addressed the status of testing for HER2 expression.

“Current IHC testing is relatively poor at distinguishing HER2-low and -ultralow cancers from HER2 0 cancers,” Krop said. “There are probably multiple reasons for this, but an important one is that the original test was designed to distinguish high levels of HER2 [IHC 3+] from all the lower levels. It was not designed to distinguish the very low levels to the even lower or 0 cancers.”

Pathologists have cited outdated practices as one of the main reasons for IHC testing inconsistencies. When IHC was originally created as an extension of a special stain in anatomic pathology, regulating the test as a stain was a logical choice; however, a lot has changed since the inception of the testing.4 In the 1990s, this pertinent issue became more pressing as IHC methods were adapted to expand testing for estrogen receptors and progesterone receptors.

Following the 1998 FDA approval of trastuzumab (Herceptin), the call to action amplified as new HER2-targeted agents began to enter the treatment paradigm, leading to a new era of companion diagnostics. In turn, there was a need for more rigorous methods of analytic standardization. Now, nearly 30 years and countless FDA approvals later, the issue is still present.

“If you give 100 different pathologists the same exact slide, you’re going to get different answers reading it, particularly for something like HER2,” Taylor told OncLive. “In my view, [understanding HER2-low and -ultralow cancers] is impossible. You may do it in a single lab that’s carefully calibrated internally by the pathologist, but to expect a second lab to get the same answer, in my view, is a bridge too far. It's not possible. It does a disservice to the whole process because you’re going to get poor answers.”

Future Implications for IHC Testing in Breast Cancer

Inconsistent IHC testing could have ramifications for both patients and pathologists due to the potential impact on patient outcomes and financial toxicities. As such, pathologists continue to call for regulatory reform to establish stringent standards for IHC testing in HER2-positive breast cancer.

“It’s essential that [regulatory agencies] get involved,” Taylor said. “Pathologists take pride in doing the best we can, but it's not our mandate or prerogative to necessarily make everything standard from lab to lab, but regulations help that occur. It tends to be costly to introduce regulations, and as pathologists, we’ve got this split issue.”

As of late, pathologists have faced the issue of stretching parameters in IHC testing.² Although IHC has traditionally aided in making diagnostic decisions, newer FDA-approved assays now extend this function to recommending optimal therapies. These companion diagnostic tests are challenging pathologists to make difficult decisions on subjective parameters in largely non-reproducible assays.

“We need to have tests be designed in such a way that if they’re intended to be read by pathologists, they need to optimize the pathologist’s ability to reproduce [the results], and we’ve certainly seen that’s not the case with the current tests,” Rimm said. “In my opinion, we should be moving past having them read; instead, [we need to] measure results. I define reading as one pathologist giving an expert opinion, but it’s subjective compared with a measurement, which is a standardized approach.”

References

1. IHC (ImmunoHistoChemistry) Tests. BreastCancer.org. Updated January 2, 2024. Accessed September 11, 2024. https://www.breastcancer.org/screening-testing/ihc-immunohistochemistry-tests
2. Rimm DL, Dacic S, Schnitt SJ. The Pathologists' Conundrum. Arch Pathol Lab Med. 2023;147(1):17-18. doi:10.5858/arpa.2022-0226-ED
3. Bell M, Chiriboga L, Chlipala E, et al. Immunohistochemistry as an assay. J Histotechnol. 2023;46(4):156-157. doi:10.1080/01478885.2023.2278384
4. Magnani B, Taylor CR. Immunohistochemistry Should Be Regulated as an Assay. Arch Pathol Lab Med. 2023;147(11):1229-1231. doi:10.5858/arpa.2023-0048-ED
5. FDA grants regular approval to fam-trastuzumab deruxtecan-nxki for breast cancer. FDA. May 4, 2022. Accessed September 11, 2024. https://www.fda.gov/drugs/resources-information-approved-drugs/fda-grants-regular-approval-fam-trastuzumab-deruxtecan-nxki-breast-cancer
6. FDA approves fam-trastuzumab deruxtecan-nxki for HER2-low breast cancer. FDA. August 5, 2022. Accessed September 11, 2024. https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-fam-trastuzumab-deruxtecan-nxki-her2-low-breast-cancer
7. Curigliano G, Hu X, Dent RA, et al. Trastuzumab deruxtecan (T-DXd) vs physician’s choice of chemotherapy (TPC) in patients (pts) with hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-low or HER2-ultralow metastatic breast cancer (mBC) with prior endocrine therapy (ET): primary results from DESTINY-Breast06 (DB-06). J Clin Oncol. 2024;42(suppl 17):LBA1000. doi:10.1200/JCO.2024.42.17_suppl.LBA1000