HER2-Low Breast Carcinoma

A Collaboration of Oncology, Pathology, Companion Diagnostic Test Manufacturers, and Pharma.
How Did We Get Here, Where Are We Currently, and Where Are We Headed?

Hardly a day goes by without a question from an oncologist or an inquisitive patient, an update to a pathology guideline, a push alert announcing a new therapy, or a clinical update on management of HER2-low breast carcinoma. Pathologists who specialize in solid tumor pathology are aware that patients with unresectable or metastatic breast carcinoma (mBCA) with HER2 IHC score 1+ or IHC 2+/HER2 FISH negative, referred to in the literature as HER2-low, are candidates for new antibody drug conjugate (ADC) therapy, specifically the FDA-approved drug trastuzumab deruxtecan (T-DXd).^1,2 The introduction of third-generation ADC therapy, in what are collectively called HER2 negative tumors as defined by ASCO/CAP guideline criteria, represents a paradigm shift from previous treatment protocols. In those protocols, patients without HER2 overexpression or gene amplification were deemed ineligible for monoclonal antibody therapy.

This summary is not written to provide guidelines on reporting HER2-low breast cancer, as this topic has been addressed in articles in CAP TODAY, in the CAP biomarker breast reporting template, and in the updated HER2 testing guidelines.^3-4 Rather, the point of this review is to provide a history of targeted therapy in breast cancer and focus more specifically on the sequential development ADC therapy in mBCA as established in the underlying clinical trials studies. Understanding the history of targeted therapy and the associated clinical trials highlights the critical role played by pathology, oncology, test manufacturers, and pharma in the expanding field of precision medicine. Pathologists need to understand the processes by which new therapies are derived and the crucial role they play in test interpretation, test implementation, and patient education.

Paul Ehrlich first proposed the concept of targeted drug therapy 100 years ago.⁵ A significant development in this field was noted with the introduction of targeted monoclonal antibody therapy in metastatic HER2 positive invasive breast cancer in 1998 with the approval of the drug trastuzumab. Subsequent monoclonal antibody therapies followed as noted by the approval of pertuzumab in 2012.

As a monoclonal antibody, trastuzumab works by binding to the HER2 protein receptor on tumor expressing cells and inhibits HER2 homodimerization, thus preventing HER2-mediated signaling.⁶ Pertuzumab on the other hand inhibits hetero-dimerization of HER2 with HER3, a related growth factor receptor.⁶ Both of these monoclonal antibodies are used in first-line treatment of unresectable or metastatic HER2 positive breast cancer and may also be administered in earlier stage disease.⁸

The HER2 gene is amplified in up to 20% of breast cancer cases.⁷ These amplified cases by definition are candidates for monoclonal antibody therapy as previously described. Currently, trastuzumab and pertuzumab in combination with a taxane is the current standard treatment for newly diagnosed HER2-positive metastatic breast cancer.^7,8 Use of this therapy has resulted in marked improvement in prognosis in patients with advanced and earlier-stage HER2-positive breast cancer.⁹

Despite the success of monoclonal antibody and taxane as first-line therapy with its impact on improved prognosis in mBCA patients, a subgroup of these patients fails to respond or experiences disease recurrence after this standard first-line therapy. Efforts were made to evaluate a more robust second-line therapy that could be used in this group of patients. Research led to the development of the ADC trastuzumab emtansine as an alternative to the current second-line therapy of lapatinib plus capecitabine.

Antibody drug conjugates function as tumoral antigen-specific antibodies connected via a linker to a potent cytotoxic agent known as the payload.¹⁰ The ADC trastuzumab emtansine incorporates the human epidermal growth factor receptor 2 (HER2)-targeted antitumor properties of trastuzumab with the cytotoxic activity of the microtubule-inhibitory agent DM1.¹¹ Use of this ADC therapy not only allows for targeted receptor binding and subsequent disruption of intracellular signaling pathways, but also for targeted transport of cytotoxic chemotherapy drugs specifically into cancer cells.⁵ This mode of action is the primary mechanism upon which ADCs function. The EMILIA clinical trial showed that trastuzumab emtansine (T-DM1) significantly prolonged progression-free and overall survival with less toxicity than the standard lapatinib plus capecitabine in patients with HER2-positive advanced breast cancer previously treated with trastuzumab and a taxane.¹¹

Treatment guidelines evolved based on these findings and use of the second-generation ADC T-DM1 in HER2-positive metastatic breast cancer became the standard second-line treatment for patients with disease progression following standard first-line therapy with trastuzumab and a taxane.^11,12,13

Despite advances in treatment and improved prognosis in the HER2-positive population, nearly 80% of metastatic or locally advanced invasive breast cancers are assessed as HER2-negative. These tumors historically were not candidates for anti-HER2 therapy and patients faced a poor prognosis if standard endocrine therapy or chemotherapy were no longer effective. Additionally, some HER2-positive patients became refractory to treatment with the second-generation ADC trastuzumab emtansine.

This led to subsequent research and the development of T-DXd, a third-generation ADC composed of anti-HER2 monoclonal antibody linked to a topoisomerase I inhibitor payload through a tetrapeptide-based cleavable linker.¹⁴ This ADC is notable for its high drug-to-antibody ratio of 8 and its cleavage by lysosomal enzymes that may lead to reduced systemic toxic effects.¹⁴ Preclinical results demonstrated that this new ADC was effective in T-DM1-insensitive, HER2-positive patient-derived xenograft (PDX) models and demonstrated antitumor efficacy against several breast cancer PDX models with low HER2 expression.¹⁴ A representation of T-DXd and its mode of action is available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8125530/

A series of clinical trials in humans, specifically the DESTINY clinical trial series, was initiated to evaluate use of this new third-generation ADC and its potential applications including in HER2-low breast cancer specimens in humans.

The phases of clinical trial development are noted below, followed by a summary of the DESTINY clinical trial series which led to the approval of T-DXd in HER2-low breast cancer.

Biomedical clinical trials of an experimental drug, treatment, device, or behavioral intervention proceed through four phases:¹⁵

Other clinical trials, including the phase 2 DAISY trial, focused on use of trastuzumab deruxtecan in metastatic breast cancer with variable HER2 expression and examined treatment response and resistance through biomarker analysis of tumor samples at different timepoints.¹⁹ The study showed the greatest objective response rate in cohort 1 (HER2 overexpressing tumors defined as IHC 3+ or ISH positive), followed by cohort 2 (HER2-low tumors, defined as IHC 2+/ISH-negative or IHC 1+). Interestingly, an objective response rate of 29.7% was also noted in cohort 3 (HER2 non-expressing defined as IHC 0) tumors.¹⁹ This suggests that very low levels of HER2 could allow for receptor binding of T-DXd and that the definition of HER2-low may need to be expanded to include HER2-Ultra low cases that show faint, barely perceptible staining that is greater than 0% but less than or equal to 10% (currently considered IHC 0).

In summary, the optimal treatment protocols, HER2 IHC scoring, nomenclature, and testing modalities in HER2-negative metastatic or locally advanced breast carcinoma will continue to evolve with additional trial data. Concerns regarding reproducibility between pathologists for separating HER2 0 and HER2 1+ IHC persist.²⁰ Pathologists will need to be prepared to consider alternative assays or alternative testing modalities to better discriminate low levels of HER2 protein expression. Future findings could lead to changes in HER2 result algorithms and reporting descriptors as new data is published. At this time, however, the reporting guidelines for reporting HER2 results remain unchanged with the caveat that a comment for HER2 1+ tumors may be considered “HER2-low” for purposes of precision-guided therapy.⁴ The evolution of precision medicine in the sphere of low-expression HER2 breast carcinoma cases will require a collaborative effort between pathologists, oncologists, test manufacturers, and pharmaceutical companies.

1. Food & Drug Administration. FDA approves fam-trastuzumab deruxtecan-nxki for HER2-low breast cancer. https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-fam-trastuzumab-deruxtecan-nxki-her2-low-breast-cancer
2. Modi S, Jacot T, Yamashita J, et al. Trastuzumab deruxtecan in previously treated HER2-low advanced breast cancer. N Engl J Med. 2022;387:9-20.
3. College of American Pathologists. Template for Reporting Results of Biomarker Testing of Specimens from Patients With Carcinoma of the Breast. https://documents.cap.org/documents/Breast.Bmk_1.5.0.1.REL_CAPCP.pdf
4. College of American Pathologists. HER2 Testing in Breast Cancer - 2023 Guideline Update.
   https://www.cap.org/protocols-and-guidelines/cap-guidelines/current-cap-guidelines/recommendations-for-human-epidermal-growth-factor-2-testing-in-breast-cancer
5. Xiao T, Ali S, Mata DG, et al. Antibody-drug conjugates in breast cancer: Ascent to destiny and beyond-A 2023 review. Curr Oncol. 2023;30:6447-6461.
6. Greenblatt K, Khaddour K. Trastuzumab. In: StatPearls. StatPearls Publishing; 2024. Trastuzumab - StatPearls - NCBI Bookshelf (nih.gov)
7. Cortes J, Kim S-B, Chung W-P, et al. Trastuzumab deruxtecan versus trastuzumab emtansine for breast cancer. N Engl J Med. 2022;386:1143-1154.
8. National Comprehensive Cancer Network. Breast cancer, version 1.202. https://www.nccn.org/professionals/physician_gls/pdf/breast.pdf
9. Shirman Y, Lubovsky S, Shai Ayelet. HER2-low breast cancer: current landscape and future prospects. Breast Cancer. 2023;15:605-616.
10.  Fu Z, Li S, Han S et al. Antibody drug conjugate: The “biological missile” for targeted cancer therapy. Sig Transduct Target Ther. 2022;7:93.
11. Verma S, Miles D, Gianni L, et al. Trastuzumab emtansine for HER2-positive advanced breast cancer. N Engl J Med. 2012;367:1783-91.
12. Amiri-Kordestani L, Blumenthal GM, Xu QC, et al. FDA approval: ado-trastuzumab emtansine for the treatment of patients with HER2-positive metastatic breast cancer. Clin Cancer Res. 2014;20(17):4436-4441. doi: 10.1158/1078-0432.CCR-14-0012.
13. National Comprehensive Cancer Network. Breast cancer, version 1.2021 https://www.nccn.org/professionals/physician_gls/pdf/breast.pdf
14. Ogitani Y, Aida T, Hagihara K, et al. DS-8201a, a novel HER2-targeting ADC with a novel DNA topoisomerase I inhibitor, demonstrates a promising antitumor efficacy with differentiation from T-DM1. Clin Cancer Res. 2016;22:5097-108.
15. National Institutes of Health. NIH Central Resource for Grants and Funding-Glossary. https://grants.nih.gov/grants/glossary.htm#ClinicalTrial
16. Modi S, Saura C, Yamashita T, et al. DESTINY-Breast01 Investigators. Trastuzumab deruxtecan in previously treated HER2-Positive breast cancer. N Engl J Med. 2020;382(7):610-621.
17. Andre F, Park YH, Kim S-B, et al. Trastuzumab deruxtecan versus treatment of physician’s choice in patients with HER2-positive metastatic breast cancer (DESTINY-Breast02): a randomized, open-label, multicentre, phase 3 trial. Lancet. 2023;401:1773-1785.
18. National Library of Medicine. Clinicaltrials.gov https://clinicaltrials.gov/study/NCT04622319?term=DESTINY%20Breast-05&rank=1#locations
19. Mosele F, Deluche E, Lusque A, et al. Trastuzumab deruxtecan in metastatic breast cancer with variable HER2 expression: the phase 2 DAISY trial. Nat Med. 2023;29(8):2110-2120.
20. Fernandez AI, Liu M, Bellizzi A, et al. Examination of Low ERBB2 Protein Expression in Breast Cancer Tissue. JAMA Oncol. 2022;8(4):607–610.

Ronald Paler Jr., MD, FCAP, serves as an anatomic and cytopathology board-certified pathologist. He attended Wayne State University School of Medicine in Detroit and performed residency training at Los Angeles County + USC Medical Center in Los Angeles. He has practiced pathology in the commercial reference laboratory setting for 24 years. Earlier in his career, he worked as Chief Pathologist and Laboratory Medical Director at CND Life Sciences and as Medical Director and Surgical Pathologist at Labcorp Oncology (formerly Impath and Genzyme Genetics) for 21 years, the last eight years as Medical Director in Phoenix. His accomplishments include implementation of PD-L1 companion diagnostic testing, HPV mRNA ISH testing, digital pathology, novel IHC markers, EGFR, KRAS, and NRAS molecular testing, and the launch of new LIS systems. He has served on various committees within the College of American Pathologists, including the 15189 Committee and the CAP Personalized Healthcare Committee, as well as the House of Delegates. Dr. Paler is an ardent champion of laboratory quality and ISO 15189 principles.