Comparison of HercepTest™ mAb pharmDx (Dako Omnis, GE001) with Ventana PATHWAY anti-HER-2/neu (4B5) in breast cancer: correlation with HER2 amplification and HER2 low status

HER2-low across solid tumours: different incidences and definitions

Antibody-drug conjugates, particularly trastuzumab deruxtecan (T-DXd), have emerged as effective therapies for various solid tumours. Clinical trials show that T-DXd improves survival in both HER2-positive and HER2-low breast cancer patients. Additionally, it improves survival in HER2-positive gastro-oesophageal cancer and elicits objective responses in HER2-low tumours. Responses have also been noted in lung and gynaecological cancers with HER2 expression, although subgroup analyses for HER2-low cases are lacking. This review assesses HER2 protein expression levels and gene amplification across solid tumours where T-DXd shows potential benefits. We focus on the accuracy and limitations of HER2 testing methods, particularly for identifying HER2-low cancer. A semi-systematic approach was employed, searching EMBASE, Medline, Cochrane, and PubMed databases. We calculated median incidences of HER2-positive, HER2-low, and HER2-0 by immunohistochemistry (IHC), and HER2 amplification by in situ hybridisation (ISH). A total of 144 studies were included, covering breast (n=57), gastro-oesophageal (n=33), lung (n=17), gynaecological (n=24), and various other carcinomas (n=13). The median incidences of HER2-low were 52%, 16%, 58%, and 17% in breast, gastro-oesophageal, endometrial, and ovarian cancers, respectively, with unknown incidences in lung and cervical cancers. Factors influencing HER2-low detection include tumour heterogeneity, antibody clones, observer variability, and lack of validated scoring criteria. Given the significant proportion of HER2-low cases, many patients could benefit from T-DXd, but limitations in detection accuracy necessitate further research and standardisation in diagnostic methods and criteria to advance the clinical utility of T-DXd for HER2-low tumours.

Best pathology practices for HER2 testing interpretation, including HER2-low and ultralow, in the era of HER2-directed antibody drug conjugates (ADCs), aligned with the 2018 CAP/ASCO HER2 guidelines

With the emergence of studies showing clinical response to novel human epidermal growth factor receptor 2(HER2)-targeted treatments among HER2-low and HER2-ultralow metastatic breast cancer patients, this gives hope to patients failing to respond to conventional treatment. Hence, the interest in identifying these group of patients is becoming important. Pathologists' interpretation and scoring of HER2 immunohistochemistry (IHC) was challenged from a dichotomous way to a ternary way of reporting. But HER2 IHC testing is affected by a number of pre-analytic, analytic and post-analytic challenges that could affect interpretation at the lower HER2 protein expression range. Interobserver variability among pathologists and inter-assay variabilities likewise posses a challenge since prior guidelines' emphasis were focused on recognizing HER2-overexpressed from HER2-non-overexpressed tumours. In consideration of these clinical trials, the American Society of Clinical Oncologists and College of American Pathologists (ASCO/CAP) updated and reaffirmed the guideline on HER2 testing in 2018 and in 2023, with the criteria for HER2 categories redefined. Here, we reviewed the definition and differences among HER2-low, HER2-ultralow and HER2-null breast cancer cases as well as the different challenges on HER2 IHC testing encountered and reported, including best pathology practice recommendations to avaoid or minimize these challenges.

Redefining Human Epidermal Growth Factor Receptor 2 Testing in Breast Cancer in the Era of Novel Antibody-Drug Conjugates

The advent of next-generation antibody-drug conjugates (ADCs), particularly trastuzumab deruxtecan (T-DXd), has transformed our understanding of human epidermal growth factor receptor 2 (HER2) targetability for breast cancer (BC) treatment. Historically categorized as HER2-positive or HER2-negative on the basis of trastuzumab eligibility, this classification has evolved significantly over the past 5 years. The DESTINY-Breast04 trial marked the entry of anti-HER2 therapies for patients with HER2-low BC, while DESTINY-Breast06 demonstrated the potential for earlier and broader use of T-DXd. The latter trial revealed that even minimal HER2 expression in tumors previously classified as HER2-0 might be clinically relevant and targetable with T-DXd. This has led to further refinement of HER2 classification, introducing the concepts of HER2-ultralow (HER2-0 with staining) and HER2-null BC (HER2-0 without staining). With these findings, most patients with metastatic BC are currently considered eligible for T-DXd. Accurately identifying candidates for these therapies has highlighted the limitations of current HER2 diagnostic practices, on the basis of immunohistochemistry (IHC)/in situ hybridization assessment. IHC assay, optimized to detect high levels of HER2 protein, faces limitations in discriminating finer variations at the lower end of the HER2 expression spectrum. This is further complicated by the heterogeneity of HER2 expression. To overcome these barriers, new approaches may be required. Quantitative methods for HER2 membrane assessment, genomic and transcriptomic evaluations of HER2, and the integration of artificial intelligence into tissue analysis hold promise and are currently under investigation. Additionally, noninvasive strategies, such as analysis of circulating tumor DNA or circulating tumor cells, may enable real-time HER2 status assessment and better patient selection for ADC. However, these techniques require rigorous validation to ensure their clinical utility. This evolving landscape underscores the need for improvement of diagnostic approaches to support the expanding role of ADCs in BC treatment.

HER2-Low Breast Cancer-Current Knowledge and Future Directions

The concept of binary classification of HER2 status has recently been challenged following the DESTINY-Breast trial data showing a clinically meaningful response to antibody-drug conjugates (ADCs) in invasive breast cancer expressing low levels of HER2. HER2-low breast cancer is defined as an immunohistochemistry (IHC) score of 1+ and 2+ without HER2 gene amplification. While HER2-low breast cancer does not represent a biological entity, it encompasses both hormone receptor-positive and triple-negative breast cancer. Differences exist between this group and HER2-null breast cancer. In this review, we provide an update on HER2-low and HER2-ultralow breast cancer, including background trial data, the evolution of HER2-low expression, current clinical guidelines, quality issues, and future directions.

Systematic Analysis of Factors Affecting Human Epidermal Growth Factor Receptor 2 Interpretation Consistency: Staining Protocols, Artificial Intelligence-Based Image Standardization, and Classification Criteria

The efficacy of human epidermal growth factor receptor 2 (HER2)-targeting antibody-drug conjugates has underscored the critical need for precise HER2 diagnostics in breast cancer treatment. Despite the clinical importance, variability in immunohistochemical (IHC) staining protocols and interobserver inconsistencies challenge the reliability of HER2 status assessment, which is critical for guiding patient treatment strategies. To investigate the factors affecting HER2 interpretation consistency, tissue microarrays from 1063 breast carcinoma cases underwent 3 distinct IHC protocols, and a novel artificial intelligence (AI) model was developed to standardize HER2-stained images. A total of 5 sets of tissue microarrays (Nordi QC, protocol 1, protocol 2, protocol 1 AI, and protocol 2 AI) were independently reviewed by 8 pathologists. The Fleiss Kappa value and overall agreement rate measured interobserver agreement, with logistic regression analyzing the impact of variables on diagnostic accuracy. Our results showed that the Nordi QC protocol had the highest interobserver agreement (Kappa 0.754). AI-based image normalization notably enhanced consistency, particularly for HER2 low cases, aligning scores toward the Nordi QC standard. Logistic regression analysis indicated that both staining protocol and AI-based image standardization significantly influenced diagnostic accuracy (P < .001). The American Society of Clinical Oncology/College of American Pathologists 2018 binary criteria demonstrated the highest HER2 interobserver consistency (Kappa > 0.95). Compared with the American Society of Clinical Oncology/College of American Pathologists 2023 criteria, the newly proposed null, ultra-low/low, positive criteria, merging HER2 low and ultra-low categories, demonstrated improved reliability and agreement, especially in distinguishing the challenging HER2-ultra-low cases, which showed an exceedingly low interobserver agreement (Kappa < 0.20) across all protocols. Overall, variability in IHC staining protocols and HER2 classification criteria significantly affect the diagnostic consistency among pathologists. The integration of an AI model for image standardization and the adoption of the null, ultra-low/low, positive criteria may refine diagnostic precision and bolster clinical decision-making in breast cancer treatment.

Clinicopathological characteristics and prognostic significance of HER2 status evaluation in patients with urothelial carcinoma: a retrospective single-center experience in China

Due to the lack of an in-depth evaluation of urothelial carcinoma (UC) HER2 expression in both primary and recurrent UC, we examined possible clinical and pathological variables associated with altered UC HER2 expression. The HER2 status and clinicopathological features of primary UCs and matched recurrences from 2015 to 2023 (n = 1352) were compared by evolution category (stable or altered). In 1352 UC patients, HER2 protein expression was linked with age, histological grade, and recurrence. HER2 was expressed in 794 UC patients (58.7%) (1 + , 2 + , and 3 +). High-level (HER2 scored 2 + and 3 +) HER2 expression does not correlate with gene amplification. Significant differences in HER2 expression exist between initial and recurrent tumors (p < 0.05), including a shift from positive to negative expression. Recurrent UC patients showed associations between HER2 expression, histological grade (p < 0.05), and time to surgical recurrence (p = 0.008). HER2 expression in high-grade UC correlated positively with PD-L1 expression in tumor cells (TC) (rs = 0.222, p = 0.047) and tumor-associated immune cells (IC) (rs = 0.238, p = 0.032), especially in recurrent patients (rs = 0.464, p = 0.022). HER2 expression was associated with the biological behavior and prognosis of UC patients. Increased HER2 expression in recurrent UC indicates a poorer prognosis. Consequently, retesting for HER2 is essential in cases of recurrent UC. The association between HER2 and PD-L1 expression in high-grade, recurrent UC suggests the potential for utilizing ADC alongside immunotherapy.

Interobserver consistency and diagnostic challenges in HER2-ultralow breast cancer: a multicenter study

BACKGROUND

Recent advancements in novel antibody-drug conjugates (ADCs) have demonstrated efficacy in patients with human epidermal growth factor receptor 2 (HER2)-ultralow breast cancer (BC), expanding the eligibility for anti-HER2 targeted therapy to include some patients previously categorized as HER2 immunohistochemistry (IHC) 0. This expansion underscores the need for pathologists to accurately differentiate HER2-null and HER2-ultralow.

MATERIALS AND METHODS

Thirty-six pathologists from four centers nationwide conducted microscopic visual assessments on HER2 IHC slides from 50 consecutive BC surgical specimens, all previously diagnosed as HER2 IHC 0.

RESULTS

The interobserver consistency in differentiating HER2-null from HER2-ultralow, measured by Fleiss κ, was only 0.230-lower than the consistency for combined HER2 IHC 0 cases (Fleiss κ = 0.344) and binary classification (HER2-null versus HER2-non-null; Fleiss κ = 0.292). High agreement for HER2-null versus HER2-ultralow differentiation was achieved in only 4% of cases, while combining them into HER2 IHC 0 raised high agreement cases to 32%, higher than the 18% seen in the binary classification. Consensus among the 36 pathologists aligned with historical scores in 72% of cases; however, when subdividing HER2 IHC 0 into HER2-null and HER2-ultralow, the consistency dropped to 54%.

CONCLUSIONS

The low consistency among pathologists in distinguishing HER2-null, -ultralow, and 1+ cases may impact patient eligibility for new ADC therapies. To address this challenge, there is a need for improved detection methods, artificial intelligence-assisted quantitative assessments, and larger clinical datasets to refine the definition of HER2-ultralow.

Exploring the heterogeneity of HER2 gene status and expression in non-positive breast cancer patients: insights from immunohistochemistry and fluorescence in situ hybridization

Breast cancer became the most prevalent malignancy among women, and HER2 expression status is critical for treatment decisions. With the emergence of ADC drugs, HER2 low-expressing patients who previously did not respond well to traditional anti-HER2 therapies may now benefit. In this study, immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) were applied to assess HER2 expression in 349 patients with HER2-non-positive breast cancer. Our analysis revealed that HER2-low tumors exhibited fewer grade III tumors (39.74% and 55.65%, respectively, P = 0.005) and higher positivity for estrogen receptor (ER, 88.89% vs. 61.74%, P < 0.001) and progesterone receptor (PR, 84.62% vs. 57.39%, P < 0.001) compared to HER2-ZERO tumors. Of the 349 cases, IHC was ultimately evaluated in 327, the antibodies demonstrated only 64.22% (95% CI: 58.76-69.42%) agreement between clone 4B5 and clone EP3. Pathologist 1, who had more extensive working experience, demonstrated higher consistency (94.19%) with the gold standard when using clone EP3, compared to Pathologist 2 (74.31%). FISH analysis revealed significant differences in HER2/CEP17 ratio and average HER2 copy numbers between HER2-ZERO and HER2-low tumors, but no clear cut-off value could be identified. Notably, HER2/CEP17 ratio mostly between 1 and 2, with HER2-ZERO tumors primarily ≤ 1.4, and average HER2 copy numbers were mostly ≥ 2 and < 4, with HER2-ZERO tumors primarily ≤ 2.5. Despite distinct clinicopathological features, FISH remains inadequate for distinguishing HER2-low from HER2-ZERO expression. Further studies are needed to improve HER2 assessment in this challenging subset of patients.

Evaluation of trophoblast cell surface antigen-2 (TROP2) protein expression in chemotherapy-resistant and metastatic breast carcinomas

Trophoblast cell-surface antigen 2 (TROP2), a transmembrane receptor expressed in many carcinomas, is a target for novel antibody-drug conjugates such as sacituzumab govitecan. TROP2-targeted therapy is used for unresectable locally advanced or metastatic triple-negative and hormone receptor-positive, HER2-negative breast cancers. The role of TROP2 as a predictive marker is yet unclear. Standardized interpretation criteria for TROP2 immunohistochemistry (IHC) are lacking. Here, we compared three antibody clones and two methods for semi-quantitative assessment, aiming to establish reproducible evaluation criteria. First, TROP2 IHC was performed on normal tissues and nine breast cancers, using the BSB-148, EPR20043 and SP293 clones. EPR20043 was selected for subsequent evaluation in 69 breast cancers without pathological complete response to neoadjuvant chemotherapy (NAC). Four pathologists applied the ASCO/CAP guidelines for HER2 IHC testing (designated as the 'membrane score') and the H-score. All H-scores were categorized as low (0-100), intermediate (101-200) and high (201-300). Although the membrane scores strongly correlated with the categorized H-scores, the latter showed higher interobserver variability. Next, TROP2 IHC was performed on 94 breast cancer metastases and evaluated by six pathologists, confirming the strong correlation between the membrane scores and H-scores. In metastases, the interobserver variability was similar for both methods. Our observations support the application of the HER2 ASCO/CAP guidelines for semi-quantitative evaluation of membranous TROP2 protein expression, as this method strongly correlates with the H-score and is less prone to interobserver variability in post-NAC breast resections. Future studies should investigate the association between the TROP2 membrane score and response to TROP2-targeted therapy.

The "lows": Update on ER-low and HER2-low breast cancer

ER-low and HER2-low breast cancers have emerged as clinically significant subtypes that challenge traditional diagnostic categories and treatment paradigms. These subtypes, representing a spectrum of disease, exhibit distinct biological behaviors, therapeutic responses, and prognostic outcomes. HER2-low breast cancer, defined by low HER2 protein expression (IHC score of 1+ or 2+ without HER2 gene amplification), has achieved clinical significance, particularly following the DESTINY-Breast trials, which demonstrated the efficacy of trastuzumab deruxtecan (T-DXd) in the population of patients with advanced HER2-low disease. Similarly, ER-low breast cancer, characterized by low estrogen receptor expression (in 1%-10 % invasive tumor cells), poses unique challenges due to its intermediate biological behavior and uncertain response to endocrine therapies. The identification of these subtypes is further complicated by inconsistencies in testing methodologies, which can lead to misclassification and impact treatment decisions. As our understanding of these subtypes improves, the need for standardized diagnostic approaches and individualized therapeutic decisions becomes increasingly urgent. Ongoing research and collaboration between pathologists and oncologists are essential for refining diagnostic criteria and improving outcomes for patients with breast cancers characterized by low expression of these theragnostic biomarkers. This review aims to consolidate current knowledge on HER2-low and ER-low breast cancers, focusing on the challenges associated with their identification, the implications for treatment, and future directions in clinical management. By examining recent studies and interlaboratory assessments, this review emphasizes the critical need for accurate and reproducible testing and reporting, and for the development of tailored therapeutic strategies for these "low" expression cancers.

The efficacy of trastuzumab-deruxtecan for the treatment of patients with advanced HER2-low breast cancer

INTRODUCTION

Until recently, the available human receptor epidermal growth factor 2 (HER2) targeted agents were ineffective for treating patients with HER2-low expressing breast cancer (defined as immunohistochemical expression of 1+ or 2+ without amplification). The development of novel and potent HER2-directed antibody-drug conjugates, affective at treating HER2-low expressing breast cancers, have changed the way we think about HER2-low expression and expanded the treatment options for many patients with advanced disease.

AREAS COVERED

In this review, we summarize the current management of HER2-low breast cancer and commonly encountered challenges such as treatment sequencing and toxicity management.

EXPERT OPINION

trastuzumab deruxtecan (T-DXd) is a treatment option for patients with advanced, HER2-low breast cancer, irrespective of the hormone receptor status. The current optimal place in treatment algorithms is after the first line of chemotherapy, both in HR-positive and triple-negative breast cancer; however, other agents are available in this setting and risks and benefits for each should be considered in shared decision making. Up to 10-15% of patients receiving T-DXd develop interstitial lung disease. Patient and clinician education are key to safely implement T-DXd in clinical practice.

Comparing the Sensitivity of HER2 Epitope Detection of HercepTest mAb pharmDx (Dako Omnis, GE001) and Ventana PATHWAY Anti-HER-2/neu (4B5) Using IHC Calibrators

Accurate assessment of HER2 expression levels is paramount for determining eligibility for targeted therapies. HER2 immunohistochemistry provides a semiquantitative measurement of HER2 protein overexpression. Historically, little focus has been on the lower end of the HER2 expression range. The advent of novel therapeutic molecules that require fewer membrane epitopes to be effective has prompted a reevaluation of the current immunohistochemistry testing protocols, with special emphasis on the detection limit. Here, we have used Boston Cell Standards technology to determine the sensitivity of 2 commercially available HER2 immunohistochemistry assays, including a lower limit of detection.

Clinical characterization, prognostic, and predictive values of HER2-low in patients with early breast cancer in the PALLAS trial (ABCSG-42/AFT-05/BIG-14-13/PrE0109)

BACKGROUND

Bidirectional crosstalk between HER2 and estrogen receptor (ER) pathways may influence outcomes and the efficacy of endocrine therapy (ET). Low HER2 expression levels (HER2-low) have emerged as a predictive biomarker in patients with breast cancer (BC).

METHODS

PALLAS is an open, international, phase 3 study evaluating the addition of palbociclib for 2 years to adjuvant ET in patients with stage II-III ER-positive/HER2-negative BC. To assess the impact of HER2 expression on patient outcomes in the phase III PALLAS trial, we analyzed (1) the association between rate of HER2-low with demographic and clinicopathological parameters, (2) the prognostic value of HER2-low status on invasive disease-free survival (iDFS), distant relapse-free survival (DRFS), and overall survival (OS) and (3) HER2 expression's value as a predictive biomarker of response to palbociclib. HER2-low was defined as HER2 immunohistochemistry (IHC) 1 + or IHC 2 + with negative in situ hybridization (ISH). All pathologic evaluation was performed locally. Prognostic and predictive power of HER2 were assessed with Cox models.

RESULTS

From the original PALLAS intention-to-treat population (N = 5753), 5304 patients (92.2%) were included in this analysis. Among these, 2254 patients (42.5%) were classified as having HER2 IHC 0 (HER2-0), and 3050 (57.5%) as having HER2-low disease (1838 with IHC 1 + and 1212 with IHC 2 +). Median follow-up was 59.8 months. HER2-low prevalence varied significantly across 21 participating countries (range 16.7% to 75.6%; p < 0.001) and was more frequent in patients enrolled in North America (63.1%) than in Europe (53.4%) or other regions (53.4%) (p < 0.001). HER2 status was not significantly associated with iDFS in a multivariable Cox model (hazard ratio 0.93, 95% confidence interval 0.81 - 1.06). No significant interaction was observed between treatment arm and HER2 status for iDFS (p = 0.43). Similar results were obtained for DRFS and OS.

CONCLUSIONS

In this large, prospective, global patient cohort, no differences were observed in clinical parameters, prognosis, or differential benefit from palbociclib between HER2-0 and HER2-low tumors. Significant geographic variability was observed in the prevalence of HER2-low status, suggesting a high degree of variation in pathologic assessment of HER2 expression without impact on outcomes.

Best practices for achieving consensus in HER2-low expression in breast cancer: current perspectives from practising pathologists

AIMS

Human epidermal growth factor receptor 2 (HER2) expression is an important biomarker in breast cancer (BC). Most BC cases categorised as HER2-negative (HER2-) express low levels of HER2 [immunohistochemistry (IHC) 1+ or IHC 2+/in-situ hybridisation not amplified (ISH-)] and represent a clinically relevant therapeutic category that is amenable to targeted therapy using a recently approved HER2-directed antibody-drug conjugate. A group of practising pathologists, with expertise in breast pathology and BC biomarker testing, outline best practices and guidance for achieving consensus in HER2 IHC scoring for BC.

METHODS AND RESULTS

The authors describe current knowledge and challenges of IHC testing and scoring of HER2-low expressing BC and provide best practices and guidance for accurate identification of BCs expressing low levels of HER2. These expert pathologists propose an algorithm for assessing HER2 expression with validated IHC assays and incorporate the 2023 American Society of Clinical Oncology and College of American Pathologist guideline update. The authors also provide guidance on when to seek consensus for HER2 IHC scoring, how to incorporate HER2-low into IHC reporting and present examples of HER2 IHC staining, including challenging cases.

CONCLUSIONS

Awareness of BC cases that are negative for HER protein overexpression/gene amplification and the related clinical relevance for targeted therapy highlight the importance of accurate HER2 IHC scoring for optimal treatment selection.

Consensus Guidelines on Human Epidermal Growth Factor Receptor 2 (HER2)-Low Testing in Breast Cancer in Malaysia

Breast cancer is one of the most common cancers in Malaysia. Recently, a new nomenclature was introduced for breast cancers with human epidermal growth factor receptor 2 (HER2) immunohistochemistry (IHC) 1+, or 2+ with negative in situ hybridization (ISH), i.e., HER2-low breast cancer. In current clinical practice, these breast cancers are reported as HER2-negative. Clinical trials have shown that HER2-low breast cancer benefits from targeted therapy with anti-HER2 antibody-drug conjugates. Unfortunately, various challenges and obstacles are faced by local pathologists in HER2 testing, which may jeopardize the standard of care for patients with HER2-low breast cancer. This consensus guideline aims to elucidate standard practices pertaining to HER2 testing and HER2-low interpretation in Malaysia. Topics discussed among a panel of local experts include tissue sampling and handling, assay and antibody selection, result interpretation and reporting, and quality assurance. Practice recommendations made in this consensus guideline reflect current international guidelines and, where appropriate, adapted to the Malaysian landscape.

Performance of HER2 DAKO HercepTest and Ventana 4B5 immunohistochemical assays on detecting HER2 gene-amplification in uterine serous carcinomas

We compared the performance of two commonly-used HER2 immunohistochemistry (IHC) assays in uterine serous carcinomas (USC), correlating with HER2 gene amplification by fluorescence in-situ hybridization (FISH). Sixty-five USCs were stained by both HercepTest™ and PATHWAY 4B5 assays. FISH was performed by HER2 IQFISH pharmDx. Consensus HER2 IHC scoring was performed, and HER2 testing results were evaluated using USC-specific criteria. Complete concordance between HercepTest and 4B5 assays was achieved in 44/65 tumors (68%). The overall HER2 IHC/FISH concordance was 94% (45/48) by HercepTest and 91% (42/46) by 4B5. All HER2 IHC 3+ cases with HercepTest (n = 6) and 4B5 (n = 4) were gene-amplified, corresponding to specificities of 100%. For cases with IHC 2+, 41% (7/17) by HercepTest and 42% (8/19) by 4B5 had HER2 gene amplification. The sensitivity for HercepTest and 4B5 were 38% and 25%, respectively, at a cut-off of IHC 3+ (P = 0.50), and were 81% and 75%, respectively, at a cut-off of IHC 2+ (P > 0.99). Among HER2 IHC 0-1+ cases, 3/42 cases by HercepTest and 4/42 cases by 4B5 showed amplified FISH results, corresponding to overall false negative rates of 19% for HercepTest and 25% for 4B5. By using USC-specific IHC scoring criteria, both HercepTest and 4B5 assays showed high specificities (100%) for HER2 gene amplification in IHC 3+ cases, high IHC/FISH concordance, and comparable sensitivity for detecting HER2 gene amplification. The notable false negative rates using IHC 2+ as a cut-off for reflexing FISH analysis may warrant consideration for performing FISH in IHC 1+ cases until more data become available.

Trastuzumab deruxtecan for human epidermal growth factor receptor 2-low advanced or metastatic breast cancer: recommendations from the Japanese Breast Cancer Society Clinical Practice Guidelines

The Japanese Breast Cancer Society Clinical Practice Guidelines are published as timely guidance on clinical issues in breast cancer treatment in Japan. In the recent edition of these guidelines, we addressed a new clinical question 34 (CQ 34, systemic treatment part) "Is trastuzumab deruxtecan recommended for patients with unresectable or metastatic HER2-low breast cancer?" and a new future research question 7 (FRQ 7, pathological diagnosis part) "How is HER2-low breast cancer diagnosed for the indication of trastuzumab deruxtecan?". These questions address use of trastuzumab deruxtecan in patients with unresectable or metastatic HER2-low breast cancer who have previously received chemotherapy for metastatic disease. The strengths of evidence and recommendation were determined through a quantitative and qualitative systematic review using multiple outcomes, including efficacy and safety. We conclude that trastuzumab deruxtecan is recommended for this patient population (strength of recommendation: 1; strength of evidence: moderate; CQ34) and that HER2-low expression for the indication of trastuzumab deruxtecan should be diagnosed using companion diagnostics based on appropriate criteria (FRQ7).

Open questions, current challenges, and future perspectives in targeting human epidermal growth factor receptor 2-low breast cancer

Approximately 60% of traditionally defined human epidermal growth factor receptor 2 (HER2)-negative breast cancers express low levels of HER2 [HER2-low; defined as immunohistochemistry (IHC) 1+ or IHC 2+/in situ hybridization (ISH)-]. HER2-low breast cancers encompass a large percentage of both hormone receptor-positive (up to 85%) and triple-negative (up to 63%) breast cancers. The DESTINY-Breast04 trial established that HER2-low tumors are targetable, leading to the approval of trastuzumab deruxtecan (T-DXd) as the first HER2-directed therapy for the treatment of HER2-low breast cancer in the United States and Europe. This change in the clinical landscape results in a number of questions and challenges-including those related to HER2 assessment and patient identification-and highlights the need for careful assessment of HER2 expression to identify patients eligible for T-DXd. This review provides context for understanding how to identify patients with HER2-low breast cancer with respect to sample types, scoring and reporting HER2 status, and testing methods and assays. It also discusses management of important T-DXd-related adverse events. Available evidence supports the efficacy of T-DXd in patients with any history of IHC 1+ or IHC 2+/ISH- scores; however, future research may further refine the population who could benefit from T-DXd or other HER2-directed therapies and identify novel methods for patient identification. Because HER2 expression can change with disease progression or treatment, and variability exists in scoring and interpretation of HER2 status, careful re-evaluation in certain scenarios may help to identify more patients who may benefit from T-DXd.

Predictive and prognostic biomarkers in breast tumours

In the age of precision medicine, extensive research has investigated tumour biomarkers to predict the behaviour of cancer and/or response to treatment in order to better understand the prognosis and treatment of disease. In breast cancer, significant progress has been made to categorise a common disease into subtypes defined by intrinsic tumour biology, measured by tumour biomarkers. This review encompasses the established biomarkers within breast cancer with the most up-to-date information regarding their understanding and clinical use as predictive and/or prognostic markers of breast cancer.

Emerging Landscape of Targeted Therapy of Breast Cancers With Low Human Epidermal Growth Factor Receptor 2 Protein Expression

CONTEXT.—

Human epidermal growth factor receptor 2 (HER2) status in breast cancer is currently classified as negative or positive for selecting patients for anti-HER2 targeted therapy. The evolution of the HER2 status has included a new HER2-low category defined as an HER2 immunohistochemistry score of 1+ or 2+ without gene amplification. This new category opens the door to a targetable HER2-low breast cancer population for which new treatments may be effective.

OBJECTIVE.—

To review the current literature on the emerging category of breast cancers with low HER2 protein expression, including the clinical, histopathologic, and molecular features, and outline the clinical trials and best practice recommendations for identifying HER2-low-expressing breast cancers by immunohistochemistry.

DATA SOURCES.—

We conducted a literature review based on peer-reviewed original articles, review articles, regulatory communications, ongoing and past clinical trials identified through ClinicalTrials.gov, and the authors' practice experience.

CONCLUSIONS.—

The availability of new targeted therapy potentially effective for patients with breast cancers with low HER2 protein expression requires multidisciplinary recognition. In particular, pathologists need to recognize and identify this category to allow the optimal selection of patients for targeted therapy.

Standardized pathology report for HER2 testing in compliance with 2023 ASCO/CAP updates and 2023 ESMO consensus statements on HER2-low breast cancer

Since the release of the DESTINY-Breast04 (DB-04) trial findings in June 2022, the field of pathology has seen a renaissance of HER2 as a predictive biomarker in breast cancer. The trial focused on patients with metastatic breast cancer who were classified as "HER2-low," i.e., those with immunohistochemistry (IHC) HER2 1 + or 2 + and negative in situ hybridization (ISH) results. The study revealed that treating these patients with trastuzumab deruxtecan (T-DXd) instead of the oncologist's chosen chemotherapy led to outstanding improvements in survival. This has challenged the existing binary HER2 pathological classification system, which categorized tumors as either positive (overexpression/amplification) or negative, as per the ASCO/CAP 2018 guideline reaffirmed by ASCO/CAP 2023 guideline update. Given that DB-04 excluded patients with HER2 IHC score 0 status, the results of the ongoing DB-06 trial may shed further light on the potential benefits of T-DXd therapy for these patients. Roughly half of all breast cancers are estimated to belong to the HER2-low category, which does not represent a distinct or specific subtype of cancer. Instead, it encompasses a diverse group of tumors that exhibit clinical, morphological, immunohistochemical, and molecular variations. However, HER2-low offers a distinctive biomarker status that identifies a specific therapeutic regimen (i.e., T-DXd) linked to a favorable prognosis in breast cancer. This unique association emphasizes the importance of accurately identifying these tumors. Differentiating between a HER2 IHC score 0 and score 1 + has not been clinically significant until now. To ensure accurate classification and avoid misdiagnosis, it is necessary to adopt standardized procedures, guidelines, and specialized training for pathologists in interpreting HER2 expression in the lower spectrum. Additionally, the utilization of artificial intelligence holds promise in supporting this endeavor. Here, we address the current state of the art and unresolved issues in assessing HER2-low status, with a particular emphasis on the score 0. We explore the dilemma surrounding the exclusion of HER2-zero patients from potentially beneficial therapy based on traditional HER2 testing. Additionally, we examine the clinical context, considering that DB-04 primarily involved heavily pretreated late-stage metastatic breast cancers. We also delve into emerging evidence suggesting that extrapolating HER2-low status from the original diagnosis may lead to misleading results. Finally, we provide recommendations for conducting high-quality testing and propose a standardized pathology report in compliance with 2023 ASCO/CAP updates and 2023 ESMO consensus statements on HER2-low breast cancer.

[Comparative immunohistochemistry study of different antibodies clones for detection of breast cancer markers (estrogen receptor, progesterone receptor, HER2/neu, Ki-67)]

OBJECTIVE

A comparative study of detection of breast cancer markers (estrogen receptors, progesterone receptors, HER2/neu, Ki-67) by immunohistochemical method with antibodies produced by PrimeBioMed (Russia) and antibodies produced by Roche Ventana (USA).

MATERIAL AND METHODS

Surgical specimens and biopsies from 37 patients with invasive breast cancer were used. Sections were stained with antibodies of clones ER SP1 and GM030, PR 1E2 and PBM-5B8, HER2/neu 4B5 and PBM-46A6, Ki-67 30-9 and GM010.

RESULTS

There was a high positive and significant correlation between the immunohistochemistry results and antibodies of the clones ER-SP1 and GM030, PR1E2 and PBM-5B8, HER2/neu4B5 and PBM-46A6, Ki-67 30-9 and GM010.

CONCLUSION

The study showed the possibility of using antibodies of clones GM030, HER2/neu 4B5, PBM-46A6, GM010 (PrimeBioMed) on the Ventana Bench Marck Ultra automatic immunostainer using the detection system UltraView Universal DAB Detection Kit.

Prognostic Impact of HER2-Low and HER2-Zero in Resectable Breast Cancer with Different Hormone Receptor Status: A Landmark Analysis of Real-World Data from the National Cancer Center of China

BACKGROUND

The prognostic impact of HER2-low on overall survival (OS) and disease-free survival (DFS) in patients with resectable breast cancer (BC) remains controversial, partly resulting from the hormone receptor (HR) status.

OBJECTIVE

To investigate the prognostic impact of HER2-low in different HR subgroups.

PATIENTS AND METHODS

We retrospectively retrieved medical records of treatment-naive primary HER2-low and HER2-zero BC patients who were diagnosed with invasive ductal carcinoma and underwent surgery in the Cancer Hospital of the Chinese Academy of Medical Sciences from January 2009 to September 2017 (n = 7371). We compared the clinicopathologic features and performed Cox regression and landmark survival analyses to explore the prognostic impact of HER2-low on survival outcomes during distinct post-surgery intervals-36 months, 60 months, and 120 months.

RESULTS

HER2-low BC, compared to HER2-zero BC, exhibited less aggressive clinicopathologic features, such as smaller invasion size, lower grade, increased nerve invasion, higher HR positivity, and a higher proportion of low-Ki67 cases. In the HR-positive subgroup, HER2-low demonstrated improved OS (p = 0.046) and DFS (p = 0.026) within 60 months. Conversely, HER2-low displayed worse DFS (p = 0.046) in the HR-negative subgroup after 36 months from surgery. The findings remained robust in uni- and multi-variable Cox models.

CONCLUSIONS

HER2-low BCs manifested less aggressive clinicopathologic features than the HER2-zero cases. The prognostic impact of HER2-low in resectable BCs exhibits variability contingent upon the patients' HR status.

The prevalence and clinical significance of HER2 expression in prostate adenocarcinoma

AIMS

Abnormalities in HER2 are well-established oncogenic drivers and are approved therapeutic targets in various malignancies. Prior studies evaluating HER2 expression in prostate cancer (PCa) have yielded variable results. Most of these studies used immunohistochemistry scoring systems based on breast cancer data. The goal of this study was to determine the prevalence and clinical significance of HER2 expression using a scoring system that better reflects the HER2 staining pattern observed in PCa.

METHODS

We randomly selected similar numbers of localized low risk (AJCC stage I), locally advanced (AJCC stages II & III), and metastatic (AJCC stage IV) PCa patients treated at the DC VA Medical Center between 2000 and 2022. Among them, we included patients who had sufficient PCa tissue samples and clinical information required for this analysis. Two experienced pathologists independently scored HER2 expression (Ventana Pathway anti-HER2) according to a modified gastric cancer HER2 scoring system.

RESULTS

Out of the 231 patients included, 85 % self-identified as Black. 58.9 % of patients expressed HER2 (1+: 35.5 %; 2+: 18.2 %; 3+: 5.2 %). Validity of the results was confirmed using the HercepTest antibody. Higher HER2 expression was associated with a higher Gleason Score/Grade Group and advanced disease.

CONCLUSIONS

Our findings support the adverse prognostic impact on HER2 in PCa. We propose the use of a modified scoring system to evaluate HER2 expression in PCa. The observed high prevalence of HER2 expression supports the consideration of novel HER2-targeted therapies addressing even low levels of HER2 expression in future PCa trials.

Think "HER2" different: integrative diagnostic approaches for HER2-low breast cancer

This work explores the complex field of HER2 testing in the HER2-low breast cancer era, with a focus on methodological aspects. We aim to propose clear positions to scientific societies, institutions, pathologists, and oncologists to guide and shape the appropriate diagnostic strategies for HER2-low breast cancer. The fundamental question at hand is whether the necessary tools to effectively translate our knowledge about HER2 into practical diagnostic schemes for the lower spectrum of expression are available. Our investigation is centered on the significance of distinguishing between an immunohistochemistry (IHC) score 0 and score 1+ in light of the clinical implications now apparent, as patients with HER2-low breast cancer become eligible for trastuzumab-deruxtecan treatment. Furthermore, we discuss the definition of HER2-low beyond its conventional boundaries and assess the reliability of established diagnostic procedures designed at a time when therapeutic perspectives were non-existent for these cases. In this regard, we examine potential complementary technologies, such as gene expression analysis and liquid biopsy. Ultimately, we consider the potential role of artificial intelligence (AI) in the field of digital pathology and its integration into HER2 testing, with a particular emphasis on its application in the context of HER2-low breast cancer.

High inter-laboratory variability in the assessment of HER2-low breast cancer: a national registry study on 50,714 Danish patients

BACKGROUND

Considering the recent advancements in the treatment of breast cancer with low expression of human epidermal growth factor receptor 2 (HER2), we aimed to examine inter-laboratory variability in the assessment of HER2-low breast cancer across all Danish pathology departments.

METHODS

From the Danish Breast Cancer Group, we obtained data on all women diagnosed with primary invasive breast cancer in 2007-2019 who were subsequently assigned for curatively intended treatment.

RESULTS

Of 50,714 patients, HER2 score and status were recorded for 48,382, among whom 59.2% belonged to the HER2-low group (score 1+ or 2+ without gene amplification), 26.8% had a HER2 score of 0, and 14.0% were HER2 positive. The proportion of HER2-low cases ranged from 46.3 to 71.8% among pathology departments (P < 0.0001) and from 49.3 to 65.6% over the years (P < 0.0001). In comparison, HER2 positivity rates ranged from 11.8 to 17.2% among departments (P < 0.0001) and from 12.6 to 15.7% over the years (P = 0.005). In the eight departments with the highest number of patients, variability in HER2-low cases increased from 2011 to 2019, although the same immunohistochemical assay was used. By multivariable logistic regression, the examining department was significantly related to both HER2 score 0 and HER2 positivity (P < 0.0001) but showed greater dispersion in odds ratios in the former case (range 0.25-1.41 vs. 0.84-1.27).

CONCLUSIONS

Our data showed high inter-laboratory variability in the assessment of HER2-low breast cancer. The findings cast doubt on whether the current test method for HER2 is robust and reliable enough to select HER2-low patients for HER2-targeted treatment in daily clinical practice.

New treatment options for metastatic HER2-low breast cancer : Consequences for histopathological diagnosis

The overexpression of HER2 in breast cancer is a classic example for molecular targeted therapy, and it has been shown that classical anti-HER2 therapeutics were only effective in patients with HER2 overexpressing tumors. Therefore, in recent decades, pathologists have been focused on the reliable identification of HER2 overexpressing tumors. Based on the results of recent clinical trials in metastatic breast cancer with antibody-drug conjugates (ADCs), this diagnostic strategy for evaluation of HER2 is currently changing. It has been shown that the ADC trastuzumab-deruxtecan is effective not only against tumors with classical HER2 overexpression, but also against HER2-low tumors. These clinical trial results lead to a paradigm shift in the treatment of patients whose tumours were previously classified as HER2 negative. In addition to the identification of HER2 (score 3+) overexpressing tumors, it is necessary to identify HER2-low expressing tumors (defined as an immunohistochemistry (IHC) score of 1+ or IHC2+ with negative in situ hybridization).Due to the therapeutic consequences, it is important to quickly adapt the diagnostic workup and reporting to the new requirements. In addition, the new therapeutic options for anti-HER2 therapy lead to new challenges for standardization as well as to new scientific questions for the characterization of tumors with low HER2 expression.

The Significance of Recognition of Human Epidermal Growth Factor 2 Low in Breast Cancer Therapy

In response to recent clinical trials that demonstrate the clinical benefit of antibody-drug conjugate drug therapy in breast cancer (BC) with human epidermal growth factor 2 (HER2) immunohistochemical scores of 1+ or 2+ and negative in situ hybridization results, a new concept of "HER2-low BC" has emerged to describe this newly relevant therapeutic category of BC. Clinical recognition of HER2-low BC has caused a paradigm shift in the therapeutic landscape and management of patients with BC and resulted in rapid changes in clinical practice guidelines. In addition the American Society of Clinical Oncology-College of American Pathologists (ASCO-CAP) recently updated their HER2 Guidelines Recommendations to specifically address HER2-low BC. A literature search in PubMed of peer-reviewed articles, regulatory communications, and relevant practice guidelines pertaining to HER2-low BC was conducted. In this review, we have summarized current published knowledge regarding the clinicopathologic and molecular features, diagnostic criteria, and most current guideline recommendations regarding HER2-low BC, and also highlight ongoing practical and diagnostic challenges when identifying HER2-low BC in routine clinical practice.

Impact of the OncotypeDX score and HER2 RNA PCR levels on HER2-low IHC levels in primary and metastasized tumors

PURPOSE

One-half of hormone receptor-positive (HR +) breast cancer (BC) patients have low expression of HER2 (HER2-low) and may benefit from trastuzumab deruxtecan (TDXd). This study aimed to identify parameters associated with HER2-low levels in primary and metastatic tumors. We specifically sought to determine whether OncotypeDX and HER2 mRNA levels could identify patients who would otherwise be considered HER2-negative by immunohistochemistry (IHC).

METHODS

This retrospective analysis of all consecutive HR + patients who underwent OncotypeDX from January 2004 to December 2020 was conducted in a single medical center (n = 1429). We divided HER2-negative cases into HER2-low (IHC = 1 + or 2 + and non-amplified fluorescent situ hybridization) and HER2-0 (IHC = 0). HER2 RT-PCR was evaluated from the OncotypeDX results.

RESULTS

HER2-low cases exhibited significantly higher HER2 RT-PCR scores (p = 2.1e-9), elevated estrogen receptor (ER) levels (p = 0.0114), and larger tumor sizes compared to HER2-0 cases (> 2 cm; 36.6% vs. 22.1%, respectively, p < 0.00001). Primary tumors > 2 cm were more likely to be HER2-low (OR = 2.07, 95% CI: 1.6317 to 2.6475, p < 0.0001). Metastatic BCs expressed higher HER2 IHC scores compared with primary BCs (Wilcoxon signed-rank, p = 0.046). HER2 IHC scores were higher for low-risk vs. medium-risk OncotypeDX (p = 0.0067). No other clinical or pathological parameters were associated with the increase in HER2 levels in the metastatic samples.

CONCLUSION

It might be beneficial to use clinical data from the primary tumor, including the HER2 RT-PCR score, to determine a HER2-low status.

HER2 Equivocal (Score = 2+) Breast Carcinoma Cases Identified by Immunohistochemistry at a South African Hospital. What is the Impact of Fluorescent In Situ Hybridization Testing?

The American Society of Clinical Oncology and the College of American Pathologists (ASCO/CAP) guidelines are used for human epidermal growth factor receptor 2 (HER2) reporting in breast carcinoma. Cases that demonstrate weak to moderate complete membrane immunohistochemical staining in >10% of the tumor are scored as 2+ (equivocal). This study aimed to determine what proportion of HER2 immunohistochemistry (IHC) score = 2+ breast carcinomas were confirmed to be positive by HER2 fluorescent in situ hybridization (FISH). There were 241 HER2 IHC score = 2+ breast carcinomas included. Most (74.3%) carcinomas were estrogen and progesterone receptor-positive. Invasive breast carcinoma of no special type (89.2%) was the commonest histologic subtype. Most tumors were grade 2 (64.3%). As per the FISH report, at the time of diagnosis, 27 cases (11.2%) were HER2 FISH positive. All HER2 FISH equivocal cases and one FISH positive case assessed using the 2013 ASCO/CAP HER2 criteria were reclassified to HER2 FISH negative when the 2018 criteria were applied. There was a high level of agreement (κ = 0.979) between HER2 FISH results obtained using the 2013 and the 2018 criteria. This study provides insight into the frequency of HER2 FISH positivity (11.2%) among HER2 IHC score = 2+ breast carcinomas and the impact of modifications to the ASCO/CAP HER2 guidelines. Elimination of the HER2 FISH equivocal category by the 2018 guidelines has reduced the need for repeat testing and simplified clinical management. Reclassification of previous HER2 FISH positive to negative has resulted in some patients being ineligible for costly anti-HER therapy.

Low and Ultra-Low HER2 in Human Breast Cancer: An Effort to Define New Neoplastic Subtypes

HER2-low and ultra-low breast cancer (BC) have been recently proposed as new subcategories of HER2 BC, supporting a re-consideration of immunohistochemical negative scores of 0, 1+ and the 2+/in situ hybridization (ISH) negative phenotype. In the present review, we outline the criteria needed to exactly distinguish HER2-low and ultra-low BC. Recent clinical trials have demonstrated significant clinical benefits of novel HER2 directing antibody-drug conjugates (ADCs) in treating these groups of tumors. In particular, trastuzumab-deruxtecan (T-Dxd), a HER2-directing ADC, has been recently approved by the US Food and Drug Administration as the first targeted therapy to treat HER2-low BC. Furthermore, ongoing trials, such as the DESTINY-Breast06 trial, are currently evaluating ADCs in patients with HER2-ultra low BC. Finally, we hope that new guidelines may help to codify HER2-low and ultra-low BC, increasing our knowledge of tumor biology and improving a targetable new therapeutical treatment.

ESMO expert consensus statements (ECS) on the definition, diagnosis, and management of HER2-low breast cancer

Human epidermal growth factor receptor 2 (HER2)-low breast cancer has recently emerged as a targetable subset of breast tumors, based on the evidence from clinical trials of novel anti-HER2 antibody-drug conjugates. This evolution has raised several biological and clinical questions, warranting the establishment of consensus to optimally treat patients with HER2-low breast tumors. Between 2022 and 2023, the European Society for Medical Oncology (ESMO) held a virtual consensus-building process focused on HER2-low breast cancer. The consensus included a multidisciplinary panel of 32 leading experts in the management of breast cancer from nine different countries. The aim of the consensus was to develop statements on topics that are not covered in detail in the current ESMO Clinical Practice Guideline. The main topics identified for discussion were (i) biology of HER2-low breast cancer; (ii) pathologic diagnosis of HER2-low breast cancer; (iii) clinical management of HER2-low metastatic breast cancer; and (iv) clinical trial design for HER2-low breast cancer. The expert panel was divided into four working groups to address questions relating to one of the four topics outlined above. A review of the relevant scientific literature was conducted in advance. Consensus statements were developed by the working groups and then presented to the entire panel for further discussion and amendment before voting. This article presents the developed statements, including findings from the expert panel discussions, expert opinion, and a summary of evidence supporting each statement.

HER2-low breast cancer: evolution of HER2 expression from primary tumor to distant metastases

BACKGROUND

Breast cancer (BC) with low human epidermal growth factor receptor 2 (HER2) expression is attracting much attention due to the breakthrough progress of novel anti-HER2 antibody-drug conjugates. HER2 expression is examined in patients with HER2-low BC and their distant metastases in this study, so as to further clarify the dynamic characteristics of HER2 low status in the process of disease progression.

METHODS

Patients diagnosed with HER2 low breast cancer (defined as IHC1+ or IHC2+/ISH-) between 2012 and 2021 were included in this study. We evaluated HER2 expression of primary sites and metastatic sites, compared the impact of different clinicopathological parameters on HER2 status of metastases and compared the overall survival and disease-free survival of patients with different HER2 status in metastases.

RESULTS

Ninety-eight patients were included. All HER2 IHC scores were confirmed and the consistent rate with the original pathological report was 81.1%. 27.6% of the patients showed different HER2 status in metastases. The HER2 discordance rate differed among different metastatic sites (p = 0.040). The higher the T stage of the primary BC, the higher the rate of HER2 discordance was observed (p = 0.042). For the specimen type of metastasis, HER2 discordant rate was higher in surgical specimen than biopsy (p = 0.050). No difference of HER2 discordance rate was found between HER2-1+ and HER2-2+ patients. But comparing HER2 IHC score, HER2-2+ patients were less likely to have consistent metastatic HER2 levels than HER2-1+ patients (p = 0.006). No difference in survival outcomes was observed between patients with different HER2 status in metastases.

CONCLUSIONS

There is a possibility of HER2 expression alteration in the metastases of HER2-low breast cancer. And the rate of altered HER2 low expression was different among different metastatic sites, different T stages of primary BC and specimen type of metastasis. No prognostic significance was observed.

Anticipating changes in the HER2 status of breast tumours with disease progression-towards better treatment decisions in the new era of HER2-low breast cancers

BACKGROUND

HER2 expression is often negative or low in primary breast cancers (BCs) but its changes with disease progression remain poorly known. We aimed to estimate them between primary and recurrent tumours, and identify predictive factors.

METHODS

We compared the HER2 status, and clinical and pathological characteristics by its evolution category (stable or changed), between all primary BCs and matched recurrences registered in our database in 2000-2020 (n = 512).

RESULTS

HER2-low tumours were the most prevalent at diagnosis (44.9%), followed by HER2-negative tumours (39.3%). HER2 status significantly changed in 37.3% of recurrences, mainly of HER2-negative and HER2-low tumours. HER2-negative tumours which relapsed as HER2-low significantly more frequently expressed oestrogen receptors (ER) and recurred later than stably HER2-negative tumours. Changed HER2 status in distant metastases correlated with lower proliferation rates and higher ER expression in primary tumours, and among metastases of hormone receptor-positive (HR+) tumours-with weak progesterone receptor (PR) expression in primary tumours.

CONCLUSIONS

HER2 status changes with BC progression, with enrichment of HER2-low tumours in advanced stages. The ER+/PR- status, low proliferation index and time to late recurrence correlated with these changes. These findings highlight the need of retesting recurrences, especially of HR + primary tumours, to identify candidates for new anti-HER2 therapies.

Intra-patient and inter-metastasis heterogeneity of HER2-low status in metastatic breast cancer

INTRODUCTION

Anti-HER2 antibody-drug conjugates (ADCs) have shown important efficacy in HER2-low metastatic breast cancer (mBC). Criteria for receiving ADCs are based on a single assay on the primary tumour or a small metastatic biopsy. We assessed the intra-patient inter-metastasis heterogeneity of HER2-low status in HER2-negative mBC.

PATIENTS AND METHODS

We included samples of 10 patients (7 ER-positive and 3 ER-negative) donated in the context of our post-mortem tissue donation program UPTIDER. Excisional post-mortem biopsies of 257 metastases and 8 breast tumours underwent central HER2 immunohistochemistry (IHC), alongside 41 pre-mortem primary or metastatic samples. They were classified as HER2-zero, HER2-low (HER2-1+ or HER2-2+, in situ hybridisation [ISH] negative) or HER2-positive (HER2-3+ or HER2-2+, ISH-positive) following ASCO/CAP guidelines 2018. HER2-zero was further subdivided into HER2-undetected (no staining) and HER2-ultralow (faint staining in ≤10% of tumour cells).

RESULTS

Median post-mortem interval was 2.5 h. In 8/10 patients, HER2-low and HER2-zero metastases co-existed, with the proportion of HER2-low lesions ranging from 5% to 89%. A total of 32% of metastases currently classified as HER2-zero were HER2-ultralow. Intra-organ inter-metastasis heterogeneity of HER2-scores was observed in the liver in 3/6 patients. Patients with primary ER-positive disease had a higher proportion of HER2-low metastases as compared to ER-negative disease (46% versus 8%, respectively). At the metastasis level, higher percentages of ER-expressing cells were observed in HER2-low or -ultralow as compared to HER2-undetected metastases.

CONCLUSIONS

Important intra-patient inter-metastasis heterogeneity of HER2-low status exists. This questions the validity of HER2-low in its current form as a theranostic marker.

Concordance of HER2-low scoring in breast carcinoma among expert pathologists in the United Kingdom and the republic of Ireland -on behalf of the UK national coordinating committee for breast pathology

BACKGROUND

Recent clinical evidence showed that breast cancer with low HER2 expression levels responded to trastuzumab deruxtecan therapy. The HER2-low cancers comprise immunohistochemistry (IHC) score 1+ and 2+ ISH non-amplified tumours, currently classified as HER2 negative. Little data exists on the reproducibility of pathologists reporting of HER2-low cancer.

PATIENT AND METHODS

Sixteen expert pathologists of the UK National Coordinating Committee for Breast Pathology scored 50 digitally scanned HER2 IHC slides. The overall level of agreement, Fleiss multiple-rater kappa statistics and Cohen's Kappa were calculated. Cases with low concordance were re-scored by the same pathologists after a washout period.

RESULTS

Absolute agreement was achieved in 6% of cases, all of which scored 3+. Poor agreement was found in 5/50 (10%) of cases. This was due to heterogeneous HER2 expression, cytoplasmic staining and low expression spanning the 10% cut-off value. Highest concordance (86%) was achieved when scores were clustered as 0 versus others. Improvement in kappa of overall agreement was achieved when scores 1+ and 2+ were combined. Inter-observer agreement was moderate to substantial in the whole cohort but fair to moderate in the HER2-low group. Similarly, consensus-observer agreement was substantial to almost perfect in the whole cohort and moderate to substantial in the HER2-low group.

CONCLUSION

HER2-low breast cancer suffers from lower concordance among expert pathologists. While most cases can reproducibly be classified, a small proportion (10%) remained challenging. Refining the criteria for reporting and consensus scoring will help select appropriate patients for targeted therapy.

HER2-Low Breast Cancer-Diagnostic Challenges and Opportunities for Insights from Ongoing Studies: A Podcast

Breast cancer has been traditionally classified as either human epidermal growth factor receptor 2 (HER2)-positive or HER2-negative based on immunohistochemistry (IHC) scoring and/or gene amplification. HER2-positive breast cancer (defined as IHC 3+ or IHC 2+ and in situ hybridization [ISH]+) is routinely treated with HER2-targeted therapies, while HER2-negative breast cancer (defined as IHC 0, IHC 1+, or IHC 2+/ISH-) was not previously eligible for HER2-targeted therapy. Some tumors traditionally defined as HER2-negative express low levels of HER2 (i.e., HER2-low breast cancer, defined as IHC 1+ or IHC 2+/ISH-). Recently reported results from the DESTINY-Breast04 trial demonstrated that the HER2-targeted antibody-drug conjugate trastuzumab deruxtecan (T-DXd) improved survival in patients with previously treated advanced or metastatic HER2-low breast cancer, leading to the approval of T-DXd in the US and EU for patients with unresectable or metastatic HER2-low breast cancer after prior chemotherapy in the metastatic setting or disease recurrence within 6 months of adjuvant chemotherapy. As the first HER2-targeted therapy approved for the treatment of HER2-low breast cancer, this represents a change in the clinical landscape and presents new challenges, including identifying patients with HER2-low breast cancer. In this podcast, we discuss the strengths and limitations of current methodologies for classifying HER2 expression and future research that will help refine the identification of patients expected to benefit from HER2-targeted therapy, such as T‑DXd or other antibody-drug conjugates. Although current methodologies are not optimized to identify all patients with HER2-low breast cancer who may potentially benefit from HER2-targeted antibody-drug conjugates, they are likely to identify many. Ongoing studies-including the DESTINY-Breast06 trial evaluating T-DXd in patients with HER2-low breast cancer and those with tumors expressing very low levels of HER2 (IHC > 0 to < 1+)-will provide insights that may improve the identification of patient populations expected to benefit from HER2-targeted antibody-drug conjugates. Supplementary file1 (MP4 123466 KB).

HER2-low breast cancer: insights on pathological testing

Human epidermal growth factor receptor 2 (HER2) is an important biomarker for predicting prognosis and effectiveness of HER2-targeted therapy in breast cancer. The emergence of novel HER2 antibody-drug conjugate has led to a shift from the binary categorization of HER2 status (i.e., negative and positive) to a ternary categorization gradually (i.e., HER2 0, HER2 low expression and HER2 positive). The heterogeneity of HER2 low expression in breast cancer has also recently aroused widespread concern, and the heterogeneity of tumors has led to differences in the efficacy of HER2-targeted therapy. Therefore, it is crucial to accurately identify the HER2 expression status of breast cancer, which can provide a basis for patients to formulate personalized treatment strategies. In recent years, artificial intelligence (AI) has developed rapidly and been widely used in the pathological accurate diagnosis of breast cancer. The research results show that AI can significantly improve the consistency and accuracy of HER2 interpreted by pathologists in breast cancer. This has provoked many discussions on HER2-low breast cancer, such as quality control prior to HER2-low expression detection, the latest progress of tumor heterogeneity, and the application of AI. In this paper, we discuss the latest testing guidelines and advances on HER2-low breast cancer, aiming to standardize and improve the pathological testing of HER2-low breast cancer.

How I treat HER2-low advanced breast cancer

INTRODUCTION

Targeting low levels of human receptor epidermal growth factor 2 (HER2) expression has reshaped the treatment paradigm for half of the patients with advanced breast cancer. HER2-low is currently defined as a HER2 immunohistochemical expression of 1+ or 2+ without amplification by in-situ hybridization. Until recently, HER2-targeted agents were ineffective in treating patients with HER2-low disease.

AREAS COVERED

In this narrative review, we summarize the current management of HER2-low breast cancer. We highlight the findings of the DESTINY-Breast 04 phase 3 trial, which confirmed the efficacy of trastuzumab-deruxtecan (T-DXd) for the treatment of patients with advanced, pretreated HER2-low breast cancer. We also discuss how to implement this new treatment option in treatment algorithms of hormone receptor (HR)-positive and triple-negative tumors, as well as how to optimally manage selected toxicities of T-DXd.

EXPERT OPINION

T-DXd is currently the standard of care for patients with advanced, pretreated, HER2-low breast cancer. Based on the design of the DESTINY-Breast04 trial, the current optimal place in treatment algorithms is after the first line of chemotherapy, both in HR-positive and triple-negative breast cancer. Up to 10-15% of the patients receiving T-DXd are expected to develop interstitial lung disease, which in 1-2% of the cases can be fatal. Adequate monitoring and prompt management are required to minimize the impact of ILD and to safely implement T-DXd in clinical practice.

[New treatment options for metastatic HER2-low breast cancer : Consequences for histopathological diagnosis]

The overexpression of HER2 in breast cancer is a classic example for molecular targeted therapy, and it has been shown that classical anti-HER2 therapeutics were only effective in patients with HER2 overexpressing tumors. Therefore, in recent decades, pathologists have been focused on the reliable identification of HER2 overexpressing tumors. Based on the results of recent clinical trials in metastatic breast cancer with antibody-drug conjugates (ADCs), this diagnostic strategy for evaluation of HER2 is currently changing. It has been shown that the ADC trastuzumab-deruxtecan is effective not only against tumors with classical HER2 overexpression, but also against HER2-low tumors. These clinical trial results lead to a paradigm shift in the treatment of patients whose tumours were previously classified as HER2 negative. In addition to the identification of HER2 (score 3+) overexpressing tumors, it is necessary to identify HER2-low expressing tumors (defined as an immunohistochemistry (IHC) score of 1+ or IHC2+ with negative in situ hybridization).Due to the therapeutic consequences, it is important to quickly adapt the diagnostic workup and reporting to the new requirements. In addition, the new therapeutic options for anti-HER2 therapy lead to new challenges for standardization as well as to new scientific questions for the characterization of tumors with low HER2 expression.