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Aims: This study aimed to develop an AI algorithm for automated HER2 scoring in urothelial bladder cancer (UBCa) and assess the interobserver agreement using both manual and AI-assisted methods based on breast cancer criteria. Methods and Results: We utilized 330 slides from two institutions for initial AI development and selected 200 slides for the ring study, involving six pathologists (3 senior, 3 junior). Our AI algorithm achieved high accuracy in two independent tests, with accuracies of 0.94 and 0.92. In the ring study, the AI-assisted method improved both accuracy (0.66 vs 0.94) and consistency (kappa=0.48; 95 % CI, 0.443–0.526 vs kappa=0.87; 95 % CI, 0.852–0.885) compared to manual scoring, especially in HER2-low cases (F1-scores: 0.63 vs 0.92). Additionally, in 62.3 % of heterogeneous HER2-positive cases, the interpretation accuracy significantly improved (0.49 vs 0.93). Pathologists, particularly junior ones, experienced enhanced accuracy and consistency with AI assistance. Conclusions: This is the first study to provide a quantification algorithm for HER2 scoring in UBCa to assist pathologists in diagnosis. The ring study demonstrated that HER2 scoring based on breast cancer criteria can be effectively applied to UBCa. Furthermore, AI assistance significantly improves the accuracy and consistency of interpretations among pathologists with varying levels of experience, even in heterogeneous cases.

Breast cancer has become the most prevalent malignant tumor worldwide and remains one of the leading causes of cancer-related mortality among women globally. The prognosis for patients with metastatic breast cancer remains poor, necessitating the exploration of novel therapeutic strategies to improve survival rates. In the era of precision medicine, antibody-drug conjugates (ADCs) have gained significant attention as a targeted therapeutic strategy in breast cancer treatment. ADCs, a relatively new treatment for breast cancer, deliver cytotoxic drugs (payloads), directly into the tumor space, turning chemotherapy into a targeted agent, which enables patients to experience significant improvements with manageable drug toxicity. For the treatment of breast cancer, there are three ADCs approved for breast cancer treatment: Trastuzumab emtansine (T-DM1), Trastuzumab Deruxtecan (T-Dxd) targeting HER-2, and Sacituzumab Govitecan (SG) targeting Trop-2. Recent clinical studies have demonstrated that the benefits of ADC therapies extend beyond HER2-positive breast cancer toinclude hormone receptor (HR)-positive breast cancer, triple-negative breast cancer (TNBC), and HER2-low expressing breast cancer. Notably, the DESTINY-Breast series of studies, particularly focusing on T-Dxd, encompass neoadjuvant, adjuvant, and multiple lines of therapy for advanced breast cancer. This marks the advent of a comprehensive ADC era in breast cancer treatment. This review summarizes the efficacy and adverse effects of ADC therapies that have completed or are currently undergoing phase I-III clinical trials. Additionally, it analyzes potential combination strategies to overcome ADC resistance, aiming to provide clinicians with a comprehensive clinical guide to the use of ADCs in breast cancer treatment.

For the past 2 decades, pathologists have been accustomed to reporting the HER2 status of breast cancer as either positive or negative, based on HER2 IHC. Today, however, there is a clinical imperative to employ a 3-tier approach to interpreting HER2 IHC that can also identify tumours categorised as HER2-low. Meeting this need for a finer degree of discrimination may be challenging, and in this article, we consider the potential for the integration of computational approaches to support pathologists in achieving accurate and reproducible HER2 IHC scoring as well as outlining some of the practicalities involved.

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.

Treatment of breast cancer with low expression of human epidermal growth factor receptor 2 (HER2; HER2-low) has drawn much attention in recent years. With the proven therapeutic effect of trastuzumab deruxtecan (T-DXd) in patients with HER2-low (immunohistochemistry [IHC] 1+, or IHC2+/in situ hybridization [ISH]-) breast cancer, HER2-low may become a new subtype of targeted therapy for breast cancer. The expert committee formulated this consensus based on the current clinical studies and clinical medication experience. The current consensus is the collaborative work of an interdisciplinary working group, including experts in the fields of pathology and oncology. The purpose of this consensus was to guide the clinical diagnosis and treatment of HER2-low breast cancer, thereby prolonging the overall survival of patients.

Background: HER2-low has emerged as a new predictive biomarker in metastatic breast cancer. However, its prognostic value in early-stage carcinomas needs to be revisited. We aimed to evaluate the association of HER2-low carcinomas with PAM50 risk groups combined with clinicopathological variables in early breast cancer. Methods: We conducted a retrospective analysis of 332 patients with early-stage breast cancer that underwent PAM50 signature analysis between 2015 and 2021at Hospital Universitario 12 de Octubre (Madrid, Spain). Clinical and pathological variables were collected from medical records. After adjusting for potential confounders, we estimated Odds Ratio (OR) and 95% confidence interval for high-risk PAM50 subgroup, comparing HER2-low versus HER2-zero carcinomas by multivariable logistic regression. P values below 0.05 were deemed statistically significant. Results: 192 (57%) patients were classified as HER2-low carcinomas. Median follow-up was 34 months. Adjusted OR for high-risk PAM50 when comparing HER2-low versus HER2-zero carcinomas was 1.31 (95% CI: 0.75–2.30, p = 0.33). The multivariable model detected significant associations for Ki-67% (≥20% vs.

Background: CDK4/6 inhibitors combined with endocrine therapy have significantly improved treatment outcomes for metastatic hormone receptor-positive (HR+) breast cancer patients. However, the impact of low HER2 expression on treatment response and progression-free survival (PFS) remains unclear. Methods: This multicenter retrospective study included 204 HR+ breast cancer patients treated with a combination of CDK4/6 inhibitor and endocrine therapy. HER2-zero disease was detected in 138 (68%) and HER2-low disease in 66 (32%) patients. Treatment-related characteristics and clinical outcomes were analyzed, with a median follow-up of 22 months. Results: The objective response rate (ORR) was 72.7% in the HER2 low group and 66.6% in the HER2 zero group (p = 0.54). Median PFS was not significantly different between the HER2-low and HER2 zero groups (19 months vs.18 months, p = 0.89), although there was a trend toward longer PFS in the HER2-low group for first-line treatment (24 months progression-free survival rate 63% vs 49%). In recurrent disease, the median PFS was 25 months in the HER2-low group and 12 months in the HER2-zero group (p = 0.08), while in de novo metastatic disease, the median PFS was 18 months in the HER2-low group and 27 months in the HER2-zero group (p = 0.16). The order of CDK4/6 inhibitor use and the presence of visceral metastasis were identified as independent variables affecting PFS. Conclusion: Low HER2 expression did not significantly impact treatment response or PFS in HR+ breast cancer patients treated with a CDK4/6 inhibitor and endocrine therapy. Because of the conflicting results in the literature, further prospective studies are needed to evaluate the clinical significance of HER2 expression in HR+ breast cancer.

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.

Background: A considerable number of patients with breast cancer will suffer from brain metastasis in the advanced setting. The HER2 status serves as a significant prognostic factor and the reference of applying treatment for patients with breast cancer brain metastasis (BCBM). Methods: Between January 2010 and July 2021, patients with BCBM who had available HER2 status were identified. The patients with HER2 1+ in immunohistochemistry (IHC) or IHC 2+ and fluorescence in situ hybridization (FISH) negative were categorized as HER2-low. Comparisons were conducted between the HER2-low and HER2-zero population. The primary endpoint was overall survival (OS) after the diagnosis of BCBM. Survival outcomes were assessed using Kaplan-Meier curves with log-rank test and Cox proportional hazards model. Results: In this study, we analyzed 71 patients with the HER2-low breast cancer subtype and 64 patients with the HER2-zero subtype. Despite the limited sample size, our findings revealed a significantly better OS for patients with HER2-low cancer compared to their HER2-zero counterparts (26 m vs 20 m, p = 0.0017). This trend was particularly notable in the HR-negative group (26 m vs 13 m, p = 0.0078), whereas no significant difference was observed among the HR-positive patients. Furthermore, Cox regression analysis revealed that the HER2-low status was an independent prognostic factor for better survival in the HR-negative patients (p = 0.046 in multivariate analysis). Conclusions: Patients diagnosed with HER2-low BCBM exhibited a more favorable prognosis than those with HER2-zero BCBM, particularly within the HR-negative subgroup. The low expression of HER2 is supposed to be linked to the prolonged survival of BCBM patients.

The human epidermal growth factor receptor 2 (HER2) plays a central role in breast cancer (BC). Therefore, it is critical to develop a method that can capture its spatial and temporal heterogeneity. Nowadays, therapeutic decisions for BC patients relies on evaluation of HER2 status from tissue biopsies using immunohistochemistry and in situ hybridization. Nevertheless, considering the technical and logistical challenges associated with tissue biopsies, there is an unmet need for a non-invasive and accurate approach to obtain real-time assessment of HER2 status. In this context, circulating biomarkers, particularly circulating tumor cells (CTCs), emerged as promising candidates. HER2 assessment on CTCs can be performed at genomic, transcriptomic, and protein levels on both bulk CTCs and at the single-cell resolution. However, the main limitation of the literature to date is the lack of a consistent definition of HER2-positive CTCs, which poses a major challenge for both, future research and clinical applications. Several studies revealed discordance in HER2 status between the primary tumor and corresponding CTCs. For instance, HER2-positive CTCs have been detected among patients with HER2-negative BC and vice versa. As a result, researchers have evaluated the prognostic and predictive value of HER2 status in CTCs, both in the early and metastatic settings, to increase the possibility of using anti-HER2 therapy also for these patients and to dissect mechanisms of treatment resistance. This review aims to provide an overview of the methods to determine HER2 status in CTCs and to summarize the evidence and future perspective on how CTCs-HER2 assessment can be integrated into the clinical management of BC patients.

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.

Background: With the introduction of investigational human epidermal growth factor receptor 2 (HER2) targeting treatments, thorough understanding of breast cancer with different HER2 expression levels is critical. The aim of this study was to compare clinicopathologic characteristics and survival of patients with metastatic breast cancer according to the level of HER2 expression. Methods: Women with distant metastatic breast cancer during 2008–2016 were selected from PALGA, the Dutch Pathology Registry, and linked to the PHARMO Database Network. Breast cancer samples were categorised as HER2 immunohistochemistry score 0 (IHC0), HER2-low or HER2+. Results: Among women with hormone receptor (HR) positive metastatic breast cancer (n = 989), 373 (38%) cancers were HER2 IHC0, 472 (48%) were HER2-low and 144 (15%) were HER2+. Among HR negative patients (n = 272), the proportion of HER2 IHC0, HER2-low and HER2+ was 110 (40%), 104 (38%) and 58 (21%) respectively.Within the HR + cohort, patients with HER2 IHC0 or HER2-low cancer were significantly older compared to HER2+ patients. This age difference was not seen in the HR-cohort. The localisation of distant metastases differed significantly between HER2 IHC0 or HER2-low versus HER2+ cases. Survival rates did not differ markedly by subtypes. Conclusion: Substantial proportion of patients had a HER2-low breast cancer. No clear differences in survival were found when comparing HER2 and HR status. Getting more granular insights in the level of HER2 expression and addressing HER2-low as a separate category could help to assess the impact of emerging treatment strategies. Therefore, more detailed information on HER2 expression should be routinely reported.

Introduction: Reclassification of HER2-negative breast cancers to HER2 low-level expression allowed targeted anti-HER2 therapy in about 60% of patients, improving outcome. The high recurrence rates and often dismal outcomes with current therapies of high-grade Mullerian carcinomas, offers opportunity to explore anti-HER2 therapies in the gynecologic tract carcinomas. We investigated HER2 low expression as currently defined in breast carcinomas. Methods: We reviewed all high-grade Mullerian cancers between 2016 and 2021, where HER2 by IHC and/or FISH tests were available. Additional clinical information was recorded, and statistical analysis was performed using SPSS (version 27). Results: Forty (49.4%) tumors were endometrial, 20 (24.7%) ovarian, 16 (19.8%) fallopian tubal, and 5 (6.2%) primarily peritoneal. Overall, 17 (21.0%) were HER2 positive (IHC 3+/IHC 2+/FISH amplified), 31 (38.3%) HER2 low (IHC 1+/2+/FISH non-amplified), and 30 (37.0%) were HER2 negative (IHC = 0). HER2 low expression was noted in 15% ovarian, 25% fallopian tubal, 53% endometrial, and 60% peritoneal tumors; 34% and 21% of serous carcinomas, 63% and 13% of carcinosarcomas, and 67% and 33% of endometrioid carcinomas were HER2 low and HER2 positive respectively. HER2 negative and HER2 low expression had a significant association with primary tumor location (p = 0.001); endometrium and peritoneal tumors were more likely to be HER2 low and HER2 negative. During a mean follow-up of 13.2 months (range: 1–34), 5% of the patients were deceased. Conclusions: Based on the current HER2-low recommendations in the breast, about one-third of patients with high-grade Mullerian carcinomas might qualify for anti-HER2 therapy with a potential for improved progression-free and overall survival.

Summary: Background: HER2-low cancers are heterogeneous with different degrees of HER2 expression and hormone receptor (HR) status. Currently, its analysis is mostly focused on the standard clinic-pathologic features or common biomarkers expression, without considering the heterogeneity within the category. A further characterization and understanding of this cancer subgroup will facilitate its management. Methods: A large cohort of HER2-negative cancers (N = 1464) was included. The HER2-low (N = 412) and HER2-zero cancers (N = 1052) were compared and correlated with a comprehensive panel of clinico-pathologic features and biomarker expression according to different HER2 expressions and HR statuses. The prognostic values of these features in HER2-low cancers were also evaluated. Findings: The characteristics of HER2-low breast cancers, as compared to HER2-zero, varied with the HR status. HER2-low luminal cancers were associated with younger age, larger tumor, high pAKT and high HLA expression. Among TNBCs, opposite trends in age and tumor size were found. Additionally, HER2-low TNBC showed less necrosis, higher pN, lower c-kit and CK14 than HER2-zero cancers. Nonetheless, regardless of HR status, HER2-low status was associated with increased COX2 and AR expression, implicated in the biology of HER2-low cancers. HER2-low cancers showed high expression of HLAs in tumors and PD-L1 in immune cells. In particular, the co-expression of HLAs was found to be associated with better survival in HER2-low cancers. Interpretation: This study revealed further characteristic of HER2-low breast cancers as compared to HER2-zero cancers, provided further insights into its prognostication and therapeutic strategies. Funding: Health and Medical Research Fund (08190586), Cheng Yue Pui Charity Foundation and CUHK direct grant.

Introduction: Antibody-drug conjugates (ADCs) trastuzumab-deruxtecan (T-DXd) and sacituzumab-govitecan (SG) provided significant progression-free survival (PFS) and overall survival (OS) improvements over chemotherapy (CT) in pretreated hormone receptor-positive (HR+) and triple-negative (TN)/HER2-low metastatic breast cancer (MBC). However, no direct comparison between the two exists, nor with the more recent datopotamab-deruxtecan (Dato-DXd)., Methods: We conducted a network meta-analysis (NMA) to compare efficacy and safety of T-DXd and SG in CT-pretreated HR+ and TN/HER2-low MBC and assess their benefit over standard CT, exploring also a comparison with Dato-DXd. Hazard ratios (HRs) with 95 % confidence intervals (CI) were calculated for PFS/OS. P-score was used for treatment ranking., Results: Three RCTs (956 patients) were included in the primary analysis and 5 (1,445) in the exploratory NMA with Dato-DXd. In HR+/HER2-low, T-DXd showed no significant difference in PFS and OS when compared to SG. Similarly, in TN/HER2-low, PFS and OS did not differ significantly between the two ADCs. The P-score analysis favored T-DXd over SG in HR+/HER2-low in PFS (0.90 vs. 0.60) and OS (0.89 vs. 0.60). SG was favored over T-DXd in OS in TN/HER2-low (0.80 vs. 0.69). Similar results were obtained for HR+ MBC when including Dato-Dxd, which showed the worst performance, while T-DXd was the only ADC significantly outperforming CT in OS. The ADCs showed significantly better PFS and OS than CT in HR+/HER2-low and TN/HER2-low (all p < 0.001). SG had higher rates of neutropenia, diarrhea and alopecia vs. T-DXd, which showed more thrombocytopenia, fatigue and nausea. Pneumonitis and cardiotoxicity were typically T-DXd-related, and T-DXd showed more toxicity-related discontinuations., Conclusions: Similar efficacy with T-DXd and SG in HER2-low MBC was observed, regardless of HR status. Safety profile, local drug-approval criteria and guidelines, patients' preferences and overall quality of evidence should ultimately guide therapeutic decision-making. Dato-DXd role remains uncertain., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: F. Schettini reports honoraria from Novartis, Gilead and Daiichy-Sankyo for educational events/materials and travel expenses from Novartis, Gilead and Daiichy-Sankyo. T. Pascual has received honoraria for speaker activities from Pfizer, AstraZeneca, Novartis, Veracyte and Argenetics and has held an advisory role with Novartis. O. Martínez-Sáez reports advisory/consulting fees from Reveal Genomics, Roche and Astrazeneca and lecture fees from Daiichi Sankyo, Pfizer, Novartis and Eisai and travel expenses from Gilead and Novartis. R. Sánchez-Bayona reported receiving travel grants from Pfizer, Gilead, AstraZeneca, and Novartis; receiving honoraria for speaker or advisory board participation from Novartis, Lilly, AstraZeneca, Daiichi Sankyo, Gilead, Roche, GlaxoSmithKline, Clovis Oncology, Seagen, and Accord; and having nonfinancial interests as a member of the ESMO YOC and scientific secretary of the Spanish Society of Medical Oncology. B. Conte reports speaker fees from Veracyte and payment for educational events from Medsite and Novartis.G. Buono received speaker’s honoraria, consulting honoraria, and advisory board honoraria from: Novartis, GSK, Eli-Lilly, Pfizer, AstraZeneca, Roche, Daiichi Sankyo, Seagen, Gilead, Exact Science and Genetic.M. Lambertini reported having an advisory role for Roche, Lilly, Novartis, AstraZeneca, Pfizer, Seagen, Gilead, MSD, Menarini and Exact Sciences; receiving speaker honoraria from Roche, Lilly, Novartis, Pfizer, Sandoz, Libbs, Daiichi Sankyo, Takeda, Knight, Ipsen, Menarini and AstraZeneca; receiving travel grants from Gilead and Daiichi Sankyo; receiving research funding (to his institution) from Gilead; and having nonfinancial interests as the chair of the European Society for Medical Oncology (ESMO) Young Oncologists Committee (YOC) and as a member of the national council of the Italian Association of Medical Oncology. K. Punie reported receiving research grants (to his institution) from MSD and Sanofi; speaker fees and honoraria for consultancy and advisory board functions from AstraZeneca, Eli Lilly, Exact Sciences, Focus Patient, Gilead, Menarini, MSD, Novartis, Pfizer, Roche, and Seagen; speaker fees and honoraria for consultancy and advisory board functions (to his institution) from AstraZeneca, Eli Lilly, Exact Sciences, Gilead, MSD, Novartis, Pfizer, Roche, and Seagen; stock options from Need Inc; and travel grants from AstraZeneca, Novartis, Pfizer, PharmaMar, and Roche. J.M. Cejalvo reports speakers' fees from Daiichy Sankyo and Pfizer Inc. outside the submitted work.G. Arpino reports personal fees for research and medical writing: AstraZeneca; advisory boards, travel grants, activities as a speaker, consultancy: AstraZeneca, Daiichi Sankyo, Eisai, Eli Lilly, Gilead, Exact Science, Novartis, Roche, Seagen, Viatris.D. Generali declares personal fees for educational events by Novartis, Lilly, Pfizer, Daiichy-Sankyo, Roche; research funds from AstraZeneca, Novartis and LILT. E. Ciruelos reports consulting fees from Novartis, Lilly, Pfizer, Roche, AstraZeneca, and Daiichi Sankyo; speaker's bureau from Lilly, Pfizer, AstraZeneca, and Daiichi Sankyo; and travel and accommodations from Pfizer and Roche. J. Cortés reports consulting for Roche, Celgene, Cellestia, AstraZeneca, Seattle Genetics, Daiichi Sankyo, Erytech, Athenex, Polyphor, Lilly, Merck Sharp&Dohme, GSK, Leuko, Bioasis, Clovis Oncology, Boehringer Ingelheim, Ellipses, Hibercell, BioInvent, Gemoab, Gilead, Menarini, Zymeworks, Reveal Genomics, Expres2ion Biotechnologies; honoraria from Roche, Novartis, Celgene, Eisai, Pfizer, Samsung Bioepis, Lilly, Merck Sharp&Dohme, Daiichi Sankyo.; research funding to the Institution from Roche, Ariad pharmaceuticals, AstraZeneca, Baxalta GMBH/Servier Affaires, Bayer healthcare, Eisai, F. Hoffman-La Roche, Guardanth health, Merck Sharp&Dohme, Pfizer, Piqur Therapeutics, Puma C, Queen Mary University of London; stock shares for MEDSIR, Nektar Pharmaceuticals, Leuko (relative); Travel, accommodation, expenses from Roche, Novartis, Eisai, Pfizer, Daiichi Sankyo, Astrazeneca, Gilead; and Patents: Pharmaceutical Combinations of A Pi3k Inhibitor And A Microtubule Destabilizing Agent. Javier Cortés Castán, Alejandro Piris Giménez, Violeta Serra Elizalde. WO 2014/199294 A. ISSUED Her2 as a predictor of response to dual HER2 blockade in the absence of cytotoxic therapy. Aleix Prat, Antonio Llombart, Javier Cortés. US 2019/0338368 A1. LICENSED.A. Gennari received advisory role from AstraZeneca, Daiichi, Eisai, Lilly, Novartis, Pfizer, Roche, Seagen, Gilead, Teva, and Gentili; lecture honoraria from Novartis, Pfizer, Gilead, Roche, Eisai, Seagen, Teva, and Gentili; and research support from Roche, Eisai, Gilead, and Pharmanutra.M. Vidal has declared honoraria for presentations from Novartis, Roche, Pfizer, and Daichii, and travel expenses from Roche and Pfizer. Additionally, she has participated on a Data Safety Monitoring Board or Advisory Board for Novartis and Roche. S.M. Tolaney reports consulting or advisory roles for Novartis, Pfizer (SeaGen), Merck, Eli Lilly, AstraZeneca, Genentech/Roche, Eisai, Sanofi, Bristol Myers Squibb, CytomX Therapeutics, Daiichi Sankyo, Gilead, Zymeworks, Zentalis, Blueprint Medicines, Reveal Genomics, Sumitovant Biopharma, Umoja Biopharma, Artios Pharma, Menarini/Stemline, Aadi Bio, Bayer, Incyte Corporation, Jazz Pharmaceuticals, Natera, Tango Therapeutics, Systimmune, eFFECTOR, Hengrui USA, Cullinan Oncology, Circle Pharma, and Arvinas; research funding from Genentech/Roche, Merck, Exelixis, Pfizer, Lilly, Novartis, Bristol Myers Squibb, Eisai, AstraZeneca, Gilead, NanoString Technologies, Seattle Genetics, and OncoPep; and travel support from Eli Lilly, Sanofi, Gilead, and Jazz Pharmaceuticals. A. Prat reports advisory and consulting fees from AstraZeneca, Roche, Pfizer, Novartis, Daiichi Sankyo, and Peptomyc, lecture fees from AstraZeneca, Roche, Novartis, and Daiichi Sankyo, institutional financial interests from AstraZeneca, Novartis, Roche, and Daiichi Sankyo; stockholder and employee of Reveal Genomics; patents filed PCT/EP2016/080056, PCT/EP2022/086493, PCT/EP2023/060810, EP23382703 and EP23383369. G. Villacampa has received a speaker’s fee from Pfizer, MSD, GSK and Pierre Fabrer, has held an advisory role with AstraZeneca and received consultant fees from Reveal Genomics. The other authors have nothing to declare., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: Recent data suggest that human epidermal growth factor receptor 2 (HER2)-low breast cancer may represent a distinct entity. We aimed to compare disease characteristics and outcomes between HER2-low and HER2-0 in estrogen receptor (ER) positive, early-stage breast cancer. Methods: A single center retrospective study comprising all women with ER positive, HER2 negative early breast cancer, for whom an Oncotype DX test was performed between 2005 and 2012. Women were grouped to HER2-low (immunohistochemistry +1 or +2 and in situ hybridization not amplified) or HER2-0. Clinico-pathological features and Oncotype recurrence score (RS) were collected. Data on overall-survival (OS), disease-free survival (DFS) and distant disease-free survival (DDFS) were evaluated according to HER2 expression status. Results: 608 women were included, of which 304 women had HER2-0 and 304 had HER2-low disease. Lobular subtype was significantly more common in HER-0 compared to HER2-low disease (17% vs. 8%, p = 0.005). The prevalence of other clinic-pathological characteristics and long-term prognosis were comparable between both groups. For women with high genomic risk (RS > 25), HER2-low expression was associated with significantly favorable OS (HR = 0.31, 95% CI 0.11–0.78, p = 0.01), DFS (HR = 0.40, 95% CI 0.20–0.82, p = 0.01) and DDFS (HR = 0.26, 95% CI 0.11–0.63, P = 0.002) compared to women with HER2-0. For women with low genomic risk (RS ≤ 25), long-term prognosis was unrelated to HER2 expression. Conclusion: The prognostic impact of HER2-low expression in early-stage luminal disease varies across the genomic risk, with significant favorable outcomes of HER2-low expression compared to HER2-0 in women with high genomic risk.

Background: Trastuzumab deruxtecan (T-DXd) is approved for human epidermal growth factor receptor 2 (HER2)-positive and HER2-low advanced breast cancer (ABC). T-DXd has shown encouraging intracranial activity in HER2-positive ABC patients with stable or active brain metastases (BMs); however, its efficacy in patients with HER2-low ABC with BMs is not well established yet., Methods: DEBBRAH is a single-arm, five-cohort, phase II study evaluating T-DXd in patients with central nervous system involvement from HER2-positive and HER2-low ABC. Here, we report results from patients with heavily pretreated HER2-low ABC and active BMs, enrolled in cohorts 2 (n = 6, asymptomatic untreated BMs) and 4 (n = 6, progressing BMs after local therapy). Patients received 5.4 mg/kg T-DXd intravenously once every 21 days. The primary endpoint was intracranial objective response rate per Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) for both cohorts., Results: Intracranial objective response rate per RANO-BM was 50.0% [3/6 patients; 95% confidence interval (CI) 11.8% to 88.2%] and 33.3% [2/6 patients; 95% CI 4.3% to 77.7%; P = 0.033 (one-sided)] in cohorts 2 and 4, respectively. All responders had partial responses. Median time to intracranial response was 2.3 months (range, 1.5-4.0 months) and median duration of intracranial response was 7.2 months (range, 2.8-16.8 months). Median progression-free survival per RECIST v.1.1. was 5.4 months (95% CI 4.1-10.0 months). Treatment-emergent adverse events occurred in all patients included (16.7% grade 3). Three patients (25.0%) had grade 1 interstitial lung disease/pneumonitis., Conclusions: T-DXd demonstrated promising intracranial activity in pretreated HER2-low ABC patients with active BMs. Further studies are needed to validate these results in larger cohorts. This trial is registered with ClinicalTrials.gov, NCT04420598., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: Data on the pathological responses and survival outcomes after neoadjuvant chemotherapy (NACT) in human epidermal growth factor receptor-2 (HER2)-low breast cancer (BC) are lacking. This study aims to investigate this topic in the real world., Methods: Clinicopathological data from 819 HER2-negative BC patients who underwent NACT between 2010 and 2020 were retrospectively retrieved from the Shanghai Jiaotong University Breast Cancer Database. These patients were categorized into HER2-low and HER2-0 groups. Logistic analyses were conducted to identify predictors of complete pathological response (pCR) and breast pCR. Cox regression analyses were conducted to assess the factors associated with disease-free survival (DFS) and overall survival (OS). Kaplan-Meier (K-M) curves were generated to compare DFS and OS between HER2-low BC and HER2-0 BC., Results: Of the 819 BC patients, 669 (81.7%) had HER2-low tumors, and 150 (18.3%) had HER2-0 tumors. HER2-low BC had a significantly higher ratio of ER ≥ 10%, PR ≥ 20%, and Ki67 ≥ 15% than HER2-0 BC. A significantly higher breast pCR rate was observed in HER2-low BC than in HER2-0 BC (13.6% and 7.3%, respectively, P = .036). Age, HER2 status (low or 0), Ki67, and surgery options were associated with breast pCR in HER2-negative BC. In HER2-low BC, the pCR rate of ER ≥ 10% BC was significantly lower than that of ER < 10% BC, but the DFS and OS of ER 10% BC were significantly higher. The K-M curve showed no significant differences in DFS or OS between HER2-low and HER2-0 BC. Cox regression revealed that ER expression and histological grade (III vs. I∼II) were significantly associated with survival in HER2-low BC., Conclusions: In this real-world data (RWD) study, a significantly higher breast pCR rate was found in HER2-low BC than in HER2-0 BC, although there was no significant difference in survival. Moreover, ER expression had a significant prognostic impact on HER2-low BC., Competing Interests: Conflicts of interest The authors declare that they have no competing interests., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Background: The characterization and comparison of gene expression and intrinsic subtype (IS) changes induced by neoadjuvant chemotherapy (NACT) and endocrine therapy in hormone receptor-positive (HR+)/human epidermal growth factor receptor 2 (HER2)-low versus HR+/HER2-0 breast cancer (BC) has not been conducted so far. Most evidence on the association of HER2 status with pathologic responses and prognosis in HR+/HER2-negative BC is controversial and restricted to NACT-treated disease. Similarly, a temporal heterogeneity in HER2 status has been described only with NACT., Methods: We retrospectively recruited a consecutive cohort of 186 patients with stage I-IIIB HR+/HER2-negative BC treated with neoadjuvant therapy (NAT). Available diagnostic biopsies and surgical samples were characterized for main pathological features, PAM50 IS and ROR-P score, and gene expression. Associations with pathologic complete response, residual cancer burden-0/I, event-free survival (EFS) and overall survival (OS) based on HER2 status were assessed. Pre/post pathologic/molecular changes were analyzed in matched samples., Results: The HER2-low (62.9%) and HER2-0 (37.1%) cohorts did not differ significantly in main baseline features, treatments administered, breast-conserving surgery, pathologic complete response and residual cancer burden-0/I rates, EFS, and OS. NAT induced, regardless of HER2 status, a significant reduction of estrogen receptor/progesterone receptor and Ki67 levels, a down-regulation of PAM50 proliferation- and luminal-related genes/signatures, an up-regulation of selected immune genes, and a shift towards less aggressive IS and lower ROR-P. Moreover, 25% of HER2-0 changed to HER2-low and 34% HER2-low became HER2-0. HER2 shifts were significant after NACT (P < 0.001), not neoadjuvant endocrine therapy (P = 0.063), with consistent ERBB2 mRNA level dynamics. HER2 changes were not associated with EFS/OS., Conclusions: HER2-low and HER2-0 status change after NAT in ∼30% of cases, mostly after NACT. Targeted adjuvant strategies should be investigated accordingly. Molecular downstaging with current chemo/endocrine agents and immunotherapy should not rely on HER2 immunohistochemical levels in HR+/HER2-negative BC. Instead, HER2-low-targeted approaches should be explored to pursue more effective and/or less toxic dimensional downstaging., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: Breast cancer (BC) is the second most common cancer that metastasizes to the brain. Particularly up to half of patients with human epidermal growth factor receptor 2 (HER2)-positive (HER2+) metastatic breast cancer (mBC) may develop brain metastases over the course of the disease. Nevertheless, little is known about the prevalence and the outcome of brain and leptomeningeal metastases (BLMM) in HER2-low BC. We compared the cumulative incidence of BLMM and associated outcomes among patients with HER2-low, HER2-negative (HER2-) and HER2+ mBC., Patients and Methods: This cohort study was conducted from the Epidemiological Strategy and Medical Economics (ESME) mBC database and included patients treated for mBC between 2012 and 2020 across 18 French comprehensive cancer centers and with known HER2 and hormone receptor (HR) status. The cumulative incidence of BLMM after metastatic diagnosis was estimated using a competing risk methodology with death defined as a competing event., Results: 19 585 patients were included with 6118 (31.2%), 9943 (50.8%) and 3524 (18.0%) being HER2-low, HER2- and HER2+ mBC, respectively. After a median follow-up of 48.6 months [95% confidence interval (CI) 47.7-49.3 months], BLMM were reported in 4727 patients: 1192 (25.2%) were diagnosed with BLMM at first metastatic diagnosis and 3535 (74.8%) after metastatic diagnosis. Multivariable analysis adjusted for age, histological grade, metastases-free interval and HR status showed that the risk of BLMM at metastatic diagnosis was similar in patients with HER2- compared to HER2-low mBC [odds ratio (OR) (95% CI) 1.00 (0.86-1.17)] and higher in those with HER2+ compared to HER2-low [OR (95% CI) 2.23 (1.87-2.66)]. Similar results were found after metastatic diagnosis; the risk of BLMM was similar in HER2- compared to HER2-low [subdistribution hazard ratio (sHR) (95% CI) 1.07 (0.98-1.16)] and higher in the HER2+ group [sHR (95% CI) 1.56 (1.41-1.73)]., Conclusions: The prevalence and evolution of BLMM in HER2-low mBC are similar to those in patients with HER2- tumors. In contrast to patients with HER2+ mBC, the prognosis of BLMM remains dismal in this population., Competing Interests: Disclosure WJ declares grants: AstraZeneca, Daiichi Sankyo; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events: AstraZeneca, Eisai, BMS, Lilly France, Daiichi Sankyo, MSD, Novartis, Pfizer, Roche, Seagen; support for attending meetings and/or travel: AstraZeneca, Novartis, Chugai Pharma, Pfizer, Eisai, Pierre Fabre, Glaxo Smithkline, Roche, Lilly France, Sanofi Aventis; participation on a data safety monitoring board or advisory board : AstraZeneca, Eisai, BMS, Lilly France, Daiichi Sankyo, MSD, Novartis, Pfizer, Roche, Seagen, Gilead. MA declares payment or honoraria for lectures, presentations, speaker’s bureaus, manuscript writing or educational events: Gilead, Daiichi Sankyo, Pfizer, Lilly, AstraZeneca, Novartis; support for attending meetings and/or travel: Gilead, Pfizer, Novartis; participation on a data safety monitoring board or advisory board: AstraZeneca, Novartis, Pfizer. FLD declares consulting fees: Novartis, Gilead; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events: Amgen, Daiichi, Lilly, Novartis, Seagen, AZ; support for attending meetings and/or travel: Novartis, Pfizer, Seagen; participation on a data safety monitoring board or advisory board: Daiichi, Lilly, Seagen, Pfizer, Novartis, Roche, Exact Sciences; receipt of equipment, materials, drugs, medical writing, gifts or other services : Lilly. TF declares payment or honoraria for lectures, presentations, speaker’s bureaus, manuscript writing or educational events: Roche, Jansen, Lilly. BP received fees as advisor/consultant from Pierre Fabre (self), Daiichi Sankyo (self), Merck Sharp & Dohme (institution), Seattle Genetics (institution), Eli Lilly (institution) and Novartis (institution); funding to institution for research support from Daiichi Sankyo and AstraZeneca; and travel expenses from AstraZeneca, Pfizer and Gilead. JSF declares consulting fees: Pfizer, Lilly, Novartis, AstraZeneca, Clovis Oncology, GSK, Gilead, Daiichi Sankyo, Seagen, Exact Science, MSD; payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events: Lilly, Novartis, AstraZeneca, Gilead, Daiichi Sankyo, Seagen, MSD; support for attending meetings and/or travel: Pfizer, Lilly, Novartis, AstraZeneca, Clovis Oncology, GSK, Gilead, Daiichi Sankyo, Seagen, MSD. All other authors have declared no conflicts of interest., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

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., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: Human epidermal growth factor receptor 2 (HER2)-low is a newly defined category with HER2 1+ or 2+ expression by immunohistochemistry (IHC) and lack of HER2 gene amplification measured by in situ hybridization (ISH). Much remains unknown about the HER2-low status across tumor types and changes in HER2 status between primary and metastatic samples., Patients and Methods: HER2 expression by IHC was evaluated in 4701 patients with solid tumors. We have evaluated the HER2 expression by IHC and amplification by ISH in paired breast and gastric/gastroesophageal (GEJ) primary and metastatic samples. HER2 expression was correlated with ERBB2 genomic alterations evaluated by next-generation sequencing (NGS) in non-breast, non-gastric/GEJ samples., Results: HER2 expression (HER2 IHC 1-3+) was found in half (49.8%) of the cancers, with HER2-low (1 or 2+) found in many tumor types: 47.1% in breast, 34.6% in gastric/GEJ, 50.0% in salivary gland, 46.9% in lung, 46.5% in endometrial, 46% in urothelial, and 45.5% of gallbladder cancers. The concordance evaluation of HER2 expression between primary and metastatic breast cancer samples showed that HER2 3+ remained unchanged in 87.1% with a strong agreement between primary and metastatic samples, with a weighted kappa (Κ) of 0.85 (95% confidence interval 0.79-0.91). ERBB2 alterations were identified in 117 (7.5%) patients with non-breast, non-gastric/GEJ solid tumors who had NGS testing. Of 1436 patients without ERBB2 alterations, 512 (35.7%) showed any level HER2 expression by IHC., Conclusion: Our results show that HER2-low expression is frequently found across tumor types. These findings suggest that many patients with HER2-low solid tumors might benefit from HER2-targeted therapies., (Copyright © 2023. Published by Elsevier Ltd.)

Background: Approximately 80% of all breast cancers (BCs) are currently categorized as human epidermal growth factor receptor 2 (HER2)-negative [immunohistochemistry (IHC) 0, 1+, or 2+/in situ hybridization (ISH) negative]; approximately 60% of BCs traditionally categorized as HER2-negative express low levels of HER2. HER2-low (IHC 1+ or IHC 2+/ISH-) status became clinically actionable with approval of trastuzumab deruxtecan to treat unresectable/metastatic HER2-low BC. Greater understanding of patients with HER2-low disease is urgently needed., Patients and Methods: This global, multicenter, retrospective study (NCT04807595) included tissue samples from patients with confirmed HER2-negative unresectable/metastatic BC [any hormone receptor (HR) status] diagnosed from 2014 to 2017. Pathologists rescored HER2 IHC-stained slides as HER2-low (IHC 1+ or IHC 2+/ISH-) or HER2 IHC 0 after training on low-end expression scoring using Ventana 4B5 and other assays at local laboratories (13 sites; 10 countries) blinded to historical scores. HER2-low prevalence and concordance between historical scores and rescores were assessed. Demographics, clinicopathological characteristics, treatments, and outcomes were examined., Results: In rescored samples from 789 patients with HER2-negative unresectable/metastatic BC, the overall HER2-low prevalence was 67.2% (HR positive, 71.1%; HR negative, 52.8%). Concordance was moderate between historical and rescored HER2 statuses (81.3%; κ = 0.583); positive agreement was numerically higher for HER2-low (87.5%) than HER2 IHC 0 (69.9%). More than 30% of historical IHC 0 cases were rescored as HER2-low overall (all assays) and using Ventana 4B5. There were no notable differences between HER2-low and HER2 IHC 0 in patient characteristics, treatments received, or clinical outcomes., Conclusions: Approximately two-thirds of patients with historically HER2-negative unresectable/metastatic BC may benefit from HER2-low-directed treatments. Our data suggest that HER2 reassessment in patients with historical IHC 0 scores may be considered to help optimize selection of patients for treatment. Further, accurate identification of patients with HER2-low BC may be achieved with standardized pathologist training., Competing Interests: Disclosure All authors received nonfinancial support (assistance with manuscript preparation) from Articulate Science, LLC, and Helios Medical Communications Ltd funded by AstraZeneca. Additional disclosures are as follows: GV reports consulting or advisory fees from Agilent, AstraZeneca, Daiichi Sankyo, and Roche; participation in speakers bureaus and data safety monitoring advisory boards for Agilent, AstraZeneca, Daiichi Sankyo and MSD Oncology, and Roche; personal travel fees from AstraZeneca and Roche; and grants from Roche. MB reports consulting or advisory fees from AstraZeneca and Roche Canada; and contracted research funding from Pfizer Canada. NN reports grants from Chugai, Daiichi Sankyo, Eisai, Lilly, Mochida, Novartis, and Pfizer; and participation in speakers bureaus for AstraZeneca, Chugai, Daiichi Sankyo, Eisai, Lilly, Novartis, and Pfizer. ET reports consulting or advisory fees from Daiichi Sankyo and Lilly; and fees for non-CME services from AstraZeneca, Daiichi Sankyo, and Lilly. SB reports consulting or advisory fees from AstraZeneca, Lilly, MSD, Novartis, Pfizer, Roche, and Sanofi; and contracted research funding from AstraZeneca, Lilly, GSK, MSD, Novartis, Pfizer, and Roche. FPL reports consulting or advisory fees from Agendia, AstraZeneca, Daiichi Sankyo, Eisai, Exact Sciences, GSK, MammaPrint, MSD, Myriad Genetics, Novartis, Pfizer, Roche, Seagen, and Veracyte; contracted research funding from AstraZeneca, MammaPrint, MSD, Myriad Genetics, Roche, and Veracyte; and personal travel fees from AstraZeneca and Seagen. NH reports participation in speakers bureaus for AstraZeneca, Chugai, Genomic Health, Kirin, Novartis, and Pfizer. JS reports research funding from AstraZeneca, Boehringer Ingelheim, Daiichi Sankyo, GSK, Eli Lilly, MSD, Novartis, Pfizer, Roche, and Sanofi; and partnerships with Roche. RTS reports consulting and non-CME service fees from AstraZeneca, Gilead, Lilly, Novartis, and Pfizer. AMB reports consulting or advisory fees from AbbVie, Agendia, Bayer, Biotheranostics, Coherus BioSciences, Daiichi Sankyo, Lilly, Eisai, Genentech/Roche, General Electric, Gilead Sciences, Immunomedics, Merck, Michael J. Hennessy Associates, Myriad Genetics, Novartis, OncLive, Pfizer, Puma Biotechnology, Seagen, and Tyme; and contracted research funding from AstraZeneca, Daiichi Sankyo, Lilly, Genentech/Roche, Gilead Sciences, Merck, Novartis, and Puma Biotechnology. CSO reports consulting or advisory fees from Roche and Seagen; and participation in speakers bureaus for Lilly. FS reports no conflict of interests. GH reports being a shareholder in Rhythm Biosciences Limited. DV reports a consulting or advisory role with OPEN Health; employment with AstraZeneca and may own stock or stock options in that company. GDJ reports a consulting or advisory role with AstraZeneca/MedImmune. AM is an employee of Daiichi Sankyo and may own stock or stock options in that company. AL and VGM are employees of AstraZeneca and may own stock or stock options in that company. Data sharing Data underlying the findings described in this manuscript may be obtained in accordance with AstraZeneca’s data sharing policy described at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure. Data for studies directly listed on Vivli can be requested through Vivli at www.vivli.org. Data for studies not listed on Vivli could be requested through Vivli at https://vivli.org/members/enquiries-about-studies-not-listed-on-the-vivli-platform/. AstraZeneca Vivli member page is also available outlining further details: https://vivli.org/ourmember/astrazeneca/. Disclaimers Medical writing support was funded by AstraZeneca in accordance with Good Publication Practice (GPP) guidelines (http://www.ismpp.org/gpp-2022). The manuscript was reviewed for medical accuracy by AstraZeneca and Daiichi Sankyo; however, the authors retained full control of the content and made the final decisions for all aspects of this article., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: Human epidermal growth factor receptor 2 (HER2)-low expression in breast cancer has been recently identified as a new therapeutic target. However, it is unclear if HER2-low status has an independent impact on prognosis., Materials and Methods: A systematic literature research was carried out to identify studies comparing survival outcomes of patients affected by HER2-low versus HER2-zero breast cancer. Using random-effects models, pooled hazard ratios (HRs) and odds ratios (ORs) with 95% confidence intervals (CIs) were calculated for progression-free survival (PFS) and overall survival (OS) in the metastatic setting as well as disease-free survival (DFS), OS and pathological complete response (pCR) in the early setting. Subgroup analyses by hormone receptor (HoR) status were carried out. The study protocol is registered on PROSPERO (n.CRD42023390777)., Results: Among 1916 identified records, 42 studies including 1 797 175 patients were eligible. In the early setting, HER2-low status was associated with significant improved DFS (HR 0.86, 95% CI 0.79-0.92, P < 0.001) and OS (HR 0.90, 95% CI 0.85-0.95, P < 0.001) when compared to HER2-zero status. Improved OS was observed for both HoR-positive and HoR-negative HER2-low populations, while DFS improvement was observed only in the HoR-positive subgroup. HER2-low status was significantly associated with a lower rate of pCR as compared to HER2-zero status both in the overall population (OR 0.74, 95% CI 0.62-0.88, P = 0.001) and in the HoR-positive subgroup (OR 0.77, 95% CI 0.65-0.90, P = 0.001). In the metastatic setting, patients with HER2-low breast cancers showed better OS when compared with those with HER2-zero tumours in the overall population (HR 0.94, 95% CI 0.89-0.98, P = 0.008), regardless of HoR status. No significant PFS differences were found., Conclusions: Compared with HER2-zero status, HER2-low status appears to be associated with a slightly increased OS both in the advanced and early settings, regardless of HoR expression. In the early setting, HER2-low tumours seem to be associated to lower pCR rates, especially if HoR-positive., Competing Interests: Disclosure CM reports support to attend medical conferences from Gilead and honoraria from Novartis and Lilly (all outside the submitted work). EA reports consultancy fees/honoraria from Eli Lilly, Sandoz, AstraZeneca, research grant to her institution from Gilead and support for attending medical conferences from Novartis, Roche, Eli Lilly, Genetic, Istituto Gentili, Daiichi Sankyo, AstraZeneca. GNM reports support to attend medical conferences from Roche and Bayer (all outside the submitted work). AP reports advisory and consulting fees from Roche, Pfizer, Novartis, Amgen, BMS, Puma, Oncolytics Biotech, MSD, Guardant Health, Peptomyc and Lilly, lecture fees from Roche, Pfizer, Novartis, Amgen, BMS, Nanostring Technologies and Daiichi Sankyo, institutional financial interests from Boehringer, Novartis, Roche, Nanostring, Sysmex Europe GmbH, Medica Scientia inno. Re-search, SL, Celgene, Astellas and Pfizer; and shares ownership and a leadership role in Reveal Genomics, SL. EB reports funding to her institution from Gilead Science. FS declares personal fees for educational activities from Novartis and Gilead and travel expenses from Gilead, Novartis and Daiichy Sankyo. GV received honoraria for advisory boards and consulting fees from Roche, AstraZeneca, Daiichi Sankyo, MSD Oncology and Pfizer. LDM reports institutional research grant from Eli Lilly, Novartis, Roche, Daiichi Sankyo and Seagen, consulting fees from Eli Lilly; honoraria from Roche, Novartis, Pfizer, Eli Lilly, AstraZeneca, Merck Sharp and Dohme, Seagen, Gilead, Pierre Fabre, Eisa, Exact Sciences and Ipsen and support for attending meetings from Roche, Pfizer and Eisai; and fees for participation on a data safety monitoring board or advisory board from Novartis, Roche, Eli Lilly, Pfizer, Daiichi-Sankyo, Exact Sciences, Gilead, Pierre Fabre, Eisai, AstraZeneca and Agendia. ML played an advisory role for Roche, Lilly, Novartis, Astrazeneca, Pfizer, Seagen, Gilead, MSD and Exact Sciences and received speaker honoraria from Roche, Daiichi Sankyo, Lilly, Novartis, Pfizer, Sandoz, Libbs and Takeda and travel grants from Gilead outside the submitted work. EDA received honoraria and/or participated to advisory board from Roche/GNE, Novartis, Seattle Genetics, Zodiac, Libbs and Pierre Fabre; received travel grants from Roche/GNE and GSK/Novartis; and received research grant to his institution from Roche/GNE, AstraZeneca, GSK/Novartis and Servier. All other authors have declared no conflicts of interest., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Background: Few studies have focused on converting ER-low-positive and HER2-low status following neoadjuvant therapy (NAT). We aimed to assess the evolution in ER and HER2 status after NAT in breast cancer patients., Patients and Methods: Our study included 481 patients with residual invasive breast cancer after NAT. ER and HER2 status were assessed in the primary tumor and residual disease, and associations between ER and HER2 conversion and clinicopathological factors were explored., Results: In primary tumors, 305 (63.4%) cases were ER-positive (including 36 cases of ER-low-positive), 176 (36.6%) were ER-negative. In residual disease, ER status changed in 76 (15.8%) cases, of which 69 cases switched from positive to negative. ER-low-positive tumors (31/36) were the most likely to change. In primary tumors, 140 (29.1%) tumors were HER2-positive, and 341 (70.9%) were HER2-negative (including 209 cases of HER2-low and 132 cases of HER2-zero). In residual disease, 25 (5.2%) cases had HER2 conversion between positive and negative. Considering HER2-low status, 113 (23.5%) cases had HER2 conversion, mostly driven by cases switching either to or from HER2-low. ER conversion had a positive correlation with pretreatment ER status (r = 0.25; P = .00). There was a positive correlation between HER2 conversion and HER2-targeted therapy (r = 0.18; P = .00)., Conclusion: Conversion of ER and HER2 status was observed in some breast cancer patients after NAT. Both ER-low-positive and HER2-low tumors showed high instability from the primary tumor to residual disease. ER and HER2 status should be retested in residual disease for further treatment decisions, especially in ER-low-positive and HER2-low breast cancer., Competing Interests: Disclosure We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Effects of NAT on ER and HER2 status in breast cancer.”, (Copyright © 2023 Elsevier Inc. All rights reserved.)

Background: The prognostic differences between HER2-zero and HER2-low breast cancer (BC) remain unclear. Purpose of this meta-analysis is to investigate the differences between HER2-low and HER2-zero in terms of clinicopathological factors and survival outcomes in early-stage BC., Methods: We searched major databases and congress proceedings until November 1, 2022 to identify studies comparing HER2-zero and HER2-low in early-stage BC. HER2-zero immunohistochemically (IHC) was defined as score 0, while HER2-low was defined as IHC 1+ or 2+/in situ hybridization negative., Result: A total of 23 retrospective studies involving 636,535 patients were included. HER2-low rate was 67.5% in the hormone receptor (HR)-positive group, while this rate was 48.6% in the HR-negative group. In the analysis of clinicopathological factors by HR status, the proportion of premenopausal patients within the HR-positive group was greater in the HER2-zero arm (66.5% vs 61.8%), whereas grade 3 tumors (74.2% vs 71.5%), patients younger than 50 years of age (47.3% vs 39.6%), and T3-T4 tumors (7.7% vs 6.3%) within the HR-negative group was higher in the HER2-zero arm. In both the HR-positive and HR-negative groups, the HER2-low arm showed significantly improved results for disease-free survival (DFS) and overall survival (OS). The hazard ratios for DFS and OS in the HR-positive group were 0.88 (95% CI 0.83-0.94) and 0.87 (95% CI 0.78-0.96), respectively. In the HR-negative group, the hazard ratios for DFS and OS were 0.87 (95% CI 0.79-0.97) and 0.86 (95% CI 0.84-0.89), respectively., Conclusion: In early-stage BC, HER2-low is associated with better DFS and OS compared to HER2-zero, regardless of HR status., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Background: Triple-negative breast cancer (TNBC) is associated with poor prognosis, and new treatment options are urgently needed. About 34%-39% of primary TNBCs show a low expression of human epidermal growth factor receptor 2 (HER2-low), which is a target for new anti-HER2 drugs. However, little is known about the frequency and the prognostic value of HER2-low in metastatic TNBC., Patients and Methods: We retrospectively included patients with TNBC from five European countries for this international, multicenter analysis. Triple-negativity had to be shown in a metastatic site or in the primary breast tumor diagnosed simultaneously or within 3 years before metastatic disease. HER2-low was defined as immunohistochemically (IHC) 1+ or 2+ without ERBB2 gene amplification. Survival probabilities were calculated by the Kaplan-Meier method, and multivariable hazard ratios (HRs) were estimated by Cox regression models., Results: In total, 691 patients, diagnosed between January 2006 and February 2021, were assessable. The incidence of HER2-low was 32.0% [95% confidence interval (CI) 28.5% to 35.5%], with similar proportions in metastases (n = 265; 29.8%) and primary tumors (n = 425; 33.4%; P = 0.324). The median overall survival (OS) in HER2-low and HER2-0 TNBC was 18.6 and 16.1 months, respectively (HR 1.00; 95% CI 0.83-1.19; P = 0.969). Similarly, in multivariable analysis, HER2-low had no significant impact on OS (HR 0.95; 95% CI 0.79-1.13; P = 0.545). No difference in prognosis was observed between HER2 IHC 0/1+ and IHC 2+ tumors (HR 0.89; 95% CI 0.69-1.17; P = 0.414)., Conclusions: In this large international dataset of metastatic TNBC, the frequency of HER2-low was 32.0%. Neither in univariable nor in multivariable analysis HER2-low showed any influence on OS., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The recent decade has witnessed a vigorous prosper of antibody-drug conjugates (ADCs) in solid tumors including breast cancer. Integrating the specificity of monoclonal antibodies and potency of cytotoxic drugs, ADCs are capable of delivering cytotoxic agents directly to tumor cells and surrounding accomplices with heterogeneous antigen expression by exerting the distinctive bystander effect. Up till now, three ADCs (T-DM1, T-DXd and SG) have attained the official approval and stepped into clinical practices in breast cancer, with numerous promising products in the pipeline. As an unprecedented breast cancer subgroup identified following solidified drug benefit, the cognitive and therapeutic paradigm of HER2-low population which was previously thought lacking definite targets and efficacious regimens has been thoroughly rewritten by ADCs, and several encouraging achievements are expected in ongoing trials. Herein, we discuss the contrived knowledge, latest advancements and future perspectives of ADCs in HER2-overexpressing and HER2-low breast cancer., Competing Interests: Declaration of Competing Interest The authors declare no potential conflicts of interest., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Metastatic triple-negative breast cancer (mTNBC) is a poor prognostic disease with limited treatments and uncertain therapeutic algorithms. We performed a systematic review and multiple Bayesian network meta-analyses according to treatment line to establish an optimal therapeutic sequencing strategy for this lethal disease. We included 125 first-line trials (37,812 patients) and 33 s/further-lines trials (11,321 patients). The primary endpoint was progression-free survival (PFS). Secondary endpoints included overall response rates (ORR), overall survival (OS) and safety, for first and further lines, separately. We also estimated separate treatment rankings for the first and subsequent lines according to each endpoint, based on (surface under the cumulative ranking curve) SUCRA values. No first-line treatment was associated with superior PFS and OS than paclitaxel ± bevacizumab. Platinum-based polychemotherapies were generally superior in terms of ORR, at the cost of higher toxicity.. PARP-inhibitors in germline-BRCA1/2-mutant patients, and immunotherapy + chemotherapy in PD-L1-positive mTNBC, performed similar to paclitaxel ± bevacizumab. In PD-L1-positive mTNBC, pembrolizumab + chemotherapy was better than atezolizumab + nab-paclitaxel in terms of OS according to SUCRA values. In second/further-lines, sacituzumab govitecan outperformed all other treatments on all endpoints, followed by PARP-inhibitors in germline-BRCA1/2-mutant tumors. Trastuzumab deruxtecan in HER2-low mTNBC performed similarly and was the best advanced-line treatment in terms of PFS and OS after sacituzumab govitecan, according to SUCRA values. Moreover, comparisons with sacituzumab govitecan, talazoparib and olaparib were not statistically significant. The most effective alternatives or candidates for subsequent lines were represented by nab-paclitaxel (in ORR), capecitabine (in PFS) and eribulin (in PFS and OS)., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Dr Giuliano, Arpino and De Placido S have declared honoraria from Roche, Pfizer, Astra-Zeneca, Novartis, Celgene, Eli Lilly, Amgen, and Eisai, outside of the submitted work. Dr Lambertini has declared personal fees (advisory role and/or speaker honoraria) from Roche, Takeda, Sandoz, Eli Lilly, Pfizer, AstraZeneca, Novartis Exact Sciences and Ipsen, outside of the submitted work. Dr Bartsch has declared honoraria from AstraZeneca, Daiichy Sankyo, Eli Lilly, Novartis, Pfizer, Pierre Fabre, Roche, consulting or advisory role for AstraZeneca, Daiichi, Eisai, Eli Lilly, MSD Oncology, Novartis, Pfizer, Pierre Fabre, Puma Biotechnology, Roche, research funding from Daiichi-Sankyo, travel, accommodations, expenses from Roche, Pfizer, outside of the submitted work. Dr Pinato has received lecture fees from ViiV Healthcare and Bayer Healthcare and travel expenses from BMS and Bayer Healthcare; consulting fees for Mina Therapeutics, EISAI, Roche, and Astra Zeneca; research funding (to institution) from MSD and BMS, outside of the submitted work. Dr Harbeck has declared stocks and other ownership interests in the West German Study Group, honoraria from Roche, Novartis, Daiichi-Sankyo, Amgen, Pfizer, Exact Sciences, AstraZeneca, Pierre Fabre, SeaGen; consulting or advisory role for Roche/Genentech, Novartis, Pfizer, Eli Lilly, Sandoz, Daiichi Sankyo, AstraZeneca, Merck Sharp & Dohme, Sandoz, SeaGen, Pierre Fabre and research funding to institution from Roche/Genentech, Eli Lilly, Merck Sharp & Dohme, outside of the submitted work. Dr Lüftner has received honoraria from Amgen, AstraZeneca, Celgene, Eisai, Genomic Health, Eli Lilly, Loreal, MSD, Novartis, Pierre Fabre, Pfizer, Tesaro, Teva, outside of the submitted work. Dr Denys has reported consulting fees (advisory role) paid to her institution by Pfizer, Roche, PharmaMar, AstraZeneca, Eli Lilly, Novartis, Amgen, Tesaro; grants from Research Funding institutional Roche; and other fees (travel, accommodations, expenses) by Pfizer, Roche, PharmaMar, Teva, AstraZeneca, outside of the submitted work. Dr Zaman has reported travel, accommodation and expenses from Roche, Pierre Fabri and MSD Oncology, research funding from Roche/Genentech consulting/advisory role for Roche, Eli Lilly, MSD Oncology, Mylan, Novartis, Daiichy and Pierre Fabre, and other relationships with Pierre Fabre, all outside the submitted work. Dr Gligorov has declared speakers’ bureau honoraria from Roche-Genentech, Novartis, Eisai, Genomic Health, Ipsen, Pfizer, Mylan, Eli Lilly, Sandoz, and Pierre-Fabre; consultant/advisory board member for Roche-Genentech, Novartis, Onxeo, Daichii Sanyo, MSD, Eisai, Genomic Health, Ipsen, Macrogenics, Pfizer, Mylan, Eli Lilly, Immunomedics, Pierre-Fabre; grant support from Roche-Genetech, Eisai, Genomic Health, Pfizer, Mylan; travel, accommodation paid by Roche-Genentech, Novartis, Eisai, Genomic Health, Pfizer, Eli Lilly, Pierre-Fabre; personal fees and non-financial support from Daichii Sanyo, MSD, outside of the submitted work. Dr Awada reports advisory roles, travel grants, and speaker fees from Roche, Eli Lilly, Amgen, ESAI, BMS, Pfizer, Novartis, MSD, Ipsen, and Leopharma. Dr Campone has declared honoraria from Novartis, Eli Lilly, GT1, Consulting or Advisory Role from Novartis, Servier, Menarini, Sanofi, Eli Lilly, Pfizer, AstraZeneca/MedImmune, Abbvie, Pierre Fabre, Accord Healthcare, Sandoz-Novartis, Seattle Genetics, Daiichi Sankyo Europe GmbH, Speakers' Bureau from Novartis, Amgen, Research Funding from Novartis, Travel, Accommodations, Expenses from Novartis, AstraZeneca, Pfizer, Other Relationship from Roche, outside of the submitted work. Dr Wildiers has declared Consulting or Advisory Role from Roche, Eli Lilly, Pfizer, Sirtex Medical, Orion Corporation, Puma Biotechnology, AstraZeneca, Biocartis, Novartis, Daiichi Sankyo, Research Funding from Roche, Novartis, Travel, Accommodations, Expenses from Pfizer, Roche, outside of the submitted work. Dr Gennari has declared consulting/advisory role for Roche, MSD, Eli Lilly, Pierre Fabre, EISAI, and Daichii Sankyo; speakers bureau for Eisai, Novartis, Eli Lilly, Roche, Teva, Gentili, Pfizer, Astra Zeneca, Celgene, and Daichii Sankyo; research funds from EISAI, Eli Lilly, and Roche, outside of the submitted work. Dr Tjan-Heijnen has declared Honoraria and Travel expenses from Novartis, Roche, Eli Lilly, Pfizer and Honoraria from Daichii Sankyo, as well as Research Funding from Roche, Eisai, Pfizer, Novartis, Eli Lilly, AstraZeneca and Daichii Sankyo, outside of the submitted work. Dr Cortes has declared Stock and Other Ownership Interests from MedSIR, Honoraria from Novartis, Eisai, Celgene, Pfizer, Roche, Samsung, Eli Lilly, Merck Sharp & Dohme, Daiichi Sankyo, Consulting or Advisory Role from Celgene, Cellestia Biotech, AstraZeneca, Biothera, Merus, Roche, Seattle Genetics, Daiichi Sankyo, ERYTECH Pharma, Polyphor, Athenex, Eli Lilly, Servier, Merck Sharp & Dohme, GlaxoSmithKline, Leuko, Clovis Oncology, Bioasis, Boehringer Ingelheim, Research Funding from ARIAD, Astrazeneca, Baxalta, Bayer, Eisai, Guardant Health, Merck Sharp & Dohme, Pfizer, Puma Biotechnology, Queen Mary University of London, Roche, Piqur, Travel, Accommodations, Expenses from Roche, Pfizer, Eisai, Novartis, Daiichi Sankyo, outside of the submitted work. Dr Del Mastro acted as a consultant for Roche, Novartis, MSD, Pfizer, Ipsen, AstraZeneca, Genomic Health, Eli Lilly, Seattle Genetics, Eisai, Pierre Fabre, and Daiichi Sankyo, received speaker honoraria from Roche, Novartis, Eli Lilly and MSD, and travel grants from Roche, Pfizer and Celgene, outside the submitted work. Dr Martin has declared Honoraria from Roche/Genentech, Eli Lilly, Pfizer, Novartis, Pierre Fabre, Consulting or Advisory Role from Roche/Genentech, Novartis, Pfizer, Eli Lilly, AstraZeneca, Taiho Pharmaceutical, PharmaMar, Speakers' Bureau from Eli Lilly/ImClone, Roche/Genentech, Pierre Fabre, Research Funding from Novartis, Roche, Puma Biotechnology, outside of the submitted work. Dr Curigliano has reported personal fees from Roche, Pfizer, Novartis, Eli Lilly, Foundation Medicine, BMS, Samsung, Astra Zeneca, Daichii Sankyo Boeringer, GSK, Seagen, non-financial support from Roche, Pfizer, grants from Merck, other from Ellipsis, outside the submitted work. Dr Jerusalem has reported grants, personal fees and non-financial support from Novartis, Roche and Pfizer, personal fees and non-financial support from Eli Lilly, Amgen, BMS and Astra-Zeneca, personal fees from Daiichi Sankyo and Abbvie, non-financial support from Medimmune and MerckKGaA, outside the submitted work. Dr Prat reports grants and personal fees from Pfizer, grants and personal fees from Eli Lilly, personal fees from Nanostring tecnologies, grants and personal fees from Amgen, grants from Roche, personal fees from Oncolytics Biotech, Daiichi Sankyo, PUMA, BMS, consulting/advisory role for NanoString Technologies, Amgen, Roche, Novartis, Pfizer and Bristol-Myers Squibb and patent PCT/EP2016/080056, outside of the submitted work. Dr Prat is also an equity stockholder, founder and consultant of Reveal Genomics and has a patent HER2DX and WO 2018/103834 licensed to Reveal Genomics, all outside of the submitted work. Dr Generali has declared Honoraria from Novartis and Eli Lilly, Research Funding from Novartis and Astra-Zeneca, outside of the submitted work. All other authors declared nothing to disclose., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

HER2, a human epidermal growth factor, being activated by amplification, is a negative prognostic factor in breast cancer. HER2 is the target of anti-HER2 antibodies (Trastuzumab, Pertuzumab…). For more than 10 years, breast cancers have been classified into HER2 positive and HER2 negative. However, the advent of new cytotoxic drugs combined with anti-HER2 antibodies, such as TDM1 or trastuzumab déruxtécan, have shown very promising therapeutic activity in patients with low HER2 expression breast cancer. These new therapeutic perspectives encourage a better identification of low HER2 tumours in order to identify patients who could benefit from them. Thus, the classification of breast tumours evolves to individualize HER2-negative tumours (score 0), HER2-positive tumours (score 3+ and 2+ amplified) and HER2-low tumours (scores 1+ and 2+ not-amplified). HER2-low tumours are common and represent more than half of all breast cancers. To identify these HER2-low tumours, pathology laboratories should not change their usual technique calibrated according to ASCO/CAP and GEFPICS recommendations. Until more clinical data about response to these new treatment strategies are available, GEFPICS does not require pathologists to identify this HER2-low category. Nevertheless, this designation will allow clinicians to identify patients whose tumours fall into this category in the very short term and offer them new treatment options., (Copyright © 2021 Société FranÇaise du Cancer. Publié par Elsevier Masson SAS. Tous droits réservés.)

The identification of Human epidermal growth factor receptor 2 (HER2) as a target in breast cancer and the subsequent development of HER2-targeted therapies has revolutionized the treatment of patients with HER2-positive breast cancer. However, there is an increasing awareness of how frequently tumors have low or heterogeneous expression of HER2. It is now recognized that this impacts the degree of benefit from HER2-targeted therapies. With the advent of novel and more potent antibody drug conjugates, targeting HER2 in traditional HER2-negative tumors with "HER2-low" expression is becoming possible. It is essential to refine the nomenclature around HER2 expression to enable clinicians to optimize treatment for patients across the HER2 expression spectrum in breast cancer. HER2 heterogeneity can be detected by conventional IHC, gene expression profiling or other methods and numerous studies have documented the correlation between the presence of HER2 heterogeneity and shorter disease-free survival (DFS) and overall survival (OS). Validation of techniques to identify HER2 heterogeneity in the clinic and concurrent development of agents to effectively treat tumors with non-uniform HER2 expression is needed., (Copyright © 2021 Elsevier Ltd. All rights reserved.)