HER2-Low Breast Cancer: Pathological and Clinical Landscape

HER2-low status and response to neoadjuvant chemotherapy in HER2 negative early breast cancer

PURPOSE

Knowledge on whether low expressions of HER2 have prognostic impact in early-stage breast cancer (BC) and on its response to current chemotherapy protocols can contribute to medical practice and development of new drugs for this subset of patients, changing treatment paradigms. This study aims to evaluate the impact of HER2-low status on response to neoadjuvant chemotherapy (NACT) and survival outcomes in early-stage HER2-negative BC.

METHODS

Records from all BC patients treated with NACT from January 2007 to December 2018 in a single cancer center were retrospectively reviewed. HER2-negative (immunohistochemistry [IHC] 0, + 1, or + 2 non-amplified by in situ hybridization [ISH]) patients were included. HER2-low was defined by IHC + 1 or + 2 ISH non-amplified and HER2-0 by IHC 0. The coprimary objectives were to compare pathological complete response (pCR) and relapse-free survival (RFS) between luminal/HER2-low versus luminal/HER2-0 populations and between triple negative (TNBC)/HER2-low versus TNBC/HER2-0.

RESULTS

In total, 855 HER2-negative patients were identified. The median follow-up was 59 months. 542 patients had luminal subtype (63.4%) and 313 had TNBC (36.6%). 285 (33.3%) were HER2-low. Among luminal patients, 145 had HER2 IHC + 1 (26.8%) and 91 had IHC + 2/ISH non-amplified (16.8%). In TNBC, 36 had HER2 IHC + 1 (11.5%) and 13 had IHC + 2/ISH non-amplified (4.2%). Most patients had locally advanced tumors, regardless of subtype or HER2-low status. For luminal disease, pCR was achieved in 13% of HER2-low tumors versus 9.5% of HER2-0 (p = 0.27). Similarly, there was no difference in pCR rates among TNBC: 51% versus 47% in HER2-low versus HER2-0, respectively (p = 0.64). HER2-low was also not prognostic for RFS, with 5-year RFS rates of 72.1% versus 71.7% (p = 0.47) for luminal HER2-low/HER2-0, respectively, and 75.6% versus 70.8% (p = 0.23) for TNBC HER2-low/HER2-0.

CONCLUSION

Our data does not support HER2-low as a biologically distinct BC subtype, with no prognostic value on survival outcomes and no predictive effect for pCR after conventional NACT.

FDA Approval Summary: Fam-Trastuzumab Deruxtecan-Nxki for the Treatment of Unresectable or Metastatic HER2-Positive Breast Cancer

On December 20, 2019, the FDA granted accelerated approval to fam-trastuzumab deruxtecan-nxki [DS-8201a; T-DXd; tradename ENHERTU (Daiichi Sankyo)] for the treatment of adult patients with unresectable or metastatic HER2-positive breast cancer who have received two or more prior anti-HER2-based regimens in the metastatic setting. Approval was based on data from study DS8201-A-U201 (DESTINY-Breast01) with supportive safety data from study DS8201-A-J101. The primary efficacy endpoint in DESTINY-Breast01 was overall response rate (ORR) based on confirmed responses by blinded independent central review (ICR) using RECIST v1.1 in all participants who were assigned to receive the recommended dose of 5.4 mg/kg while secondary endpoints included duration of response (DoR). The confirmed ORR based on ICR in these 184 patients was 60.3% [95% confidence interval (CI): 52.9-67.4] and the median DoR was 14.8 months (95% CI: 13.8-16.9). Interstitial lung disease, including pneumonitis, was experienced in patients treated with T-DXd and can be severe, life threatening, or fatal. In addition, neutropenia and left ventricular dysfunction were included as Warnings and Precautions in labeling. Other important common adverse reactions were nausea, fatigue, vomiting, alopecia, constipation, decreased appetite, anemia, diarrhea, and thrombocytopenia. Overall, the totality of efficacy and safety data supported the accelerated approval of T-DXd for the intended indication.

Implementing antibody-drug conjugates (ADCs) in HER2-positive breast cancer: state of the art and future directions

The development of anti-HER2 agents has been one of the most meaningful advancements in the management of metastatic breast cancer, significantly improving survival outcomes. Despite the efficacy of anti-HER2 monoclonal antibodies, concurrent chemotherapy is still needed to maximize response. Antibody-drug conjugates (ADCs) are a class of therapeutics that combines an antigen-specific antibody backbone with a potent cytotoxic payload, resulting in an improved therapeutic index. Two anti-HER2 ADCs have been approved by the FDA with different indications in HER2-positive breast cancer. Ado-trastuzumab emtansine (T-DM1) was the first-in-class HER2-targeting ADC, initially approved in 2013 for metastatic patients who previously received trastuzumab and a taxane, and the label was expanded in 2019 to include adjuvant treatment of high-risk patients with residual disease after neoadjuvant taxane and trastuzumab-based therapy. In 2020, trastuzumab deruxtecan (T-DXd) was the second approved ADC for patients who had received at least 2 lines of anti-HER2-based therapy in the metastatic setting. The success of these two agents has transformed the treatment of HER2-positive breast cancer and has re-energized the field of ADC development. Given their advanced pharmaceutical properties, next-generation HER2-targeted ADCs have the potential to be active beyond traditional HER2-positive breast cancer and may be effective in cells with low expression of HER2 or ERBB2 mutations, opening a spectrum of new possible clinical applications. Ongoing challenges include improving target-specificity, optimizing the toxicity profile, and identifying biomarkers for patient selection. The aim of this review is to summarize the principal molecular, clinical, and safety characteristics of approved and experimental anti-HER2 ADCs, contextualizing the current and future landscape of drug development.

Sacituzumab govitecan and trastuzumab deruxtecan: two new antibody-drug conjugates in the breast cancer treatment landscape

BACKGROUND

Two new antibody-drug conjugates (ADCs) containing a topoisomerase I inhibitor payload have recently emerged in the breast cancer (BC) treatment landscape. Sacituzumab govitecan-hziy (SG) is a first-in-class anti-trophoblast cell-surface antigen 2 ADC approved for pretreated metastatic triple-negative breast cancer (mTNBC) and trastuzumab deruxtecan (T-DXd) gained approval for human epidermal growth factor receptor-2 (HER2)-positive advanced BC (aBC). We aim to provide a contemporary review and the current clinical trial landscape of SG and T-DXd in BC.

MATERIALS AND METHODS

We conducted a literature search from Medline database through PubMed, major conference proceedings [abstracts from European Society for Medical Oncology (Breast) Congress, American Society of Clinical Oncology annual meeting, San Antonio Breast Cancer Symposium] and ClinicalTrials.gov with search terms 'sacituzumab govitecan', 'IMMU-132', 'trastuzumab deruxtecan' and 'DS-8201a' up to 21 March 2021.

RESULTS

We assessed 293 records for eligibility, of which 153 were included in this review after screening and exclusion. For SG, efficacy and safety data are available from a phase III trial in pretreated mTNBC and from a phase I/II basket study in mTNBC and hormone receptor-positive/HER2-negative aBC. Thirteen trials with pending primary analysis are ongoing with SG as single agent or in combination, of which 11 are enrolling (2/11 in the early setting). For T-DXd, efficacy/safety data are available as single agent in pretreated HER2-positive (phase Ib and phase II) and in HER2-low aBC (phase Ib), and in combination with nivolumab in HER2-low/positive aBC (phase Ib). Of 23 ongoing trials with T-DXd, 12 are open for enrollment and 3 phase III trials have completed recruitment. The distinct safety profiles of both drugs and their management are discussed.

CONCLUSION

Given their robust single-agent activity, SG and T-DXd are expected to substantially impact treatment standards, both in and far beyond the currently approved indications. Several trials are investigating new treatment settings for both drugs, including a transition to earlier lines and combinations with other anticancer treatments such as immune checkpoint inhibitors.

Prognostic effect of low-level HER2 expression in patients with clinically negative HER2 status

PURPOSE

Assessment of HER2 overexpression using immunohistochemistry (IHC) and/or in situ hybridisation (ISH) for the detection of HER2 amplifications is standard to identify patients for established HER2-directed treatments. Patients with lower HER2 expression levels have recently also become candidates for novel therapies targeting HER2. This study aimed to assess tumour and patient characteristics and prognosis in patients with advanced breast cancer (aBC), relative to low HER2 expression levels.

METHODS

PRAEGNANT is a prospective aBC registry (NCT02338167), focusing on molecular biomarkers. Patients in all therapy lines receiving any kind of treatment are eligible. This analysis includes patients with conventionally HER2-negative aBC. Clinical outcome was compared in the groups with no (IHC score 0) or with low HER2 expression (IHC 1+, or IHC 2+/ISH negative).

RESULTS

Low HER2 expression levels in triple-negative aBC patients did not influence progression-free survival. Overall survival appeared poorer in patients with IHC 2+ compared with patients with no HER2 expression in the unadjusted analysis (hazard ratio 2.24, 95% confidence interval 0.1.12-4.47). However, this effect was not maintained in the adjusted analysis. In HER2-negative, hormone receptor-positive patients, low HER2 expression appeared to have no effect on prognosis, neither progression-free survival nor overall survival.

CONCLUSIONS

We could not demonstrate that HER2 expression at a low level and assessed in clinical routine can differentiate patients into prognostic groups. However, the prevalence of patients with a low expression makes this population interesting for clinical trials with potentially active treatments using HER2 as a target.

Quantification of Human Epidermal Growth Factor Receptor 2 by Immunopeptide Enrichment and Targeted Mass Spectrometry in Formalin-Fixed Paraffin-Embedded and Frozen Breast Cancer Tissues

BACKGROUND

Conventional HER2-targeting therapies improve outcomes for patients with HER2-positive breast cancer (BC), defined as tumors showing HER2 protein overexpression by immunohistochemistry and/or ERBB2 gene amplification determined by in situ hybridization (ISH). Emerging HER2-targeting compounds show benefit in some patients with neither HER2 protein overexpression nor ERBB2 gene amplification, creating a need for new assays to select HER2-low tumors for treatment with these compounds. We evaluated the analytical performance of a targeted mass spectrometry-based assay for quantifying HER2 protein in formalin-fixed paraffin-embedded (FFPE) and frozen BC biopsies.

METHODS

We used immunoaffinity-enrichment coupled to multiple reaction monitoring-mass spectrometry (immuno-MRM-MS) to quantify HER2 protein (as peptide GLQSLPTHDPSPLQR) in 96 frozen and 119 FFPE BC biopsies. We characterized linearity, lower limit of quantification (LLOQ), and intra- and inter-day variation of the assay in frozen and FFPE tissue matrices. We determined concordance between HER2 immuno-MRM-MS and predicate immunohistochemistry and ISH assays and examined the benefit of multiplexing the assay to include proteins expressed in tumor subcompartments (e.g., stroma, adipose, lymphocytes, epithelium) to account for tissue heterogeneity.

RESULTS

HER2 immuno-MRM-MS assay linearity was ≥103, assay coefficient of variation was 7.8% (FFPE) and 5.9% (frozen) for spiked-in analyte, and 7.7% (FFPE) and 7.9% (frozen) for endogenous measurements. Immuno-MRM-MS-based HER2 measurements strongly correlated with predicate assay HER2 determinations, and concordance was improved by normalizing to glyceraldehyde-3-phosphate dehydrogenase. HER2 was quantified above the LLOQ in all tumors.

CONCLUSIONS

Immuno-MRM-MS can be used to quantify HER2 in FFPE and frozen BC biopsies, even at low HER2 expression levels.

HER2-Low Breast Cancer: Molecular Characteristics and Prognosis

BACKGROUND

We aimed to determine the distribution of intrinsic subtypes within HER2-low breast cancer (BC), and to describe the prognostic impact of HER2-low status on survival outcomes.

METHODS

This is a retrospective, observational study of primary BC extracted from The Cancer Genome Atlas dataset. We described the distribution of PAM50 intrinsic subtypes within HER2-low BC subtype according to hormonal receptor status (positive (HR+) and negative (HR-)). Secondly, we assessed the impact of HER2-low on survival outcomes (progression-free interval (PFI), disease-free interval (DFI), and overall survival (OS)).

RESULTS

We analyzed 804 primary BCs, including 410 (51%) HER2-low BCs (336 HR+ and 74 HR-). The proportion of HER2-enriched tumors was higher in the HER2-low/HR- group compared to HER2-low/HR+ (13.7% versus 1.2%, respectively). HER2-enriched tumors were more frequent in HER2-low/HR- and HER2-low/HR+ subtypes, compared to HER2-negative/HR- and HER2-negative/HR+ subtypes, respectively (13.7% versus 1.6% and 1.2% versus 0.5%, respectively). We observed no significant differences in PFI, DFI, and OS between HER2-low subtypes and each non-HER2-low subtype paired by HR status.

CONCLUSIONS

Our characterization of PAM50 intrinsic subtypes within HER2-low breast cancer may explain the different clinical behaviors and responses to treatment, and ultimately support further investigation of new treatment strategies in the HER2-low category. Moreover, it highlights the importance of considering HR status in the HER2-low category.

Current and Future Management of HER2-Positive Metastatic Breast Cancer

Human epidermal growth factor receptor 2 (HER2) is overexpressed and/or amplified in approximately 20% of breast cancers, conferring an aggressive tumor behavior but also an opportunity for targeted therapies. In the advanced setting, the prognosis of patients suffering from this disease has greatly improved after the introduction of new anti-HER2 drugs beyond trastuzumab. For most patients, a taxane combined with trastuzumab and pertuzumab in the first-line setting, followed by trastuzumab-emtansine in second line, should be considered the standard of care today. However, chemo-free anti-HER2 strategies in hormone receptor-positive, HER2-positive breast cancer could also be considered in selected patients. In the third-line setting and beyond, several emerging anti-HER2 therapies are becoming available, including tucatinib, fam-trastuzumab deruxtecan-nxki (DS-8201a), neratinib, and margetuximab-cmkb. In addition, new compounds and combinations are showing promising results in the late-line setting. The treatment landscape of HER2-positive advanced disease is evolving constantly, active drugs such as pertuzumab and trastuzumab-emtansine are moving to early-stage, many biomarkers, including quantification of HER2 itself, are being explored to improve patient selection, and patient populations with specific needs are emerging, such as those with brain metastasis. Here, we provide an overview of the current and future management of HER2-positive advanced breast cancer.

Unlocking the potential of antibody-drug conjugates for cancer therapy

Nine different antibody-drug conjugates (ADCs) are currently approved as cancer treatments, with dozens more in preclinical and clinical development. The primary goal of ADCs is to improve the therapeutic index of antineoplastic agents by restricting their systemic delivery to cells that express the target antigen of interest. Advances in synthetic biochemistry have ushered in a new generation of ADCs, which promise to improve upon the tissue specificity and cytotoxicity of their predecessors. Many of these drugs have impressive activity against treatment-refractory cancers, although hurdles impeding their broader use remain, including systemic toxicity, inadequate biomarkers for patient selection, acquired resistance and unknown benefit in combination with other cancer therapies. Emerging evidence indicates that the efficacy of a given ADC depends on the intricacies of how the antibody, linker and payload components interact with the tumour and its microenvironment, all of which have important clinical implications. In this Review, we discuss the current state of knowledge regarding the design, mechanism of action and clinical efficacy of ADCs as well as the apparent limitations of this treatment class. We then propose a path forward by highlighting several hypotheses and novel strategies to maximize the potential benefit that ADCs can provide to patients with cancer.

Discordance Between Immunohistochemistry and In Situ Hybridization to Detect HER2 Overexpression/Gene Amplification in Breast Cancer in the Modern Age: A Single Institution Experience and Pooled Literature Review Study

BACKGROUND

Human epidermal growth factor 2 (HER2) amplification and/or overexpression occurs in 12% to 25% of breast cancers. Accurate detection of HER2 is critical in predicting response to HER2-targeted therapy. Both immunohistochemistry (IHC) and in situ hybridization (ISH) are FDA-approved methods for detecting HER2 status because its protein overexpression is largely attributable to gene amplification. However, variable discordant results between IHC and ISH have been reported.

METHODS

We determined the frequency of HER2 IHC/ISH discordance in these patients and also performed a pooled literature review analysis.

RESULTS

Of the 1125 consecutive primary or metastatic breast cancers with HER2 IHC and ISH performed simultaneously between 2015 and 2020, 84.6% had an unequivocal HER2 status. Discordance was found in 30 cases from 26 patients, including 13 IHC^-/ISH⁺ and 17 IHC⁺/ISH^-, representing 1.6% and 11.9% of IHC^- and IHC⁺ cases, respectively. Review of the literature between 2001 and 2020 identified 46 relevant studies, with a total of 43,468 cases with IHC and ISH performed. The IHC^-/ISH+ and IHC⁺/ISH^- discordances were seen in all antibody clones and ISH methods used. The IHC⁺/ISH^- discordance was significantly higher than IHC^-/ISH⁺ (13.8% vs. 3%, P < .0001). The overall discordance constituted 4% of all cases and 5.4% of those with an unequivocal IHC status. Significantly lower incongruities for both IHC^-/ISH⁺ and IHC⁺/ISH^- were found in those published after 2018. The discordances probably reflect altered biology of HER2 oncogene/oncoprotein. Routinely performing both IHC and ISH may uncover such cases to prevent denial of potentially beneficial targeted therapy.

Precision medicine in breast cancer: From clinical trials to clinical practice

INTRODUCTION

Breast cancer (BC) is the most common cancer in women and, despite the undeniable improvements in the outcome of these patients obtained in the last decade, the discovery and the validation of new actionable molecular targets represent a priority. ESCAT permits to rank molecular alterations in different classes according to their evidence of actionability in a specific cancer type, assisting clinicians in their therapeutical decisions. MAIN: ERBB2, PIK3CA and germline BRCA1/2 alterations are biomarkers prospectively validated in BC, driving the selection of targeted therapies, and are therefore classified in the highest level of evidence (Ia). Agnostic biomarkers, namely microsatellite instability, NTRK fusions and high tumor mutational burden, demonstrated similar activity across different tumor types and are consequently ranked in tier Ic. In tier II are classified alterations that still need confirmatory prospective studies but for which evidence of efficacy is available. Somatic BRCA1/2 mutations, germline PALB2 mutations, HER2-low expression, ERBB2 mutations, PTEN deletions, AKT1 mutations, ESR1 resistance mutations satisfy the requirements to be classified in this tier. In tier III are ranked various molecular alterations for which there is evidence of actionability in other tumors (IIIa) or that have similar functional impact in the same gene or pathway of a tier I alteration, without clinical data (IIIb). In tier IV are listed the molecular alterations for which only preclinical studies are available.

CONCLUSION

In this review we report the most relevant molecular targets in BC, ordered pursuant to their pathway and classified in concordance with ESCAT.

Targeting HER2 in breast cancer: new drugs and paradigms on the horizon

About 15-20% of all breast cancers (BCs) are defined human epidermal growth factor receptor 2 (HER2)-positive, based on the overexpression of HER2 protein and/or amplification of ERBB2 gene. Such alterations lead to a more aggressive behavior of the disease, but also predict response to treatments targeting HER2. Indeed, several anti-HER2 compounds have been developed and approved in the last two decades, significantly improving our ability to cure patients in the early setting, and greatly extending their survival in the advanced setting. However, recent evolutions in this field promise to improve outcomes even further, through advancements in established HER2-targeting strategies, as well as the exploration of novel strategies. In particular, the engineering of new antibody-drug conjugates, with higher drug-to-antibody ratios (DARs) and cleavable linkers, has already led to the development of a highly effective drug, namely trastuzumab deruxtecan, recently approved by the Food and Drug Administration (FDA) and European Medicines Agency (EMA) for the treatment of advanced HER2-positive (HER2+) BC, and currently in study in the early setting. Moreover, the novel tyrosine kinase inhibitor tucatinib was recently approved by FDA and EMA, showing to improve survival of HER2+ advanced BC patients, particularly in those with brain metastasis. Immunotherapy is also being investigated in the HER2+ subtype, through immune-checkpoint inhibition, cancer vaccines and adoptive-cell therapies. Overall, the enlarging arsenal of promising anti-HER2 compounds is expected to deliver significant improvements in the prognosis of both early and advanced HER2+ BC in the years to come. Moreover, some of such agents are showing encouraging activity in the much wider population of HER2-low advanced BC patients, challenging current BC classifications. If confirmed, this new paradigm would potentially expand the population deriving benefit from HER2-targeted treatments to up to 70% of all advanced BC patients, leading to a revolution in current treatment algorithms, and possibly to a redefinition of HER2 classification.

Trastuzumab deruxtecan in HER2-positive metastatic breast cancer and beyond

INTRODUCTION

Despite the substantial improvements made in human epidermal growth factor receptor 2 (HER2)-targeted therapies since the approval of trastuzumab more than 20 years ago, there is still considerable unmet need in patients with HER2-expressing breast cancer (BC) and other solid tumors. Trastuzumab deruxtecan (T-DXd) is a newer antibody-drug conjugate approved for the treatment of metastatic breast cancer (BC) and gastric cancer (GC) and is under active investigation in other solid tumors, including non-small cell lung cancer, colorectal cancer, and HER2-low tumors.

AREAS COVERED

The current treatment and investigational landscape of HER2-positive and HER2-low metastatic BC (mBC) and the preclinical and clinical trials investigating T-DXd. To identify relevant literature, a search was performed on English-language publications and congress abstracts.

EXPERT OPINION

T-DXd is likely to become the standard of care for second-line treatment of HER2-positive mBC, and it may play a role in the treatment of hormone receptor-positive and triple-negative mBC with HER2-low expression. Because it was recently approved in the United States and Japan to treat HER2-positive metastatic GC, it holds promise for the treatment of other HER2-positive solid tumors, including colorectal cancer, non-small cell lung cancer, and HER2-low BC.

Predictors of Nodal Pathological Complete Response in Asian Women with Stage II-III Node-Positive Breast Cancer

BACKGROUND

Neoadjuvant chemotherapy (NAC) is increasingly used to treat node-positive (N+) breast cancer. Predictors of nodal pathological complete response (pCR) in Asian women are poorly described and there is variety in the management of the axilla after NAC. We evaluated predictors of nodal pCR and axillary management in a cohort of Asian N+ patients.

METHODS

Consecutive biopsy-proven N+ breast cancer patients treated with NAC were identified from the Shanghai Ruijin Hospital in China. Axillary lymph node dissection was performed on all patients, irrespective of the nodal response to NAC.

RESULTS

A total of 323 patients were included. Nodal pCR was achieved in 105 patients (33%), 15% of HR+/HER2- tumors, 38% of HR+/HER2+ tumors, 49% of HR-/HER2+ tumors, and 42% of HR-/HER2-tumors (p < 0.001). Factors associated with nodal pCR were (1) receptor status (HR+/HER2- [referent]: OR 3.42, 95% CI 1.43-8.16, p = 0.006 for HR+/HER2+; OR 4.19, 95% CI 1.85-9.50, p = 0.001 for HR-/HER2+; and OR 2.94, 95% CI 1.11-7.74, p = 0.029 for HR-/HER2-), (2) breast pCR (no pCR [referent]: OR 15.22, 95% CI 6.29-36.79, p < 0.001), and (3) absence of lymphovascular invasion (LVI [referent]: OR 9.04, 95% CI 2.09-39.18, p = 0.003).

CONCLUSION

This study confirmed expected predictors of nodal pCR in Asian women and the benefit of NAC in downstaging the axilla independently of ethnicity.

Targeting HER2 heterogeneity in early-stage breast cancer

PURPOSE OF REVIEW

HER2-positive (HER2+) breast cancer is clinically and biologically a heterogenous disease and not all patients benefit to the same extent from current anti-HER2 therapies.

RECENT FINDINGS

Among HER2+ breast cancer, molecular intrinsic subtypes, PIK3CA mutation status, levels of HER2 gene/protein, immune infiltration, or intratumor heterogeneity modulate HER2-treatment sensitivity. HER2-enriched carcinomas, with high levels of HER2 and tumor-infiltrating lymphocytes (TILs) are highly sensitive to anti-HER2 therapies, regardless of chemotherapy. Luminal A/B tumors are more estrogen receptor-dependent than HER2-dependent, harbor higher rates of PIK3CA mutations, and are less responsive to anti-HER2 treatment. HER2 intratumoral heterogeneity that exists in approximately 10% of HER2+ disease may also cause treatment resistance. Early changes occur during neoadjuvant anti-HER2 therapy that can predict response. Importantly, HER2 expression is not a binary but rather a continuous variable. Overall, 34-63% of HER2-negative breast cancers express HER2, and HER2-low tumors have become a new entity, for which novel targeted therapies may be effective.

SUMMARY

Although much of what is discussed currently remains investigational, it is clear that HER2+ breast cancer is a complex disease comprising different entities. Future strategies to escalate or de-escalate treatment in early-stage HER2+ disease should consider other biomarkers beyond HER2 and estrogen receptor status, including intrinsic subtype, HER2 levels, and TILs; and evaluate different treatment strategies among patients with estrogen receptor-positive/HER2+ and estrogen receptor-negative/HER2+ diseases.

Immunotherapy as a partner for HER2-directed therapies

Introduction: Existing HER2-targeted therapies modulate the tumor microenvironment and the immunologic response cancer in a favorable way. While these therapies have made dramatic improvements in the treatment and prognosis of HER2-overexpressing malignancies, additional treatment options are still needed.Areas covered: This review covers the immunomodulatory effects of approved HER2-targeted therapies. We discuss the preclinical data that demonstrate an additive effect of the combination of trastuzumab or other HER2-targeting agents with immunomodulatory drugs. Finally, we report the initial studies on the combination of HER2-targeted agents together with immune checkpoint inhibitors or cancer vaccines in breast cancer.Expert opinion: Preclinical data suggest a synergistic effect of HER2-targeted therapy together with both checkpoint inhibitor and cancer vaccine immunotherapy. Results from initial trials with PD-1/PD-L1-blocking therapy together with HER2-targeted therapy have been negative, but responses were seen in patients with PD-L1+ breast cancer. Trastuzumab together with HER2-targeted cancer vaccination has shown benefits in triple negative breast cancer. Further trials are necessary and warranted to confirm the benefit of these combinations.

Clinical Development of New Antibody-Drug Conjugates in Breast Cancer: To Infinity and Beyond

Metastatic breast cancer remains an incurable disease, and new therapies are needed. One major limitation of chemotherapy is the toxicity associated with higher dose exposure. Antibody-drug conjugates (ADCs) are a complex and evolving class of agents specifically designed with the objective of delivering antineoplastic medicines in the most precise and selectively targeted way. ADCs are composed of four key components: (1) the target antigen, (2) an antibody construct, (3) a payload (most commonly a cytotoxic agent), and (4) a linker moiety that couples the payload and the antibody. In this review, we discuss the clinical development of ADCs for the treatment of breast cancer, focusing on two recently FDA-approved agents, trastuzumab deruxtecan and sacituzumab govitecan, and discuss the ongoing efforts exploring new agents. Finally, we summarize the current portfolio of clinical trials that could change the algorithm of treatment for early and advanced breast cancer.

Triple-negative breast cancer: promising prognostic biomarkers currently in development

Introduction: Triple-negative breast cancer (TNBC) is an aggressive type of breast cancer associated with poor prognosis and limited treatment options. Validated prognostic and predictive biomarkers are needed to guide treatment decisions and prognostication.Areas covered: In this review, we discuss established and developing prognostic and predictive biomarkers in TNBC and associated emerging and approved therapies. Biomarkers reviewed include epidermal growth factor receptor (EGFR), vascular endothelial growth factors (VEGF), fibroblast growth factor receptor (FGFR), human epidermal growth factor receptor 2 (HER2), androgen receptor, NOTCH signaling, oxidative stress/redox signaling, microRNAs, TP53 mutation, breast cancer susceptibility gene 1 or 2 (BRCA1/2) mutation/homologous recombination deficiency (HRD), NTRK gene fusion, PI3K/AKT/mTOR, immune biomarkers (programmed death-ligand 1 (PDL1), tumor-infiltrating lymphocytes (TILs), tumor mutational burden (TMB), neoantigens, defects in DNA mismatch repair proteins (dMMR)/microsatellite instability-high (MSI-H)), circulating tumor cells/cell-free DNA, novel targets of antibody-drug conjugates, and residual disease.Expert opinion: Biomarker-driven care in the management of TNBC is increasing and has helped expand options for patients diagnosed with this subtype of breast cancer. Research efforts are ongoing to identify additional biomarkers and targeted treatment options with the ultimate goal of improving clinical outcomes and survivorship.

The evolving paradigm of biomarker actionability: Histology-agnosticism as a spectrum, rather than a binary quality

Precision medicine is progressively revolutionizing oncology, through the identification of biomarkers predictive of treatment response in cancer patients. For three of such biomarkers, namely NTRK-fusions, microsatellite instability and high tumor mutational burden, drugs have been approved by regulatory agencies regardless of tumor histology, realizing the paradigm of histology-agnostic actionability. Several additional biomarkers are being studied in a histology-agnostic manner, and may in the future expand this list. However, most available evidence suggest that histology-agnosticism may be the extreme of a continuous spectrum of actionability, rather than a binary quality. The present review recapitulates such evidence, highlighting opportunities and challenges posed by the emergence of the spectrum of biomarker actionability in the context of a prevalently histology-based oncology.

Assessment of HER2 using the 2018 ASCO/CAP guideline update for invasive breast cancer: a critical look at cases classified as HER2 2+ by immunohistochemistry

In 2018, the American Society of Clinical Oncology/College of American Pathologists revised the criteria for HER2 immunohistochemistry (IHC) equivocal (2+) classification in their updated guideline. We reviewed invasive breast cancer specimens originally classified as equivocal (2+) under the 2018 guideline that underwent HER2 fluorescence in situ hybridization (FISH) testing from August 2018 to August 2019 at our Canadian reference hospital to investigate cases with ambiguous staining patterns between the 1+ and 2+ definitions. Demographics, pathologic features, and pre-analytic conditions were recorded. The H&E and corresponding HER2 IHC slides were reviewed to confirm tumor type and grade, and classify as HER2 indeterminate, 0, 1+, 2+, or "Intermediate" (staining features between the 1+ and 2+ classifications). FISH testing was performed on 289 cases and 273 met inclusion criteria. The FISH-amplified rate was 12.1%. Upon IHC review, 44.7% (122/273) of cases were reclassified as Intermediate. These cases had incomplete staining with moderate intensity (43/122, 35.3%) and/or <10% complete weak or moderate staining (102/122, 83.6%). Intermediate cases had a significantly lower frequency of amplified FISH results than 2+ cases (p < 0.0001), with only four (3.3%) FISH positive and two (1.6%) FISH heterogeneous. Our study highlights the ambiguity in the current guideline for classifying some HER2 IHC patterns. As the rate of gene amplification in these cases was low (4.9%), we recommend adhering to the 2018 HER2 2+ criteria for reflex FISH testing. However, cases with <10% moderate complete staining and certain heterogeneous patterns warrant special consideration. Further descriptive clarification of 1+ criteria is needed.

[ISH-based HER2 diagnostics]

A prerequisite for all HER2 directed therapies is the demonstration of HER2 receptor protein overexpression and/or gene amplification by in situ hybridization (ISH). ASCO and CAP have published several HER2 test guidelines over the past 15 years for both breast and gastric cancer. The latest version for breast cancer (2018) focuses on special issues of ISH related to the definitions of special diagnostic groups (1-5). The guidelines for gastroesophageal adenocarcinoma (2017), essentially based on ToGA trial data, are now also being used for other tumors such as pancreas, gallbladder, and non-small-cell lung cancer. For colorectal cancer, a modified testing procedure has been proposed. Recently, besides overexpression and amplification, a third type of HER gene alteration, namely mutation, has gained much interest. Next-generation sequencing (NGS) allows detection of both amplification and mutation of the HER2 gene providing new options of therapy especially in the case of activating mutations.

ISH-based HER2 diagnostics

A prerequisite for all HER2 directed therapies is the demonstration of HER2 receptor protein overexpression and/or gene amplification by in situ hybridization (ISH). ASCO and CAP have published several HER2 test guidelines over the past 15 years for both breast and gastric cancer. The latest version for breast cancer (2018) focuses on special issues of ISH related to the definitions of special diagnostic groups (1-5). The guidelines for gastroesophageal adenocarcinoma (2017), essentially based on ToGA trial data, are now also being used for other tumors such as pancreas, gallbladder, and non-small-cell lung cancer. For colorectal cancer, a modified testing procedure has been proposed. Recently, besides overexpression and amplification, a third type of HER gene alteration, namely mutation, has gained much interest. Next-generation sequencing (NGS) allows detection of both amplification and mutation of the HER2 gene providing new options of therapy especially in the case of activating mutations.

Margetuximab for the treatment of HER2-positive metastatic breast cancer

INTRODUCTION

No specific standard treatment is currently recommended for HER2-positive advanced breast cancer (BC) patients progressing to dual HER2 blockade and to trastuzumab emtansine (TDM-1). However, several novel anti-HER2 agents are emerging and rapidly revolutionizing this setting. Among these, the FC-engineered monoclonal antibody margetuximab has recently demonstrated to slightly improve progression-free survival (PFS) compared with trastuzumab, when combined with chemotherapy for pretreated HER2-positive advanced BC.

AREAS COVERED

The present review article recapitulates the clinical development of margetuximab, critically discussing its implications in the current landscape of BC treatment algorithms.

EXPERT OPINION

The clinical role of Margetuximab can only be interpreted in view of the rapidly evolving treatment landscape for pretreated HER2-positive advanced BC. Indeed, the recently approved anti-HER2 agents tucatinib and trastuzumab deruxtecan currently represent appealing options for the post-TDM1 setting, while margetuximab may have a role after progression to the abovementioned agents, in case of a future approval. Regardless of its clinical uptake, it should be noted that the development of margetuximab has relevantly improved our biological understanding of HER2-positive BC, highlighting the implication of patient's genotype in determining treatment outcomes, as well as the relevance of antibody-dependent cellular cytotoxicity (ADCC) in the context of HER2-blockade.

Predictive markers for pathological complete response after neo-adjuvant chemotherapy in triple-negative breast cancer

A combination of Sox10 and GATA3 was previously identified as a marker for metastatic triple-negative breast cancer (TNBC), but it is uncertain whether their expression is associated with pathological complete response (pCR) after neoadjuvant chemotherapy (NAC). This study investigates the predictive value of clinicopathological characteristics, as well as protein expression of Sox10, GATA3, p53 and p63, in a consecutive series of TNBC patients treated with NAC. Archived hematoxylin & eosin stained slides of core biopsies and resection specimens from 35 TNBC patients were reviewed. The following clinicopathological characteristics were determined at the biopsy level: age at diagnosis, cancer type, Nottingham grade, lympho-vascular invasion, syncytial growth, necrosis, clear cell differentiation, myxoid peritumor stroma, stromal tumor-infiltrating lymphocytes (sTILs) and presence of an in situ component. The MD Anderson residual cancer burden (RCB) score and corresponding RCB class were determined. Immunohistochemistry for Sox10, p53, GATA3 and p63 was performed at the biopsy level. sTILs, either as a continuous or as a dichotomous variable, were the only parameter that was significantly associated with pCR in univariable and multivariable analyses. Assessment of sTILs showed moderate to good interobserver agreement. High sTILs (≥40%) were significantly associated with increased pCR rates, and this association was observer-independent. This retrospective study of a consecutive community-based cohort of TNBC patients confirms that sTILs are a robust, observer-independent predictor for therapeutic response after NAC. The combination of Sox10, GATA3 and p53 immunoreactivity is unlikely to harbor any predictive value for pCR in TNBC.