Emerging role of human epidermal growth factor 2-low status in the prognosis and management of triple-negative breast cancer: a narrative review

Introduction

Breast cancer is the most frequently diagnosed malignancy among women worldwide and remains one of the leading causes of female cancer-related morbidity and mortality (1-3). Breast cancer is not a single pathological entity but rather a highly heterogenous disease, incorporating variations in cell origin, molecular makeup, tumor extrinsic factors, aggressiveness and treatment resistance, amongst others (4-8). To account for this heterogeneity, various stratifications of breast cancer have been put forward, with the most commonly used molecular classifications recognizing major subtypes based on the expression of estrogen (ER) and progesterone (PR) hormone (HR) receptors and human epidermal growth factor 2 (HER2) (4,9-12). Triple-negative breast cancer (TNBC), which accounts for 15–20% of all breast cancers, lacks expression of these biomarkers and is generally regarded as the most aggressive subtype, with the least favourable patient outcomes (10-16). TNBC itself is highly heterogeneous, leading to its sub-stratifications into various subtypes (5-8,17,18).

There has been a significant research effort to identify clinically actionable and pharmacologically targetable biomarkers in breast cancer in general and TNBC in particular to expand the arsenal of current treatments. These efforts led to the development of new targeted treatments, such as with poly(ADP-ribose) polymerase (PARP) inhibitors in metastatic and high-risk early-stage patients with germline mutated BRCA1 and BRCA2 (breast cancer genes 1 and 2) tumors (19-25); sacituzumab govitecan, an antibody drug conjugate (ADC) targeting trophoblast cell surface antigen 2 (Trop-2) linked to the active metabolite of irinotecan, in metastatic TNBC patients (26,27); and immunotherapy, such as the immune checkpoint inhibitor pembrolizumab (28-31).

In the search for clinically actionable biomarkers in TNBC, there has been a renewed interest in the subgroup of TNBC patients whose tumors express HER2 at lower levels. Since patients with HER2-positive breast cancers benefit from a range of anti-HER2-targeted therapies, including tyrosine kinase inhibitors (TKIs), lapatinib and tucatinib; anti-HER2 monoclonal antibodies, trastuzumab and pertuzumab; and ADC of trastuzumab with cytotoxic agents, such as emtansine (T-DM1) or trastuzumab deruxtecan (T-DXd) (32,33), targeting breast cancers with low levels of HER2 expression with anti-HER2-directed approaches appears to represent an attractive targeted therapeutic approach. A breakthrough in this direction has been recently achieved based on several clinical trials establishing that, in HER2-low expressing breast cancer, T-DXd treatment confers good antitumor activity (34,35) and survival benefits in patients with metastatic disease (36-38). This paradigm shift in management of traditionally-HER2-negative breast cancers not only has paved a new and exciting area of further research, but also highlighted important questions regarding definitions and classifications of breast cancers and its subtypes, HER2 tests sensitivities and specificities, and identification of patient subgroups that would benefit from the anti-HER2-directed treatment and others.

The objective of this review is to provide a comprehensive overview of clinical research on HER2-low disease and its implications for the management of breast cancer with the focus on the TNBC subtype. We summarize current evidence on the epidemiologic and biologic characteristics of HER2-low TNBC, along with recent advances, ongoing challenges, and emerging targeted treatments and therapeutic strategies in this category of patients. We present this article in accordance with the Narrative Review reporting checklist (available at https://tcr.amegroups.com/article/view/10.21037/tcr-2026-1-0370/rc).

Methods

We queried PubMed, Google Scholar and other databases (Table 1) for studies published up to March 2026, using a set of selected search terms (such as TNBC, breast cancer, HER2-low, HER2-ultralow, trastuzumab deruxtecan, T-DXd, T-DM1), and selected studies based on their relevance to the topic to identify themes, controversies, and knowledge gaps, especially pertinent to the TNBC context (Table 1). We also searched conference abstracts and hand searched reference lists of relevant papers to provide the most updated and comprehensive review of the topic as possible. Finally, we also searched “ClinicalTrials.gov” for clinical and investigational studies exploring anti-HER2 directed treatment in HER2-low and HER2-ultralow, with an emphasis on TNBC. Additional studies were searched in March of 2026 using the same search strategies, for revisions to the paper, and to add updated information from recently published studies if available.

HER2-low in TNBC

Definition of HER2-low

HER2 status is considered positive if immunohistochemistry (IHC) staining score is 3+ or if it is 2+ (equivocal) with a positive in situ hybridization (ISH) test (39,40). Negative HER2 status is assigned if IHC score is 0 or 1+ or if it is equivocal on IHC with a negative ISH test (39,40). However, in recent years, a subgroup of tumors termed HER2-low has gained recognition as a clinically significant category (41), largely based on clinical trials that show therapeutic benefits of anti-HER2 ADC T-DXd in patients with breast cancer with low levels of HER2 expression (36,37,42). Therefore, the American Society of Clinical Oncology (ASCO)—College of American Pathologists (CAP) 2023 guideline update (41) notes that HER2-negative breast cancers with IHC 1+ or IHC 2+ and without amplification by ISH (HER2-low), while are not proven to represent a separate subtype, are clinically important to distinguish within the HER2-negative category (41). Additionally, the importance of distinguishing another category as HER2-ultralow (IHC 0 with faint membrane staining in >0% but ≤10% of tumor cells) from true HER2-null disease is emphasized, as this distinction may influence eligibility for HER2-directed ADCs (43). Thus, categorization of the HER2 status to HER2-low (44-46) and HER2-ultralow (42,47) within the HER2-negative breast cancer category bears important prognostic and therapeutic implications.

Prevalence of HER2-low status in TNBC

The reported prevalence of HER2 low status in TNBC patients varies but can reach as high as half of the patients, and may be explained by variation in populations studied including the stage of the disease, type of sample (biopsy vs. surgical resection) and ethnicity (44,46,48-50). A meta-analysis in patients with HER2-low and HER2 IHC 0 (HER2-0) early-stage breast cancer receiving neoadjuvant therapy showed that 61.3% had HER2-low status and 52.4% were HR-positive (51). A pooled analysis of 2,310 patients from 4 prospective neoadjuvant trials, excluding HER2-positive tumors, reported a slightly lower prevalence of 47.5% of HER2-low tumor status, 36% of which were HR-negative (44). In the retrospective evaluation of 638 patients with TNBC who were treated with neoadjuvant chemotherapy, the prevalence of tumors with HER2-low status was reported to be 53.6% (48). In a study of Asian females with TNBC, 47.72% had a HER2-low status (52). Retrospective analysis of 691 patients with TNBC reported the incidence of HER2-low status of 32.0% (primary site 33.4% and metastatic site 29.8%) (50). Thus, while there is variability in reporting frequency of HER2-low status in TNBC patients, it is overall relatively prevalent.

HER2-low status as a prognostic factor

The data regarding the relationship between clinical and pathological parameters and HER2-low status in general is disparate, but overall points to some prognostic significance of this status compared to HER2 IHC 0 (HER2-0) breast cancers.

Several studies report no significant difference in overall survival (OS), disease-free survival (DFS), relapse-free survival or pathological complete response (pCR) rate in TNBC patients with HER2-low compared to HER2-0 tumors (46,50,52,53). However, subgroup analysis of some of these studies still suggests that HER2-low status might have prognostic significance. For example, in a retrospective study of non-metastatic TNBC patients, while no significant differences in OS or the relapse-free survival of patients with HER2-0 and HER2-low status were observed, patients with HER2 IHC score of 2+, compared to those with a score of 0 or 1+, had worse relapse-free survival (46). Another retrospective study of TNBC patients, while failing to confirm that HER2-low status influences recurrence-free survival (RFS) in the entire cohort, showed that in patients with the TNBC luminal androgen receptor (LAR) subtype and HER2-low vs. HER2-0 status, the former was associated with significantly better RFS (53).

Other studies also point to the clinical prognostic value of HER2-low status in breast cancer patients. In a pooled analysis of prospective neoadjuvant trials, patients with HER2-low tumors, compared to HER2-0 ones, had a significantly longer DFS and OS (44). In a subgroup analysis, patients with HR-negative tumors with the HER2-low status also had longer DFS and OS, in both univariate and multivariate analyses (44). In a retrospective study, longer OS was also observed in HER2-low vs. HER2-0 TNBC patients who were treated with neoadjuvant chemotherapy (48). In another retrospective cohort study, patients with HR-negative (but not HR-positive) breast cancer with HER2-low vs. HER2-0 status were shown to have a longer OS (54).

Observations that HER2-low status bears prognostic significance are further confirmed by meta-analyses. A meta-analysis including over a million patients from 42 studies reported that in an early setting of breast cancer, HER2-low vs. HER2-0 status was associated with improved DFS and OS, with the latter also seen in both HR-positive and HR-negative subgroups, and the former in the HR-positive population only (55). In metastatic breast cancer patients with the HER2-low status, compared to those with HER2-0 tumors, a better OS regardless of HR status has been observed (55). A meta-analysis of 38 studies in patients with HER2-low and HER2-0 early-stage breast cancer receiving neoadjuvant therapy showed that patients with HER2-low status tumors have a worse pCR rate, compared to those with HER2-0 status (51). These conclusions were also reached when analyzing a subset of TNBC patients (51). Another meta-analysis looking at the survival outcomes of patients with HER2-low, compared to those with HER2-0, breast cancers after neoadjuvant treatment, showed that HER2-low tumors elicit a lower pCR rate regardless of the HR status (56). In these patients, HER2-low, compared to HER2-0 status, was associated with improvements in OS and DFS in the overall cohort, and improvement in DFS in the HR-positive, but not in the HR-negative subgroup (56).

Thus, based on the number of studies and large meta-analyses, HER2-low status appears to be associated with improved OS and DFS, but negatively correlates with pCR rate. Interestingly, HER2-low, compared with HER2-0, tumors in TNBC patients show lower tumor grade (46), Ki-67 proliferation index and rate of BRCA1 mutations (52,53). Retrospective analysis of 261 TNBC patients who received neoadjuvant chemotherapy also reported a lower Ki-67 index in HER2-low compared to HER2-0 tumors (57). These observations, at least in part, might explain the lower pCR rate in HER2-low TNBC patients, since slow growing, less aggressive tumours appear to respond less to chemotherapy than aggressive ones (58). Thus, while more research is required, HER2-low status appears to bear an important prognostic significance.

HER2-low as a separate biological subtype

Controversies exist as to whether tumors with the HER2-low status should be considered a distinct molecular subtype. In one study, HER2-low breast cancers failed to be shown to represent a distinct biological entity, where investigators showed an overall similar genomic landscape in HER2-low and HER2-0 breast cancers, except for a higher ERBB2 (Erb-B2 receptor tyrosine kinase 2, encodes for HER2) copy count and a lower rate of ERBB2 hemi-deletions in HER2-low compared to HER2-0 tumors (59). These conclusions are supported by other studies and question the designation of HER2-low breast cancers to a separate subtype category (60).

However, others suggest that breast tumors with HER2-low status should be considered a distinct molecular subtype (61). Using RNA sequencing (RNA-seq) and machine-learning algorithms, a transcriptomic Breast Cancer Classifier has been developed that incorporates HER2-low breast cancer as a distinct subtype (61). This classification showed prognostic similarities between basal subtypes, which commonly include TNBC and HER2-low tumors (61). Furthermore, based on multiomics analysis, HER2-low TNBC can be further stratified into subgroups with distinct features and therapeutic vulnerabilities (45).

Overall, despite a debate around whether HER2-low is a distinct biological and phenotypical subtype or not, recent clinical trials demonstrate the importance of the utility of this sub-classification for anti-HER2 targeted treatment selection.

Accuracy of defining HER2-low

Variability in interpretation and tumor heterogeneity of HER2-low status poses diagnostic challenges. Various factors might impact the accuracy of defining HER2-low or ultralow status in breast cancer samples. These include the sensitivity and specificity of the assays used, variability in interpretation of test results, and the type of breast cancer specimen (e.g., biopsy vs. surgical specimen and/or primary vs. metastatic site) (43,49,62,63). In terms of the accuracy of test results, an expert consensus has been developed addressing concerns regarding assay variability, reproducibility and quality assurance standards in detecting low and ultralow levels of HER2 expression (43). While beyond the scope of this review, these expert recommendations provide an evolving framework for improving diagnostics for HER2-low and ultralow categories, mandating adherence to international guidelines with the utilization of appropriate quality assurance practices and standardization of testing and reporting protocols (43). Furthermore, notable interobserver variability in assigning a HER2-low status might be overcome by pathologist training and potentially the use of artificial intelligence to ensure reproducibility of the results (43,62).

Aside from the diagnostic test and analytical considerations, breast cancer molecular heterogeneity might, at least in part, play a role in observed discordances of HER2-low or ultralow status. HER2-low expression appears to be very unstable during the disease progression (49). For example, in a retrospective study of TNBC patient samples, discordance rates ranging from 26% to 44% have been reported between matched core-surgical, early-metastatic and metastatic site samples, with the highest discordance observed between the primary and metastatic biopsies (63). Relatively high rates of discordances in ascertaining HER2 status have also been reported between matched primary and recurrent tumors (49). Furthermore, in patients with multiple resected breast tumors, 37% of the patients had differences in the IHC score between two tumours (64). The differences in HER2-low vs. HER2-0 status might also be related to the type of specimen (e.g., radiological biopsy vs. surgical resection) (49,63,64). While these findings might be partly explained by the issues of the reliability in HER2 testing, they also likely represent the molecular heterogeneity of breast cancer and its changes during natural and/or treatment-driven disease evolution (49,63,64). Within that context, repeat biopsies and testing for HER2 status in multiple tumour sites might increase chances of detecting HER2-low or ultralow breast cancers that would have important implications in terms of patient treatment (49,63,64). In light of the latter, further improvements in diagnostic tools and approaches are highly desired.

HER2-targeting therapies in HER2-positive breast cancers

In the 1980s, the HER2/neu oncogene was identified, characterized, and its gene amplification linked to worse survival parameters in breast cancer (65-67). Shortly after, a monoclonal antibody against HER2 was shown to have an antiproliferative effect in breast cancer (68). Subsequently, trastuzumab was developed as an anti-HER2 monoclonal antibody that was “humanized” to minimize a human anti-mouse antibody response (69). In a number of clinical trials in patients with HER2-overexpressing breast cancers, trastuzumab showed significant clinical benefits in the adjuvant, neoadjuvant and metastatic settings (70-72). These successes led to the development and regulatory approvals for clinical use of other HER2-targeted agents, such as anti-HER2 monoclonal antibody pertuzumab (73,74) and TKIs, such as tucatinib (75), lapatinib (76) and neratinib (77), in various settings of breast cancer (Figure 1).

Figure 1 FDA-approved anti-HER2 targeted therapies for breast cancer. Created with BioRender (Biorender.com). ADC, antibody-drug conjugate; FDA, Food and Drug Administration; HER2, human epidermal growth factor 2; TKI, tyrosine kinase inhibitor.

In the late 2000s, to increase the efficacy of anti-HER2 targeting, the first antibody-cytotoxic drug conjugate, T-DM1, was developed, showing greater preclinical anticancer activity than trastuzumab alone in HER2-overexpressing breast cancer cells (78). T-DM1 is an ADC of trastuzumab with the microtubule-depolymerizing maytansinoid (emtansine) cytotoxic chemotherapeutic agent (78), allowing targeted delivery of the chemotherapeutic agent directly to the cells that express HER2. Subsequent trials with T-DM1 showing substantial clinical benefits led to its regulatory approvals for use in patients with HER2-overexpressing unresectable early-stage, locally advanced and metastatic settings of breast cancer (79-81).

Another anti-HER2 ADC, T-DXd, which consists of trastuzumab linked to topoisomerase I inhibitor deruxtecan by a cleavable tetrapeptide linker, showed in preclinical studies a potent anticancer activity against HER2-positive cancer cells of various origins, including breast (82,83). T-DXd was also effective in T-DM1 insensitive and high HER2-expressing patient-derived xenograft models (82). Moreover, T-DXd, as opposed to T-DM1, elicited anticancer activity in breast cancer patient-derived organoids with low HER2 expression [IHC 1+/fluorescence in situ hybridization (FISH)-negative] (82). The superiority of the anticancer effects of T-DXd compared to T-DM1 was subsequently shown in clinical trials, establishing T-DXd as a new standard of care ADC targeting the HER2 protein (84-86).

In the DESTINY-Breast01 and 02 trials, T-DXd showed anticancer activity and improved survival in patients with HER2-positive metastatic breast cancer previously treated with T-DM1 (84,85). A phase I trial (ClinicalTrials.gov identifier NCT02564900) also reported preliminary antitumor activity of T-DXd in T-DM1 pretreated patients with HER2-positive breast cancer (87). DESTINY-Breast03 clinical trial directly compared the efficacy and safety of T-DXd to T-DM1 in HER2-positive metastatic breast cancer patients who previously received trastuzumab and a taxane, and reported superiority of the former in terms of the risk of disease progression and OS (86,88). Furthermore, analysis of the DESTINY-Breast05 Phase III randomized clinical trial (NCT04622319) that compared T-DXd to T-DM1 in patients with residual disease after neoadjuvant treatment in patients with high-risk HER2-positive breast cancer, indicated that T-DXd reduced the risk of recurrence compared to its counterpart (89).

Overall, while several anti-HER2 targeted therapeutics exist and are approved for clinical use, T-DXd shows advantages in the treatment of HER2-positive and HER2-low breast cancers. The increased chemo-payload and its release upon the attachment of the ADC to the HER2 receptor provides a biological mechanistic justification for the broader local antitumor activity of T-DXd, impacting surrounding cancer cells and the tumor microenvironment cells in the vicinity (bystander effect) (90,91).

HER2-targeting therapies in breast cancers with low HER2 expression

Numerous studies confirmed trastuzumab’s clinical efficacy in HER2-overexpressing breast cancers (70-72). Preclinical studies also indicate that it is effective in cells with overexpressed HER2 but is less effective when HER2 is expressed at lower levels (69,92). In a retrospective study of patients enrolled in various trastuzumab trials in metastatic breast cancer patients with HER2 IHC score of 2+ and 3+, 22% of specimens were FISH negative, of which 72% had an IHC score of 2+ (93). Analysis of the efficacy of trastuzumab in these patients stratified by the FISH status indicated that its clinical benefits are restricted to patients with FISH-positive, as opposed to FISH-negative tumors (93). Similar results were reported in a phase II clinical trial, indicating that clinical benefits of trastuzumab are more pronounced in patients with HER2 IHC 3+ overexpressing or FISH-positive metastatic breast cancers (94).

Similarly, in preclinical studies, T-DM1 did not show significant anticancer activity in breast cancer patient-derived organoids with low HER2 expression, as compared to those with high HER2 expression (82). However, T-DXd elicited anticancer activity in both HER2 high- and low-expressing breast cancer models (82). The differences between these two ADCs are likely explained by the higher drug-to-antibody ratio and thus higher chemotherapeutic payload of T-DXd compared to T-DM1 (82,95). Moreover, the released payload of T-DXd (due to the cleavable linker), as opposed to T-DM1, potentially leads to a cytotoxic effect on surrounding tumor cells, even if they do not express HER2, and the shorter half-life of the released chemotherapeutics minimizes systemic side effects (90). These properties of T-DXd set it apart from other currently clinically approved anti-HER2 drugs as a potential treatment for patients with HER2-low tumor status.

In 2015, a phase I study was initiated to assess the safety and tolerability of T-DXd in patients with advanced HER2-expressing or HER2-mutated solid malignancies, including patients with advanced breast cancer whose tumor had a HER2-low status (NCT02564900) (Table 2). The study reported that in the heavily pretreated patients with HER2-low expressing breast cancer (85.3% of which were HR-positive), T-DXd showed an acceptable safety profile and conferred good anticancer activity with a high overall response rate (34,35).

These encouraging results led to the DESTINY-Breast04 (NCT03734029) Phase III randomized clinical trial of T-DXd vs. physician’s choice standard treatment (no anti-HER2 therapy) in metastatic or unresectable HER2-low-expressing breast cancer patients. HER2-low expression was defined as IHC score 1+ or 2+ with negative ISH (36). The trial included heavily pretreated metastatic breast cancer patients, and 11.3% (N=63) of those who were randomized had HR-negative disease. In the analysis of all patients, significant improvements in progression-free survival (PFS) and OS were observed in the T-DXd treatment group (36). Patients with HR+ status in the T-DXd treatment showed significant improvement in PFS (median 10.1 vs. 5.4 months in the physician’s choice cohort) and OS (23.9 vs. 17.5 months in the physician’s choice group) (36). In the HR-negative subgroup the median PFS and OS were 8.5 [95% confidence interval (CI): 4.3–11.7] and 18.2 months (95% CI: 13.6 to not evaluable) in the T-DXd and 2.9 (95% CI: 1.4–5.1) and 8.3 months (95% CI: 5.6–20.6) in the physician’s choice of treatment group, respectively (36). These results suggest clinically meaningful benefits of T-DXd in metastatic or unresectable HR-negative breast cancer patients. The trial also reported a manageable safety profile of T-DXd in these populations (36,37).

In October 2025, long-term survival analysis of the DESTINY-Breast04 trial was reported, showing that median OS in the T-DXd arm was 22.9 vs. 16.8 months in the physician’s choice treatment arm, confirming the survival benefits observed after previous analysis (37). In the extended analysis of the patients with HR-negative tumors, median OS was 17.1 months (95% CI: 13.6–23.0) in the T-DXd and 8.3 months (95% CI: 5.6–20.4) in the physician’s choice of treatment arm (37). Overall, the exploratory efficacy and subgroup analyses suggested that T-DXd might be as effective in the ER-low-positive and HR-negative as in the HR-positive subgroups and that its efficacy is independent of the tumor expression levels of ER (37).

Based on the results of the DESTINY-Breast04 trial, the Food and Drug Administration (FDA) approved T-DXd in August 2022 for “adult patients with unresectable or metastatic HER2-low (IHC 1+ or IHC 2+/ISH−) breast cancer who have received a prior chemotherapy in the metastatic setting or developed disease recurrence during or within six months of completing adjuvant chemotherapy” (98).

In 2020, a Phase III randomized clinical trial DESTINY-Breast06 (NCT04494425) was initiated to assess the efficacy of T-DXd compared to physician’s choice chemotherapy in patients with HER2-low expressing HR-positive breast cancer patients who, in the metastatic setting, had disease progression on endocrine therapy (42). In this trial of 713 patients who had HER2-low breast cancer, and 153 patients who had HER2-ultralow disease, T-DXd treatment prolonged PFS compared to chemotherapy (42). In addition, T-DXd preserved the quality-of-life indicators while delaying deterioration in various patient-reported outcomes (96). This trial served a basis for the January 2025 FDA approval of T-DXd in the category of patients with unresectable/metastatic HR-positive HER2-low or HER2-ultralow breast cancer that “has progressed on one or more endocrine therapies in the metastatic setting” (99).

A phase II clinical trial (DAISY, NCT04132960) of T-DXd in patients with pretreated metastatic breast cancer (including HR-positive and HR-negative) showed that, in the study arms, the endpoint of the objective response rate was met, including patients with HER2-overexpressing (70.6%, 95% CI: 58.3–81%), as well as HER2-low tumors (37.5%, 95% CI: 26.4–49.7%) (91).

Thus, overall T-DXd shows significant benefits in breast cancer patients with advanced or metastatic disease and HER2-low or ultralow expression status (Table 2). In addition to the metastatic setting, T-DXd treatment might be of benefit in the neoadjuvant/post-neoadjuvant/adjuvant regimens and is a promising future research direction (44,46,48) (Table 3). These recent revolutionary developments in drug discovery and clinical management in breast cancer with HER2-low status has opened the door for new pharmaceutical, translational and clinical investigations, with several clinical trials and non-interventional studies initiated to assess clinical efficacy of T-DXd in various settings, including in TNBC (Table 3).

Challenges and future directions in management of patients with low HER2 expressing breast cancers

Despite significant successes in treating breast cancer patients whose tumor is expressing low levels of HER2, several challenges exist, including accuracy in diagnosing HER2-low and HER2-ultralow breast cancers and inherited or acquired treatment resistance.

Challenges and future directions in diagnostics of breast cancers with low HER2 expression

The testing for HER2 status is currently based on the ASCO/CAP guidelines (41). The choice of IHC assays for detecting HER2-low expression is important for the current stratification of patients for HER2-directed treatments (100). Based on DESTINY-Breast04 and DESTINY-Breast06 clinical trials, the FDA approved Ventana’s PATHWAY anti-HER2 (4B5) rabbit monoclonal primary antibody IHC assay as a companion diagnostic tool for HER2-ultralow and HER2-low categories (99). While IHC and ISH remain gold standards, other assays might be potentially studied and utilized to identify patients with HER2-low and HER2-ultralow breast cancers who might benefit from targeted therapies in the future. Moreover, results from the DAISY trial suggest that IHC may not be the optimal test for identifying the HER2 expression cutoff for selecting patients who would benefit from T-DXd in metastatic breast cancer patients (91).

IHC and ISH are protein- and DNA-based tests to detect protein expression and gene amplification, respectively. Detection of HER2 messenger RNA (mRNA) expression is a potential new avenue that can be explored in assessing the HER2 expression status. Analysis of early breast cancer trials’ patients with the HER2-low status showed that this status is prevalent and associated with higher, compared to HER2-0 status, HER2 mRNA expression (60). Thus, mRNA-detection-based assays that include Oncotype DX’s ERBB2 mRNA score, RNA-seq or RNA-based ISH (RNAscope) might hold clinical promise (101-103).

Functional HER2 assays might also have a clinical utility. A recent study indicated that a subset of breast cancer patients with HER2-negative status by ISH/IHC might benefit from anti-HER2 treatment (104). In this study, the authors assessed HER2 functional activity in addition to the HER2 protein quantification and, using both HER2-negative and HER2-positive breast cancer models, reported that HER2 signaling status better predicted anti-HER2 treatment’s anticancer effects than HER2 receptor status (104). In addition, almost one quarter of patient tumor samples with HER2-negative status elicited abnormal HER2 signaling, suggesting that functional biomarker assays for HER2-related signaling dysfunction might be a useful tool for stratifying patients who would benefit from anti-HER2 targeted treatment (104). Last but not least, spatial assessment of HER2 distribution might be of value in assessing the efficacy of T-DXd treatment (91,105), opening an avenue of application for spatial omics technologies in studies of clinical efficacy of T-DXd.

Resistance to T-DXd

Efficacy of T-DXd correlates with the levels of HER2 expression (91,106). Results of the DAISY trial indicate that T-DXd efficacy is determined by HER2 expression, as was supported by several lines of evidence, including the differences in T-DXd uptake of tumors and PFS rates of patients with different HER2 expression levels and the decrease in HER2 expression at treatment resistance (91). Hence, tumors can develop resistance to T-DXd via downregulation of HER2. In T-DXd-treated metastatic breast cancer patients, 49% of cases had a major decrease or complete loss (52%) of HER2 expression at disease progression when comparing treatment specimens before and after T-DXd treatment (107). This study showed that the cancer cell resistance to T-DXd can develop through HER2 downregulation or loss of its expression and mutations at the HER2 protein binding sites that disrupt T-DXd binding (107). However, other compensatory mechanisms might be in play during the development of resistance and can include tumor intrinsic and extrinsic (e.g., tumor microenvironment) factors (91,106,108). For example, the SLX4 gene that encodes a protein important for DNA damage repair, has been reported in the DAISY trial, if mutated or depleted to be associated with the development of resistance to T-DXd in breast cancer (91).

Combination treatments with T-DXd

One of the foundational strategies to tackle breast cancer treatment resistance involves the use of combination treatments that can act synergistically to increase the anticancer efficacy of the drug or to suppress activation of the specific drug-resistance mechanisms (109,110). Combination treatments, if used in lower doses, compared to the single-agent treatment, can also provide benefits of reduced drug-associated toxicities (109). Hence, there are a number of clinical trials, assessing combinations of T-DXd with chemotherapeutic, other targeted treatments and immunotherapeutics, in patients with HER2-low and HER2-ultralow HR-positive or HR-negative breast cancers (Table 3).

In immunocompetent HER2-expressing colon cancer mouse models, T-DXd treatment has been shown to have anticancer activity and to enhance antitumor immunity, including increasing the presence of dendritic and CD8⁺ T cells in the tumor microenvironment and enhancing programmed death-ligand 1 (PD-L1) expression on tumor cells (111). The combination of anti-programmed cell death protein 1 (anti-PD-1) antibody with T-DXd was more effective compared to a single-agent treatment (111). Consequently, various immunotherapy approaches are being explored in HER2-positive breast cancers in combination with anti-HER2-targeted therapies (112). On December 15, 2025, the FDA approved T-DXd with pertuzumab in patients with unresectable or metastatic HER2-positive (IHC 3+ or ISH-positive) breast cancer (113). This approval is based on the results of the DESTINY-Breast09 trial (NCT04784715) showing that T-DXd in combination with pertuzumab significantly improved the risk of progression or death in patients with HER2-positive advanced or metastatic breast cancer, compared to taxane, trastuzumab and pertuzumab treatment (114). Pembrolizumab in combination with T-DXd is being investigated in a Phase Ib clinical trial in patients with HER2-positive and HER2-low locally advanced/metastatic breast cancer (NCT04042701) (115) (Table 3). In heavily pretreated patients with HER2-low metastatic breast cancer, investigators observed prolonged median PFS and the duration of response (115). Promising efficacy, safety and tolerability were observed in a Phase I trial (NCT03523572) including patients with metastatic HER2-positive or HER2-low breast cancers treated with a combination of T-DXd and immunotherapeutic nivolumab (116). These studies provide encouraging results for combination treatment strategies including T-DXd and immunotherapy drugs in HER2-low-expressing breast cancers, with several trials aiming to explore T-DXd combination with investigational and clinically-approved immunotherapeutics (e.g., NCT05765851, NCT05795101 and NCT05868226, Table 3).

Combinations of T-DXd with chemotherapeutic, targeted or antihormonal agents also appear to be promising (97,117). A phase Ib DESTINY-Breast08 study (NCT04556773) in patients with HER2-low metastatic breast cancer demonstrated preliminary clinical activity and acceptable safety profiles of combination of T-DXd with capecitabine, capivasertib (an inhibitor of AKT, a key component of the PI3K signalling pathway), anastrozole, or fulvestrant (97).

Combinations of T-DXd with TKIs in certain subgroups of low-expressing HER2 breast cancers might also be promising. Multiomics analysis of HER2-low TNBC identified 3 subgroups within this category of tumors, including the tyrosine kinase-relevant subgroup that showed activation of HER2 signalling, suggesting that combinations of TKIs with T-DXd might be beneficial in this category of patients (45). Currently, anlotinib (NCT06331169) is being investigated in a Phase 1b clinical trial with HER2-low or HER2-ultralow advanced or metastatic breast cancers, including TNBC (Table 3). Given potential signaling alterations in phosphoinositide 3-kinase (PI3K) and associated pathways and the increased rate of PIK3CA in patients with TNBC HER2-low breast cancers (45,52,53), respective inhibitors, such as inavolisib, in combination with T-DXd might provide an effective combination approach in this patient population and is being investigated in a clinical trial including HER2-low metastatic or locally advanced breast cancers with PIK3CA mutations (NCT03424005).

Other combinations of T-DXd with clinically-approved or investigational targeted drugs, such as vascular endothelial growth factor (VEGF) inhibitor, bevacizumab (NCT07150208); enhancer of zeste homologs 1 and 2 (EZH1/2) inhibitor, valemetostat (NCT05633979); RNA polymerase I inhibitor, pidnarulex (NCT07137416); and WEE1 kinase inhibitor, azenosertib (NCT06364410), are being investigated in early-stage clinical trials in patients with HER2-low and/or HER2-ultralow breast cancers (Table 3). Various combinations of T-DXd with endocrine therapies in HR-positive HER2-low or HER2-ultralow breast cancer are also being investigated in clinical trials (e.g., NCT07198724 and NCT04553770).

Combination treatments of T-DXd with another ADC are also being explored. Trop-2-directed ADC sacituzumab govitecan is clinically used in TNBC (26,27). Another Trop-2-directed ADC that is FDA-approved for use in inoperable/metastatic HR-positive HER2-negative breast cancers is datopotamab deruxtecan (Dato-DXd) (118). Dato-DXd delivery of the same payload of T-DXd is independent of HER2-expression, suggesting that this alternative DXd delivery mechanism might be useful when HER2 is lost (for example, due to resistance development to T-DXd treatment) (107). This combination treatment was tested in preclinical models, showing the same anticancer efficacy at lower doses (compared to higher single-agent doses) of T-DXd and Dato-DXd, and that combination treatment had suppressed tumor growth more strongly than monotherapy due to the increased tumor payload delivery (107). These findings suggest that a combination of these ADCs might tackle HER2-loss-mediated T-DXd resistance (107). A phase 2 clinical trial (NCT06533826, TRADE DXd) is assessing the efficacy of administering T-DXd after Dato-DXd (or vice versa) in patients with HER2-low and HER2-0 advanced unresectable or metastatic HR-positive or HR-negative breast cancer (Table 3).

Aside from combination treatments, other strategies are being investigated to increase the clinical benefits of T-DXd in HER2-low and HER2-ultralow breast cancer patients, including TNBC. The benefits of T-DXd treatment in HER2-low cancers might also be increased by precision targeting in specific populations that would better respond to treatment. This approach is being utilized in a phase 2 clinical trial (NCT05953168, Table 3) of patients with HER2-low locally advanced or metastatic LAR subtype of TNBC, which has been shown to have better RFS compared to patients with HER2-0 status (53). New delivery methods of T-DXd are also being investigated to increase its efficacy in HER2-low breast cancers. In a phase II clinical study (NCT04420598, DEBBRAH trial) of pretreated HER2-positive or HER2-low breast cancer patients with brain and/or leptomeningeal metastases, intracranial delivery methods of T-DXd have been shown to have intracranial activity with acceptable toxicity profile and quality of life indicators (119), with further research in these populations ongoing (NCT07134153, Table 3).

New drugs and treatment approaches

The success of T-DXd in oncology paralleled the development of new anti-HER2 ADCs (108). ARX788 is an ADC containing an antibody against HER2 and a non-cleavable drug-linker with a cytotoxic payload of the antimitotic compound monomethyl auristatin F [MMAF, Amberstatin 269 (AS269), that disrupts tubulin polymerization] with a drug-to-antibody ratio of 1.9 (108,120). It showed superior activity to T-DM1 in in vitro and in vivo preclinical models of breast and other cancers expressing higher and lower levels of HER2 (120,121). ARX788 also shows antitumor effects in T-DM1-resistant HER2-expressing breast and gastric cancer preclinical models (120,121). In HER2-positive metastatic breast cancer patients, ARX788 overall had a manageable toxicity profile (122,123). Results of clinical trials (such as ACE-Breast-02 and ACE-Breast-06) in HER2-positive breast cancer patients with advanced or metastatic disease show clinical efficacy of ARX788, including in progressive disease after trastuzumab (123,124). ARX788 in patients with HER2-low status is also being investigated in a clinical trial (NCT06224673).

ADC BL-M07D1, consisting of a humanized anti-HER2 antibody, a cathepsin B cleavable linker, and a topoisomerase I inhibitor Ed-04, showed promise in terms of anticancer efficacy and adequate safety and tolerability in a phase I trial (NCT05461768) in patients with locally advanced or metastatic solid tumors, including pretreated HER2-positive or HER2-low breast cancers (125). Similar promising results were reported for ADC disitamab vedotin (RC48), an anti-HER2-antibody-drug conjugate (ADC) with a cleavable linker and chemo-payload of monomethyl auristatin E (126), in a phase II study (NCT05331326) in patients with pretreated HER2-positive or HER2-low metastatic breast cancer that elicited abnormal activation of the PI3K/AKT/mechanistic target of rapamycin (mTOR) pathway (127). Numerous other HER2-directed ADCs have been developed in recent years, showing promise in preclinical models and are being investigated in clinical trials in patients with HER2-expressing breast cancers [reviewed in (108,128)].

HER2-low status might also have a biomarker utility for patient selection for specific chemotherapy regimens. In TNBC patients who did not achieve pCR after neoadjuvant treatment and who received capecitabine, had superior DFS and OS compared to those who did not receive it, not only in the overall patient comparison but also within a subgroup of HER2-low (but not HER2-0) TNBC patients (129). This study provides support for further investigations of precision oncology-based stratification of patients who will benefit from capecitabine and the utilization of HER2-low status as a treatment biomarker, not only for anti-HER2 targeting therapies.

Conclusions

The development of new HER2-directed ADCs is revolutionizing the treatment and management of breast cancer patients. Historically, clinically-considered HER2-negative breast cancers are now categorized into HER2-low, HER2-ultralow and HER2-negative molecular therapeutically-relevant entities. Rapid evolution of this domain of research prompts improvements in the detection and definition of clinically meaningful HER2-positive signals through the application of more specific and sensitive approaches and technologies. Beyond the limitations of current assays in reliably detecting low or ultralow HER2 expression, the marked molecular heterogeneity in breast cancer, especially TNBC, poses significant diagnostic challenges. In this context, false-negative HER2 results may preclude the administration of potentially life-saving HER2-directed ADC treatments. At this early stage of advancements in this field, T-DXd remains the only FDA-approved ADC for breast cancer patients with HER2-low and HER2-ultralow tumors. Notably, clinical benefits of T-DXd are more pronounced in HR-positive vs. HR-negative (or traditionally TNBC) tumors, which represent distinct biological, molecular and clinical entities. Thus, more studies of T-DXd and similar ADCs restricted to TNBC patients with HER2 low-expressing status might shed further light on their efficacy in these patient subpopulations. Another cause of concern is the development of treatment resistance to T-DXd, which, at least in part, can be addressed by combination treatments with various chemotherapeutic, immunotherapeutic and targeted drugs. Rapid developments in drug design and discovery, with resulting promising preclinical and clinical studies, have expanded the list of HER2-directed ADCs that might soon find their place in clinical practice. Results of numerous ongoing or recently completed trials in patients with HER2-low and HER2-ultralow breast cancers are being eagerly awaited. Overall, modern scientific advancements have unearthed an exciting field of breast cancer research and clinical innovations that will hopefully continue to positively impact outcomes of patients suffering from breast cancer and one of its most clinically challenging subtypes, TNBC.

Acknowledgments

We thank Ms. Anna Bonvissuto-Pin for reviewing the final version of the manuscript.

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://tcr.amegroups.com/article/view/10.21037/tcr-2026-1-0370/rc

Peer Review File: Available at https://tcr.amegroups.com/article/view/10.21037/tcr-2026-1-0370/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tcr.amegroups.com/article/view/10.21037/tcr-2026-1-0370/coif). H.M received honoraria for educational events by AstraZeneca, Gilead Sciences, Eli Lily and Novartis; and has received travel grants for attending scientific conferences by Daiichi Sankyo and Gilead Sciences. E.A.G. reports honoraria from AstraZeneca for an educational presentation. A.P. reports research support from Breast Cancer Canada; the Clinician Scientist Award, Department of Surgery, Western University, London, ON, Canada; and the Academic Medical Organization of Southwestern Ontario (AMOSO) Innovation Fund of the Alternative Funding Plan of the Academic Health Sciences Centres of Ontario (No. INN24-017), Canada. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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