Abstract
Globally, breast cancer is the most frequently diagnosed malignancy in women, with the HER2-low subtype (IHC 1+ or IHC 2+/ISH−) accounting for approximately 45%–55% of all invasive breast cancer cases. Conventional HER2-targeted therapies are ineffective for this subtype, and treatment has long relied on endocrine therapy or chemotherapy, with a median overall survival of less than 18 months after resistance development. This narrative review synthesizes evidence from clinical trials identified through systematic searches of the PubMed, ASCO, and ESMO databases until December 2025. Antibody-drug conjugates (ADCs) combine targeted delivery with potent cytotoxicity. The next-generation ADC trastuzumab deruxtecan (T-DXd) has achieved a breakthrough in the treatment of HER2-low breast cancer, attributed to its high drug-to-antibody ratio (DAR = 8), robust bystander effect, and non-classical killing mechanisms mediated by the tumor microenvironment. The landmark DESTINY-Breast04 trial provided the first definitive evidence that T-DXd significantly prolongs progression-free survival (PFS) and overall survival (OS). The DESTINY-Breast06 study further expanded the population that may benefit to include patients with HER2-ultralow expression and moved treatment to an earlier line, after endocrine therapy failure and before chemotherapy initiation. Chinese ADCs, such as disitamab vedotin (objective response rate [ORR], 33.3%), SHR-A1811, and MRG002, have also shown promising efficacy. However, resistance mechanisms, including target loss, impaired endocytosis, payload efflux, abnormalities in DNA repair, and suppression of the immune microenvironment, are emerging. Combination strategies involving endocrine therapy, immune checkpoint inhibitors, and HER2 tyrosine kinase inhibitors are expected to overcome resistance and enhance the efficacy of treatment. This review provides a comprehensive overview of ADC development in advanced HER2-low breast cancer, summarizes key clinical data, resistance mechanisms, and combination strategies, and discusses future directions, including biomarker development and optimal treatment sequencing, to guide clinical practice and future research in this area.
Keywords: low expression of her2, advanced breast cancer, antibody-drug conjugate, trastuzumab deruxtecan, mechanisms of drug resistance, combination therapy
Introduction
Globally, breast cancer is the most frequently diagnosed malignancy in women. In 2022, an estimated 2.3 million new cases and 670,000 deaths from breast cancer occurred worldwide, posing a serious threat to women’s health and survival.1 Based on the expression patterns of ER, PR, and HER2, breast cancer can be divided into Luminal, HER2‑positive, and triple‑negative subtypes. Historically, a binary classification system for HER2 expression was used in clinical diagnosis and treatment: only tumors with IHC 3+ or IHC 2+ accompanied by ISH amplification were classified as HER2-positive, whereas all others were considered HER2-negative. In 2021 that the concept of HER2-low expression was formally recognized by the Chinese Society of Clinical Oncology (CSCO), shifting the paradigm from binary to ternary classification and establishing HER2-low breast cancer as a distinct therapeutic subgroup. Notably, the HER2-low subtype (defined as IHC 1+ or IHC 2+ with negative ISH results) accounts for 45%–55% of all invasive breast cancer cases, representing the largest subgroup of patients with this malignancy.2 This subtype is highly heterogeneous; the majority of cases are hormone receptor (HR)-positive (~85%) and are more prone to endocrine resistance and visceral metastasis, whereas the HR-negative (triple-negative) subgroup exhibits a more aggressive biological behavior and poorer prognosis. Furthermore, the development of domestic ADCs in China is not only a scientific endeavor but is also driven by the need to improve patient access and address regional pharmacoeconomic considerations in China.
Prior clinical investigations have demonstrated that traditional anti‑HER2 targeted agents, including trastuzumab and pertuzumab, fail to confer survival advantages to individuals diagnosed with HER2‑low breast cancer. For an extended period, therapeutic management of these patients has adhered to endocrine interventions for HR‑positive disease or chemotherapy protocols for triple‑negative disease, with the majority encountering a therapeutic impasse due to the scarcity of available treatments following the onset of endocrine resistance.3 Antibody‑drug conjugates (ADCs) are an innovative class of biopharmaceutical agents constructed via the conjugation of targeted monoclonal antibodies, cleavable linker molecules, and potent cytotoxic warheads, integrating both highly accurate targeting capabilities and robust antitumor cytotoxic effects.4 In 2022, the DESTINY-Breast04 study provided the first conclusive evidence that the new-generation ADC drug trastuzumab deruxtecan (T-DXd) can prolong PFS and OS in patients with advanced HER2-low breast cancer. It has overcome the longstanding lack of effective targeted therapies for HER2-low breast cancer and established HER2-low as a distinct clinically actionable therapeutic subgroup.5 In recent years, various innovative ADC drugs have achieved landmark progress in the field of HER2-low breast cancer. Simultaneously, in-depth research has been continuously conduced on drug resistance mechanisms and combined treatment strategies, which has comprehensively reshaped the diagnosis and treatment patterns of HER2-low breast cancer. This narrative review systematically reviews the latest research advances in ADC drugs for the treatment of advanced HER2-low breast cancer.
Methods
A comprehensive literature search was conducted using PubMed and the conference proceedings of the American Society of Clinical Oncology (ASCO) and European Society for Medical Oncology (ESMO) until December 2025. Search terms included “HER2-low breast cancer,” “antibody-drug conjugate,” “trastuzumab deruxtecan,” “disitamab vedotin,” “resistance mechanisms,” and “combination therapy.” Phase I–III clinical trials, systematic reviews, meta-analyses, and high-impact mechanistic studies published in peer-reviewed journals were prioritized. The reference lists of the retrieved articles were screened to identify additional relevant studies. Given the narrative nature of this review, a formal quantitative meta-analysis was not performed; instead, we aimed to provide a comprehensive and critical synthesis of available evidence.
Definition and Biological Characteristics of HER2-Low Breast Cancer
Definition and Diagnostic Criteria
In 2021, the CSCO officially proposed an explicit definition of HER2-low expression for the first time in its Guidelines for the Diagnosis and Treatment of Breast Cancer. Specifically, this definition refers to a score of 1+ on IHC staining or an IHC score of 2+ in the absence of HER2 gene amplification detected through in situ hybridization (ISH) testing.6 In 2023, the ASCO/CAP breast cancer HER2 testing guidelines were updated, further standardizing the interpretation criteria for low HER2 expression and clearly defining it as an independent treatment subgroup. Simultaneously, the concept of HER2 ultra-low expression (IHC0 with focal and weak cell membrane staining) was proposed, providing a basis for the expansion of the benefit population of ADC drugs.7
Importantly, accurate identification of HER2-low and ultralow expression remains challenging. Interobserver variability in IHC scoring is a substantial concern, with studies reporting only moderate concordance among pathologists, particularly in distinguishing IHC 0 from IHC 1+. This variability directly affects patient eligibility for ADC therapy and highlights the urgent need for more standardized testing protocols and the development of quantitative and automated scoring methods. The evolving diagnostic landscape, including emerging digital pathology and artificial intelligence-based approaches, may help address these challenges.
Although HER2-low is now a clinically actionable category, debate continues as to whether it represents a distinct biological subtype or is primarily a therapeutically defined entity. This distinction has important implications for understanding disease biology, identifying predictive biomarkers, and developing therapeutic strategies. Current evidence suggests that HER2-low breast cancer is biologically heterogeneous and may not constitute a single distinct entity but rather a spectrum of tumors unified by their responsiveness to novel ADC therapies.
Biological Characteristics
HER2-low breast cancer does not merely represent a quantitative distinction in HER2 expression levels but also exhibits distinct biological behaviors and molecular traits. In contrast to HER2-positive breast cancer, the vast majority of HER2-low cases are hormone receptor (HR)-positive, with this proportion reaching approximately 85%, and are more prone to visceral metastasis and more heterogeneous tumors. HER2 expression often changes dynamically between primary and metastatic lesions and before and after treatment.8 Compared with breast cancer with zero HER2 expression, breast cancer with low HER2 expression has a higher proliferation index and a greater risk of endocrine resistance. The biological behavior of patients with low ER expression (IHC1%-10%) is more similar to that of triple-negative breast cancer, and the prognosis is worse. Furthermore, immunosuppression is frequently observed in the tumor microenvironment of breast cancer with low HER2 expression, characterized by enhanced infiltration of tumor-associated macrophages and elevated PD-L1 expression. This distinctive feature lays a theoretical foundation for the combined use of ADC and immunotherapies.9
Clinical Diagnosis and Treatment Dilemmas
Breast cancer with low HER2 expression has not been regarded as an independent subtype in the past, and the treatment strategy lacks targeted options. For patients with HR-positive and HER2 low expression, CDK4/6 inhibitors combined with endocrine therapy are the first line treatments. However, after drug resistance, second-line and later-line treatments can only choose single-agent chemotherapy, the objective response rate (ORR) is less than 30%, and the median PFS is only 3–4 months.10 For patients with HR-negative and HER2 low expression (triple-negative), the treatment options are more limited, with an ORR of only approximately 20% and a median OS of less than 12 months for later-line chemotherapy. Therefore, there is an urgent need to develop new therapeutic drugs that do not rely on HER2 signaling pathway blockade, and the emergence of ADC drugs has completely resolved this dilemma.
Structure and Antitumor Mechanism of HER2-Targeted ADC Drugs
The Core Structure of the ADC Drug
HER2-targeting ADCs consist of three key structural components (see Figure 1): ① Targeting antibody: With trastuzumab or newly developed humanized anti-HER2 monoclonal antibodies serving as their backbone, these antibodies can selectively identify the HER2 protein expressed on tumor cell membranes, thereby achieving accurate targeted delivery, and at the same time preserving the antibody-dependent cell-mediated cytotoxicity (ADCC) activity; ② Linkers: They are classified into cleavable and non-cleavable types. Cleavable linkers remain highly stable in the bloodstream and can be specifically cleaved by certain proteases or the acidic environment upon entering the lysosomes of tumor cells, thereby enabling the precise release of the cytotoxic payload. They represent the mainstream choice for the new generation of ADCs; and ③ Cytotoxic load: most of these drugs are nanomolar potent cytotoxic drugs, including topoisomerase I inhibitors (such as DXd) and tubulin inhibitors (such as MMAE and DM1), which can efficiently kill tumor cells in the proliferation cycle.11
Figure 1.
Schematic diagram of the structure and antitumor mechanism of ADC drugs targeting HER2.
Classical Antitumor Mechanisms of Action
The conventional anti-tumor mechanism of ADC agents can be divided into five core stages: ① Targeted binding: The antibody component of ADCs specifically binds to the HER2 antigen present on the membrane of tumor cells, thereby forming an ADC-antigen complex; ② Endocytic internalization: The formed complex is taken up into tumor cells via clathrin-mediated endocytosis, which leads to the formation of early endosomes; ③ Lysosomal trafficking: Early endosomes mature into late endosomes and eventually fuse with lysosomes; ④ Payload liberation: The acidic microenvironment within lysosomes and proteases cleave apart the linker, liberating the free cytotoxic payload; and ⑤ Cell killing: loading exerts cytotoxic effects, blocking the tumor cell cycle, and inducing apoptosis by inhibiting topoisomerase I or tubulin synthesis.12
Key Synergistic Mechanisms Against HER2-Low Tumors
Bystander Effect
The bystander effect is the core mechanism of ADC in the treatment of HER2-low and heterogeneous tumors. The new generation of ADC, represented by T-DXd, adopts cytotoxic loading with cleavable linkers and high membrane permeability. After the loading is released in the target cells, it can penetrate the cell membrane and spread to neighboring tumor cells, achieving high killing efficiency regardless of whether these cells express HER2.13 This mechanism addresses the problem of heterogeneity in HER2-low tumors, and even if only some cells within the tumor express HER2, comprehensive killing of the entire tumor can be achieved through bystander effects.
High Drug-Antibody Ratio (DAR) and High Payload
A new generation of ADC drugs achieves higher and more uniform DAR values by optimizing the coupling technology. The DAR value of T-DXd is stable at 8, which is much higher than the DAR value of 3.5 of the first-generation ADCT-DM1. A single antibody molecule can carry a greater cytotoxic load, and even in the case of low HER2 expression and limited antigen binding, it can release a sufficient concentration of load in tumor cells to achieve strong killing.14 At the same time, the new generation of ADC adopts DXd equal loading, and its cytotoxicity is 100–1000 times that of traditional chemotherapy drugs, with an IC50 at the nanomole level, and tumor cell killing can be achieved at very low concentrations, which is further adapted to the treatment needs of HER2 low expression tumors.15
Noncanonical Killing Mechanisms Mediated by the Tumor Microenvironment
A study published in Nature Communications in 2025 was the first to reveal the non-classical mechanism of action of T-DXd in HER2-low/ultra-low tumors. Cathepsin L (CTSL), which is highly expressed in the breast cancer microenvironment, can directly cleave the tetrapeptide-linker of T-DXd outside the cell, releasing free DXd, and this process is completely independent of HER2 binding and endocytosis. This perfectly explains the efficacy of T-DXd in HER2 ultra-low expression and even some IHC0 patients.16 Simultaneously, DXd can induce immunogenic cell death of tumor cells, release ATP, HMGB1, and other danger signals, promote dendritic cell maturation and antigen presentation, coordinate with the Fc segment of antibody-mediated macrophage phagocytosis, activate adaptive anti-tumor immunity, and realize the synergistic effect of targeted killing and immunotherapy.
Clinical Research Progress of ADC Drugs in Advanced Breast Cancer with Low HER2 Expression
Trastuzumab Deruxtecan (T-DXd)
T-DXd is a next-generation HER2-targeting ADC developed jointly by Daiichi Sankyo and AstraZeneca. It is composed of trastuzumab, a cleaved tetrapeptide linker, and topoisomerase I inhibitor DXd, DAR=8, with high membrane permeability and a strong bystander effect. It is the most evidence-based and widely used ADC drug for HER2 low expression breast cancer.
Landmark Phase III Study: DESTINY-Breast04
DESTINY-Breast04 (NCT03734029) is a worldwide, multi-institutional, randomized, open-label Phase III clinical study—the first of its kind to confirm that ADCs can deliver substantial survival advantages to patients diagnosed with advanced HER2-low breast cancer. In this study, 557 individuals with advanced HER2‑low breast cancer were enrolled, all of whom had received one or two prior chemotherapy regimens.17 Among them, 494 tested positive for hormone receptors (HR+), while the remaining 63 were HR‑negative. These participants were randomly split into two groups at a 2:1 ratio: one group received T‑DXd (5.4 mg/kg every three weeks), and the other received a chemotherapy regimen chosen by the treating physician (TPC), which included drugs such as capecitabine or paclitaxel.
Focusing on the HR‑positive subset, those treated with T‑DXd achieved a median progression‑free survival (PFS) of 9.6 months, markedly surpassing the 4.2 months observed in the chemotherapy arm. Their median OS was 23.9 months with T‑DXd, which translated into a survival gain of 6.3 months relative to standard chemotherapy. Looking at the entire participant cohort, the median PFS was 8.8 months for the T‑DXd arm versus 4.2 months for the TPC arm, while median OS values were 22.9 and 16.8 months, respectively. Even within the HR‑negative (ie, triple‑negative) subgroup, T‑DXd yielded a median OS of 17.1 months, more than twice the 8.3 months seen in the chemotherapy group.
In terms of treatment response, the ORR for T‑DXd was 52.6%, which was far higher than the 16.3% recorded for the TPC group. The complete response (CR) rate was 16.1%, and the disease control rate (DCR) exceeded 80%.
From a safety perspective, T‑DXd was generally well tolerated. The most common side effects were gastrointestinal, including nausea, fatigue, and vomiting. The frequency of grade 3 or higher treatment‑related adverse events (TRAEs) was lower in the T‑DXd arm than in the chemotherapy arm. Interstitial lung disease (ILD) or pneumonitis occurred in 12.1% of cases overall, with only 2.2% being grade ≥3. No treatment-related deaths were reported, confirming that T‑DXd has an acceptable and clinically manageable safety profile.
Forward Line of Treatment and Population Expansion: DESTinY-BREAST 06
The global, multicenter, open‑label, randomized Phase III study known as DESTinY‑Breast06 (NCT04494425) was conducted to evaluate the efficacy and safety profile of T‑DXd in individuals with HR‑positive advanced breast cancer showing either low or ultra‑low HER2 levels, whose disease had advanced following endocrine treatment. In this trial, 153 participants with ultra‑low HER2 expression—none of whom had received chemotherapy during the metastatic phase—were divided equally (1:1 ratio) into two arms: one receiving T‑DXd and the other undergoing standard chemotherapy.
Within the main analysis population, the T‑DXd arm achieved a medianPFS of 13.2 months, significantly exceeding the 8.1 months seen in the chemotherapy arm, which represented a 38% drop in the likelihood of disease progression or mortality. The ORR for T‑DXd was 57.3%, nearly double that of the chemotherapy group (31.2%).
A preplanned subgroup analysis revealed that the PFS advantage associated with HER2‑low tumors was comparable to that observed in tumors with ultra‑low HER2 expression. More specifically, among those with ultra‑low HER2 status, the median PFS was 13.2 months in the T‑DXd group versus 8.3 months in the chemotherapy group. These results provide the first evidence that ADCs can provide substantial therapeutic benefits for patients with ultra‑low HER2 expression.18
Disitamab Vedotin (RC48)
RC48 is a HER2-targeted ADC developed independently by RemeGen. It consists of a recombinant humanized anti-HER2 monoclonal antibody, a cleavable VC linker, and the microtubule inhibitor MMAE, with a DAR of four. It has demonstrated favorable antitumor activity and a manageable safety profile. Notably, disitamab vedotin is the first domestically developed ADC to receive regulatory approval in China, specifically for the treatment of HER2-low breast cancer.19
Previous clinical studies have shown that in heavily pretreated patients with HER2-low advanced breast cancer, disitamab vedotin monotherapy achieved an ORR of 33.3% and a median PFS of 5.1 months. In patients with liver metastases, disitamab vedotin demonstrated superior antitumor activity compared to conventional chemotherapy, with a manageable safety profile.20 Although SHR-A1811 and MRG002 are promising candidates, it is important to note that they remain investigational agents, with clinical data derived primarily from early-phase trials. Therefore, their regulatory status and clinical maturity are not comparable to those of disitamab vedotin. The approval of disitamab vedotin has helped address unmet clinical needs in China, particularly among patients with HER2-low breast cancer and liver metastases, and has improved treatment accessibility while also demonstrating China’s growing capabilities in ADC development.
Previous clinical studies have shown that in patients with HER2 low expression advanced breast cancer who have received multiple lines of treatment, the ORR of RC48 monotherapy reaches 33.3%, and the median PFS reaches 5.1 months. For patients with liver metastasis, RC48 has better anti-tumor activity than traditional chemotherapy, and its safety is controllable.21,22 Relevant studies have shown that RC48 combined with the PD-1 inhibitor penpulimab can achieve a pCR of 25.00% in the neoadjuvant treatment of HER2 low expression breast cancer, with good safety, which provides an evidence-based basis for the combined treatment strategy.23
The approval of RC48 fills the gap in the treatment of breast cancer patients with HER2 low expression and liver metastasis in China, provides Chinese patients with more accessible domestic ADC treatment options, and also marks that China has reached an internationally advanced level in the field of ADC drug research and development.
Other HER2-Targeted ADC Agents in Development
Shr-A1811
SHR-A1811 consists of trastuzumab, a cleavable tetrapeptide linker, and a novel topoisomerase I inhibitor, SHR169265, with a drug-to-antibody ratio (DAR) of 8, exhibiting high antitumor potency and low pulmonary toxicity. Findings from a Phase I clinical trial demonstrated that among patients with advanced solid tumors (including breast cancer) harboring HER2 expression or mutations, the median PFS of SHR-A1811 monotherapy was 11.0 months, and the incidence of ILD was 2.5%, which was lower than that of similar ADC drugs, and the overall safety was better.24
Mrg002
MRG002, an HER2-targeted ADC independently developed by Mabpharm in China, comprises a monoclonal antibody targeting HER2, a cleavable VC-type linker, and monomethyl auristatin E (MMAE). Results from its Phase II clinical study revealed that in patients with advanced HER2-low breast cancer in severe clinical conditions, the ORR was 39.3%, the median duration of response (DoR) was 6.1 months, and the safety profile was well-controllable. Only one patient developed grade 2 ILD, and no ILD events of grade 3 or above were reported in the study. Moreover, MRG002 shows considerable antitumor activity in patients with disease progression after T-DXd treatment, providing a new therapeutic alternative for individuals with drug-resistant tumors.25
Trastuzumab Duocarmazine (SYD985)
SYD985 is a HER2-targeted ADC consisting of trastuzumab coupled to the DNA alkylating agent dukaromycin, with a DAR of 2.8 and a significant bystander effect. Initial Phase 1 clinical trials demonstrated that among patients with advanced breast cancer characterized by low HER2 expression, the ORR of the HR-positive subgroup was 27%, while the ORR of the triple-negative subgroup was 40%, showing anti-tumor activity.26
Sacituzumab Govitecan
Although not a HER2-targeted ADC, the Trop-2-directed ADC sacituzumab govitecan has demonstrated promising activity in patients with HER2-low breast cancer. In the TROPiCS-02 study, sacituzumab govitecan achieved a median PFS of 5.5 months and an ORR of 21% in the HER2-low subgroup, providing an additional treatment option beyond HER2-targeted approaches. These findings highlight the potential of alternative ADC targets in HER2-low settings. In addition, other HER2-targeted ADCs under development, such as ZW49 and ARX788, are being investigated, although data specific to HER2-low breast cancer are more limited than those available for T-DXd and disitamab vedotin. The key characteristics and clinical data of the major HER2-targeted ADCs discussed above are summarized in Table 1.
Table 1.
Key HER2-Targeted Antibody-Drug Conjugates (ADCs) in HER2-Low Breast Cancer
| ADC Name | Target | Linker | Payload | DAR | Key Trial(s) and NCT Number(s) | Key Findings in HER2-Low Breast Cancer |
|---|---|---|---|---|---|---|
| Trastuzumab deruxtecan (T-DXd) | HER2 | Cleavable tetrapeptide | DXd (topoisomerase I inhibitor) | 8 | DESTINY-Breast04 (NCT03734029); DESTINY-Breast06 (NCT04494425) | Median PFS: 8.8–13.2 months; ORR: 52.6%–57.3%; effective in both HR+ and HR- populations |
| Disitamab vedotin (RC48) | HER2 | Cleavable VC | MMAE (microtubule inhibitor) | 4 | NCT03500380; NCT04400695 | ORR: 33.3%; median PFS: 5.1 months; promising activity in patients with liver metastases |
| SHR-A1811 | HER2 | Cleavable tetrapeptide | SHR169265 (topoisomerase I inhibitor) | 8 | NCT04513223 (Phase I) | Median PFS: 11.0 months (pooled analysis of HER2+ and HER2-low); ILD incidence: 2.5% |
| MRG002 | HER2 | Cleavable VC | MMAE (microtubule inhibitor) | 4 | NCT05652868 (Phase II) | ORR: 39.3%; median DoR: 6.1 months; retains activity after T-DXd resistance |
| Trastuzumab duocarmazine (SYD985) | HER2 | Cleavable | Duocarmycin (DNA alkylating agent) | 2.8 | NCT02277717 (Phase I) | ORR: 27% (HR+), 40% (TNBC) |
Drug Resistance Mechanisms and Coping Strategies of ADC Drugs
Although ADC drugs have made a breakthrough in HER2 low expression advanced breast cancer, with the popularization of clinical application, the problem of drug resistance has gradually become prominent, which has become the core obstacle limiting the long-term benefit of patients. Current studies have shown that the drug resistance mechanism of ADC is complex, involving multiple dimensions such as targets, drug transport, loading activity, DNA damage repair, and immune microenvironment, and the drug resistance profiles of different ADC drugs are different.
Core Resistance Mechanisms
Target-Related Drug Resistance
Target-related resistance is one of the most important mechanisms of ADC resistance, with the following mechanisms supported by varying levels of evidence. Clinical evidence: ① Downregulation or loss of HER2 expression. Long-term ADC exposure has been clinically observed to induce reduced or complete loss of HER2 protein expression in tumor cells from patients with disease progression on T-DXd, thereby impairing effective ADC binding and internalization in tumor cells. ② Intratumoral HER2 heterogeneity and clonal selection. The selective proliferation of HER2-null clones has been documented in post-progression biopsies, contributing to treatment failure.27,28 Preclinical evidence: ① Abnormal HER2 structures. Point mutations and truncated expression of the ERBB2 gene have been shown to affect antibody binding and reduce ADC internalization efficiency in cell-line models. ② Mucin overexpression. Overexpression of mucins such as MUC1 and MUC4 has been demonstrated in vitro to mask HER2 epitopes and hinder ADC binding to its target. However, these mechanisms require further clinical validation.
Endocytosis and Intracellular Transport Disorders
The cytotoxic effect of ADC is highly dependent on receptor-mediated endocytosis and lysosomal degradation, and abnormalities in this process can directly lead to drug resistance. Studies have shown that tumor cells can inhibit the internalization of the ADC-HER2 complex by downregulating the expression of endocytosis-related aptamers or hindering linker cleavage and load release by lysosomal alkalinization and downregulation of lysosomal protease expression.29 In addition, HER2 heterodimerization with EGFR/HER3 can lead to abnormal transport, which prevents the efficient entry of ADC into lysosomes and ultimately leads to drug resistance.
Load-Related Resistance
Payload-related resistance mainly involves two mechanisms. Clinical evidence: ① Increased drug efflux. The overexpression of ABC transporters, such as ABCB1 and ABCG2, has been associated with resistance in patient samples. These transporters can actively efflux intracellular payloads, thereby reducing the intracellular drug concentration. Microtubule inhibitors, such as MMAE and DM1, are typical substrates of ABC transporters, whereas DXd is less susceptible to transporter-mediated efflux.30 Preclinical evidence: ② Reduced sensitivity of tumor cells to ADC payloads. Downregulation or mutation of topoisomerase I has been identified in resistant cell lines, resulting in reduced tumor cell sensitivity to DXd treatment. Similarly, alterations in tubulin isoform expression or tubulin mutations have been shown in vitro to confer resistance to microtubule inhibitors, such as MMAE and DM1. The clinical relevance of these alterations requires further investigation in future studies.
Abnormal DNA Damage Repair Pathways
Topoisomerase I inhibitors, such as DXd, play a central role in inducing DNA double-strand breaks in tumor cells; therefore, abnormal activation of DNA damage repair pathways is an important resistance mechanism. Studies have shown that abnormal activation of the ATM/ATR-Chk1/Chk2 pathway, homologous recombination repair (HRR) pathway, and non-homologous end joining (NHEJ) pathway can rapidly repair DNA damage induced by the load, leading to DXd resistance in tumor cells.31
Immune Microenvironment Related Drug Resistance
The anti-tumor effect of ADC is closely related to the immune microenvironment, and an immunosuppressive microenvironment can impair the efficacy of ADC. Studies have shown that increased infiltration of M2-type tumor-associated macrophages (TAM) and regulatory T cells (Tregs) and overexpression of immune checkpoint molecules (PD-L1 and TIM-3) in the tumor microenvironment can inhibit ADC-induced anti-tumor immune responses and mediate drug resistance.32 In addition, overexpression of CD47 on tumor cells can inhibit the phagocytosis of macrophages on tumor cells and weaken the immunomodulatory effect of ADC, which is also an important drug resistance mechanism (Figure 2).
Figure 2.
ADC Drug Resistance Mechanisms.
Strategies to Address Drug Resistance
Numerous studies have explored the drug resistance mechanisms of ADC, leading to the development of explorations have been carried out and diverse coping strategies. Sequential treatment of ADC with different mechanisms of action: For patients with topoisomerase I inhibitor ADC (such as T-DXd) resistance, microtubule inhibitor ADC (such as vedicitumumab and MRG002) can be used. The loading mechanisms of the two drugs are different, and there is no cross-resistance. Several clinical studies have confirmed that they still have certain anti-tumor activities in patients with T-DXd resistance.33
Combination therapy to reverse drug resistance: Combination with endocrine therapy, TKI, immune checkpoint inhibitors, and PARP inhibitors can reverse drug resistance through a synergistic effect. For example, PI3K inhibitors can upregulate HER2 expression in tumor cells and enhance the binding and internalization of ADC. PARP inhibitors can inhibit DNA damage repair and enhance the cytotoxic effects of DXd. Anti-cd47 antibody can restore the phagocytic function of macrophages and enhance the immunomodulatory effect of ADC.
Research and development of novel ADC drugs: The development of the next generation of ADC drugs, such as dual-target, novel loading, and transmembrane transport ADC, is the core direction to overcome drug resistance. For example, targeting HER2/HER3 dual-target ADC reduces resistance mediated by target loss; ADC with pH-sensitive release properties can overcome drug resistance caused by the lysosomal transport barrier. ADC loaded with non-P-gp substrates can avoid drug efflux-mediated resistance.
Optimization of dose and administration regimen: By adjusting the dose and interval of administration or adopting a metronic administration mode, the effective drug concentration in tumor tissue can be maintained, the proliferation of drug-resistant clones can be inhibited, and the incidence of adverse reactions can be reduced, which provides a new drug administration strategy for drug-resistant patients.
Exploration of Combined Treatment Strategy of ADC Drugs
Although monotherapy employing ADCs delivers substantial clinical benefits, a subset of patients still fails to attain therapeutic remission and ultimately develops drug resistance and disease progression. To further elevate treatment efficacy and postpone the onset of resistance, numerous clinical trials have been initiated to explore ADC-based combination strategies. These studies investigated the synergistic actions of ADCs in conjunction with endocrine agents, immunotherapies, and targeted therapies, with the goal of refining and optimizing treatment regimens for advanced breast cancer characterized by low HER2 expression.
Combination Endocrine Therapy
Endocrine intervention is the cornerstone of clinical management for individuals with advanced breast cancer with HR-positive and low HER2 expression. This therapeutic modality can suppress the ER signaling cascade, promote elevated expression of HER2 in neoplastic cells, and enhance the target binding and internalization of ADC. ADC can effectively kill tumor cells resistant to endocrine therapy and overcome endocrine resistance. Relevant clinical investigation findings (NCT03500380) demonstrated that among individuals with disease progression after CDK4/6 inhibitor and endocrine treatment, the verified ORR of T-DXd in combination with fulvestrant (n=20) reached 40.0% (95% CI: 19.1%-63.9%), and the median PFS remained immature at the time of analysis. The ORR of combination therapy with anastrozole (aromatase inhibitor, n=21) was 71.4% (95% CI: 47.8%-88.7%). The combination regimen was generally well tolerated; however, compared with T-DXd monotherapy, the incidence of treatment-related adverse events of grade ≥3 was 55.0%, and the incidence of interstitial lung disease was 25.0% (all grade 2) in the combination fulvestrant group, and no new safety signals were observed.34 This study provides an important evidence-based basis for ADC combined with endocrine therapy, which is expected to promote this combination regimen as a first-line treatment and further improve the survival outcomes of patients.
Combined with Immune Checkpoint Inhibitors
ADC agents can convert immunologically quiescent “cold tumors” into active “hot tumors” by triggering immunogenic cell death and remodeling the tumor immune microenvironment, and they have a significant synergistic anti-tumor effect when combined with immune checkpoint inhibitors.35 Numerous clinical trials have verified the remarkable therapeutic effectiveness of this combination strategy. Corresponding research outcomes reveal that the ORR of T-DXd plus durvalumab reaches 50.0% in patients with advanced breast cancer exhibiting low HER2 expression, which is distinctly better than the historical data of single-agent therapy.36
Combined with HER2TKI
HER2TKIs (such as neratinib and tucatinib) irreversibly inhibit the HER2 tyrosine kinase activity, block downstream signaling pathways, and up-regulate HER2 expression in tumor cells, which has a dual synergistic effect when combined with ADC.In contrast, TKI can inhibit HER2 signaling pathway and enhance the anti-tumor effect of ADC. On the other hand, TKI can upregulate HER2 expression, increase ADC target binding, and overcome drug resistance mediated by target downregulation.37 Studies have shown that T-DXd combined with neratinib can effectively inhibit tumor growth and delay the occurrence of drug resistance in breast cancer models with low HER2 expression.38
Other Combination Treatment Strategies
In addition to the above combination regimens, combination therapy with ADCs and PI3K/AKT/mTOR pathway inhibitors, CDK4/6 inhibitors, PARP inhibitors, and antiangiogenic drugs is also being explored. Preclinical studies have shown that PI3K inhibitors can reverse ADC drug resistance mediated by the abnormal activation of the PI3K pathway. CDK4/6 inhibitors can enhance DXd-induced DNA damage and synergistically kill tumor cells. PARP inhibitors can inhibit DNA damage repair and have a significant synergistic effect with DXd.39 Currently, several clinical trials are underway to explore the efficacy and safety of these combination regimens, which are expected to provide more diversified treatment options for HER2-low advanced breast cancer.
Conclusion
HER2-low breast cancer, one of the most prevalent breast cancer subtypes, has long been characterized by a lack of effective targeted therapies and poor clinical outcomes. The advent of next-generation ADCs, particularly T-DXd, has fundamentally reshaped the treatment landscape. The DESTINY-Breast04 and DESTINY-Breast06 trials have established T-DXd as a new standard of care, demonstrating a median PFS of 8.8–13.2 months and an ORR of 52.6%–57.3% in heavily pretreated patients. In clinical practice, this evidence is now supporting the use of T-DXd in earlier lines of therapy, following progression on endocrine therapy and potentially before chemotherapy, particularly in patients with HR-positive disease. The approval of domestically developed ADCs such as disitamab vedotin has improved treatment accessibility for patients in China, while ongoing research on SHR-A1811 and MRG002 holds promise for further expanding therapeutic options.
Despite these advances, several challenges remain unresolved. There are currently no head-to-head trials comparing T-DXd with other HER2-targeted ADCs or with sacituzumab govitecan. The optimal sequencing of these agents and the identification of patients most likely to benefit from each remaining clinical questions. Furthermore, the management of long-term toxicities, particularly ILD, and the need to overcome acquired resistance in clinical settings represent urgent unmet needs.
To address these challenges, we propose the following priorities. For clinicians: T-DXd should be considered a preferred option over chemotherapy for patients with HER2-low advanced breast cancer who have progressed on endocrine therapy. Careful monitoring for ILD and proactive management of gastrointestinal toxicities are essential. For researchers: Priority should be given to identifying robust predictive biomarkers beyond HER2 IHC to optimize patient selection, elucidating the mechanisms underlying primary and acquired resistance, and developing rational combination strategies to enhance therapeutic efficacy. For policymakers: Standardization of HER2 testing to accurately identify low and ultralow expression is imperative to ensure appropriate patient selection for ADC therapy. With the continued development of next-generation ADCs, deeper elucidation of resistance mechanisms, optimization of combination strategies, and discovery of predictive biomarkers, the treatment of HER2-low breast cancer will continue to advance toward greater precision and personalization. Future efforts should focus on moving ADCs into earlier lines of treatment and expanding their application across the disease continuum, from metastatic to early-stage disease, with the ultimate goals of achieving long-term disease control and potentially curing patients with HER2-low breast cancer.
Funding Statement
There is no funding to report.
Data Sharing Statement
The data used during the current study are available from the corresponding author.
Ethics Declaration
This study was approved by the Ethics Committee of the First Affiliated Hospital of Zhejiang Chinese Medical University.
Consent for Publication
All have consented to the submission and publication of this manuscript.
Disclosure
The authors report no conflicts of interest in this work.
References
- 1.Bray F, Laversanne M, Sung H, et al. Global cancer statistics 2022: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries. CA Cancer J Clin. 2024;74(3):229–12. doi: 10.3322/caac.21834 [DOI] [PubMed] [Google Scholar]
- 2.Wei T, Wang D, Gao S, et al. Clinicopathologic characteristics and prognostic significance of HER2-low expression in patients with early breast cancer: a systematic review and meta-analysis. Front Oncol. 2023;13:1100332. doi: 10.3389/fonc.2023.1100332 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Cil I, Kucukarda A, M AM, et al. Efficacy and safety of trastuzumab emtansine in older patients with HER2-positive advanced breast cancer: a real-world study. Tumori. 2022;108(1):19–25. doi: 10.1177/03008916211037739 [DOI] [PubMed] [Google Scholar]
- 4.J CRV, L MM, C WW. Antibody–drug conjugates: an emerging concept in cancer therapy. Angew Chem Int Ed. 2014;53(15):3796–3827. doi: 10.1002/anie.201307628 [DOI] [PubMed] [Google Scholar]
- 5.Modi S. Trastuzumab deruxtecan in previously treated HER2-low advanced breast cancer: a plain language summary of the DESTINY-Breast04 study. Future Oncol. 2025;21(4):367–380. doi: 10.1080/14796694.2024.2434388 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Li J, Jiang Z. Interpretation of key updates to the 2021 Chinese Clinical Oncology Society breast cancer diagnosis and treatment guidelines. Chin Med J. 2021;101(24):1835–1838. [Google Scholar]
- 7.Update G, Somerfield MR, Dowsett M. Human epidermal growth factor receptor 2 testing in breast cancer. Arch Pathol Lab Med. 2023;147:993–1000. doi: 10.5858/arpa.2023-0950-SA [DOI] [PubMed] [Google Scholar]
- 8.Tarantino P, Hamilton E, M TS, et al. HER2-low breast cancer: pathological and clinical landscape. J Clin Oncol. 2020;38(17):1951–1962. doi: 10.1200/JCO.19.02488 [DOI] [PubMed] [Google Scholar]
- 9.Basem Y, Ata A, Sherif A, et al. Mechanisms of resistance and synergy: the role of tumor microenvironment in her2-low breast cancer therapy. Pharmaceuticals. 2026;19(4):541. doi: 10.3390/ph19040541 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Denkert C, Seither F, Schneeweiss A, et al. Clinical and molecular characteristics of HER2-low-positive breast cancer: pooled analysis of individual patient data from four prospective, neoadjuvant clinical trials. Lancet Oncol. 2021;22(8):1151–1161. doi: 10.1016/S1470-2045(21)00301-6 [DOI] [PubMed] [Google Scholar]
- 11.F WA, El-Tanani M, Talath S, et al. Antibody–drug conjugates and beyond: next-generation targeted therapies for breast cancer. Cancers. 2025;17(24):3943. doi: 10.3390/cancers17243943 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Yao X, Lu Y, Shi S, et al. Antibody-drug conjugates (ADCs): smart missiles targeting cancer therapy. Colloids Surf B Biointerfaces. 2025;257:115161. doi: 10.1016/j.colsurfb.2025.115161 [DOI] [PubMed] [Google Scholar]
- 13.Wang Y, Cheng X, Li X, et al. Bystander effect in antibody-drug conjugates: navigating the fine line in tumor heterogeneity. Crit Rev Oncol Hematol. 2025;216:104979. doi: 10.1016/j.critrevonc.2025.104979 [DOI] [PubMed] [Google Scholar]
- 14.Lim J, Park S, Hwang S, et al. Recent trends in conjugation methods of antibody drug conjugate. Yakhak Hoeji. 2025;69(1):1–15. doi: 10.17480/psk.2025.69.1.1 [DOI] [Google Scholar]
- 15.M GD, M SR. Sacituzumab govitecan, a novel, third-generation, antibody-drug conjugate (ADC) for cancer therapy. Expert Opin Biol Ther. 2020;20(8):871–885. doi: 10.1080/14712598.2020.1757067 [DOI] [PubMed] [Google Scholar]
- 16.C TL, S WJ, Ma X, et al. Effective extracellular payload release and immunomodulatory interactions govern the therapeutic effect of trastuzumab deruxtecan (T-DXd). Nat Commun. 2025;16(1):3167. doi: 10.1038/s41467-025-58266-8 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Modi S, Jacot W, Yamashita T, et al. Trastuzumab deruxtecan in previously treated HER2-low advanced breast cancer. N Engl J Med. 2022;387(1):9–20. doi: 10.1056/NEJMoa2203690 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18.Bardia A, Hu X, Dent R, et al. Trastuzumab deruxtecan after endocrine therapy in metastatic breast cancer. 2024;391(22):2110–2122. doi: 10.1056/NEJMoa2407086 [DOI] [PubMed] [Google Scholar]
- 19.Yang S, Chai C, Wang H. Research progress of antibody-drug conjugates targeting human epidermal growth factor receptor in breast cancer. Med Recapitul. 2023;29(14):2788–2793. [Google Scholar]
- 20.Wang J, Liu Y, Zhang Q, et al. Disitamab vedotin, a HER2-directed antibody-drug conjugate, in patients with HER2-overexpression and HER2-low advanced breast cancer: a phase I/Ib study. Cancer Commun. 2024;44(7):833–851. doi: 10.1002/cac2.12577 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Wang J, Liu Y, Zhang Q, et al. Disitamab vedotin, a her2-directed antibody drug-conjugate, in patients with her2-positive and her2-low advanced breast cancer: a phase 1/1b study. 2023. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Guo Q, Zhang J. Progress in targeted therapy for HER2-low expression breast cancer. Chin Oncol. 2023;33(2):181–190. [Google Scholar]
- 23.Liu X, Zhuang C, Song Y, et al. An exploratory clinical study investigating the neoadjuvant treatment of HER2-low expressing, stage II-III breast cancer with disitamab vedotin combined with penpulimab (RCVDBCIIR005). 2025.
- 24.Yao H, Yan M, Tong Z, et al. SHR-A1811, a novel HER2-targeting antibody-drug conjugate, in advanced solid tumors (HORIZON-X): a global phase 1 trial. Signal Transduct Target Ther. 2026;11(1):104. doi: 10.1038/s41392-026-02612-9 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25.Zhang S, Sun T, Wang X, et al. Efficacy and safety of MRG002 monotherapy in treating patients with locally advanced or metastatic breast cancer with low HER2 expression: a multi-center, non-randomized, open-label phase II clinical trial[J]. Eur J Cancer. 2025;230:115794. doi: 10.1016/j.ejca.2025.115794 [DOI] [PubMed] [Google Scholar]
- 26.Saura C, Thistlethwaite F, Banerji U, et al. A phase I expansion cohorts study of SYD985 in heavily pretreated patients with HER2-positive or HER2-low metastatic breast cancer. J Clin Oncol. 2018;36:1014. doi: 10.1200/JCO.2018.36.15_suppl.1014 [DOI] [Google Scholar]
- 27.Murase Y, Nanjo S, Ueda T, et al. Mechanisms of resistance to antibody–drug conjugates in cancers[J]. Respir Investig. 2025;63(4):693–698. doi: 10.1016/j.resinv.2025.05.012 [DOI] [PubMed] [Google Scholar]
- 28.Saleh K, Khoury R, Khalife N, et al. Mechanisms of action and resistance to anti-HER2 antibody-drug conjugates in breast cancer. Cancer Drug Resist. 2024;7:22. doi: 10.20517/cdr.2024.06 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29.Hao Y, Song Z. Mechanisms of resistance to antibody-drug conjugates in cancer therapy: molecular basis and therapeutic strategies. Cancer Drug Resist. 2025;8:59. doi: 10.20517/cdr.2025.148 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30.S RJ, Guo H, Chen L, et al. Identification of antibody-drug conjugate payloads that are substrates of ATP-binding cassette drug efflux transporters. Cancer Drug Resist. 2026;9:2. doi: 10.20517/cdr.2025.151 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 31.Wen X, Wang W, Song J, et al. Resistance mechanisms and post-trastuzumab deruxtecan (t-dxd) strategies in her2+ and her2-low breast cancer: from biology to clinical practice. Crit Rev Oncol Hematol. 2026;221:105179. doi: 10.1016/j.critrevonc.2026.105179 [DOI] [PubMed] [Google Scholar]
- 32.Valle I, Grinda T, Antonuzzo L, et al. Antibody–drug conjugates in breast cancer: mechanisms of resistance and future therapeutic perspectives. NPJ Breast Cancer. 2025;11(1):102. doi: 10.1038/s41523-025-00829-5 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 33.Pourjamal N, Le Joncour V, Vereb G, et al. Disitamab vedotin in preclinical models of HER2-positive breast and gastric cancers resistant to trastuzumab emtansine and trastuzumab deruxtecan. Transl Oncol. 2025;53:102284. doi: 10.1016/j.tranon.2025.102284 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 34.Jhaveri K, Loi S, Hamilton E, et al. DESTINY-Breast08: a phase 1b study of trastuzumab deruxtecan in combination with other anticancer therapies in patients with HER2-low metastatic breast cancer. Clin Cancer Res. 2026;32:1046–1058. doi: 10.1158/1078-0432.CCR-25-0874 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 35.Wang Y, Lu K, Xu Y, et al. Antibody-drug conjugates as immuno-oncology agents in colorectal cancer: targets, payloads, and therapeutic synergies. Front Immunol. 2025;16:1678907. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 36.Tamura K, Tsurutani J, Takahashi S, et al. Trastuzumab deruxtecan (DS-8201a) in patients with advanced HER2-positive breast cancer previously treated with trastuzumab emtansine: a dose-expansion, phase 1 study. Lancet Oncol. 2019;20(6):816–826. doi: 10.1016/S1470-2045(19)30097-X [DOI] [PubMed] [Google Scholar]
- 37.Olson D, Taylor J, Willis K, et al. HER2-selective and reversible tyrosine kinase inhibitor tucatinib potentiates the activity of T-DM1 in preclinical models of HER2-positive breast cancer. Cancer Res Commun. 2023;3(9):1927–1939. doi: 10.1158/2767-9764.CRC-23-0302 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 38.Arshad M, Azad A, K CPY, et al. Neratinib could be effective as monotherapy or in combination with trastuzumab in HER2-low breast cancer cells and organoid models. Br J Cancer. 2024;130(12):1990–2002. doi: 10.1038/s41416-024-02665-z [DOI] [PMC free article] [PubMed] [Google Scholar]
- 39.M RC, B GA, Devineni A, et al. CDK4/6 inhibitors promote PARP1 degradation and synergize with PARP inhibitors in non-small cell lung cancer. Transl Oncol. 2025;52:102231. doi: 10.1016/j.tranon.2024.102231 [DOI] [PMC free article] [PubMed] [Google Scholar]
Associated Data
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Data Availability Statement
The data used during the current study are available from the corresponding author.