Trastuzumab emtansine: mechanisms of action and drug resistance

Tucatinib approval by EMA expands options for HER2-positive locally advanced or metastatic breast cancer

Does adjuvant therapy reduce postmetastatic survival?

The current review describes the rationale and current clinical development of antibody drug conjugates (ADCs), along with their perspectives for the future. Trastuzumab emtansine was the first ADC approved by the U.S. Food and Drug Administration for the treatment of human epidermal growth factor receptor 2 positive metastatic breast cancer in the second-line setting, with a high efficacy and a favorable safety profile. ADC represents an exciting new class of cancer therapeutics that combines a targeted approach for delivering cytotoxic agents. About 30 new ADCs are currently under investigation in oncology. ADCs are empowered antibodies designed to exploit the targeting ability of monoclonal antibodies (mAbs) by linking them to cytotoxic agents, giving them higher tumor selectivity, and potentially an increased therapeutic window, as compared with cytotoxic agents alone. The key components of ADCs include a mAb, a stable linker and a cytotoxic agent. In linking mAbs with cytotoxic agents, the aim is to optimize the properties of both components, bringing their complementary features together. Trastuzumab emtansine has been the first ADC to be marketed in human epidermal growth factor receptor 2 positive metastatic breast cancer. Current clinical development of ADCs includes a variety of targets, as well as combinations with other therapeutic agents, such as chemotherapy, radiotherapy, targeted therapies, and immune checkpoint inhibitors.

Ado-trastuzumab emtansine (Kadcyla—Genentech) has received FDA approval for the management of human epidermal growth factor receptor 2 (HER2)–positive metastatic breast cancer. The therapy is specifi cally indicated for those who have already undergone unsuccessful treatment with trastuzumab (Herceptin—Genentech) and a taxane. Trastuzumab emtansine is an antibody–drug conjugate comprising the antibody trastuzumab and the chemotherapy drug DM1 attached using a stable linker.

The advent of antibody-drug conjugates (ADCs) represents a landmark advance in cancer therapy, permitting targeted delivery of a potent cytotoxic agent to tumor cells with minimal damage to surrounding cells. Although ADCs can induce sustained therapeutic responses in heavily pretreated patients, they can also cause significant toxicity and thus require careful monitoring. The prototype ADC, ado-trastuzumab emtansine (T-DM1) is comprised of a humanized, monoclonal human epidermal growth factor receptor 2 (HER2)-directed antibody, trastuzumab, linked to the cytotoxic agent, DM1, and is used for the treatment of early-stage and advanced HER2-positive breast cancer. Liver toxicities, including transaminitis and nodular regenerative hyperplasia resulting in portal hypertension have been described. We report a case series of four patients who developed hepatopulmonary syndrome (HPS) during treatment with T-DM1. HPS is characterized by hypoxemia, portal hypertension, and intrapulmonary shunting, and it can be associated with severe hypoxic respiratory failure. HPS secondary to noncirrhotic portal hypertension occurring with long-term exposure to T-DM1 has not previously been reported. Four patients who received T-DM1 in our institutional cohort (n=230) developed HPS, which can be associated with severe hypoxic respiratory failure. Each patient diagnosed with HPS received >50 doses of T-DM1. Only one patient at diagnosis had resting hypoxia, while the other three patients became hypoxic with exertion only. Discontinuation of T-DM1 led to clinical improvement in hypoxia in three of the four patients. The spectrum of liver injury that occurs with long-term use of T-DM1 remains incompletely defined. As T-DM1 is approved for use in the management of early-stage operable and advanced breast cancer, awareness of HPS as a potential complication of long-term administration of T-DM1 is necessary. The emergence of dyspnea alone or combined with low oxygen saturation and signs of hypoxemia (clubbing or elevated hemoglobin) should raise clinical suspicion and prompt evaluation for HPS. Cancer care team members should be vigilant regarding the potential for new and serious side effects associated with novel targeted therapies, which may emerge years beyond initial regulatory approval.

Antibody-drug conjugates (ADCs) represent a transformative class of targeted therapies designed to deliver potent cytotoxic agents specifically to tumor cells, minimizing systemic toxicity. This review provides a comprehensive overview of ADCs, detailing their mechanisms of action, design strategies, and clinical advancements. ADCs utilize monoclonal antibodies to selectively bind tumor-associated antigens, enabling the precise delivery of toxic payloads to cancer cells. The review explores the critical components of ADCs, including the antibody, linker, and payload, and highlights how these elements can be optimized to improve efficacy and minimize off-target effects. We examine the evolution of ADC design from early constructs to the latest innovations and the development of novel payloads that extend therapeutic possibilities beyond traditional cytotoxic agents. Additionally, we discuss the clinical success of ADCs, with examples from approved therapies such as gemtuzumab ozogamicin, brentuximab vedotin, and trastuzumab emtansine, which have redefined the treatment landscape for various cancers. Despite their success, ADCs face challenges such as tumor heterogeneity, resistance mechanisms, and toxicity, which are actively being addressed through ongoing research. The review concludes with an outlook on the future of ADCs, highlighting emerging strategies in conjugation technology, payload design, and combination therapies that are poised to enhance their therapeutic potential across oncology and other disease areas.

Overcoming Challenges in the Development of Anticancer ADCs

Antibody-drug conjugates (ADCs), comprised of cancer cell-specific antibodies linked to cytotoxic drugs, represent an innovative and promising class of anticancer agents. ADCs often rely on high receptor numbers for target binding, followed by internalization and release of the payload to exert a tumor-targeted, intracellular effect. Determination of the exact number of receptors per cell is therefore highly valuable in the context of ADCs. Here, we focused on the quantification of the human epidermal growth factor receptor 2 (HER2), since expression of this target is often amplified in various tumor types. Two HER2-targeting ADCs, i.e., ado-trastuzumab emtansine (T-DM1) and fam-trastuzumab deruxtecan-nxki (T-DXd), have already been approved for clinical use, while roughly twenty HER2-targeting ADCs are currently under investigation. The development and improvement of ADCs could benefit from in vitro studies investigating the internalization, cytotoxic effect and possible bystander effect of ADCs related to receptor numbers. The number of HER2 per cell was quantified in a panel of thirteen cancer cell lines originating from different cancer types. The cell lines BxPC-3, CAL-27, COLO 205, HCC1954, HEC-251, K-562, MOLM-13, NCI-H23, RT-112, SK-BR-3, SK-OV-3, SW837 and U-87MG ATCC were cultured for one week in the supplier’s recommended medium. Quantification of the number of surface-expressed receptors was performed with flow cytometry using Quantibrite PE beads and PE-conjugated ErbB2/HER2 antibody. The effect of T-DM1 and T-DXd on the viability of HER2 high-, intermediate- and low-expressing cells was determined in proliferation assays in 9-point dose ranges. Three independent flow cytometry experiments revealed that the breast and ovarian cancer cell lines HCC1954, SK-BR-3, and SK-OV-3 expressed the highest number of HER2 per cell, ranging between 20, 000 and 53, 000 receptors. Intermediate expression was observed on the COLO 205, HEC-251, RT-112 and SW837 cell lines, with a range of 1800 to 3800 receptors per cell. BxPC-3, CAL-27, K-562, MOLM-13, NCI-H23 and U-87MG ATCC belong to the group with the lowest or no expression of HER2, ranging up to 400 receptors per cell. T-DM1 and T-DXd induced efficient cell death in cells expressing high receptor numbers, while little or no effect was observed in cell lines with intermediate, low or no expression of HER2. Since high target expression is correlated with higher efficacy for several ADCs, as shown for T-DM1 and T-DXd is our study, this indicates that HCC1954, SK-BR-3 or SK-OV-3 cells could serve as an attractive model to study HER2 internalization. Moreover, co-cultures of HER2-positive and HER2-negative cells could aid in the investigation of ADC bystander effect. This research demonstrates the importance of receptor number quantification in the context of ADCs and may therefore contribute to future ADC development. Citation Format: Mandy W. Drossaert-Smeets, Eef F. Smits, Milan M. Bolwerk, Janneke J. Melis, Dimitri Pappaioannou, Guido J. Zaman. Receptor quantification in the context of human epidermal growth factor receptor 2 (HER2) antibody-drug conjugates [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1564.

To review data relating to anthracyclines in the adjuvant treatment of early breast cancer. This is a report from a seminar in which the future of anthracyclines in the adjuvant treatment of breast cancer was considered. In particular, the question of whether anthracyclines should now be discarded and replaced by taxanes was addressed. Accumulating data from large randomized trials indicate that genetic markers may have a role in predicting sensitivity to cytotoxic drugs. However, no reliable, validated test is available for predicting sensitivity to anthracyclines in particular. Topoisomerase IIalpha amplification and/or deletion, especially in conjunction with human epidermal growth factor receptor-2 amplification, has been proposed to fulfill this role but more data are needed. Currently, only one published trial has shown that a taxane-based regimen may be superior to an anthracycline-based regimen, but several trials indicate that combinations including both anthracyclines and taxanes may be better still. Further studies aimed at optimizing anthracyclines and taxanes in combination, and integrating biologic agents, seem to be the way forward. There is no validated test that can determine whether anthracyclines can be of greater benefit than other agents for individual patients. Anthracyclines have been extensively tested in clinical trials spanning several decades; currently, there are insufficient data to recommend replacing them in the adjuvant treatment of breast cancer.

The HER2 receptor tyrosine kinase is amplified in approximately 20% of human breast cancer and is associated with poor clinical outcome. The humanized antibodies trastuzumab and pertuzumab are approved for use in both early and metastatic HER2-positive breast cancer, and are most often given with chemotherapy. Antibody-drug conjugates (ADCs) are anti-tumor agents designed to deliver potent cytotoxic drugs selectively to target-expressing tumor cells. Trastuzumab emtansine is a HER2-directed ADC comprised of trastuzumab covalently linked to the microtubule inhibitor DM1, through the stable MCC linker. Trastuzumab emtansine is approved for use in HER2-positive metastatic breast cancer as a single agent in patients who have received prior trastuzumab and a taxane. We are now exploring new HER2-directed ADCs (‘2nd generation ADCs’) with different mechanisms of action (MOA) than trastuzumab emtansine by investigating ADCs utilizing DNA-damaging agents, such as pyrrolobenzodiazepine (PBD) dimers and cyclopropylbenzindole (CBI) dimers, as the cytotoxic drug components. These agents have been conjugated to either trastuzumab or the humanized anti-HER2 antibody 7C2 (hu7C2) using both uncleavable and cleavable linkers. As free drugs and ADCs, the PBDs and CBIs show similar or greater potency in cell proliferation assays in vitro compared to DM1 and trastuzumab emtansine. However, unlike DM1, these agents are not strong substrates of Pgp/MDR1. Moreover, the PBDs and CBIs are active on non-dividing cells, whereas microtubule inhibitors such as DM1 do not affect non-dividing cells. Robust anti-tumor activity was observed in vivo in the fo5 HER2 transgenic tumor transplant model with the 2nd generation ADCs. Efficacious doses resulting in tumor stasis or regression ranged from 0.25-3 mg/kg administered as a single injection. In contrast, doses of trastuzumab emtansine required for stasis/regression in this model are 10 and 15 mg/kg, respectively. Efficacious doses were well-tolerated in the mouse xenograft models. Further tolerability studies of the 2nd generation ADCs were performed in rats. As rats are a non-binding species for trastuzumab and hu7C2, these studies assessed antigen-independent toxicities. Maximum tolerated doses for the different ADCs ranged from 2.5-15 mg/kg administered as a single injection, compared to 46 mg/kg for trastuzumab emtansine (Poon et al., 2013), likely reflecting both the different MOA and greater potency of the cytotoxic agents utilized in the 2nd generation HER2-directed ADCs. Overall, our findings demonstrate robust in vitro and in vivo activity of HER2 ADCs comprised of DNA-active agents, allowing for further development of a HER2-directed ADC distinct from trastuzumab emtansine. Citation Format: Gail D. Lewis Phillips, Guangmin Li, Jun Guo, Jeffrey Lau, Shang-Fan Yu, Thomas Pillow, Byoung-Chul Lee, Jack Sadowsky, Melissa Schutten, Carter Fields, Mark X. Sliwkowski. Preclinical development of 2nd generation HER2-directed antibody-drug conjugates. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1207.

Antibody-drug conjugates (ADCs) employ overexpressed cell surface antigens to deliver cytotoxic payloads inside cancer cells. However, the relationship between target expression and ADC efficacy remains ambiguous. In this manuscript, we have addressed a part of this ambiguity by quantitatively investigating the effect of antigen expression levels on ADC exposure within cancer cells. Trastuzumab-valine-citrulline-monomethyl auristatin E was used as a model ADC, and four different cell lines with diverse levels of human epidermal growth factor receptor 2 (HER2) expression were used as model cells. The pharmacokinetics (PK) of total trastuzumab, released monomethyl auristatin E (MMAE), and total MMAE were measured inside the cells and in the cell culture media following incubation with two different concentrations of ADC. In addition, target expression levels, target internalization rate, and cathepsin B and MDR1 protein concentrations were determined for each cell line. All the PK data were mathematically characterized using a cell-level systems PK model for ADC. It was found that SKBR-3, MDA-MB-453, MCF-7, and MDA-MB-468 cells had ∼800,000, ∼250,000, ∼50,000, and ∼10,000 HER2 receptors per cell, respectively. A strong linear relationship (R 2 > 0.9) was observed between HER2 receptor count and released MMAE exposure inside the cancer cells. There was an inverse relationship found between HER2 expression level and internalization rate, and cathepsin B and multidrug resistance protein 1 (MDR1) expression level varied slightly among the cell lines. The PK model was able to simultaneously capture all the PK profiles reasonably well while estimating only two parameters. Our results demonstrate a strong quantitative relationship between antigen expression level and intracellular exposure of ADCs in cancer cells. SIGNIFICANCE STATEMENT: In this manuscript, we have demonstrated a strong linear relationship between target expression level and antibody-drug conjugate (ADC) exposure inside cancer cells. We have also shown that this relationship can be accurately captured using the cell-level systems pharmacokinetics model developed for ADCs. Our results indirectly suggest that the lack of relationship between target expression and efficacy of ADC may stem from differences in the pharmacodynamic properties of cancer cells.

Effects of third-generation aromatase inhibitors on bone

Bone metastases in breast cancer

To report a case of atypical corneal lesions presumably induced by trastuzumab emtansine, an antibody-drug conjugate that is designed to selectively deliver cytotoxic agents to human epidermal growth factor receptor 2 (HER2)-positive breast cancer cells. A 64-year-old Japanese woman developed bilateral corneal epithelial abnormalities that originated from the limbus. The corneal lesions covered the superior area in the right eye and both superior and inferior areas including the visual axis in the left eye. The patient had advanced ductal carcinoma of the left breast and had been receiving anticancer treatment with trastuzumab emtansine for 15 months. After switching the chemotherapy from trastuzumab emtansine monotherapy to the combination of docetaxel, trastuzumab, and pertuzumab, the abnormal corneal lesions showed gradual improvement. As corneal epithelial cells express human epidermal growth factor receptor 2 under normal conditions, such cells may also be targeted by trastuzumab emtansine and lead to corneal epithelial lesions.

Introduction: Trastuzumab emtansine (T-DM1) is an antibody–drug conjugate (ADC) that combines intracellular delivery of the potent cytotoxic agent, DM1 (a derivative of maytansine) with the antitumor activity of trastuzumab. While there are several ADCs in Phase III development, T-DM1 is the only one in which the targeting antibody has antitumor properties. T-DM1 is also the only ADC that is directed toward the human EGFR 2 (HER2). Effective therapies are limited in HER2-positive advanced or metastatic breast cancer (MBC), particularly following progression on available HER2-targeted therapies.Areas covered: The mechanisms of action, preclinical efficacy and clinical profile of T-DM1 are reported. The latest preclinical and clinical data for T-DM1 are examined.Expert opinion: T-DM1 has significant antitumor potency in vitro and in vivo, which is maintained in tumors resistant to trastuzumab or lapatinib. In Phase I and II trials, T-DM1 provided objective tumor responses and was well tolerated across various lines of therapy in patients with HER2-positive MBC. In addition, it showed similar efficacy to trastuzumab plus docetaxel in first-line MBC. Ongoing trials (including two Phase III studies) are investigating T-DM1 as single-agent therapy or combined with other chemotherapeutic or biologic agents, and the results should help to define the place of T-DM1 within current treatment algorithms for HER2-positive disease.