3D layered co-culture model enhances Trastuzumab Deruxtecan sensitivity and reveals the combined effect with G007-LK in HER2-positive non-small cell lung cancer.

Human epidermal growth factor receptor 2 (HER2) aberrations are observed in various cancers. In non-small cell lung cancer, genetic alterations activating HER2, mostly exon 20 insertion mutations, occur in approximately 2-4% of cases. Trastuzumab deruxtecan (T-DXd), a HER2-targeted antibody-drug conjugate has been approved as the first HER2-targeted drug for HER2-mutant lung cancer. However, some cases are not responsive to T-DXd and the primary resistant mechanism remains unclear. In this study, we assessed sensitivity to T-DXd in JFCR-007, a patient-derived HER2-mutant lung cancer cell line. Although JFCR-007 was sensitive to HER2 tyrosine kinase inhibitors, it showed resistance to T-DXd in attachment or spheroid conditions. Accordingly, we established a three-dimensional (3D) layered co-culture model of JFCR-007, where it exhibited a lumen-like structure and became sensitive to T-DXd. In addition, an in-house inhibitor library screening revealed that G007-LK, a tankyrase inhibitor, was effective when combined with T-DXd. G007-LK increased the cytotoxicity of topoisomerase-I inhibitor, DXd, a payload of T-DXd and SN-38. This combined effect was also observed in H2170, an HER2-amplified lung cancer cell line. These results suggest that the proposed 3D co-culture system may help in evaluating the efficacy of T-DXd and may recapitulate the tumor microenvironment.

Clinical benefit of post-trastuzumab deruxtecan treatment in patients with HER 2-positive unresectable or metastatic breast cancer: A single-institution retrospective observational study.

PURPOSE: Trastuzumab deruxtecan (T-DXd) can improve the prognosis of patients with human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (MBC). However, data on treatment recommendations after T-DXd are lacking. Thus, this study aimed to evaluate the treatment options after T-DXd and their effectiveness. METHODS: Patients with HER2-positive MBC were included in this study. Data from clinical records were retrospectively analyzed. The primary outcome was time to treatment failure (TTF). Secondary endpoints were TTF of each treatment and first-line treatment after interstitial lung disease (ILD) and overall survival (OS). RESULTS: A total of 29 patients were included. Among them, 18 (62%) were hormone receptor-positive. All patients had a median TTF (mTTF) of 3.5 months (95% confidence interval (CI) 2.1-10.03). The mTTF of each treatment, including HER2 tyrosine kinase inhibitor (HER2 TKI), other anti-HER2 treatments, and other treatments, was 2.6, 8.8, and 3.8 months, respectively. No significant differences were observed between treatments, but regimens that include trastuzumab showed a longer TTF than TKI. However, the mTTF among patients who developed T-DXd-related ILD was 2.33 months (95% CI 0.7-not reached), which was shorter than that among those who did not develop ILD (3.83 months, 95% CI 2.1-10.03, hazard ratio: 2.046, 95% CI 0.760-5.507, p = 0.258). The median OS was 14.9 months (95% CI 11.07-29.17). CONCLUSION: Treatments after T-DXd showed a shorter mTTF. Regimens that include trastuzumab may be more effective post-T-DXd treatment than HER2 TKI. Further data are needed to establish the best sequential treatment after T-DXd.

Correlation of HER2 Protein Level With mRNA Level Quantified by RNAscope in Breast Cancer.

Trastuzumab deruxtecan (T-DXd) has been approved by the US Food and Drug Administration (FDA) to treat patients with metastatic HER2-positive and HER2-low breast cancer, and clinical trials are examining its efficacy against early-stage breast cancer. Current HER2 immunohistochemical (IHC) assays are suboptimal in evaluating HER2-low breast cancers and identifying which patients would benefit from T-DXd. HER2 expression in 526 breast cancer tissue microarray (TMA) cores was measured using the FDA-approved PATHWAY and HercepTest IHC assays, and the corresponding RNA levels were evaluated by RNAscope. HER2 protein levels by regression analysis using a quantitative immunofluorescence score against cell line arrays with known HER2 protein levels determined by mass spectrometry were available in 48 of the cores. RNAscope was also performed in 32 metastatic biopsies from 23 patients who were subsequently treated with T-DXd, and the results were correlated with response rate. HER2 RNA levels by RNAscope strongly correlated with HER2 protein levels (P < .0001) and with HER2 IHC H-scores from the PATHWAY and HercepTest assays (P < .0001). However, neither protein levels nor RNA levels significantly differed between cases scored 0, ultralow, and 1+ by PATHWAY and HercepTest. The RNA levels were significantly higher (P = .030) in responders (6.4 ± 8.2 dots/cell, n = 12) than those in nonresponders (2.6 ± 2.2, n = 20) to T-DXd. RNAscope is a simple assay that can be objectively quantified and is a promising alternative to current IHC assays in evaluating HER2 expression in breast cancers, especially HER2-low cases, and may identify patients who would benefit from T-DXd.

Real-world effectiveness and safety of trastuzumab-deruxtecan in Japanese patients with HER2-positive advanced gastric cancer (EN-DEAVOR study).

BACKGROUND: Trastuzumab-deruxtecan (T-DXd) was approved for the treatment of HER2-positive patients with advanced gastric cancer in Japan based on the results of the DESTINY-Gastric01 trial. This study aimed to collect real-world data and evaluate the effectiveness and safety of T-DXd. METHODS: Patients aged ≥ 20 years at the start of T-DXd administration with a histopathologically confirmed diagnosis of HER2-positive unresectable advanced or recurrent gastric or gastroesophageal junction (GEJ) adenocarcinoma that had worsened after chemotherapy were enrolled in this retrospective cohort study. Key outcomes included T-DXd treatment status, overall survival (OS), real-world progression-free survival (rwPFS), time to treatment failure (TTF), objective response rate and frequency of grade ≥ 3 adverse events (AEs). RESULTS: Of the 312 patients included in the analysis, 75.3% were male, the median (range) age was 70.0 (27.0-89.0) years, 12.2% had an ECOG PS ≥ 2, 43.3% had ascites and the initial T-DXd dose was > 5.4- ≤ 6.4 mg/kg in 78.2% of patients. The median (95% confidence interval) OS, rwPFS and TTF (months) was 8.9 (8.0-11.0), 4.6 (4.0-5.1) and 3.9 (3.4-4.2), respectively. The response rate was 42.9% in patients with a target lesion. In total, 48.4% of patients experienced a grade ≥ 3 AE, 2.6% experienced grade 5 AEs and 60.9% experienced AEs leading to T-DXd dose adjustments (reduction: 36.9%, interruption: 34.0% or discontinuation: 23.7%). No new safety signals were detected. CONCLUSIONS: T-DXd was effective and had a manageable safety profile as a third- or later-line treatment for patients with HER2-positive gastric or GEJ cancer in Japanese clinical practice. CLINICAL TRIAL REGISTRATION: UMIN000049032.

Cardiac toxicity of HER-2 targeting antibody-drug conjugates: overview and clinical implications.

Antibody-drug conjugates (ADCs) have recently emerged as a promising therapeutic option that combine the specificity of monoclonal antibodies and the cytotoxic effect of chemotherapy. With numerous ADCs approved and on the market, a particular concern of ADCs that target HER-2 has been their cardiac side effects, in view of the crucial role of HER-2 in cardiac development and physiology. While rarely toxic and generally safe, numerous publications have outlined the consistent association of trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd) with the development of cardiac toxicity. Despite not being clinically relevant in most cases, cardiac baseline evaluation, monitoring and early detection of cardiac adverse events remain pivotal with HER-2 targeting ADCs. This review aims to summarize and better characterize the complete cardiac toxicity profile of HER-2 ADCs, with the goal of improving clinical understanding of this adverse event, leading to better recognition, monitoring and management.

[Box: see text].

Opportunities and Challenges for a Histology-Agnostic Utilization of Trastuzumab Deruxtecan.

PURPOSE OF REVIEW: This review delves into the prospects and challenges offered by a potential pan-histological utilization of trastuzumab deruxtecan (T-DXd) in patients with advanced solid tumors. RECENT FINDINGS: The HER2-targeted antibody-drug conjugate (ADC) T-DXd has shown broad activity across cancer types, with current indications for patients with biomarker-selected breast, gastric, and non-small-cell lung cancer and relevant activity observed in multiple histology-specific trials. Moreover, two recently reported phase 2 trials (DESTINY-Pantumor02 and HERALD) have supported the potential for a pan-cancer utilization of this ADC in patients with advanced cancers expressing HER2 or with HER2 amplifications. By improving the delivery of cytotoxic chemotherapy, ADCs have allowed for meaningful clinical advantages in broad populations of cancer patients, often leading to survival advantages over conventional chemotherapy. Notably, the broad spectrum of activity of certain ADCs has led to the hypothesis of a histology-agnostic utilization based on detecting specific biomarkers, similar to what is already established for certain targeted treatments and immunotherapy. To date, T-DXd has shown the broadest activity across cancer types, with current approvals in breast, gastric, and lung cancer, and relevant antitumor activity observed in a multiplicity of additional cancer types. The optimization of the drug dose, identification of predictive biomarkers, and clarification of mechanisms of resistance will be critical steps in view of a pan-histological expansion in the use of T-DXd.

The efficacy and safety of trastuzumab deruxtecan (T-DXd) in HER2-expressing solid tumours: a single-arm meta-analysis.

OBJECTIVE: We performed a meta-analysis to assess the efficacy and safety of T-DXd in the treatment of HER2-expressing solid tumours. METHODS: We systematically searched PubMed, Web of Science, Embase and the Cochrane Library and collected studies published before March 17, 2023, on T-DXd for HER2-expressing tumours for a meta-analysis. We performed a subgroup analysis based on the different cancer types and the doses used. RESULTS: There were 11 studies including 1349 HER2-expressing patients in this meta-analysis. The pooled ORR was 47.91%, and the pooled DCR was 87.01%. The mPFS and mOS combined were 9.63 and 10.71 months, respectively. The most common adverse reactions in grades 1-2 were decreased appetite (49.3%) and vomiting (43.0%). The netropemia (31.2%) and leukopenia (31.2%) were the most common grade 3 and higher adverse reactions. Subgroup analysis showed that breast cancer had the best ORR and DCR, with 66.96 and 96.52%, respectively. CONCLUSIONS: Overall, the efficacy of T-DXd in treating HER2-expressing solid tumours is encouraging, especially breast and non-small cell lung cancers, and has an acceptable safety profile. However, concerns remain about potentially serious treatment adverse events (e.g. interstitial lung disease/pneumonia). More well-designed, large-scale randomized controlled trials are needed to demonstrate our study.

Transition of average drug-to-antibody ratio of trastuzumab deruxtecan in systemic circulation in monkeys using a hybrid affinity capture liquid chromatography-tandem mass spectrometry.

Trastuzumab deruxtecan (T-DXd, DS-8201a) is an antibody-drug conjugate, comprising an anti-HER2 antibody at a drug-to-antibody ratio of 7-8 with the topoisomerase I inhibitor DXd. In this study, the concentrations of antibody-conjugated DXd and total antibody were determined and observed to decrease over time following intravenous administration of T-DXd to monkeys. The drug-to-antibody ratio of T-DXd also decreased in a time-dependent manner, which reached approximately 2.5 in 21 days after administration. It was suggested that antibody-conjugated DXd of T-DXd was relatively stable in vivo compared with that of other reported antibody-drug conjugates.

HER2 Low Breast Cancer: A New Subtype or a Trojan for Cytotoxic Drug Delivery?

Despite the great progress made in the understanding of the biological behavior of certain types of invasive breast cancer, there is still no single histological or molecular classification that encompasses such diversity and accurately predicts the clinical course of distinct breast cancer subtypes. The long-lasting classification of breast cancer as HER2-positive vs. HER2-negative has recently come into question with the discovery of new antibody drug conjugates (ADC), which are proven to be remarkably efficient in treating HER2-low breast cancer. The HER2-low paradigm has challenged the traditional understanding of HER2 overexpression and emphasized the need for more robust HER2 testing in order to encompass HER2 intratumoral heterogeneity and spatial distribution more accurately. It is yet to be seen if low HER2 will remain merely a marker of HER2-equipped tumors targetable with ADCs or if distinctive molecular and phenotypic groups within HER2-low tumors will eventually be discerned.

Trastuzumab deruxtecan for HER2+ advanced breast cancer.

Trastuzumab deruxtecan (T-DXd, DS-8201), an anti-HER2 antibody-drug conjugate, has shown significant clinical benefits in HER2+ metastatic breast cancer patients. In the phase 2 DESTINY-Breast01 trial, T-DXd demonstrated an objective response of 60.9% and median progression-free survival of 16.4 months, laying the foundation for accelerated approval in HER2+ metastatic breast cancer patients who have received two or more prior anti-HER2-based regimens in the metastatic setting. Moreover, T-DXd exhibited promising antitumor efficacy against HER2-low-expressing metastatic breast cancer. Its distinctive side effect was pneumonitis, with a 13.6% incidence. It is approved in the US with boxed warnings for interstitial lung disease and embryo-fetal toxicity. This review focuses on preclinical, pharmacokinetic and pharmacodynamic data on T-DXd and clinical evidence of its antitumor activity (both as monotherapy and in combination) and tolerability in metastatic breast cancer.

Lay abstract Breast cancer can be grouped based on its HR and HER2 status. For patients with HER2+ breast cancer, treatments that fight HER2 portion are adopted. Trastuzumab deruxtecan (DS-8201) is a new drug that consists of two parts: a cytotoxic drug and anti-HER2 antibody. It can selectively target HER2-expressing tumor cells. In a recent clinical trial, trastuzumab deruxtecan demonstrated tumor shrinkage in six of ten patients. The time to increase in tumor size or death was 16 months. Its antitumor effect was also demonstrated in low-HER2-expressing breast cancer. Approximately 13% of patients experience lung inflammation following trastuzumab deruxtecan treatment. In these cases, the drug should be promptly halted and the doctor can start steroid therapy.

HER2-low breast cancers: Current insights and future directions.

In light of the significant clinical benefits of novel HER2-targeting antibody-drug conjugates in advanced HER2-low expressing breast cancers in recent phases I and III clinical trials, particularly trastuzumab-deruxtecan (T-Dxd), the new "HER2-low" category in breast cancers (breast cancer with a HER2 IHC score of 1+, or 2+ without gene amplification) has gained increasing attention. In the past year, "HER2-low" breast cancers have been under active investigation by both oncologists and pathologists. In this current review, we update the recent cutting-edge research on HER2-low breast cancers, with a focus on the biology of HER2-low breast cancers, the issues on the identification of HER2-low breast cancers by immunohistochemistry in current practice of pathology, and the future directions in this emerging category in breast cancers.

Trastuzumab deruxtecan for the treatment of HER2-positive advanced gastric cancer: a clinical perspective.

Human epidermal growth factor receptor 2 (HER2)-positive gastric cancer is a subtype for which new drugs and specific treatment strategies should be developed. Trastuzumab deruxtecan (T-DXd) is a novel HER2-targeted antibody-drug conjugate containing topoisomerase I inhibitor as a payload. In the randomized phase 2 study (DESTINY-Gastric01) for HER2-positive advanced gastric or gastroesophageal junction cancer (AGC), patients treated with T-DXd showed a significantly higher response rate compared with the chemotherapy of physician's choice, associated with remarkably prolonged progression-free and overall survival. T-DXd also exhibits anti-tumor activity to HER2-negative tumor cells close to HER2-positive cells (so-called bystander killing effect). T-DXd was effective even for HER2-low expressing breast and gastric cancer in several clinical studies. Taking advantage of these strong points and synergism with other cytotoxic, molecular-targeted and immunological agents, it is expected that T-DXd will bring further progression in treatment both for strongly and weakly HER2 positive AGC in various treatment settings including perioperative chemotherapy.

HER2-Positive Gastric Cancer and Antibody Treatment: State of the Art and Future Developments.

Despite a decreasing incidence in Western countries, gastric cancer is among the most common cancer subtypes globally and is associated with one of the highest tumor-related mortality rates. Biomarkers play an increasing role in the treatment against gastric cancer. HER2 was one of the first biomarkers that found its way into clinical practice. Since the ToGA trial, trastuzumab has been part of first-line palliative chemotherapy in metastatic or unresectable gastric cancer. HER2-targeting agents, such as the tyrosine kinase inhibitor lapatinib, the antibody drug conjugate (ADC) trastuzumab-emtansine or dual HER2 inhibition (pertuzumab and trastuzumab), have been investigated in the second-line setting but led to negative study results. More recently, the ADC trastuzumab-deruxtecan was authorized after the failure of trastuzumab-based treatment. However, further improvements in HER2-directed therapy are required as resistance mechanisms and HER2 heterogeneity limit the existing treatment options. This review aims to give an overview of the current standard-of-care HER2-directed therapy in gastric cancer, as well as its challenges and future developments.

Effectiveness of trastuzumab deruxtecan rechallenge in a patient with HER2-positive metastatic breast cancer: a case report.

Trastuzumab deruxtecan (T-DXd), a high-payload antibody drug conjugate, has been reported to exert potent antitumor effects and has recently shown promising efficacy against human epidermal growth factor receptor 2 (HER2)-positive adenocarcinoma. Despite its high efficacy, interstitial lung disease (ILD) is a severe adverse event (AE) associated with T-DXd. This report describes a patient who was successfully treated with a dose-reduced T-DXd challenge after recovery from ILD. Little disease progression was observed during the treatment interruption period; thus, the effect of T-DXd was considered to have been maintained. T-DXd may induce ILD, and re-administration under careful observation is considered an important option for treating patients with HER2-positive breast cancer.

EGFR-directed antibodies promote HER2 ADC internalization and efficacy.

Trastuzumab deruxtecan (T-DXd) is a human epidermal growth factor receptor 2 (HER2)-targeting antibody drug conjugate that has remarkable activity in HER2-positive cancers. However, the degree of benefit of T-DXd is not uniform among solid tumors even with high levels of HER2. Despite high HER2 expression, the HER2/T-DXd complex may not always undergo internalization and payload release dependent on the receptor's conformation and context. We hypothesize that epidermal growth factor receptor (EGFR), a dimerization partner of HER2, can modulate HER2 trafficking through endocytic pathways and affect T-DXd uptake. We demonstrate that elevated EGFR expression levels can promote EGFR/HER2 heterodimer formation and suppress T-DXd internalization and efficacy. Knockdown of EGFR expression or pharmacologic stimulation of EGFR endocytosis with EGFR monoclonal antibodies restores T-DXd trafficking and antitumor activity in EGFR-overexpressing cancers in vivo. Our results reveal EGFR overexpression to be a potential mechanism of resistance to T-DXd, which can be overcome by combination therapy strategies targeting EGFR.

Lung toxicity induced by anti-HER2 antibody - drug conjugates for breast cancer.

Human epidermal growth factor receptor 2 (HER2) serves as both a prognostic indicator and a therapeutic target for breast cancer. Therefore, anti-HER2 therapy plays a crucial role in the treatment of HER2-positive cancer. Antibody-drug conjugates (ADCs) are composed of a monoclonal antibody, a chemical linker and a payload, wherein their aim is to reduce the toxicity associated with chemotherapy drugs by utilizing specific antibodies. Among the anti-HER2 ADCs currently approved for clinical use, trastuzumab emtansine(T-DM1) and trastuzumab deruxtecan (T-Dxd) have demonstrated remarkable efficacy in treating HER2-positive breast cancer. However, it is essential to emphasize the occurrence of lung toxicity during the treatment process, which can be life-threatening. In this review, we provide an overview of the new epidemiological features associated with interstitial lung disease (ILD) related to anti-HER2 ADCs in breast cancer. We also summarize the potential pathogenesis and explore the diagnosis and treatment strategies within this field.

Trastuzumab deruxtecan in patients with previously treated HER2-low advanced breast cancer and active brain metastases: the DEBBRAH trial.

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

Trastuzumab deruxtecan in human epidermal growth factor receptor 2-positive breast cancer brain metastases: a systematic review and meta-analysis.

BACKGROUND: Trastuzumab deruxtecan (T-DXd) has shown promising results in patients with breast cancer brain metastases (BCBMs). We conducted a systematic review and meta-analysis to evaluate the effectiveness and safety of T-DXd in the human epidermal growth factor receptor 2 (HER2)-positive BCBM population. PATIENTS AND METHODS: We searched PubMed, Embase, and Cochrane Library databases as well as American Society of Clinical Oncology (ASCO), European Society for Medical Oncology (ESMO), and San Antonio Breast Cancer Symposium (SABCS) websites for clinical trials (CTs) and observational studies evaluating T-DXd in patients with HER2-positive BCBM. Heterogeneity was assessed with I2 statistics. Random effects models were used for all statistical analyses, which were carried out using R software (version 4.2.2). RESULTS: Ten studies were included, six CTs (n = 189) and four observational studies (n = 130), with a total of 319 patients. The median progression-free survival was 15 months [95% confidence interval (CI) 13.9-16.1 months]. The objective response rate (ORR) was 61% (95% CI 52% to 70%), and the intracranial (IC)-ORR was 61% (95% CI 54% to 69%). No significant differences in ORR and IC-ORR were observed between CTs and observational studies (P = 0.31 and 0.58, respectively). The clinical benefit rate (CBR) was 80% (95% CI 52% to 94%), and the IC-CBR was 70% (95% CI 54% to 82%). The ORR was 68% (95% CI 57% to 77%) in the subgroup of patients with stable BMs and 60% (95% CI 48%-72%) in patients with active BM, with no significant difference between groups (P = 0.35). CONCLUSIONS: Our systematic review and meta-analysis supports the IC activity of T-DXd in patients with stable BM and active BM. TRIAL REGISTRATION: International Prospective Register of Systematic Reviews (PROSPERO) under the protocol number CRD42023422589.

Trastuzumab deruxtecan (DS-8201a), a HER2-targeting antibody-drug conjugate, demonstrates in vitro and in vivo antitumor activity against primary and metastatic ovarian tumors overexpressing HER2.

High-grade serous ovarian cancer (HGSOC) and ovarian clear cell carcinoma (CC), are biologically aggressive tumors endowed with the ability to rapidly metastasize to the abdominal cavity and distant organs. About 10% of HGSOC and 30% of CC demonstrate HER2 IHC 3 + receptor over-expression. We evaluated the efficacy of trastuzumab deruxtecan (T-DXd; DS-8201a), a novel HER2-targeting antibody-drug conjugate (ADC) to an ADC isotype control (CTL ADC) against multiple HGSOC and CC tumor models. Eleven ovarian cancer cell lines including a matched primary and metastatic cell line established from the same patient, were evaluated for HER2 expression by immunohistochemistry and flow cytometry, and gene amplification by fluorescence in situ hybridization assays. In vitro experiments demonstrated T-DXd to be significantly more effective against HER2 3 + HGSOC and CC cell lines when compared to CTL ADC (p < 0.0001). T-DXd induced efficient bystander killing of HER2 non-expressing tumor cells when admixed with HER2 3 + cells. In vivo activity of T-DXd was studied in HER2 IHC 3 + HGSOC and CC mouse xenograft models. We found T-DXd to be significantly more effective than CTL ADC against HER2 3 + HGSOC (KR(CH)31) and CC (OVA10) xenografts with a significant difference in tumor growth starting at day 8 (p = 0.0003 for KR(CH)31, p < 0.0001 for OVA10). T-DXd also conferred a survival advantage in both xenograft models. T-DXd may represent an effective ADC against primary and metastatic HER2-overexpressing HGSOC and CC.

The DNA repair pathway as a therapeutic target to synergize with trastuzumab deruxtecan in HER2-targeted antibody-drug conjugate-resistant HER2-overexpressing breast cancer.

BACKGROUND: Anti-HER2 therapies, including the HER2 antibody-drug conjugates (ADCs) trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd), have led to improved survival outcomes in patients with HER2-overexpressing (HER2+) metastatic breast cancer. However, intrinsic or acquired resistance to anti-HER2-based therapies remains a clinical challenge in these patients, as there is no standard of care following disease progression. The purpose of this study was to elucidate the mechanisms of resistance to T-DM1 and T-DXd in HER2+ BC patients and preclinical models and identify targets whose inhibition enhances the antitumor activity of T-DXd in HER2-directed ADC-resistant HER2+ breast cancer in vitro and in vivo. METHODS: Targeted DNA and whole transcriptome sequencing were performed in breast cancer patient tissue samples to investigate genetic aberrations that arose after anti-HER2 therapy. We generated T-DM1 and T-DXd-resistant HER2+ breast cancer cell lines. To elucidate their resistance mechanisms and to identify potential synergistic kinase targets for enhancing the efficacy of T-DXd, we used fluorescence in situ hybridization, droplet digital PCR, Western blotting, whole-genome sequencing, cDNA microarray, and synthetic lethal kinome RNA interference screening. In addition, cell viability, colony formation, and xenograft assays were used to determine the synergistic antitumor effect of T-DXd combinations. RESULTS: We found reduced HER2 expression in patients and amplified DNA repair-related genes in patients after anti-HER2 therapy. Reduced ERBB2 gene amplification in HER2-directed ADC-resistant HER2+ breast cancer cell lines was through DNA damage and epigenetic mechanisms. In HER2-directed ADC-resistant HER2+ breast cancer cell lines, our non-biased RNA interference screening identified the DNA repair pathway as a potential target within the canonical pathways to enhance the efficacy of T-DXd. We validated that the combination of T-DXd with ataxia telangiectasia and Rad3-related inhibitor, elimusertib, led to significant breast cancer cell death in vitro (P < 0.01) and in vivo (P < 0.01) compared to single agents. CONCLUSIONS: The DNA repair pathways contribute to HER2-directed ADC resistance. Our data justify exploring the combination treatment of T-DXd with DNA repair-targeting drugs to treat HER2-directed ADC-resistant HER2+ breast cancer in clinical trials.