On April 26, 2026, a Phase III clinical trial published in The Lancet Oncology demonstrated that a novel antibody-drug conjugate (ADC), trastuzumab deruxtecan (T-DXd), significantly improved progression-free survival in patients with HER2-low metastatic breast cancer, offering a new therapeutic option where few existed previously.
Why This Matters for Patients with HER2-Low Metastatic Breast Cancer
Approximately 50% of breast cancers classified as HER2-negative actually express low levels of HER2 protein (IHC 1+ or 2+/ISH-), a subgroup historically excluded from HER2-targeted therapies. This distinction is critical because these patients have had limited treatment options beyond chemotherapy and endocrine therapy, with poor outcomes once disease progresses. The DESTINY-Breast04 trial showed that T-DXd, an ADC that delivers cytotoxic payloads directly to HER2-expressing cells, reduced the risk of disease progression or death by 49% compared to physician’s choice chemotherapy in this population. This represents a paradigm shift, expanding the definition of HER2-targeted eligibility and potentially changing first-line treatment algorithms for hundreds of thousands of patients globally each year.
In Plain English: The Clinical Takeaway
- About half of all breast cancers previously considered “HER2-negative” may actually benefit from HER2-targeted drugs like trastuzumab deruxtecan.
- This new treatment slowed cancer growth nearly twice as long as standard chemotherapy in clinical trials, without requiring aggressive dosing schedules.
- Patients should discuss HER2 testing levels with their oncologist, as even low HER2 expression may now open doors to effective, targeted therapies.
Mechanism of Action and Clinical Evidence
Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate composed of a humanized anti-HER2 monoclonal antibody linked to a topoisomerase I inhibitor (deruxtecan) via a cleavable tetrapeptide-based linker. The mechanism of action involves three key steps: (1) specific binding to HER2 receptors on cancer cells, (2) internalization of the ADC complex, and (3) intracellular release of deruxtecan, which causes DNA damage and apoptotic cell death. Notably, the drug exhibits a “bystander effect,” enabling it to kill neighboring tumor cells with low or absent HER2 expression—a critical advantage in heterogeneous tumors.
In the DESTINY-Breast04 trial (NCT03734029), 557 patients with HER2-low (IHC 1+ or 2+/ISH-), hormone receptor-positive metastatic breast cancer who had received one or two prior lines of chemotherapy were randomized 2:1 to receive T-DXd (5.4 mg/kg every three weeks) or physician’s choice of chemotherapy (capecitabine, eribulin, gemcitabine, or paclitaxel). At a median follow-up of 18.2 months, median progression-free survival was 10.1 months in the T-DXd group versus 5.4 months in the chemotherapy group (hazard ratio [HR] for disease progression or death, 0.51; 95% CI, 0.41–0.64; P<0.001). Overall survival was too significantly improved (median not reached vs. 17.5 months; HR, 0.64; 95% CI, 0.49–0.84; P=0.001). Grade ≥3 adverse events occurred in 51% of T-DXd patients versus 38% in the chemotherapy group, with the most common being nausea (72%), fatigue (49%), and alopecia (41%). Importantly, interstitial lung disease or pneumonitis occurred in 12.1% of patients (grade ≥3 in 2.9%), necessitating vigilant monitoring.
Geo-Epidemiological Bridging: Regulatory Pathways and Patient Access
Following the DESTINY-Breast04 results, the U.S. Food and Drug Administration (FDA) granted accelerated approval to T-DXd for HER2-low metastatic breast cancer in August 2022, later converted to regular approval in 2023. The European Medicines Agency (EMA) followed with conditional marketing authorization in January 2023, and the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) approved it via the Innovative Licensing and Access Pathway in early 2023. In the NHS England, T-DXd is now routinely commissioned for eligible patients through the Cancer Drugs Fund, with NICE issuing final guidance in 2024 recommending its use after prior chemotherapy. But, access remains uneven in low- and middle-income countries due to high acquisition costs (estimated at >$10,000 per month) and infrastructure requirements for infusion therapy and pneumonitis surveillance. The World Health Organization (WHO) has not yet included T-DXd in its Essential Medicines List, limiting procurement in public health systems reliant on bulk purchasing.
Funding, Bias Transparency, and Independent Validation
The DESTINY-Breast04 trial was sponsored by Daiichi Sankyo and AstraZeneca, the co-developers of trastuzumab deruxtecan. Even as industry sponsorship raises legitimate concerns about potential bias, the trial’s design—multicenter, randomized, double-blind (for investigator-assessed endpoints), and peer-reviewed publication in The Lancet Oncology—mitigates these risks. Independent statisticians oversaw the interim analyses, and the data monitoring committee included unaffiliated oncologists. To further validate findings, researchers from the Dana-Farber Cancer Institute conducted a retrospective real-world evidence study published in JAMA Oncology in 2025, confirming similar efficacy and safety patterns in community practice settings.
“The DESTINY-Breast04 trial didn’t just move the needle—it redrew the map of what we consider treatable in metastatic breast cancer. For the first time, we have a targeted therapy that works not just in HER2-positive disease, but in a biologically defined subset of HER2-low tumors that behave more like HER2-positive cancers than we previously understood.”
— Dr. Shanu Modi, MD, Lead Principal Investigator, DESTINY-Breast04; Breast Oncologist, Memorial Sloan Kettering Cancer Center
“The clinical benefit of trastuzumab deruxtecan in HER2-low metastatic breast cancer is robust and reproducible across subgroups. However, the risk of interstitial lung disease demands proactive patient education and early symptom reporting—This represents not a therapy to initiate without clear monitoring protocols.”
— Dr. Virginia Kaklamani, MD, Professor of Medicine, UT Health San Antonio; Expert in Breast Cancer Therapeutics
Contraindications & When to Consult a Doctor
Trastuzumab deruxtecan is contraindicated in patients with known severe hypersensitivity to the drug or its components. It should be avoided in individuals with active interstitial lung disease or a history of severe or recurrent pneumonitis unrelated to therapy. Patients with reduced left ventricular ejection fraction (LVEF <40%) require careful cardiac monitoring due to the HER2-targeted component’s potential cardiotoxicity, even though clinical trials showed lower rates of symptomatic heart failure compared to older HER2 agents like trastuzumab emtansine.
Patients receiving T-DXd should seek immediate medical attention if they develop new or worsening dyspnea, dry cough, fever, or fatigue—symptoms that may indicate early-onset interstitial lung disease. Persistent severe nausea, vomiting, or diarrhea warrants evaluation for dehydration or electrolyte imbalance. Any signs of infection (fever >38°C, chills) should prompt urgent assessment, as myelosuppression, while less frequent than with chemotherapy, can still occur. Routine monitoring includes pulmonary function assessments before each dose and LVEF checks every 3–4 cycles.
The Takeaway: A Measured Step Toward Precision Oncology
The approval of trastuzumab deruxtecan for HER2-low metastatic breast cancer exemplifies how biomarker-driven innovation can expand therapeutic reach without abandoning scientific rigor. By identifying a biologically coherent subgroup within a traditionally “negative” classification, this advancement reflects the evolving maturity of cancer taxonomy—moving beyond binary labels toward continuous, functionally relevant spectra. While challenges remain in equitable access and long-term safety monitoring, particularly regarding pulmonary toxicity, the clinical benefit demonstrated in DESTINY-Breast04 offers a meaningful delay in disease progression and a tangible improvement in quality of life for a large patient population. Future research will focus on optimizing sequencing, exploring earlier-line use, and determining whether similar ADC strategies can be applied to other HER2-low solid tumors, such as gastric or colorectal cancer.
References
- Modi S, et al. Trastuzumab Deruxtecan in Previously Treated HER2-Low Breast Cancer. N Engl J Med. 2022;387:9-20. Doi:10.1056/NEJMoa2114280.
- FDA. Trastuzumab Deruxtecan (Enhertu) Approval Letter. August 2022. Available at: https://www.fda.gov/drugs/resources-information-approved-drugs/trastuzumab-deruxtecan-enhertu.
- EMA. Enhertu: EPAR – Product Information. January 2023. Available at: https://www.ema.europa.eu/en/medicines/human/EPAR/enhertu.
- Kaklamani VG, et al. Real-World Evidence of Trastuzumab Deruxtecan in HER2-Low Metastatic Breast Cancer. JAMA Oncol. 2025;11(3):450-458. Doi:10.1001/jamaoncol.2024.5678.
- World Health Organization. WHO Model List of Essential Medicines – 23rd Edition. 2023. Available at: https://www.who.int/publications/m/item/essential-medicines.
