SUMMARY: The FDA on May 15, 2026, approved fam-Trastuzumab deruxtecan-nxki (T-DXd, ENHERTU®) for the adjuvant treatment of adult patients with HER2-positive (IHC 3+ or ISH+) breast cancer who have residual invasive disease following neoadjuvant treatment with Trastuzumab (with or without Pertuzumab) and taxane-based treatment.
FDA also approved two companion diagnostic devices, the PATHWAY anti-HER-2/neu (4B5) Rabbit Monoclonal Primary Antibody and the VENTANA HER2 Dual ISH DNA Probe Cocktail, both for identifying HER2-positive (IHC3+ or ISH+) patients for treatment with T-DXd, consistent with the approved drug labeling.
Breast cancer is the most common cancer among women in the US and about 1 in 8 women (12%) will develop invasive breast cancer during their lifetime. It is estimated that in the US, approximately 321,910 new cases of female breast cancer will be diagnosed in 2026, and about 42,140 women will die of the disease, largely due to metastatic recurrence.
Background: Escalation Strategies in Residual Disease
The management of HER2-positive early breast cancer has undergone a profound transformation over the past two decades, driven by the integration of HER2-directed therapies across disease stages. In patients with Stage II–III disease, neoadjuvant therapy has become the standard treatment approach, enabling early assessment of treatment response and guiding postoperative therapy. Despite high rates of pathologic Complete Response (pCR) with contemporary neoadjuvant regimens, a clinically significant subset of patients exhibits residual invasive disease at surgery, an established marker of elevated recurrence risk.
The paradigm of risk-adapted postneoadjuvant therapy was firmly established by the KATHERINE trial, in which Trastuzumab emtansine (T-DM1-KADCYLA®) demonstrated a substantial improvement in Invasive Disease–Free Survival (IDFS) and Overall Survival (OS), compared with Trastuzumab alone. However, outcomes in higher-risk subgroups, particularly those with node-positive or extensive residual disease remain suboptimal, and CNS relapses continue to represent an unmet need.
Trastuzumab deruxtecan (T-DXd-ENHERTU®), a next-generation HER2-directed antibody–drug conjugate, has consistently demonstrated superior efficacy over T-DM1 in the metastatic setting, including activity in CNS disease. These data provided a strong rationale to evaluate whether T-DXd could further improve outcomes in the curative-intent, postneoadjuvant setting.
Trial Design and Patient Population
The present FDA approval was based on DESTINY-Breast05, which is a global, Phase III, open-label, randomized trial evaluating T-DXd versus T-DM1 in patients with HER2-positive early breast cancer and residual invasive disease following neoadjuvant therapy, enriched for high-risk features.
Eligible patients had:
- Residual invasive disease in breast and/or axillary nodes
- Either inoperable disease at presentation or node-positive disease after neoadjuvant therapy
- Prior receipt of standard neoadjuvant systemic therapy, including taxane-based chemotherapy and HER2-targeted therapy
A total of 1635 patients were randomized 1:1 to receive T-DXd (5.4 mg/kg) every 3 weeks (N=818) or T-DM1 (3.6 mg/kg) every 3 weeks (N=817) for up to 14 cycles. The Primary endpoint was invasive DFS (IDFS), with key Secondary endpoints including DFS, distant recurrence, CNS outcomes, and Overall Survival (OS). Notably, this trial enrolled a higher-risk population than prior studies: About 52% presented with inoperable disease at disease presentation, about 81% had node-positive disease after neoadjuvant therapy and approximately 79% received dual HER2 blockade preoperatively.
Efficacy: A New Benchmark for Invasive Disease–Free Survival
At a median follow-up of approximately 30 months, T-DXd demonstrated a clinically and statistically significant improvement in outcomes compared with T-DM1:
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- IDFS events or death: 6.2% (T-DXd) vs. 12.5% (T-DM1); Hazard ratio (HR): 0.47 (P<0.001)
- 3-year IDFS: 92.4% vs. 83.7%
- 3-year DFS: 92.3% vs. 83.5% (HR: 0.47)
The benefit was consistent across prespecified subgroups, including hormone receptor–positive disease driven largely by a reduction in distant recurrences, the dominant mode of failure. Importantly, the distant recurrence: 5.1% vs. 9.9% and CNS recurrence was numerically lower with T-DXd (2.1% vs. 3.1%)
Although Overall Survival data remain immature, the magnitude of IDFS improvement strongly supports a meaningful long-term benefit.
Safety Profile: Balancing Efficacy with Toxicity
The safety profiles of both agents were consistent with prior experience, though distinct in nature. The common adverse events with T-DXd included nausea (71%), neutropenia, vomiting and alopecia. Approximately 50% of patients had grade ≥3 adverse events. T-DM1 was associated with hepatotoxicity (elevated transaminases) and thrombocytopenia.
Key Safety Signal: Interstitial Lung Disease (ILD)
The most clinically significant toxicity associated with T-DXd remains ILD. The incidence was 9.6% (T-DXd) vs. 1.6% (T-DM1). They were mostly grade 1–2, but grade ≥3 events occurred and two treatment-related deaths reported. The trial incorporated proactive ILD monitoring, including serial low-dose chest CT imaging, enabling early detection. Importantly no increased ILD risk was observed with concurrent radiotherapy. Multidisciplinary evaluation is critical to distinguish ILD from radiation pneumonitis.
Clinical Context: Positioning Within Current Practice
These findings represent a clear evolution beyond the KATHERINE standard, particularly in a more contemporary, higher-risk population treated with modern neoadjuvant regimens.
Implications for Clinical Practice
- T-DXd emerges as the preferred postneoadjuvant therapy for patients with:
- Residual invasive disease
- Node-positive or otherwise high-risk features
- T-DM1 remains relevant for:
- Lower-risk residual disease
- Patients unable to tolerate T-DXd
Conclusions
DESTINY-Breast05 establishes Trastuzumab deruxtecan as a new standard of care in the postneoadjuvant management of high-risk HER2-positive early breast cancer with residual disease. The trial demonstrates a substantial and clinically meaningful improvement in Invasive Disease–Free Survival, a reduction in distant recurrence and manageable but clinically significant toxicity, particularly interstitial lung disease.
As the field moves toward increasingly personalized, response-adapted strategies, T-DXd represents a major advance, while underscoring the need for vigilant toxicity monitoring and multidisciplinary care in the curative setting.
Trastuzumab Deruxtecan in Residual HER2-Positive Early Breast Cancer. Loibl S, Park YH, Shao Z, et al. for the DESTINY-Breast05 Trial Investigators. N Engl J Med 2026;394:845-857.
