Pivotal Trial for Enhertu in TNBC Population
The pivotal trial for fam-trastuzumab deruxtecan (Enhertu) in triple-negative breast cancer is DESTINY-Breast04 (NCT03734029), which established T-DXd as the first effective HER2-targeted therapy for HER2-low TNBC patients. 1
DESTINY-Breast04: The Landmark Study
DESTINY-Breast04 was a Phase III, multicenter, open-label, randomized trial that fundamentally changed the treatment landscape for HER2-low breast cancer, including TNBC. The trial enrolled 557 patients with HER2-low metastatic breast cancer (defined as IHC 1+ or IHC 2+/ISH-negative) who had received one or two previous lines of chemotherapy. 1
Key Trial Design Elements:
- Randomization: 2:1 ratio to receive T-DXd 5.4 mg/kg every 3 weeks versus physician's choice of chemotherapy
- Population: 88.7% hormone receptor-positive (494 patients) and 11.3% hormone receptor-negative/TNBC (63 patients)
- HER2-low definition: IHC 1+ or IHC 2+ with negative in situ hybridization 1
Efficacy Results in the Overall Population:
Among all patients (including TNBC subset):
- Median PFS: 9.9 months with T-DXd vs 5.1 months with chemotherapy (HR 0.50; P<0.001)
- Median OS: 23.4 months with T-DXd vs 16.8 months with chemotherapy (HR 0.64; P=0.001) 1
Safety Profile:
- Grade ≥3 adverse events occurred in 52.6% with T-DXd versus 67.4% with chemotherapy
- Critical toxicity: Adjudicated drug-related interstitial lung disease/pneumonitis occurred in 12.1% of patients receiving T-DXd, with 0.8% experiencing grade 5 (fatal) events 1
Clinical Context and Mechanism
The success of T-DXd in HER2-low TNBC represents a paradigm shift. The mechanism of action in HER2-low disease is primarily related to delivery of cytotoxic molecules (the topoisomerase I inhibitor payload) rather than blockade of the HER2 pathway itself. 2 This explains why traditional anti-HER2 antibodies (trastuzumab, pertuzumab) failed in HER2-low disease, while ADCs with potent payloads succeeded.
Why DESTINY-Breast04 Was Pivotal:
- First randomized Phase III data demonstrating survival benefit in HER2-low breast cancer, including TNBC 2
- Established HER2-low as a targetable entity despite not being a distinct molecular subtype 2
- Led to FDA and EMA regulatory approval for HER2-low metastatic breast cancer 3
- Changed clinical practice guidelines from ASCO, ESMO, and NCCN to incorporate HER2-low testing and T-DXd treatment 4, 5
Current Treatment Position
T-DXd is now recognized as a preferred second-line option for HER2-low TNBC after failure of first-line chemotherapy, based on DESTINY-Breast04 results. 5 However, for hormone receptor-negative/TNBC patients specifically, sacituzumab govitecan remains the preferred second-line option overall, with T-DXd reserved for the HER2-low subset. 5
Critical Clinical Caveat:
Accurate HER2 testing is essential. The clinical benefit of T-DXd requires proper identification of HER2-low expression (IHC 1+ or IHC 2+/ISH-negative), which necessitates standardized testing protocols, validated assays, and pathologist training to distinguish between IHC 0 (null), IHC 0+ (ultralow), and HER2-low. 3 Misclassification can lead to inappropriate treatment selection.
Monitoring Requirements:
Given the 12.1% incidence of ILD/pneumonitis, vigilant pulmonary monitoring is mandatory, with protocol-guided management to mitigate this potentially fatal toxicity while preserving therapeutic benefit. 1, 6 This includes baseline pulmonary assessment and regular clinical surveillance for respiratory symptoms.