Background
Phase III, open-label RCT. 524 patients with HER2+ unresectable or metastatic breast cancer who had received prior trastuzumab and taxane. Head-to-head comparison of T-DXd vs T-DM1. This trial directly challenged T-DM1 (established standard after trastuzumab/taxane) and compared two anti-HER2 ADCs with different payloads, DAR, and linker technology.
Interventions and follow up
Arm A: T-DXd (trastuzumab deruxtecan) 5.4mg/kg IV q21d
Arm B: T-DM1 (trastuzumab emtansine) 3.6mg/kg IV q21d
Primary endpoint: Progression-free survival (PFS)
mFollow up: 28.4 month
Arm B: T-DM1 (trastuzumab emtansine) 3.6mg/kg IV q21d
Primary endpoint: Progression-free survival (PFS)
mFollow up: 28.4 month
Results
PFS: 28.8 vs 6.8mo, HR 0.33, 95% CI 0.26–0.43, P<.001
12-mo PFS rate: 75.8% vs 34.1%
OS (updated): Not reached vs 37.5mo, HR 0.64, 95% CI 0.47–0.87, P=.004
12-mo PFS rate: 75.8% vs 34.1%
OS (updated): Not reached vs 37.5mo, HR 0.64, 95% CI 0.47–0.87, P=.004
Adverse events
Pulmonary: ILD any grade 15.2% (T-DXd) vs 3.1% (T-DM1); grade ≥3 2.7% vs 0.8%
Gastrointestinal: Grade ≥3 nausea 9.6% vs 2.3%
Hematologic: Thrombocytopenia grade ≥3 7.7% (T-DXd) vs 24.9% (T-DM1)
Dermatologic: Alopecia any grade 37.7% vs 4.0%
Overall: Discontinuation 22.3% (T-DXd) vs 18.3% (T-DM1)
Gastrointestinal: Grade ≥3 nausea 9.6% vs 2.3%
Hematologic: Thrombocytopenia grade ≥3 7.7% (T-DXd) vs 24.9% (T-DM1)
Dermatologic: Alopecia any grade 37.7% vs 4.0%
Overall: Discontinuation 22.3% (T-DXd) vs 18.3% (T-DM1)
Conclusions
T-DXd demonstrated a dramatically superior PFS (HR 0.33) and OS benefit vs T-DM1 in HER2+ mBC after prior trastuzumab and taxane. DESTINY-Breast03 represents one of the largest PFS hazard ratios reported in an oncology phase III trial and established T-DXd as the unequivocal second-line standard in HER2+ mBC.
Key Limitations
Key Limitations: ILD (15.2% any-grade) requires systematic monitoring and mandatory ILD algorithm management; fatal ILD occurred in 0.8% of patients. The magnitude of the PFS difference (HR 0.33) may reflect the relatively poor performance of T-DM1 in this population (T-DM1 median PFS 6.8 months) as much as T-DXd efficacy. Long-term effects of extended T-DXd therapy are not fully characterized. Patients with active ILD or poor pulmonary reserve are not suitable candidates.
Clinical Context
DESTINY-Breast03 led to FDA approval of T-DXd for HER2+ mBC after prior anti-HER2 therapy (2022). ESMO-MCBS score: 5. T-DXd has since become the preferred second-line option in all major guidelines. DB09 is now testing T-DXd vs THP in the first-line setting — if positive, T-DXd may become the new first-line standard. ILD monitoring protocols are well-established and should be implemented proactively.
References
References: Cortés J et al, NEJM 2022