Background
Phase III, 3-arm (1:1:1), open-label RCT. N=1157 with previously untreated HER2-positive metastatic or locally recurrent unresectable breast cancer. Arms: T-DXd + pertuzumab, T-DXd monotherapy, THP. Primary comparison T-DXd+pertuzumab vs THP. ~50% HR-positive.
Interventions and follow up
Arm A: T-DXd 5.4mg/kg IV q3wk + pertuzumab 840mg load then 420mg IV q3wk (N=383)
Arm B: THP — trastuzumab 8mg/kg load then 6mg/kg q3wk + pertuzumab 840mg load then 420mg q3wk + taxane (docetaxel 75mg/m² q3wk or paclitaxel 80mg/m² qwk) (N=387)
Primary endpoint: PFS (T-DXd+P vs THP) by BICR
mFollow up: NR at data cutoff
Arm B: THP — trastuzumab 8mg/kg load then 6mg/kg q3wk + pertuzumab 840mg load then 420mg q3wk + taxane (docetaxel 75mg/m² q3wk or paclitaxel 80mg/m² qwk) (N=387)
Primary endpoint: PFS (T-DXd+P vs THP) by BICR
mFollow up: NR at data cutoff
Results
PFS: 40.7 vs 26.9mo, HR 0.56, 95% CI 0.44–0.71, P<.00001
OS: immature at data cutoff
T-DXd monotherapy vs THP: NR at primary analysis
OS: immature at data cutoff
T-DXd monotherapy vs THP: NR at primary analysis
Adverse events
Overall (grade ≥3, T-DXd+P vs THP): 63.5% vs 62.3%; SAEs 27.0% vs 25.1%
Hematologic (grade ≥3, T-DXd+P): neutropenia 23.9%, anemia 8.4%
Metabolic (grade ≥3, T-DXd+P): hypokalemia 10.2%
Pulmonary: ILD any grade 12.1% (T-DXd+P); grade 5 ILD 0.5% vs 1.0%
Neuro: peripheral neuropathy grade ≥3 lower with T-DXd+P than THP taxane arm
Hematologic (grade ≥3, T-DXd+P): neutropenia 23.9%, anemia 8.4%
Metabolic (grade ≥3, T-DXd+P): hypokalemia 10.2%
Pulmonary: ILD any grade 12.1% (T-DXd+P); grade 5 ILD 0.5% vs 1.0%
Neuro: peripheral neuropathy grade ≥3 lower with T-DXd+P than THP taxane arm
Conclusions
T-DXd + pertuzumab significantly improved PFS vs THP (HR 0.56; 40.7 vs 26.9mo), the largest PFS improvement demonstrated in first-line HER2-positive mBC. OS data awaited.
Key Limitations
OS immature; survival benefit unconfirmed. Open-label design. THP (CLEOPATRA) has 12+ years of OS data vs early T-DXd+P. ILD ~12% requires systematic monitoring. No data in active brain metastases. Long-term cardiac safety needs further characterization.
Clinical Context
Establishes T-DXd + pertuzumab as candidate new first-line standard in HER2-positive mBC, potentially replacing THP (CLEOPATRA, 2012). FDA review anticipated pending OS maturity. ESMO-MCBS pending. The 40.7-month median PFS is the longest reported in this setting.