Background
Phase III, open-label RCT. 1095 patients with HER2+ locally advanced or metastatic breast cancer with no prior treatment for advanced disease. Three-arm comparison of T-DM1-based regimens vs standard pertuzumab+trastuzumab+taxane (THP), designed to test whether T-DM1's ADC mechanism could replace or improve upon chemotherapy in first-line HER2+ mBC.
Interventions and follow up
Arm A: T-DM1 3.6mg/kg q21d + pertuzumab 420mg q21d (loading 840mg)
Arm B: T-DM1 3.6mg/kg q21d + placebo
Primary endpoint: PFS of Arm A vs Arm C and Arm B vs Arm C
mFollow up: 35 month
Arm B: T-DM1 3.6mg/kg q21d + placebo
Primary endpoint: PFS of Arm A vs Arm C and Arm B vs Arm C
mFollow up: 35 month
Results
Arm C (control): Trastuzumab + taxane (docetaxel or paclitaxel at investigator choice)
PFS (Arm A vs C): 15.2 vs 13.7mo, HR 0.87, 95% CI 0.69–1.08 — not significant
PFS (Arm B vs C): 14.1 vs 13.7mo, HR 0.91, 95% CI 0.73–1.13 — not significant
PFS (Arm A vs C): 15.2 vs 13.7mo, HR 0.87, 95% CI 0.69–1.08 — not significant
PFS (Arm B vs C): 14.1 vs 13.7mo, HR 0.91, 95% CI 0.73–1.13 — not significant
Adverse events
Hematologic: Grade ≥3 thrombocytopenia Arm A 7.3% vs C 0.5%; grade ≥3 neutropenia Arm A 2.5% vs C 46.1%
Neurologic: Grade ≥3 peripheral neuropathy Arm A 2.7% vs C 6.7%
Overall: T-DM1 had a better neutropenia profile but more thrombocytopenia and neuropathy; total AE burden shifted but not clearly reduced vs standard chemo
Neurologic: Grade ≥3 peripheral neuropathy Arm A 2.7% vs C 6.7%
Overall: T-DM1 had a better neutropenia profile but more thrombocytopenia and neuropathy; total AE burden shifted but not clearly reduced vs standard chemo
Conclusions
T-DM1 ± pertuzumab was not superior to trastuzumab + taxane in first-line HER2+ mBC. MARIANNE was definitively negative for both T-DM1 arms vs control, establishing that T-DM1 does not replace chemotherapy-containing regimens in the first-line setting.
Key Limitations
Key Limitations: The control arm (trastuzumab + taxane alone) was substandard by current standards — CLEOPATRA established pertuzumab + trastuzumab + taxane as first-line standard at roughly the same time, making the comparator arm underpowered clinically. Had the trial compared T-DM1 vs THP (dual HER2 blockade), results might differ. The PFS values in all arms were similar (~14–15 months), suggesting MARIANNE failed to show superiority despite comparable activity. No biomarker stratification identified T-DM1 responders.
Clinical Context
MARIANNE established that T-DM1 should not be used in the first-line HER2+ mBC setting. T-DM1's subsequent role shifted to the second-line (EMILIA) and adjuvant residual-disease settings (KATHERINE). With T-DXd now dominant in both second-line mBC (DESTINY-Breast03) and adjuvant residual disease (DESTINY-Breast05), T-DM1's clinical utility has further narrowed. MARIANNE is a clinically important negative trial demonstrating that superior mechanisms in vitro do not always translate to improved survival outcomes.
References
References: Ellis PA et al, JCO 2017