Abstract
BACKGROUND: Most patients with human epidermal growth factor receptor 2 (HER2)-positive breast cancer develop resistance or relapse. Zanidatamab is a novel, humanized, dual-HER2-targeted bispecific antibody with antitumor activity and a manageable safety profile as monotherapy in HER2-positive cancers. This trial evaluated the efficacy and safety of zanidatamab with docetaxel as first-line treatment in HER2-positive breast cancer. METHODS: Cohort 1 of this open-label, multicenter, phase Ib/II trial enrolled adult patients from China or South Korea with histologically or cytologically confirmed unresectable, locally advanced, recurrent or metastatic HER2-positive breast cancer. Patients received intravenous zanidatamab 30 mg/kg with docetaxel 75 mg/m(2) or a flat dose of zanidatamab 1800 mg with docetaxel 75 mg/m(2) once every 3 weeks. Primary objectives were to evaluate the preliminary antitumor activity, safety, and tolerability of zanidatamab with docetaxel. RESULTS: At data cut-off (7 December 2023), 38 patients were enrolled in cohort 1; median study follow-up was 24.8 months. The confirmed objective response rate was 90.9%, disease control rate was 97.0%, and median duration of response was 23.5 months. Median time to response was 5.9 weeks. Median progression-free and overall survival were 22.1 months and 36.9 months, respectively. All patients experienced one or more treatment-emergent adverse events (TEAE), and 71.1% experienced grade ≥3 TEAEs. All patients had one or more treatment-related AE (TRAE), and 97.4% experienced zanidatamab-related TRAEs. Serious TEAEs were reported for 31.6% of patients: 18.4% had serious TRAEs, all of which were zanidatamab related. One death due to respiratory failure was recorded but was assessed as not related to study treatment. TEAEs and TRAEs leading to treatment discontinuation were recorded for 10.5% and 7.9% of patients, respectively. CONCLUSION: Zanidatamab demonstrated efficacy and a manageable and tolerable safety profile with docetaxel as first-line treatment in patients with HER2-positive breast cancer. These data support the further development of zanidatamab in this patient population.