Efficacy and safety of margetuximab plus chemotherapy

Background: Trastuzumab is the recommended first-line treatment for human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (MBC) patients in China, but therapeutic resistance to trastuzumab and other early-line treatments is common and late-line treatment options are limited. Derived from the same murine precursor antibody, margetuximab has enhanced anti-tumor activity compared with trastuzumab and may be an effective late-line treatment. However, data regarding the use of margetuximab in pre-treated Chinese patients are scarce. This study aimed to evaluate the efficacy and safety of margetuximab plus chemotherapy Methods: In this randomized, open-label, multicenter, phase II bridging study, eligible Chinese patients pretreated with ≥2 lines of anti-HER2 therapies were randomized 1:1 by stratified block randomization to margetuximab (15 mg/kg over at least 120 minutes) or trastuzumab (6 mg/kg over at least 30 minutes), each administered intravenously once every 21-day cycle and plus chemotherapy. Disease assessment was conducted once every two treatment cycles (6 weeks ± 7 days). The primary endpoint was progression-free survival (PFS) by blinded independent central review (BICR). Secondary endpoints included overall survival (OS), investigator-assessed PFS, objective response rate (ORR), duration of response (DoR), clinical benefit rate (CBR), and the incidence and severity of treatment-emergent adverse events (TEAEs). Results: Between February 4, 2020 and February 23, 2021, 123 patients were randomized to the margetuximab (n=62) and trastuzumab (n=61) arms. Among them, 15 and 7 patients, respectively, were still on study treatment as of data cut-off (September 3, 2021). Overall, 99.2% were female, median age was 53 years old. All patients were pretreated with trastuzumab, and 83.7% and 25.2%, respectively, were pretreated with tyrosine kinase inhibitors (TKIs) and pertuzumab. Baseline characteristics were numerically balanced between arms. BICR-assessed median PFS (mPFS) was 5.5 months in the margetuximab arm and 4.1 months in the trastuzumab arm, with a hazard ratio (HR) of 0.69 [95% confidence interval (CI): 0.43-1.12], which met the consistency criterion (HR <0.88) for bridging success. Median investigator-assessed PFS was 5.5 months in the margetuximab arm and 4.0 months in the trastuzumab arm (HR =0.63; 95% CI: 0.41-0.96). Median OS (mOS) was not yet reached. Both ORR and CBR were greater in the margetuximab arm (25.5% Conclusions: Margetuximab was effective and well-tolerated in this study, supporting its clinical use in pretreated HER2-positive MBC patients in China. Trial Registration: ClinicalTrials.gov Identifier: NCT04262804.