Efficacy and safety of margetuximab plus chemotherapy vs. trastuzumab plus chemotherapy in Chinese patients with pretreated HER2-positive advanced metastatic breast cancer: results from a randomized, open-label, multicenter, phase II bridging study

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 vs. trastuzumab plus chemotherapy in Chinese patients, and to determine whether the results are consistent with the clinical benefit of margetuximab observed in the pivotal global phase III study. 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% vs. 12.5%; 32.7% vs. 14.3%). Safety results were numerically comparable between the two arms. Anti-HER2 treatment-related infusion-related reactions (IRRs) were more common in the margetuximab arm than in the trastuzumab arm (12.9% vs. 1.7%). All IRRs could be resolved. 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.