Potentiation of trastuzumab emtansine (T-DM1)-driven antitumor activity by pertuzumab in a HER2-positive gastric cancer model.

Abstract

e13502 Background: The efficacy and safety of T-DM1 combined with pertuzumab in HER2-positive breast cancer is currently being investigated in a phase III clinical trial (MARIANNE). Anti-HER2 therapy with T-DM1 combined with pertuzumab in HER2-positive gastric cancer is of great interest. In a previous study in a HER2-positive gastric cancer xenograft model, T-DM1 in combination with pertuzumab demonstrated enhanced antitumor activity compared with each agent alone, and potentiated ADCC and apoptosis activity (AACR, 2011). In this study, we investigated the mechanisms underlying the enhanced activity observed with these two drugs. Methods: The binding of T-DM1 to cell surface HER2 was measured in a HER2-positive human gastric cancer cell line (NCI-N87) by flow cytometry using Alexa Fluor 647-labeled T-DM1. The phosphorylation status of HER signaling molecules was assessed by Western blot, and cellular localization of T-DM1 was determined by fluorescence microscopy using labeled T-DM1 in the presence/absence of pertuzumab. Results: Binding of T-DM1 to cell surface HER2 was significantly increased in the presence of pertuzumab. Concomitant treatment with T-DM1 and pertuzumab led to a reduction of phosphorylated (p)EGFR or pHER3 in EGF or heregulin-stimulated cells, respectively, resulting in suppression of downstream pERK or pAkt pathways. Fluorescence microscopy revealed that pertuzumab facilitated the internalization of HER2-bound T-DM1 from the cell surface into the cytoplasm. Conclusions: Results suggest that the enhanced antitumor activity observed with the combination of T-DM1 and pertuzumab may be attributed to 1) increased binding of T-DM1 to HER2, which could potentially augment ADCC and diminish downstream HER2-signaling, and 2) increased T-DM1 internalization into the cytoplasm, which may potentiate tumor cell death caused by DM1. These results suggest that T-DM1 in combination with pertuzumab may provide clinical benefit for patients with HER2-positive gastric cancer.