Antitumor activities against breast cancers by an afucosylated anti-HER2 monoclonal antibody H2Mab-77-mG2a-f

Breast cancer patients with high levels of human epidermal growth factor receptor 2 (HER2) expression have worse clinical outcomes. Anti-HER2 monoclonal antibody (mAb) is the most important therapeutic modality for HER2-positive breast cancer. We previously immunized mice with the ectodomain of HER2 to create the anti-HER2 mAb, H₂Mab-77 (mouse IgG₁, kappa). This was then altered to produce H₂Mab-77-mG_2a-f, an afucosylated mouse IgG_2a. In the present work, we examined the reactivity of H₂Mab-77-mG_2a-f and antitumor effects against breast cancers in vitro and in vivo. BT-474, an endogenously HER2-expressing breast cancer cell line, was identified by H₂Mab-77-mG_2a-f with a strong binding affinity (a dissociation constant [K_D]: 5.0 × 10⁻⁹ M). H₂Mab-77-mG_2a-f could stain HER2 of breast cancer tissues in immunohistochemistry and detect HER2 protein in Western blot analysis. Furthermore, H₂Mab-77-mG_2a-f demonstrated strong antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) for BT-474 cells. MDA-MB-468, a HER2-negative breast cancer cell line, was unaffected by H₂Mab-77-mG_2a-f. Additionally, in the BT-474-bearing tumor xenograft model, H₂Mab-77-mG_2a-f substantially suppressed tumor development when compared with the control mouse IgG_2a mAb. In contrast, the HER2-negative MDA-MB-468-bearing tumor xenograft model showed no response to H₂Mab-77-mG_2a-f. These findings point to the possibility of H₂Mab-77-mG_2a-f as a treatment regimen by showing that it has antitumor effects on HER2-positive breast tumors.