A FcγRIII-engaging bispecific antibody expands the range of HER2-expressing breast tumors eligible to antibody therapy

// Marc Turini 1,2,3,4 , Patrick Chames 1,2,3,4 , Pierre Bruhns 5,6 , Daniel Baty 1,2,3,4,* and Brigitte Kerfelec 1,2,3,4,* 1 INSERM, U1068, CRCM, Marseille, France 2 Institut Paoli-Calmettes, Marseille, France 3 Aix-Marseille Universite, UM105, Marseille, France 4 CNRS, UMR7258, CRCM, Marseille, France 5 Departement d’Immunologie, Laboratoire Anticorps en Therapie et Pathologie, Institut Pasteur, Paris, France 6 INSERM, U760, Paris, France * Senior co-authors Correspondence: Brigitte Kerfelec, email: // Keywords : bispecific antibody, breast cancer, FcγRIIIA polymorphism, HER2, trastuzumab Received : March 20, 2014 Accepted : June 10, 2014 Published : June 11, 2014 Abstract Trastuzumab is established as treatment of HER2 high metastatic breast cancers but many limitations impair its efficacy. Here, we report the design of a Fab-like bispecific antibody (HER2bsFab) that displays a moderate affinity for HER2 and a unique, specific and high affinity for FcγRIII. In vitro characterization showed that ADCC was the major mechanism of action of HER2bsFab as no significant HER2-driven effect was observed. HER2bsFab mediated ADCC at picomolar concentration against HER2 high , HER2 low as well as trastuzumab-refractive cell lines. In vivo HER2bsFab potently inhibited HER2 high tumor growth by recruitment of mouse FcγRIII and IV-positive resident effector cells and more importantly, exhibited a net superiority over trastuzumab at inhibiting HER2 low tumor growth. Moreover, FcγRIIIA-engagement by HER2bsFab was independent of V/F158 polymorphism and induced a stronger NK cells activation in response to target cell recognition. Thus, taking advantage of its epitope specificity and affinity for HER2 and FcγRIIIA, HER2bsFab exhibits potent anti-tumor activity against HER2 low tumors while evading most of trastuzumab Fc-linked limitations thereby potentially enlarging the number of patients eligible for breast cancer immunotherapy.