Unbiased Combinatorial Screening Identifies a Bispecific IgG1 that Potently Inhibits HER3 Signaling via HER2-Guided Ligand Blockade.

HER2-driven cancers require phosphatidylinositide-3 kinase (PI3K)/Akt signaling through HER3 to promote tumor growth and survival. The therapeutic benefit of HER2-targeting agents, which depend on PI3K/Akt inhibition, can be overcome by hyperactivation of the heregulin (HRG)/HER3 pathway. Here we describe an unbiased phenotypic combinatorial screening approach to identify a bispecific immunoglobulin G1 (IgG1) antibody against HER2 and HER3. In tumor models resistant to HER2-targeting agents, the bispecific IgG1 potently inhibits the HRG/HER3 pathway and downstream PI3K/Akt signaling via a "dock & block" mechanism. This bispecific IgG1 is a potentially effective therapy for breast cancer and other tumors with hyperactivated HRG/HER3 signaling.