A recombinant human immunoglobulin G1 (IgG1) bispecific antibody directed against the human epidermal growth factor receptor 2 (HER2) and the negative immunoregulatory human cell receptor programmed cell death protein 1 (PD-1; PDCD1; CD279), with potential immune checkpoint inhibitory and antineoplastic activities. Upon administration, fidasimtamab simultaneously targets, binds to and inhibits HER2 and PD-1 and their downstream signaling pathways, and bridges PD-1-expressing T-cells to HER2-expressing tumor cells. This may inhibit tumor cell proliferation of HER2-overexpressing cells. Inhibition of PD-1-mediated signaling may restore immune function through the activation of T-cells and T-cell-mediated immune responses against the HER2-expressing tumor cells. PD-1, a transmembrane protein in the immunoglobulin superfamily (IgSF) expressed on T-cells, functions as an immune checkpoint that negatively regulates T-cell activation and effector function when activated by its ligands programmed cell death-1 ligand 1 (PD-L1) or 2 (PD-L2); it plays an important role in tumor evasion from host immunity.