Narlumosbart (also known as Narusuobai Monoclonal Antibody) is a humanized IgG4-kappa monoclonal antibody that specifically targets receptor activator of nuclear factor-κB ligand (RANKL), a key cytokine involved in bone metabolism and tumor-bone interactions. As an IgG4 isotype, it is engineered to minimize effector functions (e.g., ADCC, CDC) and focus on ligand blockade. By binding to RANKL with high affinity, narlumosbart prevents RANKL from interacting with its receptor RANK on osteoclast precursors and mature osteoclasts, thereby inhibiting osteoclast differentiation, activation, and survival.