Bispecific Antibody Binding To RANKL and Osteonectin with Enhanced Localization to the Bone

Therapeutics reducing bone turnover, such as denosumab (Dmab), an anti-RANKL antibody, can provide treatments for patients with bone destruction. However, some patients with osteoporosis or localized primary bone tumors and many patients with various types of bone-metastatic cancer display unsatisfactory responses to Dmab. For achieving greater efficiency of RANKL neutralization in the bone microenvironment by enhancing the distribution of Dmab to the bone, we reengineered Dmab by fusing with single-chain variable fragments of an antibody specific for osteonectin (On), which is abundantly expressed in osseous tissues. The bispecific antibody, Dmab-FvOn, showed a similar activity as Dmab in inhibiting RANKL as examined in an osteoclast differentiation assay. When administered to mice, Dmab-FvOn was found to localize in increased proportions at the endosteum of the bone where osteonectin is abundant. Our study suggests that by linking anti-RANKL with an osteonectin-targeting moiety, a greater proportion of the therapeutic effector can be distributed in the bone. Future studies are needed to investigate whether the bispecific antibody can achieve higher therapeutic efficacy and lower toxicity.