ELISA Methodology for Detection of Modified Osteoprotegerin in Clinical Studies

Abstract

Osteoprotegerin (OPG), also known as osteoclast inhibitory factor, is a soluble receptor of the tumor necrosis factor receptor superfamily. The protein is secreted as a covalent, disulfide-linked homodimer, which is the predominant extracellular form (1), and is expressed in multiple tissues (1)(2)(3). OPG-mediated pathways might have a role in osteoporosis (3)(4)(5)(6) because estrogen increases OPG gene expression (4)(5). OPG maintains the structure of healthy bone and inhibits osteoclast activation and differentiation (3)(7). In the vascular system, OPG inhibits pathological calcification in the media intima (3). OPG has been proposed for therapy of osteopenic disorders, such as postmenopausal osteoporosis, Paget disease, rheumatoid arthritis, hypercalcemia, and lytic bone metastases (8). Initially, we developed an antibody-based ELISA method with an anti-human OPG monoclonal antibody for capture and an anti-human OPG polyclonal antibody for detection. Yano et al. (9) raised the concern for us that we may not detect the active dimeric OPG with antibody capture because they reported that serum OPG increased with age and that the monomer was the predominant form of OPG in human serum. Although they used a different antibody-dependent ELISA method (monoclonal capture and detection), the results reported by Yano et al. (9) do not correspond with the work performed at Amgen (1)(3)(4)(5)(7)(8)(10)(11)(12). We reasoned that OPG ligand (OPGL) (2)(7)(8)(10)(11)(12)(13)(14)(15)(16), also …