Regulation of bone metabolism in osteoporosis. Novel drugs for osteoporosis in development

Bone is continuously regenerated and remodeled as an adaptation to mechanical load. Bone mass and fracture resistance are maintained by a balanced equilibrium between bone formation and bone resorption. Regeneration and response to mechanical load are, however, impaired in osteoporosis and during aging. Bone resorption is enhanced by chronic inflammation while bone formation is altered by rising levels of inhibitors in the aging organism. Core molecular principles of the regulation of bone metabolism in health and disease have been characterized and developed as therapeutic targets. The receptor activator of nuclear factor kappaB ligand (RANKL) and osteoclast-derived protease cathepsin K are important regulators and effectors of osteoclast differentiation and bone resorption. Bone formation is stimulated by bone morphogenetic proteins (BMP) and via the parathyroid hormone receptor and the Wnt signaling pathway. The principles of osteoclast inhibition using bisphosphonates have now been known for almost three decades. Based on more recent knowledge RANKL and cathepsin K have been developed as new therapeutic targets to inhibit bone resorption. While denosumab, a RANKL antibody, has already been introduced into routine treatment strategies, the cathepsin K antagonist odanacatib is currently in the licensing process. Bone formation can also be stimulated by local administration of BMPs, by systemic treatment with the parathyroid hormone fragment teriparatide and by using antibodies targeting the Wnt inhibitor sclerostin. The latter are presently being tested in phase III clinical studies. In the near future a panel of traditional and novel treatment strategies will be available that will enable us to meet the individual clinical needs during aging and for the treatment of osteoporosis.