Update in osteoporosis and metabolic bone disorders

Considerable progress has been made in the development and testing of agents to treat osteoporosis. Most impressive are reports on new antiresorptive agents-both bisphosphonates ( ibandronate and zoledronic acid) and monoclonal antibodies (MAbs) (denosumab) directed against receptor activator of nuclear factor kappa B-ligand, a key molecule in the control of commitment and activation of osteoclasts. Bisphosphonates promise convenience and potency at slowing bone loss, whereas denosumab offers powerful suppression of resorption and rapid offset of action. Attention is also shifting from the osteoclast as a target for new therapies to the osteoblast and the osteocyte, with its complex network within the depths of bone. Wnt signaling through the frizzled receptor and its coreceptor, the low-density lipoprotein receptor related protein-5, appears from both molecular and in vivo evidence to be a pivotal pathway for modulating osteoblastic activity, bone formation, and bone strength. The recently identified product of the SOST gene or sclerostin has also been shown to block Wnt signaling. Sclerostin is produced by the osteocytes buried in the bone and is a new target to treat bone loss. Clinical trial reports indicate that the calcimimetic cinacalcet can effectively treat PTH hypersecretion due to primary and secondary hyperparathyroidism and parathyroid carcinoma. Lastly, it is now recognized that the matrix protein dentin matrix protein-1 enhances the release of the phosphate-regulating factor fibroblast growth factor 23 and that mutations in dentin matrix protein-1 play a causative role in a form of hypophosphatemic rickets.