Parathyroid hormone: An anabolic treatment for osteoporosis

Osteoporosis is a disease characterised by low bone mass, structural deteri oration of bone and increased risk of fracture. The prevalence, and cost, o f osteoporosis is increasing dramatically with our ageing population and th e World Health Organization now considers it to be the second-leading healt hcare problem. All currently approved therapies for osteoporosis leg., estr ogen, bisphosphonates, calcitonin and selective estrogen receptor modulator s) are anti-resorptive agents that act on osteoclasts to prevent further bo ne loss. A new class of bone anabolic agent capable of building mechanicall y strong new bone in patients with established osteoporosis is in developme nt. While the parathyroid hormone (PTH) is classically considered to be a b one catabolic agent, when delivered intermittently at low doses PTH potentl y stimulates cortical and trabecular bone growth in animals humans. The nat ive hPTH-(1-84) and its osteogenic fragment, hPTH-(1-34), have already ente red Phase III clinical trials. Understanding the mechanism of PTH's osteoge nic actions has led to the development of smaller PTH analogues which can a lso build mechanically normal bone in osteopenic rats. These new PTH analog ues are promising candidates for treating osteoporosis in humans as they ar e as efficacious as hPTH-(1-84) and hPTH-(1-34), but there is evidence that they may have considerably less ability to induce hypercalcemia, the major side effect of PTH therapy. In addition to treating osteoporosis, PTHs may be used to promote fracture healing, to restore bone loss in immobilized p atients, or following excessive glucocorticoid or prolonged spaceflight, an d to treat psoriasis.