Evaluation of Factors That Affect Skeletal Responses to PTH

In women with postmenopausal osteoporosis, PTH increases bone mineral density more than anti-resorptive agents, and its use markedly reduces the incidence of new spine and non-spine fractures. Still, PTH is not a cure for osteoporosis in many patients because PTH-stimulated bone formation declines as PTH therapy continues. Biochemical analyses suggest that bone formation and resorption peak after 6 to 9 months of daily PTH therapy and then decline progressively. The study will last 18 months. Blood, urine, and bone density tests will occur at screening. At the start of the study, participants will be randomly assigned to one of two PTH dose regimens. Patients will go to Massachusetts General Hospital at Months 0, 3, 6, 9, 12, 15, and 18 for blood and urine collection. In addition, bone density tests by DXA will be performed at Months 0, 6, 12, and 18, and by quantitative CT scans at Months 0 and 18. Approximately 6 weeks after any change in PTH dose, each participant's blood calcium will be checked 4 to 6 hours after that day's PTH injection, and her 24-hour urine calcium excretion will also be checked. Participants may enroll in optional substudies that will test whether reduced skeletal responses to long-term treatment with PTH are accompanied by changes in its absorption and/or destruction and whether reduced skeletal responses to long-term treatment with PTH are accompanied by parallel reductions in kidney responses to PTH.