Safety and Efficacy Study to Evaluate Denosumab Compared With Zoledronic Acid in Postmenopausal Women With Osteoporosis

Inclusion Criteria: * Ambulatory postmenopausal women. * Age 55 years or older * Subject has provided informed consent prior to any study specific procedures * Received oral bisphosphonate therapy for osteoporosis at least 2 years prior to screening visit * Screening BMD (g/cm²) values at the lumbar spine, total hip or femoral neck values of equal to or less than those listed in the protocol. * At least 2 lumbar vertebrae and one hip must be evaluable by dual energy x-ray absorptiometry (DXA) at the screening visit Exclusion Criteria: * Received other osteoporosis treatment or bone active treatment * Evidence of history of any of the following: * hyperthyroidism (stable on antithyroid therapy is allowed) * hypothyroidism (stable on thyroid replacement therapy is allowed) * hypo- or hyperparathyroidism * hypo- or hypercalcemia based on the central laboratory reference ranges * Recent tooth extraction (within 6 months of screening visit) * Paget disease of bone (subject report or chart review) * other bone diseases which affect bone metabolism (eg, osteopetrosis, osteogenesis imperfecta) (chart review) * Abnormalities of the following per central laboratory reference ranges: * vitamin D deficiency (25\[OH\] vitamin D level \< 20 ng/mL), repletion will be allowed and subjects may be re-screened * hypercalcemia * elevated transaminases ≥ 2.0 x upper limits of normal (ULN) * History of any solid organ or bone marrow transplant * Malignancy (except nonmelanoma skin cancers, cervical or breast ductal carcinoma in situ) within the last 5 years * Known intolerance to calcium or vitamin D supplements * Self-reported alcohol or drug abuse within 12 months prior to screening * Currently receiving treatment in another investigational device or drug study, or less than 30 days since ending treatment on another investigational device or drug study(s) * History or evidence of any other clinically significant disorder, condition or disease that in the opinion of the Investigator or Amgen physician, if consulted, would pose a risk to subject safety or interfere with the study evaluation, procedures or completion