Effects of denosumab on bone density, mass and strength in women with postmenopausal osteoporosis

Denosumab is a human monoclonal antibody which specifically blocks receptor activator of nuclear factor kappa B ligand and is a very potent antiresorptive drug. Its efficacy in reducing the risk of vertebral, hip and nonskeletal fracture has been proven in a large prospective, randomized multicenter study of 7808 postmenopausal women with osteoporosis [Fracture Reduction Evaluation of Denosumab in Osteoporosis Every 6 Months (FREEDOM) trial]. Denosumab causes somewhat greater increases in bone mineral density (BMD) than the class of bisphosphonate antiresorptives. Denosumab also causes an increase in bone mass and bone strength in the spine, ultradistal and diaphysis of the radius, proximal tibia and the hip. Recently long-term treatment with denosumab has been shown to cause a continued almost linear increase in total hip and femoral neck BMD beyond 3 years up to 8 years. In this respect, denosumab seems to differ from the bisphosphonate group in which the rate of improvement of BMD diminishes and for some drugs becomes negative after 3-4 years when the process of secondary mineralization flattens out. This unique property of an antiresorptive drug points towards mechanisms of action which differ from the bisphosphonate group. Both types of antiresorptives decrease cortical porosity but contrary to bisphosphonates the reduction in cortical porosity continues with denosumab which, in addition, also seems to cause a slight continuous modeling-based formation of new bone despite suppression of bone remodeling. The net effect is an increase in cortical thickening and bone mass, and increased strength of cortical bone. This may contribute substantially to the significant further reduction of the nonvertebral fracture risk which was found in the long-term denosumab arm of the FREEDOM extension trial during years 4-7.