Denosumab improves density and strength parameters as measured by QCT of the radius in postmenopausal women with low bone mineral density

Background: Bone strength is determined by both cortical and trabecular bone compartments and can be evaluated radiologically through measurement of bone density and geometry. Quantitative computed tomography (QCT) separately assesses cortical and trabecular bone reliably at various sites, including the distal radius where there is a gradation of cortical and trabecular bone. We evaluated the effect of denosumab, a fully human monoclonal antibody that inhibits RANK ligand, on distal radius QCT in women with low bone mass to assess the impact of this novel therapy separately on trabecular and cortical bone.

Methods: Postmenopausal women (n=332) with spine areal bone mineral density (BMD) T-scores between -1.0 and -2.5 received denosumab 60 mg or placebo every 6 months during the 24-month study. QCT measurements along the distal radius were made using a whole-body computed tomography scanner and were used to determine the percentage change from baseline in volumetric BMD; volumetric bone mineral content (BMC); cortical thickness; volume; circumference; and density-weighted polar moment of inertia (PMI), a derived index of bone strength.

Results: Denosumab treatment significantly increased total BMD and BMC along the radius (proximal, distal, and ultradistal sections). At 24 months, the ultradistal region had the greatest percentage increase in total BMD (4.7% [95% CI, 3.6-5.7]; P<0.001) and total BMC (5.7% [95% CI, 4.8-6.6]; P<0.001) over placebo. When cortical and trabecular bone at the proximal and distal regions were separately assessed, cortical bone had significant (P<0.001) increases in BMD, BMC, and thickness, and trabecular bone had a significant increase in BMD relative to placebo (P<0.05). Bone strength, estimated by density-weighted PMI, significantly increased compared with placebo after 6 months of treatment, with the largest percentage increase occurring at 24 months in the ultradistal region (6.6% [95% CI, 5.6-7.6]; P<0.0001).

Conclusions: QCT measurements demonstrated that denosumab significantly increased BMD, BMC, and PMI along the radius over 24 months. Additionally, denosumab prevented the decrease in QCT-measured cortical thickness observed in the placebo group. These data extend the evidence from previous dual-energy X-ray absorptiometry studies for a positive effect of denosumab on both the cortical and trabecular bone compartments and propose a possible mechanism for the reduction in fracture risk achieved with denosumab therapy.