Impact of bone mineral density in reducing fracture risk in patients receiving alendronate plus alfacalcidol therapy

Backgroud: Changes in bone mineral density (BMD) are a potential surrogate marker for fracture endpoints in clinical trials. However little is known whether the increase in BMD in response to combination treatment with alendronate plus alfacalcidol is associated with fracture risk reduction. We aimed to evaluate the impact of BMD on fracture risk in osteoporosis patients, using the data from the randomized clinical trial comparing alendronate plus alfacalcidol with alendronate alone. Methods: We selected 412 patients with two or more prevalent vertebral fractures and who had BMD measurements at baseline and after 6, 12, and/or 24 months out of 2022 patients from the database of the Japanese Osteoporosis Intervention Trial. Patients in this subset who received combination treatment with alendronate plus alfacalcidol had shown a lower risk of fracture than patients treated with alendronate alone. We used Poisson regression model analysis to calculate the proportion of treatment effect (PTE) that was attributable to BMD increases in patients receiving combination treatment. Results: The highest PTE attributable to changes in BMD was 1.2% in patients with a BMD increase of 3% or more in the lumbar spine. For BMD measurements of the radius, the highest PTE was 2.8% with a BMD increase of 0% or more. For BMD measurements of the metacarpal bone, the highest PTE was 1.2% with a BMD increase of 3% or more. In patients with a BMD greater than or equal to 70% of the young adult mean in the lumbar spine, the PTE attributable to BMD was 0.2%. In patients with a BMD greater than or equal to 70% of the young adult mean in the radius, the PTE attributable to BMD was 0.3%. Conclusions: The additional effects of alfacalcidol in reducing fracture risk do not likely result from increased BMD; other mechanisms remain a possibility.