Serum osteoprotegerin and its ligand in Paget's disease of bone -: Relationship to disease activity and effect of treatment with bisphosphonates

Objective. To test the following hypotheses: 1) osteoprotegerin (OPG) and receptor activator of nuclear factor kappaB ligand (RANKL) serum levels in patients with Paget's disease are related to disease activity and are different from those in healthy individuals; 2) interleukin-6 (IL-6), a cytokine that has been shown to have higher levels in Paget's disease, modulates these factors; and 3) the antiresorptive effect of bisphosphonates in Paget's disease of bone may be mediated through these local factors. Methods. The study group comprised 31 patients with Paget's disease who received 400 mg/day of oral tiludronate for 3 months. Serum levels of OPG, RANKL, IL-6, bone alkaline phosphatase (AP), N-terminal type I procollagen propeptide, urinary N-terminal crosslinking telopeptide of type I collagen, and urinary alpha-C-terminal crosslinking telopeptide of type I collagen were measured at baseline and I month after the end of therapy. In addition, the RANKL:OPG ratio was calculated, and disease activity was evaluated at baseline by quantitative bone scintigraphy. Results. Mean baseline OPG values were higher in patients with Paget's disease than in healthy control subjects (P < 0.005), but RANKL and IL-6 values and RANKL:OPG ratios in the 2 groups were similar. OPG concentrations decreased significantly after treatment with tiludronate (P < 0.005), whereas no significant changes were observed in serum RANKL values. No correlation was found between either bone markers or quantitative scintigraphic indices and serum levels of OPG, RANKL, IL-6, and RANKL:OPG ratios. Serum OPG decreased significantly only in those patients with baseline OPG values >4.1 pM/liter. Conclusion. Serum OPG increases in Paget's disease and decreases after treatment with tiludronate, especially in patients with the highest OPG values. In contrast, RANKL serum levels and RANKL:OPG ratios are unmodified in patients with Paget's disease. Although serum OPG, RANKL, and IL-6 values were unrelated to disease activity, the increase in OPG may reflect a protective mechanism of the skeleton to compensate for increased bone resorption.