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 kappa B 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 1 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.