101 UTILITY OF BONE TURNOVER MARKERS IN GUIDING ZOLEDRONIC ACID THERAPY AFTER STOPPING DENOSUMAB

Abstract

Abstract Background Denosumab is commonly used to treat osteoporosis, but drug holidays are not an option, with a risk of rebound bone loss and vertebral fractures on stopping. However, data on therapy safety and efficacy is lacking beyond 10-years and switching treatment to bisphosphonates may be appropriate for some patients. Recent guidelines suggest treatment with Zoledronic Acid if stopping Denosumab, with consideration for a second dose based on subsequent Bone Turnover Markers (BTM). We aimed to assess BTM of older patients transitioned to Zoledronic acid after stopping Denosumab. Methods Patients who received Zoledronic acid 6 months after their last and final Denosumab injection were identified at our bone health clinic. We compared BTM on Denosumab therapy versus at 3 months post Zoledronic acid administration. BTM measured was cross linked C-Telopeptide of type I collagen (CTX ) (ng/ml). Results 48 patients were included (mean age 70.6 years and 90% female, mean Denosumab treatment 4.6 years). Mean CTX at baseline was 0.09 and 0.29 at 3-months post Zoledronic acid (mean difference 0.20, P <0.001). 40% of patients (19) had a CTX >0.30 (above the therapeutic range) at 3 months and in those, mean CTX was 0.45. Duration of prior Denosumab therapy, baseline CTX or age did not predict either change in CTX or CTX rises above 0.30. Conclusion All patients had a significant elevation in BTM at 3-months after transitioning to Zoledronic acid. However, a high proportion (40%) had a rise in CTX above the therapeutic range. Recent guidelines suggest treatment of such patients with a second infusion of Zoledronic acid to suppress bone resorption and bone loss. This highlights the value of using BTM in guiding therapy post Denosumab cessation. We did not find any predictors of greater rebound bone turnover but sample size was small and likely to be underpowered.