Long-term biochemical response after bisphosphonate therapy in Paget's disease of bone. Proposed intervals for monitoring treatment.

To monitor the long-term evolution of Paget's disease activity after treatment with tiludronate by using serum total alkaline phosphatase (TAP) and more sensitive markers such as bone alkaline phosphatase (BAP), procollagen type I N propeptide (PINP) and urinary N-terminal cross-linking telopeptide of type I collagen (NTX); to analyse the predictors of long-term response to therapy; and to study the most appropriate intervals of time for monitoring the response to therapy. Thirty-two patients with Paget's disease were included in the study. All received 400 mg of oral tiludronate daily for 3 months. A total of 21 patients completed the study. In these patients, serum TAP, BAP and PINP and urinary NTX were measured at baseline and at 1, 6, 12 and 24 months after discontinuation of therapy. Quantitative bone scintigraphy was performed at baseline and at 6 and 24 months after the end of treatment, obtaining a scintigraphic activity index (SAI). Patients were classified into two groups depending on the long-term response to treatment: Group 1, patients who presented a persistent and significant decrease in disease activity at this time, n = 12 (57%) and Group 2, patients who presented a relapse in the activity of the disease at 24 months after treatment, n = 9 (43%). The relapse of disease activity was defined as a significant increase of SAI (>13%) between 6 and 24 months after the end of treatment, whereas the response to therapy was defined as a significant reduction in SAI (>13%) at 6 months after the end of treatment. In addition, these results were compared with the biochemical evolution of bone markers. Biochemical markers and SAI decreased significantly after therapy and the nadir response was observed at 6 months. At this time 100% of patients responded to therapy. The persistent long-term response was associated with lower baseline indices of bone turnover (serum BAP<60 ng/ml or TAP<600 IU/l). The intervals of time for monitoring depended on the marker used: no patient from Group 1 presented a biochemical relapse in serum TAP at 1 and 2 yr after the end of treatment whereas 33 and 45% of these patients showed relapsed serum BAP at these time points. Moreover, all patients from Group 2 presented a biochemical relapse of serum BAP at 2 yr whereas in only 33% of these patients did serum TAP relapse at this time. Most of the Pagetic patients treated with tiludronate presented a long-term response, which persisted 2 yr after the end of treatment. The nadir response to treatment was observed 6 months after discontinuation of therapy whereas the relapse of disease activity was already observed 1 yr after the end of therapy and depended on both the baseline disease activity and the bone marker used in the evaluation.