An evaluation of serum osteocalcin in Paget's disease of bone and its response to diphosphonate treatment

Drug treatment strategies for Paget's disease: relieving pain and preventing progression

INTRODUCTION

Paget's disease of bone (PDB) is a focal bone disorder caused by a marked dysregulation of osteoblasts and osteoclasts in basic multicellular units, leading to abnormal and disorganized deposition of collagen fibers (the so-called "woven bone"). Therefore, pagetic bones are increased in size, and at increased risk for bone pain, deformities, fractures, osteoarthritis, and, more rarely, neoplastic degeneration.

AREAS COVERED

In this review we revise the available information concerning the pharmacological treatment of PDB.

EXPERT OPINION

PDB progresses slowly within the affected skeletal sites and, if untreated, often leads to bone overgrowth, with bone pain, deformity and a likely increased risk of complications. Thus, the primary goal of treatment is the restoration of a normal bone turnover, in order to relieve bone pain or other symptoms and possibly prevent the complications. PDB long remained a poorly treatable disorder until the discovery of antiresorptive agents such as calcitonin first and bisphosphonates (BPs) later. With the recent development of potent intravenous BPs like zoledronate, allowing a better control of disease activity over the long term with a single infusion, has contributed to a marked improvement of the clinical management of this invalidating disorder.

Adult Paget's disease of bone: a review

Adult PD of bone is the second commonest metabolic bone condition after osteoporosis. The condition is characterized by increased bone cell activity, with bone-resorbing osteoclasts often larger and containing more nuclei than normal, and osteoblasts producing increased amounts of disorganized bone. This leads to expanded bone of poor quality possessing both sclerotic and lytic areas. PD of bone has a strong genetic element, with a family history being noted in 10-20% of cases. A number of genetic defects have been found to be associated with the condition. The most common disease-associated variants identified affect the SQSTM1 gene, providing insights into disease aetiology, with the clinical value of knowledge of SQSTM1 mutation status currently under active investigation. The diagnosis may be suggested by an isolated raised total ALP without other identifiable causes. This can be confirmed on plain X-rays and the extent determined by isotope bone scan. The mainstays of treatment are the bisphosphonates, especially i.v. zoledronate, which results in long-term suppression of bone turnover. ALP is the usual means of monitoring the condition, although more specific bone turnover markers can be helpful, especially in coincident liver disease. Patients should be followed up to monitor for biochemical relapse or development of complications, which may require medical or surgical intervention.

Current options for the treatment of Paget's disease of the bone

Paget’s disease of bone (PDB) is a chronic bone remodelling disorder characterized by increased osteoclast-mediated bone resorption, with subsequent compensatory increases in new bone formation, resulting in a disorganized mosaic of woven and lamellar bone at affected skeletal sites. This disease is most often asymptomatic but can be associated with bone pain or deformity, fractures, secondary arthritis, neurological complications, deafness, contributing to substantial morbidity and reduced quality of life. Neoplastic degeneration of pagetic bone is a relatively rare event, occurring with an incidence of less than 1%, but has a grave prognosis. Specific therapy for PDB is aimed at decreasing the abnormal bone turnover and bisphosphonates are currently considered the treatment of choice. These treatments are associated with a reduction in plasma alkaline phosphatase (ALP) activity and an improvement in radiological and scintigraphic appearance and with a reduction in bone pain and bone deformity, Recently, the availability of newer, more potent nitrogen-containing bisphosphonates has improved treatment outcomes, allowing a more effective and convenient management of this debilitating disorder.

Biochemical assessment of Paget's disease of bone

Biochemical measurements of bone turnover provide an objective assessment of disease activity and the response to treatment. Alkaline phosphatase is the best characterized of the bone turnover markers and reflects the extent and activity of Paget's disease. However, in addition to bone-specific alkaline phosphatase (Bone ALP), there is also osteocalcin (OC) and procollagen type 1 N-terminal propeptide (P1NP) as formation markers. A variety of telopeptides (C-terminal telopeptide of type I collagen, [CTX], N-telopeptide of type I collagen [NTX]) or cross-link breakdown products of type 1 collagen can be used to assess bone resorption. Total alkaline phosphatase (Total ALP), Bone ALP, and P1NP all perform similarly in diagnosis and in evaluating the response to treatment, but the general availability, low interassay variation, and inexpensiveness of Total ALP makes it the best test for routine use. Measurement of the biological variability of the different markers in stable, untreated Paget's disease indicates how great a change (critical difference) is needed to define a true alteration in disease activity. Bone ALP, P1NP, and NTX show the highest therapy induced change/critical difference ratio during antiresorptive treatment. Some of the resorption markers show more complex changes in response to treatment. Pyridinoline (PYD) or deoxypyridinoline (DPD) cross-links of type 1 collagen are excreted in urine either as free or as peptide bound moieties, but it is the latter which decrease by the greatest amount in response to bisphosphonate therapy. Newly formed type 1 collagen contains an aspartyl-glycine motif (alphaCTX), which undergoes spontaneous isoaspartyl formation to betaCTX as the bone ages. In untreated Paget's disease, the alphaCTX is raised proportionately more (16-fold) than betaCTX (3-fold) and decreases in response to bisphosphonate therapy to a greater extent than betaCTX (measured in the sCTX assay). As bisphosphonates have become more potent, the aim of treatment has shifted toward the achievement of a rate of bone turnover in the lower part of the reference range. This is important because the duration of remission of disease activity is strongly determined by the post treatment nadir bone turnover.

Maladie de Paget, exploration et surveillance

Paget's disease is a progressive bone disease, characterized by bone hypertrophy and increased bone resorption. The pain and deformity that characterize its clinical course are not simple to assess, but it has a characteristic radiographic appearance. In most cases, treatment can be monitored through biological markers of bone turnover. Total alkaline phosphatase is the best marker and measurement of collagen breakdown products is useful in difficult cases. The goal of treatment is the normalization of these biological markers, to prevent complications of the disease.

Paget's disease of bone: benefits of neridonate as a first treatment and in cases of relapse after clodronate

This study assessed the efficacy of 200 mg of aminohexane bisphosphonate (neridronate) administered by intravenous infusion in a single dose or in two separate doses on consecutive days in 32 patients (16 males and 16 females, average age 66 years) affected by active Paget's disease of bone. Fifteen patients had never been treated with any antiresorptive agent and 17 had had unsatisfactory results from a prior clodronate treatment. All of the latter patients had failed to enter a remission stage (i.e., normalization of bone turnover was not reported at any time during treatment) and had had a full relapse within 6 months after clodronate infusion. In the present study bone-specific alkaline phosphatase (bAp), deoxypyridinoline (dPyr), and N- and C-terminal polypeptide of collagen type 1 (Ntx, Ctx) were determined before neridronate administration and at 1, 3, 6, and 12 months thereafter. Basal values of bAp were 51.7 +/- 2.3 microg/L, range 31.7-92.5 (normal range 6.2-23.6). No statistical differences in markers of bone turnover were evident in the basal state between new pagetic patients (bAp = 55.1 +/- 4.1) and those suffering a relapse after clodronate (bAp = 48.8 +/- 2.6). Neridronate induced an average percent change from baseline in excess bAp of 68.0 +/- 4.3 and in excess dPyr, Ntx, and Ctx of 68.1 +/- 11, 60.6 +/- 8.5, and 86.7 +/- 7.8, respectively. Markers of bone resorption declined more slowly in patients treated previously with clodronate, although the average change in percent decrement from baseline in excess bAp as well as in excess of bone resorption markers was not different from that registered in untreated pagetic patients. Response to treatment, defined as a percent decrement from baseline in excess bAp of 50% or more at any time during the 12-month follow-up, was observed in 27 patients (84.4%). Remission (a drop in bAp to within normal range) was achieved in 21 patients (65.6%) and was maintained in 12 at 12-month follow-up, with no significant differences between either 1- or 2-day infusions, or between new pagetic patients and those relapsing after clodronate. In 15 of 21 patients requiring analgesics to alleviate bone pain, pain was reduced or completely alleviated in 8. A slight, short-lived acute phase reaction (fever and/or arthromyalgia) occurred in 6 patients. To summarize, 200 mg of intravenous neridronate, in one or two doses, significantly reduced the biochemical indices of disease activity in the majority of patients, showing a normalization of bAp in more than 60%. We conclude that neridronate can be used safely in the treatment of patients with Paget's disease of bone either as a first bisphosphonate treatment or as retreatment for patients relapsing after clodronate.

Elimination and biochemical responses to intravenous alendronate in postmenopausal osteoporosis

Postmenopausal women with established vertebral osteoporosis were studied for 2 years to determine the terminal elimination half-life and the duration of response to treatment with intravenous alendronate (30 mg) given over 4 days. The urinary excretion of alendronate followed a multiexponential decline. Approximately 50% of the total dose was excreted over the first 5 days, and a further 17% was excreted in the succeeding 6 months. Thereafter, there was a much slower elimination phase with an estimated mean terminal half-life of greater than 10 years (n = 11). Urinary excretion of hydroxyproline and calcium decreased significantly from pretreatment values by day 3, reaching a nadir by 1 week (40% and 67% decrease, respectively). Thereafter, hydroxyproline remained suppressed for the following 2 years. In contrast, urinary calcium excretion returned gradually toward pretreatment values over the first year and during the second year was comparable to pretreatment values. Serum activity of alkaline phosphatase activity decreased over 3 months (23% reduction), increased gradually thereafter, and returned to pretreatment values at month 24. Bone mineral density measured at the spine increased by approximately 5% during the first year and remained significantly higher than pretreatment values at 2 years. We conclude that a short course of high doses of intravenous alendronate is associated with a prolonged skeletal retention of the agent. This open study also suggests that this regimen has a sustained effect on bone turnover persisting for at least 1 year.

Paget's disease of bone: diagnosis and management

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Sustained response to intravenous alendronate in postmenopausal osteoporosis

We studied the effects of alendronate (amino-hydroxybutylidene bisphosphonate) on biochemical indices of bone turnover and on lumbar spinal bone mineral density in 15 postmenopausal women with vertebral osteoporosis. Alendronate 7.5 mg daily was administered intravenously as a slow infusion for four consecutive days. Treatment was associated with a significant decrease in serum calcium (p < 0.01), fasting urinary calcium excretion (p < 0.01) and hydroxyproline excretion within several days followed a later decrease in serum alkaline phosphatase activity that showed a significant reduction at two months after treatment (p < 0.05). Serum calcium reverted to pretreatment values by the second week after infusion, but the decrease in alkaline phosphatase, urinary calcium, and hydroxyproline excretion persisted to six months after infusion. There was a 3% mean increase in lumbar bone mineral density at six months (p < 0.01). A transient lymphopenia or leucopenia was noted in eight patients and a short-lived fever in six. No other side effects were observed. This study demonstrates that shortterm exposure to high intravenous doses of alendronate induces suppression of bone resorption in osteoporosis that persists for at least 6 months after infusion. We conclude that a short exposure to high intravenous doses induces sustained effects on bone turnover in much the same manner as that observed in Paget's disease of bone.

Comparison of serum osteocalcin with total and bone specific alkaline phosphatase and urinary hydroxyproline:creatinine ratio in patients with Paget's disease of bone

Osteocalcin or bone Gla protein (BGP) is secreted by osteoblasts, and its serum concentration is elevated in a number of conditions with high bone turnover. A comparison of serum osteocalcin with total (TALP) and bone specific alkaline phosphatase (BALP) and urinary hydroxyproline/creatinine (OHP/Cr ratio) was performed in 13 patients with active Paget's disease of bone. BGP did not correlate significantly with either BALP or TALP, but did show a significant correlation with OHP/Cr ratio (r = 0.76; P < 0.01). BGP does not appear to be as sensitive a marker as BALP or TALP in Paget's disease.

Drugs used in the treatment of metabolic bone disease. Clinical pharmacology and therapeutic use

Osteoporosis is the most important metabolic bone disease and places an increasing burden on the healthcare system. The condition can be prevented by the early introduction of hormone replacement therapy. The role of bisphosphonates in achieving the same result is being actively explored. The attraction of preventing bone loss is that it preserves the micro-architecture of bone, and therefore its mechanical integrity. The great problem of treating the established condition is that substantial bone loss is accompanied by architectural disintegration. Replacing lost bone may not necessarily restore mechanical integrity and protect against fractures. The management of Paget's disease has been quite revolutionised by the introduction of the bisphosphonates. The condition is a result of a primary increase in osteoclastic bone resorption which can be corrected by bisphosphonates, with considerable symptomatic improvement. The increasing potency and safety margin of the newer agents has meant that the threshold for treatment has fallen. There is now potential for long term control of bone turnover with the hope of preventing late complications. Hypercalcaemia of malignancy is usually the result of both increased bone destruction and decreased urinary calcium excretion. These two components of hypercalcaemia demand different approaches to management. The general availability of an ever-expanding range of increasingly potent bisphosphonates has resulted in a dramatic improvement in the treatment of increased bone resorption associated with malignancy. Many types of tumour, either directly or indirectly, compromise the ability of the kidney to eliminate a calcium load derived from increased bone destruction. Calcitonin is the only agent which is currently available to counter this process.

Differential response of serum alkaline phosphatase and serum osteocalcin in Paget's osteosarcoma

Serum osteocalcin did not show any response to the onset of osteosarcoma in Paget's disease of bone whereas serum alkaline phosphatase increased rapidly. This suggests that osteocalcin is not useful in the diagnosis and management of Paget's osteosarcoma and does not reflect the same osteoblastic processes in bone as serum alkaline phosphatase.

A double-blind study of deflazacort and prednisone in patients with chronic inflammatory disorders

Deflazacort and prednisone were given to 26 patients with rheumatoid arthritis, polymyalgia rheumatica, or other chronic inflammatory diseases, in a double-blind study. Deflazacort rapidly and effectively suppressed disease activity in a manner supporting its assumed therapeutic potency of 83% that of prednisone. Prednisone induced a rapid increase in the level of daily calcium excretion that was not evident with deflazacort. Cortisol secretion was acutely inhibited by prednisone, but not by deflazacort. Neither corticosteroid had a significant effect on glucose metabolism, at the doses studied. Treatment with deflazacort may be an effective alternative to prednisone treatment, with fewer adverse effects on levels of calcium and cortisol, in patients with severe inflammatory conditions warranting the use of glucocorticoids.

Advances in the management of Paget's disease of bone

The advent of potent new bisphosphonates (diphosphonates) now makes it possible to restore and maintain normal bone turnover in many patients with Paget's disease of bone (osteitis deformans). This has necessitated a reappraisal of the indications for treatment, the ways in which disease activity and response are assessed, as well as the place of existing therapies. Measurements of urinary hydroxyproline and serum alkaline phosphatase remain the most useful markers of disease activity. Pyridinium crosslinks may prove to be more specific than hydroxyproline in the assessment of bone resorption but osteocalcin has been disappointing in monitoring the effect of treatment on bone formation. Etidronic acid (disodium etidronate), the first bisphosphonate introduced for clinical use, is a potent inhibitor of osteoclastic bone resorption but its potential is limited by the development of defective mineralisation with high dosage (10 to 20 mg/kg/day). The newer bisphosphonates, clodronic acid (clodronate) and pamidronic acid (pamidronate, APD), are free from this problem and appear able to control a wide range of disease activity. A small number of patients appear resistant to the agents but the underlying mechanism is unclear. The efficacy and safety of these bisphosphonates makes it likely that the threshold for treating asymptomatic patients will fall in the hope of preventing long term complications. These developments will lead to a reappraisal of the role of calcitonin which can now be administered by both the parenteral and intranasal routes. One focus of interest will be on the quality of the bone laid down during treatment. Meticulous radiographic studies have shown that calcitonin improves bone architecture and this may have particular relevance to the treatment of lytic disease. The relative merits of the different forms of therapy for Paget's disease need further evaluation, particularly with respect to the identification of specific advantages of individual drugs.

Circulating osteocalcin in primary biliary cirrhosis following liver transplantation and during treatment with ciclosporin

Circulating osteocalcin, a non-collagen bone protein is considered a useful indirect index of turnover. Osteocalcin levels were measured by radioimmunoassay in 57 female patients with primary biliary cirrhosis (PBC) from two centres, 52 normal female controls, 11 female patients following liver transplantation for PBC and seven female patients with PBC treated with Ciclosporin. Serum levels were significantly less in the PBC group (median 10 ng/ml) compared to normals (median 15 ng/ml: p less than 0.001). Patients with PBC treated with Ciclosporin had significantly higher levels than untreated patients (median 23 ng/ml: p less than 0.001). Levels also increased significantly following transplantation when all patients were receiving Ciclosporin (median 40 ng/ml: p less than 0.001). The results suggest that the abnormal bone metabolism of PBC is significantly altered by Ciclosporin and liver transplantation.