Clinical usefulness of biochemical resorption markers in osteoporosis

Assessing Heterogeneity in the N-Telopeptides of Type I Collagen by Mass Spectrometry

Collagen cross-links created by the lysyl oxidase and lysyl hydroxylase families of enzymes are a significant contributing factor to the biomechanical strength and rigidity of tissues, which in turn influence cell signaling and ultimately cell phenotype. In the clinic, the proteolytically liberated N-terminal cross-linked peptide of collagen I (NTX) is used as a biomarker of bone and connective tissue turnover, which is altered in several disease processes. Despite the clinical utility of these collagen breakdown products, the majority of the cross-linked peptide species have not been identified in proteomic datasets. Here we evaluate several parameters for the preparation and identification of these peptides from the collagen I-rich Achilles tendon. Our refined approach involving chemical digestion for protein solubilization coupled with mass spectrometry allows for the identification of the NTX cross-links in a range of modification states. Based on the specificity of the enzymatic cross-linking reaction we utilized follow-up variable modification searches to facilitate identification with a wider range of analytical workflows. We then applied a spectral library approach to identify differences in collagen cross-links in bovine pulmonary hypertension. The presented method offers unique opportunities to understand extracellular matrix remodeling events in development, aging, wound healing, and fibrotic disease that modulate collagen architecture through lysyl-hydroxylase and lysyl-oxidase enzymes.

Regulatory Perspectives in Pharmacometric Models of Osteoporosis

Osteoporosis is a disorder of the bones in which they are weakened to the extent that they become more prone to fracture. There are various forms of osteoporosis: some of them are induced by drugs, and others occur as a chronic progressive disorder as an individual gets older. As the median age of the population rises across the world, the chronic form of the bone disease is drawing attention as an important worldwide health issue. Developing new treatments for osteoporosis and comparing them with existing treatments are complicated processes due to current acceptance by regulatory authorities of bone mineral density (BMD) and fracture risk as clinical end points, which require clinical trials to be large, prolonged, and expensive to determine clinically significant impacts in BMD and fracture risk. Moreover, changes in BMD and fracture risk are not always correlated, with some clinical trials showing BMD improvement without a reduction in fractures. More recently, bone turnover markers specific to bone formation and resorption have been recognized that reflect bone physiology at a cellular level. These bone turnover markers change faster than BMD and fracture risk, and mathematically linking the biomarkers via a computational model to BMD and/or fracture risk may help in predicting BMD and fracture risk changes over time during the progression of a disease or when under treatment. Here, we discuss important concepts of bone physiology, osteoporosis, treatment options, mathematical modeling of osteoporosis, and the use of these models by the pharmaceutical industry and the Food and Drug Administration.

Study of the distribution by age group of serum cross-linked C-terminal telopeptide of type I collagen and procollagen type I N-propeptide in healthy Japanese women to establish reference values

Osteoporosis prevention is an important public health goal. Bone turnover markers are clinically measured to assess bone strength. C-terminal telopeptide of type I collagen (CTX) is released when collagens degrade and serves as an indicator of bone resorption. Simple CTX immunoassays are now available. However, serum CTX (sCTX) reference ranges for Japanese women are lacking. Procollagen type I N-propeptide (intact P1NP) reflects osteoblast activity, serving as a marker of bone formation. Because sCTX and intact P1NP are clinically applied as bone turnover markers, we determined reference ranges for both sCTX and intact P1NP in healthy Japanese women. We collected 228 blood samples from healthy Japanese women aged 19-83 years, grouped by age and menopausal status. We measured sCTX and intact P1NP and examined their correlation. sCTX values differed significantly between the two consecutive decade groups encompassing 19-39 years of age, intact P1NP values between 20 and 30 s, between post-menopausal 50 and 60 s, and between pre-and post-menopausal women in their 50 s. The mean sCTX of 91 healthy pre-menopausal women was 0.255 (0.100-0.653) ng/mL, the intact P1NP in 90 women 33.2 (17.1-64.7) μg/L. Corresponding values for post-menopausal women were 0.345 (0.115-1.030) ng/mL and 41.6 (21.9-79.1) μg/L. sCTX correlated with intact P1NP. Bone resorption markers are measured to assess anti-resorption agents, bone formation markers to assess the effects of bone-forming agents. The sCTX and intact P1NP reference values determined herein, in healthy Japanese women, are expected to be useful for osteoporosis treatment, assessment of fracture risk, and other clinical applications.

The amino- and carboxyterminal cross-linked telopeptides of collagen type I, NTX-I and CTX-I: a comparative review

Bone diseases such as osteoporosis or bone metastases are a continuously growing problem in the ageing populations across the world. In recent years, great efforts have been made to develop specific and sensitive biochemical markers of bone turnover that could help in the assessment and monitoring of bone turnover. The amino- and carboxyterminal cross-linked telopeptides of type I collagen (NTX-I and CTX-I, respectively) are two widely used bone resorption markers that attracted great attention due to their relatively high sensitivity and specificity for the degradation of type I collagen, and their rapid adaptation to automated analyzers. However, the clinical performance of both markers differs significantly depending on the clinical situation. These differences have caused considerable confusion and uncertainty. If used correctly, both markers have great potential to improve the management of many bone diseases. We here review the biochemistry, analytical background and clinical performance of NTX-I and CTX-I, as documented in the accessible literature until March 2008.

Hypocitraturia: a risk factor for reduced bone mineral density in idiopathic hypercalciuria?

The association between idiopathic hypercalciuria (IH) and reduced bone mineral density (BMD) has been described in adults and children. Frequently, hypocitraturia (HC) is an associated condition. To determine the effect that HC may have on bone metabolism of these patients, we studied 88 children with IH at diagnosis, divided into the following groups: group 1-44 (50%) patients with associated HC; group 2-44 (50%) patients without HC; group 3 (29 subjects), a healthy control group. Urinary and blood electrolytes, as long as urinary N-telopeptide, were measured. Lumbar spine (L2-L4) and femoral neck bone mineral density (BMD) and bone mineral content (BMC) were measured by dual energy X-ray absorptiometry. There was no difference in age between the three groups (P=0.80), but weight, height, body mass index, and bone age were lower (P<0.01) and serum intact parathyroid hormone (iPTH) was higher (P<0.05) in group 1 than in groups 2 and 3. N-telopeptide, measured in urine, did not differ between groups. The following bone densitometry parameters: lumbar spine BMC, BMC adjusted for height (BMCh), BMC adjusted for width of vertebrae (BMCw) and BMD, as well as femoral neck BMD, were significantly lower in group 1 than in groups 2 and 3 (P<0.01). When we corrected densitometry parameters for height, BMC was lower in group 1 and not in group 2 when compared with controls.

CONCLUSIONS

Children with IH and associated HC may have a higher risk of bone mass loss and consequent osteopenia. Further studies are needed to assess the role that hypocitraturia may have in this form of bone disease.

The use of biochemical markers in osteoporosis

Osteoporosis is a skeletal disease in which there is a loss of, or de-crease in, bone mass with a deterioration of the microarchitecture of bone tissue. The disease is progressive, taking place over a period of years, and involves derangements in the processes of bone turnover. These derangements can be classified as those in which osteoclast activity (resorption) is stimulated so that more bone is re-moved than formed or in which osteoblast activity (formation) is hindered such that refilling of the resorption cavity is incomplete. Regardless of the process, a key pathologic development is the net loss of bone mass. This article reviews the use of biochemical markers in osteoporosis.

Bone alterations in children with idiopathic hypercalciuria at the time of diagnosis

Some children with idiopathic hypercalciuria (IH) develop bone alterations at some stage of the disease. The aims of this study were to evaluate bone mass in 88 children with IH (G1) at the time of diagnosis and to compare the findings with data for a control group of 29 normal children (G2). Kidney and bone metabolism markers were measured in both groups, and bone densitometry was performed. Serum alkaline phosphatase, intact parathyroid hormone, urinary calcium and uric acid were significantly higher in G1, whereas urinary volume and urinary citrate excretion were lower. The following densitometric parameters were significantly lower in G1: (1) lumbar spine (L(2)-L(4)) bone mineral density (BMD), bone mineral content (BMC), BMC corrected for height and for width of the vertebra, volumetric BMD (BMDvol), and Z score; (2) whole-body BMD; (3) femoral neck BMD. Lumbar spine BMDvol was reduced (osteopenia) in 35% of the patients compared with G2. N telopeptide, a urinary marker of bone resorption, was significantly higher in G1 than in G2, and was negatively correlated with lumbar spine BMD and BMDvol. Children with urinary lithiasis or idiopathic hyperuricosuria associated with IH showed no significant differences in bone metabolism compared with children without these associations. We conclude that (1) there is an altered bone metabolism in IH, with osteopenia already present at diagnosis in 35% of the patients; (2) N telopeptide is one of the most useful markers of bone alterations in IH, especially at an early stage of the disease; (3) investigation of bone metabolism is necessary in IH to prevent future serious consequences such as osteoporosis and bone fractures.

Monitoring hormone replacement therapy by biochemical markers of bone metabolism in menopausal women

Biochemical markers of bone metabolism are divided into two groups: formation and resorption markers. Bone turnover is a dynamic process, which increases in postmenopausal period. Hormone replacement therapy (HRT) can diminish this increased bone turnover. Biochemical markers reflect acute changes in bone metabolism. Therefore, they may be very useful for the prediction of subsequent bone mineral density changes after HRT in menopausal women. Both oral and transdermal routes of HRT are efficacious in decreasing the levels of biochemical markers. However, markers do not replace bone mineral density measurement. Collagen type I cross linked N-telopeptide, collagen type I cross linked C-telopeptide, and osteocalcin are the most promising markers.

Diurnal, week-to-week, and long-term variation in urine deoxypyridinoline cross-link excretion in healthy older women

OBJECTIVES

To establish a reference range for morning and afternoon excretion of urinary deoxypyridinoline (DPD) in apparently healthy older women selected from a volunteer database. To assess the extent of diurnal variation and short and long-term within-subject longitudinal variation.

DESIGN

Prospective, observational, cohort study.

SETTING

Clinical Age Research Unit, King's College School of Medicine, London, United Kingdom.

PARTICIPANTS

Forty-two women aged 68 to 89 (median age 75) selected from a volunteer database.

METHODS

Subjects completed an osteoporosis risk factor questionnaire and a physical examination and had a measurement of the broadband ultrasound attenuation and speed of sound of their right heel. Subjects provided six urine samples: morning and afternoon at baseline and 1 week and 60 weeks later for measurement of DPD.

RESULTS

The mean baseline values for DPD of morning and afternoon samples were 7.2 nM/mM and 6.0 nM/mM creatinine, respectively. The majority of subjects showed diurnal variation, with mean afternoon values 15% lower than morning values (P <.0001 for afternoon vs morning values). The mean difference in DPD after 60 weeks was 1.67 nM/mM for morning and 1.34 nM/mM for afternoon creatinine. This difference was not significant. Some individuals displayed marked changes in DPD excretion with no change in health status or treatment. DPD excretion in a nonfasting afternoon sample showed similar characteristics to morning void samples in terms of scatter, week-to-week variation, and long-term reproducibility.

CONCLUSIONS

The study was set up to provide background data to assist the development of a clinical osteoporosis service for older women. Further studies are needed to determine whether these measurements predict fracture risk and respond to treatment changes in this age group.

The values of urinary NTx in postmenopausal women undergoing HRT; the role of additional alendronate therapy

OBJECTIVE

To determine the changes in levels of urinary NTx at the end of the 6th month of oral and transdermal hormone replacement therapy (HRT) and the effects of additional alendronate therapy for osteoporotic women.

METHOD

Of 66 postmenopausal women 23 were treated with oral estradiol+norethisterone acetate (E+P), and 22 were treated with transdermal estradiol+norethisterone acetate. The third group consisted of 21 women with osteoporosis (bone mineral density < 100 mg/cm(3)) and treated with oral E+P plus alendronate 10 mg/day.

RESULT

Significant decreases of urinary NTx levels were seen after HRT in all study groups (P < 0.05). But the decline of NTx levels was not different between the oral and transdermal HRT groups (P > 0.05). There was no additional decrease in the levels of NTx with alendronate therapy (P > 0.05) but NTx excretion diminished more in patients with high baseline levels.

CONCLUSION

The decline of NTx at the end of the 6th month of HRT reflects the decrease of bone resorption and it is not related to the route of administration.

Biochemical markers of bone turnover

BACKGROUND

Osteoporosis in many countries has reached epidemic proportions. This has stimulated the development of biochemical markers to assist in the assessment of osteoporotic risk and in monitoring the efficacy of treatment. Biochemical markers of bone turnover are products released from osteoblasts and osteoclasts or collagen breakdown products.

MARKERS

Markers of bone formation include bone-specific alkaline phosphatase (BAP), osteocalcin (OC) and procollagen peptides. All of these can be measured easily by immunoassay techniques. Of these markers, OC has been extensively studied. However, OC undergoes in vitro degradation, thus, assay results are variable. BAP, on the other hand, is much more stable and shows less within-person biological variation. Bone resorption markers include tartrate-resistant acid phosphatase (TRAP) and collagen breakdown products, such as pyridinium cross-links, galactosyl hydroxylysine and cross-linked telopeptides, such as CTx and NTx. Of these, deoxypyridinium (DPD) has been extensively studied. DPD shows diurnal variation and the within-individual biological variation is large. Of the newer assays, NTx appear to show large differences at menopause.

CONCLUSIONS

Thus, serum BAP and DPD or NTx are the current choice of bone markers.

Osteopenia and osteoporosis in gastrointestinal diseases: diagnosis and treatment

An increased awareness of the higher incidence of osteopenia and osteoporosis associated with a number of gastrointestinal disease states has occurred over the last few years. High rates of bone loss have been reported in luminal diseases such as inflammatory bowel disease and celiac disease as well as in cholestatic liver diseases and in the post-liver transplant setting. The post-gastrectomy state and chronic pancreatitis are also associated with decreased bone density. Publications over the last year have provided a better understanding of the true incidence of osteoporosis and fracture risk in these gastrointestinal disease states. Dual-energy x-ray absorptiometry remains the diagnostic procedure of choice. Biochemical markers of bone resorption have a role in identifying those patients with ongoing bone loss and monitoring their response to therapy. Identification of patients at risk and initiation of measures to prevent bone loss form the optimal therapeutic strategy. This article reviews advancements in the understanding of the development and activation of osteoblasts and osteoclasts. It also reviews the recent data concerning the diagnosis and treatment of bone loss associated with various gastrointestinal disease states.

Markers of bone turnover in postmenopausal women receiving hormone replacement therapy

We investigated the effects of hormone replacement therapy (n = 27) on biochemical markers of bone turnover in a cross-sectional study of 127 postmenopausal women (according to WHO guidelines 18 patients had normal bone mineral density and 109 suffered from bone loss). Urinary excretion of free deoxypyridinoline and C- or N-telopeptide fragments of type I collagen served as bone resorption markers, serum osteocalcin as a bone formation marker. In women with no hormone replacement therapy, only C- and N-telopeptides correlated significantly with the lumbal T-score as an index for bone mineral density. Patients with bone loss receiving hormone replacement therapy exhibited significantly lower C-telopeptide, N-telopeptide and osteocalcin levels than those with no therapy (mean -45%, -43% and -26%, respectively), while deoxypyridinoline showed no significant differences. Among the markers investigated, C- and N-telopeptides seemed to be more reliable to detect therapeutic effects on bone metabolism. We present a preliminary model to evaluate bone turnover and resorption/formation rate.

Biological variability in assessing the clinical value of biochemical markers of bone turnover

Analytical and biological variability of three bone markers, deoxypyridinoline (DPD), CrossLaps (CTx) and galactosylhydroxylysine (GHYL) were compared. From 14 healthy subjects (six women, eight men; age 29-44 years) recruited from our laboratory staff, two sets of samples of early morning urine were obtained - four samples taken weekly for 4 weeks (all subjects) and three samples taken monthly for 3 months from five subjects. Data were expressed as the ratio to creatinine concentration. All the methods met the analytical goals (CV(A)< or =1/2CV(I(within-subject))) DPD 0.06, CTx 0.05 and GHYL 0.07 with CV(I(within-subject)) being 0.22, 0.19 and 0.38, respectively. The reference values were of limited usefulness particularly for CTx and GHYL, the index of individuality (II) being 0.50 and 0.48 respectively. As the index of heterogeneity (IH) was not significant, being 0.23 for CTx, 0.28 for DPD and 0.46 for GHYL, which are all <1.71 (1+2S.D.), within-subject variances can be used to calculate the reference change value (RCV): 0.58 for DPD, 0.54 for CTx and 1. 08 for GHYL. Moreover, we found constant variations in DPD and CTx, week to week and month to month. Our findings suggest that DPD and CTx provide more reliable results than GHYL, showing a lower within-subject variation, a lower and time-constant RCV allowing reliable monitoring without regard for timing.