Osteoporos Int (2008) 19:339–348 DOI 10.1007/s00198-007-0462-5
ORIGINAL ARTICLE
Bone turnover and bone collagen maturation in osteoporosis: effects of antiresorptive therapies I. Byrjalsen & D. J. Leeming & P. Qvist & C. Christiansen & M. A. Karsdal
Received: 15 March 2007 / Accepted: 9 August 2007 / Published online: 11 September 2007 # International Osteoporosis Foundation and National Osteoporosis Foundation 2007
Abstract Summary Bone collagen maturation may be important for anti-fracture efficacy as the reduction in risk is only partly explained by a concomitant increase in BMD during antiresorptive therapy. Different treatments caused diverse profiles in bone collagen degradation products, which may have implications for bone quality. Introduction The aim of the present study was to evaluate the effect of different anti-resorptive treatments on bone collagen maturation measured as the ratio between the degradation products of newly synthesized and mature isomerized C-telopeptides of type I collagen. Methods Participants were from cohorts of healthy postmenopausal women participating in double blind, placebocontrolled 2-year studies of alendronate, ibandronate, intranasal hormone replacement therapy (HRT), oral HRT, transdermal HRT, or raloxifene (n=427). The non-isomerized ααCTX and isomerized ββCTX were measured in urine samples obtained at baseline, and after 6, 12, and 24 months of therapy. Results Bone collagen maturation measured as the ratio between ααCTX and ββCTX showed that bisphosphonate treatment induced a collagen profile consistent with an older matrix with a 52% (alendronate) and 38% (ibandronate) reduction in the ratio between the two
I. Byrjalsen (*) : D. J. Leeming : P. Qvist : C. Christiansen : M. A. Karsdal Nordic Bioscience A/S, Herlev Hovedgade 207, Herlev DK-2730, Denmark e-mail:
[email protected]
CTX isoforms vs. 3% and 15% with HRT or raloxifene, respectively. Conclusions Anti-resorptive treatments had different effects on the endogenous profile of bone collagen maturation. Whether that effect on bone collagen has an impact on bone strength independent on the treatmentdependent effect on BMD should be investigated. Keywords Bone quality . Bone turnover . Collagen . Fracture risk . Matrix proteins
Introduction Bone is a dynamic tissue being continuously remodeled throughout life not only to maintain the calcium homeostasis but also to maintain mechanical strength by replacing fatigued bone by new, mechanically sound bone [1]. Bone formation and resorption are two highly coupled metabolic processes [2–6], although the balance between resorption and formation differs in different phases of life. The loss of ovarian sex steroids at menopause results in accelerated bone turnover with a predominance of bone resorption over bone formation [7]. The related negative calcium balance promotes bone loss, increases bone fragility, and thereby the risk of future fractures [8]. A rational approach to counter this altered balance is the inhibition of bone resorption, although also bone formation is inhibited due to coupling between these cellular events [9–11]. Accordingly, pronounced inhibition of osteoclast function can be expected to impair the dynamic renewal of skeletal tissue leading to alterations in the biochemical composition of bone and changed mechanical properties [12]. The inorganic phase of the bone provides the stiffness,
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i.e., the ability to resist compression, whereas the organic phase, mainly constituted of type I collagen, provides bone its flexibility, i.e., the ability to absorb energy and undergo deformation. How biochemical changes in the bone collagen are associated with fracture and bone quality remains to be investigated and understood. The bone collagen is continuously renewed and posttranslationally modified, including enzymatic cross-linking (pyridinoline and deoxypyridinoline), non-enzymatic glycation (pentosidine), and β-isomerization occurring in the DG motif of the CTX epitope (1207EKAHDGGR1214). It has been shown that the degree of isomerization reflects the skeletal age [13, 14]. Studies on in vitro aging of fetal bovine cortical bone recently demonstrated that the bone mechanical properties changed as a function of time, as did the amount of cross-linking and pentosidine, as well as the degree of β-isomerization [15]. The biochemical changes of collagen were associated with a 30% decrease in bending and compressive yield stress and a 2.5-fold increase in compressive post-yield energy absorption. In another recent study of human lumbar vertebrae the amount of pentosidine and degree of β-isomerization was related to the biomechanical properties of bone after adjustment for BMD [16]. BMD is considered the most important determinant of fracture risk with the World Health Organization (WHO) criteria of osteoporosis defined as BMD-measurements falling 2.5 standard deviation below the average of young adults (T-score ≤−2.5). However, recent studies have shown that up to one-half of patients with incident fractures have BMD above the WHO diagnostic threshold criteria [17–19], and it has also been reported that the risk of an osteoporotic fracture is approximately tenfold higher in old compared with young individuals at the same BMD level [20]. Additionally, recent data demonstrate that during antiresorptive therapy the reduction in fracture risk is only vaguely explained by the concomitant increase in BMD [21]. Even though drugs have become more and more effective in terms of increasing BMD, achievements in terms of antifracture efficacy have not followed correspondingly, e.g., the mean increase in spinal BMD to alendronate is sevenfold higher compared to that of calcitonin, yet the reductions in vertebral fracture risk are fairly comparable being 44% and 36%, respectively [21, 22] . The discrepancies in BMD response and fracture risk protection may be due to the different mode of action of different anti-resorptive treatments. To look into this aspect of anti-resorptive therapy, we investigated the effect on bone collagen age, measured as the ratio of ααCTX to ββCTX in urine samples during different types of anti-resorptive treatment regimens of bisphosphonates, hormone replacement therapy (HRT), and raloxifene (RLX).
Osteoporos Int (2008) 19:339–348
Methods Subjects and study design All studies were conducted in accordance with Helsinki Declaration II, and approved by local ethical committees. Written informed consent was obtained for all participants. Bisphosphonate - Alendronate The participants were part of a double blind, placebo-controlled, randomized 3-year study on the effects of oral alendronate in the prevention of postmenopausal osteoporosis [10]. Study participants who completed the first 2-year study period and who received daily 10 mg of alendronate (n=14), 20 mg of alendronate (n=13), or placebo (n=13) were included. Bisphosphonate – Ibandronate The participants were part of a double blind, placebo-controlled, randomized 2-year study on the effects of oral ibandronate in the prevention of postmenopausal osteoporosis [23]. A randomly selected subgroup of study participants receiving daily continuous oral 2.5 mg of ibandronate daily (n=36), intermittent oral 20 mg of ibandronate every 2nd day for 24 days every 3 months (n=36), or placebo (n=26) was included. After the first year of therapy, the placebo group was crossedover to receive active treatment. HRT - intranasal estradiol The participants were part of a double blind, placebo-controlled, randomized 2-year study on the effects of daily intranasal 17β-estradiol for prevention of bone loss in early postmenopausal women [24]. A randomly selected subgroup of study participants receiving daily 300 μg of 17β-estradiol (n=50), or placebo (n=25) was included. HRT - oral estradiol The participants were part of a double blind, placebo-controlled, randomized 2-year study on the effects of 17β-estradiol continuously combined with drospirenone on the safety and efficacy for prevention of postmenopausal osteoporosis [25]. A randomly selected subgroup of study participants receiving daily 1 mg of 17βestradiol continuously combined with 1 mg or 2 mg of drospirenone (n=49), or placebo (n=33) was included. HRT - transdermal estradiol The participants were part of a double blind, placebo-controlled, randomized 2-year study on the effects of daily transdermal 17β-estradiol continuously combined with transdermal levonorgestrel on the safety and efficacy for prevention of postmenopausal osteoporosis [26]. A randomly selected subgroup of study participants receiving daily 45 μg of 17β-estradiol contin-
Osteoporos Int (2008) 19:339–348
uously combined with 40 μg of levonorgestrel (n=35), or placebo (n=20) was included.
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Results Demographic data
Raloxifene (RLX) The participants were part of a double blind, placebo-controlled, randomized 2-year study on the effects of raloxifene on bone mineral density in postmenopausal women [27]. A randomly selected subgroup of study participants receiving daily 60 mg of raloxifene (n=30), or placebo (n=47) was included. From all studies second void fasting urine samples were obtained at baseline, after 6 months, 12 months, and 24 months of therapy. Biochemical markers and bone mineral density Urine samples were stored at −20°C until analysis. The urinary excretion of ααCTX was measured using the ALPHA CrossLaps ELISA [14], and the urinary excretion of ββCTX was measured using the Serum CrossLaps One Step ELISA (urine samples were pre-dilution 1:10 in assay buffer before measurement)(Nordic Bioscience Diagnostics, Herlev, Denmark) [14, 28]. Urinary creatinine was measured by routine chemistry method and used for calculation of creatinine-corrected marker levels. The urine samples were measured at same time in all subgroups in present study. Bone mineral density (BMD) was measured by dualenergy X-ray absorptiometry at the spine (BMDspine), and hip (BMDhip) with Hologic QDR-1000, or QD-R2000 densitometers (Hologic, Waltham, MA, USA). Statistical analysis To assess longitudinal changes, the values were calculated for each person and expressed as the percentage of the initial baseline value. The data of the creatinine-corrected values of ααCTX and ββCTX and the relative changes were logarithmically transformed to obtain normality and symmetry of variances. Analysis of variance (ANOVA) was used for comparison of baseline data between study groups with Tukey–Kramer adjusted significance levels in the multiple comparisons. The two-tailed Student’s t-test was applied for pair-wise comparisons of data from the active treatment group and placebo within each study and for comparison of selected subgroup with non-selected subgroup within each study. Regression analysis was used to assess relation of baseline characteristics with age, and to calculate the changes in BMD during treatment. For all tests p
