Antiresorptive Therapy and the Bone Turnover Markers—Assessed Fracture Risk in Postmenopausal Women CURRENT

Purpose/Introduction : The aim of this study was to determine relationship of the bone markers levels with the fracture risk and treatment monitoring in patients with osteoporosis. Bone markers may point out to on specific aspects of bone quality, detecting changes of bone mineral density, thus providing prognostic perspective and accounting for a substantial proportion of fracture risk reduction. Methods : The case-control study comprised data from 55 patients undergoing evaluation for osteoporosis at Medicus Universalis Polyclinic in Krusevac. Densitometric findings, P1NP, CTX and osteocalcin levels were determined in all patients twice – at the first assessment and 6 months after. While 30 patients took no medical therapy, 25 of them were treated with ibandronate. Results : No convincing difference in densitometric measurements between patients with and without prevalent fractures were noted, while mean osteocalcin and P1NP levels were significantly lower (p<0.05) in osteoporotic patients who suffered fractures. A significant correlation between those bone turnover markers and T-score was established, especially in the second measurement and in patients treated with ibandronate. Conclusion : In postmenopausal women and individuals with low BMD, the presence of increased bone turnover markers suggests an increased risk of fractures. Furthermore, these metabolic markers are useful in the monitoring of patients receiving antiresorptive therapy, wherein fast decline of their levels indicate favorable course. Their determination after 6 months offers the remarkable advantage in assessing the effectiveness of medical treatment comparing to 12–24 months required to document changes by BMD.


Introduction
With extended life expectancy, aging population in different regions of the world is facing almost epidemic incidence of osteoporosis. Causes are numerous; along with increased longevity, certain medications such as corticosteroids may have direct impact on bone mass; the same can be said about frequent routine obstetric interventions aiming to induce iatrogenic (surgical) menopause and changes in the way of life.
Estimated probability for future fractures in individuals over 50 is about 40% for female and 13% in male population. The biggest socio-epidemiological problem is hip fracture, taking in consideration high mortality rate, frequent sequels, and impaired quality of life; in addition, this type of fracture burdens society with high expenses of medical treatment (20)(21)(22)(23). Osteoporosis, clinically well defined by low bone density (low BMD) and disrupted bone microarchitecture, (established by DXA measurements, with T-score below -2,5), leads to enhanced bone fragility, and consequent suspectibility for fractures (28). Fractures most often arise in locations characterized by previous quantitative and qualitative deterioration of trabecular bone.
Technological developments in bone mineral density (BMD) measurements have led to diagnostic criteria that are widely applied. The World Health Organization diagnostic criteria for osteoporosis includes a BMD that lies 2.5 standard deviations or more below the average value for young healthy women (a T-score of <-2.5 SD) [4,5,15,16].
Markers of bone turnover are biochemical products released during bone formation or reabsorption.
Commonly used bone resorption markers are degradation products of type I collagen 1 (5,6), but noncollagenous proteins such as osteoclast-derived enzyme tartrate-resistant acid phosphatase 5b (TRACP) have also been investigated as resorption markers.
These compounds may be easily determined (usually in blood or urine). They reflect bone metabolic activity although they themselves have no function whatsoever in regulation of skeletal metabolism.
Markers of bone formation are direct or indirect products of the osteoblasts' activity expressed during various phases of their maturation and function. Type I collagen is an important component of bone matrix, and osteoblasts secrete its precursor -procollagen during the bone formation. The terminal peptides at each end of the procollagen molecule, procollagen type 1 pro-peptide (P1NP) and procollagen type I C pro-peptide (PICP), are cleaved by enzymes during bone matrix formation and released into the circulation (9)(10)(11).
Osteocalcin is another very sensitive and specific bone formation marker, also produced by osteoblasts. It is main non-collagen product of bone matrix. Osteocalcin demonstrates high calcium binding properties and serve as a messenger for calcitriol and leukocyte esterase inhibitor. It is excreted by the kidneys and its fragments may also be measured in urine. Newly formed osteoid undergoes maturation followed by mineralization. During this phase osteoblasts secrete alkaline phosphatase (ALP) into the extracellular fluid that may be detected in serum. In healthy adults, however, about half of the measured ALP is of hepatic origin, while the other half derived from the bone. Bone-derived isoform (BALP) is now widely available. Bone-derived alkaline phosphatase is very specific and sensitive marker of enhanced bone metabolism and reliable index of effectiveness of the antiresorptive therapy (34).
The pyridinium cross-links, pyridinoline (PYD) and deoxypyridinoline (DPD), formed during the maturation of bone collagen, are present in significant amounts in bone and dentine. These compounds are released during bone resorption and excreted in urine in the free and peptide-bound forms without being metabolized. (36) The peptide-bound forms of PYD and DPD include the Cterminal and N-terminal cross-linking telopeptides (CTX, NTX) of the type I collagen. These molecules are also released into the circulation and subsequently excreted in urine (9). Estrogen deficiency, associated with menopause, results in age related increase in bone remodeling and imbalance between bone formation and resorption.

Aim
The aim of this study was to determine relationship of the bone markers levels with the fracture risk and treatment monitoring in patients with osteoporosis and to present our experience in overall research agenda.

Methods
The case-control study comprised analytical data gathered from 55 patients undergoing evaluation and treatment protocols for osteoporosis at Medicus Universalis Polyclinic in Krusevac. All patients were previously diagnosed and staged with secondary osteoporosis by the experienced rheumatologist. Study was approved by Ethical Committee at Medical Faculty, University of Pristina -K.Mitrovica.
A diagnostic, therapeutic and follow-up protocol was formed for each patient. They were divided in 2 groups determined by presence of the osteoporotic fractures. Densitometric findings, P1NP, CTX and osteocalcin levels were determined in all patients twice -at the first assessment and 6 months after.
While 30 patients took no medical therapy, 25 of them were treated with ibandronate. Within this subgroup the densitometry and blood samples were obtained at the first assessment and 6 months after the initiation of therapy.
For the verification of secondary osteoporosis, we used KSA GE -LUNOR densitometer; the densitometry was performed at spinal and hip osteoporosis predilection sites.
Bio-humoral indicators were obtained by chemiluminescence, using Roche ELECSYS Immunoassay Analyzer.
Statistical analysis: Descriptive statistical parameters -arithmetic mean and standard deviation (SD) as well as determination of frequencies by observation characteristics were used to summarize the obtained information.
Testing of statistical significance of differences between mean values for data with normal distribution was performed by Student's t-test, while Kruskal-Wallis and Man-Whitney's U test were used for data that did not fit normal distribution. The Wilcoxon equivalent pair test was used to test for statistical significance of differences between dependent samples. For non-ordinal and non-parametric data, differences between the frequencies were evaluated by the Hi square test.
Pearson's linear correlation coefficient was used to calculate correlations.
The software programs SPSS and Instat were used for statistical processing.
Results T-score, CTX, osteocalcin and P1NP levels were obtained from the group of 54 female and one male patient ( Table 1), age 57.85 ± 0.9. Mean BMI was slightly above the normal (26.04 ± 4.18 kg/m 2 ).
About 9% of female patients were in pre-menopause, 15% had menopause with premature onset, while the rest had normal menopause. Densitometry revealed normal bone mass in 2 patients, osteopenia in 35 of them, while 18 patients had osteoporosis manifested on densitometry. About one -tenth of patients had prevalent (pre-existing) fractures.
Power analysis for the given data sample (55 patients) showed the following results: >0.85 for osteocalcin mean values, >0.95 for P1NP mean values and >0.80 for osteodensitometric measurements distribution (maximal power of the analysis is 1). These data confirmed the adequate sample size for the specified analyses.
There was no significant difference in densitometric measurements in patients with and without prevalent fractures; mean osteocalcin level (p<0.05) and P1NP level was significantly higher in patients suffering pre-existing fractures. (t=2.530, DF=53, p< 0.05), while beta-CTX levels showed no significant differences (Table 2). Table 3 delineates correlation coefficients between T-score and biochemical markers (CTX, osteocalcin and P1NP) in all patients with osteoporosis, regardless of ibandronate application. In the first assessment the correlation between T-score and biochemical markers of osteoporosis was weak and insignificant (r 0.17-0.20, p>0.05); in the second measurement performed six months after the first one, T score values were significantly related to CTX, osteocalcin and P1NP levels (r between 0.31 and 0.39, p<0.01 and 0.05 respectively).
In patients not treated with ibandronate, the correlation between T-score and osteocalcin, P1NP and CTX in the first measurement was insignificant, whereas a significant (low-to-middle level) correlation between T-score and those BTMs was estimated in the second measurement (Table 4). Also, middleto-high correlation between bone turnover markers was found in first as well as in second assessment.
In patients treated by ibandronate, the correlation between T-score and osteocalcin, P1NP and CTX in the first measurement (Table 5) was insignificant, similarly to those not receiving therapy. Also, CTX, osteocalcin and P1NP values were positively associated among themselves in this group. In the second assessment in patients previously treated with ibandronate T-score values were still not significantly related to BTM levels, while the biochemical markers themselves correlated significantly (except for lack of correlation between CTX and P1NP).

Discussion
Although BMD is considered golden standard in osteoporosis monitoring, it may reveal only 66-74 % osteoporotic changes. (9); moreover, normal BMD does not exclude individual fracture probability.
Bone markers level reflect total bone turnover, rather than resorption/formation balance in individual bone remodeling unit (35). Bone turnover influences microarchitecture of matrix, mineral content and mineralization process. There are some misconceptions in discerning bone strength from bone density: strong bone doesn't always mean optimal density. (12) Bone quality is not precisely definedit is described as the combination of all those properties that make bone resistant to fractures, independently of BMD. Bone strength is determined in vivo by bone markers turnover assessment, while bone geometry and microarchitecture may be estimated by evaluation of biochemical markers levels or using (more invasive) histomorphometric analysis of materials from bone biopsies. BTM might be a useful tool in predicting fracture risk and for monitoring antiresorptive treatment.
Increased bone turnover leads to demineralization and bone mass reduction and deteriorates bone microarchitecture and strength, thus enhancing fracture susceptibility.

Conclusion
In postmenopausal women and individuals with low BMD, the presence of increased bone turnover markers suggests an increased risk of fractures. Furthermore, these metabolic markers are useful in the monitoring of patients receiving antiresorptive therapy, wherein fast decline of their levels indicate favorable course. Their determination after 6 months offers the remarkable advantage in assessing the effectiveness of medical treatment comparing to 12-24 months required to document changes by BMD.