Association of Changes in Bone Remodeling and Coronary Calcification in Hemodialysis Patients: A Prospective Study Daniela Veit Barreto, MD,1 Fellype de Carvalho Barreto, MD,1 Aluízio Barbosa de Carvalho, MD,1 Lilian Cuppari, PhD,1 Sérgio Antonio Draibe, MD,1 Maria Aparecida Dalboni, PhD,1 Rosa Maria Affonso Moyses, MD,2 Kátia Rodrigues Neves, MD,2 Vanda Jorgetti, MD,2 Márcio Miname, MD,3 Raul D. Santos, MD,3 and Maria Eugênia F. Canziani, MD1 Background: Vascular calcification is common and constitutes a prognostic marker of mortality in the hemodialysis population. Derangements of mineral metabolism may influence its development. The aim of this study is to prospectively evaluate the association between bone remodeling disorders and progression of coronary artery calcification (CAC) in hemodialysis patients. Study Design: Cohort study nested within a randomized controlled trial. Setting & Participants: 64 stable hemodialysis patients. Predictor: Bone-related laboratory parameters and bone histomorphometric characteristics at baseline and after 1 year of follow-up. Outcomes: Progression of CAC assessed by means of coronary multislice tomography at baseline and after 1 year of follow-up. Baseline calcification score of 30 Agatston units or greater was defined as calcification. Change in calcification score of 15% or greater was defined as progression. Results: Of 64 patients, 26 (40%) had CAC at baseline and 38 (60%) did not. Participants without CAC at baseline were younger (P ⬍ 0.001), mainly men (P ⫽ 0.03) and nonwhite (P ⫽ 0.003), and had lower serum osteoprotegerin levels (P ⫽ 0.003) and higher trabecular bone volume (P ⫽ 0.001). Age (P ⫽ 0.003;  coefficient ⫽ 1.107; 95% confidence interval [CI], 1.036 to 1.183) and trabecular bone volume (P ⫽ 0.006;  coefficient ⫽ 0.828; 95% CI, 0.723 to 0.948) were predictors for CAC development. Of 38 participants who had calcification at baseline, 26 (68%) had CAC progression in 1 year. Progressors had lower bone-specific alkaline phosphatase (P ⫽ 0.03) and deoxypyridinoline levels (P ⫽ 0.02) on follow-up, and low turnover was mainly diagnosed at the 12-month bone biopsy (P ⫽ 0.04). Low-turnover bone status at the 12-month bone biopsy was the only independent predictor for CAC progression (P ⫽ 0.04;  coefficient ⫽ 4.5; 95% CI, 1.04 to 19.39). According to bone histological examination, nonprogressors with initially high turnover (n ⫽ 5) subsequently had decreased bone formation rate (P ⫽ 0.03), and those initially with low turnover (n ⫽ 7) subsequently had increased bone formation rate (P ⫽ 0.003) and osteoid volume (P ⫽ 0.001). Limitations: Relatively small population, absence of patients with severe hyperparathyroidism, short observational period. Conclusions: Lower trabecular bone volume was associated with CAC development, whereas improvement in bone turnover was associated with lower CAC progression in patients with high- and low-turnover bone disorders. Because CAC is implicated in cardiovascular mortality, bone derangements may constitute a modifiable mortality risk factor in hemodialysis patients. Am J Kidney Dis 52:1139-1150. © 2008 by the National Kidney Foundation, Inc. INDEX WORDS: Hemodialysis; renal osteodystrophy; vascular calcification; cardiovascular disease.
C
ardiovascular disease is the leading cause of death in the chronic kidney disease (CKD) population.1 There is accumulating evidence that these individuals show a greater burden of vascular calcification than the general population,2 and this excess calcification has been reported as a strong and independent prognostic marker of mortality in patients with CKD.3
The impact of vascular calcification on cardiac mortality is caused by distinct consequences: (1) the complication of atherosclerotic lesions, ensuing reduced myocardial perfusion, and (2) the increment in arterial stiffness, which produces hemodynamic changes that may result in increased afterload, left ventricular hypertrophy, and decreased coronary artery perfusion.4
From the 1Department of Internal Medicine, Division of Nephrology, Federal University of São Paulo; 2Department of Internal Medicine, Division of Nephrology, University of São Paulo; and 3The Lipid Clinic of the Instituto do Coração (InCor, Heart Institute), University of São Paulo, São Paulo, Brazil. Received February 20, 2008. Accepted in revised form June 24, 2008. Originally published online as doi: 10.1053/j.ajkd.2008.06.024 on September 29, 2008.
Address correspondence to Maria Eugênia F. Canziani, MD, Rua Pedro de Toledo, 282, CEP 04039-000, São Paulo, SP, Brazil. E-mail:
[email protected] © 2008 by the National Kidney Foundation, Inc. 0272-6386/08/5206-0015$34.00/0 doi:10.1053/j.ajkd.2008.06.024
American Journal of Kidney Diseases, Vol 52, No 6 (December), 2008: pp 1139-1150
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Recent studies have shown that the development of vascular calcification is the result of an active process, similar to ossification.5-7 After specific triggering, vascular smooth muscle cells undergo phenotypic changes characteristic of osteoblast-like cells, which include vesicle formation, matrix production, and attraction of local factors that take part in the mineralization process. In physiological conditions, this process is regulated by several circulating and local inhibitors, such as osteoprotegerin (OPG), matrix Gla protein (MGP), and fetuin-A (␣2–HeremansSchmid glycoprotein).8-10 Conversely, in the uremic milieu, a variety of stimuli seem to be capable of disrupting this physiological inhibition of vascular calcification. High serum phosphorus, calcium, and calcium-phosphorus product levels are pointed out as important promoters of vascular smooth muscle cell differentiation.11 Renal osteodystrophy is virtually ubiquitous in patients with CKD and is characterized by a spectrum of bone histological abnormalities that may result in hyperphosphatemia and hypercalcemia as a consequence of either excessive bone resorption, which characterizes osteitis fibrosa, or the inability of bone to buffer the excess calcium load in patients with adynamic bone disease.12 Thus, it is possible to hypothesize that there is a causative pathological link between renal osteodystrophy and vascular calcification in patients with CKD. The present study is part of a randomized controlled trial comparing the effects of sevelamer with those of calcium acetate on bone metabolism in long-term hemodialysis patients. There was no difference in coronary artery calcification (CAC) progression or changes in bone remodeling between patients randomly assigned to calcium acetate or sevelamer treatment.13 Baseline data have been published elsewhere.14 The present analysis was undertaken to prospectively evaluate the association between bone remodeling disorders and progression of CAC in hemodialysis patients.
METHODS Participants and Study Design The present data consist of a secondary analysis of a randomized clinical trial comparing the effects of sevelamer (Renagel, 800-mg tablets; Genzyme Co, Cambridge, MA) and calcium acetate (PhosLo, 667-mg tablets; Fresenius
Figure 1. Patient distribution. Abbreviation: MsCT, multislice computed tomography.
Medical Care, Waltham, MA) on CAC. Patient distribution is shown in Fig 1. One hundred fifty-three patients on hemodialysis therapy for at least 3 months were screened at 4 dialysis centers in São Paulo, Brazil. All patients received 4-hour hemodialysis sessions 3 times/wk using hollow-fiber polysulfone or acetate membranes. Exclusion criteria included serious gastrointestinal disease, ethanol or drug abuse, active malignancy, human immunodeficiency virus infection, chronic inflammatory disease, use of steroids, severe hyperparathyroidism (defined as intact parathyroid hormone [iPTH] ⬎ 1,000 pg/mL), body weight greater than 100 kg, continuous use of antiarrhythmic or seizure drugs, pregnancy or breast-feeding, previous myocardial revascularization, uncontrolled diabetes mellitus, or hypertension (as deemed by the investigator). After screening, subjects underwent a 2-week washout period in which all phosphate binders were withheld. Patients with hyperphosphatemia (serum phosphorus ⬎ 5.5 mg/dL) at the end of the washout period were eligible. One hundred one patients were then randomly assigned to receive either sevelamer or calcium acetate for 12 months. None of the randomized patients had ever used sevelamer before this trial began. The present analysis includes 64 patients who completed the 12-month protocol and had baseline and end-of-study multislice coronary tomography and bone biopsy data available. Thirty-seven patients were excluded for the following reasons: kidney transplantation (12 patients), parathyroidectomy (2 patients), refusal to undergo bone biopsy (9 patients), death (9 patients), and change of dialysis modality or referral to other dialysis facility (5 patients). There were no differences between excluded patients and those who completed the study regarding age, time on dialysis therapy, smoking, and presence of diabetes mellitus or hypertension. Baseline CAC and bone histomorphometric parameters were also similar.
Bone Remodeling and Vascular Calcification The initial dose of phosphate binder was calculated for each patient according to the usual label recommendations. Thereafter, dosage was adjusted monthly based on results of laboratory tests. Study end points were serum phosphorus level of 3.5 to 5.5 mg/dL, ionized calcium level of 1.11 to 1.40 mmol/L, and iPTH level of 150 to 300 pg/mL. Investigators were also encouraged to alter calcium dialysate concentration and vitamin D treatment during the study based on baseline bone biopsy diagnosis. Therefore, patients with low-turnover bone disease were withdrawn from vitamin D treatment and shifted to a 2.5-mEq/L calcium dialysate concentration regardless of iPTH levels. Daily phosphorus intake and oral calcium load were estimated from a 3-day diet inquiry at baseline and 6 and 12 months, as described elsewhere.15 Blood samples were drawn periodically to verify changes in mineral metabolism. To determine CAC status, patients underwent multislice coronary tomography at baseline and after 1 year. All patients signed an informed consent form. The study protocol was reviewed and approved by the internal review board.
Laboratory Measurements Whole blood was collected from all participants at the respective study sites in a fasting state the morning before the first hemodialysis session of the week. Laboratory evaluation included monthly measurements of ionized calcium and phosphorus. Serum iPTH (Immulite Assay; DPC, Los Angeles, CA; reference range, 10 to 65 pg/mL) was determined every second month. OPG (enzyme-linked immunosorbent assay; Immundiagnostik Laboratory, Bensheim, Germany; reference range, 30.45 ⫾ 12.1 pg/mL), soluble receptor activator of nuclear factor-B ligand (enzymelinked immunosorbent assay; Immundiagnostik Laboratory; detection limit, 1.5 pg/mL), and 25-hydroxy vitamin D (radioimmunoassay; DiaSorin, Stillwater, MN; reference range, 18 to 62 ng/mL) levels were determined at baseline and 12 months. As specific serum markers of bone turnover, bone-specific alkaline phosphatase (enzyme immunoassay; Metra Biosystem Inc, Mountain View, CA; reference range, 11.6 to 42.7 U/L for men and 15 to 41.3 U/L for women) levels were determined at baseline and 6 and 12 months, and deoxypyridinoline levels (enzyme immunoassay; Quidel Corp, San Diego, CA; reference range, 3.25 ⫾ 0.66 nmol/L for men and 3.43 ⫾ 0.64 nmol/L for women) were determined at baseline and 12 months.
Imaging Procedure Calcification score was determined using 16-slice multislice coronary tomography (Somatron Volum Zoom; Siemens AG, Erlhagen, Germany). A chest radiographic image without contrast was acquired while the subject was in apnea to determine initial and final scan levels. Images of each section were acquired during a 150-millisecond exposure with a distance of 3 mm between each slice. Timing of image acquisition was coordinated with the diastolic phase of the cardiac cycle at 60% of the RR interval with the use of electrocardiographic monitoring. All scans were analyzed using Workstation software (Indigo 02 SGI; Mountain View, CA) to determine calcium score. This software can detect calcified lesions with density of at least 130 Housfield units
1141 and minimal surface area of 0.5 mm2. Total calcium score calculation was based on measurement of the total volume and area of calcified lesions, as well as measurements of mean and peak calcium density. Total score was the sum of each coronary score, expressed in modified Agatston units (AU).16,17 A single experienced investigator who was blinded to all other patient data read all scans. The reported intertest variability using similar methods was 15% or less, and the SD of interscan variability was approximately 10%. Additionally, the ability to track CAC progression is most accurate in patients with intermediate to higher scores because the absolute error in CAC measurement approximates actual CAC scores in patients with low scores (ie, CAC, 1 to 30 AU).18 Therefore, based on baseline CAC scores, patients were divided into 2 groups: the noncalcified (baseline calcium score ⬍ 30 AU) and calcified groups (baseline calcium score ⱖ 30 AU). Subsequently, the relative progression of CAC was calculated for the calcified group, subdividing this group into the nonprogressor group (patients with relative change in calcium score ⬍ 15% in 1 year), and progressor group (patients with relative change in calcium score ⱖ 15% in 1 year). Absolute progression of CAC was calculated as the difference between the 12-month and baseline scores, and relative progression was calculated by using the ratio: (absolute progression/baseline score) ⫻ 100.
Bone Biopsy Baseline and 12-month bone specimens were obtained alternatively from the right or left iliac crest to avoid interference of repair processes from the previous biopsy. The procedure was conducted using a trephine with a 7-mm inner diameter adapted to an electrical drill (Gauthier Medical, Rochester, MN). All patients were prelabeled with oral tetracycline (20 mg/kg/d for 3 days) administered over 2 separated periods 10 days apart. Bone fragments were submitted for the usual processing and histological studies.19 Bone histomorphometric analysis was conducted using the semiautomatic method contained in the Osteomeasure software (Osteometrics Inc, Atlanta, GA). Histomorphometric parameters were those suggested by the American Society of Bone and Mineral Research histomorphometry nomenclature committee.20 Reference ranges used for static parameters were obtained from local controls,21 whereas dynamic parameters followed those described elsewhere.22 Renal osteodystrophy was classified into one of the classic types according to the following criteria: (1) predominant hyperparathyroid bone disease, defined as bone formation rate/bone surface (BFR/BS; reference range, 0.13 ⫾ 0.07 3/2/d for men and 0.07 ⫾ 0.03 3/2/d for women), as well as either osteoblast surface/bone surface (reference range, 1.2% ⫾ 1.4% for men and 1.2% ⫾ 3.2% for women) or osteoclast surface/bone surface (reference range, 0.03% ⫾ 0.11% for men and 0.03% ⫾ 0.06% for women) more than 1 SD greater than the normal range, osteoid volume trabecular bone volume (OV/BV; reference range, 2.9% ⫾ 2.7% for men and 1.55% ⫾ 1.9% for women) within or greater than the normal range and marrow fibrosis greater than 0.5%; (2) adynamic bone disease, defined as BFR/BS and OV/BV greater than 1 SD less than the normal range and marrow fibrosis less than 0.5%; (3) osteomalacia, defined as BFR/BS
1142 greater than 1 SD less than the normal range and OV/BV greater than 1 SD greater than the normal range; and (4) mixed uremic osteodystrophy, defined as BFR/BS, OV/BV, and mineralization lag time (reference range, 21.3 ⫾ 2.3 days for men and 23.7 ⫾ 2.7 days for women) greater than 1 SD greater than the normal range and marrow fibrosis greater than 0.5%. Thereafter, these types were grouped into 1 of 2 major patterns: high-turnover bone status (predominant hyperparathyroid bone disease or mixed uremic osteodystrophy) or low-turnover bone status (osteomalacia or adynamic bone disease). According to the recently proposed Turnover, Mineralization and Volume classification for bone histomorphometry,23 selected parameters for bone turnover (bone formation rate), mineralization (osteoid volume and mineralization lag time), and volume (trabecular bone volume) were presented in comparisons between groups.
Statistical Analysis Data are presented as mean ⫾ SD, except for calcium score, which is presented as median and range because of its non Gaussian distribution. Time-average means were calculated for all variables, and the resulting values are presented in comparisons between groups. Demographic, laboratory, and bone histological characteristics were compared between groups using 2 or Fisher exact test for categorical variables and t-test or Mann-Whitney test for continuous variables. Changes from baseline to end-of-study CAC were compared within groups by using Wilcoxon signed rank test. Missing laboratory values were not imputed. Logistic regression analysis was conducted to determine factors independently related to CAC development and progression. All probability values are 2 tailed. P ⱕ 0.05 is considered statistically significant. Analyses were conducted using SPSS (SPSS Inc, Chicago, IL), version 13.0, for Windows (Microsoft Corp, Redmond, WA).
Units Values for the following parameters are given throughout in the first unit listed and may be converted to the second unit shown by multiplying by the conversion factor provided: 25-hydroxy vitamin D (ng/mL to nmol/L; 2.496), calcium ion (mmol/L to mEq/L; 2), and inorganic phosphorus (mg/dL to mmol/L; 0.3229). iPTH levels given in pg/mL and ng/L are equivalent.
RESULTS The 64 patients analyzed were relatively young (47 ⫾ 12 years) and predominantly men (66%) and white (62.5%). Average time on hemodialysis therapy was 37 ⫾ 25 months. Most had hypertension (66%) and 12.5% had diabetes as a comorbid condition. Thirty-seven patients received sevelamer and 27 received calcium acetate as phosphate-binders. Compared with the calcified group (Table 1), the noncalcified group was significantly younger and predominantly men
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and nonwhite. There were no differences between groups regarding ionized calcium, phosphorus, bone-specific alkaline phosphatase, deoxypiridinoline, or iPTH levels, but the noncalcified group had significantly lower serum OPG levels at baseline. Regarding bone histomorphometry, patients from the noncalcified group had significantly higher trabecular bone volume at baseline, whereas histomorphometric parameters of bone turnover and mineralization, as well as bone histological diagnosis, did not differ between groups (data not shown). On follow-up (Table 2), the noncalcified group persisted with lower serum OPG levels, in addition to higher trabecular bone volume. Other laboratory and histomorphometric parameters did not differ between groups. There were no differences between groups regarding the drug used as phosphate binder during the study (calcium acetate or sevelamer), estimated oral daily calcium load, or frequency of calcitriol treatment. Logistic regression analysis showed age (P ⫽ 0.003;  coefficient ⫽ 1.107; 95% confidence interval [CI], 1.036 to 1.183) and trabecular bone volume (P ⫽ 0.006;  coefficient ⫽ 0.828; 95% CI, 0.723 to 0.948) as independent predictors for CAC development. When patients in the calcified group who did not experience progression (nonprogressor group) were compared with those with CAC that progressed in 1 year (progressor group; Table 3), there were no significant differences regarding age, ethnicity, sex, hypertension, or diabetes status. Baseline serum ionized calcium, phosphorus, iPTH, bone-specific alkaline phosphatase, and deoxypyridinoline levels were similar between groups, as were bone histomorphometric parameters. Follow-up data are listed in Table 4. Estimated oral daily calcium load, dialysate calcium concentration, frequency of patients treated with calcitriol, and drug used as phosphate binder during the study (calcium acetate or sevelamer) did not differ between groups. Both bone-specific alkaline phosphatase and deoxypyridinoline levels were significantly lower in patients from the progressor group on follow-up, whereas no differences were observed between groups regarding other serum markers of bone turnover or histomorphometric parameters.
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Table 1. Characteristics of the Noncalcified (baseline CAC < 30 AU) and Calcified (baseline CAC > 30 AU) Groups at Baseline Noncalcified (n ⫽ 26)
Calcified (n ⫽ 38)
P
0 (0-27)
309 (35-5,014)
N/A
39.9 ⫾ 10.0 50 46 39.3 ⫾ 28.9 23.7 ⫾ 4.0
51.4 ⫾ 12.2 24 74 36.3 ⫾ 22.3 25.3 ⫾ 3.7
⬍0.001 0.03 0.03 0.6 0.1
58 4 12
71 18 32
0.3 0.1 0.06
7.4 ⫾ 2.7 1.24 ⫾ 0.99 32.4 ⫾ 43.9 397.9 ⫾ 409.1 158.7 ⫾ 215.3 31.2 ⫾ 16.4 134.6 ⫾ 37.9 5.3 ⫾ 10.4
7.3 ⫾ 1.25 1.23 ⫾ 0.08 28.7 ⫾ 21.0 393.6 ⫾ 316.8 91.3 ⫾ 78.0 33.6 ⫾ 15.0 191.5 ⫾ 68.5 5.6 ⫾ 8.0
0.8 0.7 0.3 0.9 0.3 0.5 ⬍0.001 0.8
0.042 ⫾ 0.045
0.039 ⫾ 0.057
0.3
4.29 ⫾ 4.54 107.6 ⫾ 123.0
3.80 ⫾ 3.60 148.1 ⫾ 132.6
0.9 0.1
Coronary calcification Coronary calcium score (AU) Demographic characteristics Age (y) Men (%) White (%) Time on hemodialysis therapy (mo) Body mass index (kg/m2) Comorbid conditions Hypertension (%) Diabetes mellitus (%) Smoking (%) Laboratory measurements Phosphorus (mg/dL) Ionized calcium (mmol/L) Bone-specific alkaline phosphatase (U/L) Intact parathyroid hormone (pg/mL) Deoxypyridoline (nmol/L) 25-Hydroxy vitamin D3 (ng/mL) Osteoprotegerin (pg/mL) sRANKL (pg/mL) Histomorphometric parameters Turnover BFR/BS (3/2/d) Mineralization OV/BV (%) Mlt (d) Volume BV/TV (%)
20.9 ⫾ 7.1
15.3 ⫾ 5.0
0.001
Note: Reference ranges: BFR/BS, 0.07 ⫾ 0.03 / /d for women and 0.13 ⫾ 0.07 / /d for men; BV/TV, 21.8% ⫾ 7.2% for women and 24.0% ⫾ 6.1% for men; Mlt, 23.7 ⫾ 2.7 days for women and 21.3 ⫾ 2.3 days for men; OV/BV, 1.55% ⫾ 1.9% for women and 2.9% ⫾ 2.7% for men. Reference values from Dos Reis et al20 and Melsen and Mosekilde.21 Inorganic phosphorus in mg/dL may be converted to mmol/L by multiplying by 0.3229. Intact parathyroid hormone levels in pg/mL and ng/L are equivalent. 25-Hydroxy vitamin D in ng/mL may be converted to nmol/L by multiplying by 2.496. Calcium ion in mmol/L may be converted to mEq/L by multiplying by 2. Abbreviations: AU, Agatston units; BFR/BS, bone formation rate/bone surface; BV/TV, trabecular bone volume/tissue volume; CAC, coronary artery calcification; Mlt, mineralization lag time; N/A, not applicable; OV/BV, osteoid volume/ trabecular bone volume; sRANKL, soluble receptor activator of nuclear factor-B ligand. 3
2
Taking into consideration baseline bone histological status, high or low turnover, as shown in Fig 2, significantly more patients from the progressor group persisted with low turnover at the 12-month bone biopsy (58% versus 17%; P ⫽ 0.01). Logistic regression analysis showed diagnosis of low-turnover bone state at the 12-month bone biopsy as the only independent predictor for CAC progression (P ⫽ 0.04;  coefficient ⫽ 4.5; 95% CI, 1.04 to 19.39). Figure 3 shows the comparison of absolute changes in histomorphometric parameters between the nonprogressor (n ⫽ 5) and progressor groups (n ⫽ 9) in patients classified as having
3
2
high-turnover bone status at baseline. Patients from the nonprogressor group showed a significant decrease in bone formation rate after 1 year compared with the progressor group (BFR/BS, ⫺0.124 ⫾ 0.101 versus 0.075 ⫾ 0.139 3/2/d; P ⫽ 0.03). Changes in trabecular bone volume/ tissue volume (3.92% ⫾ 2.39% versus 0.18% ⫾ 6.09% for the nonprogressor and progressor groups, respectively; P ⫽ 0.2) and either static (OV/BV, ⫺0.76% ⫾ 7.17% versus ⫺0.5% ⫾ 3.06% for the nonprogressor and progressor groups, respectively; P ⫽ 0.9) or dynamic parameters of mineralization lag time (162 ⫾ 121 versus 34 ⫾ 218 days for the nonprogressor and
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Table 2. Characteristics of the Noncalcified (baseline CAC < 30 AU) and Calcified (baseline CAC > 30 AU) Groups on Follow-up Noncalcified (n ⫽ 26)
Calcified (n ⫽ 38)
P
0 (0-33)
546 (44-6,204)
N/A
42 54 50 880.8 ⫾ 624.7
42 37 63 810.8 ⫾ 635.6
0.9 0.2 0.3 0.9
5.4 ⫾ 0.93 1.27 ⫾ 0.08 35.8 ⫾ 28.0 411.5 ⫾ 324.4 166.0 ⫾ 161.4 28.2 ⫾ 11.8 219.4 ⫾ 80.9 7.1 ⫾ 10.3
5.8 ⫾ 1.23 1.28 ⫾ 0.07 32.4 ⫾ 15.8 439.9 ⫾ 319.0 109.6 ⫾ 92.6 29.3 ⫾ 11.7 297.3 ⫾ 107.6 9.5 ⫾ 12.8
0.1 0.5 0.9 0.7 0.1 0.7 0.003 0.3
0.092 ⫾ 0.094
0.062 ⫾ 0.094
0.3
4.21 ⫾ 3.41 127.1 ⫾ 135.7
3.63 ⫾ 2.92 136.3 ⫾ 132.9
0.6 0.5
21.1 ⫾ 6.6
16.6 ⫾ 5.6
Coronary calcification Coronary calcium score (AU) Medical interventions Calcitriol use (%)* Dialysate calcium 2.5 mEq/L (%)† Phosphate binder sevelamer (%) Calcium load (mg/d) Laboratory measurements Phosphorus (mg/dL) Ionized calcium (mmol/L) Bone-specific alkaline phosphatase (U/L) Intact parathyroid hormone (pg/mL) Deoxypyridoline (nmol/L) 25-Hydroxy vitamin D3 (ng/mL) Osteoprotegerin (pg/mL) sRANKL (pg/mL) Histomorphometric parameters Turnover BFR/BS (3/2/d) Mineralization OV/BV (%) Mlt (d) Volume BV/TV (%)
0.005
Note: Follow-up values represent the mean of all measures of each parameter during the study. For histomorphometric parameters and CAC, follow-up stands for 12-month measures. Reference ranges: BFR/BS, 0.07 ⫾ 0.03 3/2/d for women and 0.13 ⫾ 0.07 3/2/d for men; BV/TV, 21.8% ⫾ 7.2% for women and 24.0% ⫾ 6.1% for men; Mlt, 23.7 ⫾ 2.7 days for women and 21.3 ⫾ 2.3 days for men; OV/BV, 1.55% ⫾ 1.9% for women and 2.9% ⫾ 2.7% for men. Reference values from Dos Reis et al20 and Melsen and Mosekilde.21 Inorganic phosphorus in mg/dL may be converted to mmol/L by multiplying by 0.3229. Intact parathyroid hormone levels in pg/mL and ng/L are equivalent. 25-Hydroxy vitamin D in ng/mL may be converted to nmol/L by multiplying by 2.496. Calcium ion in mmol/L may be converted to mEq/L by multiplying by 2. Abbreviations: AU, Agatston units; BFR/BS, bone formation rate/bone surface; BV/TV, trabecular bone volume/tissue volume; CAC, coronary artery calcification; Mlt, mineralization lag time; N/A, not applicable; OV/BV, osteoid volume/ trabecular bone volume; sRANKL, soluble receptor activator of nuclear factor-B ligand. *Patients who received calcitriol for at least 1 month during follow-up. †Patients treated with dialysate calcium of 2.5 mEq/L for at least 3 months during follow-up.
progressor groups, respectively; P ⫽ 0.2) did not differ significantly between groups. Figure 4 shows the comparison of absolute changes in histomorphometric parameters between the nonprogressor (n ⫽ 7) and progressor groups (n ⫽ 17) in patients classified as having low-turnover bone status at baseline. Patients from the nonprogressor group had a significant increase in bone formation rate (BFR/BS, 0.109 ⫾ 0.104 versus 0.005 ⫾ 0.014 3/2/d; P ⫽ 0.003), osteoid volume (OV/BV, 2.88% ⫾ 3.51& versus ⫺1.06% ⫾ 1.55% for the nonprogressor and progressor groups, respectively; P ⫽ 0.001), and tendency to decreasing mineralization lag time after 1 year compared with the progressor group
(mineralization lag time, ⫺163 ⫾ 145 versus ⫺25 ⫾ 107 days for the nonprogressor and progressor groups, respectively; P ⫽ 0.09). Changes in trabecular bone volume were similar between groups (trabecular bone volume/tissue volume, ⫺2.95% ⫾ 8.88% versus 2.68% ⫾ 5.04% for the nonprogressor and progressor groups, respectively; P ⫽ 0.2).
DISCUSSION Cardiovascular disease is still the most common cause of death in patients with CKD. The cumulative prevalence of traditional risk factors is insufficient to explain the increased cardiovascular mortality in this population. Many other factors associated with CKD and its therapy have
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Table 3. Characteristics of the Nonprogressor (relative CAC progression < 15%) and Progressor Groups (relative CAC progression > 15%) at Baseline Nonprogressors (n ⫽ 12)
Progressors (n ⫽ 26)
P
471 (57-1,581)
244 (35-5,014)
N/A
49.7 ⫾ 13.4 33 67 40.5 ⫾ 18.4 24.3 ⫾ 4.0
52.2 ⫾ 11.8 19 77 34.4 ⫾ 24.0 25.8 ⫾ 3.5
0.6 0.4 0.7 0.4 0.3
67 17 8
73 19 42
0.7 0.9 0.06
7.29 ⫾ 1.29 1.28 ⫾ 0.75 32.5 ⫾ 17.4 437.7 ⫾ 400.0 107.2 ⫾ 69.9 33.0 ⫾ 14.4 191.4 ⫾ 76.5 9.4 ⫾ 11.6
7.30 ⫾ 1.25 1.21 ⫾ 0.80 26.8 ⫾ 22.6 373.3 ⫾ 277.1 83.9 ⫾ 81.6 33.9 ⫾ 15.5 191.6 ⫾ 66.1 3.8 ⫾ 4.85
0.9 0.03 0.2 0.6 0.1 0.9 0.9 0.1
0.06 ⫾ 0.09
0.028 ⫾ 0.031
0.8
3.49 ⫾ 4.9 164.4 ⫾ 144.4
3.94 ⫾ 2.9 140.6 ⫾ 129.1
0.2 0.8
14.9 ⫾ 4.6
0.6
Coronary calcification Coronary calcium score (AU) Demographic characteristics Age (y) Men (%) White (%) Time on hemodialysis therapy (mo) Body mass index (kg/m2) Comorbid conditions Hypertension (%) Diabetes mellitus (%) Smoking (%) Laboratory parameters Phosphorus (mg/dL) Ionized calcium (mmol/L) Bone-specific alkaline phosphatase (U/L) Intact parathyroid hormone (pg/mL) Deoxypyridoline (nmol/L) 25-Hydroxy vitamin D3 (ng/mL) Osteoprotegerin (pg/mL) sRANKL (pg/mL) Histomorphometric parameters Turnover BFR/BS (3/2/d) Mineralization OV/BV (%) Mlt (d) Volume BV/TV (%)
16.2 ⫾ 6.0
Note: Reference ranges: BFR/BS, 0.07 ⫾ 0.03 / /d for women and 0.13 ⫾ 0.07 / /d for men; BV/TV, 21.8% ⫾ 7.2% for women and 24.0% ⫾ 6.1% for men; Mlt, 23.7 ⫾ 2.7 days for women and 21.3 ⫾ 2.3 days for men; OV/BV, 1.55% ⫾ 1.9% for women and 2.9% ⫾ 2.7% for men. Reference values from Dos Reis et al20 and Melsen and Mosekilde.21 Inorganic phosphorus in mg/dL may be converted to mmol/L by multiplying by 0.3229. Intact parathyroid hormone levels in pg/mL and ng/L are equivalent. 25-Hydroxy vitamin D in ng/mL may be converted to nmol/L by multiplying by 2.496. Calcium ion in mmol/L may be converted to mEq/L by multiplying by 2. Abbreviations: AU, Agatston units; BFR/BS, bone formation rate/bone surface; BV/TV, trabecular bone volume/tissue volume; CAC, coronary artery calcification; Mlt, mineralization lag time; N/A, not applicable; OV/BV, osteoid volume/ trabecular bone volume; sRANKL, soluble receptor activator of nuclear factor-B ligand. 3
2
been proposed as possible contributing factors, such as chronic inflammation, oxidative stress, hyperhomocysteinemia, and disorders of mineral metabolism. Since Block et al24 first described phosphatemia and calcium-phosphorus product as positively associated with all-cause and specific cardiovascular mortality in hemodialysis patients, many efforts have been made to elucidate the mechanisms involved because these alterations potentially are preventable. Several studies have been performed to address whether there is an association between vascular calcification and various markers of mineral metabolism
3
2
in uremic patients, but the reported results are conflicting.25-28 In the present study, there was no association between CAC development or progression and phosphatemia or calcium-phosphorus product. Because this protocol was designed to compare the effects of 2 phosphate binders on CAC, phosphatemia was a controlled parameter, restricting observations regarding its association with CAC development and progression. However, a clear relationship between CAC development and other mineral metabolism markers was shown, including bone remodeling analyzed by using histomorphometric evaluation. This study showed an association between lower
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Table 4. Characteristics of the Nonprogressor (relative CAC progression < 15%) and Progressor Groups (relative CAC progression > 15%) on Follow-up Nonprogressors (n ⫽ 12)
Progressors (n ⫽ 26)
P
491 (44-1,743)
546 (71-6,204)
N/A
42 42 67 692.2 ⫾ 491.2
42 35 62 865.5 ⫾ 694.1
0.9 0.7 0.8 0.4
5.8 ⫾ 1.1 1.31 ⫾ 0.08 38.3 ⫾ 12.6 465.5 ⫾ 276.2 148.7 ⫾ 94.1 28.2 ⫾ 11.8 333.3 ⫾ 134.9 13.7 ⫾ 14.6
5.8 ⫾ 1.30 1.27 ⫾ 0.07 29.7 ⫾ 16.6 428.1 ⫾ 341.5 91.5 ⫾ 87.8 29.3 ⫾ 11.7 280.6 ⫾ 90.6 7.5 ⫾ 11.6
0.8 0.3 0.03 0.7 0.01 0.2 0.2 0.06
0.07 ⫾ 0.09
0.06 ⫾ 0.09
0.5
4.85 ⫾ 3.1 137.0 ⫾ 131.6
3.07 ⫾ 2.7 136.1 ⫾ 136.1
0.08 0.8
16.1 ⫾ 6.2
16.8 ⫾ 5.4
0.6
Coronary calcification Coronary calcium score (AU) Medical interventions Calcitriol use (%)* Dialysate calcium 2.5 mEq/L (%)† Phosphate binder sevelamer (%) Calcium load (mg/d) Laboratory parameters Phosphorus (mg/dL) Ionized calcium (mmol/L) Bone-specific alkaline phosphatase (U/L) Intact parathyroid hormone (pg/mL) Deoxypyridoline (nmol/L) 25-Hydroxy vitamin D3 (ng/mL) Osteoprotegerin (pg/mL) sRANKL (pg/mL) Histomorphometric parameters Turnover BFR/BS (3/2/d) Mineralization OV/BV (%) Mlt (d) Volume BV/TV (%)
Note: Follow-up values represent the mean of all measures of each parameter during the study. For histomorphometric parameters and CAC, follow-up stands for 12-month measures. Reference ranges: BFR/BS, 0.07 ⫾ 0.03 3/2/d for women and 0.13 ⫾ 0.07 3/2/d for men; BV/TV, 21.8% ⫾ 7.2% for women and 24.0% ⫾ 6.1% for men; Mlt, 23.7 ⫾ 2.7days for women and 21.3 ⫾ 2.3 days for men; OV/BV, 1.55% ⫾ 1.9% for women and 2.9% ⫾ 2.7% for men. Reference values from Dos Reis et al20 and Melsen and Mosekilde.21 Inorganic phosphorus in mg/dL may be converted to mmol/L by multiplying by 0.3229. Intact parathyroid hormone levels in pg/mL and ng/L are equivalent. 25-Hydroxy vitamin D in ng/mL may be converted to nmol/L by multiplying by 2.496. Calcium ion in mmol/L may be converted to mEq/L by multiplying by 2. Abbreviations: AU, Agatston units; BFR/BS, bone formation rate/bone surface; BV/TV, trabecular bone volume/tissue volume; CAC, coronary artery calcification; Mlt, mineralization lag time; N/A, not applicable; OV/BV, osteoid volume/ trabecular bone volume; sRANKL, soluble receptor activator of nuclear factor-B ligand. *Patients who received calcitriol for at least 1 month during follow-up. †Patients treated with dialysate calcium of 2.5 mEq/L for at least 3 months during follow-up.
trabecular bone volume and CAC. This observation supports the relationship between arterial calcification and osteopenia formerly documented by Braun et al26 in hemodialysis patients assessed by using electron beam computed tomography. Similarly, in the general population, elderly women who had osteopenia detected by means of bone densitometry were at greater risk of advanced atherosclerosis, as well as vascular calcification.29,30 Because CKD directly produces a loss of skeletal anabolic potential by either diminishing skeletal growth factor or producing anabolic inhibitors,31 osteoporosis occurs frequently in patients with CKD, as previously reported.32
Interestingly, other traditional risk factors for osteoporosis were associated with CAC development in the present study, such as older age, white race, and female sex. There is mounting evidence that vascular calcification and osteoporosis frequently occur together and share many of the same risk factors.33,34 Additionally, in postmenopausal women, progression of vascular calcification closely parallels bone loss, suggesting that as bone is lost in the skeleton, there is concurrent formation of bone in vessels.35 Thus, it has been proposed that an imbalance in calcium allocation allows its movement from bone to the vascular wall through mechanisms that involve OPG, a soluble decoy receptor of the
Bone Remodeling and Vascular Calcification
1147
turnover decreased CAC progression. It must be considered that this high-turnover state is driven by secondary hyperparathyroidism, which is characterized by poorly differentiated osteoblast precursors manifesting a fibroblastic phenotype and by increased osteoclastic activity. This results in net bone resorption, fibrosis of the bone marrow space, and release of calcium and phosphate into
Figure 2. Evolution of low turnover in the nonprogressor and progressor groups. Significantly more patients from the progressor group persisted with low turnover at the 12-month bone biopsy (58% versus 17%; P ⫽ 0.01).
tumor necrosis factor superfamily, being a crucial tie between the bone and vascular systems.36 In alignment with these observations, in the present study, the presence of CAC was directly associated with serum OPG level. It was already shown that greater OPG levels are related to ageing and CKD.37,38 It is possible that this increase in OPG levels represents a homeostatic response to limit the bone loss associated with other bone resorption factors.39 It must be emphasized that in the studied population, there was a subgroup of young patients with preserved trabecular bone volume who did not develop CAC, even after 1 year of observation. These observations confirm the important link between bone status and vascular calcification and suggest a role of specific genetic characteristics (ie, calcification inhibitors) on triggering CAC development, which were not addressed in the present study. This study also shows that patients with highturnover bone status who had a decreased bone formation rate during the 1 year observational period were at lower risk of CAC progression. Because of the prospective design of this study, it is possible to infer that in the group of patients with high-turnover bone status, lowering high
Figure 3. Comparison of absolute changes in histomorphometric parameters between patients from the nonprogressor (closed square) and progressor groups (open square) with a diagnosis of high-turnover bone status at baseline. Abbreviations: BFR/BS, bone formation rate/ bone surface; BV/TV, trabecular bone volume/tissue volume; Mlt, mineralization lag time; OV/BV, osteoid volume/ trabecular bone volume.
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Figure 4. Comparison of absolute changes in histomorphometric parameters between patients from the nonprogressor (closed square) and progressor groups (open square) with a diagnosis of low-turnover bone status at baseline. Abbreviations: BFR/BS, bone formation rate/ bone surface; BV/TV, trabecular bone volume/tissue volume; Mlt, mineralization lag time; OV/BV, osteoid volume/ trabecular bone volume.
the extracellular fluid.40 Both ions could function as promoters of vascular smooth-muscle cell phenotypic differentiation into osteoblast-like cells and initiate the vascular calcification process. Concerning the role of secondary hyperparathyroidism on vascular calcification, Neves et al41 showed that normal and uremic rats submitted to
Barreto et al
parathyroidectomy that subsequently received PTH replacement in supraphysiological doses developed intense aortic medial calcification, with some animals showing coronary calcification. These findings suggested that high PTH levels induced high bone turnover and medial calcification independent of uremia. In the present study, there was no association between CAC progression and iPTH levels, which may occur in part because patients with severe hyperparathyroidism (iPTH ⬎ 1,000 pg/mL) were not included. At the other end of the renal osteodystrophy spectrum, adynamic bone disorder is characterized by quiescent osteoblasts and osteoclasts with markedly decreased bone turnover. In this situation, reduction of the rapidly diffusible ion pool of calcium and phosphorus associated with the reduction of bone mineralization front reduces the skeleton’s ability to buffer extracellular calcium and phosphate, resulting in increased postprandial fluctuations.31,42 In the present study, patients with a diagnosis of low-turnover bone status at baseline and who had shown higher bone formation rate and osteoid volume on follow-up had lower CAC progression. Corroborating these findings, patients who had lower CAC progression also had greater deoxypyridinoline and bonespecific alkaline phosphatase levels on follow-up. As addressed, because this is a prospective study, it is feasible to infer that improvement in bone turnover is associated with lower CAC progression. The present findings concur with a former experimental study by Davies et al43 that showed the reversal of both adynamic bone disease and vascular calcification by treatment with bone morphogenetic protein-7 (BMP-7), a skeletal anabolic factor. Altogether, the improvement in bone turnover was associated with lower CAC progression in patients with high- and low-turnover bone disorders. Considering that CAC is implicated in greater cardiovascular and all-cause mortality, renal osteodystrophy may constitute an additional and potentially modifiable mortality risk factor. Major limitations of this study include the brief observational period (1 year) and a relatively small number of patients. Furthermore, because this protocol was originally designed to compare the effect of different phosphate binders on CAC and bone remodeling in hemodialysis patients, this may have biased observations regarding specific conditions, such as the role of hyperphosphatemia in CAC
Bone Remodeling and Vascular Calcification
development. In addition, because of the absence of participants with severe hyperparathyroidism, the effect of high bone turnover on CAC progression might have been underestimated. In conclusion, this is the first study to prospectively analyze the relationship between CAC progression and changes in bone remodeling in a hemodialysis population. Nephrologists must keep in mind that low trabecular bone volume and low bone turnover state may be associated with the development and progression of CAC in patients with CKD, and measures taken to normalize bone remodeling could at the same time prevent the progression of CAC. Further research is needed to investigate whether these findings have a causal relationship or both bone remodeling disorders and vascular calcification reflect an interrelated pathological response to some as yet unidentified environmental trigger, depending on a specific genetic background.
ACKNOWLEDGEMENTS Support: Information on funding sources is listed in the financial disclosure. Financial Disclosure: Genzyme Corp (manufacturer of Renagel) provided the funding for this trial. The investigators were solely responsible for the design, conduct, analysis, and publication of the trial. There were no restrictions on publications, and all data were maintained and analyzed solely by the authors. Drs Canziani, Carvalho, Jorgetti, and Moyses report having received consulting fees and research grant from Genzyme.
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1150 26. Braun J, Oldendorf M, Moshage W, et al: Electron beam computed tomography in the evaluation of cardiac calcification in chronic dialysis patients. Am J Kidney Dis 27:394-401, 1996 27. Goodman WG, Goldin J, Kuizon BD, et al: Coronaryartery calcification in young adults with end-stage renal disease who are undergoing dialysis. N Engl J Med 342:1478-1483, 2000 28. Stompor T, Pasowicz M, Sullowicz W, et al: An association between coronary artery calcification score, lipid profile, and selected markers of chronic inflammation in ESRD patients treated with peritoneal dialysis. Am J Kidney Dis 41:203-211, 2003 29. Tanko LB, Bagger YZ, Christiansen C: Low bone mineral density in the hip as a marker of advanced atherosclerosis in elderly women. Calcif Tissue Int 73:15-20, 2003 30. Kiel DP, Kauppila LI, Cupples LA, Hannan MT, O’Donnell CJ, Wilson PW: Bone loss and the progression of abdominal aortic calcification over a 25 year period: The Framingham Heart Study. Calcif Tissue Int 68:271-276, 2001 31. Lund RJ, Davies MR, Brown AJ, Hruska KA: Successful treatment of an adynamic bone disorder with bone morphogenetic protein-7 in a renal ablation model. J Am Soc Nephrol 15:359-369, 2004 32. Barreto FC, Barreto DV, Moyses RM, et al: Osteoporosis in hemodialysis patients revisited by bone histomorphometry: A new insight into an old problem. Kidney Int 69:1852-1857, 2006 33. Stevenson JC: Osteoporosis and cardiovascular disease in women: Converging path. Lancet 336:1121-1122, 1990 34. Collin-Osdoby P: Regulation of vascular calcification by osteoclast regulatory factors RANKL and osteoprotegerin. Circ Res 95:1046-1057, 2004
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