Abstract
BACKGROUND
We hypothesized that high-dose active vitamin D therapy in the form of alphacalcidol (α-calcidol), used to treat secondary hyperparathyroidism in chronic kidney disease, could lead to vascular calcification and accelerated progression of aortic stiffness.
METHODS
We conducted an observational study in 85 patients on chronic hemodialysis, among which 70 were taking a weekly dose of α-calcidol of <2 µg and 15 were taking a weekly dose of ≥2 µg (pharmacological dose). Parathyroid hormone, 25-hydroxyvitamin D, fibroblast growth factor 23, and α-klotho were determined. Aortic stiffness was assessed by determination of carotid–femoral pulse wave velocity (cf-PWV) at baseline and after a mean follow-up of 1.2 years. A multivariable regression model was used to evaluate the impact of pharmacological dose of α-calcidol on the progression of aortic stiffness.
RESULTS
At baseline, clinical, biological, and hemodynamic parameters were similar. At follow-up, cf-PWV increased more in patients with pharmacological dose of α-calcidol (0.583±2.291 m/s vs. 1.948±1.475 m/s; P = 0.04). After adjustment for changes in mean blood pressure and duration of follow-up, pharmacological dose of α-calcidol was associated with a higher rate of progression of cf-PWV (0.969 m/s; 95% confidence interval = 0.111–1.827; P = 0.03), and this association persisted after further adjustments for parameters of mineral metabolism.
CONCLUSIONS
In this study, pharmacological dose of α-calcidol was associated with accelerated progression of aortic stiffness. This study suggest that the vascular safety of active vitamin D posology may need to be specifically addressed in the treatment of chronic kidney disease–related bone mineral disorder.
Keywords: α-calcidol, aortic stiffness, blood pressure, chronic kidney disease, hemodialysis, hypertension, parathyroid hormone, vitamin D.
Cardiovascular disease is the leading cause of death in patients with advanced chronic kidney disease (CKD). The risk of cardiovascular mortality in hemodialysis patients is 10–20 times higher than in the general population, even after stratification for traditional cardiovascular risk factors.1 Aortic stiffness, a nontraditional cardiovascular risk factor that results in increased central pulse pressure, cardiac workload, and left ventricular hypertrophy, has been shown to be a strong predictor for cardiovascular morbidity and mortality in CKD.2–4 Even in earlier stages of CKD, the decline in renal function is associated with a higher degree of aortic stiffness at follow-up.5,6
In a longitudinal study with repeated measurements of aortic stiffness, we have shown that the rate of progression of aortic stiffness is accelerated in patients undergoing hemodialysis. Our findings suggested that, although traditional cardiovascular risk factors may play some role in the progression of aortic stiffness before development of advanced CKD, the increased rate of progression of aortic stiffness in CKD patients on dialysis is probably determined by specific CKD-related risk factors.7 The mechanisms of aortic stiffness in CKD still remain poorly understood. However, vascular calcification seems to play a dominant role in the alteration of biomechanical properties of the arterial wall.8–10 CKD mineral and bone disorder and, more specifically, hyperphosphatemia play key roles in the transdifferentiation of vascular smooth muscle cells into an osteoblastic phenotype, therefore promoting vascular calcification.11 In CKD, because there is a lack of activation of vitamin D by a reduction in 1-α-hydroxylase activity that is usually performed by the kidney, the use of 1-α-hydroxy-vitamin D (α-calcidol) or other forms of active vitamin D have become the cornerstone of therapy for the treatment of CKD mineral and bone disorder.12–14 However, the impact of high-dose (pharmacological) active vitamin D therapy on vascular health is at best controversial. In a procalcifying media in vitro, active vitamin D therapy can stimulate vascular smooth muscle cell transdifferentiation into osteoblast-like cells and promote vascular calcification.15–21
In the context of this study, we hypothesized that pharmacological dose of active vitamin D may result in accelerated progression of aortic stiffness in CKD. Therefore, we examined the impact of active vitamin D therapy on the rate of progression of aortic stiffness in a group of hemodialysis patients.
METHODS
Study design and population
This was a single-center, observational, longitudinal study examining the impact of active vitamin D therapy on the rate of progression of aortic stiffness, which was conducted at CHU de Québec–L’Hôtel-Dieu de Québec Hospital. Between August 2006 and January 2009, 126 patients underwent extensive evaluation for medical history, laboratory data, pharmacological treatment, and hemodynamic parameters of arterial stiffness at baseline and after a mean follow-up of 1.2±0.4 years. This cohort of patients was composed of adult patients on chronic hemodialysis (>3 months) with stable dry weight and adequate dialysis. The exclusion criteria of the cohort were any clinical conditions that would hamper hemodynamic measurements (absence of femoral pulse; systolic blood pressure (SBP) of <90mm Hg) or acute episode of illness (infection, acute heart failure, active bleeding), as previously reported.7 Because warfarin therapy may be associated with vascular calcification, we specifically excluded patients who were exposed to warfarin (n = 41) between the 2 sets of evaluations (observation period) for this study. Eighty-five subjects met the study criteria. Subjects were then divided into 2 groups based on the weekly dose of α-calcidol of 0–1.99µg/wk (<2 µg/week) or ≥2 µg/week (pharmacological dose). The cutoff of 2 µg/week of α-calcidol was arbitrarily chosen because it was assumed to be more than what is generally considered to be supplementation. α-Calcidiol was administered orally at bedtime 1–7 times per week after dialysis session. The dose of α-calcidiol was adjusted according to the treating physician during the observation period. The study was approved by the institutional review board and was conducted in accordance to the Declaration of Helsinki. All patients provided informed consent.
Objectives
The primary objective of the study was to evaluate the impact of α-calcidol dose on the rate of progression of aortic stiffness after adjustments for changes in mean blood pressure (BP) and duration of follow-up. The secondary objectives were to examine whether the impact of α-calcidol on the rate of progression of aortic stiffness is related to (i) hormones involved in CKD-related disorder of mineral metabolism such as 25-hydroxy-vitamin D (25-(OH)D), parathyroid hormone (PTH), fibroblast growth factor 23 (FGF-23), and α-Klotho; and (ii) comorbidities such as age, diabetes, and cardiovascular disease.
Arterial parameters
Hemodynamic measurements were performed after 15 minutes of rest in a supine position before the midweek dialysis session. In case of an arterio-venous fistula, measurements were performed on the contralateral arm. Brachial artery BP was recorded using an automatic sphygmomanometer BPM-100 (BP-Tru, Coquitlam, Canada). BP was recorded 6 times, with a 2-minute interval between each measurement, and the average of the last 5 measurements was used to determine the brachial SBP and diastolic BP.22 We determined carotid–femoral pulse wave velocity (cf-PWV) using Complior SP (Artech Medical, Pantin, France), as described previously.23 Three consecutive recordings were performed to determine the transit time, followed by measurement of direct distance between the 2 probes to calculate PWV by dividing the travel distance by transit time. All measurements were performed by experienced investigators, with an intrasession coefficient of variation of <3%. For the purpose of comparison with the reference values published, standard cf-PWV was obtained, taking into account differences in the transit time using the maximal upstroke algorithm and the overestimation of true distance by multiplying direct distance by 0.8.24 An abnormal cf-PWV was defined as a standard cf-PWV that is superior to the decade-specific 90th percentile of age for normal subjects.24 To assess the impact of arterial stiffness on central pulse wave profile, radial pulse wave profile was recorded by aplanation tonometry after recalibration with systolic and diastolic brachial BP (SphygmoCor system; AtCor Medical, Sydney, Australia). Three consecutive recordings were performed, and central pulse wave profile was constructed using the generalized transfer function, from which central SBP, diastolic BP, mean BP, and heart rate–adjusted central augmentation index (AIx) were derived as previously described and validated.25
Parameters of mineral metabolism and other biochemical assays
All tests were performed on blood samples obtained before the second dialysis of the week. PTH was measured with the PTH stat assay from Roche Diagnostics (Laval, QC, Canada) using 2 antibodies reactive with epitopes in the amino acid regions 26–32 and 55–64. Plasma α-klotho and FGF-23 C-terminal levels were assessed by sandwich enzyme-linked immunosorbent assay kit, according to their respective manufacturer protocol (Immuno-Biological Laboratories, Takasaki, Gunma, Japan; 2nd generation, Immutopics. San Clemente, CA). 25-(OH)D was measured by using the 25-hydroxy vitamin D Direct EIA kit (Immunodiagnostic Systems, Fountain Hills, AZ), which measures both vitamin D2 and D3. C-reactive protein was measured by an immunoturbidimetric method. Dialysis dose, lipid profile, mineral metabolism, hemoglobin, and iron reserves were performed by clinical biochemistry laboratory on predialysis samples.
Data analysis
Data are expressed as means ± SD or median (25th–75th percentiles). Mann–Whitney U and Fisher exact tests were used to compare baseline parameters between groups. Changes (Δ: follow-up – baseline) in cf-PWV were adjusted for changes in mean BP and duration of follow-up by means of linear regression. For secondary objectives, we constructed a multivariable regression model that also included sequentially dialysis vintage, 25-(OH)D, PTH, FGF-23, and α-klotho, then age, sex, diabetes, and cardiovascular disease. Data that did not follow normal distribution were log10 transformed (dialysis vintage, PTH, FGF-23, α-klotho). A 2-tailed P value <0.05 was considered to be statistically significant.
RESULTS
Patient population
The baseline characteristics of the groups are shown in Table 1. The groups were similar in terms of age, weight, and comorbidities (diabetes, hypertension, and cardiovascular disease). The proportion of patients on statins and angiotensin-converting enzyme inhibitors/angiotensin receptor blockers were not statistically different. Parameters of mineral metabolism were also similar except for nonsignificantly higher levels of PTH in the pharmacological α-calcidol group. The proportion of patients on calcium-based phosphate binders and the median dose of calcium were similar in both groups.
Table 1.
Baseline clinical and biological characteristics
| Parameters | α-Calcidol <2 µg/wk (n = 70) | α-Calcidol ≥ 2µg/wk (n = 15) | P value |
|---|---|---|---|
| Age, y | 65±16 | 58±21 | 0.12 |
| Men | 39 (56) | 12 (80) | 0.08 |
| Weight, kg | 73.7±19.6 | 75.1±15.1 | 0.80 |
| Body mass index, kg/m2 | 27.4±6.5 | 27.6±6.9 | 0.91 |
| Smoking, active or past history | 25 (36) | 5 (33) | 0.86 |
| Hypertension | 65 (93) | 13 (87) | 0.43 |
| Diabetes | 34 (49) | 7 (47) | 0.89 |
| CVD | 40 (57) | 6 (40) | 0.23 |
| Dialysis vintage, y | 3.7 (0.3–26.2) | 4.9 (0.6–19.8) | 0.24 |
| Duration of dialysis session, min | 220±26 | 222±19 | 0.78 |
| Kt/v | 1.73±0.31 | 1.67±0.22 | 0.88 |
| Dialysis catheter | 13 (19) | 2 (13) | 0.73 |
| Medication | |||
| α-calcidol, µg/wka | 0.3 (0.0–0.9) | 2.25 (2.0–5.3) | — |
| Calcium | 54 (77) | 11 (73) | 0.75 |
| Calcium daily dose, mg/d | 1,000 (500–1,500) | 1,000 (250–1,500) | 0.37 |
| ACEI/ARB | 28 (40) | 4 (27) | 0.34 |
| Statins | 41 (59) | 8 (53) | 0.71 |
| Mineral metabolism | |||
| PTH, ng/L | 220 (161–396) | 426 (203–657) | 0.14 |
| Calcium, mmol/L | 2.19±0.15 | 2.18±0.18 | 0.99 |
| Albumin, g/L | 39±3 | 40±4 | 0.14 |
| Phosphate, mmol/L | 1.45±0.41 | 1.45±0.36 | 0.62 |
| Calcium-phosphate product, mmol2/L2 | 3.23±0.96 | 3.33±0.68 | 0.67 |
| Alkaline phosphatase, U/L | 85 (67–124) | 103 (71–184) | 0.11 |
| 25-(OH)D, nmol/L | 37.1±16.3 | 35.6±8.8 | 0.62 |
| α-Klotho, pg/ml | 246 (178–352) | 368 (213–820) | 0.25 |
| FGF-23, RU/ml | 1,154 (597–2,880) | 1,578 (713–4,339) | 0.68 |
| C-reactive protein, mg/L | 5.5 (2.5–12.1) | 4.8 (2.5–12.8) | 0.93 |
| Anemia parameters | |||
| Hemoglobin, g/L | 116.2±11.9 | 121.1±7.99 | 0.10 |
| Transferin, g/L | 1.67±0.27 | 1.76±0.31 | 0.38 |
| Ferritin, µg/L | 361±310 | 323±193 | 0.78 |
| Transferrin saturation | 0.26±0.13 | 0.27±0.14 | 0.33 |
| Lipid profile | |||
| Total cholesterol, mmol/L | 3.53±1.00 | 3.28±1.51 | 0.97 |
| LDL, mmol/L | 1.79±0.69 | 1.97±0.52 | 0.26 |
| HDL, mmol/L | 1.03±0.36 | 1.00±0.34 | 0.79 |
| TG, mmol/L | 1.91±1.19 | 1.77±1.10 | 0.65 |
Results are means ± SD, no. (%), or median (25th–75th percentiles).
Abbreviations: 25-(OH)D, 25-hydroxyvitamin D; ACEI, angiotensin-converting-enzyme inhibitor; ARB, angiotensin receptor blockers; CVD, cardiovascular disease; FGF-23, fibroblast growth factor 23; HDL, high-density lipoprotein; LDL, low-density lipoprotein; PTH, parathyroid hormone; TG, triglycerides.
aIndicates the average weekly dose of α-calcidol during the observation period.
Arterial parameters are shown in Table 2. At baseline, SBP and AIx were significantly lower and cf-PWV was nonsignificantly lower in the pharmacological group. However, the proportion of patients with abnormally high cf-PWV (>90th decade-specific percentile) was similar in both groups.
Table 2.
Hemodynamic parameters at baseline and follow-up
| Parameters | α-Calcidol <2 µg/wk (n = 70) | α-Calcidol ≥2 µg/wk (n = 15) | P value |
|---|---|---|---|
| Brachial hemodynamic parameters | |||
| SBP, mm Hg | 130.4±25.1 | 116.7±17.0 | 0.01 |
| DBP, mm Hg | 67.7±14.6 | 65.7±12.8 | 0.78 |
| HR, bpm | 68.5±11.2 | 71.3±8.1 | 0.22 |
| Central hemodynamic parameters | |||
| SBP, mm Hg | 120.6±24.2 | 105.8±17.0 | 0.03 |
| DBP, mm Hg | 68.7±14.9 | 66.6±13.1 | 0.61 |
| MBP, mm Hg | 89.8±17.8 | 82.8±13.6 | 0.15 |
| Δ MBP, mm Hg | 3.6±16.2 | 12.0±19.0* | 0.17 |
| AIx@75, % | |||
| Baseline | 27.9±9.3 | 19.0±8.2 | 0.001 |
| Follow-up | 29.7±9.7 | 25.4±9.9 | 0.13 |
| Δ AIx@75 | 1.7±7.2* | 6.4±7.8** | 0.03 |
| Aortic stiffness | |||
| Baseline | |||
| cf-PWV, m/s | 13.32±3.88 | 11.56±3.59 | 0.12 |
| Standard cf-PWV, m/s | 12.43±4.18 | 10.59±4.05 | 0.12 |
| Abnormal cf-PWV | 37 (53) | 6 (40) | 0.37 |
| Follow-up | |||
| cf-PWV, m/s | 13.83±3.60 | 13.50±4.05 | 0.77 |
| Standard cf-PWV, m/s | 13.06±3.90 | 12.70±4.39 | 0.77 |
| Abnormal cf-PWV | 40 (57) | 9 (60) | 0.84 |
| Progression | |||
| Δ cf-PWV, m/s | 0.583±2.291* | 1.948±1.475** | 0.04 |
| Δ cf-PWV, % | 6.3±17.0 | 19.9±21.8** | 0.01 |
Results are means ± SD or no. (%).
Abbreviations: Δ AIx@75, changes in augmentation index adjusted for heart rate; Δ cf-PWV, changes in carotid-femoral pulse wave velocity; Δ MBP, changes in mean blood pressure; AIx@75, augmentation Index adjusted for heart rate; cf-PWV, carotid-femoral pulse wave velocity; DBP, diastolic blood pressure; HR, heart rate; MBP, mean blood pressure; SBP, systolic blood pressure.
*P < 0.05 as compared with baseline values within each group.
**P < 0.001 as compared with baseline values within each group.
Impact of α-calcidol on arterial parameters
After a mean follow-up of 1.2 years, both groups had a significant progression of cf-PWV (Table 2). The progression of cf-PWV, unadjusted and adjusted for changes in mean BP and duration of follow-up, was accelerated in the group of patients with pharmacological dose of α-calcidol (Figure 1). Simultaneously, AIx increased in both groups but to a higher extent in the pharmacological dose group. In multivariable analysis, after adjusting for age, sex, cardiovascular disease, dialysis vintage, duration of follow-up, and changes in central mean BP, pharmacological dose of α-calcidol was still significantly associated with a faster progression of AIx by 3.4% (95% confidence interval (CI) = 0.14–6.74; P = 0.04).
Figure 1.
Progression of aortic stiffness according to weekly dose of α-calcidol. Changes in aortic stiffness as measured by changes (Δ) in carotid–femoral pulse wave velocity (cf-PWV) (□) and adjusted for changes in mean blood pressure and duration of follow-up (■) in patients according to the weekly dose of α-calcidol. *P < 0.05 as compared with baseline, **P < 0.001 as compared with baseline.
To examine whether the association between pharmacological dose of α-calcidol and progression of aortic stiffness may be related to confounding effects, we performed secondary analyses. After adjustment for 25-(OH)D, PTH, FGF-23, and α-klotho, pharmacological dose of α-calcidol was still significantly associated with a higher rate of progression of aortic stiffness (Table 3, model 2). In this model, independent of α-calcidol dose, only lower PTH levels were associated with an increased rate of progression of aortic stiffness. A subsequent model that took into account other comorbidities (model 3: age, sex, diabetes, and cardiovascular disease) still showed a significant independent effect of pharmacological dose of α-calcidol on the accelerated progression of aortic stiffness.
Table 3.
Determinants of changes in aortic stiffness
| Outcome | Predictors | β | 95 % CI | P value | |
|---|---|---|---|---|---|
| ∆ cf-PWV, m/s | |||||
| Model 1 | α-Calcidol ≥2 µg/wk | 0.969 | 0.111 to 1.827 | 0.03 | |
| Model 2 | α-Calcidol ≥2 µg/wk | 1.015 | 0.219 to 1.812 | 0.01 | |
| 25-(OH)D | 0.001 | -0.024 to 0.027 | 0.92 | ||
| Log10 PTH | −1.278 | −2.434 to −0.121 | 0.03 | ||
| Log10 FGF-23 | −0.265 | −1.081 to 0.551 | 0.53 | ||
| Log10 α-klotho | 0.395 | −0.743 to 1.532 | 0.496 | ||
| Model 3 | α-Calcidol ≥2 µg/wk | 1.013 | 0.217 to 1.808 | 0.01 | |
| 25-(OH)D | −0.002 | −0.026 to 0.023 | 0.90 | ||
| Log10 PTH | −1.276 | −2.563 to 0.010 | 0.052 | ||
| Log10 FGF-23 | −0.209 | −1.227 to 0.808 | 0.69 | ||
| Log10 α-klotho | 0.366 | −0.798 to 1.530 | 0.54 | ||
| Model 4 | α-Calcidol ≥2 µg/wk | 0.939 | 0.111 to 1.767 | 0.03 | |
| 25-(OH)D | −0.002 | −0.026 to 0.022 | 0.85 | ||
| Log10 PTH | −1.098 | −2.428 to 0.232 | 0.11 | ||
| Log10 FGF-23 | −0.400 | −1.389 to 0.589 | 0.43 | ||
| Log10 α-klotho | 0.224 | −1.061 to 1.510 | 0.73 | ||
Model 1 was adjusted for mean blood pressure and duration of follow-up. Model 2 was adjusted for parameters in model 1 plus dialysis vintage. Model 3 was adjusted for parameters in model 2 plus age, sex, cardiovascular disease, and diabetes. Model 4 was adjusted for parameters in model 3 plus baseline carotid–femoral pulse wave velocity (cf-PWV).
Abbreviations: 25-(OH)D, 25-hydroxyvitamin D; FGF-23, fibroblast growth factor 23; PTH, parathyroid hormone.
As part of the sensitivity analysis, we included baseline cf-PWV into model 4 and still showed no impact on the magnitude of effect of pharmacological dose of α-calcidol on the rate of progression of aortic stiffness. We also examined and found no quadratic relationship between α-calcidol dose and the rate of progression of aortic stiffness (linear component: slope = −0.026, P = 0.90; quadratic component: slope = 0.015, P = 0.30). We then divided the patients into 3 categories (no α-calcidol; 0 < α-calcidol < 2 µg/week; and α-calcidol ≥ 2 µg/week) and examined the impact of this categorization on the rate of progression of aortic stiffness (Figure 2). Upon further examination, we found that heart rate, serum calcium, phosphate, C-reactive protein, angiotensin-converting enzyme inhibitors/angiotensin receptor blockers, and statins did not influence the impact of pharmacological dose of α-calcidol on the rate of progression of aortic stiffness.
Figure 2.
Progression of aortic stiffness according to 3 α-calcidol weekly doses. The box plot represents changes in aortic stiffness as measured by changes (Δ) in carotid–femoral pulse wave velocity (cf-PWV) adjusted for changes in mean blood pressure and duration of follow-up in patients according to the 3 categories of baseline weekly dose of α-calcidol (no α-calcidol (n=37); α-calcidol of 0–2 µg/week (n=33); ≥2 µg/week (n=15)). The whiskers represent 10th and 90th percentiles. *P < 0.05 as compared with 2 other groups.
Relationship between parameters of mineral metabolism and aortic stiffness
For comparison with previous studies that have looked at the relationship between aortic stiffness and parameters of mineral metabolism in a cross-sectional fashion, we performed exploratory analysis between baseline cf-PWV and PTH, 25-(OH)D, FGF-23, and α-klotho (Table 4). There were no significant relationships between PTH, α-klotho, and cf-PWV, whereas the negative association between baseline cf-PWV, FGF-23, and 25-(OH)D was no longer significant after correction for age.
Table 4.
Relationship between parameters of mineral metabolism and baseline aortic stiffness
| Outcome parameter | Predictor | Model | Slope (95% CI) | P value |
|---|---|---|---|---|
| cf-PWV, m/s | Log 10 PTH, ng/L | Unadjusted | 0.03 (−2.3 to 2.3) | 0.98 |
| 25-(OH)D, nmol/L | Unadjusted | −0.05 (−0.10 to −0.004) | 0.03 | |
| Adjusteda | −0.03 (−0.07 to 0.01) | 0.15 | ||
| Log 10 FGF-23, RU/ml | Unadjusted | −3.3 (−5.0 to −1.6) | <0.001 | |
| Adjusteda | −1.1 (−3.2 to 0.82) | 0.25 | ||
| α-Klotho, pg/ml | Unadjusted | −1.1 (−3.8 to 1.5) | 0.40 |
Abbreviations: 25-(OH)D: 25-hydroxyvitamin D; cf-PWV, carotid–femoral pulse wave velocity; FGF-23, fibroblast growth factor 23; PTH, parathyroid hormone.
aAdjusted for age using linear regression.
DISCUSSION
This observational study shows, for the first time, that the use of pharmacological dose of α-calcidol in hemodialysis patients is associated with an accelerated progression of aortic stiffness and a higher progression of central AIx. This effect was not related to comorbidities and other parameters of mineral metabolism.
Active vitamin D therapy has been the cornerstone of treatment of patients with CKD mineral and bone disorder. Observational studies suggest that active vitamin D or vitamin D analogs are associated with a survival benefit in dialysis population.12,14 The discovery of vitamin D receptor in cardiovascular system and initial results showing reversal of left ventricular hypertrophy by active vitamin D therapy led to the proposition that vitamin D may have cardiovascular benefits.26–28 However, a more recent randomized controlled trial failed to show an improvement in cardiac structure and function in nondialysis CKD patients by vitamin D analog paricalcitol.29 Furthermore, a recent observational study that analyzed Dialysis Outcomes and Practice Patterns Study data concluded that vitamin D was associated with a survival benefit in models prone to bias because of unmeasured confounding factors and that no improvement in mortality was observed using models that tend to be more independent of unmeasured confounding factors.30
Olgaard and Lewin have well captured the dilemma regarding indication, uses, and misuses of active vitamin D in CKD patients.31 The role of vitamin D in the process of vascular calcification still remains ambiguous in humans. In observational studies, Shroff and colleagues32 suggested that there may be a U-shaped relationship between serum calcitriol levels and vascular calcification in a pediatric CKD population. Two cross-sectional studies in non-CKD populations reported an inverse association between calcitriol and coronary artery calcification,33,34 whereas another study failed to demonstrate any associations.35 The potential role of renal and extrarenal activation of vitamin D in relation to vascular disease has been well detailed in a review article by Richart and colleagues.36 In our study, 25-(OH)D was associated with baseline aortic stiffness, but this relationship was no longer significant after correction for age. The level of 25-(OH)D was not associated in any manner with the rate of progression of aortic stiffness in either univariable or multivariable models. In our population, we failed to see a quadratic (U-shaped) relationship between α-calcidol dose and rate of progression of aortic stiffness, and the visual examination of Figure 2 does not give a signal toward such a U-shaped relationship. The noticeable effect of pharmacological dose of α-calcidol on the rate of progression of aortic stiffness is in keeping with the potentially toxic and procalcifying effects of high-dose active vitamin D therapy. Indeed, in vascular smooth muscle cell cultures, active vitamin D treatment enhances osteoblastic differentiation,15,16 although not consistently.19,37 However, in animal models of CKD, vascular calcification is consistently enhanced by active vitamin D administration.17–21
Low PTH levels and very high PTH levels have been proposed to be associated with vascular calcification, respectively, through the effects of low and high turnover bone diseases. In our study, if the impact of pharmacological dose of α-calcidol on accelerated progression of aortic stiffness was the result of an indication bias, one would have assumed that the introduction of PTH into the model would have attenuated the effects of α-calcidol on the rate of progression of aortic stiffness. But this was not the case, and in fact, incorporation of PTH into the model shows that lower PTH was associated with a greater progression of aortic stiffness independent of α-calcidol dose. This is in keeping with the observation that low PTH and low bone turnover may be associated with increased vascular calcification.38
The levels of calcium, phosphate, and calcium-phosphate product were similar in both groups, and we found no associations between these parameters and the rate of progression of aortic stiffness. This lack of association may be related to the well-controlled levels of calcium and phosphate in this cohort. However, this lack of association between markers of mineral metabolism and aortic stiffness has also been observed in community-living elderly subjects.39 Nevertheless, these observations suggesting that the impact of high-dose α-calcidol on the accelerated progression of aortic stiffness was not related to the effects of active vitamin D on serum levels of calcium and phosphate.
Since the discovery of FGF-23 and α-klotho, the understanding of CKD mineral and bone disorder has become ever more complex. FGF-23, produced by osteocytes, is a phosphate-regulating hormone. Extremely high levels of FGF-23 in CKD have been associated with increasing mortality.40 It was proposed that these high levels of FGF-23 may also have a negative effect on the cardiovascular system.41 However, the impact of FGF-23 on vascular calcification per se is at best controversial. Although an in vitro study showed that the addition of FGF-23 to the procalcifying media seems to protect vascular smooth muscle cells against vascular calcification,42 in nondialysis patients from the Chronic Renal Insufficiency Cohort study, baseline FGF-23 was not associated with the coronary artery or aortic calcium score at follow-up.43 In our study, we found a negative association between baseline degree of aortic stiffness and FGF-23, but this association was no longer significant after correction for age. Moreover, FGF-23 had no impact on either the rate of progression of aortic stiffness or on the effect of α-calcidol dose on the rate of progression of aortic stiffness.
Low circulating levels of soluble klotho, which results from post-translational cleavage of the extracellular domain of klotho, is reduced in CKD. It is believed that circulating klotho can act as a hormone, which may play a significant role in protecting against vascular calcification.44 Kitagwa and colleagues45 showed that independent of age and mean BP, klotho levels were negatively associated with ankle–brachial PWV >1,400cm/s in nondialysis CKD patients. However, others failed to show any associations between klotho gene polymorphism and vascular calcification.46 In our study, using univariable and multivariable models, we found no relationship between soluble klotho, aortic stiffness, and progression of aortic stiffness in either group of α-calcidol.
The strength of the study is that it evaluates aortic stiffness at 2 time points within each individual. In contrast with previous cross-sectional studies, the longitudinal design of the study establishes a temporal relationship between exposure to high-dose α-calcidol and progression of aortic stiffness. Furthermore, patients were similar in terms of clinical comorbidities, parameters of mineral metabolism, pharmacotherapy, and arterial parameters. We acknowledge that because of the observational nature of the study and the inherent indication bias for pharmacological dose of α-calcidol, the causality link cannot be formally established. However, we constructed various models to examine whether one of the mineral metabolism parameters could partially be accountable for the accelerated progression of aortic stiffness. By indication, a clinician is likely to use pharmacological doses of α-calcidol in patients with higher PTH levels. However, incorporation of PTH into model 2 of Table 3 shows that lower PTH is associated with a faster progression of aortic stiffness. In addition, by taking into account the PTH level, the impact of pharmacological dose of α-calcidol on the rate of progression of aortic stiffness stayed within the same order of magnitude, suggesting that at least part of the effect could be related to the dose of α-calcidol therapy.
The study has a number of limitations. First, this is a small, observational study, and the results should be confirmed by larger observational or interventional studies. Second, because of the small number of patients, the lack of a quadratic relationship between α-calcidol dose and progression of aortic stiffness does not preclude that such a relationship exists. Third, we used 2 µg/week as an arbitrary cutoff based on the general impression that a dose >2 µg/week is probably more than what is generally accepted to be a physiological supplementation. Although there are no formal data available to ascertain such an impression in dialysis population, in the Dialysis Outcomes and Practice Patterns Study (DOPPS) cohort, the mean monthly dose of active vitamin D was reported to be 10 µg/month (i.e., 2.3 µg/week).47 Fourth, because there were only 2 sets of measurements of aortic stiffness over a relatively short period of time, we could not examine whether there is a curvilinear relationship between α-calcidol and progression of aortic stiffness. Finally, because of small sample size, the study was underpowered to examine the interaction of other potential confounding such as age–vitamin D and sex–vitamin D on the rate of progression of aortic stiffness.
In summary, our results indicate that α-calcidol ≥2 µg/week is associated with accelerated progression of aortic stiffness in hemodialysis patients. Because the proposed benefits of active vitamin D therapy, if any, may not be linearly dose dependent, future studies should consider not only the impact of a specific active vitamin D or vitamin D analog but also the dose at which these drugs are administered.
DISCLOSURES
The authors declared no conflict of interest.
ACKNOWLEDGMENTS
C. Fortier and F. Mac-Way contributed equally to this work. We are grateful to the dialysis personnel for their generous contribution and kind collaboration. This work was supported by the Canadian Institute of Health Research (CIHR), New Emerging Team Grant (NET-54008), the Heart & Stroke Foundation of Canada, the Kidney Foundation of Canada, and the Canadian Diabetes Association. C. Fortier holds a scholarship from Fonds de Recherche du Québec–Santé (FRQ-S); Dr De Serres holds a Kidney Research Scientist Core Education and National Training Program scholarship supported by CIHR, Kidney Foundation of Canada and Canadian Society of Nephrology and from FRQ-S; and Dr Agharazii holds a scholarship from FRQ-S and a research chair in nephrology from Université Laval.
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