Sevelamer Use, Vitamin K Levels, Vascular Calcifications, and Vertebral Fractures in Hemodialysis Patients: Results from the VIKI Study

Association of aberrant mineral metabolic markers with fracture risk in chronic kidney disease: a comprehensive meta-analysis

BACKGROUND

This meta-analysis aims to investigate the impact of abnormalities in mineral metabolic markers, including serum phosphate and calcium, intact parathyroid hormone (iPTH), and fibroblast growth factor 23 (FGF23) on the risk of fractures in patients with chronic kidney disease (CKD).

METHODS

A systematic search was conducted across MEDLINE, Web of Science, EMBASE, ClinicalTrials.gov, and the Cochrane Central Register for Controlled Trials. The outcomes were association of mineral metabolic markers with the risk of fractures in patients with chronic kidney disease. Pooled risk estimates and 95% confidence intervals (CIs) were calculated using fixed-effects or random-effects models.

RESULTS

Thirty-two studies were included in the meta-analysis. High and low levels of serum phosphate in hemodialysis (HD) patients were both associated with an increased risk of fractures (RR = 1.08, 95% CI 1.02-1.15, P = 0.013; RR = 1.13, 95% CI 1.02-1.25, P = 0.022, respectively). Similarly, abnormal levels of iPTH in CKD patients, both high and low, were associated with increased fracture risk (RR = 1.25, 95% CI 1.20-1.31, P < 0.001; RR = 1.41, 95% CI 1.10-1.82, P = 0.007, respectively). Elevated FGF23 levels were also linked to an increased risk of fractures (RR = 1.32, 95% CI 1.06-1.66, P = 0.015). While a higher level of calcium exhibited a trend towards reduced fracture incidence without statistical significance (RR = 0.90, 95% CI 0.77-1.05, P = 0.181), lower calcium levels tended to increase fracture risk without statistical significance (RR = 1.11, 95% CI 0.99-1.24, P = 0.087). Notably, subjects treated with calcium and phosphorus modulating drugs demonstrated a statistically significant reduction in fractures among CKD patients undergoing dialysis (phosphate binders, RR = 0.79, 95% CI 0.70-0.89; cinacalcet, RR = 0.74, 95% CI 0.59-0.93; vitamin D analogues, RR = 0.82, 95% CI 0.74-0.92, respectively).

CONCLUSION

This meta-analysis underscores the association between abnormal mineral metabolic markers, including high serum phosphate, iPTH, and FGF23, and an increased risk of fractures in CKD patients. Notably, both elevated and decreased levels of phosphate and iPTH contribute to fracture risk. The efficacy of active vitamin D, phosphorus binders, and cinacalcet in preventing fractures was observed in HD patients but not in the non-dialysis CKD population.

TRIAL REGISTRATION

PROSPERO CRD42023493951.

Acquired hypophosphatemic osteomalacia: case series from a Peruvian referral center (1999-2023)

BACKGROUND

Acquired hypophosphatemic osteomalacia (AHO) is a rare metabolic bone disorder characterized by hypophosphatemia and impaired bone mineralization. Tumor-induced osteomalacia (TIO) is the most common cause of AHO, caused by phosphaturic tumors that overproduce fibroblast growth factor 23 (FGF-23).

OBJECTIVE

To present the clinical characteristics, diagnostic challenges, and outcomes of seven cases of AHO in Peruvian patients between 1999 and 2023.

METHODS

A retrospective review of seven patients diagnosed with AHO was conducted. Clinical data, including diagnostic procedures, treatments, and outcomes, were collected.

RESULTS

Seven cases of AHO were reviewed. In case one, osteomalacia did not improve despite supraphysiological doses of vitamin D (ergocalciferol/cholecalciferol), and no tumor was detected with available tests, resulting in the patient's death. Cases two and three involved tumors located in the right leg and right hemithorax, respectively, with symptom resolution following total resection. In cases four, five, and seven, exhaustive exams failed to locate tumors. Cases four, six, and seven showed elevated FGF-23 levels, while case five had inappropriately normal FGF-23 levels. Case seven was the first patient in Peru to receive burosumab treatment. In case six, a tumor in the head of the femur was identified, but the patient opted for nonsurgical management.

CONCLUSION

The diagnosis of AHO is challenging, requiring a high index of clinical suspicion and biochemical confirmation. TIO is the most common cause of AHO, emphasizing the importance of locating the phosphaturic tumor. However, in some cases, the tumor remains elusive despite exhaustive diagnostic workups. This is particularly challenging in developing countries like Peru, where resources are limited.

Serum phosphate is associated with increased risk of bone fragility fractures in haemodialysis patients

BACKGROUND

Bone fragility fractures are associated with high morbidity and mortality. This study analysed the association between the current biochemical parameters of chronic kidney disease-mineral and bone disorders (CKD-MBD) and bone fragility fractures in the COSMOS (Current management Of Secondary hyperparathyroidism: a Multicentre Observational Study) project.

METHODS

COSMOS is a 3-year, multicentre, open cohort, prospective, observational study carried out in 6797 haemodialysis patients (227 centres from 20 European countries). The association of bone fragility fractures (outcome) with serum calcium, phosphate and parathyroid hormone (PTH) (exposure), was assessed using standard Cox proportional hazards regression and Cox proportional hazards regression for recurrent events. Additional analyses were performed considering all-cause mortality as a competitive event for bone fragility fracture occurrence. Multivariable models were used in all strategies, with the fully adjusted model including a total of 24 variables.

RESULTS

During a median follow-up of 24 months, 252 (4%) patients experienced at least one bone fragility fracture (incident bone fragility fracture rate 28.5 per 1000 patient-years). In the fractured and non-fractured patients, the percentage of men was 43.7% and 61.4%, mean age 68.1 and 63.8 years and a haemodialysis vintage of 55.9 and 38.3 months, respectively. Baseline serum phosphate >6.1 mg/dL (reference value 4.3-6.1 mg/dL) was significantly associated with a higher bone fragility fracture risk in both regression models {hazard ratio (HR) 1.53 [95% confidence interval (CI) 1.10-2.13] and HR 1.44 (95% CI 1.02-2.05)}. The significant association persisted after competitive risk analysis [subHR 1.42 (95% CI 1.02-1.98)] but the finding was not confirmed when serum phosphate was considered as a continuous variable. Baseline serum calcium showed no association with bone fragility fracture risk in any regression model. Baseline serum PTH >800 pg/mL was significantly associated with a higher bone fragility fracture risk in both regression models, but the association disappeared after a competitive risk analysis.

CONCLUSIONS

Hyperphosphatemia was independently and consistently associated with an increased bone fracture risk, suggesting serum phosphate could be a novel risk factor for bone fractures in haemodialysis patients.

Real-world usage of Chronic Kidney Disease - Mineral Bone Disorder (CKD-MBD) biomarkers in nephrology practices

Background

Chronic kidney disease mineral bone disorder (CKD-MBD) is a condition characterized by alterations of calcium, phosphate, parathyroid hormone (PTH), and fibroblast growth factor 23 (FGF-23) metabolism that in turn promote bone disorders, vascular calcifications, and increase cardiovascular (CV) risk. Nephrologists' awareness of diagnostic, prognostic, and therapeutic tools to manage CKD-MBD plays a primary role in adequately preventing and managing this condition in clinical practice.

Methods

A national survey (composed of 15 closed questions) was launched to inquire about the use of bone biomarkers in the management of CKD-MBD patients by nephrologists and to gain knowledge about the implementation of guideline recommendations in clinical practice.

Results

One hundred and six Italian nephrologists participated in the survey for an overall response rate of about 10%. Nephrologists indicated that the laboratories of their hospitals were able to satisfy request of ionized calcium levels, 105 (99.1%) of both PTH and alkaline phosphatase (ALP), 100 (94.3%) of 25(OH)D, and 61 (57.5%) of 1.25(OH)2D; while most laboratories did not support the requests of biomarkers such as FGF-23 (intact: 88.7% and c-terminal: 93.4%), Klotho (95.3%; soluble form: 97.2%), tartrate-resistant acid phosphatase 5b (TRAP-5b) (92.5%), C-terminal telopeptide (CTX) (71.7%), and pro-collagen type 1 N-terminal pro-peptide (P1NP) (88.7%). As interesting data regarding Italian nephrologists' behavior to start treatment of secondary hyperparathyroidism (sHPT), the majority of clinicians used KDOQI guidelines (n = 55, 51.9%). In contrast, only 40 nephrologists (37.7%) relied on KDIGO guidelines, which recommended referring to values of PTH between two and nine times the upper limit of the normal range.

Conclusion

Results point out a marked heterogeneity in the management of CKD-MBD by clinicians as well as a suboptimal implementation of guidelines in Italian clinical practice.

Vitamin K supplementation impact in dialysis patients: a systematic review and meta-analysis of randomized trials

Vitamin K supplementation has been considered recently as a potential treatment for addressing vascular calcification in chronic kidney disease patients. We conducted a systematic review and meta-analysis to summarize the impact of vitamin K supplementation in dialysis patients. Electronic databases were searched for clinical randomized trials among patients treated with vitamin K. Random effects models were performed and risk of bias was evaluated with Cochrane tools and the search was conducted until 15 of September 2023. Eleven trials comprising 830 patients (both adult and pediatric, mainly hemodialysis) compared vitamin K with different controls: lower doses of vitamin K, standard care or placebo. Vitamin K supplementation had no effect on mortality. Vitamin K administration improved vitamin K levels and led to lower levels of dp-uc-MGP and moderately increased calcium levels [0.18 (0.04-0.32)]. Vitamin K1 proved more potency in reducing dp-uc-MGP [SMD -1.64 (-2.05, -1.23) vs. -0.56 (-0.82, -0.31)] and also raised serum vitamin K levels in comparison with vitamin K2 [5.69 (3.43, 7.94) vs. 2.25 (-2.36, 6.87)]. While it did not have a proved benefit in changing calcification scores [-0.14 (-0.37 ± 0.09)], vitamin K proved to be a safe product. There was some concern with bias. Vitamin K supplementation has no impact on mortality and did not show significant benefit in reversing calcification scores. Vitamin K1 improved vitamin K deposits and lowered dp-uc-MGP, which is a calcification biomarker more than vitamin K2. As it proved to be a safe product, additional randomized well-powered studies with improved treatment regimens are needed to establish the true impact of vitamin K in dialysis patients.

Dysbiosis in Patients with Chronic Kidney Disease: Let Us Talk About Vitamin K

PURPOSE OF REVIEW

This narrative review aimed to summarize the current evidence on the connection between dysbiosis and vitamin K deficiency in patients with chronic kidney disease (CKD). The presence of dysbiosis (perturbations in the composition of the microbiota) has been described in several non-communicable diseases, including chronic kidney disease, and it has been hypothesized that dysbiosis may cause vitamin K deficiency. Patients with CKD present both vitamin K deficiency and gut dysbiosis; however, the relationship between gut dysbiosis and vitamin K deficiency remains to be addressed.

RECENT FINDINGS

Recently, few studies in animals have demonstrated that a dysbiotic environment is associated with low production of vitamin K by the gut microbiota. Vitamin K plays a vital role in blood coagulation as well as in the cardiovascular and bone systems. It serves as a cofactor for γ-glutamyl carboxylases and thus is essential for the post-translational modification and activation of vitamin K-dependent calcification regulators, such as osteocalcin, matrix Gla protein, Gla-rich protein, and proteins C and S. Additionally, vitamin K executes essential antioxidant and anti-inflammatory functions. Dietary intake is the main source of vitamin K; however, it also can be produced by gut microbiota. This review discusses the effects of uremia on the imbalance in gut microbiota, vitamin K-producing bacteria, and vitamin K deficiency in CKD patients, leading to a better understanding and raising hypothesis for future clinical studies.

A Novel Quantitative Computer-Assisted Score Can Improve Repeatability in the Estimate of Vascular Calcifications at the Abdominal Aorta

In CKD and in the elderly, Vascular Calcifications (VC) are associated to cardiovascular events and bone fractures. VC scores at the abdominal aorta (AA) from lateral spine radiographs are widely applied (the 0-24 semiquantitative discrete visual score (SV) being the most used). We hypothesised that a novel continuum score based on quantitative computer-assisted tracking of calcifications (QC score) can improve the precision of the SV score. This study tested the repeatability and reproducibility of QC score and SV score. In forty-four patients with VC from an earlier study, five experts from four specialties evaluated the data twice using a dedicated software. Test-retest was performed on eight subjects. QC results were reported in a 0-24 scale to readily compare with SV. The QC score showed higher intra-operator repeatability: the 95% CI of Bland-Altman differences was almost halved in QC; intra-operator R2 improved from 0.67 for SV to 0.79 for QC. Inter-observer repeatability was higher for QC score in the first (Intraclass Correlation Coefficient 0.78 vs. 0.64), but not in the second evaluation (0.84 vs. 0.82), indicating a possible heavier learning artefact for SV. The Minimum Detectable Difference (MDD) was smaller for QC (2.98 vs. 4 for SV, in the 0-24 range). Both scores were insensitive to test-retest procedure. Notably, QC and SV scores were discordant: SV showed generally higher values, and an increasing trend of differences with VC severity. In summary, the new QC score improved the precision of lateral spine radiograph scores in estimating VC. We reported for the first time an estimate of MDD in VC assessment that was 25% lower for the new QC score with respect to the usual SV score. An ongoing study will determine whether this lower MDD may reduce follow-up times to check for VC progression.

Vitamin K-Dependent Carboxylation of Osteocalcin in Bone-Ally or Adversary of Bone Mineral Status in Rats with Experimental Chronic Kidney Disease?

Chronic kidney disease (CKD) commonly occurs with vitamin K (VK) deficiency and impaired bone mineralization. However, there are no data explaining the metabolism of endogenous VK and its role in bone mineralization in CKD. In this study, we measured serum levels of phylloquinone (VK1), menaquinone 4 and 7 (MK4, MK7), and VK-dependent proteins: osteocalcin, undercarboxylated osteocalcin (Glu-OC), and undercarboxylated matrix Gla protein (ucMGP). The carboxylated osteocalcin (Gla-OC), Glu-OC, and the expression of genes involved in VK cycle were determined in bone. The obtained results were juxtaposed with the bone mineral status of rats with CKD. The obtained results suggest that the reduced VK1 level observed in CKD rats may be caused by the accelerated conversion of VK1 to the form of menaquinones. The bone tissue possesses all enzymes, enabling the conversion of VK1 to menaquinones and VK recycling. However, in the course of CKD with hyperparathyroidism, the intensified osteoblastogenesis causes the generation of immature osteoblasts with impaired mineralization. The particular clinical significance seems to have a finding that serum osteocalcin and Glu-OC, commonly used biomarkers of VK deficiency, could be inappropriate in CKD conditions, whereas Gla-OC synthesized in bone appears to have an adverse impact on bone mineral status in this model.

Role of Vitamin K in Chronic Kidney Disease: A Focus on Bone and Cardiovascular Health

Chronic kidney disease (CKD) is commonly associated with vitamin K deficiency. Some of the serious complications of CKD are represented by cardiovascular disease (CVD) and skeletal fragility with an increased risk of morbidity and mortality. A complex pathogenetic link between hormonal and ionic disturbances, bone tissue and metabolism alterations, and vascular calcification (VC) exists and has been defined as chronic kidney disease–mineral and bone disorder (CKD-MBD). Poor vitamin K status seems to have a key role in the progression of CKD, but also in the onset and advance of both bone and cardiovascular complications. Three forms of vitamin K are currently known: vitamin K1 (phylloquinone), vitamin K2 (menaquinone), and vitamin K3 (menadione). Vitamin K plays different roles, including in activating vitamin K-dependent proteins (VKDPs) and in modulating bone metabolism and contributing to the inhibition of VC. This review focuses on the biochemical and functional characteristics of vitamin K vitamers, suggesting this nutrient as a possible marker of kidney, CV, and bone damage in the CKD population and exploring its potential use for promoting health in this clinical setting. Treatment strategies for CKD-associated osteoporosis and CV disease should include vitamin K supplementation. However, further randomized clinical studies are needed to assess the safety and the adequate dosage to prevent these CKD complications.

The Role of Vitamin K in CKD-MBD

PURPOSE OF REVIEW

We describe the mechanism of action of vitamin K, and its implication in cardiovascular disease, bone fractures, and inflammation to underline its protective role, especially in chronic kidney disease (CKD).

RECENT FINDINGS

Vitamin K acts as a coenzyme of y-glutamyl carboxylase, transforming undercarboxylated in carboxylated vitamin K-dependent proteins. Furthermore, through the binding of the nuclear steroid and xenobiotic receptor, it activates the expression of genes that encode proteins involved in the maintenance of bone quality and bone remodeling. There are three main types of K vitamers: phylloquinone, menaquinones, and menadione. CKD patients, for several conditions typical of the disease, are characterized by lower levels of vitamin K than the general populations, with a resulting higher prevalence of bone fractures, vascular calcifications, and mortality. Therefore, the definition of vitamin K dosage is an important issue, potentially leading to reduced bone fractures and improved vascular calcifications in the general population and CKD patients.

Vitamin K and cardiovascular complications in CKD patients

Vitamin K, well known for its role in coagulation, encompasses two major subgroups: Vitamin K1 is exclusively synthesized by plants, whereas vitamin K2 mostly originates from bacterial synthesis. Vitamin K serves as a cofactor for the enzyme γ-glutamyl carboxylase, which carboxylates and thereby activates various vitamin K dependent proteins. Several vitamin K-dependent proteins are synthesized in bone but the role of vitamin K for bone health in CKD patients, in particular the prevention of osteoporosis is still not firmly established. Here we focus on another prominent action of vitamin K, in particular vitamin K2, namely the activation of matrix Gla protein (MGP), the most potent inhibitor of cardiovascular calcifications. Multiple observational studies link relative vitamin K deficiency or low intake to cardiovascular calcification progress, morbidity and mortality. Patients with advanced chronic kidney disease (CKD) are particularly vitamin K deficient, in part because of dietary restrictions but possibly also due to impaired endogenous recycling of vitamin K. At the same time this population is characterized by markedly accelerated cardiovascular calcifications and mortality. High dose dietary supplementation with vitamin K2, in particular the most potent form menaquinone-7 (MK7), can potently reduce circulating levels of dephosphorylated uncarboxylated, i.e. inactive MGP in patients with end stage kidney disease. However, despite this compelling data basis, several randomized controlled trials with high dose MK7 supplements in patients with advanced CKD have failed to confirm cardiovascular benefits. Here we discuss potential reasons and solutions for this.

Vitamin K and vascular calcification

PURPOSE OF REVIEW

Vascular calcification is a common and important cardiovascular risk factor in patients with chronic kidney disease (CKD). Recent advances in the understanding of the biology of vascular calcification implicate vitamin K-dependent proteins as important regulators in this process. This review highlights recent key advances in vascular biology, epidemiology, and clinical trials in this rapidly evolving field.

RECENT FINDINGS

Vitamin K deficiency is associated with increasing severity of vascular calcification among patients with CKD, but the relationship with cardiovascular disease and mortality is inconsistent. Vitamin K may reduce calcification propensity by improving the activity of vitamin K-dependent calcification inhibitors or by down-regulating components of the innate immune system to reduce inflammation. However, recent randomized controlled trials in patients with diabetes, CKD, renal transplant, and on hemodialysis have failed to demonstrate improvement in vascular calcification or stiffness after vitamin K treatment.

SUMMARY

Current evidence does not support a clinically useful role for vitamin K supplementation to prevent or reverse vascular calcification in patients with CKD. Knowledge gaps remain, particularly whether higher doses of vitamin K, longer duration of supplementations, or use a vitamin K as a part of a package of measures to counteract vascular calcification might be effective.

Phosphate, Microbiota and CKD

Phosphate is a key uremic toxin associated with adverse outcomes. As chronic kidney disease (CKD) progresses, the kidney capacity to excrete excess dietary phosphate decreases, triggering compensatory endocrine responses that drive CKD-mineral and bone disorder (CKD-MBD). Eventually, hyperphosphatemia develops, and low phosphate diet and phosphate binders are prescribed. Recent data have identified a potential role of the gut microbiota in mineral bone disorders. Thus, parathyroid hormone (PTH) only caused bone loss in mice whose microbiota was enriched in the Th17 cell-inducing taxa segmented filamentous bacteria. Furthermore, the microbiota was required for PTH to stimulate bone formation and increase bone mass, and this was dependent on bacterial production of the short-chain fatty acid butyrate. We review current knowledge on the relationship between phosphate, microbiota and CKD-MBD. Topics include microbial bioactive compounds of special interest in CKD, the impact of dietary phosphate and phosphate binders on the gut microbiota, the modulation of CKD-MBD by the microbiota and the potential therapeutic use of microbiota to treat CKD-MBD through the clinical translation of concepts from other fields of science such as the optimization of phosphorus utilization and the use of phosphate-accumulating organisms.

Vitamin K in CKD Bone Disorders

Vitamin K is principally known because it is involved in blood coagulation. Furthermore, epidemiological studies showed that its deficit was associated with increased fragility fractures, vascular calcification and mortality. There are two main types of vitamin K vitamers: Phylloquinone (or PK) and Menaquinones (MKn). Vitamin K acts both as coenzyme of y-glutamyl carboxylase (GGCX) transforming undercarboxylated in carboxylated vitamin K-dependent proteins (e.g., Osteocalcin and Matrix Gla Protein) and as a ligand of the nuclear steroid and xenobiotic receptor (SXR) (in murine species Pregnane X Receptor: PXR), expressed in osteoblasts. It has been highlighted that the uremic state is a condition of greater vitamin K deficiency than the general population with resulting higher prevalence of bone fractures, vascular calcifications and mortality. The purpose of this literature review is to evaluate the protective role of Vitamin K in bone health in CKD patients.

Predictive Value of Measures of Vascular Calcification Burden and Progression for Risk of Death in Incident to Dialysis Patients

Background: Vascular calcification (VC) is a marker of cardiovascular (CV) disease and various methods allow for presence and extension assessment in different arterial districts. Nevertheless, it is currently unclear which one of these methods for VC evaluation best predict outcome and if this piece of information adds to the predictive value of traditional CV risk factors in patients receiving hemodialysis (HD). Methods: data of 184 of the 466 patients followed in the Independent study (NCT00710788) were post hoc examined to assess the association three concurrent measures of vascular calcification and all-cause survival. Specifically, coronary artery calcification (CAC) was determined by the Agatston and the volume score while abdominal aorta calcification was determined by plain X-ray of the lumbar spine (Kauppila score (KS)). Survival and regression models as well as metrics of risk recalculation were used to test the association of VC and outcome beyond the Framingham risk score. Results: Middle-age (62.6(15.8) years) men (51%) and women (49%) starting HD were analyzed. Over 36 (median 36; interquartile range: 8–36) months of follow-up 69 patients expired. Each measure of VC (CAC or KS) predicted all-cause mortality independently factors commonly associated with all-cause survival (p < 0.001). Far more importantly, each measurement of VC significantly improved risk prediction and patient reclassification (p < 0.001) beyond traditional cardiovascular risk factors. Conclusions: Overall, presence and extension of VC, irrespective of the arterial site, predict risk of all-cause of death in patients starting hemodialysis. Of note, both CAC and KS increase risk stratification beyond traditional CV risk factors. However, future efforts are needed to assess whether a risk-based approach encompassing VC screening to guide HD patient management improves survival.