Effect of paricalcitol and cinacalcet on serum phosphate, FGF-23, and bone in rats with chronic kidney disease

The 1,24,25(OH)3D3 metabolite in clinical and experimental CKD: Impact of calcitriol treatment

Calcitriol, and other vitamin D receptor activators, remain a primary treatment for elevated parathyroid hormone levels in patients with end stage kidney disease. The objective of this study was to assess the 24-hydroxylation-mediated metabolism of 25(OH)D₃ and 1,25(OH)₂D₃ in rats with experimental kidney disease treated with calcitriol and in a cross-sectional analysis of patients requiring hemodialysis. Methods: Animals were stratified by creatinine into a time control group or calcitriol (20 ng/kg/day) for 3 weeks following CKD induction using a dietary adenine model (0.25% adenine). Hemodialysis patients were recruited and demographic data including calcitriol prescription was obtained by chart review and participant interview. Vitamin D metabolites were assessed using LC-MS/MS. In the rat model, 1,25(OH)₂D₃ levels increased substantially in calcitriol-treated rats yet there was no increase in its primary metabolite: 1,24,25(OH)₂D₃. A lower ratio of 1,24,25(OH)₂D₃:1,25(OH)₂D₃ (1,25-VMR) was associated with increased calcium levels in calcitriol treated rats. In hemodialysis patients (N = 86), the level of 1,25(OH)₂D₃ was substantially higher in calcitriol-treated patients yet there was no difference between groups in 1,24,25(OH)₃D₃, resulting in a marked decrease in the 1,25-VMR in calcitriol treated patients. In hemodialysis patients treated with calcitriol, 1,25(OH)₂D₃ and a lower ratio between 1,24,25(OH)₃D₃ and 1,25(OH)₂D₃ were associated with higher serum calcium levels. Impaired metabolism of exogenous calcitriol may contribute to the adverse effects associated with this treatment. A better understanding of the uniquely dysfunctional catabolic vitamin D profile in CKD may guide more effective treatment strategies.

Assessment of the Association of Vitamin D and the Risk of Tuberculosis among End-Stage Kidney Disease Population

We investigated the role of vitamin D in the risk of tuberculosis (TB) among patients with end-stage kidney disease (ESKD). The retrospective cohort was conducted with data of 20,985 patients with kidney disease and 20,985 controls without kidney disease (1:1 matching on age of cohort entry and sex) in the duration of 1997−2010 from the Taiwan National Health insurance database. Then, by a case−cohort study, among 20,985 kidney disease, 3194 ESKD patients were identified with matched 3194 non-ESKD patients. Multivariate analyses revealed a significant association between kidney disease and tuberculosis (adjusted incidence rate ratio (IRR) 1.57 (1.33−1.86)), and the risk increased after 3 years of follow-up the (adjusted IRR 3.79 (2.55−5.62)), but after more years of follow-up no significance was observed. We also found that ESKD increases the risk of tuberculosis (adjusted IRR 3.67 (2.27−5.93)). However, vitamin D usage was not related with the tuberculosis risk in ESKD patients (p > 0.1783). Our study showed increased risk of tuberculosis in kidney disease and ESKD patients, and vitamin D was not beneficial in ESKD.

Role of Calcimimetics in Treating Bone and Mineral Disorders Related to Chronic Kidney Disease

Renal osteodystrophy is common in patients with chronic kidney disease and end-stage renal disease and leads to the risks of fracture and extraosseous vascular calcification. Secondary hyperparathyroidism (SHPT) is characterized by a compensatory increase in parathyroid hormone (PTH) secretion in response to decreased renal phosphate excretion, resulting in potentiating bone resorption and decreased bone quantity and quality. Calcium-sensing receptors (CaSRs) are group C G-proteins and negatively regulate the parathyroid glands through (1) increasing CaSR insertion within the plasma membrane, (2) increasing 1,25-dihydroxy vitamin D3 within the kidney and parathyroid glands, (3) inhibiting fibroblast growth factor 23 (FGF23) in osteocytes, and (4) attenuating intestinal calcium absorption through Transient Receptor Potential Vanilloid subfamily member 6 (TRPV6). Calcimimetics (CaMs) decrease PTH concentrations without elevating the serum calcium levels or extraosseous calcification through direct interaction with cell membrane CaSRs. CaMs reduce osteoclast activity by reducing stress-induced oxidative autophagy and improving Wnt-10b release, which promotes the growth of osteoblasts and subsequent mineralization. CaMs also directly promote osteoblast proliferation and survival. Consequently, bone quality may improve due to decreased bone resorption and improved bone formation. CaMs modulate cardiovascular fibrosis, calcification, and renal fibrosis through different mechanisms. Therefore, CaMs assist in treating SHPT. This narrative review focuses on the role of CaMs in renal osteodystrophy, including their mechanisms and clinical efficacy.

Effect of Early and Delayed Commencement of Paricalcitol in Combination with Enalapril on the Progression of Experimental Polycystic Kidney Disease

Vitamin D secosteroids are intranuclear regulators of cellular growth and suppress the renin-angiotensin system. The aim of this study was to test the hypothesis that the vitamin D receptor agonist, paricalcitol (PC), either alone or with enalapril (E) (an angiotensin-converting enzyme inhibitor), reduces the progression of polycystic kidney disease. Preventative treatment of Lewis polycystic kidney (LPK) and Lewis control rats with PC (0.2 μg/kg i.p. 5 days/week) or vehicle from postnatal weeks 3 to 10 did not alter kidney enlargement. To evaluate the efficacy in established disease, LPK rats received either PC (0.8 μg/kg i.p; 3 days/week), vehicle, E (50 mg/L in water) or the combination of PC + E from weeks 10 to 20. In established disease, PC also did not alter the progression of kidney enlargement, kidney cyst growth or decline in renal function in LPK rats. Moreover, the higher dose of PC was associated with increased serum calcium and weight loss. However, in established disease, the combination of PC + E reduced systolic blood pressure and heart-body weight ratio compared to vehicle and E alone (p < 0.05). In conclusion, the combination of PC + E attenuated cardiovascular disease but caused hypercalcaemia and did not alter kidney cyst growth in LPK rats.

Phosphate and fibroblast growth factor 23 in diabetes

Diabetes is associated with a strongly elevated risk of cardiovascular disease, which is even more pronounced in patients with diabetic nephropathy. Currently available guideline-based efforts to correct traditional risk factors are only partly able to attenuate this risk, underlining the urge to identify novel treatment targets. Emerging data point towards a role for disturbances in phosphate metabolism in diabetes. In this review, we discuss the role of phosphate and the phosphate-regulating hormone fibroblast growth factor 23 (FGF23) in diabetes. We address deregulations of phosphate metabolism in patients with diabetes, including diabetic ketoacidosis. Moreover, we discuss potential adverse consequences of these deregulations, including the role of deregulated phosphate and glucose as drivers of vascular calcification propensity. Finally, we highlight potential treatment options to correct abnormalities in phosphate and FGF23. While further studies are needed to more precisely assess their clinical impact, deregulations in phosphate and FGF23 are promising potential target in diabetes and diabetic nephropathy.

Ap2s1 mutation causes hypercalcaemia in mice and impairs interaction between calcium-sensing receptor and adaptor protein-2

Adaptor protein 2 (AP2), a heterotetrameric complex comprising AP2α, AP2β2, AP2μ2 and AP2σ2 subunits, is ubiquitously expressed and involved in endocytosis and trafficking of membrane proteins, such as the calcium-sensing receptor (CaSR), a G-protein coupled receptor that signals via Gα₁₁. Mutations of CaSR, Gα₁₁ and AP2σ2, encoded by AP2S1, cause familial hypocalciuric hypercalcaemia types 1–3 (FHH1–3), respectively. FHH3 patients have heterozygous AP2S1 missense Arg15 mutations (p.Arg15Cys, p.Arg15His or p.Arg15Leu) with hypercalcaemia, which may be marked and symptomatic, and occasional hypophosphataemia and osteomalacia. To further characterize the phenotypic spectrum and calcitropic pathophysiology of FHH3, we used CRISPR/Cas9 genome editing to generate mice harboring the AP2S1 p.Arg15Leu mutation, which causes the most severe FHH3 phenotype. Heterozygous (Ap2s1^+/L15) mice were viable, and had marked hypercalcaemia, hypermagnesaemia, hypophosphataemia, and increases in alkaline phosphatase activity and fibroblast growth factor-23. Plasma 1,25-dihydroxyvitamin D was normal, and no alterations in bone mineral density or bone turnover were noted. Homozygous (Ap2s1^L15/L15) mice invariably died perinatally. Co-immunoprecipitation studies showed that the AP2S1 p.Arg15Leu mutation impaired protein–protein interactions between AP2σ2 and the other AP2 subunits, and also with the CaSR. Cinacalcet, a CaSR positive allosteric modulator, decreased plasma calcium and parathyroid hormone concentrations in Ap2s1^+/L15 mice, but had no effect on the diminished AP2σ2-CaSR interaction in vitro. Thus, our studies have established a mouse model that is representative for FHH3 in humans, and demonstrated that the AP2S1 p.Arg15Leu mutation causes a predominantly calcitropic phenotype, which can be ameliorated by treatment with cinacalcet.

Skeletal and mineral metabolic effects of risedronate in a rat model of high-turnover renal osteodystrophy

INTRODUCTION

High-turnover bone disease is a major consequence of SHPT and may explain the high risk for fracture in patients with advanced chronic kidney disease (CKD). Bisphosphonates suppress bone turnover and improve bone strength, but their effects have not been fully characterized in advanced CKD with severe SHPT. Bisphosphonates also increase 1,25-dihydroxyvitamin D levels in normal and uremic rats, but the underlying mechanism remains to be determined.

MATERIALS AND METHODS

We investigated the skeletal and mineral metabolic effects of RIS, a pyridinyl bisphosphonate, in rats with severe SHPT induced by 5/6 nephrectomy plus a high phosphate diet.

RESULTS

Nephrectomized rats developed severe SHPT, along with hyperphosphatemia, low 1,25-dihydroxyvitamin D, and markedly increased FGF23. Moreover, these rats exhibited characteristic features of high-turnover renal osteodystrophy, including increased indices of trabecular bone turnover, decreased cortical bone thickness, inferior cortical biomechanical properties, and a prominent increase in peritrabecular fibrosis. RIS treatment increased bone volume and partially attenuated trabecular bone remodeling, cortical bone loss, and mechanical properties, whereas it produced a marked improvement in peritrabecular fibrosis along with a corresponding decrease in osteogenic gene markers. RIS treatment also suppressed the elevation of FGF23, which was associated with increased 1,25-dihydroxyvitamin D.

CONCLUSIONS

In a rat model of severe SHPT, treatment with RIS partially attenuated histological manifestations of high-turnover bone disease. RIS treatment also suppressed the elevation of FGF23, which may explain the increased 1,25-dihydroxyvitamin D production during the treatment.

Regulation and Effects of FGF23 in Chronic Kidney Disease

Chronic kidney disease (CKD) is a global health epidemic that accelerates cardiovascular disease, increases risk of infection, and causes anemia and bone disease, among other complications that collectively increase risk of premature death. Alterations in calcium and phosphate homeostasis have long been considered nontraditional risk factors for many of the most morbid outcomes of CKD. The discovery of fibroblast growth factor 23 (FGF23), which revolutionized the diagnosis and treatment of rare hereditary disorders of FGF23 excess that cause hypophosphatemic rickets, has also driven major paradigm shifts in our understanding of the pathophysiology and downstream end-organ complications of disordered mineral metabolism in CKD. As research of FGF23 in CKD has rapidly advanced, major new questions about its regulation and effects continuously emerge. These are promoting exciting innovations in laboratory, patient-oriented, and epidemiological research and stimulating clinical trials of new therapies and repurposing of existing ones to target FGF23.

Direct bone effects of calcimimetics in chronic kidney disease?

Calcimimetics are widely used in patients on dialysis to treat secondary hyperparathyroidism. Our current view is that bone effects are only indirect through parathyroid hormone suppression. However, because bone cells express the calcium-sensing receptor, direct calcium-sensing receptor-mediated effects are also possible. New experimental data demonstrate direct anabolic bone actions of calcimimetics, independent of parathyroid hormone suppression. Because these effects could be clinically useful, further studies are necessary to confirm the validity of this observation.

Vitamin D and Calcimimetics in Cardiovascular Disease

Cardiovascular disease has earned its place as one of the leading noncommunicable diseases that has become a modern-day global epidemic. The increasing incidence and prevalence of chronic kidney disease (CKD) has added to this enormous burden, given that CKD is now recognized as an established risk factor for accelerated cardiovascular disease. In fact, cardiovascular disease remains the leading cause of death in the CKD population, with significant prognostic implications. Alterations in vitamin D levels as renal function declines has been linked invariably to the development of cardiovascular disease beyond a mere epiphenomenon, and has become an important focus in recent years in our search for new therapies. Another compound, cinacalcet, which belongs to the calcimimetic class of agents, also has taken center stage over the past few years as a potential cardiovasculoprotective agent. However, given limited well-designed randomized trials to inform us, our clinical practice for the management of cardiovascular disease in CKD has not been adequately refined. This article considers the biological mechanisms, regulation, and current experimental, clinical, and trial data available to help guide the therapeutic use of vitamin D and calcimimetics in the setting of CKD and cardiovascular disease.

Phosphate Toxicity in CKD: The Killer among Us

Maintenance of a normal serum phosphate level depends on absorption in the gut, reabsorption and excretion by the kidney, and the flux between the extracellular and skeletal pools. Phosphate homeostasis is a coordinated, complex system of crosstalk between the bone, intestine, kidney, and parathyroid gland. Dysfunction of this system has serious clinical consequences in healthy individuals and those with conditions, such as CKD, in which hyperphosphatemia is associated with increased risks of cardiovascular morbidity and mortality. The last half-century of renal research has helped define the contribution of the parathyroid hormone, calcitriol, fibroblast growth factor 23, and Klotho in the regulation of phosphate. However, despite new discoveries and insights gained during this time, what remains unchanged is the recognition that phosphate retention is the initiating factor for the development of many of the complications observed in CKD, namely secondary hyperparathyroidism and bone and cardiovascular diseases. Controlling phosphate load remains the primary goal in the treatment of CKD. This review discusses the clinical effects of dysregulated phosphate metabolism, particularly in CKD, and its association with cardiovascular disease. The importance of early control of phosphate load in the treatment of CKD is emphasized, and the latest research in the treatment of phosphate retention is discussed.

Vitamin D in patients with chronic kidney disease: a position statement of the Working Group "Trace Elements and Mineral Metabolism" of the Italian Society of Nephrology

In the late 1970s, calcitriol was introduced into clinical practice for the management of secondary renal hyperparathyroidism in chronic kidney disease (CKD). Since then, the use of calcifediol or other native forms of vitamin D was largely ignored until the publication of the 2009 Kidney Disease Improving Global Outcomes (KDIGO) recommendations. The guidelines suggested that measurement of circulating levels of 25(OH)D (calcifediol) and its supplementation were to be performed on the same basis as for the general population. This indication was based on the fact that the precursors of active vitamin D had provided to CKD patients considerable benefits in survival, mainly due to their pleiotropic effects on the cardiovascular system. However, despite the long-term use of various classes of vitamin D in CKD, a clear definition is still lacking concerning the most appropriate time for initiation of therapy, the best compound to prescribe (active metabolites or analogs), the proper dosage, and the most suitable duration of therapy. The aim of this position statement is to provide and critically appraise the current plentiful evidence on vitamin D in different clinical settings related to CKD, particularly focusing on outcomes, monitoring and treatment-associated risks. However, it should be taken in account that position statements are meant to provide guidance; therefore, they are not to be considered prescriptive for all patients and, importantly, they cannot replace the judgment of clinicians.

Calcitriol Suppression of Parathyroid Hormone Fails to Improve Skeletal Properties in an Animal Model of Chronic Kidney Disease

BACKGROUND

Chronic kidney disease (CKD) leads to complex metabolic changes and an increased risk of fracture. Currently, calcitriol is the standard of care as it effectively suppresses parathyroid hormone (PTH) levels in CKD patients. While calcitriol and its analogs improve BMD and reduce fractures in the general population, the extension of these benefits to patients with advanced kidney disease is unclear. Here, the impact of calcitriol on the skeleton was examined in the setting of reduction in PTH.

METHODS

Male Cy/+ rats, a PKD-like CKD model, were treated with either vehicle or calcitriol for 5 weeks. Their normal littermates served as controls. Animals were assessed for changes in mineral metabolism and skeletal parameters (microCT, histology, whole bone mechanics and bone quality).

RESULTS

PTH levels were significantly higher (12-fold) in animals with CKD compared to normal controls. CKD animals also exhibited negative changes in bone structural and mechanical properties. Calcitriol treatment resulted in a 60% suppression of PTH levels in animals with CKD. Despite these changes, it had no impact on bone volume (cortical or cancellous), bone turnover, osteoclast number or whole bone mechanical properties.

CONCLUSIONS

These data indicate that while calcitriol effectively lowered PTH in rats with CKD, it did little to prevent the negative effects of secondary hyperparathyroidism on the skeleton.

FGF23 from bench to bedside

There is a strong association between elevated circulating fibroblast growth factor-23 (FGF23) levels and adverse outcomes in patients with chronic kidney disease (CKD) of all stages. Initially discovered as a regulator of phosphate and vitamin D homeostasis, FGF23 has now been implicated in several pathophysiological mechanisms that may negatively impact the cardiovascular and renal systems. FGF23 is purported to have direct (off-target) effects in the myocardium, as well as canonical effects on FGF receptor/α-klotho receptor complexes in the kidney to activate the renin-angiotensin-aldosterone system, modulate soluble α-klotho levels, and increase sodium retention, to cause left ventricular hypertrophy (LVH). Conversely, FGF23 could be an innocent bystander produced in response to chronic inflammation or other processes associated with CKD that cause LVH and adverse cardiovascular outcomes. Further exploration of these complex mechanisms is needed before modulation of FGF23 can become a legitimate clinical target in CKD.

The calcium-sensing receptor in bone--mechanistic and therapeutic insights

The extracellular calcium-sensing receptor, CaSR, is a member of the G protein-coupled receptor superfamily and has a critical role in modulating Ca(2+) homeostasis via its role in the parathyroid glands and kidneys. New evidence suggests that CaSR expression in cartilage and bone also directly regulates skeletal homeostasis. This Review discusses the role of CaSR in chondrocytes, through which CaSR contributes to the development of the cartilaginous growth plate, as well as in osteoblasts and osteoclasts, through which CaSR has effects on skeletal development and bone turnover in young and mature animals. The interaction of skeletal CaSR activation with parathyroid hormone (PTH), which is secreted by the parathyroid gland, can lead to net bone formation in trabecular bone or net bone resorption in cortical bone. Allosteric modulators of CaSR are beneficial in some clinical conditions, with effects that are mediated by the ability of these agents to alter levels of PTH and improve Ca(2+) homeostasis. However, further insights into the action of CaSR in bone cells might lead to CaSR-based drugs that maximize not only the effects of the receptor on the parathyroid glands and kidneys but also on bone.

Changes in bone mineral metabolism parameters, including FGF23, after discontinuing cinacalcet at kidney transplantation

Little is known about the effects of the administration of cinacalcet in dialytic patients who are scheduled for kidney transplantation, and in particular about the changes in FGF23 and other mineral metabolism parameters after surgery compared with recipients not on cinacalcet at kidney transplantation. We performed a prospective observational cohort study with recruitment of consecutive kidney transplant recipients at our institution. Patients were classified according to whether they were under treatment with cinacalcet before transplantation. Bone mineral metabolism parameters, including C-terminal FGF23, were measured at baseline, on day 15, and at 1, 3, and 6 months after transplantation. In previously cinacalcet-treated patients, cinacalcet therapy was discontinued on the day of surgery and was not restarted after transplantation. A total of 48 kidney transplant recipients, 20 on cinacalcet at surgery and 28 cinacalcet non-treated patients, completed the follow-up. Serum phosphate declined significantly in the first 15 days after transplantation with no differences between the two groups, whereas cinacalcet-treated patients showed higher FGF23 levels, although not significant. After transplantation, PTH and serum calcium were significantly higher in cinacalcet-treated patients. We conclude that patients receiving cinacalcet on dialysis presented similar serum phosphate levels but higher PTH and serum calcium levels during the initial six months after kidney transplantation than cinacalcet non-treated patients. The group previously treated with cinacalcet before transplantation showed higher FGF23 levels without significant differences, so further studies should investigate its relevance in the management of these patients.

Effect of Cinacalcet and Vitamin D Analogs on Fibroblast Growth Factor-23 during the Treatment of Secondary Hyperparathyroidism

BACKGROUND AND OBJECTIVES

Cinacalcet and vitamin D are often combined to treat secondary hyperparathyroidism (SHPT) in patients on dialysis. Independent effects on fibroblast growth factor-23 (FGF-23) concentrations in patients on hemodialysis administered cinacalcet or vitamin D analogs as monotherapies during treatment of SHPT are evaluated.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A multicenter, randomized, open-label study to compare the efficacy of cinacalcet versus traditional vitamin D therapy for management of secondary hyperparathyroidism among subjects undergoing hemodialysis (PARADIGM) was a prospective, phase 4, multicenter, randomized, open-label study conducted globally. Participants (n=312) were randomized 1:1 to cinacalcet (n=155) or vitamin D analog (n=157) for 52 weeks. Levels of FGF-23 were measured at baseline and weeks 20 and 52. The absolute and percentage changes from baseline in plasma FGF-23, parathyroid hormone (PTH), calcium (Ca), phosphorus (P), and calcium-phosphorus product (Ca×P) were assessed. Correlations and logistic regression were used to explore relationships between changes in FGF-23 and changes in PTH, Ca, P, and Ca×P from baseline to week 52 by treatment arm.

RESULTS

Median (quartiles 1, 3) decrease in FGF-23 concentrations was observed in the cinacalcet arm (-40%; -63%, 16%) compared with median increase in the vitamin D analog arm (47%; 0%, 132%) at week 52 (P<0.001). Changes in FGF-23 in both arms were unrelated to changes in PTH (cinacalcet: r=0.17, P=0.11; vitamin D analog: r=-0.04, P=0.70). Changes in FGF-23 in the vitamin D analog but not the cinacalcet arm were correlated with changes in Ca (cinacalcet: r=0.11, P=0.30; vitamin D analog: r=0.32, P<0.01) and P (cinacalcet: r=0.19, P=0.07; vitamin D analog: r=0.49, P<0.001). Changes in FGF-23 were correlated with changes in Ca×P in both arms (cinacalcet: r=0.26, P=0.01; vitamin D analog: r=0.57, P<0.001). Independent of treatment arm, participants with reductions in P or Ca×P were significantly more likely to show reductions in FGF-23.

CONCLUSIONS

During treatment of SHPT, cinacalcet use was associated with a decrease in FGF-23 concentrations, whereas vitamin D analogs were associated with an increase. The divergent effects of these treatments on FGF-23 seem to be independent of modification of PTH. It is possible that effects of cinacalcet and vitamin D analogs on FGF-23 may be mediated indirectly by other effects on bone and mineral metabolism.

Long-term clinical practice experience with cinacalcet for treatment of hypercalcemic hyperparathyroidism after kidney transplantation

Within this prospective, open-label, self-controlled study, we evaluated the long-term effects of the calcimimetic cinacalcet on calcium and phosphate homeostasis in 44 kidney transplant recipients (KTRs) with hypercalcemic hyperparathyroidism by comparing biochemical parameters of mineral metabolism between pre- and posttreatment periods. Results are described as mean differences (95% CIs) between pre- and posttreatment medians that summarize all repeated measurements of a parameter of interest between the date of initial hypercalcemia and cinacalcet initiation (median of 1.6 (IQR: 0.6-3.8) years) and up to four years after treatment start, respectively. Cinacalcet was initiated after 1.8 (0.8-4.7) years posttransplant and maintained for 6.2 (3.9-7.6) years. It significantly decreased total serum calcium (-0.30 (-0.34 to -0.26) mmol/L, P < 0.001) and parathyroid hormone levels (-79 (-103 to -55) pg/mL, P < 0.001). Serum levels of inorganic phosphate (Pi) and renal tubular reabsorption of phosphate to glomerular filtration rate (TmP/GFR) increased simultaneously (Pi: 0.19 (0.15-0.23) mmol/L, P < 0.001, TmP/GFR: 0.20 (0.16-0.23) mmol/L, P < 0.001). In summary, cinacalcet effectively controlled hypercalcemic hyperparathyroidism in KTRs in the long-term and increased low Pi levels without causing hyperphosphatemia, pointing towards a novel indication for the use of cinacalcet in KTRs.

[FGF23 and the heart]

The prevalence of chronic kidney disease (CKD) has now reached epidemic proportions and it is very likely that it will continue to rise with the increasing prevalence of juvenile diabetes mellitus, hypertension and aging population. CKD is a risk factor for cardiovascular disease (CVD) and cardiovascular disease can lead to CKD. It is also well known that patients with CKD have a higher risk of death from CVD than of progressing to end-stage renal disease that requires renal replacement therapy. In patients with CKD, there is a higher mortality from sudden cardiac death and congestive heart failure than coronary artery disease, which is not the case in the general population. The high prevalence of congestive heart failure in CKD is due to cardiac remodeling which progresses from concentric remodeling to concentric and eccentric hypertrophy, leading to left ventricular hypertrophy with both systolic and diastolic dysfunction. Recent studies have suggested that, in patients with chronic kidney disease, common traditional risk factors for cardiovascular disease such as hypertension, hyperlipidemia and obesity may not be the main determinants of cardiovascular disease. Among the various non-traditional cardiovascular risk factors present in patients with chronic kidney disease, abnormalities of CKD related mineral and bone disorder, which includes elevated fibroblast growth factor 23 (FGF23) have been one of the most extensively studied. However, after many years of research, the debate over the exact pathways by which FGF23 may lead to increased CVD still continues. FGF23 may have both direct and indirect effects on the cardiovascular system. Better understanding of the most relevant pathophysiologic pathways for FGF23 may lead to therapeutic interventions against cardiovascular disease in patients with CKD.

The use of fibroblast growth factor 23 testing in patients with kidney disease

The emergence of fibroblast growth factor 23 as a potentially modifiable risk factor in CKD has led to growing interest in its measurement as a tool to assess patient risk and target therapy. This review discusses the analytical and clinical challenges faced in translating fibroblast growth factor 23 testing into routine practice. As for other bone mineral markers, agreement between commercial fibroblast growth factor 23 assays is poor, mainly because of differences in calibration, but also, these differences reflect the variable detection of hormone fragments. Direct comparison of readout from different assays is consequently limited and likely hampers setting uniform fibroblast growth factor 23-directed targets. Efforts are needed to standardize assay output to enhance clinical use. Fibroblast growth factor 23 is robustly associated with cardiovascular and renal outcomes in patients with CKD and adds value to risk assessments based on conventional risk factors. Compared with most other mineral markers, fibroblast growth factor 23 shows better intraindividual temporal stability, with minimal diurnal and week-to-week variability, but substantial interindividual variation, maximizing discriminative power for risk stratification. Conventional therapeutic interventions for the CKD-mineral bone disorder, such as dietary phosphate restriction and use of oral phosphate binders or calcimimetics, are associated with variable efficacy at modulating circulating fibroblast growth factor 23 concentrations, like they are for other mineral metabolites. Dual therapy with dietary phosphate restriction and noncalcium-based binder use achieves the most consistent fibroblast growth factor 23-lowering effect and seems best monitored using an intact assay. Additional studies are needed to evaluate whether strategies aimed at reducing levels or antagonizing its action have beneficial effects on clinical outcomes in CKD patients. Moreover, a better understanding of the mechanisms driving fibroblast growth factor 23 elevations in CKD is needed to inform the use of therapeutic interventions targeting fibroblast growth factor 23 excess. This evidence must be forthcoming to support the use of fibroblast growth factor 23 measurement and fibroblast growth factor 23-directed therapy in the clinic.

Circulating FGF23 levels in response to acute changes in plasma Ca(2+)

The regulation of fibroblast growth factor 23 (FGF23) synthesis and secretion is still incompletely understood. FGF23 is an important regulator of renal phosphate excretion and has regulatory effects on the calciotropic hormones calcitriol and parathyroid hormone (PTH). Calcium (Ca) and phosphate homeostasis are closely interrelated, and it is therefore likely that Ca is involved in FGF23 regulation. It has recently been reported that dietary Ca influenced FGF23 levels, with high Ca increasing FGF23. The mechanism remains to be clarified. It remains unknown whether acute changes in plasma Ca influence FGF23 levels and whether a close relationship, similar that known for Ca and PTH, exists between Ca and FGF23. Thus, the aim of the present study was to examine whether acute hypercalcemia and hypocalcemia regulate FGF23 levels in the rat. Acute hypercalcemia was induced by an intravenous Ca infusion and hypocalcemia by infusion of ethylene glycol tetraacetic acid (EGTA) in normal and acutely parathyroidectomized rats. Intact plasma FGF23 and intact plasma PTH and plasma Ca(2+) and phosphate were measured. Acute hypercalcemia and hypocalcemia resulted as expected in adequate PTH secretory responses. Plasma FGF23 levels remained stable at all plasma Ca(2+) levels; acute parathyroidectomy did not affect FGF23 secretion. In conclusion, Ca is not a regulator of acute changes in FGF23 secretion.

Cinacalcet treatment decreases plasma fibroblast growth factor 23 concentration in haemodialysed patients with chronic kidney disease and secondary hyperparathyroidism

OBJECTIVE

Recent clinical studies suggest that fibroblast growth factor 23 (FGF23) is important in the pathogenesis of calcium-phosphate abnormalities in patients with chronic kidney disease and that increased plasma FGF23 concentration is a cardiovascular risk factor in these patients. The aim of this prospective, single-arm, open-label clinical study was to assess the influence of 6-month cinacalcet treatment on plasma FGF23 concentration in haemodialysed patients with secondary hyperparathyroidism (sHPT).

DESIGN, PATIENTS AND MEASUREMENTS

In 58 haemodialysed patients with sHPT (parathormone PTH > 300 ng/l), serum PTH, FGF23, calcium and phosphate concentrations were assessed before the first dose of cinacalcet and after 3 and 6 months of treatment.

RESULTS

Serum PTH concentration decreased significantly after 3 and 6 months of treatment, and the mean serum calcium and phosphate concentrations remained stable during the treatment period. Plasma FGF23 concentration (geometric mean with 95% confidence index) decreased after 3 and 6 months of treatment from 354 (261-481) ng/l to 295 (204-428) ng/l; P = 0·099 and to 183 (117-285) ng/l; P = 0·015, respectively. FGF23 concentration decreased in 52% of patients. In multivariate regression analysis, plasma FGF23 concentration changes were explained by the changes in serum phosphate, but not by serum PTH or calcium changes or by the dose of cinacalcet.

CONCLUSIONS

1. Cinacalcet treatment decreases plasma FGF23 concentration in haemodialysed patients with secondary hyperparathyroidism. 2. The decrease in plasma FGF23 concentration seems to be related to the decrease in serum phosphate concentration.

Cardiovascular Risk Factors and Chronic Kidney Disease-FGF23: A Key Molecule in the Cardiovascular Disease

Patients with chronic kidney disease (CKD) are at increased risk of mortality, mainly from cardiovascular disease. Moreover, abnormal mineral and bone metabolism, the so-called CKD-mineral and bone disorder (MBD), occurs from early stages of CKD. This CKD-MBD presents a strong cardiovascular risk for CKD patients. Discovery of fibroblast growth factor 23 (FGF23) has altered our understanding of CKD-MBD and has revealed more complex cross-talk and endocrine feedback loops between the kidney, parathyroid gland, intestines, and bone. During the past decade, reports of clinical studies have described the association between FGF23 and cardiovascular risks, left ventricular hypertrophy, and vascular calcification. Recent translational reports have described the existence of FGF23-Klotho axis in the vasculature and the causative effect of FGF23 on cardiovascular disease. These findings suggest FGF23 as a promising target for novel therapeutic approaches to improve clinical outcomes of CKD patients.

Phosphate: an old bone molecule but new cardiovascular risk factor

Phosphate handling in the body is complex and involves hormones produced by the bone, the parathyroid gland and the kidneys. Phosphate is mostly found in hydroxyapatite. however recent evidence suggests that phosphate is also a signalling molecule associated with bone formation. Phosphate balance requires careful regulation of gut and kidney phosphate transporters, SLC34 transporter family, but phosphate signalling in osteoblasts and vascular smooth muscle cells is likely mediated by the SLC20 transporter family (PiT1 and PiT2). If not properly regulated, phosphate imblanace could lead to mineral disorders as well as vascular calcification. In chronic kidney disease-mineral bone disorder, hyperphosphataemia has been consistently associated with extra-osseous calcification and cardiovascular disease. This review focuses on the physiological mechanisms involved in phosphate balance and cell signalling (i.e. osteoblasts and vascular smooth muscle cells) as well as pathological consequences of hyperphosphataemia. Finally, conventional as well as new and experimental therapeutics in the treatment of hyperphosphataemia are explored.

Calcium regulates FGF-23 expression in bone

Calcium has recently been shown to regulate fibroblast growth factor 23 (FGF-23), a bone-derived phosphate and vitamin D-regulating hormone. To better understand the regulation of FGF-23 by calcium, phosphorus, 1,25 dihydroxyvitamin D3 [1,25(OH)2D], and PTH, we examined FGF-23 expression under basal conditions and in response to PTH, doxercalciferol, or high-calcium diet treatment in Gcm2(-/-) and Cyp27b1(-/-) mutant mice. Gcm2(-/-) mice exhibited low serum PTH and 1,25(OH)2D concentrations, hypocalcemia, and hyperphosphatemia, whereas Cyp27b1(-/-) mice had high PTH, undetectable 1,25(OH)2D, hypocalcemia, and hypophosphatemia. Serum FGF-23 levels were decreased in both mutant models. Doxercalciferol administration increased serum FGF-23 levels in both mutant models. PTH administration to Gcm2(-/-) mice also increased serum FGF-23 levels, in association with an increase in both 1,25(OH)2D and calcium concentrations. Multiple regression analysis of pooled data indicated that changes in FGF-23 were positively correlated with serum calcium and 1,25(OH)2D but not related to changes in serum phosphate concentrations. A high-calcium diet also increased serum FGF-23 concentrations in Cyp27b1(-/-) mice in the absence of 1,25(OH)2D and in Gcm2(-/-) mice with low PTH. The addition of calcium to the culture media also stimulated FGF-23 message expression in MC3T3-E1 osteoblasts. In addition, FGF-23 promoter activity in cultured osteoblasts was inhibited by the L-calcium-channel inhibitor nifedipine and stimulated by calcium ionophores. The effects of chronic low calcium to prevent 1,25(OH)2D and PTH stimulation of FGF-23 in these mutant mouse models suggest that suppression of FGF-23 plays an important physiological adaptive response to hypocalcemia.

Calcimimetics and outcomes in CKD

In the past decade, several experimental studies demonstrated an inhibitory effect of calcimimetics on the progression of vascular calcification in animals with chronic kidney disease (CKD), in keeping with the expression of the calcium-sensing receptor (CaR) in vascular tissue. In addition, calcimimetics were also found to prevent the arterial remodeling caused by CKD and to slow the progression of atherosclerosis in uremic rats and mice, respectively. The mode of action of these CaR modulators could be both via a better control of secondary hyperparathyroidism and direct effects on the vessel wall. Two main clinical trials, ADVANCE and EVOLVE, recently evaluated in patients with CKD stage 5D the effects of the calcimimetic cinacalcet on the progression of vascular calcification and hard cardiovascular outcomes, respectively. Both trials missed their respective primary end point by intent-to-treat analysis although by other prespecified analyses, including adjustment for baseline characteristics, there was strong suggestive evidence in favor of reductions in risk, in agreement with numerous experimental studies. Further clinical trials are needed to settle this issue definitively.

Velcalcetide (AMG 416), a novel peptide agonist of the calcium-sensing receptor, reduces serum parathyroid hormone and FGF23 levels in healthy male subjects

CONTEXT

Velcalcetide, also known as AMG 416, is a novel, long-acting selective peptide agonist of the calcium sensing receptor. It is being developed as an intravenous treatment of secondary hyperparathyroidism (SHPT) in hemodialysis patients with chronic kidney disease-mineral and bone disorder.

OBJECTIVE

To assess the safety, tolerability, pharmacokinetics and pharmacodynamics of velcalcetide in healthy male volunteers.

METHODS

The study was a double-blind, randomized, placebo-controlled, single-dose, dose-escalation study in healthy males aged 18-45 years conducted at a single center. Each cohort included eight subjects randomized 6:2 to velcalcetide or placebo.

INTERVENTION

Velcalcetide at 0.5, 2, 5 and 10 mg or placebo was administered intravenously.

OUTCOMES

Measurements included plasma ionized calcium (iCa), serum total calcium, intact parathyroid hormone (iPTH), phosphorus and fibroblast growth factor-23 (FGF23), 1,25-dihydroxyvitamin D, calcitonin and urine creatinine, calcium and phosphorus and plasma pharmacokinetics for velcalcetide. Vital signs, safety biochemical and hematological indices, and adverse events were monitored throughout the study.

RESULTS

Intravenous administration of velcalcetide was well tolerated with no adverse reaction of nausea, vomiting or diarrhea reported. Velcalcetide mediated dose-dependent decreases in serum iPTH at 30 min, FGF23 at 24 h and iCa at 12 h post dose (P<0.05) and in urine fractional excretion of phosphorus and increases in tubular reabsorption of phosphorus. Velcalcetide plasma exposure increased in a dose-related manner and the terminal elimination of half-life was comparable across the dose range evaluated and ranged from 18.4 to 20.0 h.

CONCLUSION

Single IV doses of velcalcetide were well tolerated and associated with rapid, sustained, dose-dependent reductions in serum PTH. The results support further evaluation of velcalcetide as a treatment for SHPT in hemodialysis patients.

Mechanistic analysis for time-dependent effects of cinacalcet on serum calcium, phosphorus, and parathyroid hormone levels in 5/6 nephrectomized rats

This study investigates the time-dependent effects of cinacalcet on serum calcium, phosphorus, and parathyroid hormone (PTH) levels in 5/6 nephrectomized (NX) rats with experimental chronic renal insufficiency. In this study, 5/6 NX male, Sprague-Dawley rats were treated with vehicle or cinacalcet (10 mg/kg, oral, 1× daily). On Day 0 (before treatment), Day 12 and 13 after treatment (to approximate the clinical practice), and also at 0, 1, 4, 8, 16, and 24 hours after the last dosing, blood was collected for analysis. After 12 or 13 days of cinacalcet treatment, modest changes were observed in serum Ca and phosphorus (Pi), while PTH decreased by >45% to Sham levels (152 ± 15 pg/mL). Detailed mapping found that cinacalcet caused a significant time-dependent decrease in serum Ca following dosing, reaching a lowest point at 8 hours (decrease by 20% to 8.43 ± 0.37 mg/dL), and then returning to normal at 24 hours. Cinacalcet also caused a significant increase in serum Pi levels (by 18%). To investigate the potential mechanism of action, a broad approach was taken by testing cinacalcet in a panel of 77 protein-binding assays. Cinacalcet interacted with several channels, transporters, and neurotransmitter receptors, some of which are involved in brain and heart, and may impact Ca homeostasis. Cinacalcet dose-dependently increased brain natriuretic peptide (BNP) mRNA expression by 48% in cardiomyocytes, but had no significant effects on left ventricular hypertrophy and cardiac function. The results suggest that cinacalcet's hypocalcemic effect may be due to its nonspecific interaction with other receptors in brain and heart.

Which vitamin D in CKD-MBD? The time of burning questions

Vitamin D is a common treatment against secondary hyperparathyroidism in renal patients. However, the rationale for the prescription of vitamin D sterols in chronic kidney disease (CKD) is rapidly increasing due to the coexistence of growing expectancies close to unsatisfactory evidences, such as (1) the lack of randomized controlled trials (RCTs) proving the superiority of any vitamin D sterol against placebo on patients centered outcomes, (2) the scanty clinical data on head to head comparisons between the multiple vitamin D sterols currently available, (3) the absence of RCTs confirming the crescent expectations on nutritional vitamin D pleiotropic effects even in CKD patients, (4) the promising effects of vitamin D receptors activators (VDRA) against proteinuria and myocardial hypertrophy in diabetic CKD cohorts, and (5) the conflicting data on the impact on mortality of VDRA versus calcimimetic centered regimens to control CKD-MBD. The present review arguments these issues focusing on the opened questions that nephrologists should consider dealing with the prescription of nutritional vitamin D or VDRA and with the choice of a VDRA versus a calcimimetic based regimen in CKD-MBD patients.

The role of fibroblast growth factor-23 in cardiorenal syndrome

Abnormalities in chronic kidney disease-related bone and mineral metabolism (CKD-MBD) have emerged as novel risk factors in excess cardiovascular mortality in patients with CKD and end-stage renal disease (ESRD). The pathophysiological links between CKD-MBD and adverse cardiovascular events in this patient population are unclear. Hyperphosphatemia through induction of vascular calcifications and decreased active vitamin D production leading to activation of the renin angiotensin system (RAS) along with defects in innate immunity are purported to be the proximate cause of CKD-MBD-associated mortality in CKD. Recently, this view has been challenged by the observation that fibroblast growth factor-23 (FGF23), a newly discovered hormone produced in the bone that regulates phosphate and vitamin D metabolism by the kidney, is a strong predictor of adverse cardiovascular outcomes in patients with CKD and ESRD. Whether these associations between elevated circulating FGF23 levels and cardiovascular outcomes are causative, and if so, the mechanisms mediating the effects of FGF23 on the cardiovascular system are not clear. The principal physiological functions of FGF23 are mediated by activation of FGF receptor/α-klotho coreceptor complexes in target tissues. Elevated FGF23 has been associated with left ventricular hypertrophy (LVH), and it has been suggested that FGF23 may induce myocardial hypertrophy through a direct effect on cardiac myocytes. A direct 'off target' effect of FGF23 on LVH is controversial, however, since α-klotho (which is believed to be indispensable for the physiologic actions of FGF23) is not expressed in the myocardium. Another possibility is that FGF23's effect on the heart is mediated indirectly, via 'on target' regulation of hormonal pathways in the kidney, which include suppression of angiotensin-converting enzyme 2, Cyp27b1and α-klotho, which would be predicted to act on circulating factors known to regulate RAS, 1,25(OH)2D production and ion transport in the myocardium. Understanding of FGF23's pathophysiology and mechanisms of action responsible for its negative effects will be necessary to develop therapeutic strategies to treat CKD-MBD.

Vitamin D receptor activation and cardiovascular disease

Vitamin D has been recently associated with several renal, cardiovascular and inflammatory diseases, beyond mineral metabolism and bone health. This is due in part to widespread expression of vitamin D receptor (VDR) on tissues and cells such as heart, kidney, immune cells, brain and muscle. In chronic kidney disease (CKD) and other chronic disorders, vitamin D deficiency [serum 25(OH)D <20 ng/mL] is very common and is associated with adverse outcomes. Paricalcitol, a selective activator of VDR, has demonstrated in several experimental and clinical studies of diabetic and non-diabetic CKD a favourable profile compared to other VDR activators, alone or as add-on to standard therapy. These beneficial effects are mediated by different actions such as reduction of oxidative stress, inflammation, downregulation of cardiac and renal renin expression, downregulation of calcifying genes and direct vascular protective effects. Furthermore, paricalcitol beneficial effects may be independent of baseline serum parathyroid hormone (PTH), calcium and phosphate levels. These benefits should be confirmed in large and well-designed ongoing clinical trials.

Fibroblast growth factor-23: what we know, what we don't know, and what we need to know

Traditional risk factors of cardiovascular morbidity and mortality such as hypertension, hypercholesterolemia and obesity are paradoxically associated with better outcomes in dialysis patients, and the few trials of interventions targeting modifiable traditional risk factors have yielded disappointing results in this patient population. Non-traditional risk factors such as inflammation, anemia and abnormalities in bone and mineral metabolism have been proposed as potential explanations for the excess mortality seen in patients with chronic kidney disease (CKD) and end-stage renal disease (ESRD), but without clear understanding of what the most important pathophysiologic mechanisms of these risk factors are, which ones might be ideal treatment targets and which therapeutic interventions may be effective and safe in targeting them. Among the novel risk factors, fibroblast growth factor-23 (FGF23) has recently emerged as one of the most powerful predictors of adverse outcomes in patients with CKD and ESRD. FGF23 is a hormone produced by osteoblasts/osteocytes in bone that acts on the kidney to regulate phosphate and vitamin D metabolism through activation of FGF receptor/α-Klotho co-receptor complexes. It is possible that elevated FGF23 may exert its negative impact through distinct mechanisms of action independent from its role as a regulator of phosphorus homeostasis. Elevated circulating FGF23 concentrations have been associated with left ventricular hypertrophy (LVH), and it has been suggested that FGF23 exerts a direct effect on the myocardium. While it is possible that 'off target' effects of FGF23 present in very high concentrations could induce LVH, this possibility is controversial, since α-klotho is not expressed in the myocardium. Another possibility is that FGF23's effect on the heart is mediated indirectly, via 'on target' activation of other humoral pathways. We will review the physiology and pathophysiology of FGF23, the outcomes associated with elevated FGF23 levels, and describe putative mechanisms of action responsible for its negative effects and potential therapeutic strategies to treat these.

Cinacalcet attenuates hypercalcemia observed in mice bearing either Rice H-500 Leydig cell or C26-DCT colon tumors

Excessive secretion of parathyroid hormone-related protein (PTHrP) by tumors stimulates bone resorption and increases renal tubular reabsorption of calcium, resulting in hypercalcemia of malignancy. We investigated the ability of cinacalcet, an allosteric modulator of the calcium-sensing receptor, to attenuate hypercalcemia by assessing its effects on blood ionized calcium, serum PTHrP, and calcium-sensing receptor mRNA in mice bearing either Rice H-500 Leydig cell or C26-DCT colon tumors. Cinacalcet effectively decreased hypercalcemia in a dose- and enantiomer-dependent manner; furthermore, cinacalcet normalized phosphorus levels, but did not affect serum PTHrP. Ribonuclease protection assay results demonstrated presence of PTHrP receptor, but not calcium-sensing receptor mRNA in C26-DCT tumors. The mechanism by which cinacalcet lowered serum calcium was investigated in parathyroidectomized rats (i.e., without PTH) made hypercalcemic by PTHrP. Cinacalcet attenuated PTHrP-mediated elevations in blood ionized calcium, which were accompanied by increased plasma calcitonin. Taken together these results suggest that the cinacalcet-mediated decrease in serum calcium is not the result of a direct effect on tumor cells, but rather is the result of increased calcitonin release. In summary, cinacalcet effectively reduced tumor-mediated hypercalcemia and corrected hypophosphatemia in mice. Further investigation of cinacalcet for treatment of hypercalcemia of malignancy is warranted.

Vitamin D receptor activation in a diabetic-like environment: potential role in the activity of the endothelial pro-inflammatory and thioredoxin pathways

High blood and tissue concentrations of glucose and advanced glycation end products (AGEs) are thought to play an important role in the development of diabetic vascular complications. Thioredoxin interacting protein (TXNIP) is up-regulated in response to high levels of glucose and is an endogenous inhibitor of thioredoxin (TRX), and may play a contributory role in the occurrence of diabetic-related vascular diseases. Vitamin D inhibits endothelial proliferation and is a cardiovascular protective agent. The present study evaluated the impact of paricalcitol and calcitriol on the endothelial inflammatory and TXNIP pathways in cultured endothelial cells exposed to a diabetic-like environment. Fresh human umbilical vein cord endothelial cells (HUVEC) were treated for 24h with 200 μg/ml AGE-HSA and 250 mg/dl glucose concentrations, with paricalcitol or calcitriol. IL6, IL8, NFκB (p50/p65), receptor of AGE (RAGE), TXNIP, and TRX expressions were evaluated at the levels of mRNA, protein, and TRX activity. Calcitriol and paricalcitol significantly down-regulated the markers involved in the inflammatory responses. Only paricalcitol induced a significant decrease in TXNIP mRNA and protein expressions. Neither paricalcitol nor calcitriol affected TRX reductase activity or TRX mRNA and protein expressions. Our findings indicate that in an endothelial diabetic-like environment, paricalcitol and calcitriol significantly decreased the expression of genes involved in the inflammatory pathway. In this in vitro study, it seems that the TRX antioxidant system was not involved. The different effects found between paricalcitol and calcitriol might reflect the selectivity of vitamin D receptor (VDR) activation.

Primary hyperparathyroidism in children and adolescents

Primary hyperparathyroidism (PHPT) is a common endocrine disorder in adults in whom the typical presentation is incidentally discovered as asymptomatic hypercalcemia. PHPT is much less common in children and adolescents, but has greater morbidity in this age group, as most young patients with PHPT will have symptomatic hypercalcemia or complications such as kidney stones, abdominal pain, and skeletal fragility. An important feature of PHPT in younger patients is the relatively high prevalence of germline inactivating mutations of the CASR gene, which encodes the calcium-sensing receptor. Biallelic CASR mutations cause neonatal severe hyperparathyroidism, a life-threatening condition that presents within days of life with marked hypercalcemia, respiratory distress, failure to thrive, and skeletal demineralization. By contrast, more common heterozygous CASR mutations are generally associated with a benign variant of PHPT termed familial hypocalciuric hypercalcemia. Appropriate management of PHPT in children and adolescents requires distinction between familial hypocalciuric hypercalcemia, which generally requires no specific treatment, and other forms of PHPT that are best treated by parathyroidectomy.

Vitamin D receptor agonists increase klotho and osteopontin while decreasing aortic calcification in mice with chronic kidney disease fed a high phosphate diet

Vascular calcification is common in chronic kidney disease, where cardiovascular mortality remains the leading cause of death. Patients with kidney disease are often prescribed vitamin D receptor agonists (VDRAs) that confer a survival benefit, but the underlying mechanisms remain unclear. Here we tested two VDRAs in a mouse chronic kidney disease model where dietary phosphate loading induced aortic medial calcification. Mice were given intraperitoneal calcitriol or paricalcitol three times per week for three weeks. These treatments were associated with half of the aortic calcification compared to no therapy, and there was no difference between the two agents. In the setting of a high phosphate diet, serum parathyroid hormone and calcium levels were not significantly altered by treatment. VDRA therapy was associated with increased serum and urine klotho levels, increased phosphaturia, correction of hyperphosphatemia, and lowering of serum fibroblast growth factor-23. There was no effect on elastin remodeling or inflammation, however, the expression of the anti-calcification factor, osteopontin, in aortic medial cells was increased. Paricalcitol upregulated osteopontin secretion from mouse vascular smooth muscle cells in culture. Thus, klotho and osteopontin were upregulated by VDRA therapy in chronic kidney disease, independent of changes in serum parathyroid hormone and calcium.

Paricalcitol does not improve glucose metabolism in patients with stage 3-4 chronic kidney disease

Patients with chronic kidney disease are often insulin resistant and glucose intolerant--abnormalities that promote cardiovascular disease. Administration of 1,25-dihydroxyvitamin D (calcitriol) has improved glucose metabolism in patients with end-stage renal disease. We conducted a randomized, placebo-controlled clinical trial to test whether paricalcitol, a 1,25-dihydroxyvitamin D analog, changes glucose tolerance in earlier stages of chronic kidney disease. In a crossover design, 22 nondiabetic patients with estimated glomerular filtration rates of stage 3-4 chronic kidney disease and fasting plasma glucose of 100-125 mg/dl were given daily oral paricalcitol for 8 weeks and matching placebo for 8 weeks, separated by an 8-week washout period. The order of interventions was random and blinded to both participants and investigators. Paricalcitol significantly reduced serum concentrations of parathyroid hormone, 1,25-dihydroxyvitamin D, and 25-hydroxyvitamin D while significantly increasing serum concentrations of fibroblast growth factor-23 and 24,25-dihydroxyvitamin D. Paricalcitol, however, had no significant effect on glucose tolerance (the primary outcome measure), insulin sensitivity, beta-cell insulin response, plasma free fatty acid suppression, or urinary F2-isoprostane excretion. Thus, despite substantial effects on vitamin D metabolism, paricalcitol did not improve glucose metabolism in nondiabetic patients with stage 3-4 chronic kidney disease.

The consequences of chronic kidney disease on bone metabolism and growth in children

Growth retardation, decreased final height and renal osteodystrophy (ROD) are common complications of childhood chronic kidney disease (CKD), resulting from a combination of abnormalities in the growth hormone (GH) axis, vitamin D deficiency, hyperparathyroidism, hypogonadism, inadequate nutrition, cachexia and drug toxicity. The impact of CKD-associated bone and mineral disorders (CKD-MBD) may be immediate (serum phosphate/calcium disequilibrium) or delayed (poor growth, ROD, fractures, vascular calcifications, increased morbidity and mortality). In 2012, the clinical management of CKD-MBD in children needs to focus on three main objectives: (i) to provide an optimal growth in order to maximize the final height with an early management with recombinant GH therapy when required, (ii) to equilibrate calcium/phosphate metabolism so as to obtain acceptable bone quality and cardiovascular status and (iii) to correct all metabolic and clinical abnormalities that can worsen bone disease, growth and cardiovascular disease, i.e. metabolic acidosis, anaemia, malnutrition and 25(OH)vitamin D deficiency. The aim of this review is to provide an overview of the mineral, bone and vascular abnormalities associated with CKD in children in terms of pathophysiology, diagnosis and clinical management.

Update on fibroblast growth factor 23 in chronic kidney disease

Chronic kidney disease (CKD) is a public health epidemic that affects millions of people worldwide. Presence of CKD predisposes individuals to high risks of end-stage renal disease, cardiovascular disease and premature death. Disordered phosphate homeostasis with elevated circulating levels of fibroblast growth factor 23 (FGF23) is an early and pervasive complication of CKD. CKD is likely the most common cause of chronically elevated FGF23 levels, and the clinical condition in which levels are most markedly elevated. Although increases in FGF23 levels help maintain serum phosphate in the normal range in CKD, prospective studies in populations of pre-dialysis CKD, incident and prevalent end-stage renal disease, and kidney transplant recipients demonstrate that elevated FGF23 levels are independently associated with progression of CKD and development of cardiovascular events and mortality. It was originally thought that these observations were driven by elevated FGF23 acting as a highly sensitive biomarker of toxicity due to phosphate. However, FGF23 itself has now been shown to mediate “off-target,” direct, end-organ toxicity in the heart, which suggests that elevated FGF23 may be a novel mechanism of adverse outcomes in CKD. This report reviews recent advances in FGF23 biology relevant to CKD, the classical effects of FGF23 on mineral homeostasis, and the studies that established FGF23 excess as a biomarker and novel mechanism of cardiovascular disease. The report concludes with a critical review of the effects of different therapeutic strategies targeting FGF23 reduction and how these might be leveraged in a future randomized trial aimed at improving outcomes in CKD.

Efficacy and safety of cinacalcet for the treatment of secondary hyperparathyroidism in patients with advanced chronic kidney disease before initiation of regular dialysis

AIM

To evaluate the compassionate use of cinacalcet for the management of secondary hyperparathyroidism in patients who are not on dialysis.

METHODS

Patients with stage 4-5 chronic kidney disease (CKD) who were not on dialysis, had an intact parathyroid hormone (iPTH) level greater than 300 pg/mL, and had not responded satisfactorily to treatment with phosphate binders and vitamin D were prospectively studied. Patients received 6 months of compassionate treatment with cinacalcet, which was initiated at a dose of 30 mg/day orally and flexibly dosed thereafter based on iPTH levels.

RESULTS

Twenty-six patients with a mean age±standard deviation (SD) of 58.8±16.1 years were enrolled in the study and included in the statistical analysis. The mean percentage change in iPTH levels from baseline after 6 months of treatment was -67.9±17.0%, with 92.3% (95% confidence interval (CI), 75.9-97.9) of patients showing an iPTH level within the limits recommended by Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines. The mean serum calcium concentrations had decreased significantly at the end of the study (-8.0±6.9%), while the mean serum phosphorus concentration had significantly increased (+8.3±17.0%).

CONCLUSION

Our results suggest that cinacalcet may be a useful alternative for the treatment of secondary hyperparathyroidism in pre-dialysis patients who are unresponsive to other treatments. The hypocalcemia and hyperphosphatemia reported in previous studies may not occur if a moderate dose of calcimimetics is used in patients with marginal glomerular filtration rates, especially if combined with vitamin D analogues and calcium-based phosphate binders.

The calcium-sensing receptor complements parathyroid hormone-induced bone turnover in discrete skeletal compartments in mice

Although the calcium-sensing receptor (CaSR) and parathyroid hormone (PTH) may each exert skeletal effects, it is uncertain how CaSR and PTH interact at the level of bone in primary hyperparathyroidism (PHPT). Therefore, we simulated PHPT with 2 wk of continuous PTH infusion in adult mice with deletion of the PTH gene (Pth(-/-) mice) and with deletion of both PTH and CaSR genes (Pth(-/-)-Casr (-/-) mice) and compared skeletal phenotypes. PTH infusion in Pth(-/-) mice increased cortical bone turnover, augmented cortical porosity, and reduced cortical bone volume, femoral bone mineral density (BMD), and bone mineral content (BMC); these effects were markedly attenuated in PTH-infused Pth(-/-)-Casr(-/-) mice. In the absence of CaSR, the PTH-stimulated expression of receptor activator of nuclear factor-κB ligand and tartrate-resistant acid phosphatase and PTH-stimulated osteoclastogenesis was also reduced. In trabecular bone, PTH-induced increases in bone turnover, trabecular bone volume, and trabecular number were lower in Pth(-/-)-Casr(-/-) mice than in Pth(-/-) mice. PTH-stimulated genetic markers of osteoblast activity were also lower. These results are consistent with a role for CaSR in modulating both PTH-induced bone resorption and PTH-induced bone formation in discrete skeletal compartments.

Symmetrical craniofacial hypertrophy in patients with tertiary hyperparathyroidism and high-dose cinacalcet exposure

We are reporting on a series of two patients with end-stage renal disease on hemodialysis, presented for surgical parathyroidectomy secondary refractory hyperparathyroidism. Both patients had failed maximized medical managements, including higher-than-usual doses of the calcimimetic cinacalcet (270 and 180 mg/day, respectively). On physical exam, both patients had marked symmetrical craniofacial hypertrophy with coarse distortion of facial features, similar in appearance to past reports of Sagliker syndrome. On X-ray and computed tomographic exam, they had peculiar areas of bone absorption on the skull, imitating the radiologic appearance of multiple myeloma. Bone biopsy of the maxilla, however, did not show the expected brown tumor, but rather described only fibrosis and reactive bone formations. This phenotype developed while being on cinacalcet, progressed despite escalation of therapy, and improved only after parathyroidectomy. Both patients developed massive "hungry bone syndrome" after parathyroidectomy necessitating prolonged i.v. calcium infusion. This pattern of severe facial distortion likely represented an adverse consequence of severe tertiary hyperparathyroidism, along with supraphysiologic dose of cinacalcet administration and 25-hydroxy vitamin D deficiency in sensitive individuals. The genetic base of this observation remained unexplained.

Impact of long-term cinacalcet, ibandronate or teriparatide therapy on bone mineral density of hemodialysis patients: a pilot study

BACKGROUND

Insufficient evidenced-based information is available for the treatment of osteoporosis in hemodialysis (HD) patients.

METHODS

In 102 HD patients, bone mineral density (BMD) was measured twice 16 ± 3 months apart. In the second BMD measurement 66 of them had a femoral neck (FN) T-score <-2.5. Of these 66 patients, 38 consented to a bone biopsy. Depending on both the bone biopsy findings and parathyroid hormone levels, patients were assigned to treatment groups. Eleven patients with osteitis fibrosa and iPTH >300 pg/ml received cinacalcet, 11 with osteitis fibrosa and iPTH <300 pg/ml received ibandronate, 9 with adynamic bone disease received teriparatide, and 7 with mild abnormalities received no treatment. A third BMD measurement was done after an average treatment period of 13-16 months. We compared the annual percent change of FN and lumbar spine (LS) BMD before and during treatment.

RESULTS

FN and LS BMD decreased significantly in the cinacalcet group, with an annual change of 3.6 and 3.4% before treatment to -4.2% (p = 0.04) and -7.7% (p = 0.02) during treatment, respectively. In the teriparatide group, FN and LS BMD increased, although not significantly, with an annual change of -5.4 and -2.6% before treatment to 2.7 and 4.9% during treatment, respectively. In both the ibandronate and the no treatment groups, BMD change rate remained negative during the whole study.

CONCLUSIONS

Teriparatide administration improved BMD in HD patients with adynamic bone disease, although these results did not reach statistical significance. In HD patients with osteitis fibrosa, ibandronate did not improve BMD while cinacalcet reduced BMD.

Vitamin D therapy in chronic kidney disease and end stage renal disease

Vitamin D has garnered much research and debate about supplementation in recent years, not only as it pertains to patients with kidney disease but also to those in the general population. This review discusses observational and available clinical trial evidence about the effects of both calcitriol and vitamin D analogs (active) and ergocalciferol and cholecalciferol (nutritional) vitamin D in patients with CKD and ESRD.