The calcium-sensing receptor: a key factor in the pathogenesis of secondary hyperparathyroidism

Pathophysiology and therapies of CKD-associated secondary hyperparathyroidism

Uremic secondary hyperparathyroidism (SHP) refers to the biochemical abnormalities that characterize CKD-MBD. However, historically parathyroid hormone (PTH) is identified as the key culprit hormone and the essential biomarker of secondary hyperparathyroidism. SHP represents the adaptive response to several mineral abnormalities that initiate and maintain increased PTH secretion through classical mineral derangements and more recently elucidated hormonal dysregulations. Among classic factors involved in the pathogenesis of SHP, phosphate, calcium, and calcitriol have a prominent role. The discovery of new pathogenetic factors involved in the development of SHP (and the eventual CKD-MBD) including fibroblast growth factor-23 (FGF23) and klotho provides new hypothesis and perspectives to our understanding of this complex metabolic disturbance. Recently more than serum phosphate a critical role in regulating FGF23 synthesis and the progression of CKD is ascribed to phosphate pool, reflected by production of glycerol-3-phosphate and the formation of excessive CPP-2. Finally, also skeletal resistance to PTH action, due to dysregulation of the Wnt-β-catenin system and intestinal dysbiosis, affecting the PTH actions on bone are causal factor of SHP. Identifying all the actors at play is mandatory to allow the most precise therapeutic prescription in the individual patient. This paper aims to review, in particular, the pathophysiology of SHP, which is essential to envisage the eventual therapeutic options for the associated MBD.

Current therapeutic approach of chronic kidney disease-mineral and bone disorder

Chronic kidney disease (CKD) has emerged as one of the leading noncommunicable diseases affecting >10% of the population worldwide. Bone and mineral disorders are a common complication among patients with CKD resulting in a poor life quality, high fracture risk, increased morbidity and cardiovascular mortality. According to Kidney Disease: Improving Global Outcomes, renal osteodystrophy refers to changes in bone morphology found in bone biopsy, whereas CKD-mineral and bone disorder (CKD-MBD) defines a complex of disturbances including biochemical and hormonal alterations, disorders of bone and mineral metabolism and extraskeletal calcification. As a result, the management of CKD-MBD should focus on the aforementioned parameters, including the treatment of hyperphosphatemia, hypocalcemia, abnormal PTH and vitamin D levels. Regarding the bone fragility fractures, osteoporosis and renal osteodystrophy, which constitute the bone component of CKD-MBD, anti-osteoporotic agents constitute the mainstay of treatment. However, a thorough elucidation of the CKD-MBD pathogenesis is crucial for the ideal personalized treatment approach. In this paper, we review the pathology and management of CKD-MBD based on the current literature with special attention to recent advances.

Large library docking identifies positive allosteric modulators of the calcium-sensing receptor

Positive allosteric modulator (PAM) drugs enhance the activation of the calcium-sensing receptor (CaSR) and suppress parathyroid hormone (PTH) secretion. Unfortunately, these hyperparathyroidism-treating drugs can induce hypocalcemia and arrhythmias. Seeking improved modulators, we docked libraries of 2.7 million and 1.2 billion molecules against the CaSR structure. The billion-molecule docking found PAMs with a 2.7-fold higher hit rate than the million-molecule library, with hits up to 37-fold more potent. Structure-based optimization led to nanomolar leads. In ex vivo organ assays, one of these PAMs was 100-fold more potent than the standard of care, cinacalcet, and reduced serum PTH levels in mice without the hypocalcemia typical of CaSR drugs. As determined from cryo-electron microscopy structures, the PAMs identified here promote CaSR conformations that more closely resemble the activated state than those induced by the established drugs.

Surgical Management of Secondary and Tertiary Hyperparathyroidism

Secondary hyperparathyroidism (SHPT) often arises from kidney disease and is characterized by elevated parathyroid hormone (PTH) levels. The reported optimal PTH level to balance the compensatory physiologic response in SHPT with the pathologic morbidity and mortality has changed over time with our evolving understanding. Parathyroidectomy for kidney-related hyperparathyroidism requires consideration of the patient's dialysis status, potential for kidney transplantation, and medical history. Extent of parathyroidectomy and intraoperative decision-making requires consideration to maximize cure with the risk of permanent hypoparathyroidism. Parathyroidectomy for kidney-related hyperparathyroidism can provide a reduction in morbidity, mortality, and improved kidney allograft function and survival.

Vitamin D: A Bridge between Kidney and Heart

Chronic kidney disease (CKD) and cardiovascular disease (CVD) are highly prevalent conditions, each significantly contributing to the global burden of morbidity and mortality. CVD and CKD share a great number of common risk factors, such as hypertension, diabetes, obesity, and smoking, among others. Their relationship extends beyond these factors, encompassing intricate interplay between the two systems. Within this complex network of pathophysiological processes, vitamin D has emerged as a potential linchpin, exerting influence over diverse physiological pathways implicated in both CKD and CVD. In recent years, scientific exploration has unveiled a close connection between these two prevalent conditions and vitamin D, a crucial hormone traditionally recognized for its role in bone health. This article aims to provide an extensive review of vitamin D's multifaceted and expanding actions concerning its involvement in CKD and CVD.

Small vs. Large Library Docking for Positive Allosteric Modulators of the Calcium Sensing Receptor

Drugs acting as positive allosteric modulators (PAMs) to enhance the activation of the calcium sensing receptor (CaSR) and to suppress parathyroid hormone (PTH) secretion can treat hyperparathyroidism but suffer from side effects including hypocalcemia and arrhythmias. Seeking new CaSR modulators, we docked libraries of 2.7 million and 1.2 billion molecules against transforming pockets in the active-state receptor dimer structure. Consistent with simulations suggesting that docking improves with library size, billion-molecule docking found new PAMs with a hit rate that was 2.7-fold higher than the million-molecule library and with hits up to 37-fold more potent. Structure-based optimization of ligands from both campaigns led to nanomolar leads, one of which was advanced to animal testing. This PAM displays 100-fold the potency of the standard of care, cinacalcet, in ex vivo organ assays, and reduces serum PTH levels in mice by up to 80% without the hypocalcemia typical of CaSR drugs. Cryo-EM structures with the new PAMs show that they induce residue rearrangements in the binding pockets and promote CaSR dimer conformations that are closer to the G-protein coupled state compared to established drugs. These findings highlight the promise of large library docking for therapeutic leads, especially when combined with experimental structure determination and mechanism.

Inhibition of the calcium-sensing receptor by extracellular phosphate ions and by intracellular phosphorylation

As both a sensor of extracellular calcium (Ca2+o) concentration and a key controller of Ca2+o homeostasis, one of the most interesting properties of the calcium-sensing receptor (CaR) is its sensitivity to, and modulation by, ions and small ligands other than Ca2+. There is emerging evidence that extracellular phosphate can act as a partial, non-competitive CaR antagonist to modulate parathyroid hormone (PTH) secretion, thus permitting the CaR to integrate mineral homeostasis more broadly. Interestingly, phosphorylation of certain intracellular CaR residues can also inhibit CaR responsiveness. Thus, negatively charged phosphate can decrease CaR activity both extracellularly (via association with arginine) and intracellularly (via covalent phosphorylation).

Bone Disease in Chronic Kidney Disease and Kidney Transplant

Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD) comprises alterations in calcium, phosphorus, parathyroid hormone (PTH), Vitamin D, and fibroblast growth factor-23 (FGF-23) metabolism, abnormalities in bone turnover, mineralization, volume, linear growth or strength, and vascular calcification leading to an increase in bone fractures and vascular disease, which ultimately result in high morbidity and mortality. The bone component of CKD-MBD, referred to as renal osteodystrophy, starts early during the course of CKD as a result of the effects of progressive reduction in kidney function which modify the tight interaction between mineral, hormonal, and other biochemical mediators of cell function that ultimately lead to bone disease. In addition, other factors, such as osteoporosis not apparently dependent on the typical pathophysiologic abnormalities resulting from altered kidney function, may accompany the different varieties of renal osteodystrophy leading to an increment in the risk of bone fracture. After kidney transplantation, these bone alterations and others directly associated or not with changes in kidney function may persist, progress or transform into a different entity due to new pathogenetic mechanisms. With time, these alterations may improve or worsen depending to a large extent on the restoration of kidney function and correction of the metabolic abnormalities developed during the course of CKD. In this paper, we review the bone lesions that occur during both CKD progression and after kidney transplant and analyze the factors involved in their pathogenesis as a means to raise awareness of their complexity and interrelationship.

Vitamin D and Secondary Hyperparathyroidism in Chronic Kidney Disease: A Critical Appraisal of the Past, Present, and the Future

The association between vitamin D deficiency and especially critical shortage of active vitamin D (1,25-dihydroxyvitamin D, calcitriol) with the development of secondary hyperparathyroidism (sHPT) is a well-known fact in patients with chronic kidney disease (CKD). The association between sHPT and important clinical outcomes, such as kidney disease progression, fractures, cardiovascular events, and mortality, has turned the prevention and the control of HPT into a core issue of patients with CKD and on dialysis. However, vitamin D therapy entails the risk of unwanted side effects, such as hypercalcemia and hyperphosphatemia. This review summarizes the developments of vitamin D therapies in CKD patients of the last decades, from calcitriol substitution to extended-release calcifediol. In view of the study situation for vitamin D insufficiency and sHPT in CKD patients, we conclude that the nephrology community has to solve three core issues: (1) What is the optimal parathyroid hormone (PTH) target level for CKD and dialysis patients? (2) What is the optimal vitamin D level to support optimal PTH titration? (3) How can sHPT treatment support reduction in the occurrence of hard renal and cardiovascular events in CKD and dialysis patients?

The American Association of Endocrine Surgeons Guidelines for the Definitive Surgical Management of Secondary and Tertiary Renal Hyperparathyroidism

OBJECTIVE

To develop evidence-based recommendations for safe, effective, and appropriate treatment of secondary (SHPT) and tertiary (THPT) renal hyperparathyroidism.

BACKGROUND

Hyperparathyroidism is common among patients with chronic kidney disease, end-stage kidney disease, and kidney transplant. The surgical management of SHPT and THPT is nuanced and requires a multidisciplinary approach. There are currently no clinical practice guidelines that address the surgical treatment of SHPT and THPT.

METHODS

Medical literature was reviewed from January 1, 1985 to present January 1, 2021 by a panel of 10 experts in SHPT and THPT. Recommendations using the best available evidence was constructed. The American College of Physicians grading system was used to determine levels of evidence. Recommendations were discussed to consensus. The American Association of Endocrine Surgeons membership reviewed and commented on preliminary drafts of the content.

RESULTS

These clinical guidelines present the epidemiology and pathophysiology of SHPT and THPT and provide recommendations for work-up and management of SHPT and THPT for all involved clinicians. It outlines the preoperative, intraoperative, and postoperative management of SHPT and THPT, as well as related definitions, operative techniques, morbidity, and outcomes. Specific topics include Pathogenesis and Epidemiology, Initial Evaluation, Imaging, Preoperative and Perioperative Care, Surgical Planning and Parathyroidectomy, Adjuncts and Approaches, Outcomes, and Reoperation.

CONCLUSIONS

Evidence-based guidelines were created to assist clinicians in the optimal management of secondary and tertiary renal hyperparathyroidism.

Phosphate balance during dialysis and after kidney transplantation in patients with chronic kidney disease

PURPOSE OF REVIEW

In patients with chronic kidney disease (CKD), hyperphosphatemia is associated with several adverse outcomes, including bone fragility and progression of kidney and cardiovascular disease. However, there is a knowledge gap regarding phosphate balance in CKD. This review explores its current state, depending on the stage of CKD, dialysis modalities, and the influence of kidney transplantation.

RECENT FINDINGS

Adequate phosphate control is one of the goals of treatment for CKD-mineral and bone disorder. However, ongoing studies are challenging the benefits of phosphate-lowering treatment. Nevertheless, the current therapy is based on dietary restriction, phosphate binders, and optimal removal by dialysis. In the face of limited adherence, due to the high pill burden, adjuvant options are under investigation. The recent discovery that intestinal absorption of phosphate is mostly paracellular when the intraluminal concentration is adequate might help explain why phosphate is still well absorbed in CKD, despite the lower levels of calcitriol.

SUMMARY

Future studies could confirm the benefits of phosphate control. Greater understanding of the complex distribution of phosphate among the body compartments will help us define a better therapeutic strategy in patients with CKD.

Vitamin D Analogues and Fracture Risk in Chronic Kidney Disease: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Vitamin D receptor agonists (VDRAs) are commonly prescribed in chronic kidney disease (CKD). However, their protective effects on bone remain controversial. We performed a systematic review and meta-analysis of randomized controlled trials (RCTs) to evaluate the effect of VDRAs on fracture risk and bone mineral density (BMD) in adult patients with CKD. We searched MEDLINE, EMBASE, CENTRAL, ClinicalTrials.gov, and the WHO's International Clinical Trials Registry Platform databases from inception to June 19, 2021. We included RCTs comparing VDRAs, to placebo or another medication, in adults with CKD requiring or not dialysis. Conference abstracts and trials involving kidney transplant recipients and/or comparing VDRAs to antiresorptive or anabolic bone therapy were excluded. Primary outcome was fracture at any anatomical site. Secondary outcomes were BMD at femoral neck, lumbar spine, and/or total hip. Prespecified subgroup analyses were conducted according to baseline demographics, overall risk of bias, and follow-up time. From 6868 references retrieved, eight RCTs were eligible: five reported fracture, two reported BMD, and one reported both outcomes. As comparator, one study used no VDRAs, one used nutritional intervention and no medication, and six used placebo. In meta-analysis, VDRAs were not associated with a significant reduction in total fractures in overall (risk ratio = 0.79, 95% confidence interval 0.38-1.65, I2 = 0%, six trials, 1507 participants, 27 fractures) or in prespecified subgroup analyses. Three trials reported BMD at different sites and with different BMD measurements; thus, a meta-analysis could not be performed. Two RCTs were at high risk of bias, notably because of deviations from the intended interventions. As limitation, we have to mention the low total number of fractures included in our meta-analysis. In conclusion, current evidence from RCTs is insufficient to associate VDRAs with bone protection in CKD. Further large and long-term studies specifically designed to evaluate the efficacy of VDRAs on bone outcomes are thus required. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

An optogenetic approach for regulating human parathyroid hormone secretion

Parathyroid hormone (PTH) plays crucial role in maintaining calcium and phosphorus homeostasis. In the progression of secondary hyperparathyroidism (SHPT), expression of calcium-sensing receptors (CaSR) in the parathyroid gland decreases, which leads to persistent hypersecretion of PTH. How to precisely manipulate PTH secretion in parathyroid tissue and underlying molecular mechanism is not clear. Here, we establish an optogenetic approach that bypasses CaSR to inhibit PTH secretion in human hyperplastic parathyroid cells. We found that optogenetic stimulation elevates intracellular calcium, inhibits both PTH synthesis and secretion in human parathyroid cells. Long-term pulsatile PTH secretion induced by light stimulation prevented hyperplastic parathyroid tissue-induced bone loss by influencing the bone remodeling in mice. The effects are mediated by light stimulation of opsin expressing parathyroid cells and other type of cells in parathyroid tissue. Our study provides a strategy to regulate release of PTH and associated bone loss of SHPT through an optogenetic approach.

Parathyroid hormone (PTH) plays a role in maintaining calcium and phosphorus homeostasis, and in secondary hyperparathyroidism excess PTH secretion contributes to bone loss. Here the authors report an optogenetic approach to inhibit PTH secretion in human hyperplastic parathyroid cells, and prevented hyperplastic parathyroid tissue-induced bone loss in mice.

Tertiary and Postrenal Transplantation Hyperparathyroidism

Patients who have undergone kidney transplantation (KTx) (KTxps) are a distinctive population characterized by the persistence of some metabolic anomalies present during end-stage renal disease. Mineral metabolism (MM) parameters are frequently altered after KTx. These alterations involve calcium, phosphorus, vitamin D, and parathormone (PTH) disarrangements. At present, there is little consensus about the correct monitoring and management of PTH disorders in KTxps. This article presents the prevalence and epidemiologic and clinical impact of post-KTx hyper-PTH. The principal biochemical and instrumental investigations and the therapeutic options for these conditions are also reported.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia, or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Osteoporosis in Patients with Chronic Kidney Diseases: A Systemic Review

Chronic kidney disease (CKD) is associated with the development of mineral bone disorder (MBD), osteoporosis, and fragility fractures. Among CKD patients, adynamic bone disease or low bone turnover is the most common type of renal osteodystrophy. The consequences of CKD-MBD include increased fracture risk, greater morbidity, and mortality. Thus, the goal is to prevent the occurrences of fractures by means of alleviating CKD-induced MBD and treating subsequent osteoporosis. Changes in mineral and humoral metabolism as well as bone structure develop early in the course of CKD. CKD-MBD includes abnormalities of calcium, phosphorus, PTH, and/or vitamin D; abnormalities in bone turnover, mineralization, volume, linear growth, or strength; and/or vascular or other soft tissue calcification. In patients with CKD-MBD, using either DXA or FRAX to screen fracture risk should be considered. Biomarkers such as bALP and iPTH may assist to assess bone turnover. Before initiating an antiresorptive or anabolic agent to treat osteoporosis in CKD patients, lifestyle modifications, such as exercise, calcium, and vitamin D supplementation, smoking cessation, and avoidance of excessive alcohol intake are important. Managing hyperphosphatemia and SHPT are also crucial. Understanding the complex pathogenesis of CKD-MBD is crucial in improving one's short- and long-term outcomes. Treatment strategies for CKD-associated osteoporosis should be patient-centered to determine the type of renal osteodystrophy. This review focuses on the mechanism, evaluation and management of patients with CKD-MBD. However, further studies are needed to explore more details regarding the underlying pathophysiology and to assess the safety and efficacy of agents for treating CKD-MBD.

Severe chronic kidney disease environment reduced calcium-sensing receptor expression in parathyroid glands of adenine-induced rats even without high phosphorus diet

Background

Chronic kidney disease (CKD) disrupts mineral homeostasis and its main underlying cause is secondary hyperparathyroidism (SHPT). We previously reported that calcium-sensing receptor (CaSR) mRNA and protein expression in parathyroid glands (PTGs) significantly decreased in a CKD rat model induced by a 5/6 nephrectomy that were fed a high phosphorus diet. However, there was a significant difference in the severity of CKD between high phosphorus and adequate phosphorus diet groups. Thus, it was unclear whether CKD environment or the high phosphorus diet influenced CaSR expression, and the underlying mechanism remains largely unknown.

Methods

CKD was induced in rats with 0.75% adenine-containing diet. CKD and control rats were maintained for 5 days and 2 weeks on diets with 0.7% or 1.3% phosphorus. For gene expression analysis, quantitative real-time polymerase chain reaction was performed with TaqMan probes. Protein expression was analyzed by immunohistochemistry.

Results

PTG CaSR expression significantly decreased in the presence of a severe CKD environment, even without the high phosphate load. Ki67 expressing cells in PTGs were significantly higher only in the CKD rats fed a high phosphorus diet. Furthermore, among the many genes that could affect CaSR expression, only vitamin D receptor (VDR) and glial cells missing 2 (Gcm2) showed significant changes. Moreover, Gcm2 was significantly reduced at an early stage without significant changes in serum calcium, phosphorus and 1,25(OH)₂ vitamin D, and there was no significant reduction in CaSR and VDR expressions. Then, significantly elevated Ki67-positive cell numbers were also only observed in the early CKD PTGs with high-phosphorus diets.

Conclusions

Our data suggest that the cause of the decreased PTG CaSR expression is the reduction in VDR and Gcm2 expression; Gcm2 may play a role in the onset and progression of SHPT.

A comparative analysis of the efficacy and safety of paricalcitol versus other vitamin D receptor activators in patients undergoing hemodialysis: A systematic review and meta-analysis of 15 randomized controlled trials

Paricalcitol, a new vitamin D receptor activator (VDRA), is reported to be more effective than other VDRAs in reducing calcium and phosphorus levels in patients undergoing hemodialysis. However, the efficacy and safety of paricalcitol remain controversial. This analysis compares paricalcitol with other VDRAs in patients undergoing hemodialysis. We searched the Cochrane Library, PubMed, EMBASE, Web of Science, and CNKI up to April 22, 2019. Standardized mean difference (SMD), risk ratio (RR) and 95% confidence interval (CI) values were estimated to compare the outcomes of the groups. Two reviewers extracted data and assessed trial quality independently. All statistical analyses were performed using the standard statistical procedures of RevMan 5.2 and Stata 12.0. Fifteen studies (N = 110,544) were included in this meta-analysis. Of these studies, 11 were randomized controlled trials (RCTs) and 4 were non-randomized studies of interventions (NRSIs). Patients receiving paricalcitol experienced better overall survival (OS) than patients receiving other VDRAs, with a pooled hazard ratio of 0.86 (95% CI 0.80–0.91; P < 0.00001). Intact parathyroid hormone (iPTH) levels were significantly reduced in the paricalcitol group compared to the group receiving other VDRAs, with a pooled SMD of -0.53 (95% CI -0.89– -0.16; P = 0.004). There was a significant increase in serum calcium levels from baseline in the paricalcitol group compared to the other VDRAs group when limiting the analysis to RCTs, with a pooled SMD of 2.14 (95% CI 0.90–3.38; P = 0.0007). Changes in serum calcium levels were significantly lower in the paricalcitol group when the analysis was limited to NRSIs, with a pooled SMD of -0.85 (95% CI -1.34–-0.35; P = 0.0008). The NSRI analysis also showed a significant reduction in serum phosphorus levels in the paricalcitol group, with a pooled SMD of -0.57 (95% CI -1.00–-0.13; P = 0.01). No significant differences were observed in the incidence of hypercalcemia, hyperphosphatemia, or adverse events. Generally, paricalcitol seems superior to other VDRAs in reducing mortality and iPTH levels in patients undergoing hemodialysis. However, the comparative effectiveness of paricalcitol in reducing serum calcium and phosphorus levels needs further exploration. No significant difference was found in the rate of adverse events.

Evaluating extended-release calcifediol as a treatment option for chronic kidney disease-mineral and bone disorder (CKD-MBD)

Introduction: Extended-release calcifediol (ERC) is an orally administered prohormone of active vitamin D (1,25-dihydroxyvitamin D [1,25D]) designed to safely and sufficiently increase serum total 25-hydroxyvitamin D (25D) to reduce elevated parathyroid hormone (PTH) in patients with non-dialysis-chronic kidney disease (ND-CKD). ERC is currently approved in the United States and Canada.Areas covered: Herein, key clinical data relating to the pharmacokinetics, pharmacodynamics, efficacy and safety of ERC are reviewed.Expert opinion: Currently available treatment options for secondary hyperparathyroidism (SHPT) in ND-CKD have limitations: the effectiveness of nutritional vitamin D supplements for reduction of PTH levels is unproven and active (1α-hydroxylated) vitamin D analogues elevate serum calcium, which increases the risk of hypercalcemia and vascular calcification. Clinical studies show that ERC is an effective, well tolerated treatment for SHPT in ND-CKD. ERC gradually raises serum 25D levels, resulting in physiologically regulated increases in serum 1,25D and sustained reductions in PTH, while avoiding clinically meaningful increases in serum phosphorus, calcium and fibroblast growth factor 23. ERC offers a new, effective and well tolerated treatment option for the early management of SHPT in patients with ND-CKD.

Phosphate acts directly on the calcium-sensing receptor to stimulate parathyroid hormone secretion

Extracellular phosphate regulates its own renal excretion by eliciting concentration-dependent secretion of parathyroid hormone (PTH). However, the phosphate-sensing mechanism remains unknown and requires elucidation for understanding the aetiology of secondary hyperparathyroidism in chronic kidney disease (CKD). The calcium-sensing receptor (CaSR) is the main controller of PTH secretion and here we show that raising phosphate concentration within the pathophysiologic range for CKD significantly inhibits CaSR activity via non-competitive antagonism. Mutation of residue R62 in anion binding site-1 abolishes phosphate-induced inhibition of CaSR. Further, pathophysiologic phosphate concentrations elicit rapid and reversible increases in PTH secretion from freshly-isolated human parathyroid cells consistent with a receptor-mediated action. The same effect is seen in wild-type murine parathyroid glands, but not in CaSR knockout glands. By sensing moderate changes in extracellular phosphate concentration, the CaSR represents a phosphate sensor in the parathyroid gland, explaining the stimulatory effect of phosphate on PTH secretion.

Elevated inorganic phosphate levels promote excessive parathyroid hormone secretion, which contributes to the aetiology of secondary hyperparathyroidism. Here, the authors show that phosphate directly inhibits the calcium-sensing receptor, the main regulator of parathyroid hormone secretion.

Controversies in the Management of Secondary Hyperparathyroidism in Chronic Kidney Disease

Secondary hyperparathyroidism is a frequent complication of chronic kidney disease that begins early in the course of renal insufficiency as an adaptive response to maintain mineral homeostasis. This complex disorder affects the bone, leading to an increase in fracture risk and is associated with increased risks of vascular calcification and mortality. PURPOSE OF REVIEW: In this review, we examine the different strategies available to manage secondary hyperparathyroidism. Particularly, we focus on the adequate control of serum phosphorus by restricting intake and the use of phosphate binders, correction of hypocalcemia while minimizing calcium burden, and reduction in PTH levels through the use of vitamin D sterols and calcimimetics. RECENT FINDINGS: It was observed that although numerous agents directed at the correction of these abnormalities have demonstrated effectiveness on biochemical markers, there is still a relative scarcity of studies demonstrating treatment effectiveness as measured by hard clinical outcomes. In addition, most agents have side effects that may limit their use, even in patients in which the treatment has demonstrated efficacy in controlling these parameters. There is still controversy as to what therapeutic regimens to choose for a particular patient and what parameter should be used to follow their effects, including outcomes, side effects, pill burden, and costs, among others. In the present article, we analyze controversial aspects of the different therapeutic agents available. Although many tools and regimens are available, no one by itself is enough for an adequate management of the patient. But rather, combined therapy and individualization of approaches are recommended for better results. We suggest that new studies analyzing the effectiveness of therapeutic approaches to the management of secondary hyperparathyroidism should be directed not only to controlling parathyroid hormone levels but also to the evaluation of long-term outcomes, based on modification of morbidity, mortality, and end organ impact, while reducing side effects and controlling costs, among others.

Impact of fluorescence and autofluorescence on surgical strategy in benign and malignant neck endocrine diseases

Fluorescence and autofluorescence have been shown by several recent studies to be valuable adjuncts in identifying parathyroid glands during thyroidectomy and parathyroidectomy. The aim of this chapter is to review the impact of this new technology on surgical strategy concerning identification and preservation of parathyroid glands during thyroidectomy, identification of parathyroid glands in hyperparathyroidism, and the potential role in thyroid cancer surgery.

The effectiveness of cinacalcet: a randomized, open label study in chronic hemodialysis patients with severe secondary hyperparathyroidism

Background: Secondary hyperparathyroidism (SHPT) is associated with high incidences of cardiovascular disease, bone fracture, and mortality. This study was conducted to demonstrate the effectiveness of cinacalcet treatment on chronic kidney disease-mineral bone disorder (CKD-MBD) markers in chronic hemodialysis patients with severe SHPT.

Methods: In phase 1, 30 adult HD patients were randomized to cinacalcet or control groups for 12 weeks to explore the achievement of >30% reduction of iPTH. In phase 2, 45 patients were participated to further explore the effect of cinacalcet on CKD-MBD parameters for 24-week follow up and 12 additional weeks after cinacalcet discontinuation.

Results: In phase 1, the baseline serum iPTH levels were not different [1374 (955, 1639) pg/mL in the control group vs. 1191 (1005, 1884) pg/mL in the cinacalcet group], the percentage of patients achieving iPTH target were significantly higher in the treatment group [80% vs. 13%, p = .001]. In phase 2, the significant reductions of iPTH, FGF-23, tartrate-resistant acid phosphatase 5b, and slightly decreased size of parathyroid gland and stabilized vascular calcification were observed at 24-week follow up and markedly rebounded after discontinuation of cinacalcet.

Conclusions: The effectiveness of cinacalcet were still obviously demonstrated even in chronic HD patients with severe SHPT. In addition, the improvements of bone markers and FGF-23, and stabilization of vascular calcification were observed. Therefore, cinacalcet can provide salutary effects on CKD-MBD in severe SHPT and might be an initially effective PTH-lowering therapy prior to surgical parathyroidectomy as well as an alternative treatment in the patients unsuitable for surgery.

Clinical trial registration: ClinicalTrials.gov: NCT02056730. Date of registration: February 4, 2014.

Management of Post-transplant Hyperparathyroidism and Bone Disease

Significant advances in immunosuppressive therapies have been made in renal transplantation, leading to increased allograft and patient survival. Despite improvement in overall patient survival, patients continue to require management of persistent post-transplant hyperparathyroidism. Medications that treat persistent hyperparathyroidism include vitamin D, vitamin D analogues, and calcimimetics. Medication side effects such as hypocalcemia or hypercalcemia, and adynamic bone disease, may lead to a decrease in the drugs. When medical management fails to control persistent post-transplant hyperparathyroidism, treatment is a parathyroidectomy. Surgical techniques are not uniform between centers and surgeons. Undergoing the surgery may include a subtotal technique or a technique including total parathyroid gland resection with partial heterotopic gland reimplantation. In addition, there are possible post-surgical complications. The ideal treatment for persistent post-transplant hyperparathyroidism is the treatment and prevention of the condition while patients are being managed for their late-stage chronic kidney disease and end-stage renal disease.

Efficacy and safety of paricalcitol in patients undergoing hemodialysis: a meta-analysis

Background

The elevated calcium and phosphorus levels in patients undergoing hemodialysis may increase the risk of all-cause mortality. Paricalcitol, as a new vitamin D receptor activator (VDRA), seemed to be effective in reducing the calcium and phosphorus levels.

Objectives

The aim of this study was to compare the efficacy and safety of paricalcitol with other VDRAs in patients undergoing hemodialysis.

Methods

PubMed, Embase, and Web of Science database were systematically reviewed.

Selection criteria

Studies that focused on the use of paricalcitol for hemodialysis patients were eligible for inclusion.

Data collection and analysis

Two independent investigators performed the literature search, data extraction, and assessment of methodological quality. The outcomes were expressed with standard mean difference (SMD), HR, or risk ratio (RR) with 95% CI.

Results

Thirteen studies involving 112,695 patients were included in this meta-analysis. Among these studies, four studies were cohort studies and nine studies were randomized controlled trials (RCTs). For cohort studies, they were regarded as being of high quality; for RCTs, only one was classified as being at low risk of bias; and the remaining eight studies were at being unclear risk of bias. Compared with other VDRAs, paricalcitol significantly improved the overall survival (HR =0.86, 95% CI: 0.80, 0.92; P<0.001) and reduced the intact parathyroid hormone (iPTH) (SMD =-0.53, 95% CI: -0.90, -0.17; P=0.004). Paricalcitol offered similar effect with other VDRAs in the control of calcium (SMD =0.32, 95% CI: -0.04, 0.67; P=0.078) and phosphorus (SMD =0.06, 95% CI: -0.26, 0.37; P=0.727) levels. However, the serum change in calcium phosphate product was greater in the paricalcitol group than in the other VDRA group (SMD =2.13, 95% CI: 0.19, 4.07; P=0.031). There was no significant difference in the incidence of adverse events between the two groups (RR =1.02, 95% CI: 0.93, 1.12; P=0.674).

Conclusion

Paricalcitol was crucial in reducing the mortality in patients undergoing hemodialysis. Moreover, both paricalcitol and other VDRAs were effective in control of the serum iPTH, calcium, and phosphorus levels. Given the potential limitations in this study, more prospective large-scale, well-conducted RCTs are needed to confirm these findings.

The Emerging Role of Nutritional Vitamin D in Secondary Hyperparathyroidism in CKD

In chronic kidney disease (CKD), hyperphosphatemia induces fibroblast growth factor-23 (FGF-23) expression that disturbs renal 1,25-dihydroxy vitamin D (1,25D) synthesis; thereby increasing parathyroid hormone (PTH) production. FGF-23 acts on the parathyroid gland (PTG) to increase 1α-hydroxylase activity and results in increase intra-gland 1,25D production that attenuates PTH secretion efficiently if sufficient 25D are available. Interesting, calcimimetics can further increase PTG 1α-hydroxylase activity that emphasizes the demand for nutritional vitamin D (NVD) under high PTH status. In addition, the changes in hydroxylase enzyme activity highlight the greater parathyroid 25-hydroxyvitmain D (25D) requirement in secondary hyperparathyroidism (SHPT); the higher proportion of oxyphil cells as hyperplastic parathyroid progression; lower cytosolic vitamin D binding protein (DBP) content in the oxyphil cell; and calcitriol promote vitamin D degradation are all possible reasons supports nutritional vitamin D (NVD; e.g., Cholecalciferol) supplement is crucial in SHPT. Clinically, NVD can effectively restore serum 25D concentration and prevent the further increase in PTH level. Therefore, NVD might have the benefit of alleviating the development of SHPT in early CKD and further lowering PTH in moderate to severe SHPT in dialysis patients.

The calcium-sensing receptor in physiology and in calcitropic and noncalcitropic diseases

The Ca2+-sensing receptor (CaSR) is a dimeric family C G protein-coupled receptor that is expressed in calcitropic tissues such as the parathyroid glands and the kidneys and signals via G proteins and β-arrestin. The CaSR has a pivotal role in bone and mineral metabolism, as it regulates parathyroid hormone secretion, urinary Ca2+ excretion, skeletal development and lactation. The importance of the CaSR for these calcitropic processes is highlighted by loss-of-function and gain-of-function CaSR mutations that cause familial hypocalciuric hypercalcaemia and autosomal dominant hypocalcaemia, respectively, and also by the fact that alterations in parathyroid CaSR expression contribute to the pathogenesis of primary and secondary hyperparathyroidism. Moreover, the CaSR is an established therapeutic target for hyperparathyroid disorders. The CaSR is also expressed in organs not involved in Ca2+ homeostasis: it has noncalcitropic roles in lung and neuronal development, vascular tone, gastrointestinal nutrient sensing, wound healing and secretion of insulin and enteroendocrine hormones. Furthermore, the abnormal expression or function of the CaSR is implicated in cardiovascular and neurological diseases, as well as in asthma, and the CaSR is reported to protect against colorectal cancer and neuroblastoma but increase the malignant potential of prostate and breast cancers.

Mineral and Bone Disease in Kidney Transplant Recipients

PURPOSE OF REVIEW

Despite metabolic improvements following kidney transplantation, transplant recipients still often suffer from complex mineral and bone disease after transplantation.

RECENT FINDINGS

The pathophysiology of post-transplant disease is unique, secondary to underlying pre-transplant mineral and bone disease, immunosuppression, and changing kidney function. Changes in modern immunosuppression regimens continue to alter the clinical picture. Modern management includes reducing cumulative steroid exposure and correcting the biochemical abnormalities in mineral metabolism. While bone mineral density screening appears to help predict fracture risk and anti-osteoporotic therapy appears to have a positive effect on bone mineral density, more data regarding specific treatment is necessary. Patients with mineral and bone disease after kidney transplantation require special care in order to properly manage and mitigate their mineral and bone disease. Recent changes in clinical management of transplant patients may also be changing the implications on patients' mineral and bone disease.

Craniofacial Brown Tumor in Patients with Secondary Hyperparathyroidism to Chronic Renal Failure: Report of Two Cases in Cipto Mangunkusumo Hospital

Brown tumor is a bone lesion that arises in the setting of excess osteoclast activity in hyperparathyroidism. It consists of fibrous tissue, woven bone, and supporting vasculature, while contains no matrix. The characteristic of brown-colored lesion is a result of hemosiderin deposition into the osteolytic cysts. Two cases of young women aged 26 and 29 years old, respectively, are known with a history of end-stage renal disease (ESRD). Dialysis is performed two times/week over the last 7 years. Our patients presented with an intraoral mass of the hard palate since 12 months ago and decreased body height of 10 cm. The lesion causes difficulties in swallowing and talking. Laboratory workup showed elevated parathormone or PTH (3.391 pg/mL and >5.000 pg/mL). Neck ultrasound showed enlargement of the parathyroid glands. Supporting examination to diagnose brown tumor are neck ultrasound, CT of the neck, and parathyroid sestamibi scan. We performed parathyroidectomy. Pathology revealed hyperplasia of the parathyroid. The tumor regressed significantly within 2 weeks following the surgery, and we still observe tumor regression as well as reduction in PTH level. As clinicians, we should be alert to other possible causes of bony lesions. Clinical examination, laboratory finding, and imaging present important information to diagnose brown tumor.

Severe secondary hyperparathyroidism in patients on haemodialysis is associated with a high initial serum parathyroid hormone and beta-CrossLaps level: Results from an incident cohort

BACKGROUND

Secondary hyperparathyroidism (SHPT) is a frequent complication of renal disease and most commonly occurs in patients on haemodialysis (HD) with metabolic, vascular, endocrine, and bone complications. The aim of this study was to analyze the evolution of mineral metabolism parameters during the first 36 months of HD treatment and identify the initial factors associated with severe SHPT.

METHODS

Serum parathyroid hormone (PTH), calcium and phosphate levels were measured monthly; bone-specific alkaline phosphatase (b-ALP) and beta-CrossLaps (CTX) were measured biannually. Severe SHPT was defined as the need for cinacalcet treatment. Patients with less than 24 months of follow-up were excluded.

RESULTS

One hundred thirty-three incident HD patients were included. Baseline mean PTH was 275 ± 210 pg/mL. After an initial drop at the third month (172 ± 133 pg/mL), the serum PTH level progressively increased to the maximum at 36 months (367 ± 254 pg/mL). This initial drop was associated with the initial correction of both hypocalcaemia and hyperphosphataemia. Serum CTX and b-ALP revealed no significant changes over time. Severe SHPT was observed in 18% of patients and was associated with higher mean calcaemia and phosphataemia. In logistic regression, the initial factors associated with the risk of severe SHPT were: female sex, higher baseline PTH and CTX values. A receiver operation characteristic curve analysis identified a cut-off value of >374 pg/mL for baseline PTH and >1.2 μg/L for CTX for increased risk of developing severe SHPT. The relative risk of developing severe SHPT was 3.7 (1.8-7.5, p = 0.002) for high baseline CTX, 4.9 (2.4-9.7, p = 0.001) for high baseline PTH, and 7.7 (3.6-16, p< 0.0001) when both criteria were present.

CONCLUSION

After an initial drop, a progressive increase in the serum PTH level during the first 3 years of HD treatment was observed despite aggressive therapy. High baseline levels of PTH and CTX increased the risk of developing severe SHPT.

Long-Term Use of Cinacalcet in Kidney Transplant Recipients With Hypercalcemic Secondary Hyperparathyroidism: A Single-Center Prospective Study

OBJECTIVES

Persistent secondary hyperparathyroidism is common after successful kidney transplant, with concomitant hypercalcemia and hypophosphatemia potentially leading to reduced graft survival and increased cardiovascular risk. Cinacalcet, a calcimimetic agent that activates the calcium-sensing receptors in parathyroid glands, is a therapeutic option. In this study, we assessed the long-term treatment effects of cinacalcet for a period of up to 5 years in a cohort of kidney transplant recipients.

MATERIALS AND METHODS

Forty-seven patients with secondary hyperparathyroidism (intact parathyroid hormone level > 70 pg/mL or 7.43 pmol/L) and hypercalcemia (corrected calcium > 10.4 mg/dL or 2.6 mmol/L) were considered eligible for treatment with cinacalcet and were included in the analysis. Data were recorded at initiation of treatment and every 6 months up to a maximum follow-up of 60 months. A control group of patients treated with placebo, conventional treatment, or surgical treatment was not available for this study.

RESULTS

Mean follow-up time was 45 ± 16 months. Treatment with cinacalcet was initiated at a median of 25 months after renal transplant. Serum calcium decreased by 0.21 mmol/L (2.69 vs 2.48 mmol/L; 95% confidence interval, 0.08-0.345; P < .001) during the first 6 months, and this reduction was sustained during follow-up. Intact parathyroid hormone level decreased by 7.68 pmol/L (32.96 ± 36.4 vs 25.28 ± 19.5 pmol/L; 95% confidence interval, -6.42 to 21.75; P = not significant) at 6 months, whereas at the end of follow-up intact parathyroid hormone level decreased further by 20.07 pmol/L (32.96 ± 36.4 vs 12.89 ± 5.73 pmol/L; 95% confidence interval, 2.02-38.1; P < .01). Mean starting dose of cinacalcet was 33.5 ± 10 mg/day. According to the therapeutic response, cinacalcet dose increased steadily and reached 51.1 ± 33 mg/day at the end of the observation period. Mean serum phosphorus increased significantly, whereas estimated glomerular filtration rate remained virtually stable throughout follow-up. Adverse reactions were observed in 4 patients, comprising mild gastro-intestinal complaints.

CONCLUSIONS

Long-term treatment with cinacalcet in kidney transplant recipients with secondary hyperparathyroidism is effective in controlling hypercalcemia and correcting hypophosphatemia, without affecting graft function while being well-tolerated.

Parathyroid Nodular Hyperplasia and Responsiveness to Drug Therapy in Renal Secondary Hyperparathyroidism: An Open Question

The goal of the pharmacological therapy in secondary hyperparathyroidism (SHPT) is to reduce serum levels of parathyroid hormone and phosphorus, to correct those of calcium and vitamin D, to arrest or reverse the parathyroid hyperplasia. However, when nodular hyperplasia or an autonomous adenoma develops, surgery may be indicated. We reviewed the literature with the aim of defining if the echographic criteria predictive of unresponsiveness of SHPT to calcitriol therapy are valid also in the cinacalcet era and if drug therapy may reverse nodular hyperplasia of parathyroid gland (PTG). The responsiveness to therapy and regression of the nodular hyperplasia of PTG remains an open question in the calcimimetic era as well as the cutoff between medical and surgical therapy. Prospective studies are needed in order to clarify if an earlier use of cinacalcet in moderate SHPT might arrest the progression of parathyroid growth and stabilize SHPT.

[Management of bone and mineral metabolism disorders before the dialysis stage remains still perfectible. Data from the French Phosphorus and Calcium Survey Photo-Graphe]

Only limited data is available on the management of the chronic kidney disease-associated bone and mineral metabolism disorder (CKD-MBD) in the pre-dialysis stages of CKD in France. A better knowledge of current management habits could lead to an improvement in the implementation of international recommendations (KDIGO). The 3rd version of the French Phosphorus and Calcium Survey Photo-Graphe (Sanofi) included a cohort of CKD stages 4 and 5 patients, whose aim was to examine the prevalence of CKD-MBD and the quality of its management in patients under the care of 62 nephrologists from over 20 geographical regions in France. The study started in October 2011, i.e. one year after patient enrollment. We examined in particular the percentage of patients presenting with laboratory parameter abnormalities indicative of CKD-MBD who were not receiving adequate treatment. A total of 456 patients with CKD stage 4 and 154 with CKD stage 5 were studied. Their mean age was 72.9±14.2 years, and male/female ratio was 58/42. KDIGO targets of serum PTH for CKD stages 4 and 5 were not achieved in respectively 80 and 84% of the patients, for serum calcium in 8 and 22% and for serum phosphate in 12 and 46%. As a potential explanation, insufficient therapy was estimated to account for respectively 45 and 60% of insufficiently controlled secondary hyperparathyroidism, and for 36% of persistent hyperphosphatemia in stage 5. It should be noted that 55.5 and 57.5% of patients were receiving native vitamin D. In this national observatory, the management of CKD-MBD stages 4 and 5 appears suboptimal, especially as regards the control of secondary hyperparathyroidism, which remained untreated in nearly 50% of the patients. Hyperphosphatemia was also common and inadequately controlled in CKD stage 5. To improve the management of CKD-MBD, nephrologists need to be more aware of the importance of aiming for recommended laboratory targets and how this can be achieved.

The endoplasmic reticulum-associated protein, OS-9, behaves as a lectin in targeting the immature calcium-sensing receptor

The mechanisms responsible for the processing and quality control of the calcium-sensing receptor (CaSR) in the endoplasmic reticulum (ER) are largely unknown. In a yeast two-hybrid screen of the CaSR C-terminal tail (residues 865-1078), we identified osteosarcoma-9 (OS-9) protein as a binding partner. OS-9 is an ER-resident lectin that targets misfolded glycoproteins to the ER-associated degradation (ERAD) pathway through recognition of specific N-glycans by its mannose-6-phosphate receptor homology (MRH) domain. We show by confocal microscopy that the CaSR and OS-9 co-localize in the ER in COS-1 cells. In immunoprecipitation studies with co-expressed OS-9 and CaSR, OS-9 specifically bound the immature form of wild-type CaSR in the ER. OS-9 also bound the immature forms of a CaSR C-terminal deletion mutant and a C677A mutant that remains trapped in the ER, although binding to neither mutant was favored over wild-type receptor. OS-9 binding to immature CaSR required the MRH domain of OS-9 indicating that OS-9 acts as a lectin most likely to target misfolded CaSR to ERAD. Our results also identify two distinct binding interactions between OS-9 and the CaSR, one involving both C-terminal domains of the two proteins and the other involving both N-terminal domains. This suggests the possibility of more than one functional interaction between OS-9 and the CaSR. When we investigated the functional consequences of altered OS-9 expression, neither knockdown nor overexpression of OS-9 was found to have a significant effect on CaSR cell surface expression or CaSR-mediated ERK1/2 phosphorylation.

Low serum vitamin D is associated with higher cortical porosity in elderly men

BACKGROUND

Bone loss at peripheral sites in the elderly is mainly cortical and involves increased cortical porosity. However, an association between bone loss at these sites and 25-hydroxyvitamin D has not been reported.

OBJECTIVE

To investigate the association between serum levels of 25-hydroxyvitamin D, bone microstructure and areal bone mineral density (BMD) in elderly men.

METHODS

A population-based cohort of 444 elderly men (mean ± SD age 80.2 ± 3.5 years) was investigated. Bone microstructure was measured by high-resolution peripheral quantitative computed tomography, areal BMD by dual-energy X-ray absorptiometry and serum 25-hydroxyvitamin D and parathyroid hormone levels by immunoassay.

RESULTS

Mean cortical porosity at the distal tibia was 14.7% higher (12.5 ± 4.3% vs. 10.9 ± 4.1%, P < 0.05) whilst cortical volumetric BMD, area, trabecular bone volume fraction and femoral neck areal BMD were lower in men in the lowest quartile of vitamin D levels compared to the highest. In men with vitamin D deficiency (<25 nmol L^-1 ) or insufficiency [25-49 nmol L^-1 , in combination with an elevated serum level of parathyroid hormone (>6.8 pmol L^-1 )], cortical porosity was 17.2% higher than in vitamin D-sufficient men (P < 0.01). A linear regression model including age, weight, height, daily calcium intake, physical activity, smoking vitamin D supplementation and parathyroid hormone showed that 25-hydroxyvitamin D independently predicted cortical porosity (standardized β = -0.110, R² = 1.1%, P = 0.024), area (β = 0.123, R² = 1.4%, P = 0.007) and cortical volumetric BMD (β = 0.125, R² = 1.4%, P = 0.007) of the tibia as well as areal BMD of the femoral neck (β = 0.102, R² = 0.9%, P = 0.04).

CONCLUSION

Serum vitamin D is associated with cortical porosity, area and density, indicating that bone fragility as a result of low vitamin D could be due to changes in cortical bone microstructure and geometry.

Nonclinical Pharmacokinetics, Disposition, and Drug-Drug Interaction Potential of a Novel d-Amino Acid Peptide Agonist of the Calcium-Sensing Receptor AMG 416 (Etelcalcetide)

AMG 416 (etelcalcetide) is a novel synthetic peptide agonist of the calcium-sensing receptor composed of a linear chain of seven d-amino acids (referred to as the d-amino acid backbone) with a d-cysteine linked to an l-cysteine via a disulfide bond. AMG 416 contains four basic d-arginine residues and is a +4 charged peptide at physiologic pH with a mol. wt. of 1048.3 Da. The pharmacokinetics (PK), disposition, and potential of AMG 416 to cause drug-drug interaction were investigated in nonclinical studies with two single (14)C-labels placed either at a potentially metabolically labile acetyl position or on the d-alanine next to d-cysteine in the interior of the d-amino acid backbone. After i.v. dosing, the PK and disposition of AMG 416 were similar in male and female rats. Radioactivity rapidly distributed to most tissues in rats with intact kidneys, and renal elimination was the predominant clearance pathway. No strain-dependent differences were observed. In bilaterally nephrectomized rats, minimal radioactivity (1.2%) was excreted via nonrenal pathways. Biotransformation occurred primarily via disulfide exchange with endogenous thiol-containing molecules in whole blood rather than metabolism by enzymes, such as proteases or cytochrome P450s; the d-amino acid backbone remained unaltered. A substantial proportion of the plasma radioactivity was covalently conjugated to albumin. AMG 416 presents a low risk for P450 or transporter-mediated drug-drug interactions because it showed no interactions in vitro. These studies demonstrated a (14)C label on either the acetyl or the d-alanine in the d-amino acid backbone would be appropriate for clinical studies.

Separation and quantitative determination of cinacalcet metabolites in urine sample using RP-HPLC after derivation with a fluorescent labeling reagent

In this investigation, a novel strategy for separation and quantitative determination of four metabolites of cinacalcet (M2a-Glu, M2b-Glu, M7-Gly, and M8-Gly) in human urine is suggested. The analytical assay is based on a pre-column derivation procedure of cinacalcet metabolites with 1-pyrenyldiazomethane (PDAM) as a fluorescent labeling reagent, and subsequently separation and quantitative determination with reverse-phase high-performance liquid chromatography (RP-HPLC) coupled with a fluorescence detector. Metabolites were separated on a Microsorb-MV 100-5 C18 chromatography column (250×4.6mm, 5μm) using acetate buffer (pH 3.5):methanol (30:70 v/v) as mobile phase at a flow rate of 1.0mLmin(-1). The method was fully validated in terms of linearity (r(2)>0.996; 1-10ngmL(-1)), precision (both intra-day and inter-day; RSD<6.2%), accuracy (92-110%), specificity, robustness (0.15%<RSD<4.1%), limits of detection (5×10(-4) to 3×10(-3)ngmL(-1)) and quantification (2×10(-3) to 1×10(-2)ngmL(-1)). According to the results, the proposed method can be useful in the routine analysis for the determination of cinacalcet metabolites in urine samples.

Parathyroid Ultrasonography in Renal Secondary Hyperparathyroidism: An Overlooked and Useful Procedure

In secondary hyperparathyroidism (SHPT), ultrasonography (US) can accurately define the size and structure of parathyroid glands as well as differentiate diffuse and nodular hyperplasia. US may be also useful to predict the response of SHPT to vitamin D analogs and cinacalcet and to assess for regression of parathyroid glands hyperplasia by measurement of parathyroid gland volume. There is increasing evidence that US can potentially identify patients who will benefit from prompt surgical intervention. Therefore, US should be part of the diagnostic armamentarium in the treatment of SHPT in the daily clinical practice.

High Dose ESAs Are Associated with High iPTH Levels in Hemodialysis Patients with End-Stage Kidney Disease: A Retrospective Analysis

Objective

Anemia and secondary hyperparathyroidism are the two most common complications associated with chronic kidney disease. Erythropoiesis-stimulating agents (ESAs) are widely used in the management of anemia in hemodialysis patients. A reverse correlation has been established between hyperparathyroidism and hemoglobin levels. The aim of this retrospective study is to evaluate the relationship of high-dose ESAs and hyperparathyroidism in hemodialysis patients with anemia.

Methods

A total of 240 uremic patients maintained on regular hemodialysis were enrolled in this study. Among them, 142 patients were treated with Epiao^® (epoetin-alfa) and 98 patients were treated with Recormon^® (epoetin-beta). The target hemoglobin concentration was 110–130 g/L. Laboratory measurements including hemoglobin, calcium, phosphorus, albumin, intact-parathyroid hormone (iPTH), serum ferritin, and transferrin saturation were collected.

Results

Hemoglobin concentration increased as iPTH level decreased by stratification. However, no significant association between anemia and calcium or phosphorus level was found. Patients with iPTH levels within 150–300 pg/mL had the highest levels of hemoglobin, serum ferritin, and transferrin saturation. Patients treated with Recormon and Epiao had similar hemoglobin concentrations. However, the dose of Recormon for anemia treatment was significantly less than that the dose of Epiao (P < 0.05). The level of iPTH in the Recormon group was significantly lower than in the Epiao group. In patients with hemoglobin levels between 110 and 130 g/L (P < 0.05), iPTH level was found to be significantly lower in patients treated with lower doses of ESAs than in patients treated with higher doses of ESAs, no matter which ESA was used (Recormon or Epiao, P < 0.05).

Conclusion

The dose of ESAs might be positively associated with iPTH level, suggesting that a reasonable hemoglobin target can be achieved by using the lowest possible ESA dose.

Critical Cysteine Residues in Both the Calcium-Sensing Receptor and the Allosteric Activator AMG 416 Underlie the Mechanism of Action

AMG 416 is a novel D-amino acid-containing peptide agonist of the calcium-sensing receptor (CaSR) that is being evaluated for the treatment of secondary hyperparathyroidism in chronic kidney disease patients receiving hemodialysis. The principal amino acid residues and their location in the CaSR that accommodate AMG 416 binding and mode of action have not previously been reported. Herein we establish the importance of a pair of cysteine residues, one from AMG 416 and the other from the CaSR at position 482 (Cys482), and correlate the degree of disulfide bond formation between these residues with the pharmacological activity of AMG 416. KP-2067, a form of the CaSR agonist peptide, was included to establish the role of cysteine in vivo and in disulfide exchange. Studies conducted with AMG 416 in pigs showed a complete lack of pharmacodynamic effect and provided a foundation for determining the peptide agonist interaction site within the human CaSR. Inactivity of AMG 416 on the pig CaSR resulted from a naturally occurring mutation encoding tyrosine for cysteine (Cys) at position 482 in the pig CaSR. Replacing Cys482 in the human CaSR with serine or tyrosine ablated AMG 416 activity. Decidedly, a single substitution of cysteine for tyrosine at position 482 in the native pig CaSR provided a complete gain of activity by the peptide agonist. Direct evidence for this disulfide bond formation between the peptide and receptor was demonstrated using a mass spectrometry assay. The extent of disulfide bond formation was found to correlate with the extent of receptor activation. Notwithstanding the covalent basis of this disulfide bond, the observed in vivo pharmacology of AMG 416 showed readily reversible pharmacodynamics.

Establishing the clinical utility of autofluorescence spectroscopy for parathyroid detection

BACKGROUND

The inability of surgeons to identify parathyroid glands accurately during cervical endocrine surgery hinders patients from achieving postoperative normocalcemia. An intrinsic, near-infrared fluorescence method was developed for real-time parathyroid identification with high accuracy. This study assesses the clinical utility of this approach.

METHODS

Autofluorescence measurements were obtained from 137 patients (264 parathyroid glands) undergoing parathyroidectomy and/or thyroidectomy. Measurements were correlated to disease state, calcium levels, parathyroid hormone, vitamin D levels, age, sex, ethnicity, and body mass index. Statistical analysis identified which factors affect parathyroid detection.

RESULTS

High parathyroid fluorescence was detected consistently and showed wide variability across patients. Near-infrared fluorescence was used to identify 256 of 264 (97%) of glands correctly. The technique showed high accuracy over a wide variety of disease states, although patients with secondary hyperparathyroidism demonstrated confounding results. Analysis revealed body mass index (P < .01), disease state (P < .01), vitamin D (P < .05), and calcium levels (P < .05) account greatly for variability in signal intensity. Age, sex, parathyroid hormone, and ethnicity had no effect.

CONCLUSION

This intrinsic fluorescence-based intraoperative technique can detect nearly all parathyroid glands accurately in real time. Its discrimination capacity is largely unlimited by patient variables, but several factors affect signal intensity. These results demonstrate potential clinical utility of optical guidance for parathyroid detection.

The Pathophysiology of Secondary Hyperparathyroidism and the Consequences of Uncontrolled Mineral Metabolism in Chronic Kidney Disease: The Role of COSMOS

The development of secondary hyperparathyroidism (SHPT) is a common complication of chronic kidney disease. SHPT develops as a consequence of mineral metabolism disturbances and is characterized by elevated serum parathyroid hormone (PTH) and parathyroid hyperplasia. Evidence suggests that SHPT contributes to the development of vascular calcification and cardiovascular disease, as well as to the development of renal osteodystrophy. The elevated serum calcium, phosphorus, calcium–phosphorus product and PTH that accompany SHPT have been independently associated with an increased relative risk of mortality. Despite the danger that these risks represent, achieving control of mineral metabolism in SHPT is difficult. Recent evidence from the Current Management of Secondary Hyperparathyroidism: Multicentre Observational Study has shown that fewer than 1 in 10 haemodialysis patients simultaneously meet their National Kidney Foundation Kidney Disease Outcomes Quality Initiative targets for serum calcium, phosphorus, calcium–phosphorus product and PTH with standard treatments. There is therefore an urgent need for new strategies and novel pharmacologic therapies that improve control of mineral metabolism and PTH secretion in SHPT and thus reduce the mortality associated with this condition.

Clinical outcomes in secondary hyperparathyroidism and the potential role of calcimimetics

Cinacalcet, a type II calcimimetic agent that interacts with the calcium-sensing receptor on the parathyroid gland and increases its sensitivity to calcium, has proved an effective therapy for the treatment of the biochemical derangements that comprise uraemic secondary hyperparathyroidism. These patients experience high cardiovascular attrition with evidence that this is associated with vascular calcification, arterial stiffening and increased pulse wave velocity, and with some of the disturbances of bone and mineral metabolism in uraemia. Thus, it is possible that improved biochemical control in calcimimetic-treated patients might lead to better clinical outcomes. This hypothesis was investigated by retrospective analyses of randomized placebo-controlled phase 3 studies. The addition of cinacalcet to standard therapy with active vitamin D and phosphate binders was found to result in a 93% reduction in the rate of parathyroidectomy, a 54% reduction in fracture rate and 39% reduction in the rate of cardiovascular hospitalization, as well as improvements in some measures of quality of life. These encouraging results point to the need for a more robust assessment of the impact of cinacalcet on cardiovascular and skeletal outcomes.

Efficacy of early treatment with calcimimetics in combination with reduced doses of vitamin d sterols in dialysis patients

Vitamin D is an important physiologic regulator of bone and mineral metabolism. In chronic kidney disease, reduced renal production of calcitriol contributes to secondary hyperparathyroidism (SHPT). Consequently, supplementation with vitamin D sterols is an important treatment for SHPT and its associated mineral and bone disorders. However, doses of vitamin D sterols required to suppress parathyroid hormone (PTH) secretion often promote hypercalcaemia and hyperphosphataemia. Therefore, there is a trade-off between reduced serum PTH and increased levels of serum calcium, phosphorus and calcium–phosphorus product. It has been suggested that treatment of SHPT with cinacalcet, a type II calcimimetic, with reduced doses of vitamin D sterols could enhance achievement of calcium and phosphorus treatment targets while maintaining goals for PTH. Recent clinical trials have evaluated this hypothesis and demonstrated that treatment with cinacalcet in combination with reduced doses of vitamin D sterols is an effective treatment for the management of SHPT.

Oral phosphorus supplementation secondarily increases circulating fibroblast growth factor 23 levels at least partially via stimulation of parathyroid hormone secretion

Oral phosphorus supplementation stimulates fibroblast growth factor 23 (FGF23) secretion; however, the underlying mechanism remains unclear. The aim of this study was to investigate the involvement of parathyroid hormone (PTH) in increased plasma FGF23 levels after oral phosphorus supplementation in rats. Rats received single dose of phosphate with concomitant subcutaneous injection of saline or human PTH (1-34) after treatment with cinacalcet or its vehicle. Cinacalcet is a drug that acts as an allosteric activator of the calcium-sensing receptor and reduces PTH secretion. Plasma phosphorus and PTH levels significantly increased 1 h after oral phosphorus administration and returned to basal levels within 3 h, while plasma FGF23 levels did not change up to 2 h post-treatment, but rather significantly increased at 3 h after administration and maintained higher levels for at least 6 h compared with the 0 time point. Plasma PTH and FGF23 levels were significantly lower in the cinacalcet-treated rats than in the vehicle-treated rats. Plasma phosphorus levels were significantly higher in the cinacalcet-treated rats than in the vehicle-treated rats at 2, 3, 4, and 6 h after oral phosphorus administration. Furthermore, rats treated with cinacalcet+human PTH (1-34) showed transiently but significantly higher plasma FGF23 levels at 3 h after oral phosphorus administration compared with cinacalcet-treated rats. These results suggest that oral phosphorus supplementation secondarily increases circulating FGF23 levels at least partially by stimulation of PTH secretion.