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

Secondary hyperparathyroidism disease stabilization following calcimimetic therapy

Standard therapy for secondary hyperparathyroidism (SHPT) includes dietary calcium supplementation, active vitamin D, and phosphate binders; however, these are often insufficient to allow patients to achieve their serum parathyroid hormone (PTH), calcium and calcium-phosphorus product (Ca × P) targets. Recent preclinical studies have demonstrated that treatment with type II calcimimetics that increase the sensitivity of the calcium-sensing receptor (CaR) to calcium can reverse the alterations in CaR and vitamin D receptor expression and parathyroid cell proliferation that are associated with SHPT. These data suggest that calcimimetic treatment could stabilize disease progression and improve maintenance of treatment goals. In clinical trials involving SHPT patients, the calcimimetic cinacalcet has been shown to decrease PTH, calcium, phosphorus and Ca × P. Significant improvements were seen regardless of initial disease severity, and benefits were maintained over the course of long-term therapy (up to 4 years), indicating effective disease stabilization. In conclusion, preclinical and clinical data provide both theoretical and empirical support for the use of calcimimetics in moderate and advanced SHPT to effectively stabilize disease.

A new paradigm for the treatment of secondary hyperparathyroidism

The global rise in chronic kidney disease makes secondary hyperparathyroidism (SHPT) a growing medical concern. Conventional therapies for treating SHPT are limited and include calcium-based and calcium-free phosphate binders for reducing serum phosphorus and vitamin D or its analogues for simultaneous stimulation of calcium absorption and suppression of parathyroid hormone (PTH) gene expression. Control of SHPT using these therapies has typically been poor. Recent studies have demonstrated that use of calcimimetics that reduce PTH secretion by increasing the sensitivity of the parathyroid gland calcium-sensing receptor to circulating calcium allow improved control of serum PTH, calcium, phosphorus and calcium-phosphorus product. This review describes experimental data and the clinical rationale supporting novel strategies for the integration of calcimimetics with conventional therapies to improve control of SHPT.

Dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism

This review examines the dynamics of parathyroid hormone secretion in health and in various causes of secondary hyperparathyroidism. Although most studies of parathyroid hormone and calcium have focused on the modification of parathyroid hormone secretion by serum calcium, the relationship between parathyroid hormone and serum calcium is bifunctional because parathyroid hormone also modifies serum calcium. In normal animals and humans, factors such as phosphorus and vitamin D modify the basal parathyroid hormone level and the maximal parathyroid hormone response to hypocalcemia. Certain medications, such as lithium and estrogen, in normal individuals and sustained changes in the serum calcium concentration in hemodialysis patients change the set point of calcium, which reflects the serum calcium concentration at which parathyroid hormone secretion responds. Hypocalcemia increases the basal/maximal parathyroid hormone ratio, a measure of the relative degree of parathyroid hormone stimulation. The phenomenon of hysteresis, defined as a different parathyroid hormone value for the same serum calcium concentration during the induction of and recovery from hypo- and hypercalcemia, is discussed because it provides important insights into factors that affect parathyroid hormone secretion. In three causes of secondary hyperparathyroidism--chronic kidney disease, vitamin D deficiency, and aging--factors that affect the dynamics of parathyroid hormone secretion are evaluated in detail. During recovery from vitamin D deficiency, the maximal parathyroid hormone remains elevated while the basal parathyroid hormone value rapidly becomes normal because of a shift in the set point of calcium. Much remains to be learned about the dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism.

Activation of calcium-sensing receptor accelerates apoptosis in hyperplastic parathyroid cells

Calcimimetic compounds inhibit not only parathyroid hormone (PTH) synthesis and secretion, but also parathyroid cell proliferation. The aim of this investigation is to examine the effect of the calcimimetic compound NPS R-568 (R-568) on parathyroid cell death in uremic rats. Hyperplastic parathyroid glands were obtained from uremic rats (subtotal nephrectomy and high-phosphorus diet), and incubated in the media only or the media which contained high concentration of R-568 (10(-4)M), or 10% cyclodextrin, for 6h. R-568 treatment significantly suppressed medium PTH concentration compared with that of the other two groups. R-568 treatment not only increased the number of terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay-positive cells, but also induced the morphologic changes of cell death determined by light or electron microscopy. These results suggest that CaR activation by R-568 accelerates parathyroid cell death, probably through an apoptotic mechanism in uremic rats in vitro.

Successful Kidney Transplantation Reduces Hyperplastic Parathyroid Gland

INTRODUCTION

In dialysis patients, the parathyroid glands (PTGs) may increase progressively, producing abnormal bone metabolism. Changes in PTG volume among patients with hyperplastic PTGs are not well known after kidney transplantation. This study investigated PTG volume by ultrasound (US).

METHODS

US of PTG was performed immediately (US-0) and 12 months after (US-12) transplantation to identify glands in all recipients. We calculated the percentage reduction in PTG volume (R%PTG). We declared it significant when it was > or =35%. Bone biochemical markers and renal function were recorded sequentially.

RESULTS

Among engrafted patients, parathyroid US-0 was performed in 47 and US-0 and US-12 in 36. Some visible gland was observed upon US-0 in 13 recipients, a group that showed higher pretransplantation parathyroid hormone (PTH) levels than the remaining 34 patients with no visible glands (627 +/- 360.0 vs 280 +/- 240.9 pg/mL; P < .05). Of 36 recipients with US-0 and US-12, the baseline study identified PTGs in 12 patients (p+ group), while the remaining 24 had no identified glands (p- group). In the p+ group, no PTG, at US-12 were visible in four patients, and a significant R%PTG was observed in three at this time, representing a reduction in gland volume after transplantation among 58.3% of p+ patients. There was a progressive reduction in PTH among both groups. Patients with glandular volume reduction displayed better renal function: serum creatinine 1.7 +/- .79 versus 2.9 +/- .74 mg/dL (P < .05).

CONCLUSIONS

Transplantation reversed hyperparathyroidism and PTG volume among recipients who achieved nearly normal renal function.

Report on the vitamin D status of adult and pediatric patients with inflammatory bowel disease and its significance for bone health and disease

Vitamin D is a hormone responsible for calcium homeostasis and essential for bone mineralization throughout the lifespan. Recent studies revealed a high prevalence of hypovitaminosis D among healthy adults and children, especially in the northern hemisphere, and a link between this condition and suboptimal bone health. Moreover, maintenance of what are today considered optimal vitamin D stores has not been achieved throughout the year with currently recommended daily intake for vitamin D. The prevalence of hypovitaminosis D is even higher among adults with inflammatory bowel disease (IBD), a situation that may be caused by malabsorption and gastrointestinal losses through an inflamed intestine, among other factors. In children with IBD, existing reports of vitamin D status are scarce. The relationship between vitamin D status and bone health, although well-established in healthy adults and children, has been controversial among adults and children with IBD, and the reasons for this have not been investigated to date. Studies in animal models of colitis and in vitro human studies support a role of vitamin D in the regulation of the immune system of the gut and the potential of vitamin D and its derivatives as therapeutic adjuncts in the treatment of IBD. This role of vitamin D has not been investigated with translational studies to date. Currently, there are no guidelines for monitoring vitamin D status, treating hypovitaminosis D, and maintaining optimal vitamin D stores in patients with IBD. These tasks may prove particularly difficult because of malabsorption and gastrointestinal losses that are associated with IBD.

Role of calcium-sensing receptor in mineral ion metabolism and inherited disorders of calcium-sensing

The extracellular calcium-sensing receptor (CaR), a G protein-coupled receptor that resides on the parathyroid cell surface negatively regulates secretion of parathyroid hormone (PTH). The CaR is functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Further studies are needed to define fully the homeostatic roles of the CaR in tissues that are involved in systemic extracellular calcium [Ca(2+)](o) homeostasis. The role of the CaR in regulating calcium metabolism has been greatly clarified by the identification and studies of genetically determined disorders that either activate or inactivate the receptor. Antibodies to the CaR that either activate or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the CaR is significantly reduced in primary and secondary hyperparathyroidism, which could contribute to the defective [Ca(2+)](o)-sensing in these conditions. Calcimimetics act as CaR agonists or allosteric activators and thereby potentiate the effects of [Ca(2+)](o) on parathyroid cell function. This kind of pharmacological manipulation of the CaR is now used for the treatment of hyperparathyroid states, whereby the calcimimetics increase the activation of the CaR at any given level of extracellular calcium. Calcimimetics are also an effective element in the treatment of secondary hyperparathyroidism, particularly in dialysis patients, by virtue of reducing plasma levels of PTH, calcium and phosphate.

Therapeutic Strategies for Secondary Hyperparathyroidism in Dialysis Patients

Secondary hyperparathyroidism (SHPT) leads not only to bone disorders, but also to cardiovascular complications in long-term dialysis patients. Conventional treatment with calcium (Ca) supplement, phosphate (P) binders and active vitamin D analogs lead in part to amelioration of SHPT, but are simultaneously associated with unacceptable side-effects, including hypercalcemia, hyperphosphatemia, and increased Ca x P products, which are the risk factors for cardiovascular disease in dialysis patients. Conventional treatment has been unable to facilitate the attainment of optimal management of SHPT proposed in the K/DOQI guidelines. Cinacalcet HCl (cinacalcet), a novel calcimimetic compound, restores the sensitivity of the Ca-sensing receptor in parathyroid cells, and decreases serum parathyroid hormone (PTH) without introducing hypercalcemia or hyperphosphatemia. Cinacalcet treatment enables a significant number of patients to achieve the K/DOQI guideline. Based on experimental data, calcimimetics could ameliorate cardiovascular calcification and remodeling in uremic rats with SHPT. Clinical trials have shown that cinacalcet significantly reduced the risks of parathyroidectomy, fracture and cardiovascular hospitalization among long-term dialysis patients with SHPT. Parathyroid intervention therapy (parathyroidectomy and percutaneous direct injection) is also a useful alternative. In the present article, we review novel therapeutic strategies for SHPT.

Cinacalcet HCl: A Novel Treatment for Secondary Hyperparathyroidism Caused by Chronic Kidney Disease

Secondary hyperparathyroidism (SHPT) develops as a result of impaired calcium homeostasis when the failing kidneys disturb the complicated interactions between parathyroid hormone (PTH), calcium, phosphorus, and vitamin D. Twelve years ago, the calcium-sensing receptor (CaR) of the parathyroid gland was first cloned and identified as the principal regulator of PTH secretion. The activation of the CaR by small changes in extracellular calcium (ec(Ca2+)) regulates PTH, calcitonin secretion, urinary calcium excretion, and ultimately, bone turnover. The CaR became an ideal target for the development of calcimimetics, which are able to amplify its sensitivity to ec(Ca2+) suppressing PTH secretion. Cinacalcet HCl, a first-in-class calcimimetic, approved in both the United States and the European Union, offers a new therapeutic approach to the treatment of SHPT. The efficacy of cinacalcet HCl in treating SHPT in dialysis patients (n = 1,136) was studied in three similarly designed phase III clinical trials comparing patients receiving standard SHPT therapy plus cinacalcet HCl or plus placebo. Cinacalcet HCl, dosed from 30 to 180 mg/day, significantly reduced PTH while simultaneously lowering calcium, phosphorus, and calcium-phosphorus product in each of the three studies. Respective to the National Kidney Foundation-Kidney Disease Outcomes and Quality Initiative (NKF-K/DOQI) recommended targets for bone and mineral metabolism, 41% of cinacalcet HCl-treated patients achieved both PTH and calcium-phosphorus product targets, compared with only 6% in the placebo group. Results from 2 recent phase IIIb studies (TARGET and CONTROL) conducted in the United States also showed that cinacalcet HCl can significantly reduce or maintain reduction in PTH while simultaneously lowering calcium, phosphorus, and calcium-phosphorus product. In addition, patients taking vitamin D at baseline of these 2 trials were able to see significant mean reductions in vitamin D dose. Further assessment of cinacalcet HCl trial data has shown some important effects in SHPT patient clinical outcomes. A combined post-hoc analysis of clinical events using data from 4 (n = 1,184) cinacalcet HCl phase II and III studies suggests that treatment with cinacalcet HCl has a beneficial effect on relative risks of parathyroidectomy, fracture, and hospitalization for cardiovascular complications. Nausea and vomiting occurred more often in patients taking cinacalcet HCl than in those taking a placebo. There were also transient episodes of hypocalcemia in 5% of cinacalcet HCl patients versus 1% of placebo patients. However, these episodes were rarely associated with symptoms. The development of calcimimetics has already changed the treatment of SHPT in renal patients. Its effectiveness on the control of PTH secretion, along with simultaneous reductions in calcium, phosphorus, and calcium-phosphorus product, give this agent an advantage over traditional therapies in all levels of severity of SHPT.

Therapeutic use of calcimimetics

It has long been recognized that the secretion of PTH by chief cells in the parathyroid gland is regulated by extracellular ionized calcium. The molecular mechanism by which extracellular Ca2+ performs this feat was deduced by the cloning of the extracellular calcium-sensing receptor (CaSR) in 1993 in the laboratories of Brown and Hebert. The CaSR is a G protein-coupled cell surface receptor that belongs to family 3 of the GPCR superfamily. The CaSR senses the extracellular ionic activity of the divalent minerals Ca2+ and Mg2+ and translates this information, via a complex array of cellular signaling pathways, to modify cell and tissue function. Genetic studies have demonstrated that the activity of this receptor determines the steady-state plasma calcium concentration in humans by regulating key elements in the calcium homeostatic system. CaSR agonists (calcimimetics) and antagonists (calcilytics) have been identified and have provided both current and potential therapies for a variety of disorders. Calcimimetics can effectively reduce PTH secretion in all forms of hyperparathyroidism. They are likely to become a major therapy for secondary hyperparathyroidism associated with renal failure and for treatment of certain patients with primary hyperparathyroidism. On the therapeutic horizon are calcilytics that can transiently increase PTH and may prove useful in the treatment of osteoporosis.

Renal calcium handling after rapamycin conversion in chronic allograft dysfunction

To study the effect of rapamycin on calcium balance, we conducted a prospective study on transplant recipients. The patients were converted to rapamycin and observed for 6 months (C). Urinary Ca and P, ALK-p, Ca, P, and intact parathyroid hormone (iPTH) were examined before and 6 months after conversion. A nonconversion group (N) was found for comparison. Sixteen patients entered the study. There were increases of ALK-p (C: 67.4 +/- 32.9 to 79.6 +/- 37.0*; N: 67.3 +/- 25.1 to 67.8 +/- 14.7 IU) (*P < 0.05), Ca(2+) (C: 8.7 +/- 0.3 to 9.5 +/- 0.2*; N: 8.8 +/- 0.4 to 8.7 +/- 0.5 mg/dl), urinary P excretion (C: 287.6 +/- 257.1 to 439.4 +/- 260.9*; N: 233.9 +/- 117.2 to 264.8 +/- 143.4 mg/day) and iPTH (C: 133.7 +/- 149.6 to 200.6 +/- 171.5*; N: 128.4 +/- 57.1 to 136.3 +/- 40.4 pg/ml). Serum P (C: 5.3 +/- 1.4 to 3.6 +/- 0.6*; N: 5.2 +/- 0.8 to 5.1 +/- 0.9 mg/dl) and urinary Ca(2+) (C: 93.9 +/- 52.6 to 31.8 +/- 45.1*; N: 84.6 +/- 38.3 to 75.9 +/- 38.4 mg/day) were decrease. Rapamycin was associated with decreased urinary Ca(2+) and increased P excretion. The alteration might come from the increased parathyroid hormone.

Cinacalcet: An oral calcimimetic agent for the management of hyperparathyroidism

BACKGROUND

Uncontrolled hyperparathyroidism (HPT), particularly HPT resulting from chronic kidney disease (CKD), is associated with significant morbidity and cardiovascular mortality. Traditional medical therapy (eg, vitamin D sterols, calcium, phosphate binders) has been inadequate for the management of HPT and its vascular and skeletal complications.

OBJECTIVE

: The goal of this article was to review the efficacy and safety profile of cinacalcet, a second-generation calcimimetic, in the management of HPT secondary to CKD, primary HPT, and parathyroid carcinoma.

METHODS

MEDLINE, Web of Science, and International Pharmaceutical Abstracts were searched from 1995 to July 2005 using the terms cinacalcet, AMG 073, KRN 1493, calcimimetics, hypercalcemia, and hyperparathyroidism.

RESULTS

Compared with placebo, cinacalcet significantly reduced parathyroid hormone levels within 2 to 4 hours after administration (P < 0.05). In Phase III trials involving 1136 patients with secondary HPT, 56% of those who received cinacalcet achieved the National Kidney Foundation Kidney Disease Outcomes Quality Initiative target of a reduction in parathyroid hormone to <300 pg/mL, 65% achieved a calcium-phosphorus product <55 mg2/dL2, and a respective 49% and 46% achieved normalized serum calcium and phosphorus levels (P < 0.001). Cinacalcet's effects were similar regardless of patients' demographic characteristics, duration or mode of dialysis, severity of HPT, or use of concomitant medical therapy. Preliminary evidence suggests that cinacalcet may reverse cortical bone loss. Cinacalcet was well tolerated, with nausea (31%) and vomiting (27%) being the most commonly reported adverse effects. Hypocalcemia was transient in 5% of patients, was usually asymptomatic, and was corrected by dose reduction.

CONCLUSIONS

Based on the available evidence, cinacalcet is effective and well tolerated in the treatment of secondary HPT and refractory parathyroid carcinoma. Its use in primary HPT appears promising. Further investigations are needed to determine if cinacalcet can prevent the long-term complications of HPT and reduce mortality.

Regulation of parathyroid function in chronic renal failure

This review summarizes the factors involved in the development of hyperparathyroidism secondary (2nd-HPTH) to chronic kidney disease (CKD). Calcium and calcitriol act on their respective specific parathyroid cell receptors to inhibit parathyroid function. As well as the well-known effect of calcium and calcitriol on parathyroid cell function, there is experimental work that demonstrates that phosphate, changes in pH, PTHrP, estrogens, and some cytokines also have an effect on PTH secretion. These factors are relevant in patients with chronic kidney disease. However, low calcium, vitamin D deficiency, and an accumulation of phosphate due to the decrease in renal function are the main pathogenic factors involved in the pathogenesis of 2nd-HPTH in CKD patients.