Calcimimetics with potent and selective activity on the parathyroid calcium receptor

Update on the use of cinacalcet in the management of primary hyperparathyroidism

Cinacalcet is an allosteric modulator of the calcium sensing receptor acting to sensitize this receptor to the extracellular calcium. Cinacalcet has been shown to be effective in reducing or normalizing serum calcium levels in several groups of patients with primary hyperparathyroidism (PHPT), including patients with mild to moderate PHPT, intractable PHPT, and parathyroid carcinoma, and in PHPT as a part of multiple endocrine neoplasia type 1. Cinacalcet slightly reduces PTH levels and has no effects on bone mineral density. Cinacalcet at low doses is well tolerated, but side effects are more frequent and severe when relatively high doses are needed to control hypercalcemia. Cinacalcet may be of benefit in a wide spectrum of PHPT severity, offering a novel therapeutic option for the control of hypercalcemia in PHPT patients who are not able to undergo parathyroidectomy. To what extent the reduction of serum calcium particularly in patients with mild to moderate hypercalcemia, translates into a clinical benefit is currently unknown. Moreover, there are no data as to whether long-term treatment with cinacalcet can prevent the complications of PHPT. Finally, more data are needed on the long-term safety of cinacalcet, particularly at the renal level.

Effect of cinacalcet on renal electrolyte handling and systemic arterial blood pressure in kidney transplant patients with persistent hyperparathyroidism

BACKGROUND

The calcimimetic cinacalcet has recently been increasingly used for persistent hyperparathyroidism after renal transplantation. The present study investigated the short-term effects of cinacalcet on urinary electrolyte concentration and arterial blood pressure in kidney transplant patients with persistent hyperparathyroidism.

METHODS

In a prospective controlled single-center cross-over study, we examined 10 stable kidney transplant patients (mean estimated glomerular filtration rate 51±10 mL/min/1.73 m(2)) who received cinacalcet daily for persistent hyperparathyroidism. Urine specimens were collected at baseline and every 2 hr for a total study period of 6 hr after ingestion of 30 mg cinacalcet and without cinacalcet. Intact parathyroid hormone was determined at baseline and 2 hr later. Using ambulatory blood pressure measurement, arterial blood pressure was determined every 15 min.

RESULTS

Intact parathyroid hormone was significantly reduced with cinacalcet as compared with controls (-37±27.7% vs. -9.6±10.3%, P=0.009). With cinacalcet, urinary calcium and magnesium concentration were increased (P=0.042 and P=0.007, respectively) and differed significantly as compared with the control phase without cinacalcet. After 4 hr, an increased urinary sodium concentration was also found compared with the control phase (P=0.039). Systolic blood pressure was reduced with cinacalcet (P<0.001) and differed significantly from control phase (-13.7±9.9 mm Hg vs. -3.2±5.2 mm Hg after 2 hr, P=0.009; -18.1±10.8 mm Hg vs. -1.9±5.2 mm Hg after 4 hr, P=0.001).

CONCLUSIONS

In the short term, cinacalcet increases the urinary concentration of calcium, magnesium, and sodium. The observed antihypertensive effect might be beneficial in patients with a high cardiovascular risk after kidney transplantation.

Cinacalcet HCl prevents development of parathyroid gland hyperplasia and reverses established parathyroid gland hyperplasia in a rodent model of CKD

BACKGROUND

Secondary hyperparathyroidism (sHPT) represents an adaptive response to progressively impaired control of calcium, phosphorus and vitamin D in chronic kidney disease (CKD). It is characterized by parathyroid hyperplasia and excessive synthesis and secretion of parathyroid hormone (PTH). Parathyroid hyperplasia in uremic rats can be prevented by calcium-sensing receptor (CaSR) activation with the calcimimetic cinacalcet (Sensipar®/Mimpara®); however, it is unknown, how long the effects of cinacalcet persist after withdrawal of treatment or if cinacalcet is efficacious in uremic rats with established sHPT.

METHODS

We sought to determine the effect of cinacalcet discontinuation in uremic rats and whether cinacalcet was capable of influencing parathyroid hyperplasia in animals with established sHPT.

RESULTS

Discontinuation of cinacalcet resulted in reversal of the beneficial effects on serum PTH and parathyroid hyperplasia. In rats with established sHPT, cinacalcet decreased serum PTH and mediated regression of parathyroid hyperplasia. The cinacalcet-mediated decrease in parathyroid gland size was accompanied by increased expression of the cyclin-dependent kinase inhibitor p21. Prevention of cellular proliferation with cinacalcet occurred despite increased serum phosphorus and decreased serum calcium.

CONCLUSIONS

The animal data provided suggest established parathyroid hyperplasia can be reversed by modulating CaSR activity with cinacalcet and that continued treatment may be necessary to maintain reductions in PTH.

Pharmacotherapy of chronic kidney disease and mineral bone disorder

INTRODUCTION

Disturbances of the bone and mineral metabolism are a common complication of chronic kidney disease (CKD); these disturbances are known as CKD-mineral bone disorder (CKD-MBD). A better understanding of the pathophysiological mechanisms of CKD-MBD, along with its negative impact on other organs and systems, as well as on survival, has led to a shift in the treatment paradigm of this disorder. The use of phosphate binders changed dramatically over the last decade when noncalcium-containing phosphate binders, such as sevelamer and lanthanum carbonate, became possible alternative treatments to avoid calcium overload. Vitamin D receptor activators, such as paricalcitol and doxercalciferol, with fewer calcemic and phosphatemic effects, have also been introduced to control parathormone production and the interest in native vitamin D supplementation has grown. Furthermore, a new drug class, the calcimimetics, has recently been introduced into the therapeutic arsenal for treating secondary hyperparathyroidism.

AREAS COVERED

This review discusses the advantages and disadvantages of the above pharmacological options to treat CKD-MBD.

EXPERT OPINION

The individual-based use of phosphate binders, vitamin D and calcimimetics, separately or in combination, constitute a reasonable approach to treat CKD-MBD. These treatments aim to achieve a rigorous control of phosphorus and parathormone levels, while avoiding calcium overload.

Calcium-sensing receptor modulates cell adhesion and migration via integrins

The calcium-sensing receptor (CaSR) is a family C G protein-coupled receptor that is activated by elevated levels of extracellular divalent cations. The CaSR couples to members of the G(q) family of G proteins, and in the endocrine system this receptor is instrumental in regulating the release of parathyroid hormone from the parathyroid gland and calcitonin from thyroid cells. Here, we demonstrate that in medullary thyroid carcinoma cells, the CaSR promotes cellular adhesion and migration via coupling to members of the integrin family of extracellular matrix-binding proteins. Immunopurification and mass spectrometry, co-immunoprecipitation, and co-localization studies showed that the CaSR and β1-containing integrins are components of a macromolecular protein complex. In fibronectin-based cell adhesion and migration assays, the CaSR-positive allosteric modulator NPS R-568 induced a concentration-dependent increase in cell adhesion and migration; both of these effects were blocked by a specific CaSR-negative allosteric modulator. These effects were mediated by integrins because they were blocked by a peptide inhibitor of integrin binding to fibronectin and β1 knockdown experiments. An analysis of intracellular signaling pathways revealed a key role for CaSR-induced phospholipase C activation and the release of intracellular calcium. These results demonstrate for the first time that an ion-sensing G protein-coupled receptor functionally couples to the integrins and, in conjunction with intracellular calcium release, promotes cellular adhesion and migration in tumor cells. The significance of this interaction is further highlighted by studies implicating the CaSR in cancer metastasis, axonal growth, and stem cell attachment, functions that rely on integrin-mediated cell adhesion.

The vascular extracellular calcium-sensing receptor: an update

The extracellular calcium-sensing receptor (CaR) was first described in the parathyroid gland. Recent studies have shown that the CaR is also expressed in blood vessels, especially in the endothelial and adventitial layers but its physiological function is still not clear. However, an understanding of its possible role(s) in the vasculature (perivascular-neurones, heart and blood vessels) is important because of the use of synthetic positive allosteric CaR modulators in hyperparathyroidism and the potential importance of negative modulators in the treatment of osteoporosis. In this review, the effects of CaR activation and inhibition are detailed and the possible role of the CaR as both an amplifier and attenuator of myo-endothelial coupling in the vasculature is described.

High extracellular Ca2+ stimulates Ca2+-activated Cl- currents in frog parathyroid cells through the mediation of arachidonic acid cascade

Elevation of extracellular Ca(2+) concentration induces intracellular Ca(2+) signaling in parathyroid cells. The response is due to stimulation of the phospholipase C/Ca(2+) pathways, but the direct mechanism responsible for the rise of intracellular Ca(2+) concentration has remained elusive. Here, we describe the electrophysiological property associated with intracellular Ca(2+) signaling in frog parathyroid cells and show that Ca(2+)-activated Cl(-) channels are activated by intracellular Ca(2+) increase through an inositol 1,4,5-trisphophate (IP(3))-independent pathway. High extracellular Ca(2+) induced an outwardly-rectifying conductance in a dose-dependent manner (EC(50) ∼6 mM). The conductance was composed of an instantaneous time-independent component and a slowly activating time-dependent component and displayed a deactivating inward tail current. Extracellular Ca(2+)-induced and Ca(2+) dialysis-induced currents reversed at the equilibrium potential of Cl(-) and were inhibited by niflumic acid (a specific blocker of Ca(2+)-activated Cl(-) channel). Gramicidin-perforated whole-cell recording displayed the shift of the reversal potential in extracellular Ca(2+)-induced current, suggesting the change of intracellular Cl(-) concentration in a few minutes. Extracellular Ca(2+)-induced currents displayed a moderate dependency on guanosine triphosphate (GTP). All blockers for phospholipase C, diacylglycerol (DAG) lipase, monoacylglycerol (MAG) lipase and lipoxygenase inhibited extracellular Ca(2+)-induced current. IP(3) dialysis failed to induce conductance increase, but 2-arachidonoylglycerol (2-AG), arachidonic acid and 12S-hydroperoxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12(S)-HPETE) dialysis increased the conductance identical to extracellular Ca(2+)-induced conductance. These results indicate that high extracellular Ca(2+) raises intracellular Ca(2+) concentration through the DAG lipase/lipoxygenase pathway, resulting in the activation of Cl(-) conductance.

The effect of cinacalcet on bone remodeling and renal function in transplant patients with persistent hyperparathyroidism

BACKGROUND

Parathyroidectomy is associated with renal functional losses in transplant patients; cinacalcet offers an attractive alternative.

METHODS

We performed a prospective observational study in 58 patients with persisting hyperparathyroidism after renal transplantation (Ca≥2.6 mmol/L) and impaired renal transplant function (estimated glomerular filtration rate [eGFR] <50 mL/min). The patients received 30 to 90 mg cinacalcet for 12 months with the target to normalize serum Ca. We measured parathyroid hormone (PTH), serum Ca, serum phosphorus, alkaline phosphatase, bone-specific alkaline phosphatase, osteocalcin, and telopeptide at 0, 1, 2, 3, 6, 9, and 12 months of cinacalcet treatment. Fractional excretion of calcium and phosphorus (n=24) were monitored at 0 and 1 month.

RESULTS

At inclusion, creatinine was 181±70 μmol/L, eGFR 43±19 mL/min, PTH 371±279 pg/mL, and Ca 2.73±0.22 mmol/L. We observed nephrocalcinosis in 58% of biopsied patients at enrollment. After cinacalcet, Ca decreased significantly and normalized at nearly any measurement. Phosphorus increased significantly at months 1, 9, and 12. PTH decreased significantly, but only at months 9 and 12 and did not normalize. Bone-specific alkaline phosphatase increased significantly (>normal) by month 12. eGFR decreased and serum creatinine increased at all time points. The Δ(creatinine) % increase correlated significantly with the Δ(PTH) % decrease at month 1 and 12. Telopeptide and alkaline phosphatase correlated with PTH and telopeptide also correlated with serum creatinine.

CONCLUSION

Calcium-phosphorus homeostasis in hypercalcemic renal transplant patients normalizes under cinacalcet and PTH decreases, albeit not to normal. The renal functional decline could be PTH mediated, analogous to the effects observed after parathyroidectomy.

Amino acids stimulate cholecystokinin release through the Ca2+-sensing receptor

Cholecystokinin (CCK) is produced by discrete endocrine cells in the proximal small intestine and is released following the ingestion of food. CCK is the primary hormone responsible for gallbladder contraction and has potent effects on pancreatic secretion, gastric emptying, and satiety. In addition to fats, digested proteins and aromatic amino acids are major stimulants of CCK release. However, the cellular mechanism by which amino acids affect CCK secretion is unknown. The Ca(2+)-sensing receptor (CaSR) that was originally identified on parathyroid cells is not only sensitive to extracellular Ca(2+) but is activated by extracellular aromatic amino acids. It has been postulated that this receptor may be involved in gastrointestinal hormone secretion. Using transgenic mice expressing a CCK promoter driven/enhanced green fluorescent protein (GFP) transgene, we have been able to identify and purify viable intestinal CCK cells. Intestinal mucosal CCK cells were enriched >200-fold by fluorescence-activated cell sorting. These cells were then used for real-time PCR identification of CaSR. Immunohistochemical staining with an antibody specific for CaSR confirmed colocalization of CaSR to CCK cells. In isolated CCK cells loaded with a Ca(2+)-sensitive dye, the amino acids phenylalanine and tryptophan, but not nonaromatic amino acids, caused an increase in intracellular Ca(2+) ([Ca(2+)](i)). The increase in [Ca(2+)](i) was blocked by the CaSR inhibitor Calhex 231. Phenylalanine and tryptophan stimulated CCK release from intestinal CCK cells, and this stimulation was also blocked by CaSR inhibition. Electrophysiological recordings from isolated CCK-GFP cells revealed these cells to possess a predominant outwardly rectifying potassium current. Administration of phenylalanine inhibited basal K(+) channel activity and caused CCK cell depolarization, consistent with changes necessary for hormone secretion. These findings indicate that amino acids have a direct effect on CCK cells to stimulate CCK release by activating CaSR and suggest that CaSR is the physiological mechanism through which amino acids regulate CCK secretion.

Functional expression of the extracellular calcium sensing receptor (CaSR) in equine umbilical cord matrix size-sieved stem cells

BACKGROUND

The present study investigates the effects of high external calcium concentration ([Ca(2+)](o)) and the calcimimetic NPS R-467, a known calcium-sensing receptor (CaSR) agonist, on growth/proliferation of two equine size-sieved umbilical cord matrix mesenchymal stem cell (eUCM-MSC) lines. The involvement of CaSR on observed cell response was analyzed at both the mRNA and protein level.

METHODOLOGY/PRINCIPAL FINDINGS

A large (>8 µm in diameter) and a small (<8 µm) cell line were cultured in medium containing: 1) low [Ca(2+)](o) (0.37 mM); 2) high [Ca(2+)](o) (2.87 mM); 3) NPS R-467 (3 µM) in presence of high [Ca(2+)](o) and 4) the CaSR antagonist NPS 2390 (10 µM for 30 min.) followed by incubation in presence of NPS R-467 in medium with high [Ca(2+)](o). Growth/proliferation rates were compared between groups. In large cells, the addition of NPS R-467 significantly increased cell growth whereas increasing [Ca(2+)](o) was not effective in this cell line. In small cells, both higher [Ca(2+)](o) and NPS R-467 increased cell growth. In both cell lines, preincubation with the CaSR antagonist NPS 2390 significantly inhibited the agonistic effect of NPS R-467. In both cell lines, increased [Ca(2+)](o) and/or NPS R-467 reduced doubling time values.Treatment with NPS R-467 down-regulated CaSR mRNA expression in both cell lines. In large cells, NPS R-467 reduced CaSR labeling in the cytosol and increased it at cortical level.

CONCLUSIONS/SIGNIFICANCE

In conclusion, calcium and the calcimimetic NPS R-467 reduce CaSR mRNA expression and stimulate cell growth/proliferation in eUCM-MSC. Their use as components of media for eUCM-MSC culture could be beneficial to obtain enough cells for down-stream purposes.

Novel calcium sensing receptor ligands: a patent survey

INTRODUCTION

In the parathyroid gland, the calcium sensing receptor responds to small changes in circulating levels of Ca(2+), and consequently stimulates or inhibits the secretion of parathyroid hormone (PTH). Thus, ligands potentiating the action of calcium (calcimimetics) lead to decreased PTH secretion and can thus be useful for the treatment of hyperparathyroidism. On the other hand, ligands which antagonize the action of calcium (calcilytics) stimulate PTH secretion, favoring bone tissue regeneration.

AREAS COVERED

This review first discusses the rapid development of calcimimetics (only one of which has been approved for the treatment of hyperparathyroidism) followed by that of calcilytics (none of which has as yet been approved for the treatment of osteoporosis). Peer-reviewed articles generated by these patents are also surveyed.

EXPERT OPINION

The rapid progress in developing a clinically approved calcimimetic has not been matched by an identical success in finding an orally available calcilytic useful for the treatment of osteoporosis. However, the growing importance of osteoporosis as a debilitating disease is a stimulating factor in discovering such compounds.

The calcium-sensing receptor: a molecular perspective

Compelling evidence of a cell surface receptor sensitive to extracellular calcium was observed as early as the 1980s and was finally realized in 1993 when the calcium-sensing receptor (CaR) was cloned from bovine parathyroid tissue. Initial studies relating to the CaR focused on its key role in extracellular calcium homeostasis, but as the amount of information about the receptor grew it became evident that it was involved in many biological processes unrelated to calcium homeostasis. The CaR responds to a diverse array of stimuli extending well beyond that merely of calcium, and these stimuli can lead to the initiation of a wide variety of intracellular signaling pathways that in turn are able to regulate a diverse range of biological processes. It has been through the examination of the molecular characteristics of the CaR that we now have an understanding of how this single receptor is able to convert extracellular messages into specific cellular responses. Recent CaR-related reviews have focused on specific aspects of the receptor, generally in the context of the CaR's role in physiology and pathophysiology. This review will provide a comprehensive exploration of the different aspects of the receptor, including its structure, stimuli, signalling, interacting protein partners, and tissue expression patterns, and will relate their impact on the functionality of the CaR from a molecular perspective.

Effective use of cinacalcet for the treatment of secondary hyperparathyroidism in Austrian dialysis patients--results of the Austrian cohort of the ECHO study

BACKGROUND

Despite extensive use of standard therapy for secondary hyperparathyroidism (sHPT) in dialysis patients, still most patients do not achieve the recommended treatment targets. In a pan-European observational study (ECHO), the effectiveness of the calcimimetic cinacalcet for the treatment of sHPT was evaluated in real-world clinical practice. A sub-analysis of the entire Austrian study cohort is presented.

METHODS

Adult dialysis patients who had initiated cinacalcet therapy were included. Data on biochemical parameters of bone and mineral metabolism (intact parathyroid hormone [iPTH], calcium [Ca] and phosphorus [P]) and concurrent medication were collected 6 months prior to the initiation of cinacalcet, at initiation (baseline) and after up to 12 months of active treatment.

RESULTS

A total of 320 patients (mean age (±SD): 56 (±14) years) from 34 Austrian dialysis centres were enrolled. At baseline, patients presented with elevated serum iPTH (median 605 pg/ml) and hyperphosphataemia (median 2.1 mmol/l). After 12 months of cinacalcet treatment, serum iPTH (median percentage change -48%), calcium (-2%) and phosphorus (-6%) decreased. The greatest iPTH reduction (-66%) was found in patients with most severe sHPT (>800 pg/ml at baseline). The proportion of patients achieving the recommended NKF/K-DOQI(™) treatment targets increased from baseline to month 12 for iPTH (3-36%) and phosphorus (24 to 39%) and remained stable for calcium (51 to 50%), respectively. No patient had all 3 parameters simultaneously within NKF/K-DOQI(™) treatment targets at baseline, while 7% of patients achieved this treatment goal after 12 months. During the study the use of the phosphate binder sevelamer remained fairly stable, while the relative percentage use of calcium-based phosphate binders increased and the usage of aluminium-containing binders decreased; vitamin D analogue use remained stable.

CONCLUSION

Additional use of cinacalcet improved biochemical parameters of bone and mineral metabolism and enabled more patients to achieve and maintain the KDOQI(™) treatment targets for serum iPTH, calcium and phosphorus.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

The regulation of parathyroid hormone secretion and synthesis

Secondary hyperparathyroidism classically appears during the course of chronic renal failure and sometimes after renal transplantation. Understanding the mechanisms by which parathyroid hormone (PTH) synthesis and secretion are normally regulated is important in devising methods to regulate overactivity and hyperplasia of the parathyroid gland after the onset of renal insufficiency. Rapid regulation of PTH secretion in response to variations in serum calcium is mediated by G-protein coupled, calcium-sensing receptors on parathyroid cells, whereas alterations in the stability of mRNA-encoding PTH by mRNA-binding proteins occur in response to prolonged changes in serum calcium. Independent of changes in intestinal calcium absorption and serum calcium, 1α,25-dihydroxyvitamin D also represses the transcription of PTH by associating with the vitamin D receptor, which heterodimerizes with retinoic acid X receptors to bind vitamin D-response elements within the PTH gene. 1α,25-Dihydroxyvitamin D additionally regulates the expression of calcium-sensing receptors to indirectly alter PTH secretion. In 2°HPT seen in renal failure, reduced concentrations of calcium-sensing and vitamin D receptors, and altered mRNA-binding protein activities within the parathyroid cell, increase PTH secretion in addition to the more widely recognized changes in serum calcium, phosphorus, and 1α,25-dihydroxyvitamin D. The treatment of secondary hyperparathyroidism by correction of serum calcium and phosphorus concentrations and the administration of vitamin D analogs and calcimimetic agents may be augmented in the future by agents that alter the stability of mRNA-encoding PTH.

Design and synthesis of cyclic sulfonamides and sulfamates as new calcium sensing receptor agonists

The design, synthesis and calcimimetic properties of various cyclic sulfonamides and sulfamates are described. The latter were prepared from the corresponding o-alkenylarenesulfonamides via copper- or rhodium-catalyzed intramolecular aziridination. The size of the cyclic sulfonamide rings as well as the position of the crucial (R)-naphthylethylamine substituent significantly affected calcimimetic activity. The most active compounds were the six- and seven-membered sulfonamides 30a and 31a and sulfamate 34a.

Diverse roles of extracellular calcium-sensing receptor in the central nervous system

The G-protein-coupled calcium-sensing receptor (CaSR), upon activation by Ca(2+) or other physiologically relevant polycationic molecules, performs diverse functions in the brain. The CaSR is widely expressed in the central nervous system (CNS) and is characterized by a robust increase in its expression during postnatal brain development over adult levels throughout the CNS. Developmental increases in CaSR levels in brain correlate with myelinogenesis. Indeed, neural stem cells differentiating to the oligodendrocyte lineage exhibit the highest CaSR expression compared with those differentiating to astrocytic or neuronal lineages. In adult CNS, CaSR has broad relevance in maintaining local ionic homeostasis. CaSR shares an evolutionary relationship with the metabotropic glutamate receptor and forms heteromeric complexes with the type B-aminobutyric acid receptor subunits that affects its cell surface expression, activation, signaling, and functions. In normal physiology as well as in pathologic conditions, CaSR is activated by signals arising from mineral ions, amino acids, polyamines, glutathione, and amyloid-beta in conjunction with Ca(2+) and other divalent cationic ligands. CaSR activation regulates membrane excitability of neurons and glia and affects myelination, olfactory and gustatory signal integration, axonal and dendritic growth, and gonadotrophin-releasing hormonal-neuronal migration. Insofar as the CaSR is a clinically important therapeutic target for parathyroid disorders, development of its agonists or antagonists as therapeutics for CNS disorder could be a major breakthrough.

Effectiveness of cinacalcet in patients with recurrent/persistent secondary hyperparathyroidism following parathyroidectomy: results of the ECHO study

BACKGROUND

Progressive secondary hyperparathyroidism (sHPT) is characterized by parathyroid gland hyperplasia which may ultimately require parathyroidectomy (PTX). Although PTX is generally a successful treatment for those patients subjected to surgery, a significant proportion develops recurrent sHPT following PTX. ECHO was a pan-European observational study which evaluated the achievement of KDOQI(TM) treatment targets with cinacalcet use in patients on dialysis. Previously published results showed that cinacalcet plus flexible vitamin D therapy lowered serum PTH, phosphorus and calcium in the clinical practice with similar efficacy as seen in phase III trials.

METHODS

This subgroup analysis of ECHO describes the real-world cinacalcet treatment effect in patients with recurrent or persistent sHPT after PTX (n = 153) compared to sHPT patients without prior history of PTX (n = 1696).

RESULTS

Both groups of patients had substantially elevated serum PTH with comparable sHPT severity at baseline. After 12 months of cinacalcet treatment, 20.3% (26/128) of patients with prior PTX and 18.2% (253/1388) of patients without prior PTX achieved serum PTH and Ca × P values within the recommended KDOQI(TM) target ranges.

CONCLUSIONS

Our data support the successful use of cinacalcet in patients with recurrent/persistent sHPT after PTX.

Beneficial effect of cinacalcet in a child with familial hypocalciuric hypercalcemia

We describe a child with familial hypocalciuric hypercalcemia (FHH) in whom the hypercalcemia seemed to interfere with tissue healing after tympanoplasty. Consequently, he was placed on cinacalcet (30 mg/day), changed after 2 weeks to 60 mg/day. The treatment resulted in a decrease in serum parathyroid hormone (PTH) from 148 to 32 pg/mL (normal 7-75) and ionized calcium from 1.48 to 1.23 mmol/L (1.13-1.34), as well as successful healing of the revised surgical scar. Over the 12-month treatment period no complications were noted. We conclude that cinacalcet may be considered a new, and currently the only, tool in treating children with symptomatic FHH.

Comparison of active vitamin D compounds and a calcimimetic in mineral homeostasis

The differential effects between cinacalcet and active vitamin D compounds on parathyroid function, mineral metabolism, and skeletal function are incompletely understood. Here, we studied cinacalcet and active vitamin D compounds in mice expressing the null mutation for Cyp27b1, which encodes 25-hydroxyvitamin D-1α-hydroxylase, thereby lacking endogenous 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]. Vehicle-treated mice given high dietary calcium had hypocalcemia, hypophosphatemia, and marked secondary hyperparathyroidism. Doxercalciferol and 1,25(OH)(2)D(3) each normalized these parameters and corrected both the abnormal growth plate architecture and the diminished longitudinal bone growth observed in these mice. In contrast, cinacalcet suppressed serum parathyroid hormone (PTH) cyclically and did not correct the skeletal abnormalities and hypocalcemia persisted. Vehicle-treated mice given a "rescue diet" (high calcium and phosphorus, 20% lactose) had normal serum calcium and PTH levels; cinacalcet induced transient hypocalcemia and mild hypercalciuria. The active vitamin D compounds and cinacalcet normalized the increased osteoblast activity observed in mice with secondary hyperparathyroidism; cinacalcet, however, increased the number and activity of osteoclasts. In conclusion, cinacalcet reduces PTH in a cyclical manner, does not eliminate hypocalcemia, and does not correct abnormalities of the growth plate. Doxercalciferol and 1,25(OH)(2)D(3) reduce PTH in a sustained manner, normalize serum calcium, and improve skeletal abnormalities.

Allosteric ligands for G protein-coupled receptors: a novel strategy with attractive therapeutic opportunities

Allosteric receptor ligands bind to a recognition site that is distinct from the binding site of the endogenous messenger molecule. As a consequence, allosteric agents may attach to receptors that are already transmitter-bound. Ternary complex formation opens an avenue to qualitatively new drug actions at G protein-coupled receptors (GPCRs), in particular receptor subtype selective potentiation of endogenous transmitter action. Consequently, suitable exploitation of allosteric recognition sites as alternative molecular targets could pave the way to a drug discovery paradigm different from those aimed at mimicking or blocking the effects of endogenous (orthosteric) receptor activators. The number of allosteric ligands reported to modulate GPCR function is steadily increasing and some have already reached routine clinical use. This review aims at introducing into this fascinating field of drug discovery and at providing an overview about the achievements that have already been made. Various case examples will be discussed in the framework of GPCR classification (family A, B, and C receptors). In addition, the behavior at muscarinic receptors of hybrid derivatives incorporating both an allosteric and an orthosteric fragment in a common molecular skeleton will be illustrated.

Allosteric modulation of the calcium-sensing receptor

The calcium (Ca²⁺)-sensing receptor (CaR) belongs to family C of the G-protein coupled receptors (GPCRs). The receptor is activated by physiological levels of Ca²⁺ (and Mg²⁺) and positively modulated by a range of proteinogenic L-α-amino acids. Recently, several synthetic allosteric modulators of the receptor have been developed, which either act as positive modulators (termed calcimimetics) or negative modulators (termed calcilytics). These ligands do not activate the wild-type receptor directly, but rather shift the concentration-response curves of Ca²⁺ to the left or right, respectively. Like other family C GPCRs, the CaR contains a large amino-terminal domain and a 7-transmembrane domain. Whereas the endogenous ligands for the receptor, Ca²⁺, Mg²⁺ and the L-α-amino acids, bind to the amino-terminal domain, most if not all of the synthetic modulators published so far bind to the 7-transmembrane domain.

The most prominent physiological function of the CaR is to maintain the extracellular Ca²⁺ level in a very tight range via control of secretion of parathyroid hormone (PTH). Influence on e.g. secretion of calcitonin from thyroid C-cells and direct action on the tubule of the kidney also contribute to the control of the extracellular Ca²⁺ level. This control over PTH and Ca²⁺ levels is partially lost in patients suffering from primary and secondary hyperparathyroidism. The perspectives in CaR as a therapeutic target have been underlined by the recent approval of the calcimimetic cinacalcet for the treatment of certain forms of primary and secondary hyperparathyroidism. Cinacalcet is the first clinically administered allosteric modulator acting on a GPCR, and thus the compound constitutes an important proof-of-concept for future development of allosteric modulators on other GPCR drug targets.

Parathyroid carcinoma

Parathyroid carcinoma is a rare endocrine malignancy. The reported incidence is from 0.5 to 5% of primary hyperparathyroidism cases in various series. The cause is unknown, but clinical correlations with different genetic syndromes exist. Mutations in the HPRT2 gene seem to play a significant role in the pathogenesis of this disease. Men and women are equally affected, usually in the fourth or fifth decade of life. Most patients will present with signs and symptoms of hypercalcaemia. Cases of non-functioning carcinoma are exceedingly rare. Surgical resection is the most effective method of treatment and palliation. A significant proportion of patients will experience recurrence, and will need further surgical and, eventually, medical management of hypercalcaemia. The disease is progressive but slow growing. Most patients will require multiple operations to resect recurrent disease. The main cause of morbidity and mortality is the sequela of uncontrolled chronic hypercalcaemia rather than tumour burden. The current paper will review the epidemiology, pathogenesis, clinical presentation and diagnostic work-up of this disease. Surgical management in different scenarios is reviewed in detail, followed by other types of treatment and management of incurable disease.

Clinical utility of calcimimetics targeting the extracellular calcium-sensing receptor (CaSR)

Calcimimetics, which activate the extracellular calcium (Ca(o)(2+))-sensing receptor in the parathyroid and other tissues participating in Ca(o)(2+) homeostasis, were the first described allosteric activators of a G-protein-coupled receptor. Cinacalcet, the only calcimimetic currently approved for human use, is used clinically for treating secondary hyperparathyroidism (e.g., overactivity of parathyroid glands) in patients being dialyzed for chronic kidney disease. By sensitizing the parathyroids to Ca(o)(2+), cinacalcet lowers the circulating parathyroid hormone (PTH) level. It also reduces serum calcium and phosphate, changes increasing the percentage of patients achieving the guidelines recommended by the National Kidney Foundation (NKF) for these minerals. Studies are underway addressing whether better adherence to these guidelines in patients receiving cinacalcet reduces cardiovascular disease and related mortality, which are both common is the dialysis population. The second approved use of cinacalcet is for treating hypercalcemia in patients with inoperable parathyroid carcinoma. In this setting, it provides the first medical therapy chronically lowering serum calcium concentration in this condition, albeit not to normal in most patients. Its effect on the long-term prognosis of these patients, if any, is presently unclear. "Off-label" administration of cinacalcet [i.e., not yet approved by the US Food and Drug Administration (FDA)] effectively lowers serum calcium and/or PTH in various other forms of hyperparathyroidism and increases serum phosphate in renal phosphate-wasting syndromes by reducing PTH-induced phosphaturia. In the future, the drug could conceivably be utilized to modulate the activity of the CaSR in other tissues (i.e., kidney, colon) in therapeutically desirable ways.

Increased receptor stimulation elicits differential calcium-sensing receptor(T888) dephosphorylation

The calcium-sensing receptor (CaR) elicits oscillatory Ca(2+)(i) mobilization associated with dynamic, inhibitory protein kinase C-mediated phosphorylation of CaR(T888). While modest CaR stimulation elicits Ca(2+)(i) oscillations, greater stimulation either increases oscillation frequency or elicits sustained responses by an unknown mechanism. Here, moderate CaR stimulation (2.5 mm Ca(2+)(o), 10 min) increased CaR(T888) phosphorylation (160-kDa mature receptor) 5-fold in CaR stably transfected HEK-293 cells, whereas 3-5 mm Ca(2+)(o) treatments were without apparent effect. Treatment with 2 mm Ca(2+)(o) caused sustained CaR(T888) phosphorylation (> or = 20 min) and oscillatory Ca(2+)(i) mobilization. However, 5 mm Ca(2+)(o) increased CaR(T888) phosphorylation only briefly while eliciting sustained Ca(2+)(i) mobilization, suggesting that greater CaR activation induces rapid CaR(T888) dephosphorylation, thus permitting sustained Ca(2+)(i) responses. Indeed, 5 mm Ca(2+)(o) stimulated protein phosphatase 2A activity and induced CaR(T888) dephosphorylation following acute phorbol ester pretreatment, the latter effect being mimicked by CaR-positive allosteric modulators (NPS-R467 and l-Phe). Finally, the phosphatase inhibitor calyculin-A reversed CaR-induced inhibition of parathyroid hormone secretion from bovine parathyroid slices and normal human parathyroid cells, demonstrating the physiological importance of phosphorylation status on parathyroid function. Therefore, high Ca(2+)(o)-stimulated protein kinase C acts in concert with high Ca(2+)(o)-induced phosphatase activity to generate and maintain CaR-induced Ca(2+)(i) oscillations via the dynamic phosphorylation and dephosphorylation of CaR(T888).

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

Calcimimetics activate the calcium-sensing receptor (CaR) and reduce parathyroid hormone (PTH) by increasing the sensitivity of the parathyroid CaR to ambient calcium. The calcimimetic, cinacalcet, is effective in treating secondary hyperparathyroidism in dialysis patients [chronic kidney disease (CKD 5)], but little is known about its effects on stage 3-4 CKD patients. We compared cinacalcet and paricalcitol in uremic rats with creatinine clearances "equivalent" to patients with CKD 3-4. Uremia was induced in anesthetized rats using the 5/6th nephrectomy model. Groups were 1) uremic control, 2) uremic + cinacalcet (U+Cin; 15 mg x kg(-1) x day(-1) po for 6 wk), 3) uremic + paricalcitol (U+Par; 0.16 microg/kg, 3 x wk, ip for 6 wk), and 4) normal. Unlike U+Par animals, cinacalcet promoted hypocalcemia and marked hyperphosphatemia. The Ca x P in U+Cin rats was twice that of U+Par rats. Both compounds suppressed PTH. Serum 1,25-(OH)(2)D(3) was decreased in both U+Par and U+Cin rats. Serum FGF-23 was increased in U+Par but not in U+Cin, where it tended to decrease. Analysis of tibiae showed that U+Cin, but not U+Par, rats had reduced bone volume. U+Cin rats had similar bone formation and reduced osteoid surface, but higher bone resorption. Hypocalcemia, hyperphosphatemia, low 1,25-(OH)(2)D(3), and cinacalcet itself may play a role in the detrimental effects on bone seen in U+Cin rats. This requires further investigation. In conclusion, due to its effects on bone and to the hypocalcemia and severe hyperphosphatemia it induces, we believe that cinacalcet should not be used in patients with CKD without further detailed studies.

Residue 826 in the calcium-sensing receptor is implicated in the response to calcium and to R-568 calcimimetic compound

Within the extracellular loops of the seven-transmembrane domain of the calcium-sensing receptor (CaR) there is a region (I819-E837) relevant for calcimimetic activity. As the naturally occurring variant Ala826Thr is within this important region, it may be postulated that this change may influence the CaR response to calcium and R-568. Human embryonic kidney (HEK-293) cells transiently transfected with three different human CaRs (wild-type [A826], variant allele [T826], and artificial mutant [W826]) were used to test the ability of calcium alone or in combination with the calcimimetic R-568 to modulate CaR activity. CaR activation was detected by flow cytometry using a fluorescent probe. Intracellular calcium changes were measured in response to changes in extracellular calcium alone or with different R-568 concentrations. The change of the alanine in the 826 position (A826) for threonine (T826) worsened calcium sensitivity, increasing the EC(50) value from 2.34 +/- 0.48 mM (A826, wild-type) to 2.96 +/- 0.75 mM (T826) (P < 0.05). The T826 receptor reached a similar response with 1 muM R-568 compared with the wild-type receptor. On the contrary, the artificial introduction of a tryptophan in the same position (W826) did not affect calcium sensitivity (EC(50) = 2.64 +/- 0.81 mM) but reduced the ability of the receptor to respond to R-568. The results demonstrate the importance of the 826 residue in the CaR response to calcium and calcimimetics. Since the A826T change was described as a natural variant, the differences in the calcium and calcimimetic responses observed between the alleles could have potential clinical impact.

Physiology and pathophysiology of the calcium-sensing receptor in the kidney

The extracellular calcium-sensing receptor (CaSR) plays a major role in the maintenance of a physiological serum ionized calcium (Ca2+) concentration by regulating the circulating levels of parathyroid hormone. It was molecularly identified in 1993 by Brown et al. in the laboratory of Dr. Steven Hebert with an expression cloning strategy. Subsequent studies have demonstrated that the CaSR is highly expressed in the kidney, where it is capable of integrating signals deriving from the tubular fluid and/or the interstitial plasma. Additional studies elucidating inherited and acquired mutations in the CaSR gene, the existence of activating and inactivating autoantibodies, and genetic polymorphisms of the CaSR have greatly enhanced our understanding of the role of the CaSR in mineral ion metabolism. Allosteric modulators of the CaSR are the first drugs in their class to become available for clinical use and have been shown to treat successfully hyperparathyroidism secondary to advanced renal failure. In addition, preclinical and clinical studies suggest the possibility of using such compounds in various forms of hypercalcemic hyperparathyroidism, such as primary and lithium-induced hyperparathyroidism and that occurring after renal transplantation. This review addresses the role of the CaSR in kidney physiology and pathophysiology as well as current and in-the-pipeline treatments utilizing CaSR-based therapeutics.

Mechanisms of multimodal sensing by extracellular Ca(2+)-sensing receptors: a domain-based survey of requirements for binding and signalling

In this article we consider the molecular basis of sensing and signalling by the extracellular calcium-sensing receptor. We consider the nature of its ligands and sensing modalities, the identities of its major protein domains and their roles in sensing, signalling and trafficking as well as the significance of receptor homo- and hetero-dimerization. Finally, we consider the current, incomplete, state of knowledge regarding the requirements for ligand-specific signalling.

Bone metabolism after cinacalcet administration in patients with secondary hyperparathyroidism

Cinacalcet, an allosteric modulator of a calcium (Ca)-sensing receptor, significantly suppresses parathyroid hormone (PTH) secretion and bone turnover rate in chronic hemodialysis (HD) patients with secondary hyperparathyroidism (SHPT). In this study, bone metabolism after cinacalcet treatment was examined, because hungry bone syndrome is sometimes experienced after parathyroidectomy in severe SHPT. We conducted a prospective observational study in 17 HD patients with SHPT. Cinacalcet was started at 25 mg/day, and the dose was increased step by step based on serum calcium level. A significant decrease in serum Ca and intact PTH concentration was found within 2 weeks. Tartrate-resistant acid phosphatase 5b, a good bone resorption marker, was significantly decreased at week 2 of the study. Serum bone alkaline phosphatase, a marker of bone formation, was increased at week 2 compared with the basal level. It became, however, gradually decreased until week 14. Only one patient whose bone turnover was considerably high had a mild numbness feeling. These results suggest that cinacalcet treatment might transiently accelerate bone formation with rapid suppression of bone resorption. This uncoupling could be involved in a mechanism by which cinacalcet decreases serum Ca level.

The calcium-sensing receptor (CaSR) defends against hypercalcemia independently of its regulation of parathyroid hormone secretion

The calcium-sensing receptor (CaSR) controls parathyroid hormone (PTH) secretion, which, in turn, via direct and indirect actions on kidney, bone, and intestine, maintains a normal extracellular ionized calcium concentration (Ca(2+)(o)). There is less understanding of the CaSR's homeostatic importance outside of the parathyroid gland. We have employed single and double knockout mouse models, namely mice lacking PTH alone (CaSR(+/+) PTH(-/-), referred to as C(+)P(-)), lacking both CaSR and PTH (CaSR(-/-) PTH(-/-), C(-)P(-)) or wild-type (CaSR(+/+) PTH(+/+), C(+)P(+)) mice to study CaSR-specific functions without confounding CaSR-mediated changes in PTH. The mice received three hypercalcemic challenges: an oral Ca(2+) load, injection or constant infusion of PTH via osmotic pump, or a phosphate-deficient diet. C(-)P(-) mice show increased susceptibility to developing hypercalcemia with all three challenges compared with the other two genotypes, whereas C(+)P(-) mice defend against hypercalcemia similarly to C(+)P(+) mice. Reduced renal Ca(2+) clearance contributes to the intolerance of the C(-)P(-) mice to Ca(2+) loads, as they excrete less Ca(2+) at any given Ca(2+)(o) than the other two genotypes, confirming the CaSR's direct role in regulating renal Ca(2+) handling. In addition, C(+)P(+) and C(+)P(-), but not C(-)P(-), mice showed increases in serum calcitonin (CT) levels during hypercalcemia. The level of 1,25(OH)(2)D(3) in C(-)P(-) mice, in contrast, was similar to those in C(+)P(-) and C(+)P(+) mice during an oral Ca(2+) load, indicating that increased 1,25(OH)(2)D(3) production cannot account for the oral Ca(2+)-induced hypercalcemia in the C(-)P(-) mice. Thus, CaSR-stimulated PTH release serves as a "floor" to defend against hypocalcemia. In contrast, high-Ca(2+)(o)-induced inhibition of PTH is not required for a robust defense against hypercalcemia, at least in mice, whereas high-Ca(2+)(o)-stimulated, CaSR-mediated CT secretion and renal Ca(2+) excretion, and perhaps other factors, serve as a "ceiling" to limit hypercalcemia resulting from various types of hypercalcemic challenges.