Stimulation of calcitonin secretion by calcium receptor activators: evaluation using a new, highly sensitive, homologous immunoradiometric assay for rat calcitonin

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

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

[Cinacalcet impact on calcium homeostasis and bone remodeling in 13 renal transplanted patients with hyperparathyroidism and hypercalcaemia]

The purpose of the study is to assess the impact of cinacalcet on calcium and bone remodeling, in post-renal transplanted patients with persistent hypercalcaemia secondary to hyperparathyroidism. Thirteen renal-transplanted adult recipients with a glomerular filtration rate over 30 ml/min/1.73 m(2), a total serum calcium>2.60 mmol/l with ionized calcium>1.31 mmol/l and a parathyroid hormone serum level over 70 pg/ml, were treated with cinacalcet for 4 months followed by a 15-day wash out. The results show that cinacalcet lowers significantly total and ionized calcium respectively from 2,73 (2,67-2,86) to 2,31 (2,26-2,37) mmol/l (P<0.05) and from 1,39 (1,37-1,47) to 1,21 (1,15-1,22) mmol/l (P<0.05) with no alteration of the 24-hour urine calcium/creatinine ratio and no significant expected PTH serum level suppression (153 [115-214,9] and 166 [122-174] pg/ml). On the other hand, fasting urine calcium was significantly decreased from 0,61 (0,27-1,02) to 0,22 (0,15-0,37) (P<0.05) and bone-specific alkaline phosphatases increased from 20,5 (13-46,6) to 33,8 (12-58,9) ng/ml, upon cinacalcet treatment. After its discontinuation, all these effects were reversible. In conclusion, cinacalcet normalizes total and ionized calcium in renal-transplanted recipients with hypercalcemia secondary to hyperparathyroidism through a mechanism that could be independent of PTH serum level suppression. The increase in bone-specific alkaline phosphatases, biochemical markers of bone accretion and the significant decrease in fasting urine calcium suggest the possibility of a beneficial impact of cinacalcet on bone remodeling.

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.

Calcium-sensing receptor gene transcription is up-regulated by the proinflammatory cytokine, interleukin-1beta. Role of the NF-kappaB PATHWAY and kappaB elements

The calcium-sensing receptor (CASR) in parathyroid, thyroid, and kidney is essential for calcium homeostasis. Hypocalcemia is common in critically ill patients having increased circulating proinflammatory cytokines, although the causes are unknown. We hypothesized that the cytokines increase CASR expression and reduce the set point for parathyroid hormone suppression by extracellular calcium, leading to hypocalcemia and hypoparathyroidism. Here, we show in vivo in the rat that parathyroid, thyroid, and kidney CASR mRNA and protein increased after injection of interleukin-1beta. This was associated with decreased circulating parathyroid hormone, calcium, and 1,25-dihydroxyvitamin D levels. Interleukin-1beta stimulated endogenous CASR gene transcripts and transfected promoter reporter activity in human thyroid C-cells (TT cells) and kidney proximal tubule (HKC) cells. Cotransfection of NF-kappaB proteins enhanced activity of the reporter constructs, whereas cotransfection with inhibitor-kappaB or application of an NF-kappaB nuclear localization sequence peptide abrogated responsiveness to cytokine or NF-kappaB proteins. Mutagenesis of some, but not all, of the potential kappaB elements in the 5' part of the CASR gene led to loss of responsiveness to cytokine. These elements conferred cytokine responsiveness to a heterologous promoter, and in electrophoretic mobility shift assays, NF-kappaB complexes formed on the same three kappaB elements. In summary, the CASR gene has several functional kappaB elements that mediate its upregulation by proinflammatory cytokines and probably contribute to altered extracellular calcium homeostasis in the critically ill.

Physiological studies in heterozygous calcium sensing receptor (CaSR) gene-ablated mice confirm that the CaSR regulates calcitonin release in vivo

Background

The calcium sensing receptor (CaSR) regulates serum calcium by suppressing secretion of parathyroid hormone; it also regulates renal tubular calcium excretion. Inactivating mutations of CaSR raise serum calcium and reduce urine calcium excretion. Thyroid C-cells (which make calcitonin) express CaSR and may, therefore, be regulated by it. Since calcium stimulates release of calcitonin, the higher blood calcium caused by inactivation of CaSR should increase serum calcitonin, unless CaSR mutations alter the responsiveness of calcitonin to calcium.

To demonstrate regulatory effects of CaSR on calcitonin release, we studied calcitonin responsiveness to calcium in normal and CaSR heterozygous-ablated (Casr^+/-) mice. Casr^+/- mice have hypercalcemia and hypocalciuria, and live normal life spans. Each mouse received either 500 μl of normal saline or one of two doses of elemental calcium (500 μmol/kg or 5 mmol/kg) by intraperitoneal injection. Ionized calcium was measured at baseline and 10 minutes, and serum calcitonin was measured on the 10 minute sample.

Results

At baseline, Casr^+/- mice had a higher blood calcium, and in response to the two doses of elemental calcium, had greater increments and peak levels of ionized calcium than their wild type littermates. Despite significantly higher ionized calcium levels, the calcitonin levels of Casr^+/- mice were consistently lower than wild type at any ionized calcium level, indicating that the dose-response curve of calcitonin to increases in ionized calcium had been significantly blunted or shifted to the right in Casr^+/- mice.

Conclusions

These results confirm that the CaSR is a physiological regulator of calcitonin; therefore, in response to increases in ionized calcium, the CaSR inhibits parathyroid hormone secretion and stimulates calcitonin secretion.

Pharmacodynamics of the Type II Calcimimetic Compound Cinacalcet HCl

Calcimimetic compounds, which activate the parathyroid cell Ca(2+) receptor (CaR) and inhibit parathyroid hormone (PTH) secretion, are under experimental study as a treatment for hyperparathyroidism. This report describes the salient pharmacodynamic properties, using several test systems, of a new calcimimetic compound, cinacalcet HCl. Cinacalcet HCl increased the concentration of cytoplasmic Ca(2+) ([Ca(2+)](i)) in human embryonic kidney 293 cells expressing the human parathyroid CaR. Cinacalcet HCl (EC(50) = 51 nM) in the presence of 0.5 mM extracellular Ca(2+) elicited increases in [Ca(2+)](i) in a dose- and calcium-dependent manner. Similarly, in the presence of 0.5 mM extracellular Ca(2+), cinacalcet HCl (IC(50) = 28 nM) produced a concentration-dependent decrease in PTH secretion from cultured bovine parathyroid cells. Using rat medullary thyroid carcinoma 6-23 cells expressing the CaR, cinacalcet HCl (EC(50) = 34 nM) produced a concentration-dependent increase in calcitonin secretion. In vivo studies in rats demonstrated cinacalcet HCl is orally bioavailable and displays approximately linear pharmacokinetics over the dose range of 1 to 36 mg/kg. Furthermore, this compound suppressed serum PTH and blood-ionized Ca(2+) levels and increased serum calcitonin levels in a dose-dependent manner. Cinacalcet was about 30-fold more potent at lowering serum levels of PTH than it was at increasing serum calcitonin levels. The S-enantiomer of cinacalcet (S-AMG 073) was at least 75-fold less active in these assay systems. The present findings provide compelling evidence that cinacalcet HCl is a potent and stereoselective activator of the parathyroid CaR and, as such, might be beneficial in the treatment of hyperparathyroidism.

Daily transient decreases in plasma parathyroid hormone levels induced by the calcimimetic NPS R-568 slows the rate of bone loss but does not increase bone mass in ovariectomized rats

Daily parathyroid hormone (PTH) injections that transiently increase plasma PTH levels within the physiological range increase bone mass in osteopenic, ovariectomized (ovx) rats. This study tested the hypothesis that repeated transient decreases in plasma PTH levels from normal, induced by the daily oral administration of the calcimimetic NPS R-568, would induce an anabolic effect in bone of ovx rats with established osteopenia and/or prevent the rapid bone loss that occurs following ovx. In the reversal study, NPS R-568 was administered orally (10 or 100 micromol/kg) for 30 days to 14-month-old retired breeder rats that were ovx 5 months earlier. NPS R-568 treatment did not increase bone formation rate (BFR) or cancellous bone area (B.Ar) in the proximal tibial metaphysis, or bone mineral density (BMD), at any femoral site. In the prevention study, 3-month-old virgin rats were ovx and given NPS R-568 for the following 28 days. The 10 micromol/kg dose prevented the increase in osteoclast number and 42% of the loss of B.Ar, without affecting the elevated osteoblast populations or BFR. Surprisingly, the 100 micromol/kg dose had fewer protective effects, despite preventing the increase in BFR in both cancellous and cortical bone. Detailed analysis of cancellous bone showed that tendency for a dose-related protection of true cancellous bone occurred, but, while the 10 micromol/kg dose prevented 88% of the loss of calcified cartilage seen in control ovx rats, the 100 micromol/kg dose increased that loss by a further 31%. The mechanism underlying these disparate effects of NPS R-568 on calcified cartilage accumulation in the tibial metaphysis is unclear, but may be related to the different effects that the two doses have on plasma Ca(2+) levels. In conclusion, transient increases in PTH levels above basal, and not simple oscillations in hormone levels below normal, appear necessary for the anabolic properties of endogenous PTH to be manifested in the bones of osteopenic ovx rats.

Antagonizing the parathyroid calcium receptor stimulates parathyroid hormone secretion and bone formation in osteopenic rats

Parathyroid hormone (PTH) is an effective bone anabolic agent, but it must be administered parenterally. An orally active anabolic agent would provide a valuable alternative for treating osteoporosis. NPS 2143 is a novel, selective antagonist (a "calcilytic") of the parathyroid cell Ca(2+) receptor. Daily oral administration of NPS 2143 to osteopenic ovariectomized (OVX) rats caused a sustained increase in plasma PTH levels, provoking a dramatic increase in bone turnover but no net change in bone mineral density. Concurrent oral administration of NPS 2143 and subcutaneous infusion of 17beta-estradiol also resulted in increased bone turnover. However, the antiresorptive action of estrogen decreased the extent of bone resorption stimulated by the elevated PTH levels, leading to an increase in bone mass compared with OVX controls or to either treatment alone. Despite the sustained stimulation to the parathyroid gland, parathyroid cells did not undergo hyperplasia. These data demonstrate that an increase in endogenous PTH secretion, induced by antagonism of the parathyroid cell Ca(2+) receptor with a small molecule, leads to a dramatic increase in bone turnover, and they suggest a novel approach to the treatment of osteoporosis.

Calcimimetic Compounds: a Direct Approach to Controlling Plasma Levels of Parathyroid Hormone in Hyperparathyroidism

The Ca2+ receptor is the primary mechanism regulating the secretion of parathyroid hormone (PTH). Ligands that activate this receptor (calcimimetics) represent a novel means of lowering plasma levels of PTH. Two mechanistically distinct classes of calcimimetics that inhibit PTH secretion have been identified: type I calcimimetics are full agonists of the Ca2+ receptor and include Ca2+ and other polyvalent inorganic and organic cations; whereas type II calcimimetics, typified by phenylalkylamine compounds, behave like positive allosteric activators and increase, in a stereoselective manner, the sensitivity of the Ca2+ receptor to activation by extracellular Ca2+. The phenylalkylamine calcimimetics are orally active and decrease the plasma levels of PTH and Ca2+ in patients with primary hyperparathyroidism (HPT), a disease that so far has resisted pharmacological intervention. Such compounds are similarly safe and effective in reducing PTH levels and preventing parathyroid cell hyperplasia in rats with HPT secondary to chronic renal insufficiency and they lower plasma levels of PTH in dialysis patients with secondary HPT. Calcimimetic compounds may provide a novel therapy for treating both primary and secondary HPT.