Activation of the Calcium Receptor by Calcimimetic Agents Is Preserved Despite Modest Attenuating Effects of Hyperphosphatemia.

BACKGROUND: Phosphorus levels in the range seen clinically among patients undergoing dialysis have been reported to attenuate calcium receptor activation and modify parathyroid hormone (PTH) release from isolated parathyroid glands in vitro. Some clinicians and providers of dialysis thus have suggested that calcimimetic agents are ineffective and should not be used to manage secondary hyperparathyroidism among those undergoing dialysis when serum phosphorus concentrations exceed certain threshold levels. METHODS: To determine whether hyperphosphatemia diminishes the therapeutic response to calcimimetic agents, we used data from large clinical trials to analyze the effects of etelcalcetide and cinacalcet to lower plasma PTH levels in individuals on hemodialysis who had secondary hyperparathyroidism and varying degrees of hyperphosphatemia. RESULTS: Plasma PTH levels declined progressively during 26 weeks of treatment with either etelcalcetide or cinacalcet without regard to the degree of hyperphosphatemia at baseline. However, with each calcimimetic agent, the decreases in PTH from baseline were less at each interval of follow-up during the trials among participants with serum phosphorus levels above one of three prespecified threshold values compared with those with serum phosphorus levels below these thresholds. CONCLUSIONS: These in vivo findings are the first in humans to support the idea that hyperphosphatemia attenuates calcium receptor activation by calcium ions and by calcimimetic agents. The effect of hyperphosphatemia on the responsiveness to calcimimetic agents appears relatively modest, however, and unlikely to be significant therapeutically. The efficacy of treatment with calcimimetic agents for lowering plasma PTH levels among those with secondary hyperparathyroidism remains robust despite substantial elevations in serum phosphorus.

Treatment of Autosomal Dominant Hypocalcemia Type 1 With the Calcilytic NPSP795 (SHP635).

Autosomal dominant hypocalcemia type 1 (ADH1) is a rare form of hypoparathyroidism caused by heterozygous, gain-of-function mutations of the calcium-sensing receptor gene (CAR). Individuals are hypocalcemic with inappropriately low parathyroid hormone (PTH) secretion and relative hypercalciuria. Calcilytics are negative allosteric modulators of the extracellular calcium receptor (CaR) and therefore may have therapeutic benefits in ADH1. Five adults with ADH1 due to four distinct CAR mutations received escalating doses of the calcilytic compound NPSP795 (SHP635) on 3 consecutive days. Pharmacokinetics, pharmacodynamics, efficacy, and safety were assessed. Parallel in vitro testing with subject CaR mutations assessed the effects of NPSP795 on cytoplasmic calcium concentrations (Ca2+i ), and ERK and p38MAPK phosphorylation. These effects were correlated with clinical responses to administration of NPSP795. NPSP795 increased plasma PTH levels in a concentration-dependent manner up to 129% above baseline (p = 0.013) at the highest exposure levels. Fractional excretion of calcium (FECa) trended down but not significantly so. Blood ionized calcium levels remained stable during NPSP795 infusion despite fasting, no calcitriol supplementation, and little calcium supplementation. NPSP795 was generally safe and well-tolerated. There was significant variability in response clinically across genotypes. In vitro, all mutant CaRs were half-maximally activated (EC50 ) at lower concentrations of extracellular calcium (Ca2+o ) compared to wild-type (WT) CaR; NPSP795 exposure increased the EC50 for all CaR activity readouts. However, the in vitro responses to NPSP795 did not correlate with any clinical parameters. NPSP795 increased plasma PTH levels in subjects with ADH1 in a dose-dependent manner, and thus, serves as proof-of-concept that calcilytics could be an effective treatment for ADH1. Albeit all mutations appear to be activating at the CaR, in vitro observations were not predictive of the in vivo phenotype or the response to calcilytics, suggesting that other parameters impact the response to the drug. © 2019 American Society for Bone and Mineral Research.

ZT-031, a cyclized analog of parathyroid hormone(1-31) for the potential treatment of osteoporosis.

ZT-031 is a cyclic 31-amino acid analog of parathyroid hormone (PTH) that is in development by Zelos Therapeutics Inc for the treatment of osteoporosis and other bone-related disorders. ZT-031 activates the PTH type 1 receptor - the molecular target of the currently marketed osteogenic peptides PTH and PTH(1-34). Daily subcutaneous injections of ZT-031 prevented bone loss and replaced bone that had already been lost in an ovariectomized rat model of osteoporosis. Daily subcutaneous injections of ZT-031 in gonad-intact monkeys increased bone mineral density (BMD) at cortical and cancellous bone sites and increased serum levels of bone formation markers. The daily subcutaneous administration of ZT-031 to postmenopausal women with osteoporosis elicited a dose-dependent increase in BMD of the lumbar spine, proximal femur and total hip area. Plasma levels of bone formation markers were significantly increased above baseline after 1 month of dosing, and prior to increases in bone resorption markers. ZT-031 was demonstrated to be safe and well tolerated; episodes of hypercalcemia were infrequent and observed with a frequency greater than with placebo only at the highest doses tested for the drug. Although available data are limited, the results obtained following treatment with ZT-031 have generally been at least as favorable as those obtained with other osteogenic PTH peptides. A novel dosing paradigm has been planned for the drug.

Functional proteins involved in regulation of intracellular Ca(2+) for drug development: the extracellular calcium receptor and an innovative medical approach to control secondary hyperparathyroidism by calcimimetics.

Circulating levels of calcium ion (Ca(2+)) are maintained within a narrow physiological range mainly by the action of parathyroid hormone (PTH) secreted from parathyroid cells. Parathyroid cells can sense small fluctuations in plasma Ca(2+) levels by virtue of a cell surface Ca(2+) receptor (CaR) that belongs to the superfamily of G-protein-coupled receptors. Calcimimetics are positive allosteric modulators that activate the CaR on parathyroid cells and thereby immediately suppress PTH secretion. Pre-clinical studies with NPS R-568, a first generation calcimimetic compound, have demonstrated that daily oral administration inhibits the elevation of plasma PTH levels and parathyroid gland hyperplasia and ameliorates impaired bone qualities in rats with chronic renal insufficiency. The results of clinical trials with cinacalcet hydrochloride, a second generation calcimimetic compound, have shown that calcimimetics possess lowering effects not only on serum PTH levels but also on serum calcium x phosphorus product levels, a hallmark of an increased risk for cardiovascular death in dialysis patients with end-stage renal disease (ESRD). Thus, calcimimetics have considerable potential as an innovative medical approach to manage secondary hyperparathyroidism associated with ESRD. Indeed, cinacalcet hydrochloride has been approved in several countries and is the first positive allosteric modulator of any G protein-coupled receptor to reach the market.

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.