Calcilytic NPSP795 Increases Plasma Calcium and PTH in an Autosomal Dominant Hypocalcemia Type 1 Mouse Model

Abstract

Calcilytics are calcium-sensing receptor (CaSR) antagonists that reduce the sensitivity of the CaSR to extracellular calcium. Calcilytics have the potential to treat autosomal dominant hypocalcemia type 1 (ADH1), which is caused by germline gain-of-function CaSR mutations and leads to symptomatic hypocalcemia, inappropriately low PTH concentrations, and hypercalciuria. To date, only one calcilytic compound, NPSP795, has been evaluated in patients with ADH1: Doses of up to 30 mg per patient have been shown to increase PTH concentrations, but did not significantly alter ionized blood calcium concentrations. The aim of this study was to further investigate NPSP795 for the treatment of ADH1 by undertaking in vitro and in vivo studies involving Nuf mice, which have hypocalcemia in association with a gain-of-function CaSR mutation, Leu723Gln. Treatment of HEK293 cells stably expressing the mutant Nuf (Gln723) CaSR with 20nM NPSP795 decreased extracellular Ca 2+-mediated intracellular calcium and phosphorylated ERK responses. An in vivo doseranging study was undertaken by administering a s.c. bolus of NPSP795 at doses ranging from 0 to 30 mg/kg to heterozygous (Casr +/ Nuf) and to homozygous (Casr Nuf/Nuf) mice, and measuring plasma PTH responses at 30 min postdose. NPSP795 significantly increased plasma PTH concentrations in a dose-dependent manner with the 30 mg/kg dose causing a maximal (≥10-fold) rise in PTH. To determine whether NPSP795 can rectify the hypocalcemia of Casr +/Nuf and Casr Nuf/Nuf mice, a submaximal dose (25 mg/kg) was administered, and plasma adjusted-calcium concentrations measured over a 6-hour period. NPSP795 significantly increased plasma adjusted-calcium in Casr +/Nuf mice from 1.87 AE 0.03 mmol/L to 2.16 AE 0.06 mmol/L, and in Casr Nuf/Nuf mice from 1.70 AE 0.03 mmol/L to 1.89 AE 0.05 mmol/L. Our findings show that NPSP795 elicits dose-dependent increases in PTH and ameliorates the hypocalcemia in an ADH1 mouse model. Thus, calcilytics such as NPSP795 represent a potential targeted therapy for ADH1.

Hypoparathyroidism: Genetics and Diagnosis

“…( 102 ) Studies using several different calcilytics in ADH1 cell and animal models have demonstrated improvement in cellular signalinmineral metabolism and prevention of renal calcifications. ( 103‐106 ) Preliminary clinical data also support the potential utility of calcilytics to treat humans with ADH1. Briefly, intravenous exposure to the calcilytic NPSP795 in five patients with ADH1 was associated with dose‐related increase in PTH and decrease in fractional calcium excretion, although without a clear increase in circulating calcium.…”

Genetics of monogenic disorders of calcium and bone metabolism

“…102,103 Preclinical and clinical studies have also indicated the potential utility of calcilytic drugs, which are CaSR negative allosteric modulators, as targeted therapies for ADH1. 100,[104][105][106] Patients with ADH1 may also exhibit ectopic calcification, and some develop a Bartter syndrome, characterized by renal salt wasting leading to volume depletion, hyper-reninaemic hyperaldosteronism, and hypokalaemic alkalosis. 61 Only a few kindreds with ADH2 have been reported and these patients have similar biochemical features to ADH1, although appear to have a milder urinary phenotype with less hypercalciuria.…”

“…Small clinical studies have reported that recombinant PTH is effective for treating symptomatic ADH1 patients without increasing urine calcium excretion 102,103 . Preclinical and clinical studies have also indicated the potential utility of calcilytic drugs, which are CaSR negative allosteric modulators, as targeted therapies for ADH1 100,104–106 . Patients with ADH1 may also exhibit ectopic calcification, and some develop a Bartter syndrome, characterized by renal salt wasting leading to volume depletion, hyper‐reninaemic hyperaldosteronism, and hypokalaemic alkalosis 61 .…”

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

“…In contrast, a preliminary study from Roberts et al [ 88 ] demonstrated that calcilytics (NPSP795, negative allosteric modulators) can rapidly stimulate PTH secretion but that they do not increase serum calcium in hypoparathyroidism because of CaSR’s gain of function (autosomal dominant hypocalcemia type 1 (ADH1)). The effect of this compound on phosphatemia remains unknown in humans, but it significantly increases phosphatemia in wild-type and ADH1 mouse models [ 89 ]. Further studies are needed to assess if calcilytics will find a place in treating phosphatemia related disorders.…”