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

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 Ca²⁺-mediated intracellular calcium and phosphorylated ERK responses. An in vivo dose-ranging 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 ± 0.03 mmol/L to 2.16 ± 0.06 mmol/L, and in Casr ^Nuf/Nuf mice from 1.70 ± 0.03 mmol/L to 1.89 ± 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. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.