Endocrine Abstracts

The G-protein coupled calcium-sensing-receptor (CaSR) regulates calcium homeostasis and inactivating mutations result in familial hypocalciuric hypercalcaemia (FHH), whilst activating mutations result in autosomal dominant hypocalcaemia with hypercalciuria (ADHH). Allosteric CaSR modulators consist of: calcimimetics, which activate the CaSR e.g. Cinacalcet, that is used to treat the hypercalcaemia of chronic renal failure and metastatic parathyroid carcinoma; and calcilytics e.g. NPS2143 that antagonise the CaSR. We have assessed NPS2143 for use in ADHH by in vitro and in vivo studies utilising HEK293 cells transiently transfected with wild-type or mutant CaSRs, and a hypocalcaemic mouse model, Nuf, that harbours an activating CaSR mutation, Leu723Gln. Mice were used in accordance with UK Home Office welfare guidelines and project license restrictions. In vitro studies using HEK293 cells demonstrated that 20 nM NPS2143 restored the significantly lower EC₅₀ (1.94 mM±0.07, P< 0.01) of the mutant CaSR (Gln723) to a value (2.79 mM ±0.19) that was indistinguishable from that of the wild-type (Leu723) (EC₅₀=2.53 mM±0.14, P=0.33). Moreover, 40 and 80 nM NPS2143 significantly raised the EC₅₀ (3.97±0.1 and 4.56±0.01 respectively, P< 0.001) of the mutant Gln723 CaSR. In vivo studies, using heterozygous Nuf and wild-type mice that were given 1.1mg of NPS2143 intraperitoneally, demonstrated that the hypocalcaemia of Nuf mice was significantly improved from 1.74±0.02 to 2.15 mmol/l±0.05 (P<0.0001) at 1 h, and 2.30 mmol/l±0.08 (P<0.0001) at 24 h, which approached the values observed in wild-type mice (2.46 mmol/l±0.03). Thus, NPS2143 has a significant effect on the EC₅₀, and hence set-point, of the Nuf mutant CaSR and NPS2143 rectifies, in vivo, the hypocalcaemia of Nuf mice. Our findings indicate that calcilytics may be a useful treatment for ADHH.