Activating mutations of the G protein-coupled receptor, calcium-sensing receptor (CaSR), cause the rare and difficult to treat autosomal dominant hypocalcemia (ADH) and Bartter syndrome type 5 (BS5).
These mutations lower the set-point for extracellular Ca2+ sensing. This leads to increased parathyroid hormone (PTH) secretion. Normally, PTH stimulates Ca2+ reabsorption in the kidney and inhibits disposal, in addition to regulating skeletal Ca2+ release and uptake in bone. PTH effects are mediated via the type-1 PTH receptor (PTH1R) that couples to the small G protein Gs and activates cAMP signalling. PTH also stimulates the 1/-hydroxylation of vitamin D compounds in the kidney, and indirectly stimulates Ca2+ uptake in the gut. The effects of PTH and vitamin D in kidney, bone, and gut elevate serum Ca2+ levels and complete the classic regulatory feedback loop of the Ca2+ homeostasis. Patients with activating mutations have absent or reduced parathyroid hormone (PTH) owing to CaSR mediated hypocalcaemia and suppression of PTH secretion, and development of hypercalciuria. The mechanism of this regulation and the involvement of [Ca2+]]i , ERK1/2, or other signalling mechanisms is under investigation but is not completely understood. Nephrocalcinosis and nephrolithiasis, an increased risk of calcifications of the basal ganglia and cataract formation and osteoporosis are frequent findings 1.
There are several established clinically effective therapeutic alternatives for the hypercalcemic disorders e.g. Familial Hypercalciuric Hypercalcaemia (FHH), as a result of inactivating mutations of the CaSR, however the hypocalcemic disorders are difficult to treat.
Vitamin D analogs and Ca2+ supplementation is standard treatment to raise serum Ca2+ levels in ADH patients. However, this inevitably aggravates the hypercalciuria which frequently limits attempts to normalize serum Ca2+. Several other treatments e.g. hydrochlorothiazide was found to temporarily reduce hypercalciuria, and replacement of PTH1–34 (teriparatide) was generally effective in reducing symptoms and/or raising serum Ca2+. Although urinary Ca2+ excretion declined or was constant despite rising serum Ca2+ levels in most patients, nephrocalcinosis can develop despite continuous PTH therapy. In ADH the kidney responds with increased Ca2+ excretion to an increased filtered Ca2+ load produced by any rise of serum Ca2+ levels. This most likely further elevates the risk of renal complications. The effects of CaSR predominate the effects of PTH on tubular reabsorption of Ca2+ in the kidney. For this reason, the activated CaSR in ADH and BS5 reduces the desired effects, and increases the adverse effects, of conventional therapeutic attempts 2.
At present then most patients remain symptomatic despite intensive therapy with all available strategies. At best, the available therapeutic options reduce symptoms without further increasing the elevated risk of tissue complications. Complete symptom relief and the reduction of long-term complications caused by the underlying disease itself are currently not achievable and none of the current therapeutic strategies corrects the underlying pathophysiology.
Future Pharmacological Intervention
There is a need to target the defect in CaSR to correct the underlying pathophysiology, and functional modulation of CaSR can be achieved by allosteric modulators. A positive allosteric CaSR modulator, the calcimimetic cinacalcet is presently used to treat FHH. Negative allosteric modulators or calcilytics directly inhibit CaSR and have been developed to stimulate endogenous PTH secretion, as an alternative to injections, to promote bone formation in osteoporosis. These drugs may be useful in reducing excessive CaSR activity in ADH and BS5 patients3.
Negative Allosteric Modulators in vitro and in rodents
To date only wild-type receptors or enzymes have been studied in vitro. To pharmacologically correct the molecular cause of ADH and BS5, the function of altered CaSR or G protein/11 (GNA11), will need to be rectified and the efficacy of calcilytics needs to be established. Over 70 activating CaSR and GNA11 mutations that cause ADH or BS5 have been described and functional in vitro tests with calcilytics have been reported for 32 activating CaSR and two activating GNA11 point mutants, and all were sensitive to at least one calcilytic in vitro. Four of these mutants (A840 V, Q245R, E228K, and E228A) were also sensitive in vivo4.
Studies to date have used amino-alcohol compounds Ronacaleret (SB-751689); NPSP795 (SB- 423562); SB-423557 (prodrug of NPSP795); MK-5442 (JTT-305) and finally the quinazolinone AXT914.
The in vivo effects of calcilytics have been studied in CaSR C129S and A843E knock-in mouse models. These models mimic the human ADH phenotype, with decreased serum Ca2+ and PTH, and increased serum phosphate levels. These mice also showed hypercalciuria and reduced urinary cAMP excretion. MK-5442 or NPS 2143 increased serum Ca2+ and PTH but stabilized or even decreased urinary Ca2+ excretion. MK-5442 also increased urinary cAMP excretion, and administration for 3 months reduced Ca2+ excretion and prevented renal calcification. By contrast, long-term subcutaneous PTH injections also increased serum Ca2+ but failed to reduce Ca2+ excretion and did not prevent renal calcification 4,5.
Negative Allosteric Modulators in Clinical Trial
In man, phase I and II clinical trials with calcilytics with a particularly short serum half-life have been developed to stimulate endogenous PTH secretion, as an oral alternative to PTH injections, to enhance bone formation in osteoporosis in postmenopausal women. Serum Ca2+ levels were normal at baseline in these participants and therefore they were presumed to have wild-type CaSR. Serum Ca2+, phosphate, and PTH levels were determined, as well as renal Ca2+ excretion 6.
All drugs were administered orally once daily, with the exception of NPSP795, which was given intravenously. These trials for were relatively unsuccessful for osteoporosis and no alteration in bone mineral density was appreciated. However, a dose-dependent increase in serum Ca2+ levels was consistently observed. Calcilytics are so far generally well tolerated with most common adverse effects being mild i.e. fatigue, headache, constipation, diarrhea, nausea, and dyspepsia.
These studies performed in wild-type CaSR patients provide valuable insight into the possible therapeutic use in ADH patients.
Calcilytics in ADH patients
A small Phase II trial with the calcilytic NPSP795 for the treatment of five ADH patients harbouring the A840 V, Q245R, E228K, and E228A mutations have been reported recently. Here, serum Ca2+ levels were maintained despite fasting and no Ca2+ or vitamin D supplementation with one participant developing hypercalcemia 7. No clinical trials have been performed in patients with BS5.
All calcilytic trials discussed reported a short-lived dose-dependent increases in PTH and Ca2+, followed by a return to baseline 6–12 h after administration. These results are in line with the rapid pharmacokinetic profiles of ronacaleret 6 and AXT914 (tmax 1–2 h, t½ 4–5 h) 8 and NPSP795 (t½ < 1h after i.v. administration, tmax 2-3h, t½ 1.4-3.7h after oral administration of the prodrug SB423557).
The results of these clinical trials are promising. Considering that different calcilytics from different chemical classes (amino-alcohol and quinazolinones) with different modes of action exert similar biochemical effects, implies that these effects are specific to calcilytics, and are not a single compound or chemical class effect, broadening the opportunity for calcilytic development. Taken together, there is still an unmet need for ADH patients and the repurposing of calcilytics for in this population is a promising approach for these difficult to treat diseases.
Blog written by Elizabeth Owen
- Mayr, B.M. et al. (2015) Genetics in endocrinology: gain and loss of function mutations of the calcium sensing receptor and associated proteins: current treatment concepts. Eur. J. Endocrinol. 174, R189–R208
- Mitchell, D.M. et al. (2012) Long-term follow-up of patients with hypoparathyroidism. J. Clin. Endocrinol. Metab. 97, 4507–4514
- Hu, J. and Spiegel, A.M. (2007) Structure and function of the human calcium-sensing receptor: insights from natural and engineered mutations and allosteric modulators. J. Cell. Mol. Med. 11, 908–922
- Dong, B. et al. (2015) Calcilytic ameliorates abnormalities of mutant calcium-sensing receptor (CaSR) knock-in mice mimicking autosomal dominant hypocalcemia (ADH). J. Bone Miner. Res. 30, 1980–1993
- Hannan, F.M. et al. (2015) The calcilytic agent NPS 2143 rectifies hypocalcemia in a mouse model with an activating calcium-sensing receptor (CaSR) mutation: relevance to autosomal dominant hypocalcemia type 1 (ADH1). Endocrinology 156, 3114–3121
- Caltabiano, S. et al. (2013) Characterization of the effect of chronic administration of a calcium-sensing receptor antagonist, ronacaleret, on renal calcium excretion and serum calcium in postmeno- pausal women. Bone 56, 154–162
- Ramnitz, M. et al. (2015) Treatment of autosomal dominant hypocalcemia with the calcilytic NPSP795. J. Bone Miner. Res. 30 (Suppl. 1), SA0002
- John, M.R. et al. (2014) AXT914 a novel, orally-active parathyroid hormone-releasing drug in two early studies of healthy volunteers and postmenopausal women. Bone 64C, 204–210