Calcium-sensing receptor activator cinacalcet for treatment of cyclic nucleotide-mediated secretory diarrheas

Cyclic nucleotide elevation in intestinal epithelial cells is the key pathology causing intestinal fluid loss in secretory diarrheas such as cholera. Current secretory diarrhea treatment is primarily supportive, and oral rehydration solution is the mainstay of cholera treatment. There is an unmet need for safe, simple and effective diarrhea treatments. By promoting cAMP hydrolysis, extracellular calcium-sensing receptor (CaSR) is a regulator of intestinal fluid transport. We studied the antidiarrheal mechanisms of FDA-approved CaSR activator cinacalcet and tested its efficacy in clinically relevant human cell, mouse and intestinal organoid models of secretory diarrhea. By using selective inhibitors, we found that cAMP agonists-induced secretory short-circuit currents (Isc) in human intestinal T84 cells are mediated by collective actions of apical membrane cystic fibrosis transmembrane conductance regulator (CFTR) and Clc-2 Cl- channels, and basolateral membrane K+ channels. 30 μM cinacalcet pretreatment inhibited all 3 components of forskolin and cholera toxin-induced secretory Isc by ∼75%. In mouse jejunal mucosa, cinacalcet inhibited forskolin-induced secretory Isc by ∼60% in wild type mice, with no antisecretory effect in intestinal epithelia-specific Casr knockout mice (Casr-flox; Vil1-cre). In suckling mouse model of cholera induced by oral cholera toxin, single dose (30 mg/kg) oral cinacalcet treatment reduced intestinal fluid accumulation by ∼55% at 20 hours. Lastly, cinacalcet inhibited forskolin-induced secretory Isc by ∼75% in human colonic and ileal organoids. Our findings suggest that CaSR activator cinacalcet has antidiarrheal efficacy in distinct human cell, organoid and mouse models of secretory diarrhea. Considering its excellent clinical safety profile, cinacalcet can be repurposed as a treatment for cyclic nucleotide-mediated secretory diarrheas including cholera.

[New scenarios in secondary hyperparathyroidism: etelcalcetide. Position paper of Nephrologists form Lombardy]

Bone mineral abnormalities (defined as Chronic Kidney Disease Mineral Bone Disorder; CKD-MBD) are prevalent and associated with a substantial risk burden and poor prognosis in CKD population. Several lines of evidence support the notion that a large proportion of patients receiving maintenance dialysis experience a suboptimal biochemical control of CKD-MBD. Although no study has ever demonstrated conclusively that CKD-MBD control is associated with improved survival, an expanding therapeutic armamentarium is available to correct bone mineral abnormalities. In this position paper of Lombardy Nephrologists, a summary of the state of art of CKD-MBD as well as a summary of the unmet clinical needs will be provided. Furthermore, this position paper will focus on the potential and drawbacks of a new injectable calcimimetic, etelcalcetide, a drug available in Italy since few months ago.

Structure, pharmacology and therapeutic prospects of family C G-protein coupled receptors

Family C of G-protein coupled receptors (GPCRs) from humans is constituted by eight metabotropic glutamate (mGlu(1-8)) receptors, two heterodimeric gamma-aminobutyric acid(B) (GABA(B)) receptors, a calcium-sensing receptor (CaR), three taste (T1R) receptors, a promiscuous L-alpha-amino acid receptor (GPRC6A), and five orphan receptors. Aside from the orphan receptors, the family C GPCRs are characterised by a large amino-terminal domain, which bind the endogenous orthosteric agonists. Recently, a number of allosteric modulators binding to the seven transmembrane domains of the receptors have also been reported. Family C GPCRs regulate a number of important physiological functions and are thus intensively pursued as drug targets. So far, two drugs acting at family C receptors (the GABA(B) agonist baclofen and the positive allosteric CaR modulator cinacalcet) have been marketed. Cinacalcet is the first allosteric GPCR modulator to enter the market, which demonstrates that the therapeutic principle of allosteric modulation can also be extended to this important drug target class. In this review we outline the structure and function of family C GPCRs with particular focus on the ligand binding sites, and we present the most important pharmacological agents and the therapeutic prospects of the receptors.

Therapeutic use of calcimimetics

It has long been recognized that the secretion of PTH by chief cells in the parathyroid gland is regulated by extracellular ionized calcium. The molecular mechanism by which extracellular Ca2+ performs this feat was deduced by the cloning of the extracellular calcium-sensing receptor (CaSR) in 1993 in the laboratories of Brown and Hebert. The CaSR is a G protein-coupled cell surface receptor that belongs to family 3 of the GPCR superfamily. The CaSR senses the extracellular ionic activity of the divalent minerals Ca2+ and Mg2+ and translates this information, via a complex array of cellular signaling pathways, to modify cell and tissue function. Genetic studies have demonstrated that the activity of this receptor determines the steady-state plasma calcium concentration in humans by regulating key elements in the calcium homeostatic system. CaSR agonists (calcimimetics) and antagonists (calcilytics) have been identified and have provided both current and potential therapies for a variety of disorders. Calcimimetics can effectively reduce PTH secretion in all forms of hyperparathyroidism. They are likely to become a major therapy for secondary hyperparathyroidism associated with renal failure and for treatment of certain patients with primary hyperparathyroidism. On the therapeutic horizon are calcilytics that can transiently increase PTH and may prove useful in the treatment of osteoporosis.