A case of a hemodialysis patient with secondary hyperparathyroidism who was resistant to etelcalcetide treatment but not to cinacalcet hydrochloride

Although both cinacalcet and etelcalcetide are calcimimetics that directly inhibit parathyroid hormone (PTH) secretion by activating the calcium (Ca)-sensing receptor (CaSR), their binding sites are different. We report a first case of a hemodialysis (HD) patient with secondary hyperparathyroidism (SHPT), in whom cinacalcet, but not etelcalcetide, could reduce serum intact PTH (i-PTH) levels. A HD patient received total parathyroidectomy (PTx) with auto-transplantation 16 years earlier. Due to SHPT relapse, cinacalcet was started at 7 years after PTx. His i-PTH levels had been controlled with both 75–100 mg of cinacalcet and 4.5 μg/week of calcitriol for a year before switching from cinacalcet to etelcalcetide. At 1 month following the switch, his serum i-PTH level increased to 716 pg/mL. The dose of etelcalcetide was gradually increased and finally reached the maximal dose of 45 mg/week. Because even the maximal dose of etelcalcetide for > 4 months did not reduce his serum i-PTH levels to < 700 pg/mL, etelcalcetide was switched to 50 mg/day of cinacalcet, which reduced the levels to 208 pg/mL at 2 months after the switch. Genomic sequencing test using whole blood revealed no mutation in the portion including Cys 482 of CaSR gene. The patient was resistant to etelcalcetide treatment but not to cinacalcet, suggesting the possibility that the enlarged parathyroid gland has some change in the portion including Cys 482 in the CaSR gene. Therefore, considering the possibility of etelcalcetide resistance during SHPT treatment should be kept in mind.

Familial hypocalciuric hypercalcemia and related disorders

Familial hypocalciuric hypercalcemia (FHH) causes hypercalcemia by three genetic mechanisms: inactivating mutations in the calcium-sensing receptor, the G-protein subunit α₁₁, or adaptor-related protein complex 2, sigma 1 subunit. While hypercalcemia in other conditions causes significant morbidity and mortality, FHH generally follows a benign course. Failure to diagnose FHH can result in unwarranted treatment or surgery for the mistaken diagnosis of primary hyperparathyroidism (PHPT), given the significant overlap of biochemical features. Determinations of urinary calcium excretion greatly aid in distinguishing PHPT from FHH, but overlap still exists in certain cases. It is important that 24-h urine calcium and creatinine be included in the initial workup of hypercalcemia. FHH should be considered if low or even low normal urinary calcium levels are found in what is typically an asymptomatic hypercalcemic patient. The calcimimetic cinacalcet has been used to treat hypercalcemia in certain symptomatic causes of FHH.

Computational analysis of the extracellular domain of the Ca²⁺-sensing receptor: an alternate model for the Ca²⁺ sensing region

The extracellular Ca(2+) sensing receptor (CaSR) belongs to Class C G-protein-coupled receptors (GPCRs) which include receptors for amino acids, γ-aminobutyric acid and glutamate neurotransmitters. CaSR has been described as having an extended sequence containing a Ca(2+) binding pocket within an extracellular amino (N)-terminal domain, called a Venus Fly Trap (VFT) module. CaSR is thought to consist of three domains: 1) a Ca(2+-)sensory domain, 2) a region containing 7 transmembrane (TM) helices, and 3) a carboxy (C)-terminal tail. We find that SPOCTOPUS (a combination of hidden Markov models and artificial neural networks) predicts that Homo sapiens CaSR contains two additional TM helices ((190)D - G(210); (262)S-E(282)), with the second TM helix containing a pore-lining region ((265)K - I(280)). This predicts that the putative Ca(2+) sensory domain is within an extracellular loop, N-terminal to the highly conserved heptahelical bundle. This loop contains both the cysteine-rich domain ((537)V - C(598)) and a 14 residue "linker" sequence ((599)I - F(612)) thought to support signal transmission to the heptahelical bundle. Thus domain 1 may contain a 189 residue N-terminal extracellular region followed successively by TM-1, a short intracellular loop, TM-2 and a 329 residue extracellular loop; rather than the proposed 620 residue VFT module based on crystallography of the N-terminal region of mGluR1. Since the topologies of the two proteins differ, the published CaSR VFT model is questionable. CaSR also contains multiple caveolin-binding motifs and cholesterol-binding (CRAC/CARC) domains, facilitating localization to plasma membrane lipid rafts. Ion sensing may involve combination of pore-lining regions from CaSR dimers and CaSR-bound caveolins to form ion channels capable of monitoring ionized Ca(2+) levels.