Etelcalcetide
AMG 416, velcalcetide, KAI-4169, etelcalcetide hydrochloride
Etelcalcetide is a small synthetic peptide built almost entirely from D-amino acids that lowers parathyroid hormone in people on haemodialysis with secondary hyperparathyroidism. Unusually for a drug, it forms a covalent disulfide bond with its own receptor. It is given intravenously at the end of a dialysis session, which sidesteps the adherence and gastrointestinal problems of the oral small-molecule calcimimetic it competes with.
Mechanism
Etelcalcetide targets the calcium-sensing receptor (CaSR), a class C G-protein-coupled receptor whose large bilobed venus flytrap extracellular domain senses extracellular ionised calcium. On the parathyroid chief cell, CaSR activation suppresses parathyroid hormone synthesis and secretion. In secondary hyperparathyroidism of chronic kidney disease, the CaSR set point drifts and parathyroid mass expands, so hormone output stays inappropriately high despite phosphate binders and vitamin D analogues.
What makes etelcalcetide distinctive is how it engages the receptor. Unlike cinacalcet, which is a small-molecule positive allosteric modulator binding within the transmembrane domain, etelcalcetide binds a site in the venus flytrap domain and then undergoes thiol-disulfide exchange with the free cysteine at position 482 of the receptor, near the hinge between the two lobes. The L-cysteine carried on the peptide is displaced and a direct covalent disulfide forms between the D-cysteine of the drug and Cys482 of the CaSR. Site-directed mutagenesis showed that the degree of that covalent bond formation tracks pharmacological activity, and that mutating Cys482 abolishes it. The result is a durable stabilisation of the active receptor conformation, lowering the calcium concentration needed to trigger signalling, so parathyroid hormone falls. The all-D-amino-acid backbone with N-acetyl and C-amide caps confers near-complete resistance to proteolysis, which is what allows a seven-residue peptide to have a multi-day half-life. In whole blood the drug also exchanges onto serum albumin to form a covalent serum albumin peptide conjugate, which acts as a circulating reservoir and contributes to the long apparent half-life. Calling etelcalcetide a calcimimetic is conventional but slightly imprecise: it is a covalent allosteric activator, a mechanism with very few parallels among approved drugs.
What the research shows
Two identically designed randomised, double-blind, placebo-controlled phase 3 trials enrolled just over a thousand haemodialysis patients with moderate to severe secondary hyperparathyroidism and treated them for 26 weeks. In both, a substantially greater proportion of etelcalcetide recipients achieved a reduction in parathyroid hormone of more than 30 per cent from baseline, with parallel falls in serum calcium, phosphate and fibroblast growth factor 23. A separate head-to-head trial randomised 683 patients to intravenous etelcalcetide or oral cinacalcet under double-dummy conditions; etelcalcetide was non-inferior on the primary endpoint of parathyroid hormone reduction and also met the pre-specified superiority criterion, with fewer patients reporting vomiting though more experiencing low calcium.
The honest limitation of this evidence base is the endpoint. Every pivotal trial was powered on a biochemical surrogate (parathyroid hormone concentration), not on fracture, cardiovascular events, need for parathyroidectomy or mortality. No trial has demonstrated that lowering parathyroid hormone with etelcalcetide reduces any of those. That gap matters more than usual here, because the class precedent is discouraging: the large EVOLVE trial of the oral calcimimetic cinacalcet, with over 3,800 haemodialysis patients and a composite cardiovascular and mortality primary endpoint, did not meet significance in its unadjusted intention-to-treat analysis. Benefit appeared only in analyses adjusted for baseline imbalance and in secondary and per-protocol analyses, which is not the same thing. Anyone reading etelcalcetide's excellent parathyroid hormone data should hold that alongside it. Hypocalcaemia is also not a marginal issue: reductions in serum calcium were common in all the phase 3 trials, and symptomatic hypocalcaemia, QT prolongation and seizures appear in the labelling as consequences. The draft version of this entry also referred to a completed paediatric phase 3 programme; no such publication was confirmed during this audit, so that claim has been removed.
Evidence assessment
High-quality evidence
Replicated, adequately powered, double-blind, placebo-controlled phase 3 randomised trials plus a double-dummy active-comparator trial, and regulator-approved labelling in the US, EU and UK. All four citations confirmed against PubMed. The tier reflects the quality of the randomised evidence for the licensed biochemical indication; it should not be read as evidence that hard clinical outcomes improve, which has not been shown.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Effect of etelcalcetide vs placebo on serum parathyroid hormone in patients receiving hemodialysis with secondary hyperparathyroidism: two randomized clinical trials Preclinical only
A substantially greater proportion of etelcalcetide than placebo recipients achieved more than 30 per cent reduction in parathyroid hormone, with concurrent falls in serum calcium, phosphate and FGF23. Hypocalcaemia was markedly more frequent on active treatment. The primary endpoint was biochemical, not clinical.
Effect of etelcalcetide vs cinacalcet on serum parathyroid hormone in patients receiving hemodialysis with secondary hyperparathyroidism: a randomized clinical trial Preclinical only
Etelcalcetide was non-inferior to cinacalcet for reduction in parathyroid hormone and also met the pre-specified superiority criterion. Vomiting was less frequent with etelcalcetide, but low serum calcium was more frequent. Again a biochemical primary endpoint.
Critical cysteine residues in both the calcium-sensing receptor and the allosteric activator AMG 416 underlie the mechanism of action Preclinical only
Established that etelcalcetide activity depends on formation of a covalent disulfide bond with cysteine 482 in the receptor's venus flytrap domain, and that the degree of bond formation correlates with pharmacological activity. Defines an unusual covalent allosteric mechanism distinct from small-molecule calcimimetics.
Effect of cinacalcet on cardiovascular disease in patients undergoing dialysis Preclinical only
The oral calcimimetic cinacalcet did not significantly reduce the composite primary endpoint in the unadjusted intention-to-treat analysis. Included here because it is the only hard-outcome trial in this drug class and directly tempers the interpretation of etelcalcetide's biochemical results. It studied cinacalcet, not etelcalcetide.
Safety
Hypocalcaemia is the defining risk and is common rather than occasional: the majority of patients in the phase 3 trials had at least one serum calcium value below the normal range, and symptomatic hypocalcaemia occurred in a meaningful minority. Consequences appearing in the labelling include paraesthesia, muscle spasm, carpopedal spasm, tetany, convulsions, and QT interval prolongation with a resulting risk of ventricular arrhythmia. Nausea, vomiting and diarrhoea are frequent, though vomiting was less common than with oral cinacalcet in the head-to-head trial. Worsening heart failure has been reported, plausibly linked to the negative inotropic consequences of reduced serum calcium. Upper gastrointestinal bleeding appears as a warning, a class observation shared with cinacalcet, and caution is described in patients with predisposing conditions or on antithrombotic therapy. Over-suppression of parathyroid hormone risks adynamic bone disease, in which bone turnover falls so low that the skeleton loses its ability to buffer calcium loads and to repair microdamage. Etelcalcetide is efficiently cleared by dialysis, which shapes when it is given. It has not been studied in patients on peritoneal dialysis, in non-dialysis chronic kidney disease, or in primary hyperparathyroidism, and is not indicated in those settings. This entry is educational and does not describe dosing, timing or administration.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | Authorised for secondary hyperparathyroidism in adults with chronic kidney disease on haemodialysis, following EU centralised approval in November 2016. It is administered as an intravenous bolus at the end of the haemodialysis session and is used within specialist renal services. Not indicated outside the haemodialysis population. |
| United States | FDA-approved in February 2017 for the treatment of secondary hyperparathyroidism in adults with chronic kidney disease on haemodialysis. It is not indicated in chronic kidney disease patients not on haemodialysis, or in primary hyperparathyroidism. Labelling carries warnings for hypocalcaemia, worsening heart failure, upper gastrointestinal bleeding and adynamic bone disease. |
| WADA (sport) | Not named on the WADA Prohibited List. Etelcalcetide suppresses parathyroid hormone secretion via the calcium-sensing receptor and has no recognised performance-enhancing action, nor does it fall within the S2 growth factor provisions. Athletes should verify against the current list. |
Questions
It is a peptide: a seven-residue chain of D-amino acids with an N-acetyl cap, a C-terminal amide, and an extra L-cysteine attached by a disulfide bond. At about 1,048 daltons it sits at the boundary between peptide and small molecule in size, but it is unambiguously peptidic in construction. Cinacalcet, its main comparator, is a true small molecule.
Mammalian proteases are stereospecific and cleave L-amino acid peptides. Building the backbone from D-enantiomers makes the molecule essentially invisible to those enzymes. Combined with N-terminal acetylation and C-terminal amidation, this is what lets a seven-residue peptide survive in circulation for days rather than minutes.
Rather than simply docking reversibly, the drug undergoes thiol-disulfide exchange with cysteine 482 in the calcium-sensing receptor's extracellular venus flytrap domain, forming a genuine covalent bond. The L-cysteine it carries is released in the process. Mutating that receptor cysteine abolishes activity. Covalent mechanisms of this kind are rare among approved drugs.
That has not been demonstrated. All the pivotal etelcalcetide trials used parathyroid hormone concentration as the endpoint. The one large hard-outcome trial in this drug class, EVOLVE, tested cinacalcet against a composite of cardiovascular events and death and did not meet significance in its primary intention-to-treat analysis. The biochemical effect of etelcalcetide is clear; the clinical consequence of that effect is not.
Etelcalcetide is efficiently removed by the dialysis membrane, so giving it during a session would strip much of it out. Administering it at the end of the session means the dose is retained, and it also removes the adherence problem that limits oral calcimimetics. The dialysis team gives it rather than the patient having to remember.