Etelcalcetide

What is Etelcalcetide?

Etelcalcetide (brand name PARSABIV, formerly AMG 416 / KAI-4169) is a synthetic peptide calcimimetic that allosterically activates the calcium-sensing receptor (CaSR) on parathyroid chief cells. It was approved by the FDA on February 7, 2017 for secondary hyperparathyroidism (SHPT) in adults with chronic kidney disease on hemodialysis.

It is the first and only peptide-based calcimimetic. The previously available calcimimetic, cinacalcet (Sensipar), is an oral small molecule. Etelcalcetide's unique feature is intravenous administration three times per week at the end of hemodialysis sessions, eliminating concerns about oral adherence and gastrointestinal tolerability.

Structure

Etelcalcetide is built almost entirely from D-amino acids, which makes it highly resistant to enzymatic degradation. The structure is:

• A core D-arginine-rich heptapeptide (D-Cys, D-Ala, D-Arg, D-Arg, D-Arg, D-Ala, D-Arg-amide)
• N-terminally acetylated
• Connected via a disulfide bond to a single L-cysteine residue (this L-Cys engages the CaSR through a covalent disulfide exchange with the receptor)

The combination of D-amino acid stability and a defined covalent receptor-binding mechanism produces a peptide with predictable pharmacokinetics and high specificity.

Mechanism of Action

• Allosteric CaSR activation — etelcalcetide binds an allosteric site on the CaSR distinct from the orthosteric calcium-binding site
• Disulfide exchange — the L-Cys residue forms a transient disulfide with a CaSR cysteine residue, distinguishing etelcalcetide's mechanism from cinacalcet's pure non-covalent binding
• Sensitization to calcium — receptor activation makes the parathyroid more responsive to ambient extracellular calcium
• Reduced PTH secretion — increased CaSR signaling tonically inhibits PTH gene transcription and exocytosis
• Reduced FGF23, calcium, and phosphate — secondary effects of normalized PTH

In SHPT, chronic uremia and phosphate retention drive parathyroid hyperplasia and excessive PTH secretion. Etelcalcetide pharmacologically restores CaSR-mediated suppression of the parathyroid.

Clinical Evidence

Phase 3 placebo-controlled trial (JAMA 2017):

• 1,023 hemodialysis patients with SHPT randomized to etelcalcetide IV vs placebo for 26 weeks
• ≥30% reduction in PTH at weeks 20-27: 77% etelcalcetide vs 11% placebo (p<0.001)
• Significant reductions in serum calcium, phosphate, and FGF23

Phase 3 head-to-head vs cinacalcet (JAMA 2017):

• 683 hemodialysis patients randomized to etelcalcetide IV vs oral cinacalcet
• Etelcalcetide non-inferior on primary endpoint
• Lower rates of nausea/vomiting (cinacalcet's main tolerability problem)
• Lower rates of dose interruption

Approval History

• February 7, 2017 — FDA approval for SHPT in adults on hemodialysis
• November 2016 — EMA approval (predates FDA)

Place in Therapy

Etelcalcetide is reserved for hemodialysis patients with SHPT inadequately controlled by phosphate binders, vitamin D analogs, and dietary measures. The IV administration eliminates oral adherence problems and the GI side effects (nausea, vomiting) that affect ~30% of patients on oral cinacalcet.

It is dosed by the dialysis center staff at the end of each dialysis session, three times per week, with titration based on PTH and calcium response.

Safety Profile

The most common adverse events reflect the on-target pharmacology:

• Hypocalcemia — most common; requires monitoring of serum calcium
• Decreased serum calcium-corrected QT interval — dose adjustment needed in cardiac comorbidity
• Muscle spasms, paresthesias, headache — generally mild

Less common: heart failure exacerbation in patients with reduced ejection fraction. Etelcalcetide is contraindicated in patients with serum calcium below the lower limit of normal at baseline.

Distinction from Cinacalcet