ATC Group: H05B Anti-parathyroid agents

Vitamin D₃ must undergo a two-step metabolic process to be active; the first step occurs in the microsomal fraction of the liver where Vitamin D is hydroxylated at position 25 (25-hydroxycholecalciferol or calcifediol); the second step takes place in the kidney where 1,25-dihydroxycholecalciferol or calcitriol is formed due to the activity of enzyme 25-hydroxycholecalciferol 1-hydroxylase. Vitamin D increases absorption of calcium and phosphorus in the intestine and improves normal bone formation and mineralization.

Calcitonin is a calciotropic hormone, which inhibits bone resorption by a direct action on osteoclasts. By inhibiting osteoclast activity via its specific receptors, salmon calcitonin decreases bone resorption.

Cinacalcet is a calcimimetic agent which directly lowers PTH levels by increasing the sensitivity of the calcium sensing receptor to extracellular calcium. The reduction in PTH is associated with a concomitant decrease in serum calcium levels.

Doxercalciferol is a synthetic vitamin D₂ analog that requires metabolic activation to form the active 1α,25-(OH)₂D₂ metabolite, which binds to the vitamin D receptor (VDR) to result in the selective activation of vitamin D responsive pathways. Vitamin D and doxercalciferol have been shown to reduce PTH levels by inhibiting PTH synthesis and secretion.

Etelcalcetide is a synthetic peptide calcimimetic agent which reduces PTH secretion through binding and activation of the calcium-sensing receptor. Etelcalcetide is indicated for the treatment of secondary hyperparathyroidism (SHPT) in adult patients with chronic kidney disease (CKD) on haemodialysis therapy.

Paricalcitol is a synthetic, biologically active vitamin D analog of calcitriol with modifications to the side chain (D₂) and the A (19-nor) ring. Unlike calcitriol, it is a selective vitamin D receptor (VDR) activator. Paricalcitol reduces parathyroid hormone (PTH) levels by inhibiting parathyroid proliferation and decreasing PTH synthesis and secretion, with minimal impact on calcium and phosphorus levels, and can act directly on bone cells to maintain bone volume and improve mineralization surfaces.