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Pharmacotherapeutic group: Vitamin D3, Cholecalciferol.
Pharmacology: Pharmacodynamics: In its biologically active form Vitamin D3 stimulates intestinal calcium absorption, incorporation of calcium into the osteoid, and release of calcium from bone tissue. In the small intestine it promotes rapid and delayed calcium uptake. The passive and active transport of phosphate is also stimulated. In the kidney, it inhibits the excretion of calcium and phosphate by promoting tubular resorption. The production of parathyroid hormone (PTH) in the parathyroids is inhibited directly by the biologically active form of Vitamin D3. PTH secretion is inhibited additionally by the increased calcium uptake in the small intestine under the influence of biologically active Vitamin D3.
Pharmacokinetics: The pharmacokinetics of Vitamin D3 is well known.
Absorption: Vitamin D3 is well absorbed from the gastro-intestinal tract in the presence of bile, so the administration with the major meal of the day might therefore facilitate the absorption of Vitamin D3.
Distribution and biotransformation: It is hydroxylated in the liver to form 25-hydroxy-colecalciferol and then undergoes further hydroxylation in the kidney to form the active metabolite 1, 25-dihydroxycolecalciferol (calcitriol).
Elimination: The metabolites circulate in the blood bound to a specific α-globulin. Vitamin D3 and its metabolites are excreted mainly in the bile and faeces.
Characteristics in Specific Groups of Subjects or Patients: A 57% lower metabolic clearance rate is reported in subjects with renal impairment as compared with that of healthy volunteers. Decreased absorption and increased elimination of Vitamin D3 occurs in subjects with malabsorption. Obese subjects are less able to maintain Vitamin D3 levels with sun exposure, and are likely to require larger oral doses of Vitamin D3 to replace deficits.
