Chemical formula: C₁₅₁H₂₂₈N₄₀O₄₅S₃ Molecular mass: 3,418.598 g/mol
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. In pharmacological studies, calcitonin has been shown to have analgesic activity in animal models.
Calcitonin markedly reduces bone turnover in conditions with an increased rate of bone resorption such as Paget’s disease and acute bone loss due to sudden immobilisation. The absence of mineralisation defect with calcitonin has been demonstrated by bone histomorphometric studies both in man and in animals.
Decreases in bone resorption as judged by a reduction in urinary hydroxyproline and deoxypyridinoline are observed following calcitonin treatment in both normal volunteers and patients with bone-related disorders, including Paget’s disease and osteoporosis.
The calcium-lowering effect of calcitonin is caused both by a decrease in the efflux of calcium from the bone to the ECF and inhibition of renal tubular reabsorption of calcium.
Single injections of calcitonin salmon caused a marked transient inhibition of the ongoing bone resorptive process. With prolonged use, there is a persistent, smaller decrease in the rate of bone resorption. Histologically, this is associated with a decreased number of osteoclasts and an apparent decrease in their resorptive activity.
In healthy adults, who have a relatively low rate of bone resorption, the administration of exogenous calcitonin salmon results in decreases in serum calcium within the limits of the normal range. In healthy children and in patients whose bone resorption is more rapid, decreases in serum calcium are more pronounced in response to calcitonin salmon.
Studies with injectable calcitonin salmon show increases in the excretion of filtered phosphate, calcium, and sodium by decreasing their tubular reabsorption.
Some evidence from studies with injectable preparations suggests that calcitonin salmon may have effects on the gastrointestinal tract. Short-term administration of injectable calcitonin salmon results in marked transient decreases in the volume and acidity of gastric juice and in the volume and the trypsin and amylase content of pancreatic juice. Whether these effects continue to be elicited after each injection of calcitonin salmon during chronic therapy has not been investigated.
Salmon calcitonin is rapidly absorbed and eliminated.
Peak plasma concentrations are attained within the first hour of administration. After subcutaneous administration, peak plasma levels are reached in about 23 minutes.
Animal studies have shown that calcitonin is primarily metabolised via proteolysis in the kidney following parenteral administration. The metabolites lack the specific biological activity of calcitonin.
Bioavailability following subcutaneous and intramuscular injection in humans is high and similar for the two routes of administration (71% and 66%, respectively).
Calcitonin has a short absorption half-life of 10-15 minutes. The elimination half-life is about 1 hour for intramuscular administration and 1 to 1.5 hours for subcutaneous administration. Salmon calcitonin is primarily and almost exclusively degraded in the kidneys, forming pharmacologically inactive fragments of the molecule. Therefore, the metabolic clearance is much lower in patients with end-stage renal failure than in healthy subjects. However, the clinical relevance of this finding is not known.
Plasma protein binding is 30 to 40%.
There is a relationship between the subcutaneous dose of calcitonin and peak plasma concentrations. Following parenteral administration of 100 IU calcitonin, peak plasma concentration lies between about 200 and 400pg/ml. Higher blood levels may be associated with increased incidence of nausea and vomiting.
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