Calcitonin

Physiology and mechanism of action

Endogenous calcitonin is a polypeptide consisting of 32 amino acids. It is secreted by C-cells (parafollicular cells) of the thyroid follicles. The physiological stimulus for increased secretion is hypercalcemia (an increase in plasma $Ca^{2+}$ levels).

The pharmacodynamics of the drug are realized through three main sites of action:

1. Bone tissue: the drug decreases osteoclast activity. This leads to a reduction in bone resorption (destruction) and suppression of decalcification processes.
2. Kidneys: there is a decrease in the reabsorption of calcium and phosphates in the proximal tubules, which promotes their excretion from the body.
3. Central nervous system (CNS): calcitonin exhibits a pronounced analgesic (pain-relieving) effect, acting as a co-analgesic.

Final effect: a rapid and sustained decrease in plasma calcium ion concentration.

Comparative characteristics of drugs

Clinical practice utilizes both natural and synthetic calcitonin preparations:

• Calcitrin (Calcitrinum): a natural preparation obtained from the thyroid glands of pigs. Administered parenterally (intramuscularly or subcutaneously).
• Miacalcic (Miacalcic): a synthetic analogue of salmon calcitonin.

Advantages of synthetic salmon calcitonin: compared to mammalian calcitonin, it has a higher affinity for human receptors. This ensures a longer and more pronounced therapeutic effect.

Indications for use

Calcitonin preparations are prescribed for conditions associated with bone mass loss or pathological bone remodeling:

• Osteoporosis: for the prevention of disease progression.
• Paget's disease (osteitis deformans).
• Traumatology: delayed fracture healing.
• Pain syndrome: bone pain caused by osteolysis (tissue destruction). In this case, calcitonin acts as an adjuvant analgesic (co-analgesic), effectively managing specific pain that standard painkillers cannot handle.

Side effects and contraindications

The safety profile is identical for both conventional calcitonin and synthetic calcitonin (Miacalcic).

Adverse effects:

• Vascular reactions: flushing sensation, increased blood pressure (BP).
• Neurological disorders: dizziness.

Contraindications:

• Baseline hypocalcemia (since the drug further reduces calcium levels).
• Age under 6 years (due to insufficient clinical experience of use).

Dosage forms and dosing rules

The drug is available in two main dosage forms:

1. Solution for injection: 1 ml ampoules (concentration 100 IU/ml). Standard prescription — 100 IU intramuscularly.
2. Nasal spray: metered aerosol in bottles. One bottle usually contains 14 doses. The concentration is 200 IU in a single dose. Prescribed at 200–400 IU per nasal passage (the daily dose can be administered at once).

Spray pharmacokinetics: the therapeutic effect occurs 1 hour after instillation into the nose. The half-life ($t_{1/2}$) is short, ranging from 16 to 43 minutes.

Physiological interactions

For a deep understanding of calcitonin pharmacology, it is important to consider its interaction with other hormones:

• Thyroid hormones (thyroxine, triiodothyronine) increase tissue sensitivity to calcitonin and parathyroid hormone, ensuring normal osteogenesis.
• Parathyroid hormone is a functional antagonist of calcitonin. A decrease in blood calcium levels (including under the action of calcitonin) is detected by the calcium-sensing receptor on the membranes of the parathyroid glands, which stimulates parathyroid hormone secretion for the reverse mobilization of calcium from the bones.
• Interesting fact: calcitonin gene-related peptide (CGRP) is released from sensory nerve endings upon axon reflex stimulation (e.g., when using mustard plasters), participating in local vasodilation.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Parathyroid hormone: mechanism of action and antagonism with calcitonin
• Co-analgesics: classification and use in specific pain syndromes
• Pharmacotherapy of osteoporosis: bisphosphonates and hormonal drugs
• The role of thyroid C-cells in endocrine regulation
• The effect of thyroid hormones on the musculoskeletal system and osteogenesis

Other topics in the «Drugs and Prescriptions» section

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