Metabolism and Bioactivation
The forms of vitamin D entering the body are initially inactive. For the drug to work, it must undergo a two-step hydroxylation process requiring specific enzymes and ultraviolet (sunlight) exposure.
- Hepatic phase. The initial substrate (cholecalciferol) is converted by the enzyme 25-hydroxylase into calcidiol (25(OH)$D_3$). This is the inactive transport form of the substance.
- Renal phase. Calcidiol is transported to the kidneys, where the enzyme 1-hydroxylase transforms it into calcitriol (1,25(OH)$_2D_3$). This specific compound is the biologically active form.
Synthesis of active metabolites sharply decreases in renal pathologies (the most frequent cause), liver diseases, and inadequate sun exposure.
Mechanism of Action and Effects
The primary cellular target of active vitamin D metabolites is the intestinal mucosal cells. At the molecular level, the substance induces the synthesis of a specific calcium-binding transport protein.
Physiological outcomes of this process:
- Ensuring efficient absorption of calcium ($Ca^{2+}$) and phosphorus ($P$) ions from the gastrointestinal lumen.
- Mineralization (calcification) of bone tissue.
- Antihypocalcemic effect (preventing drops in blood calcium levels).
- Antirachitic action.
Classification of Drugs
The choice of pharmacological agent directly depends on the patient's renal excretory function and therapeutic goals. Drugs are divided into two major groups based on their activity level:
- Inactive forms. These include ergocalciferol (vitamin $D_2$, a plant-derived provitamin) and cholecalciferol (the native form of animal origin). They are indicated for correcting dietary deficiencies and preventing hypovitaminosis in healthy individuals.
- Active forms. Representatives include calcitriol and alphacalcidol (1(OH)$D_3$). Their main advantage is the ability to bypass the renal activation step. They are effective in treating hypocalcemia and all forms of osteoporosis, particularly in the setting of chronic kidney disease where native vitamin D is completely ineffective.
Pathophysiology of Deficiency and Therapy Risks
Vitamin D deficiency leads to severe skeletal disorders. In children, it causes rickets (delayed closure of fontanelles, delayed tooth eruption, muscle weakness, bone deformities). In adults, it leads to osteomalacia (softening of bones) and osteoporosis, resulting in pathological fractures under normal stress. Common causes of deficiency in adults include gastrointestinal diseases (malabsorption), menopause, and glucocorticosteroid therapy.
Pharmacotherapy complications: Overdose of active forms leads to vitamin D hypervitaminosis, manifested by nausea, fever, and generalized weakness. The primary danger is the development of hypercalcemia, which can trigger seizures and severe cardiac arrhythmias.
Pharmacological interaction: A rational combination is vitamin D co-administered with calcium carbonate. Combining it with highly soluble salts (calcium gluconate or lactate) is not recommended due to a high risk of hypercalcemia.