1. Clinical Overview
Calcitriol is the active hormonal form of Vitamin D3 (1,25-dihydroxycholecalciferol). It is a potent regulator of calcium and phosphate homeostasis, acting primarily on the intestines, bones, kidneys, and parathyroid glands. Unlike nutritional vitamin D, calcitriol does not require hepatic or renal hydroxylation for activation, making it the drug of choice in patients with impaired renal function, such as those with chronic kidney disease (CKD) and hypoparathyroidism.
| Onset | Duration | Bioavailability |
|---|---|---|
| Intestinal calcium absorption begins within 2-6 hours. Full therapeutic effect on serum calcium levels may take days to weeks. | Approximately 3-5 days after a single oral dose, due to its genomic effects and storage in adipose tissue. | 70-75% following oral administration. |
2. Mechanism of Action
Calcitriol binds to the intracellular Vitamin D Receptor (VDR), which then heterodimerizes with the Retinoid X Receptor (RXR). This complex translocates to the nucleus, binds to Vitamin D Response Elements (VDREs) in target genes, and modulates transcription. Primary actions include: 1) Increased intestinal absorption of calcium and phosphate. 2) Promotion of bone mineralization by maintaining adequate calcium-phosphate product. 3) Suppression of Parathyroid Hormone (PTH) synthesis and secretion via direct action on parathyroid gland VDRs. 4) Regulation of cell differentiation and proliferation in various tissues.
3. Indications & Uses
- Management of hypocalcemia in patients with chronic kidney disease (CKD) on dialysis
- Treatment of hypoparathyroidism (post-surgical, idiopathic, pseudohypoparathyroidism)
- Management of renal osteodystrophy in patients with CKD
4. Dosage & Administration
Adult Dosage: Initial: 0.25 mcg/day orally. May increase by 0.25 mcg/day at 4-8 week intervals based on biochemical response. Typical maintenance: 0.25 to 1.0 mcg/day. For hypoparathyroidism: 0.5 to 2.0 mcg/day.
Administration: Take orally, preferably with or after food to enhance absorption. Can be taken in the morning. Do not crush or chew capsules. Maintain adequate dietary calcium intake (800-1000 mg/day) unless contraindicated (e.g., hypercalciuria).
5. Side Effects
Common side effects may include:
- Early signs of hypercalcemia: nausea, constipation, anorexia, polydipsia, polyuria
- Headache, somnolence
- Mild skin rash
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Thiazide Diuretics (e.g., Hydrochlorothiazide) | Increased risk of hypercalcemia due to reduced renal calcium excretion. | Major |
| Cardiac Glycosides (Digoxin) | Hypercalcemia may potentiate digitalis toxicity, leading to arrhythmias. | Major |
| Magnesium-containing Antacids/Laxatives | Increased risk of hypermagnesemia, especially in dialysis patients. | Moderate |
| Ketoconazole, Antifungals (CYP3A4 inhibitors) | May increase calcitriol levels by inhibiting its metabolism. | Moderate |
| Phenytoin, Phenobarbital (Enzyme Inducers) | May increase metabolism of calcitriol, reducing its efficacy. | Moderate |
| Calcium Supplements/Phosphate Binders (Calcium-based) | Additive hypercalcemic effect. Monitor serum calcium closely. | Moderate |
| Cholestyramine, Mineral Oil | May reduce intestinal absorption of calcitriol. | Moderate |
7. Patient Counselling
- DO take the medication exactly as prescribed, usually once daily.
- DO maintain a consistent intake of calcium as advised by your doctor (neither too high nor too low).
- DO take it with food for better absorption.
- DO keep all appointments for blood tests (calcium, phosphate, creatinine).
- DONT take over-the-counter calcium, vitamin D, or antacids without consulting your doctor.
- DONT crush or chew the capsule.
- DONT change your dose without medical advice.
8. Toxicology & Storage
Overdose: Manifests as hypercalcemia and its sequelae: Severe nausea, vomiting, constipation, pancreatitis, weakness, neuropsychiatric disturbances, polydipsia, polyuria, dehydration, renal calculi, nephrocalcinosis, cardiac arrhythmias, and coma. Chronic overdose leads to metastatic calcification.
Storage: Store below 25°C. Protect from light and moisture. Keep the container tightly closed. Do not freeze. Keep out of reach of children.