Calcitriol is a hormonally-active, synthetic, vitamin D analog prescribed for the treatment of hypocalcemia, osteoporosis, and prevention of corticosteroid-induced osteoporosis.[1] Systemic calcitriol is FDA indicated to control hypocalcemia in patients on chronic renal dialysis, secondary hyperparathyroidism in those with chronic kidney disease not yet on dialysis, and hypocalcemia in patients with hypoparathyroidism and pseudohypoparathyroidism. In 2009, the FDA approved topical calcitriol ointment for the management of mild to moderate plaque psoriasis.[2] Off-label uses for systemic calcitriol include type 1, vitamin D-dependent rickets and pseudo-vitamin D deficiency rickets. Off-label use of topical calcitriol ointment includes psoriasis in children and adolescents.

Systemically, calcitriol binds to vitamin-D receptors in the kidneys, parathyroid glands, intestines, and bones to increase serum blood calcium levels by promoting absorption in the intestines, renal tubular reabsorption in the kidneys, and release from bone. Calcitriol serves as a transcription factor to encode a calcium binding protein, which transports calcium and phosphate ions across intestinal epithelial cells simultaneously.[3] Along with parathyroid hormone, calcitriol stimulates bone resorption by activating osteoclasts through the release of receptor activator of nuclear factor kappa-B ligand (RANKL) from osteoblasts. Topically, studies have found that calcitriol significantly inhibits the proliferation of normal human epidermal keratinocytes and T lymphocytes by inducing apoptosis, as well as inhibiting gene expression of relevant chemokines and epidermal proteins involved in psoriasis.[4]

Calcitriol is administered intravenously through a catheter, orally without regards to meals, and topically for external use only. For intravenous administration, no dilution is necessary, and a bolus dose is given at the end of a hemodialysis session. During the initial titration period, serum calcium and phosphorous levels should be monitored twice weekly. Calcitriol capsules and oral solution may be taken with or without meals but should be protected from prolonged light exposure. Topical administration is performed by applying a thin film to the affected area with gentle rubbing, being careful to not apply to the face or eyes.

Hypercalcemia is the most common adverse reaction and has been reported in at least one-third of patients taking systemic calcitriol. Early signs of hypercalcemia include fatigue, weakness, nausea, vomiting, abdominal pain, constipation, diarrhea, vertigo, tinnitus, ataxia, arthralgia, and irritability.[5] Late signs of hypercalcemia include polyuria, polydipsia, cardiac arrhythmias, sensory disturbances, apathy, nephrocalcinosis, urinary tract infections, and hypertension.

Less than 10% of reported adverse reactions include headache, skin rash, polydipsia, nausea, abdominal pain, and urinary tract infection. Less than 1% of reported adverse reactions include hypertension, increased hematocrit, increased hemoglobin, drowsiness, hyperthermia, lymphocytosis, increased neutrophils, anorexia, constipation and ophthalmic conditions, such as conjunctivitis and photophobia.

Hypercalciuria and nephrolithiasis have been confirmed in patients receiving systemic calcitriol treatment, and further laboratory abnormalities associated with late signs of hypercalcemia include elevated aspartate aminotransferase (AST), alanine aminotransferase (ALT), and blood urea nitrogen (BUN) levels. Because calcitriol also increases the intestinal absorption of phosphorous, hyperphosphatemia can occur in patients with renal failure and may further contribute to elevated serum creatinine levels, ectopic calcification, secondary hyperparathyroidism, and renal osteodystrophy.

Hypersensitivity reactions, including pruritus, rash, and urticaria may occur in susceptible patients, and studies of intravenous calcitriol have found rare cases of anaphylactoid reactions attributed to systemic calcitriol.[6] Less than 1% of post-marketing case reports have found pruritus, erythema, acute generalized exanthematous pustulosis, psoriasis, and contact dermatitis to be associated with calcitriol ointment.

Hypercalcemia, vitamin D toxicity, and hypersensitivity to calcitriol or any vitamin D analog are relative contraindications to taking systemic calcitriol. Further contraindications to calcitriol are patients with increased sensitivity to calcium dysregulation, including arteriosclerosis, cardiac disease, hyperphosphatemia, renal failure, and sarcoidosis. Patients with cardiac disease, especially those taking digoxin, are at an increased risk for cardiac arrhythmias from hypercalcemia and should be dosed conservatively.[7] Patients with renal failure are susceptible to increased risk for vitamin D-induced hypercalcemia and chronic hypercalcemia may lead to soft tissue calcification, nephrocalcinosis, and other toxicities.

From a preventative lifestyle perspective, accidental exposure, prolonged ultraviolet (UV) sunlight absorption, occlusive dressing use, ocular exposure, and dehydration are contraindications to calcitriol use, as they may increase serum calcium to toxic levels. Photosensitizing agents and UV sunlight may increase the risk of skin tumor formation when combined with topical calcitriol. The topical ointment should not be applied to the face and may lead to skin irritation. For patients with recent injuries and wounds, increased calcium levels may also occur with topical administration due to the enhanced absorption of calcitriol from occlusive dressings.

Pregnancy and breastfeeding are also contraindications to calcitriol administration. Calcitriol is considered in the FDA Pregnancy Category C, meaning there are no well-controlled studies on the adverse effects of calcitriol on pregnant women. In cases where calcitriol is necessary for pregnant women, close monitoring of serum calcium levels is required since pregnancy is associated with alterations in calcium production, metabolism, and excretion.[8] Further adverse effects of mental retardation and congenital aortic stenosis in the neonate must be considered as potential outcomes of hypercalcemia during pregnancy. After pregnancy, lactating mothers must also be closely monitored for hypercalcemia due to post-partum and potential lactation-induced alterations in calcium metabolism.[9]

Further contraindications and considerations to calcitriol administration include major drug interactions that are known to induce hypercalcemia. For example, many patients who benefit from calcitriol may also be taking thiazide diuretics and serum calcium concentrations must be monitored closely. Concomitant use of thiazides and calcitriol can worsen hypercalcemia as thiazides help reabsorb calcium at the distal tubule. Patients with chronic renal failure who take aluminum-containing antacids for hyperphosphatemia must also be monitored closely as the calcium-phosphate product may affect serum calcium and phosphate levels. Other drugs, such as phenytoin and phenobarbital accelerate systemic calcitriol metabolism and drugs such as ketoconazole may inhibit catabolic enzymes of calcitriol and cause unintentional hypercalcemia.