Alfacalcidol

MECHANISM OF ACTION

Alfacalcidol is converted in the liver to its active form, calcitriol (1,25-dihydroxyvitamin D₃). Calcitriol then increases the absorption of calcium and phosphorus from the intestines, enhances calcium reabsorption in the kidneys, and facilitates calcium release from bone. Unlike natural vitamin D, alfacalcidol bypasses the kidney's activation step, making it especially beneficial for patients with chronic kidney disease or impaired renal function. It helps restore calcium and phosphate balance and supports the regulation of parathyroid hormone (PTH) levels.

PHARMACOKINETICS

Absorption

Alfacalcidol is readily absorbed in the gastrointestinal tract and quickly converted into its active form in the liver. This process is highly efficient and provides an advantage over other vitamin D supplements, particularly for individuals with kidney problems. 

Distribution

The resulting active vitamin D metabolite, calcitriol, is highly bound to a specific alpha-globulin (vitamin D-binding protein) in the plasma for transport throughout the body.

Metabolism

Alfacalcidol is quickly converted in the liver by the enzyme 25-hydroxylase into 1,25-dihydroxyvitamin D (calcitriol), the biologically active form of vitamin D. This conversion bypasses the renal activation step, making alfacalcidol particularly suitable for patients with impaired kidney function. Once activated, calcitriol helps regulate calcium and phosphate metabolism. It is then further broken down into inactive, water-soluble metabolites, which are mainly excreted via the bile.

Elimination

Alfacalcidol is primarily excreted through the bile and into the feces after being metabolized in the liver. Only small amounts of the drug and its metabolites are eliminated via the urine. 

PHARMACODYNAMICS

Alfacalcidol is a synthetic analog of vitamin D that acts as a prodrug for the active hormone calcitriol. Its pharmacodynamics are therefore largely a result of its conversion to calcitriol, which regulates calcium and phosphate homeostasis throughout the body. Alfacalcidol is particularly useful for patients with impaired kidney function, as it bypasses the final activation step that occurs in healthy kidneys. 

ADMINISTRATION

Alfacalcidol is administered orally or intravenously, with the oral route being most common in daily doses of 0.25 to 1 microgram. It is especially useful in patients with kidney disease, as it does not require renal activation. Intravenous administration is typically reserved for dialysis patients. Regular monitoring of calcium, phosphate, and PTH is essential to avoid complications like hypercalcemia. Since it is activated in the liver, caution is needed in patients with liver impairment.

DOSAGE AND STRENGTHS

Available forms and strengths:

• Oral capsules: Available in various strengths, such as 0.25 mcg, 0.5 mcg, and 1 mcg.
• Oral drops: Typically available in concentrations like 2 mcg/mL.
• Intravenous (IV) injection: Available in concentrations such as 2 mcg/mL. 

DRUG INTERACTIONS

Alfacalcidol has notable drug interactions, primarily linked to its effect on calcium levels. The risk of hypercalcemia (high blood calcium) is increased when taken with other vitamin D supplements, calcium supplements, or thiazide diuretics. Combining alfacalcidol with digoxin, a heart medication, can heighten the risk of an irregular heartbeat due to elevated calcium levels. Anticonvulsants like phenytoin and phenobarbital may reduce alfacalcidol's effectiveness by speeding up its breakdown. In patients with kidney issues, special caution is needed with magnesium or aluminum-containing antacids, as alfacalcidol can increase the absorption of these minerals to potentially toxic levels.

FOOD INTERACTIONS

You should generally take alfacalcidol with food to improve absorption, though some sources state it can be taken with or without food. It is important to avoid or limit foods and drinks that inhibit calcium absorption, such as those high in oxalate (spinach, rhubarb), phytic acid (bran, whole grains), caffeine, soft drinks, and alcohol. Additionally, since alfacalcidol works to increase calcium levels, consuming excessive amounts of calcium-rich foods while taking this medication can increase the risk of developing hypercalcemia (abnormally high calcium levels).

CONTRAINDICATIONS

Alfacalcidol is contraindicated in patients with hypercalcemia (high blood calcium levels), hyperphosphatemia (high blood phosphate levels), and hypervitaminosis D. It is also not recommended for patients who have known hypersensitivity to alfacalcidol or any of its components. 

SIDE EFFECTS

• Gastrointestinal issues: Nausea, vomiting, constipation, dry mouth, or a metallic taste.
• Weakness and fatigue: General feelings of being tired, sleepy, or weak.
• Headache.
• Increased urination and thirst.
• Loss of appetite.
• Weight loss.
• Severe hypercalcemia.
• Irregular heartbeat or palpitations.
• Kidney stones.
• Skin reactions.
• Allergic reaction.

OVERDOSE

An overdose of alfacalcidol causes hypercalcemia (high blood calcium levels), which can disrupt organ function. Symptoms include nausea, vomiting, excessive thirst, and frequent urination, while severe cases can lead to kidney problems, irregular heartbeats, and confusion. If an overdose is suspected, stop taking alfacalcidol and seek immediate medical attention. Treatment focuses on reversing hypercalcemia by discontinuing the medication, increasing hydration, and potentially using other medications or dialysis in severe instances.

TOXICITY

Alfacalcidol toxicity is primarily an overdose effect that leads to dangerously high levels of calcium (hypercalcemia) and phosphate in the blood. An overdose can cause symptoms ranging from gastrointestinal upset, like nausea, vomiting, and constipation, to more serious issues such as irregular heartbeat, kidney stones, and muscle weakness. In severe cases, this can result in kidney damage and other organ system complications.