Calcitriol (Vitamin D)

Overview

Calcitriol and 1,25 dihydroxycholecalciferol are scientific names for active vitamin D. 

The biochemistry of vitamin D is different from that of other vitamins; vitamin D is actually a hormone whose activity is linked to that of parathormone (PTH). 

Calcitriol is administered therapeutically to dogs or cats in renal failure to improve their condition.

Calcitriol enhances absorption of both calcium and phosphate from the intestines, mobilizes calcium and phosphate from the bones, and inhibits the renal excretion excrete calcium and phosphate. All of these effects cause elevation of blood calcium and phosphate levels.

In renal failure, production of calcitriol and excretion of PO4 are both reduced. The blood calcium level falls and phosphorous level rises. PTH is secreted in increased quantities (i.e. hyperparathyroidism) mobilizing calcium from the bones. Calcium combines with phosphate \to form calcium phosphate, which is deposited in soft tissues. Bones become porotic and more flexible (e.g. rubber jaw). High levels of PTH are toxic. Release of PTH can be suppressed by giving small doses of calcitriol. Doses should be sufficient to suppress PTH without substantially increasing PO4 retention.

Calcitriol has positive effects on renal patients' well-being, appetite, activity, strength, and lifespan attributable primarily to keeping PTH levels below a toxic threshold. 

Calcitriol is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.

This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

Brand Names and Other Names

This drug is registered for use in humans only.

Human formulations: Rocaltrol® (Roche), Calcijex® (Abbott), and a generic preparation.

Veterinary formulations: None

Uses of Calcitriol

Uses include: 

Control of renal secondary hyperparathyroidism associated with renal failure in dogs and cats

Management of hypocalcemia associated with hypoparathyroidism

Precautions and Side Effects

While generally safe and effective when prescribed by a veterinarian, calcitriol can cause side effects in some animals.

Calcitriol should not be used in animals with known hypersensitivity or allergy to the drug.

Calcitriol should not be used in pregnant animals. 

Patients are best treated early in renal failure, as calcitriol can be helpful in preventing disease progression. It is best to employ calcitriol before phosphorus levels reach 6.0mg/dL. Calcitriol may even be ineffective when PO4 levels are > 6.0mg/dL. 

If necessary, the phosphorus level can be reduced prior to therapy with calcitriol. Dietary PO4 should be restricted. Intestinal phosphate binders are also helpful, however, only phosphate binders that do not contain calcium should be used. 

High doses of calcitriol increase serum Ca++ levels. Too high levels of Ca++ should be avoided as they interfere with cardiac and kidney function. 

Drug Interactions

Calcitriol may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with calcitriol. 

How Calcitriol is Supplied

Calcitriol is available in 0.25 and 0.5 mcg sizes 

The oral solution is available as a 1 mcg/mL [in 15 mL bottle]. 

The injectable forms is available in 1 and 2 mcg/mL concentrations.

Dosing Information

Medication should never be administered without first consulting your veterinarian. 

Doses of calcitriol vary widely depending on the reason for prescribing.

The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

The usual initial dose is 1.5 ng/kg to 3.5 ng/kg once daily for the treatment of renal secondary hyperparathyroidism. The dose may be increased according to the clinical effect. 

During calcitriol treatment, blood tests should be performed measuring serum calcium, phosphorous, and PTH levels to determine the correct dose for the individual.