ALLOPURINOL

(for veterinary information only)

BRAND NAME: ZYLOPRIM

AVAILABLE IN
100 mg and 300 mg
TABLETS

BACKGROUND

Allopurinol is used to reduce uric acid in the blood stream. Uric acid is produced in the normal degradation of biochemicals called “purines.” We eat purines everyday and normally convert them via a multi-step process to a substance called "allantoin," which is water soluble and easily excreted in urine. In short, purines are converted to hypoxanthine then to xanthine then to uric acid and finally to allantoin.

The final step of this conversion is worthy of the most attention. When something goes wrong with the conversion of uric acid to allantoin, uric acid builds up. Uric acid, which is not as soluble in water as allantoin, begins to form crystals which can show up as kidney stones (especially in human patients), bladder stones (especially in Dalmatians and in dogs with liver shunts), joint deposits (birds or humans with gout) or as deposits in other unpleasant places.

Whether the problem is natural inability to produce allantoin (birds), inability to get uric acid into liver cells for conversion to allantoin (Dalmatians and animals with liver shunts) or excessive purine intake (humans with gout), the goal in this situation is to reduce uric acid production. Allopurinol was developed to reduce uric acid crystals for human gout patients but has been adaptable to some veterinary situations.

The enzyme which converts hypoxanthine to xanthine and on to uric acid is called “xanthine oxidase.” Allopurinol binds this enzyme so that uric acid is not produced. Instead, hypoxanthine and xanthine levels build up as they are not converted.

HOW THIS MEDICATION IS USED

Mostly, in veterinary medicine, allopurinol is used in dogs but it can also be used in birds where gout is a problem. In dogs, a therapeutic diet low in purines is generally prescribed. The low purine diet is crucial to avoid the formation of xanthine bladder stones. If the diet is not controlled, allopurinol should not be used.

Allopurinol may also have some use in the treatment of such infections as Leishmaniasis and Trypanosomiasis as noted above.

• Allopurinol works equally well whether given with food or not.
• Allopurinol is typically given twice daily.
• Allopurinol should be. stored at room temperature, protected from light.
• If a dose is accidentally skipped, do not double up on the next dose. Give the dose when it is remembered and time the next dose accordingly.

SIDE EFFECTS

Side effects are not common with allopurinol but the most common side effects related to upset stomach: diarrhea, cramping, nausea. More severe side effects have been reported in humans: bone marrow suppression, hepatitis, and vasculitis have been reported. If kidney disease is present, it is best to decrease the dose of allopurinol.

INTERACTIONS WITH OTHER DRUGS

• Allopurinol combined with Ampicillin and Amoxicillin has led to skin rashes in some humans. This has been seen in dogs as well.
• The use of diuretics (furosemide, thiazides etc.) may increase uric acid levels and interfere with the function of allopurinol. It is best not to use these medications when attempting to address uric acid stones.
• Several chemotherapy agents interact with allopurinol. Cyclophosphamide use will have a greater tendency towards bone marrow suppression if it is used concurrently with allopurinol. This effect is magnified if cyclosporine (an immunomodulator) is added to the mix. Azathioprine is not removed from the body at a normal rate when it is used with allopurinol thus increasing its potential for toxicity.
• In some human patients the combination of allopurinol and trimethoprim sulfa has led to a drop in platelet count (possibly interfering with normal blood clotting mechanisms).
• Airway dilators such as theophylline are less effective in the presence of allopurinol.

CONCERNS AND CAUTIONS

The formation of xanthine bladder stones is probably the main concern when using allopurinol. This is most likely to happen if there is cheating on the therapeutic diet that accompanies allopurinol use.

• Safe use of allopurinol in pregnancy has not been established.
• Human patients in kidney failure have developed a life-threatening liver failure with fever, skin rashes, and worsening kidney failure. If allopurinol is to be used in a pet with poor liver or kidney function, the dose absolutely must be reduced and close monitoring for any similar reaction is vital.
• For more information on Uric Acid Bladder Stones in Dogs click here.
• Allopurinol is not helpful in managing uric acid stones in patients with portosystemic (liver) shunts.

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Page last updated: 6/29/2021