allopurinol
Meaning of allopurinol: Starting it can set off an attack, because a shifting uric acid level disturbs crystals already sitting in a joint.
Definition of allopurinol
Allopurinol is a medication that lowers the level of uric acid in the blood. Uric acid is the end product of breaking down purines, building blocks found in the body's own cells and in some foods. The drug blocks xanthine oxidase, the enzyme that manufactures uric acid, so less is produced rather than more being excreted.
How does it differ from the other approach?
| Allopurinol | Uricosuric drugs | |
|---|---|---|
| Mechanism | Blocks the enzyme that produces uric acid | Make the kidney excrete more of it |
| Effect on total production | Reduced | Unchanged |
| Load on the kidney | Lower | Higher, since more passes through |
| Less suitable when | Certain hypersensitivity risks apply | Kidney function is reduced or urate stones have formed |
Uric acid is made in two steps by the same enzyme, and allopurinol blocks both. The precursors that accumulate instead are far more soluble than uric acid, so they leave in the urine without forming crystals.
What is it used for?
The main use is long term reduction of uric acid in gout, in which crystals of urate deposit in joints and provoke sudden, intensely painful inflammation. Lowering the level over months allows existing deposits to dissolve. It is also used where uric acid stones form in the kidney, and in cancer care to head off tumor lysis syndrome, in which large numbers of cancer cells break apart at the start of treatment and flood the blood with purines faster than the kidneys can clear the resulting acid.
What problems are recognized with it?
A rash is the most common reaction, and while most rashes are mild, a small number progress to a severe hypersensitivity syndrome involving fever, extensive skin loss and injury to the liver and kidneys. That reaction is strongly associated with a particular tissue type gene variant, which is much more frequent in people of Han Chinese, Thai and Korean ancestry, and testing for it is available. Allopurinol also blocks the enzyme that breaks down azathioprine and mercaptopurine, so combining them raises the level of those drugs sharply. Beginning treatment can itself trigger a flare of joint pain as deposits start to shift, which does not mean the drug is failing.
Also known as
Used in a sentence
Allopurinol was prescribed to help prevent gout flares related to the patient's chronically elevated uric acid level.
Good to know
Allopurinol reduces the production of uric acid rather than increasing its removal, a mechanism distinct from that of uricosuric medications, and beginning treatment can provoke a gout flare as existing deposits start to dissolve.