Allopurinol - How It Works

CLINICAL PHARMACOLOGY Allopurinol acts on purine catabolism, without disrupting the biosynthesis of purines.

It reduces the production of uric acid by inhibiting the biochemical reactions immediately preceding its formation.

Allopurinol is a structural analogue of the natural purine base, hypoxanthine.

It is an inhibitor of xanthine oxidase, the enzyme responsible for the conversion of hypoxanthine to xanthine and of xanthine to uric acid, the end product of purine metabolism in man.

Allopurinol is metabolized to the corresponding xanthine analogue, oxipurinol (alloxanthine), which also is an inhibitor of xanthine oxidase.

It has been shown that reutilization of both hypoxanthine and xanthine for nucleotide and nucleic acid synthesis is markedly enhanced when their oxidations are inhibited by allopurinol and oxipurinol.

This reutilization does not disrupt normal nucleic acid anabolism, however, because feedback inhibition is an integral part of purine biosynthesis.

As a result of xanthine oxidase inhibition, the serum concentration of hypoxanthine plus xanthine in patients receiving allopurinol for treatment of hyperuricemia is usually in the range of 0.3 to 0.4 mg/dL compared to a normal level of approximately 0.15 mg/dL.

A maximum of 0.9 mg/dL of these oxypurines has been reported when the serum urate was lowered to less than 2 mg/dL by high doses of allopurinol.

These values are far below the saturation levels at which point their precipitation would be expected to occur (above 7 mg/dL).

The renal clearance of hypoxanthine and xanthine is at least 10 times greater than that of uric acid.

The increased xanthine and hypoxanthine in the urine have not been accompanied by problems of nephrolithiasis.

Xanthine crystalluria has been reported in only three patients.

Two of the patients had Lesch-Nyhan syndrome, which is characterized by excessive uric acid production combined with a deficiency of the enzyme, hypoxanthineguanine phosphoribosyltransferase (HGPRTase).

This enzyme is required for the conversion of hypoxanthine, xanthine, and guanine to their respective nucleotides.

The third patient had lymphosarcoma and produced an extremely large amount of uric acid because of rapid cell lysis during chemotherapy.

Allopurinol is approximately 90% absorbed from the gastrointestinal tract.

Peak plasma levels generally occur at 1.5 hours and 4.5 hours for allopurinol and oxipurinol respectively, and after a single oral dose of 300 mg allopurinol, maximum plasma levels of about 3 mcg/mL of allopurinol and 6.5 mcg/mL of oxipurinol are produced.

Approximately 20% of the ingested allopurinol is excreted in the feces.

Because of its rapid oxidation to oxipurinol and a renal clearance rate approximately that of glomerular filtration rate, allopurinol has a plasma half-life of about 1 to 2 hours.