Allopurinol is a purine analog. It is also an isomer of hypoxanthine.
Mechanism of action:
Allopurinol inhibits xanthine oxidase enzyme which is required for the synthesis of Uric acid. This enzyme is required when purine is oxidized to Uric acid.
This is effective in the treatment of gout, which may be due to;
1. Primary hyperuricemia
2. Secondary hyperuricemia (Such as those caused from the use of chemotherapeutic agents or diseases of the kidneys).
It is also effective as an anti-protozoal agent.
It is well absorbed (approximately 70-85%) orally. One of its metabolite is alloxanthine (also called as oxypurinol) which is also effective in the inhibition of xanthine oxidase. Allopurinol takes action along with this metabolite.
The plasma half life of allopurinol is 2 hours and that of oxypurinol is 15 hours. The drug and its metabolites are excreted in the urine and feces.
Due to the long half life of oxypurinol the dosage can be maintained at the rate of 1 dose/day. Its initial dose is 100 mg/day.
Hypersensitivity reactions such as rashes may occur. GI disturbances such as nausea and diarrhea can also be there.
It may cause an increase of the effect of cyclophosphamide. It may interfere with anti-cancer drugs such as 6-mercaptopurine and the immunosuppressant such as azathioprine.