Xanthine Oxidase Inhbitors

Question 1

Give an example of a xanthin oxidate inhibitor?

Answer 1

Allopurinol

Question 2

How does allopurinol work?

Answer 2

Purines –> Xanthine –> Uric Acid
Xanthine is converted to uric acid by xanthine oxidase
Xanthine oxidase inhibitors block this conversion therefore lowering serum uric acid levels preventing gout attacks and renal stones (join uric acid build up and kidneys)

Question 3

What are the main uses of allopurinol?

Answer 3

1. Prevention of acute gout attacks
2. Prevention of uric acid and calcium oxalate build up in kidneys (Renal stones)
3. Prevent hyperuricaemia and tumour lysis syndrome in chemotherapy

Question 4

What are the contraindications of allopurinol?

Answer 4

1. People currently having an acute gout attack (those who were already on it before the attack can stay on it)
2. Recurrent skin rashes
3. Hypersensitivity reactions

Question 5

What are the cautions of using allopurinol?

Answer 5

Hepatic and Renal insufficiency (as metabolised and excreted this way)

Question 6

What are the common interactions with allopurinol?

Answer 6

1. Mercaptopurine and its pro-drug azathioprine require xanthine oxidase for metabolism - When allopurinol is prescribed with these drugs, it inhibits their metabolism and increases the risk of toxicity.
2. Ace inhibitors/ thiazide/ loop diuretics with allopurinol increase the risk of hypersensitivity reactions
3. Amoxicillin and allopurinol increase the risk of a skin rash

Question 7

What are the common side effects of using allopurinol?

Answer 7

1. Skin rash
2. Hypersensitivity reaction
3. Stephen johnson syndorme
4. Toxic Epidermal Necrolysis
5. Drug Hypersensivity syndromes

Question 8

What is drug hypersensitivity syndrome?

Answer 8

Rare but fatal reaction to allopurinol that causes; eosinophilia, fever, lymphadenopathy and skin and liver involvement

Question 9

What is a risk of using allopurinol?

Answer 9

Starting allopurinol can trigger or worsen an acute attack of gout.

Question 10

What else should be prescribed when starting a patient on allopurinol?

Answer 10

When starting allopurinol for gout, NSAID or colchicine treatment should also be prescribed and continued for at least a month after serum uric acid levels return to normal to avoid triggering an acute attack.

Question 11

Give some examples of drug induced gout attacks?

Answer 11

Treatment with thiazide or loop diuretics increases serum uric acid concentrations and can cause gout. Low-dose aspirin inhibits renal excretion of uric acid and can trigger acute attacks of gout.