Author: Vanessa Ngan, Staff Writer, 2003.

What is thioguanine?

Thioguanine (also spelled tioguanine) belongs to a group of medicines called antimetabolites. In New Zealand it is currently registered for the treatment of some cancers, primarily acute leukaemias. Another use of thioguanine, although unregistered, is the treatment of moderate to severe psoriasis. Clinical trials have shown that with either daily dosing or pulse dosing, more than 70% of patients show some degree of improvement.

Thioguanine is available as a scored 40mg tablet. The trade name in New Zealand is Lanvis®.

How to use thioguanine

For the treatment of psoriasis, thioguanine can be given either as a daily dose or via pulse dosing. Daily dosing usually starts with a low dose of 20mg daily for 4 weeks which is then increased in 20mg increments up to 80-120 mg daily until a response is seen. Pulse dosing uses higher doses at less frequent intervals and appears to help in minimising bone marrow toxicity. For psoriasis, the dose is usually 80 mg twice weekly, then increasing 20 mg every 2-4 weeks to a maximum of 160mg three times a week.

It is important to stop thioguanine at the first sign of abnormal blood counts as this may indicate bone marrow toxicity. Prior to starting and throughout therapy with thioguanine, regular (every 2-4 weeks) monitoring of blood counts is required.

Side effects of thioguanine

The main dose-limiting side effect of thioguanine is bone marrow toxicity, which is seen as anaemia (low level of haemoglobin), leucopoenia (low numbers of white cells) and thrombocytopenia (low numbers of platelets). It is important to stop thioguanine temporarily at the first sign of blood abnormalities as thioguanine may have a delayed effect. Liver function tests should also be performed as jaundice (yellowing of the skin) and hepatomegaly (abnormally enlarged liver) has been associated with thioguanine. Hyperuricaemia, which can cause gout or kidney stones, is another common problem but can be minimized by drinking more water, and by taking allopurinol to prevent uric acid build-up.

Other side effects that may occur include nausea, vomiting, diarrhoea, black tarry stools, loss of appetite, unusual bruising or bleeding and stomatitis (sores in the mouth). You should see your doctor promptly if any side effects occur.

Precautions if you are on thioguanine

Thioguanine should be avoided whenever possible during pregnancy, particularly during the first trimester. This medicine may cause birth defects if either the male or female is taking it at the time of conception. It is also recommended that women should not breast-feed whilst taking thioguanine.

Thioguanine can lower the number of white blood cells in your body temporarily, which increases the chance of getting an infection. It can also lower the number of platelets, which are necessary for proper blood clotting. If this occurs, thioguanine may be stopped temporarily or you will need to take the following precautions to reduce the risk of infection or bleeding.

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