Purinethol (6-mercaptopurine, 6-MP)

Purinethol is a drug whose active ingredient is mercaptopurine (6-MP). It reduces the activity of the immune system. It belongs to the group of drugs called thiopurines.

In gastroenterology, it can be used in some suitable patients for the treatment of inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis.

Purinethol is not a steroid (cortisone) or an antibiotic. It is not addictive.

How does Purinethol work?

In Crohn’s disease and ulcerative colitis, the immune system causes long-lasting inflammation in the bowel.

Mercaptopurine reduces the multiplication and activity of immune cells and so helps keep this inflammation under control.

The drug does not work straight away. Its effect usually becomes clear within 8–12 weeks; in some patients it can take longer.

For this reason, Purinethol is not used to quickly stop sudden, severe flare-ups of the disease.

What is it used for?

Crohn’s disease

Crohn’s disease is a chronic disease that can cause inflammation in different parts of the digestive system.

Mercaptopurine can be used in some patients to prevent the disease flaring up again after it has been brought under control with another treatment.

Today its use on its own in Crohn’s disease is limited. The benefits of treatment, its side effects and the other drug options available should be weighed together.

Ulcerative colitis

Ulcerative colitis is a chronic disease that causes inflammation of the inner lining of the large bowel.

Mercaptopurine can be used to keep the disease under control, particularly in some patients who improve with steroid (cortisone) treatment but whose disease flares up again when the steroid is reduced.

Purinethol is not used for the rapid treatment of acute severe ulcerative colitis.

Use together with biological drugs

In some patients mercaptopurine can be used together with biological drugs such as infliximab.

In certain patients this approach can reduce the chance of antibodies developing against the biological drug.

However, because using the two drugs together can increase the risk of infection and some other side effects, it is not suitable for every patient.

Use in other diseases

Mercaptopurine is also a drug used in the treatment of some types of leukaemia.

It can be used together with other cancer drugs, particularly in certain phases of treatment for acute lymphoblastic leukaemia.

The way it is used and the treatment protocols in blood diseases differ from those in gastroenterology.

Important: Depending on the country and the product’s licence, its use in Crohn’s disease and ulcerative colitis may be off-label. The decision to treat should be made by a specialist.

How is Purinethol taken?

Purinethol is taken by mouth as a tablet.

The dose is decided according to the patient’s weight, blood tests, liver and kidney function, other medicines and response to treatment.

• Take the drug at the dose your doctor recommends.
• Do not increase or reduce the dose on your own.
• Follow the product’s instructions on when to take it and how it relates to meals.
• Do not take a double dose to make up for a missed one.
• Regular blood tests should be done during treatment.

Which tests are done before treatment starts?

In some patients mercaptopurine can severely reduce the production of blood cells.

To assess this risk, enzyme and genetic characteristics called TPMT and NUDT15 can be tested.

These characteristics affect how the drug is processed in the body. In some people mercaptopurine is broken down more slowly, so serious side effects can develop.

Before treatment the following assessments are usually done:

• Full blood count
• Liver function tests
• Kidney function tests
• TPMT and, where needed, NUDT15 testing
• Assessment of infections and vaccination status

Normal results in these tests do not remove the need for regular blood tests during treatment.

What are the side effects of Purinethol?

Mercaptopurine is an effective drug, but it can cause important side effects.

More common side effects

• Nausea
• Loss of appetite
• Abdominal discomfort
• Tiredness
• Raised liver tests

Rare but important side effects

Fall in blood cells: Because of suppression of the bone marrow, white blood cells, red blood cells or platelets can fall. This can increase the risk of infection, anaemia and bleeding.

Liver damage: Liver tests can worsen and, rarely, serious liver problems can develop.

Pancreatitis: Some patients can develop acute inflammation of the pancreas, particularly in the first weeks of treatment.

Infections: Suppression of the immune system can increase the risk of some infections.

Cancer risk: Long-term thiopurine treatment can increase the risk of lymphoma and some skin cancers in particular. The risk varies with the patient’s age, other characteristics and the treatments used alongside.

These risks do not mean that everyone who takes the drug will develop a serious problem. The benefits and possible harms of treatment should be weighed together.

Which symptoms mean I should see a doctor?

The following symptoms are important in patients taking Purinethol:

• Fever or clear signs of infection
• Severe and constant abdominal pain
• Repeated vomiting
• Unexplained bruising or bleeding
• Extreme tiredness
• Yellowing of the eyes or skin
• A widespread rash

Severe abdominal pain in particular may need assessment for pancreatitis.

Which check-ups are needed during treatment?

Regular monitoring is essential in Purinethol treatment.

A full blood count and liver function tests are usually checked at regular intervals.

The checks are more frequent at the start of treatment and after dose changes. Once the patient is stable, the doctor arranges the intervals between checks.

In some patients, measuring the levels of mercaptopurine metabolites in the blood can help assess how well the treatment is working and the risk of side effects.

Does it interact with other medicines?

Yes. Mercaptopurine can have serious interactions with some medicines.

Allopurinol and febuxostat: These drugs, used to lower uric acid, can increase the effect of mercaptopurine and cause severe suppression of the bone marrow.

Using it together with allopurinol is possible in some patients only with special dose adjustment and close monitoring. Using it together with febuxostat is generally not recommended.

Mesalazine and sulfasalazine: In some patients these drugs can also increase the risk of a fall in blood cells caused by mercaptopurine.

Care is needed when it is used together with other immunosuppressive drugs, because of infection and other side effects.

Tell your doctor about all the medicines you take regularly.

Can it be used in pregnancy and breastfeeding?

The use of mercaptopurine in pregnancy should be assessed according to the type of disease and the aim of treatment.

In some patients with inflammatory bowel disease, continuing thiopurine treatment during pregnancy may be appropriate to keep the disease under control.

However, the product information for mercaptopurine in cancer treatment contains serious warnings about pregnancy.

For this reason, patients who are planning a pregnancy, find out they are pregnant or are breastfeeding should decide on treatment together with the relevant specialists.

The drug should not be stopped without a doctor’s advice.

Are Purinethol and Imuran the same drug?

No.

The active ingredient of Imuran is azathioprine, while that of Purinethol is mercaptopurine.

Azathioprine is partly converted to mercaptopurine in the body. For this reason the two drugs have similar effects and side effects.

However, their doses are not the same and they should not be swapped directly for each other.

Frequently asked questions

Is Purinethol a steroid (cortisone)?

No. It is not cortisone. It belongs to a different group of drugs that reduce the activity of the immune system.

Is Purinethol addictive?

No. It is not addictive. However, because the disease continues, long-term treatment may be needed.

How long does Purinethol take to work?

Its effect usually becomes clear in 8–12 weeks. Some patients may need longer.

Is Purinethol given to every patient with Crohn’s disease or ulcerative colitis?

No. There are other, targeted treatments available today. Mercaptopurine is used in selected patients.

Can I be vaccinated while taking Purinethol?

Many inactivated vaccines can be given. However, live vaccines can be harmful in patients whose immune system is suppressed. The vaccination plan should be assessed by the doctor.

Can I go out in the sun while taking Purinethol?

Yes, but protecting yourself from the sun is important. Because the risk of some skin cancers can rise with long-term use, using sunscreen and, where needed, skin check-ups by a dermatologist are recommended.

Important: This page has been prepared to inform patients. Purinethol is a drug that needs regular blood tests and follow-up by a specialist. The dose should not be changed and the treatment should not be stopped without a doctor’s advice.