Thiopurine Drug Response

Is Testing Right for Me? I Have My Test Results

Thiopurine Drug Response

What Are Thiopurine Drugs?

Thiopurine drugs are a group of drugs used to treat a wide variety of health problems. These drugs are grouped together because they work in the same way to "turn down" or suppress the immune system.

The Thiopurine Drugs

Drug NameAbbreviationBrand Names
6-mercatopurine 6-MP Purinethol®
Azathioprine AZA Azasan® and Imuran®
6-thioguanine 6-TG Tabloid®

Health Problems Treated with Thiopurine Drugs

Thiopurine drugs may be used to treat:
  • Some types of leukemia and lymphoma in adults and children
  • Inflammatory bowel disease, including Crohn's disease and ulcerative colitis
  • Autoimmune diseases, including rheumatoid arthritis and lupus
  • Skin conditions, including psoriasis and severe childhood eczema
  • Rejection of organ transplants

Side Effects of Thiopurine Drugs

Like other medicines, thiopurine drugs can cause side effects. The most common side effects are stomach related problems (such as nausea, vomiting or abdominal pain) and bone marrow problems. Sometimes thiopurine drugs can damage the bone marrow (called "myelosuppression"). When this happens, the bone marrow stops or slows down making white blood cells. White blood cells help fight off infections. Therefore, people with low white blood cell counts are more likely to get sick. Getting an infection can be life-threatening, especially in people who have other serious health problems.

Next: What Affects Thiopurine Drug Response

Why are you considering TPMT testing for thiopurine response?

I am considering starting a thiopurine drug to treat a health problem.

I am currently taking a thiopurine drug and I'm not having any problems with it.

I am currently taking a thiopurine drug and I'm having side effects.

None of these

TPMT testing for thiopurine drug response may be useful for you. Drug response testing may be useful for finding people who need a lower dose of thiopurine drugs or who may not benefit from it at all.

In 2011, the Clinical Pharmacogenomics Implementation Consortium issued recommendations on thiopurine dosing. The consortium emphasized the importance of testing prior to starting thiopurine therapy. If you have testing before you start thiopurine therapy, it may help your doctor tailor your dose to you. It may also help you avoid serious side effects of the drug. The table below sums up how test results can change your care.

Possible Test Results and How They May Impact You

Test Result Category Chance To Get This Result What This Result Means
Normal TPMT ~90%
  • You have a lower chance for side effects.
  • You can take the usual dose of the drug.
  • You'll need regular monitoring for toxicity, because normal results don't rule out the chance you could have side effects.
Intermediate TPMT ~10%
  • You have a higher chance for side effects.
  • You'll need a smaller dose of the drug (usually 30-70% of normal dose).
  • You'll need to take the drug less often
  • You'll need regular monitoring for toxicity.
Low or absent TPMT ~0.3%
  • You have a higher risk for side effects.
  • Your doctor may consider using a different drug if available.
  • If you do take this drug, you'll need a smaller dose (at least a 10-fold decrease from normal dose).
  • You'll need to take the drug less often.
  • Your doctor will monitor you carefully for toxicity.

If you’re considering thiopurine therapy, you may also be considering other drugs relevant to your condition. Drugs for relevant conditions are almost always given in combination, so someone with severe side effects due to low TPMT activity may have those side effects compounded by other treatments.

TPMT testing for thiopurine drug response may not be useful for you. Testing may be useful for people who are starting these drugs or who are taking the drugs and having side effects. Since you are taking the drug without any side effects, testing at this point is probably not that helpful for you.

You should continue to be monitored for side effects like everyone else who takes a thiopurine drug. The most common serious side effect of thiopurine therapy is a low blood count. Your doctor can check for this with routine lab testing.

TPMT testing for thiopurine drug response may be useful for you. Drug response testing may be useful for people are experiencing side effects like a low blood count, especially if the side effects don't go away with a lower dose.

In 2011, the Clinical Pharmacogenetics Implementation Consortium recommended testing for the TPMT activity both prior to starting thiopurine therapy, and during therapy for those experiencing side effects.

TPMT testing for thiopurine response can be done in two different ways. Genetic testing (genotyping) looks directly at the DNA within the TPMT gene. Enzyme testing (phenotyping) checks the level of TPMT activity in the blood. Genotyping is usually preferred for people who are currently taking a thiopurine drug. See How Testing Works for more information.

  • If testing finds one or two gene variants, you have a higher risk for drug toxicity. These results may explain why you are having side effects from the drug. This result may support a decision to stop taking this drug.
  • If your test results are normal, changes in the TPMT gene do not explain why you are having side effects. Other factors may be affecting the way you metabolize this drug. Your doctor will have to make a decision about whether to stop your therapy by weighing the benefits you are getting versus the risk of side effects.

TPMT testing for thiopurine drug response may not be useful for you. The main reasons for testing are listed in the Who Should Consider Testing section. Testing is not recommended for people who aren't starting or currently taking a thiopurine drug.

If you have a reason for testing that isn't covered here, talk to your doctor or a genetic counselor about whether testing might be useful for you.