Mercaptopurine is a chemotherapy and immunosuppressant medication that works by mimicking one of the building blocks of DNA, disrupting the process cells use to copy their genetic material. It belongs to a class of drugs called purine antimetabolites. Its primary approved use is in treating acute lymphoblastic leukemia (ALL), the most common childhood cancer, though it also plays a significant role in managing inflammatory bowel diseases like Crohn’s disease and ulcerative colitis.
How Mercaptopurine Works
Your body naturally produces purines, molecules it needs to build DNA and RNA. Mercaptopurine is structurally similar enough to these natural purines that it tricks the body’s enzymes into processing it as if it were the real thing. Once inside a cell, the drug gets converted into an active compound that blocks several key steps in the production of new DNA building blocks. Specifically, it interferes with the conversion of precursor molecules into the forms cells need to assemble new genetic material.
Because rapidly dividing cells depend heavily on DNA production, mercaptopurine hits them hardest. In leukemia, this means it targets the fast-growing cancer cells. In autoimmune conditions, it suppresses the overactive immune cells driving inflammation. The drug can also get incorporated directly into DNA strands, further disrupting cell replication.
What It’s Used For
The FDA-approved indication for mercaptopurine is the treatment of ALL as part of a combination chemotherapy maintenance regimen. Maintenance therapy in ALL typically lasts two to three years and aims to eliminate any remaining leukemia cells after the initial intensive rounds of treatment. Mercaptopurine is a cornerstone of this phase, taken daily by mouth as either a tablet or an oral suspension.
Beyond cancer, mercaptopurine is widely used off-label to manage inflammatory bowel disease. In Crohn’s disease and ulcerative colitis, it serves as a long-term maintenance therapy to keep the disease in remission. A Cochrane review found moderate-certainty evidence that mercaptopurine and its closely related prodrug, azathioprine, are more effective than placebo at preventing clinical relapse in Crohn’s disease patients after surgery. However, they appear to be less effective than biologic therapies targeting a specific inflammatory protein called TNF-alpha, though the evidence on that comparison remains limited.
Common and Serious Side Effects
The two most clinically significant side effects are bone marrow suppression and liver toxicity. Because mercaptopurine slows the production of new cells, it can reduce your white blood cell count (a condition called neutropenia), leaving you more vulnerable to infections. It can also lower red blood cell and platelet counts. These blood count changes are dose-dependent and usually reversible when the dose is adjusted or the drug is paused.
Liver toxicity is the other major concern. The drug produces a metabolite that, at elevated levels, can damage liver cells. Studies in children with ALL found that the timing of doses affects these risks: once-daily dosing was associated with higher rates of both severe neutropenia and liver enzyme elevations compared to twice-daily dosing, because it shifts the balance of metabolites in the body. Regular blood tests to monitor liver function and blood counts are a standard part of treatment.
Other common side effects include nausea, vomiting, loss of appetite, mouth sores, and skin rashes. Because the drug suppresses the immune system, infections are a persistent risk throughout treatment.
Genetic Testing Before Starting Treatment
Not everyone processes mercaptopurine at the same rate, and for some people, standard doses can be dangerously toxic. This is determined largely by two enzymes your body uses to break down the drug: TPMT and NUDT15. Variations in the genes coding for these enzymes can cause the drug to accumulate to harmful levels.
Clinical guidelines from the Clinical Pharmacogenetics Implementation Consortium recommend genetic testing for both TPMT and NUDT15 before starting treatment. People who are intermediate metabolizers for both enzymes need a starting dose reduced to 20% to 50% of the normal amount. Those who are poor metabolizers for either enzyme require even more dramatic reductions or may need an alternative medication entirely. If genetic testing isn’t available, doctors can measure enzyme activity directly from a blood sample.
Metabolite Monitoring During Treatment
Once you’re on mercaptopurine, doctors can measure two key metabolites in your blood to fine-tune dosing. The first, called 6-TGN, is the active metabolite responsible for the drug’s therapeutic effect. Levels between 235 and 450 (measured in picomoles per 800 million red blood cells) correlate with the best clinical response. The second metabolite, 6-MMP, is associated with liver damage when levels exceed 5,700 in the same units.
This metabolite testing is especially useful in inflammatory bowel disease, where finding the right balance between effectiveness and toxicity can take time. If 6-TGN levels are too low and 6-MMP levels are high, it suggests the drug is being shunted down the wrong metabolic pathway, and dose adjustments or combination strategies may be needed.
Important Drug Interactions
The most critical drug interaction involves allopurinol, a medication commonly prescribed for gout. Allopurinol blocks xanthine oxidase, one of the enzymes that breaks down mercaptopurine. When both drugs are taken together, mercaptopurine levels in the body can rise dramatically, shifting its metabolism toward producing more of the active (and potentially toxic) metabolite. If allopurinol is necessary, the mercaptopurine dose must be substantially reduced to avoid severe bone marrow suppression. Even with dose adjustments, nearly half of ALL patients on this combination required temporary pauses in their mercaptopurine therapy due to fever, low blood counts, or infections.
Pregnancy and Breastfeeding
Mercaptopurine poses risks during pregnancy because of its mechanism of interfering with DNA replication in dividing cells. For people with inflammatory bowel disease who become pregnant while on the drug, the decision involves weighing the risks of the medication against the risks of a disease flare, and this varies by individual.
For breastfeeding, the picture is somewhat more reassuring. Most professional guidelines for gastroenterology consider breastfeeding acceptable during mercaptopurine therapy for conditions like Crohn’s disease and ulcerative colitis. The drug does pass into breast milk, but waiting four hours after a dose before nursing markedly reduces the amount an infant would receive. Some experts recommend monitoring breastfed infants with periodic blood counts and liver function tests, though others consider this unnecessary. When mercaptopurine is being used as a cancer treatment, most sources recommend against breastfeeding, though among chemotherapy drugs, antimetabolites like mercaptopurine are considered the lowest risk to nursing infants.