Mercaptopurine (Purinethol)

Raw research findings for this medication. Mostly single studies, not curated or confirmed robust.

About Mercaptopurine

What it is

Mercaptopurine is a thiopurine, a medicine that interferes with how cells build DNA, so fast-dividing cells such as leukaemia cells and active immune cells are held back. It is a mainstay of maintenance treatment for acute lymphoblastic leukaemia, especially in children, and is also used for inflammatory bowel disease. It is sold as Purinethol, among other names; 6-MP is its short name.

How genes come into it

Two genes have strong evidence for mercaptopurine. TPMT inactivates it, and about 1 in 10 people of European ancestry carry one low-activity copy (most often *3A), and about 1 in 300 carry two. NUDT15 does a related job, and its low-activity variants are more common in people of East Asian and Hispanic ancestry. With less of either enzyme the active form builds up, and the bone marrow is affected more strongly. CPIC and DPWG have published guidelines on both genes, and the US drug label discusses them. Studies have also looked at ITPA and other genes, with weaker evidence.

Published guidelines

Clinical guidelines that name a gene for Mercaptopurine. They sort people into groups by the gene copies they carry.

CPIC guideline: NUDT15, TPMT. TPMT: Normal Metabolizer, Intermediate Metabolizer, Poor Metabolizer, Indeterminate, No Result, Possible Intermediate Metabolizer · NUDT15: Normal Metabolizer, Intermediate Metabolizer, Poor Metabolizer, Indeterminate, No Result, Possible Intermediate Metabolizer. Strong. 2026.

DPWG guideline: NUDT15. Normal Metabolizer · Intermediate Metabolizer · Poor Metabolizer · Indeterminate · Possible Intermediate Metabolizer. 2025.

DPWG guideline: TPMT. Normal Metabolizer · Intermediate Metabolizer · Poor Metabolizer · Indeterminate · Possible Intermediate Metabolizer. 2025.

US FDA: NUDT15, TPMT. TPMT: Normal Metabolizer, Intermediate Metabolizer, Poor Metabolizer, Indeterminate, No Result, Possible Intermediate Metabolizer · NUDT15: Normal Metabolizer, Intermediate Metabolizer, Poor Metabolizer, Indeterminate, No Result, Possible Intermediate Metabolizer. 2020.

What research found

Every published finding on gene variants and Mercaptopurine. Upload your DNA file to see which of these rows apply to you.

Rated evidence (41 rated variants)

PharmGKB weighs all studies of a variant and rates how strong the evidence is, from 1A (strongest) to 4.

TPMT TPMT*1, TPMT*2, TPMT*3A, TPMT*3B, TPMT*3C, TPMT*4, TPMT*5, TPMT*6, TPMT*7, TPMT*8, TPMT*9, TPMT*12, TPMT*14, TPMT*15, TPMT*16, TPMT*23: Level 1A · replicated. Metabolism/PK. 41.

TPMT TPMT*1, TPMT*2, TPMT*3A, TPMT*3B, TPMT*3C, TPMT*4, TPMT*6, TPMT*9, TPMT*12: Level 1A · replicated. Toxicity. 30.

TPMT TPMT*1, TPMT*2, TPMT*3A, TPMT*3B, TPMT*3C, TPMT*9: Level 1A · replicated. 20.

NUDT15 rs116855232: Level 1A · replicated. Dosage. 16.

Metabolism (9 findings)

Studies of how quickly the body processes this medication.

CNNM2 rs58700372: Genotypes CC + CT are associated with decreased metabolism of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to genotype TT.

ITPA rs1127354: Allele A is associated with metabolism of mercaptopurine in people with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to allele C.

ITPA rs1127354: Genotypes AA + AC is associated with decreased metabolism of azathioprine or mercaptopurine in people with Inflammatory Bowel Diseases as compared to genotype CC.

NT5C2 rs72846714: Genotypes AA + AG are associated with decreased metabolism of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to genotype GG.

Response (2 findings)

Studies of how well it worked in people carrying a variant.

TPMT TPMT intermediate activity: TPMT intermediate activity phenotype is associated with increased response to mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to TPMT high activity phenotype.

TPMT rs1142345: Allele C is associated with exposure to mercaptopurine in children with Leukemia as compared to allele T.

Dosing (68 findings)

Studies of how much of the drug people needed or tolerated.

ABCC4 rs2274407: Genotype AA is associated with decreased dose of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to genotypes AC + CC.

GNAQ rs60561071: Allele T is associated with decreased dose of mercaptopurine in children with Acute lymphoblastic leukemia as compared to allele TT.

NUDT15 NUDT15*1, NUDT15*2, NUDT15*3, NUDT15*4: NUDT15 *1/*2 + *1/*3 + *1/*4 + *2/*4 + *3/*3 is associated with decreased dose of mercaptopurine in children with Acute lymphoblastic leukemia as compared to NUDT15 *1/*1.

NUDT15 NUDT15*1, NUDT15*2, NUDT15*3, NUDT15*4, NUDT15*5: NUDT15 *1/*1 (assigned as high activity phenotype) is associated with increased dose of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to NUDT15 *1/*2 + *1/*3 + *1/*4 + *1/*5 + *3/*3 + *3/*5 + *2/*3 (assigned as intermediate and low activity phenotype) .

Side effects (4 findings)

Studies of side effects in people carrying a variant.

ITPA rs7270101: Genotypes AC + CC is associated with increased metabolism of azathioprine or mercaptopurine in people with Inflammatory Bowel Diseases or Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to genotype AA.

NUDT15 NUDT15*1, NUDT15*2, NUDT15*3: NUDT15 *2/*3 are associated with decreased dose of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to NUDT15 *1/*1.

NUDT15 NUDT15*1, NUDT15*2, NUDT15*3, NUDT15*5: NUDT15 *1/*2 + *1/*3 + *1/*5 are associated with decreased dose of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to NUDT15 *1/*1.

NUDT15 rs116855232: Genotype CT is associated with increased discontinuation of azathioprine or mercaptopurine in people with Inflammatory Bowel Diseases as compared to genotype CC.

Other findings (2 findings)

Studies that do not fall into the groups above.

TPMT TPMT*1, TPMT*3A, TPMT*3C: TPMT *1/*3A + *1/*3C + *3A/*3A is associated with increased discontinuation of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to TPMT *1/*1.

NUDT15 rs116855232: Genotypes CT + TT is not associated with discontinuation of mercaptopurine in children with Precursor Cell Lymphoblastic Leukemia-Lymphoma as compared to genotype CC.

Study findings and evidence levels: PharmGKB (CC BY-SA 4.0). Guidelines: CPIC and DPWG, as bundled in PharmCAT. Most studies were done mainly in people of European ancestry.

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Not a prescribing recommendation. This page is educational information about pharmacogenomic research and clinical guidelines - it is not medical or dosing advice, and not a prediction of how any medication will work for you. Never start, stop, or change a medication or dose based on it; talk to your doctor or pharmacist, who can order clinical-grade testing if appropriate.

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Content updated September 2026.

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