Mercaptopurine (6-MP)

Impact of genetic variation on response to therapy

TPMT and NUDT15 are enzymes involved in the catabolism of thiopurine drugs including mercaptopurine. Loss of function variant alleles in the TPMT and NUDT15 genes are associated with increased pharmacological effects of thiopurines and an increased risk of toxicity including thiopurine-related leukopenia, neutropenia, myelosuppression and alopecia.

The SmPC for mercaptopurine provides dose recommendations related to TPMT and NUDT15 gene variants which are summarised as follows:

• Patients with inherited little or no TPMT activity (poor metabolisers) are at increased risk for severe toxicity from conventional doses of mercaptopurine and generally require substantial dose reduction.
• Patients with inherited variants in the NUDT15 gene are at increased risk for severe mercaptopurine toxicity and generally require dose reduction.
• Starting or target doses of mercaptopurine should be lower in patients with reduced or absent TPMT or NUDT15 enzyme activity.

The combination of reduced function variants (intermediate metaboliser status) in both TPMT and NUDT genes will require a substantial dose reduction to mitigate toxicity.

Testing recommendations

The SmPC for mercaptopurine states that genotypic testing of NUDT15 variants may be considered before initiating mercaptopurine, and that genotypic and phenotypic tests of TPMT are available. Genotypic test results will usually be reported as star alleles (e.g. *1/*3), phenotypic tests will normally be reported as an enzyme activity (e.g. TPMT level in mU/L). Some guidelines may give dosing recommendations based on phenotypic activity rather than genotype.

Disease-specific guidelines including (but not limited to) those published by The British Society of Gastroenterology recommend that testing of TPMT activity is performed prior to initiation of mercaptopurine.

Disease-specific guidelines including (but not limited to) those published by The British Society of Gastroenterology recommend NUDT15 genotype testing prior to starting thiopurine treatment if it is available.

The British Oncology Pharmacists Association have ratified and endorsed guidelines recommending that adult patients with a confirmed diagnosis of acute lymphoblastic leukaemia (ALL) should be genotyped for TPMT and NUDT15 no function variants prior to commencing treatment with mercaptopurine.  

Therapeutic recommendations

TPMT metaboliser status unknown

• Follow disease-specific guidelines regarding testing of TPMT activity prior to commencing treatment.
• Consider TPMT enzyme activity test to assess TPMT phenotype if genotype testing is not available.
• Monitor efficacy and side effects closely and titrate dose according to disease-specific treatment guidelines.

TPMT Normal metaboliser

Examples of common TPMT genotypes (see note): *1/*1

• Normal risk of thiopurine-related leukopenia, neutropenia and myelosuppression.
• Initiate standard starting dose.
• Monitor efficacy and side effects and titrate dose according to disease-specific treatment guidelines.
• Allow 2-4 weeks to reach steady-state after each dose adjustment.

TPMT Intermediate metaboliser

Examples of common TPMT genotypes (see note): *1/*2, *1/*3A, *1/*3B, *1/*3C

• Increased risk of thiopurine-related leukopenia, neutropenia and myelosuppression compared to normal metabolisers.
• Follow disease-specific treatment guidelines where available.
• Reduce starting dose to 30-80% of normal starting dose.
• Dose reduction may not be required if the normal starting dose is <75mg/m²/day or <1.5mg/kg/day. See Further Information for details of starting dose thresholds for dose reduction.
• An alternative strategy in malignant conditions only is to start with the normal starting dose and reduce the dose if side effects occur. See Further Information for details of alternative strategy in TMPT/NUDT15 intermediate metaboliser for malignant conditions.
• Monitor efficacy and side effects and titrate dose according to disease-specific treatment guidelines.
• Allow 2-4 weeks to reach steady-state after each dose adjustment.

TPMT Poor metaboliser

Examples of common TPMT genotypes (see note): *2/*2, *2/*3A, *2/*3B, *2/*3C, *3A/*3A, *3A/*3B *3A/*3C, *3B/*3B, *3B/*3C, *3C/*3C

• Greatly increased risk of thiopurine-related leukopenia, neutropenia and myelosuppression compared to normal metabolisers.
• For non-malignant conditions, consider alternative non-thiopurine therapy.
• If no alternative therapy available, or if treatment is still indicated, reduce starting dose to 5-10% of normal starting dose or according to disease-specific treatment guidelines.
• Monitor efficacy and side effects and titrate dose according to disease-specific treatment guidelines.
• Allow 4-6 weeks to reach steady-state after each dose adjustment.

NUDT15 metaboliser status unknown

• Follow disease-specific guidelines regarding NUDT15 genotype testing prior to commencing treatment.
• If thiopurine treatment is required and NUDT15 status is unknown, monitor closely for efficacy and side effects and titrate dose according to disease-specific treatment guidelines.

NUDT15 Normal metaboliser

Examples of common NUDT15 genotypes (see note): *1/*1

• Normal risk of thiopurine-related leukopenia, neutropenia and myelosuppression.
• Initiate standard starting dose.
• Monitor efficacy and side effects and titrate dose according to disease-specific treatment guidelines.
• Allow 2-4 weeks to reach steady-state after each dose adjustment.

NUDT15 Intermediate metaboliser

Examples of common NUDT15 genotypes (see note): *1/*3

• Increased risk of thiopurine-related leukopenia, neutropenia and myelosuppression compared to normal metabolisers.
• Follow disease-specific treatment guidelines to determine starting dose.
• Reduce starting dose to 30-80% of normal starting dose.
• Dose reduction may not be required if the normal starting dose is <75mg/m²/day or <1.5mg/kg/day. See Further Information for details on starting dose thresholds for dose reduction.
• An alternative strategy in malignant conditions only is to start with the normal starting dose and reduce the dose if side effects occur. See Further Information for details of alternative strategy in TMPT/NUDT15 intermediate metaboliser for malignant conditions. 
• Monitor efficacy and side effects and titrate dose according to disease-specific treatment guidelines.
• Allow 2- 4 weeks to reach steady-state after each dose adjustment.

NUDT15 Poor metaboliser

Examples of common NUDT15 genotypes (see note): *3/*3

• Significantly increased risk of thiopurine-related leukopenia, neutropenia and myelosuppression compared to normal metabolisers.
• For non-malignant conditions, consider alternative non-thiopurine therapy.
• If no alternative therapy available, or if treatment is still indicated, reduce starting dose to 5-10% of normal starting dose or according to disease-specific treatment guidelines.
• Monitor efficacy and side effects and titrate dose according to disease-specific treatment guidelines.
• Allow 4-6 weeks to reach steady-state after each dose adjustment.

TPMT and NUDT15 genotypes known

• Evidence for recommended starting dose reduction based on compound metaboliser status of phenotypes of both genes is limited.
• Where the metaboliser status of one gene recommends a greater dose reduction than the other, start with the greater dose reduction of the two recommendations. For example, if a patient is a TPMT normal or intermediate metaboliser and a NUDT15 poor metaboliser, follow the therapeutic recommendations for NUDT15 poor metaboliser.
• For patients who are intermediate metabolisers for both TPMT and NUDT15, further dose reductions are likely to be needed compared with those who are only intermediate metabolisers with respect to one gene. Follow disease-specific guidelines where available. In the absence of disease-specific guidelines it is recommended to reduce the starting dose to 20-50% of normal starting dose in this scenario.
• Monitor efficacy and side effects and titrate dose according to disease-specific treatment guidelines.

Note: This is a limited list of examples of the most common TPMT and NUDT15 genotypes based on the most prevalent alleles in the UK population.

Further information

Testing methodology and limitations

Both genotypic and phenotypic TPMT tests are available and in routine use in the UK. Genotyping identifies genetic variations in the TPMT gene, while phenotyping measures the TPMT enzyme activity (level) in erythrocytes using a biochemical assay. Phenotyping can be less accurate than genotyping as TPMT enzyme activity can be affected by blood transfusions, renal function and plasma haemoglobin level. A limitation to genotyping for both TPMT and NUTD15 is that rare variants may not be detected by genotyping for common variants. 

Due to limitations in testing methodology, in extremely rare cases results reported as TPMT *1/*3A heterozygous genotype (intermediate metaboliser) may represent a TPMT *3B/*3C compound heterozygous genotype (poor metaboliser).  It is always recommended to monitor efficacy and side effects and titrate dose according to individual response and disease-specific treatment guidelines.

Drug interactions

The increased risk of thiopurine-related leukopenia, neutropenia and myelosuppression in individuals with inherited deficiency of TPMT and/or NUDT15 may be further exacerbated by co-administration with medicinal products that inhibit TPMT, or whose primary or secondary toxicity is myelosuppression. Refer to the SmPC for a list of drug interactions.

A possible association between decreased TPMT activity and secondary leukaemias and myelodysplasia has been reported in individuals receiving mercaptopurine in combination with other cytotoxics. Refer to the SmPC for more information.

Starting dose thresholds for dose reduction in TMPT or NUDT15 intermediate metabolisers

Guidance on starting dose thresholds for dose reduction for TPMT or NUDT15 intermediate metabolisers differs between sources.  

• The Dutch Pharmacogenetics Working Group (DPWG) recommends a 50% reduction in mercaptopurine dose for TPMT intermediate metabolisers, but dose reduction is not required for doses lower than 0.75mg/kg/day. DPWG recommend a 50% dose reduction for NUDT15 intermediate metabolisers regardless of dose.   
• The Clinical Pharmacogenetics Implementation Consortium (CPIC) recommends a 30-80% reduction for TPMT or NUDT15 intermediate metabolisers if the normal starting dose is ≥75mg/m2/day or ≥ 1.5 mg/kg/day.  

Alternative strategy in TPMT/NUDT15 intermediate metabolisers for malignant conditions

For malignant indications only, DPWG advises that an alternative strategy in TPMT or NUDT15 intermediate metabolisers is to start with the normal dose and reduce to 50% if side-effects necessitate a dose reduction.    

Clinical context of the patient in determining treatment strategy

Many factors including non-genetic factors can contribute to the variability in response to thiopurines including co-morbidities, drug interactions, renal function, and hepatic function. Genetic and non-genetic factors must be considered in the clinical context of an individual patient when applying the suggested dose modifications.

Users are advised to follow disease specific guidelines where available. In the absence of disease specific guidelines users should consider the clinical context of the patient, the indication for and urgency of immunosuppression and should closely monitor efficacy and side effects and titrate dose according to response. 

References

Nova Laboratories Ltd (2025). Xaluprine 20mg/ml oral suspension SmPC. Available at: https://www.medicines.org.uk/emc/product/2949/smpc Accessed online: 20^th May 2025.

Lamb CA, Kennedy NA, Raine T et al. British Society of Gastroenterology consensus guidelines on the management of inflammatory bowel disease in adults. Gut 2019; 68:s1-s107. doi: 10.1136/gutjnl-2019-318484

Yallop D, Chauhan D, and Palmer R. Guideline for 6-mercaptopurine dosing in adult acute lymphoblastic leukaemia based on TPMT and NUDT15 genotypes v1.0. Ratified and endorsed by the British Oncology Pharmacy Association. Available at: https://ukclinicalpharmacy.org/communities/genomics/ Accessed online: 21^st May 2025.

Clinical Pharmacogenetics Implementation Consortium (2024) Guidelines for thiopurine dosing based on TPMT and NUDT15 genotypes. Available at: https://cpicpgx.org/guidelines/guideline-for-thiopurines-and-tpmt/ Accessed online 19th May 2025.

Coenen, M.J.H., Nijenhuis, M., Soree, B. et al. Dutch Pharmacogenetics Working Group (DPWG) guideline for the gene-drug interaction between TPMT/NUDT15 and thiopurines. Eur J Hum Genet (2025).  https://doi.org/10.1038/s41431-025-01986-8 Accessed online 13^th January 2026.