[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Antimetabolites](https://durgical.com/en/pharmacology/classes/antimetabolites/) › Mercaptopurine

# Mercaptopurine

## Antimetabolites?

Antimetabolites

## High alert?

An error with this medicine can cause serious harm: double checks and protocols are used.

An oral purine analogue for maintenance treatment of acute lymphoblastic leukaemia. TPMT or NUDT15 deficiency and allopurinol make it far more toxic.

Updated Nov 1, 2026

Also called: 6-mercaptopurine, 6-MP

## Quick facts

**Group**
[Antimetabolites](https://durgical.com/en/pharmacology/classes/antimetabolites/)

**Target**
De novo purine synthesis and DNA/RNA, through thioguanine nucleotides

**Used for**
Acute lymphoblastic leukaemia

**Key effects**
Myelosuppression · Nausea · Increased infections

**Monitor**
Complete blood count regularly · Liver tests

**ATC**
L01BB02

No doses: the clinician sets them by the current guideline.

On this page

- The basics Mechanism of action
- Indications
- Adverse effects
- Pharmacokinetics
- Memory aid

- Clinical What the labels mean

- Safety Contraindications
- Key interactions
- What is monitored
- Administration and safety
- Pregnancy and breastfeeding
- Nursing care

- Study Test yourself

- Connections Connections

## Mechanism of action

Mercaptopurine is a thiol version of hypoxanthine. The salvage enzyme HGPRT converts it into thioinosine monophosphate, which is turned into thioguanine nucleotides (TGNs). TGNs are built into DNA and RNA and cause cell-cycle arrest and death; other metabolites inhibit the first steps of de novo purine synthesis. It is inactivated by two routes: methylation by thiopurine S-methyltransferase (TPMT) and oxidation by xanthine oxidase. NUDT15 also breaks down the active nucleotides.

- HGPRT converts mercaptopurine into thioinosine monophosphate
- Thioguanine nucleotides are built into DNA and RNA
- Metabolites also block de novo purine synthesis
- TPMT, NUDT15 and xanthine oxidase limit its activity

## Indications

- Acute lymphoblastic leukaemia: maintenance phase of combination chemotherapy (children and adults). [Acute leukaemia (AML and ALL) →](https://durgical.com/en/conditions/acute-leukaemia/)

## Adverse effects

### Common

- Myelosuppression
- Nausea, vomiting, loss of appetite
- Liver enzyme and bilirubin rises
- Increased infections

### Serious (rare, important)

- Severe or life-threatening myelosuppression in people with TPMT or NUDT15 deficiency
- Hepatotoxicity (raised enzymes and bilirubin, jaundice)
- Hepatosplenic T-cell lymphoma and other treatment-related cancers
- Macrophage activation syndrome

## Contraindications

- The label lists no absolute contraindications; it is avoided after previous severe hypersensitivity

## Key interactions

**Allopurinol (xanthine oxidase inhibitor)**
Blocks the oxidation of mercaptopurine: much higher exposure and severe myelosuppression unless the dose is reduced.

**Warfarin**
Weaker anticoagulant effect; INR is monitored.

**Other myelosuppressive drugs, such as trimethoprim-sulfamethoxazole**
Additive bone marrow suppression.

## What is monitored

- Complete blood count regularly
- Liver tests (transaminases, alkaline phosphatase, bilirubin)
- TPMT and NUDT15 testing, especially after severe or repeated myelosuppression

## Administration and safety

- Hazardous drug: tablets and oral suspension are handled with gloves.
- Taken consistently with respect to food and time of day, because food lowers exposure.
- Maintenance regimens combine it with other oral drugs such as methotrexate on different schedules: each drug and day is double-checked.

## Pregnancy and breastfeeding

Can cause fetal harm; effective contraception is advised. It is stopped if intrahepatic cholestasis of pregnancy develops.

## Nursing care

- Ask about allopurinol or febuxostat use before every dispensing.
- Check counts and liver tests; report fever, jaundice or dark urine.
- Support adherence: missed maintenance doses raise the risk of relapse.
- Teaching: avoid live vaccines; contraception during treatment.

## Pharmacokinetics

**Absorption**
Oral; food lowers exposure.

**Distribution**
Little entry into cerebrospinal fluid.

**Metabolism**
Inactivated by TPMT (methylation) and xanthine oxidase (oxidation); people with low TPMT or NUDT15 activity build up active nucleotides.

**Half-life**
Under 2 hours for the parent drug after an oral dose (label); its active nucleotides last much longer in cells.

## Memory aid

«6-MP needs Three checks: TPMT, NUDT15 and allopurinol (xanthine oxidase)»: miss one and the marrow pays.

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme activates mercaptopurine?

HGPRT (hypoxanthine-guanine phosphoribosyltransferase).

## Why is allopurinol dangerous with mercaptopurine?

It inhibits xanthine oxidase, one of the two routes that inactivate mercaptopurine, so levels rise and myelosuppression can be severe.

## Which genetic tests help explain severe myelosuppression on mercaptopurine?

TPMT and NUDT15 deficiency.

## In which phase of ALL treatment is mercaptopurine used?

Maintenance.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/mercaptopurine/)

[More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology)

## What the labels mean

**High alert**
An error with this medicine can cause serious harm: double checks and protocols are used.

## Connections

## Related drugs 4

- [Tioguanine](https://durgical.com/en/pharmacology/tioguanine/)
- [Azathioprine](https://durgical.com/en/pharmacology/azathioprine/)
- [Allopurinol](https://durgical.com/en/pharmacology/allopurinol/)
- [Methotrexate](https://durgical.com/en/pharmacology/methotrexate/)

## Related conditions 1

- [Acute leukaemia (AML and ALL)](https://durgical.com/en/conditions/acute-leukaemia/)

[← Back to the class: Antimetabolites](https://durgical.com/en/pharmacology/classes/antimetabolites/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - MERCAPTOPURINE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=4bae5ffb-a535-3fa0-e063-6294a90aa90a) (DailyMed (U.S. National Library of Medicine), 2026); [Mercaptopurine: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682653.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=L01BB02) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/) (World Health Organization, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/mercaptopurine/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/mercaptopurine/