Mercaptopurine

Evidence Level: L1

Predicted Indications: 10

Mercaptopurine
Mercaptopurine: From Acute Lymphoblastic Leukaemia (established use) to Myeloid Leukemia

## Pharmacist Assessment Report

Using the report already-specified template directly (no additional coding/debugging skill applies here — this is a content-generation task matching the system prompt’s own detailed spec).

Mercaptopurine: From Acute Lymphoblastic Leukaemia (established use) to Myeloid Leukemia

One-Sentence Summary

Mercaptopurine is a thiopurine antimetabolite long established as a backbone maintenance agent for acute lymphoblastic leukaemia (ALL); this dataset does not hold a formal UK-licensed indication record for it. The TxGNN model predicts it may be effective for Myeloid Leukemia (AML/APL), with 28 clinical trials and 20 publications currently associated with this direction, several of which already use mercaptopurine directly within AML/APL maintenance regimens.

Quick Overview

Item	Content
Original Indication	Not recorded as a licensed UK indication in this dataset; internationally recognised standard use is maintenance therapy for acute lymphoblastic leukaemia (ALL)
Predicted New Indication	Myeloid Leukemia
TxGNN Prediction Score	99.94%
Evidence Level	L1
UK Market Status	Not marketed (no licence on record)
Number of Marketing Authorisations	0
Recommended Decision	Proceed with Guardrails

Why is This Prediction Reasonable?

Currently, detailed mechanism of action data is not available in this evidence pack. Based on known pharmacology, mercaptopurine is a purine antimetabolite (thiopurine class) that is incorporated into DNA as thioguanine nucleotides, disrupting purine synthesis and halting proliferation of rapidly dividing haematopoietic cells. Its efficacy as a maintenance-phase agent in acute lymphoblastic leukaemia is well proven.

Myeloid and lymphoid leukaemias share the same underlying vulnerability — malignant clones with high proliferative turnover that are sensitive to antimetabolite interference with nucleotide synthesis. Notably, this is not a purely theoretical extrapolation: several completed Phase 3 trials in the evidence pack (e.g. NCT00003934, NCT00599937, NCT00866918, NCT02688140, NCT00482833) already incorporate mercaptopurine directly into ATRA-based maintenance regimens for acute promyelocytic leukaemia (APL), a recognised subtype of AML. This existing, evidence-based practice substantially strengthens the mechanistic plausibility of the TxGNN prediction for myeloid leukaemia more broadly.

Clinical Trial Evidence

Trial Number	Phase	Status	Enrolment	Key Findings
NCT00003934	Phase 3	Completed	420	Tretinoin + chemotherapy ± arsenic trioxide as consolidation, then maintenance with intermittent tretinoin ± mercaptopurine/methotrexate, in untreated APL
NCT00599937	Phase 3	Completed	576	Assessed optimal timing of chemotherapy with/after ATRA and role of maintenance therapy in APL
NCT00866918	Phase 3	Completed	106	Risk-adapted therapy with arsenic trioxide during consolidation for newly diagnosed childhood APL
NCT02688140	Phase 3	Completed	135	ATO + ATRA + idarubicin vs standard AIDA regimen in high-risk APL
NCT00482833	Phase 3	Completed	276	ATO + ATRA vs standard ATRA/anthracycline (AIDA) in non-high-risk APL
NCT00962767	Phase 3	Completed	168	Two-dose gemtuzumab ozogamicin vs 2-year ATRA + chemotherapy maintenance in intermediate/high-risk APL
NCT00492856	Phase 3	Completed	105	S0521: maintenance vs observation in previously untreated low/intermediate-risk APL
NCT00700544	Phase 3	Completed	330	Addition of androgen therapy during post-remission maintenance in elderly AML (GOELAMS SA-2002)
NCT00136084	Phase 3	Completed	238	Comparison of two multi-agent chemotherapy regimens with different cytarabine dosing in newly diagnosed AML/MDS
NCT06199557	Phase 1/2	Recruiting	48	Hydroxyurea + valproic acid, or 6-mercaptopurine + valproic acid, in AML/HR-MDS patients unfit for standard induction

Literature Evidence

PMID	Year	Type	Journal	Key Findings
26425037	2015	Clinical study	J Korean Med Sci	Oral maintenance chemotherapy with daily 6-MP and weekly methotrexate improved leukaemia-free survival in transplant-ineligible AML
10497848	1999	RCT	Int J Hematol	JALSG-AML92: randomised trial found no benefit from adding etoposide to daunorubicin/cytarabine/6-MP induction in adult AML
8174198	1994	RCT	Cancer Chemother Pharmacol	Nationwide randomised comparison of daunorubicin vs aclarubicin combined with BHAC, 6-MP and prednisolone in untreated AML
8558199	1996	RCT	J Clin Oncol	Japan Leukemia Study Group randomised trials of BHAC vs cytarabine (± ubenimex) in induction/consolidation/maintenance AML
9095207	1997	Pilot study	Cancer Invest	High-dose continuous IV 6-MP plus intermediate-dose cytarabine explored as first-remission consolidation in childhood AML
1793832	1991	Cohort study	Int J Hematol	Intensive individualised induction with BHAC, daunorubicin and 6-MP achieved 71% CR in adult AML
1657335	1991	Cohort study	Chin Med J (Free China)	Cytarabine, daunorubicin and 6-MP induction/consolidation in 34 adult AML patients
24492035	2014	Review	Rinsho Ketsueki	Overview of current therapy for AML and APL (Japanese)
4518586	1973	Cohort study	Cancer	Cytarabine in combination with 6-MP for adult AML
265178	1977	Case series	Blood	Sequential subcutaneous cytarabine and oral mercaptopurine showed haematologic response in juvenile chronic myeloid leukaemia

UK Market Information

No UK marketing authorisation is currently on record for mercaptopurine in this dataset (total_licenses = 0). Any clinical use in AML/myeloid leukaemia would currently require access via an alternative licensed product, import, or off-label prescribing under specialist supervision, subject to MHRA and local Trust governance.

Cytotoxicity

Item	Content
Cytotoxicity Classification	Conventional cytotoxic (thiopurine/purine antimetabolite)
Myelosuppression Risk	High — dose-limiting toxicity; markedly increased in patients with TPMT or NUDT15 genetic variants, per literature evidence in this evidence pack
Emetogenicity Classification	Low
Monitoring Items	Full blood count (with differential), liver function tests, renal function; consider TPMT/NUDT15 genotype or phenotype testing before/during therapy
Handling Protection	Cytotoxic drug handling precautions apply per local Trust policy, even though oral administration is typical

Safety Considerations

Please refer to the SmPC and BNF for safety information. Report suspected adverse reactions via the Yellow Card Scheme.

Supplementary note from literature evidence (not formal safety labelling data): multiple studies in this evidence pack indicate substantial interindividual variability in mercaptopurine-related myelosuppression and hepatotoxicity linked to TPMT and NUDT15 polymorphisms — this is a recognised prescribing consideration but should be confirmed against the current SmPC.

Conclusion and Next Steps

Decision: Proceed with Guardrails

Rationale: Evidence level L1 is supported by numerous completed Phase 3 RCTs, several of which already use mercaptopurine directly within AML/APL maintenance regimens, giving strong mechanistic and clinical plausibility. However, a Blocking data gap (MHRA/SmPC warnings and contraindications not available) prevents completion of the initial safety review (S1), so this cannot yet be treated as a cleared repurposing candidate.

To proceed, the following is needed:

MHRA-approved SmPC/PIL data (key warnings, contraindications) to close the Blocking data gap (DG001)
Confirmed mechanism of action detail from DrugBank (DG002)
Clarification of the original UK-licensed indication and dosage form(s), since no licences are currently on record
A pharmacogenomic (TPMT/NUDT15) testing protocol as part of any myeloid leukaemia treatment pathway
Route-of-administration compatibility check against required routes for myeloid leukaemia treatment settings
Disclaimer

This content is for research purposes only and does not constitute medical advice. Clinical validation is required before any clinical application.