Clinical data
Trade names Purinethol
AHFS/ Monograph
MedlinePlus a682653
  • D
Routes of
ATC code L01BB02 (WHO)
Legal status
Legal status
  • ℞ (Prescription only)
Pharmacokinetic data
Bioavailability 5 to 37%
Metabolism xanthine oxidase
Biological half-life 60 to 120 min., longer for its active metabolites
Excretion Kidney
Synonyms 6-mercaptopurine (6-MP)
CAS Number 50-44-2 YesY
PubChem (CID) 667490
DrugBank DB01033 YesY
ChemSpider 580869 YesY
KEGG D04931 YesY
ChEBI CHEBI:50667 YesY
Chemical and physical data
Formula C5H4N4S
Molar mass 152.177 g/mol
3D model (Jmol) Interactive image

Mercaptopurine, sold under the brand name Purinethol, is an immunosuppressive medication. It is used to treat acute lymphocytic leukemia, Crohn's disease, and ulcerative colitis.[1] It is a thiopurine.[2]

It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.[3]

Medical uses

It is used to treat acute lymphocytic leukemia, Crohn's disease, and ulcerative colitis.[1]

Side effects

Some of the adverse reactions of taking mercaptopurine may include diarrhea, nausea, vomiting, loss of appetite, fatigue, stomach/abdominal pain, weakness, skin rash, darkening of the skin, and hair loss. Serious adverse reactions include mouth sores, fever, sore throat, easy bruising or bleeding, pinpoint red spots on the skin, yellowing of eyes or skin, dark urine, and painful or difficult urination. Other more serious side effects include black or tarry stools (melena), bloody stools, and bloody urine.

Symptoms of allergic reaction to mercaptopurine include rash, itching, swelling, dizziness, trouble breathing, and inflammation of the pancreas.

In some cases, mercaptopurine may suppress the production of blood cells, both white blood cells and red blood cells. It may be toxic to bone marrow. Quarterly blood counts are necessary for people on mercaptopurine. People should stop taking the medication at least temporarily while considering alternate treatment if there is an unexplained, abnormally large drop in white blood cell count, or any other blood count.

Toxicity of mercaptopurine can be linked to genetic polymorphisms in thiopurine S-methyltransferase (TPMT) and inosine triphosphate pyrophosphatase (ITPA). People with specific allele variants will require dose adjustments. Caucasian people with a variant allele of the ITPA gene, experience higher rates of febrile neuropenia than people of other ethnic groups, due to differences in allelic frequencies among ethnicities.[4]


Mercaptopurine can lower the body's ability to fight off infection. Those taking it should get permission from a doctor to receive immunizations and vaccinations. It is also recommended that, while on the drug, one should avoid those having recently received oral polio vaccine.

This drug was formerly not recommended during pregnancy and early evidence indicated pregnant women on the drug (or the related azathioprine) showed a seven-fold incidence of fetal abnormalities as well as a 20-fold increase in miscarriage.[5] There were also anecdotal reports linking mercaptopurine with spontaneous abortion, leading to the US FDA rating both AZA and mercaptopurine as category D drugs. However, Davis et al. 1999 found mercaptopurine, compared to methotrexate, was ineffective as a single-agent abortifacient; every woman in the mercaptopurine arm of the study had fetal cardiac activity at follow-up (two weeks later) and was given a suction abortion.[6] A more recent, larger study, however, performed by the Cancers et Surrisque Associe aux Maladies inflamatoires intestinales En France (CESAME) indicated an overall rate of congenital malformations not significantly greater than the general population in France.[7] The European Crohn's and Colitis Organisation (ECCO) concluded in a consensus paper in 2010 that while AZA and mercaptopurine have an FDA rating of D, new research in both animals and humans indicates that "thiopurines are safe and well tolerated during pregnancy."[8]

Mercaptopurine causes changes to chromosomes in animals and humans, though a study in 1990[9] found, "while the carcinogenic potential of 6-MP cannot be precluded, it can be only very weak or marginal." Another study in 1999[10] noted an increased risk of developing leukemia when taking large doses of 6-MP with other cytotoxic drugs.

Drug interactions

Allopurinol inhibits xanthine oxidase, the enzyme that breaks down mercaptopurine. Those taking allopurinol (often used to prevent gout) are at risk for mercaptopurine toxicity. The dose should be reduced or allopurinol should be discontinued. Several published studies have demonstrated that the use of allopurinol in combination with low dose 6mp helps reduce 6mmp levels, which are toxic to liver tissue, whilst increasing the therapeutic levels of 6mp for some inflammatory conditions.

Mechanisms of action

Official information from the package insert for purinethol:[11]

6-MP ribonucleotide inhibits purine nucleotide synthesis and metabolism by inhibiting an enzyme called Phosphoribosyl pyrophosphate amidotransferase (PRPP Amidotransferase). PRPP Amidotransferase is the rate limiting enzyme of purine synthesis.[12] This alters the synthesis and function of RNA and DNA. Mercaptopurine interferes with nucleotide interconversion and glycoprotein synthesis.


The enzyme thiopurine S-methyltransferase (TPMT) is responsible, in part, for the inactivation of 6-mercaptopurine. TPMT catalyzes the methylation of 6-mercaptopurine into the inactive metabolite 6-methylmercaptopurine - this methylation prevents mercaptopurine from further conversion into active, cytotoxic thioguanine nucleotide (TGN) metabolites.[13][14][15] Certain genetic variations within the TPMT gene can lead to decreased or absent TPMT enzyme activity, and individuals who are homozygous or heterozygous for these types of genetic variations may have increased levels of TGN metabolites and an increased risk of severe bone marrow suppression (myelosuppression) when receiving mercaptopurine.[16] In many ethnicities, TPMT polymorphisms that result in decreased or absent TPMT activity occur with a frequency of approximately 5%, meaning that about 0.25% of people are homozygous for these variants.[16][17] However, an assay of TPMT activity in red blood cells or a TPMT genetic test can identify peoplewith reduced TPMT activity, allowing for the adjustment of mercaptopurine dose or avoidance of the drug entirely.[16][18] The FDA-approved drug label for mercaptopurine recommends testing for TPMT activity to identify people at risk for myelotoxicity.[19] Indeed, testing for TPMT activity is currently one of the few examples of pharmacogenetics being translated into routine clinical care.[20]


6 MP was discovered by Nobel Prize winning scientists Gertrude B. Elion and George H. Hitchings at Burroughs Wellcome in Tuckahoe, New York,[21] and was clinically developed in collaboration with investigators at Memorial Hospital (now Memorial Sloan Kettering Cancer Center in New York City.[22] The collaboration was initiated by Cornelius P. Rhoads who had run chemical weapons programs for the US army and had been involved in the work that led to the discovery that nitrogen mustards could potentially be used as cancer drugs, and had become the director of Memorial in 1948.[22]

See also


  1. 1 2 "Mercaptopurine". The American Society of Health-System Pharmacists. Retrieved Aug 28, 2015.
  2. Sahasranaman, S.; Howard, D.; Roy, S. (2008). "Clinical pharmacology and pharmacogenetics of thiopurines". European Journal of Clinical Pharmacology. 64 (8): 753–767. doi:10.1007/s00228-008-0478-6. PMID 18506437.
  3. "19th WHO Model List of Essential Medicines (April 2015)" (PDF). WHO. April 2015. Retrieved May 10, 2015.
  4. Esses, S. I.; Morley, T. P. (1983). "Spinal arachnoiditis". The Canadian journal of neurological sciences. Le journal canadien des sciences neurologiques. 10 (1): 2–10. PMID 6404543.
  5. Nørgård, B.; L. Pedersen; K. Fonager; S. Rasmussen; H. Sørensen (March 2003). "Azathioprine, mercaptopurine and birth outcome: a population-based cohort study". Alimentary Pharmacology & Therapeutics. 17 (6): 827–834. doi:10.1046/j.1365-2036.2003.01537.x. PMID 12641505.
  6. Davis, Anne R.; Leslie Miller; Hisham Tamimi; Allen Gown (June 1999). "Methotrexate Compared With Mercaptopurine for Early Induced Abortion". Obstetrics & Gynecology. 93 (6): 904–9. doi:10.1016/S0029-7844(98)00569-9. PMID 10362152.
  7. Coelho, J.; L. Beaugerie; J.F. Colombel; X. Hébuterne; E. Lerebours; M. Lémann; P. Baumer; et al. (February 2011). "Pregnancy outcome in patients with inflammatory bowel disease treated with thiopurines: cohort from the CESAME Study". Gut. 60 (2): 198–203. doi:10.1136/gut.2010.222893. PMID 21115547.
  8. Van Assche G, Dignass A, Reinisch W, van der Woude CJ, Sturm A, De Vos M, Guslandi M, Oldenburg B, Dotan I, Marteau P, Ardizzone A, Baumgart DC, D'Haens G, Gionchetti P, Portela F, Vucelic B, Söderholm J, Escher J, Koletzko S, Kolho KL, Lukas M, Mottet C, Tilg H, Vermeire S, Carbonnel F, Cole A, Novacek G, Reinshagen M, Tsianos E, Herrlinger K, Oldenburg B, Bouhnik Y, Kiesslich R, Stange E, Travis S, Lindsay J, et al. (February 2010). "The second European evidence-based Consensus on the diagnosis and management of Crohn's disease: Special situations". Journal of Crohn's and Colitis. 4 (1): 63–101. doi:10.1016/j.crohns.2009.09.009. PMID 21122490.
  9. Maekawa, A.; T. Nagaoka; H. Onodera; Y. Matsushima; A. Todate; M. Shibutani; H. Ogasawara; Y. Kodama; Y. Hayashi (May 1990). "Two-year carcinogenicity study of 6-mercaptopurine in F344 rats". Journal of Cancer Research and Clinical Oncology. 116 (3): 245–250. doi:10.1007/BF01612898. PMID 2370249.
  10. Bo J, Schrøder H, Kristinsson J, Madsen B, Szumlanski C, Weinshilboum R, Andersen JB, Schmiegelow K (September 1999). "Possible carcinogenic effect of 6-mercaptopurine on bone marrow stem cells: relation to thiopurine metabolism". Cancer. 86 (6): 1080–6. doi:10.1002/(SICI)1097-0142(19990915)86:6<1080::AID-CNCR26>3.0.CO;2-5. PMID 10491537.
  11. "PURINETHOL (mercaptopurine) tablet [Gate Pharmaceuticals]" (PDF). DailyMed. Gate Pharmaceuticals. August 2012. Retrieved 31 December 2013.
  12. Hansen, Barbara. "Purine and Pyrimidine Metabolism." USMLE STEP 1 Biochemistry and Medical Genetics Lecture Notes. 2010 ed. N.p.: Kaplan, 2010. 288-90. Print.
  13. Zaza G, Cheok M, Krynetskaia N, Thorn C, Stocco G, Hebert JM, McLeod H, Weinshilboum RM, Relling MV, Evans WE, Klein TE, Altman RB (September 2010). "Thiopurine pathway". Pharmcogenet Genomics. 20 (9): 573–4. doi:10.1097/FPC.0b013e328334338f. PMC 3098750Freely accessible. PMID 19952870.
  14. Stocco G, Pelin M, Franca R, De Iudicibus S, Cuzzoni E, Favretto D, Martelossi S, Ventura A, Decorti G (April 2014). "Pharmacogenetics of azathioprine in inflammatory bowel disease: a role for glutathione-S-transferase?". World J Gastroenterol. 20 (13): 3534–41. doi:10.3748/wjg.v20.i13.3534. PMC 3974520Freely accessible. PMID 24707136.
  15. Fujita K, Sasaki Y (August 2007). "Pharmacogenomics in drug-metabolizing enzymes catalyzing anticancer drugs for personalized cancer chemotherapy". Curr. Drug Metab. 8 (6): 554–62. doi:10.2174/138920007781368890. PMID 17691917.
  16. 1 2 3 Relling MV, Gardner EE, Sandborn WJ, Schmiegelow K, Pui CH, Yee SW, Stein CM, Carrillo M, Evans WE & Klein TE (March 2011). "Clinical Pharmacogenetics Implementation Consortium guidelines for thiopurine methyltransferase genotype and thiopurine dosing". Clin Pharmacol Ther. 89 (3): 387–91. doi:10.1038/clpt.2010.320. PMC 3098761Freely accessible. PMID 21270794.
  17. Mutschler, Ernst; Schäfer-Korting, Monika (2001). Arzneimittelwirkungen (in German) (8 ed.). Stuttgart: Wissenschaftliche Verlagsgesellschaft. pp. 107, 936. ISBN 3-8047-1763-2.
  18. Payne, K.; Newman, W.; Fargher, E.; Tricker, K.; Bruce, I. N.; Ollier, W. E. R. (2007). "TPMT testing in rheumatology: Any better than routine monitoring?". Rheumatology. 46 (5): 727–729. doi:10.1093/rheumatology/kel427. PMID 17255139.
  19. "Label: Mercaptopurine - mercaptopurine tablet". Retrieved 11 March 2015.
  20. Wang L, Pelleymounter L, Weinshilboum R, Johnson JA, Hebert JM, Altman RB, Klein TE (June 2010). "Very important pharmacogene summary: thiopurine S-methyltransferase". Pharmacogenet Genomics. 20 (6): 401–5. doi:10.1097/FPC.0b013e3283352860. PMC 3086840Freely accessible. PMID 20154640.
  21. Katherine Bouton for the New York Times. January 29, 1989 The Nobel Pair
  22. 1 2 Mukherjee, Siddhartha (2010). The Emperor of All Maladies: A Biography of Cancer. New York. pp. 91–92. ISBN 978-1439170915.

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