|Systematic (IUPAC) name|
|Legal status||℞ Prescription only|
|Bioavailability||5 to 37%|
|Half-life||60 to 120 min., longer for its active metabolites|
|Mol. mass||152.177 g/mol|
| (what is this?)
Mercaptopurine (also called 6-mercaptopurine, 6-MP or its brand name Purinethol) is an immunosuppressive drug.
It is used to treat leukemia, pediatric non-Hodgkin's lymphoma, polycythemia vera, psoriatic arthritis, and inflammatory bowel disease (such as Crohn's disease and ulcerative colitis).
Mechanisms of action 
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. This alters the synthesis and function of RNA and DNA. Mercaptopurine interferes with nucleotide interconversion and glycoprotein synthesis.
Adverse reactions 
Some of the adverse reactions of taking mercaptopurine will 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.
Mercaptopurine causes myelosuppression, suppressing the production of white blood cells and red blood cells. It may be toxic to bone marrow. Weekly blood counts are recommended for patients on mercaptopurine. The patient should stop taking the medication at least temporarily if there is an unexplained, abnormally large drop in white blood cell count, or any other blood count.
Patients exhibiting myelosuppression or bone marrow toxicity should be tested for thiopurine methyltransferase (TPMT) enzyme deficiency. Patients with TPMT deficiency are much more likely to develop dangerous myelosuppression. In such patients, it may be possible to continue using mercaptopurine, but at a lower dose.
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.
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. 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.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. 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."
Mercaptopurine causes changes to chromosomes in animals and humans, though a study in 1990 found, "while the carcinogenic potential of 6-MP cannot be precluded, it can be only very weak or marginal." Another study in 1999 noted an increased risk of developing leukemia when taking large doses of 6-MP with other cytotoxic drugs.
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