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6-mercaptopurine

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6-mercaptopurine

Template:Drugbox

Mercaptopurine (also called 6-mercaptopurine, 6-MP or its brand name Purinethol[1]) is an immunosuppressive drug.

It is a thiopurine.[2]

Uses

It is used to treat leukemia, pediatric non-Hodgkin's lymphoma, polycythemia vera,[3] psoriatic arthritis,[4] and inflammatory bowel disease (such as Crohn's disease and ulcerative colitis).[5]


It has demonstrated some in vitro effectiveness against Mycobacterium paratuberculosis.[6]

Pharmacokinetics

Oral absorption

Distribution

Metabolism

Genetic polymorphism of TMPT and adverse reactions

Elimination

Mechanisms of action

From the package insert for PURINETHOL:

  • Mercaptopurine (6-MP) competes with hypoxanthine and guanine for the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) and is itself converted to thioinosinic acid (TIMP).
    • This intracellular nucleotide inhibits several reactions involving inosinic acid (IMP), including the conversion of IMP to xanthylic acid (XMP) and the conversion of IMP to adenylic acid (AMP) via adenylosuccinate (SAMP).
    • In addition, 6-methylthioinosinate (MTIMP) is formed by the methylation of TIMP.
      • Both TIMP and MTIMP have been reported to inhibit glutamine-5-phosphoribosylpyrophosphate amidotransferase, the first enzyme unique to the de novo pathway for purine ribonucleotide synthesis. Experiments indicate that radiolabeled mercaptopurine may be recovered from the DNA in the form of deoxythioguanosine.
  • Some mercaptopurine is converted to nucleotide derivatives of 6-thioguanine (6-TG) by the sequential actions of inosinate (IMP) dehydrogenase and xanthylate (XMP) aminase, converting TIMP to thioguanylic acid (TGMP).
  • Animal tumors that are resistant to mercaptopurine often have lost the ability to convert mercaptopurine to TIMP. However, it is clear that resistance to mercaptopurine may be acquired by other means as well, particularly in human leukemias.
  • It is not known exactly which of any one or more of the biochemical effects of mercaptopurine and its metabolites are directly or predominantly responsible for cell death.

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.[7] 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.

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

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.[8] 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.

Precautions

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.[9] 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.[10]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.[11] 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."[12]

Mercaptopurine causes changes to chromosomes in animals and humans, though a study in 1990[13] found, "while the carcinogenic potential of 6-MP cannot be precluded, it can be only very weak or marginal." Another study in 1999[14] noted an increased risk of developing leukemia when taking large doses of 6-MP with other cytotoxic drugs.

References

External links

  • FDA Label, Mercaptopurine (PDF)
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