6-Mercaptopurine Azathioprine

The second approach to prolonged therapeutic action is based on the controlled rate of conversion of the promoiety into the active compound in vivo. This approach requires particularly detailed study of the kinetics of prodrug-drug conversion. A classic example is bioconversion of azathioprine to 6-mercaptopurine. Azathioprine is used commonly in kidney transplantation, rheumatoid arthritis, and the treatment of various skin disorders. After administration, azathioprine undergoes slow... 

In addition, one of the sulfur analogs of mercaptopurine, azathioprine, has been proposed as a cytotoxic drug, and it turned out to be more effective as an immunosuppressant. 

The ability of certain anticancer agents to suppress both humoral and cellular immunity has been exploited in the field of organ transplantation and in diseases thought to be caused by an abnormal or heightened immune response. In particular, the alkylating agents cyclophosphamide and chlorambucil have been used in this context, as have several of the antimetabolites, including methotrexate, mercaptopurine, azathioprine. 

Autoimmune hemolytic anemia Prednisone,1 cyclophosphamide, chlorambucil, mercaptopurine, azathioprine, high-dose gamma globulin Usually good... 

TPMT Mercaptopurine, azathioprine Myelosuppression, radiotherapy-related secondary tumors... 

MERCAPTOPURINE AZATHIOPRINE t risk of myelosuppression and immunosuppression. Deaths have occurred following profound myelosuppression and severe sepsis Additive myelotoxic effects. Azathioprine is metabolized to 6-mercatopurine in vivo, which results in additive myelosuppression, immunosuppression and hepatotoxicity Avoid co-administration... 

The daily dose of allopurinol is 300-600 mg. In combination with benzbromarone, the daily allopurinol dose is reduced to 100 mg. In general, allopurinol is well tolerated. The incidence of side effects is 2-3%. Exanthems, pruritus, gastrointestinal problems, and dty mouth have been observed. In rare cases, hair loss, fever, leukopenia, toxic epidermolysis (Lyell syndrome), and hqDatic dysfunction have been reported. Allopurinol inhibits the metabolic inactivation of the cytostatic dtugs azathioprine and 6-mercaptopurine. Accordingly, the administered doses of azathioprine and 6-mercaptopurine must be reduced if allopurinol is given simultaneously. 

In vivo azathioprine is rapidly converted into its active metabolite 6-mercaptopurine by the enzyme thiopurine methyltransferase (TPMT). The active agent inhibits IMPDH function. Furthermore, it also acts as antimetabolite of the RNA and DNA synthesis particularly in T-lymphocytes leading to cell death. Due to genetic polymorphism of TPMT, therapy may fail, thus it is currently discussed whether individual patients should be monitored before the use of azathioprine. 

Thiopurine S methyltransferase Low activity in about 10% of Caucasians and deficient activity in about 0.4%. High incidence of severe adverse events from azathioprine and 6-mercaptopurine in carriers of low activity. 

Azathioprine, chloramphenicol, colchicine, cyclophosphamide, cytarabine, 5-fluorodeoxyuridine, 5-fluorouracil, hydroxyurea, mercaptopurine, metformin, methotrexate, phenobarbital, phenytoin, primidone, proton pump inhibitors, pyrimethamine, sulfasalazine, and vinblastine... 

Maintenance of remission of ulcerative colitis may be achieved with oral or topical aminosalicylates. Immunosuppressants such as azathioprine or 6-mercaptopurine can be used for unresponsive patients or those who develop corticosteroid dependency. 

Maintenance of remission of Crohn s disease may be achieved with oral or topical aminosalicylate derivatives, immunosuppressants (such as azathioprine, 6-mercaptopurine, and methotrexate), or infliximab. 

Agents targeting the excessive immune response or cytokines involved in IBD are potential treatment options (Table 16-3). Azathioprine and its active metabolite 6-mercaptopurine (6-MP) are inhibitors of purine biosynthesis and reduce IBD-associated GI inflammation. They are most useful for maintaining remission of IBD or reducing the need for long-term use of corticosteroids. Use in active disease is limited by their slow onset of action, which may be as long as 3 to 12 months. Adverse effects associated with azathioprine and 6-MP include hypersensitivity reactions resulting in pancreatitis, fever, rash, hepatitis, and leukopenia.25,26... 

Immunosuppressants such as azathioprine or 6-mercaptopurine can be used for unresponsive patients or those who develop corticosteroid dependency. Remission may be maintained in up to 58% of patients after 5 years of treatment.1,25 Intermittent infliximab dosing (5 mg/kg IV every 8 weeks) may be used to maintain disease remission and reduce the need for corticosteroids in patients with moderate to severe UC. Colectomy is an option for patients with progressive disease who cannot be maintained on drug therapy alone. 

Patients with CD are at high risk for disease relapse after induction of remission. Within 2 years, up to 80% of patients suffer a relapse therefore, most patients should be evaluated for indefinite maintenance therapy. Maintenance of remission of CD may be achieved with oral or topical aminosalicylate derivatives, immunosuppressants (such as azathioprine, 6-mercaptopurine, and methotrexate), or infliximab. 

There are several important drug-drug interactions with allopurinol. The effects of both theophylline and warfarin may be potentiated by allopurinol. Azathioprine and 6-mercaptopurine are purines whose metabolism is inhibited... 

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine dmgs, such as 6-mercaptopurine (6-MP), 6-thioguanine and azathioprine, to inactive metabolites [29-32]. Thiopurines form part of the routine treatment for patients with acute lymphoblastic leukemia, rheumatoid arthritis, and autoimmune diseases such as SLE and Crohn s disease, and are used as an immunosuppressant following organ transplantation. 

Molecular diagnosis of thiopurine-S-methyltransferase deficiency genetic basis for azathioprine and mercaptopurine intolerance. Ann Intern Med 1997 126 608-614. 

Immunosuppressive agents such as azathioprine and mercaptopurine (a metabohte of azathioprine) are sometimes used for the treatment of IBD. These agents are generally reserved for cases that are refractory to steroids and may be associated with serious adverse effects such as lymphomas, pancreatitis, or nephrotoxicity. Cyclosporine has been of short-term benefit in acute, severe ulcerative colitis when used in a continuous infusion. 

The immunosuppressive agents (azathioprine and mercaptopurine) are generally limited to use in patients not achieving adequate response to standard medical therapy, or to reduce steroid doses when toxic doses are required. The usual dose of azathioprine is 2 to 3 mg/kg/day and 1 to 1.5 mg/kg/day for mercaptopurine. Up to 3 to 4 months may be required to observe a response. Starting doses are typically 50 mg/day and increased at 2-week intervals while monitoring complete blood count with differential. 

Patients deficient in thiopurine S-methyltransferase (TPMT) are at greater risk of bone marrow suppression from azathioprine and mercaptopurine. Determination of TPMT or TPMT genotype is recommended to guide dosage. 

Although the published data are not consistent, there is evidence to suggest that azathioprine, mercaptopurine, methotrexate, infliximab, and adalimumab are effective in maintaining remission in Crohn s disease. 

Metronidazole or methotrexate should not be used during pregnancy. Azathioprine and mercaptopurine may be associated with fetal deformities. 

Immunosuppressants such as azathioprine and mercaptopurine have a significant potential for adverse reactions, including bone marrow suppression, and have been associated with lymphomas (in renal transplant patients) and pancreatitis. Myelosuppression resulting in leukopenia is related to a deficiency in TPMT in some patients. 

Susceptibility to excessive myelosuppression from 6-mercaptopurine, 6-thioguanine, azathioprine in treatment of acute lymphocytic leukemia (thiopurine methyltransferase)... 

In an attempt to understand further the carcinogenicity of azathioprine, recent work in the authors laboratory has concentrated on the photodegradation of two compounds associated with the drug. l-Methyl-4-nitro-5-thioimid-azole is a metabolite and 5-hydroxy- l-methyl-4-nitroimidazole a hydrolysis product. Both gave, on irradiation at wavelengths greater than 300 nm, 1-methylparabanic acid (248) [152], The primary metabolites of azathioprine had previously been shown to be more potent than the parent drug as photosensitizers. Both 6-mercaptopurine and these imidazoles may play roles in its neoplastic action [151]. 

Hydrolysis azathioprine benorylate diazepam diphenhydramine frusemide furazolidone nitrofurantoin menaquinone-4 mercaptopurine methotrexate nalidixic acid nicotinamide nicotine nicotinic acid nifedipine... 

Increased risk of hydralazine-induced lupus, increased levels of isoniazid with an increased risk of neurotoxicity, increased risk of bladder cancer in individuals exposed to aromatic amines Increased risk of serious toxicity to mercaptopurine and azathioprine... 

The number of drugs susceptible to S-methylation is still limited but greater than the number turned over by COMT. Thiopurine methyl transferase (TPMT) is an important enzyme responsible for detoxifying mercaptopurine—a drug used to treat leukemia— as well as azathioprine —a prodrug that is metabolized to mercaptopurine (Fig. 7.12). This enzyme is polymorphic and patients who are homozygous for the deficient enzyme experience severe toxicity when given usual doses of mercaptopurine (19). Similar aromatic and heterocyclic sulfhydryls can also be substrates for TPMT. The similar thiol... 

Yates, C.R., Krynetski, E.Y., Loennechen, T., et al. (1997) Molecular diagnosis of thiopurine S-methyltransferase deficiency genetic basis for azathioprine and mercaptopurine intolerance. Ann. Intern. Med. 126, 608-614. 

Baker, D.E. (2003) Pharmacogenomics of azathioprine and 6-mercaptopurine in gastroentero-logic therapy. Rev. Gastroenterol. Disord. 3, 150-157. 

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