6-Thiouric acid

Azathioprine can be administered both orally and intravenously. It is well absorbed orally and after its rapid conversion to 6-mercaptopurine it is inactivated by xanthine oxidase which converts 6-mercaptopurine to 6-thiouric acid. This final metabolite is then excreted in the urine. In combination with the xanthine oxidase inhibitor allopurinol dose adjustments of azathioprine are needed. Renal disease also raises 6-mercaptopurine concentrations and can make dose adjustments necessary. Azathioprine is still used in organ transplantation programs and for the management of several autoimmune diseases. Its adverse effects include nausea, vomiting, diarrhea and, more seriously, bone marrow suppression and hepatotoxicity. Azathioprine is not thought to cause fetal malformation. 

Azathioprine is well absorbed following oral administration, with peak blood levels occurring within 1 to 2 hours. It is rapidly and extensively metabolized to 6-mercaptopurine, which is further converted in the liver and erythrocytes to a variety of metabolites, including 6-thiouric acid. Metabolites are excreted in the urine. The half-life of azathioprine and its metabolites in the blood is about 5 hours. 

MP is converted to an inactive metabolite (6-thiouric acid) by an oxidation reaction catalyzed by xanthine oxidase, whereas 6-TG undergoes deamination. This is an important issue because the purine analog allopurinol, a potent xanthine oxidase inhibitor, is frequently used as a supportive care measure in the treatment of acute leukemias to prevent the development of hyperuricemia that often occurs with tumor cell lysis. Because allopurinol inhibits xanthine oxidase, simultaneous therapy with allopurinol and 6-MP would result in increased levels of 6-MP, thereby leading to excessive toxicity. In this setting, the dose of mercaptopurine must be reduced by 50-75%. In contrast, such an interaction does not occur with 6-TG, which can be used in full doses with allopurinol. 

Azathioprine is well absorbed from the gastrointestinal tract and is metabolized primarily to mercaptopurine. Xanthine oxidase splits much of the active material to 6-thiouric acid prior to excretion in the urine. After administration of azathioprine, small amounts of unchanged drug and mercaptopurine are also excreted by the kidney, and as much as a twofold increase in toxicity may occur in anephric or anuric patients. Since much of the drug s inactivation depends on xanthine oxidase, patients who are also receiving allopurinol (see Chapters 36 and 54) for control of hyperuricemia should have the dose of azathioprine reduced to one-fourth to one-third the usual amount to prevent excessive toxicity. 

Mercaptopurine is used in the treatment of acute lymphoid leukemia. Maintenance therapy makes use of both methotrexate and 6-mercaptopurine. Mercaptopurine is absorbed well from the gastrointestinal tract. It is metabolized through (1) methylation of the sulfhydryl group and subsequent oxidation, and (2) conversion to 6-thiouric acid with the aid of xanthine oxidase, which is inhibited by allopurinol. Mercaptopurine may cause hyperuricemia. Its chief toxicities are hepatic damage and bone marrow depression. 

When injected, azathioprine (Imuran) is rapidly converted to 6-mercaptopurine. The half-life of azathioprine after intravenous injection is 10 to 20 min, and that of 6-mercaptopurine is somewhat longer. The cytotoxic activity of these thiopurines is due to the conversion of mercaptopurine to 6-thiouric acid, a noncarcinostatic metabolite. This action is thought to block the excess synthesis of inosinic acid from its precursors, glutamine and phosphoribosylpyrophosphate. In addition, unlike cyclophosphamide, azathioprine is a potent anti-inflammatory substance that can cause a reduction in the number of monocytes and neutrophils at inflammatory sites. Antibody responses are also inhibited by azathioprine. Studies in humans have shown that azathioprine decreases the y-globulin and antibody levels, thus influencing IgG rather than IgM production. This makes azathioprine an effective immunosuppressant in the early phases of immune responses. It is less effective or completely ineffective in altering either the effector phase or already established reactivities. 

Azathioprine is well absorbed from the gastrointestinal tract and is metabolized primarily to mercaptopurine. Xanthine oxidase splits much of the active material to 6-thiouric acid prior to... 

Enzymic oxidation pathways vary with the substrate molecule, for purine-6-thione (60), although being fully oxidized to 6-thiouric acid (62), undergoes displacement first at the 8-carbon (61) rather than at... 

Disposition in the Body. Absorbed after oral administration and distributed throughout the body. It is readily metabolised to mercaptopurine, which is the major active metabolite, and to l-methyl-4-nitro-5-(5-glutathionyl)imidazole other metabolites include l-methyl-4-nitroimidazole, l-methyl-4-nitro-5-thioimi-dazole, and 6-thiouric acid mercaptopurine is further metabolised to its ribonucleotide, thioinosinic acid, which is the active moiety. About 50% of a dose is excreted in the urine in 24 hours, mainly as thiouric acid and other metabolites with about 10% consisting of unchanged drug about 12% of a dose is eliminated in the faeces in 48 hours. 

Disposition in the Body. Readily absorbed after oral administration bioavailability about 16%and very variable. It is distributed throughout the body water and diffuses into the cerebrospinal fluid. Mercaptopurine is activated in the body by intracellular conversion to nucleotide forms including the ribonucleotide thioinosinic acid. It is metabolised by xanthine oxidase to inactive 6-thiouric acid which is excreted in the urine inorganic sulphate may also be present. About 50% of an oral dose is excreted in the urine in 24 hours, up to 8% as unchanged drug. Small amounts are excreted for up to 17 days. 

Fig. 2 Metabolism of 6-mercaptopurine (6-MP) via xanthine oxidase (XO) to the inactive metabolite 6-thiouric acid (6-TU), thiopurine S-methyltransferase (TPMT) to the inactive metabolite 6-methylmercaptopurine (6-MMP), and hypoxanthine guanine phosphoribosyl transferase (HPRT) to 6-thioinosine monophosphate (6-TIMP) which is then further metabolized to thioguanine nucleotides (6-TGN), 6-methylmercaptopurine ribonucleotides (6-MMPR) or 6-thio-inosine triphosphate (6-thio-ITP), these all being active metabolites...

Mercaptopurine is partially metabolized to 6-thiouric acid (which lacks antitumor activity) by xanthine oxidase. Allopurinol, a xanthine oxidase inhibitor used in the treatment of gout, potentiates the action of 6-mercaptopurine by preventing its conversion to 6-thiouric acid. This effect is taken into consideration in treatment. Mercaptop-urines are also inactivated by S-methylation carried out by thiopurine S-methyltransferase (TPMT), particularly in hematopoietic tissues which lack xanthine oxidase. Deficiency of TPMT due to polymorphisms causes profound toxicity with the regular therapeutic regime. This is another example of the use of pharmacogenomics. 

Amongst mammals, xanthine oxidase levels appear to be less variable than aldehyde oxidase, with the highest activity for the former enzyme observed in dog, rat and cat (see Table 3.2). This is convenient for those drugs which are potential substrates for the enzyme, as dog and rat are the species routinely employed for metabolism and toxicity studies. For example, 6-mercaptopurine undergoes substantial biotransformation to 6-thiouric acid in both man and rat [116-118]. 

An interesting example of a DDI due to the inhibition of a non-CYP enzyme that can have serious clinical consequences is the inhibition of xanthine oxidase by allopu-rinol 6-mercaptopurine (6-MP) as an antimetabolite type of antineoplastic drug. One of its indications is in the treatment of inflammatory bowel disease. Actually, 6-MP is a prodrug whose active metabolite, 6-thiogua-nine (6-TG) is responsible for its therapeutic activity. Some nonresponders to 6-MP do not form sufficient amounts of 6-TG. A complementary pathway of 6-MP metabolism is oxidation to 6-thiouric acid (6TU), which is mediated by xanthine oxidase. Inhibition of this complementary pathway by allopurinol shunts the metabolism of 6-MP favoring increased formation of 6-TG. 

PHARMACOGENETICS Favorable responses to azathioprine-mercaptopurine are seen in up to two-thirds of patients. Mercaptopurine has three metabolic fates (1) conversion by xanthine oxidase to 6-thiouric acid (2) metabolism by thiopurine methyltransferase (TPMT) to... 

Some oxidations are mediated by hepatic enzymes localized outside the microsomal system. Alcohol dehydrogenase and aldehyde dehydrogenase, which catalyse a variety of alcohols and aldehydes such as ethanol and acetaldehyde, are found in the soluble fraction of the liver. Xanthine oxidase, a cytosolic enzyme mainly found in the liver and in small intestine, but also present in kidneys, spleen and heart, oxidizes mercaptopurine to 6-thiouric acid. Monoamine oxidase, a mitochondrial enzyme found in liver, kidney, intestine and nervous tissue, oxidatively deanoinates several naturally occurring amines (catecholamines, serotonin), as well as a number of drugs. 

PO/IV. Prodrug is metabolized to 6-mercaptopurine (6MP). 6MP is converted to 6-thiouric acid by xanthine oxidase. Renal excretion. 

Azathioprine is firstly metabolised in the liver to mercaptopurine and then enzymatically oxidised in the liver and intestinal wall by xanthine oxidase to an inactive compound (6-thiouric acid), which is excreted. Allopurinol inhibits first-pass metabolism by xanthine oxidase so that mercaptopurine accumulates, blood levels rise and its toxic effects develop (leucopenia, thrombocytopenia, etc.). 

In figure 6 the influence of 6-MP and allopurinol on the tetrahydrofolate formylase in leukemic blast cells of 5 patients with acute leukemia is illustrated. 6-MP in a rather high concentration of 1.5 x 10 M has only a small inhibitory effect on this enzyme.By addition of allopurinol the inhibition of the enzyme by 6-MP was markedly increased, whereas allopurinol alone had no effect. This is explained by the fact, that allopurinol, by inhibition of the xanthine oxidase, reduces the inactivation of 6-MP to 6-thiouric acid. 

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