Thioinosinic acid

The answer is c. (Katzung, p 933.) Resistance to thioguanine occurs because of an increase in alkaline phosphatase and a decrease in hypoxanthine-guanine phosphoribosyl transferase. These enzymes are responsible, respectively, for the increase in dephosphorylation of thiopurine nucleotide and the conversion of thioguanine to its active form, 6-thioinosinic acid. 

Studies on the mechanism of action of 6-mercaptopurine are complicated by the fact that its anabolic product, thioinosinic acid, is further metabolized by oxidation to 6-thioxanthylic acid [219] and by methylation to 6-(methylthio)purine ribonucleotide [206, 296]. the effects of which could be even more important than those of thioinosinic acid itself, since the methylthio compound is about 20 times as potent as a feedback inhibitor [289]. 

Chloropurine ribonucleotide and thioinosinic acid also react covalently with GMP reductase, the enzyme that converts guanylic acid to inosinic acid and ammonia [318]. 

Azathioprine is converted in vivo to thioinosinic acid, which competitively inhibits the synthesis of in-osinic acid, the precursor to adenylic acid and guanylic acid. In this way, azathioprine inhibits DNA synthesis and therefore suppresses lymphocyte proliferation. This effectively inhibits both humoral and cell-mediated immune responses. 

Lennard L. Assay of 6-thioinosinic acid and 6-thioguanine nueleotides, aetive metabolites of 6-mercaptopurine, in human red blood cells. JChomatogr 1987 423 169-178. 

Lennard L, Singleton HJ. High-performance liquid ehromatographie assay of the methyl and nucleotide metabolites of 6-mercaptopurine quantitation of red blood eell 6-thioguanine nueleotide, 6-thioinosinic acid, and 6-methylmercaptopurine metabolites in a single sample. J Chromatogr 1992 583 83-90. 

Azathioprine is a cytotoxic inhibitor of purine synthesis effective for the control of tissue rejection in organ transplantation. It is also used in the treatment of autoimmune diseases. Its biologically active metabolite, mercaptopurine, is an inhibitor of DNA synthesis. Mercaptopurine undergoes further metabolism to the active antitumour and immunosuppressive thioinosinic acid. This inhibits the conversion of purines to the corresponding phosphoribosyl-5 phosphates and hypoxanthine to inosinic acid, leading to inhibition of cell division and this is the mechanism of the immunosuppression by azathioprine and mercaptopurine. Humans are more sensitive than other species to the toxic effects of the thiopurines, in particular those involving the haematopoietic system. The major limiting toxicity of the thiopurines is bone marrow suppression, with leucopenia and thrombocytopenia. Liver toxicity is another common toxic effect. 

Originally developed for chemotherapy, azathioprine is used today mainly as an immunosuppressive agent and rarely as an antineoplastic drug. It was introduced as an immunosuppressive agent by a British pioneer of tissue transplantation, Roy Caine. Azathioprine was used to prevent rejection after tissue transplantation as a replacement for 6-mercaptopurine because it was less toxic. In addition to tissue transplantation, it is also used for rheumatoid arthritis and Crohn s disease. Azathioprine is a prodrug which in the body is converted to its active metabolites 6-mercaptopurine and 6-thioinosinic acid. Until the discovery of cyclosporine, azathioprine in combination with steroids was the standard treatment to prevent rejection after tissue transplantation. 

Azathioprine [a zah THIO preen] has been the cornerstone of immunosuppressive therapy over the last several decades. It has a nitroimidazoloyl side chain attached to the sulfur of 6-mercap-topurine, which is removed by non-enzymatic reduction in the body by glutathione to yield 6-mercaptopurine (6-MP). The latter is then converted to the corresponding nucleotide, thioinosinic acid (TIMP), by the salvage pathway enzyme, hypoxanthine-gua-nine phosphoribosyl transferase. The immunosuppressant effects of azathioprine are due to this fraudulent nucleotide. (See pp. 380-381 for a discussion of 6-MP s mechanism of action, resistance, pharmacokinetics, and adverse effects.) Because of their rapid proliferation in the immune response, and their dependence on de novo synthesis of purines required for cell division, lymphocytes are predominantly affected by the cytotoxic effects of azathioprine. The drug has little effect on suppressing a secondary immune response. 

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. 

This drug is metabolized to 6-thioinosinic acid, which inhibits the purine nucleoside pathway. 

The drug gets converted to 6-thioinosinic acid that predominantly serves as an antimetabolite to inhibit synthesis of adenine and guanine besides, it also presents conversion of purine bases into the corresponding nucleotides. 

Fig. 42.33. Xanthine oxidase inactivation of mercaptopurine and 6-thioinosinic acid.

R.B. Meyer, T.E. Stone, and B. Ulman, 2 -G-Acyl-6-thioinosine cyclic 3, 5 -phosphate as prodrugs of thioinosinic acid, J. Med. Chem. 22 811 (1977). 

In a similar way the purine antimetabolite 6-mercapto-purine (6-MP)/ after conversion to the monophosphoribo-tide, acts as an inhibitor of this enzyme (9.10). There have been described other sites of action of 6-MP (4-7) which are marked by arrows in figure 1. We have previously demonstrated the inhibition of the enzymic formate activation (tetrahydrofolate formylase) in leukemic cells by 6-MP (12). inspite of a rather high inhibitory concentration of 6-MP between 10 and 10 M, this inhibition has some practical clinical implications for the treatment of acute leukemia/ as it is detected in sensitive leukemic cells only. In accordance with reports of several authors (8,9/10) we have postulated/ that 6-MP has to be converted into 6-thioinosinic acid for exerting its inhibitory effect on the de novo synthesis of purine-nucleotides. On the other hand DAVIDSON and WINTER have shown that both 6-MP sensitive and resistant cells con-... 

According to the experiments described in table 3 the addition of allopurinol does effect neither the inhibition of the hypoxanthine-guanine phosphoribosyltransferase by 6-MP nor the conversion of 6-MP to thioinosinic acid by the same enzyme. It is unprobable that this is due to a lack of xanthine oxidase in the cells investigated, because, in the same cell-free system the inhibitory effect of 6-MP on the formate activation was markedly increased by allopurinol (fig.6). 

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