Mercaptopurine resistance

Ki 0.4 and 4.8 yM, respectively). The enzyme is strongly inhibited by Zn2+. 5 -Nucleotidase in these cells has also been examined by Paterson and Hori (81) who found the enzyme located primarily in nuclei. Nuclei prepared from a 6-mercaptopurine-resistant subline were markedly deficient in enzymic activity. 

Resistance of neoplastic ceUs to the chemotherapeutic effect of 6-mercaptopurine would most likely involve loss or inactivation of a gene encoding... 

The incorporation of 6-mercaptopurine into DNA as 2-deoxythioguanylic acid has been observed but in cells resistant to the action of the drug, which makes the meaning of this observation unclear [201]. 

Both natural and acquired resistance constitute a serious problem to therapy with purine analogues, particularly in the case of cancer. Why one acute leukaemia responds well to methotrexate but not to 6-mercaptopurine, whereas a morphologically identical leukaemia responds to 6-mercaptopurine but not to methotrexate, and a third, seemingly identical leukaemia responds to neither is a vexing problem that has so far defied solution [449]. 

Early attempts to inhibit H.Ep.-2 cells resistant to 6-mercaptopurine [464J resulted in the finding that a number of 9-alkyl derivatives of 6-mercaptopurine were highly active in this test system. 9-Alkylhypoxanthines and adenines were less effective [465]. 

Mercaptopurine ribonucleotide is not active against cells resistant to 6-mercaptopurine, presumably because nucleotides cannot penetrate cell membranes intact [466] (its activity in sensitive cells is no doubt due to its cleavage back to 6-mercaptopurine [467]). 

Uses AML, ALL, CML Action Purine-based antimetabolite (substitutes for natural purines interfering w/ nucleotide synth) Dose 2-3 mg/kg/d X in severe renal/hepatic impair Caution [D, -] Contra Resistance to mercaptopurine Disp Tabs SE X BM (leucopenia/thrombocytopenia), NA /D, anorexia, stomatitis, rash, hyperuricemia, rare hepatotox Interactions t Bleeding W/ anticoagulants, NSAIDs, salicylates, thrombolytics EMS t Effects of anticoagulants/salicylates/ NSAIDs T risk of Infxn OD May cause NA, hypotension, and diaphoresis symptomatic and supportive... 

For the purine antagonists the most common mechanism of resistance is a deficiency or complete lack of the enzyme hypoxanthine-guanine phospho-ribosyltransferase. In addition, resistance can result from decreases in the affinity of this enzyme for its substrates. Increased levels of alkaline phosphohy-drolase can inactivate active metabolites of mercaptopurine. 

Resistance to mercaptopurine may be a result of decreased drug activation by HGPRTase or increased inactivation by alkaline phosphatase. 

Azathioprine and mercaptopurine appear to be of definite benefit in maintaining renal allografts and may be of value in transplantation of other tissues. These antimetabolites have been used with some success in the management of acute glomerulonephritis and in the renal component of systemic lupus erythematosus. They have also proved useful in some cases of rheumatoid arthritis, Crohn s disease, and multiple sclerosis. The drugs have been of occasional use in prednisone-resistant antibody-mediated idiopathic thrombocytopenic purpura and autoimmune hemolytic anemias. 

Chen ZS, Lee K, Kruh GD. Transport of cyclic nucleotides and estradiol 17-beta-D-glucuronide by multidrug resistance protein 4 resistance to 6-mercaptopurine and 6-thioguanine. J Biol Chem 2001 276 33747-33754. 

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. 

Rerson, J.L., McHutchison, J.G., Fong, T., Redeker, A.G. A case of cyclosporine-sensitive, steroid-resistant autoimmune chronic active hepatitis. J. Clin. Gastroent. 1993 17 317-320 Rratt, D.S., Flavin, D.R., Kaplan, M.M. The successful treatment of autoimmune hepatitis with 6-mercaptopurine after failure with azathioprine. Gastroenterology 1996 110 271-274 Rahaman, SM., Chira, R, Koff, R.S. Idiopathic autoimmune chronic hepatitis triggered by hepatitis A. Amer. J. Gastroent. 1994 89 106-108... 

Thioguanine is u.sed in treating acute leukemia, especially in comhination with cytarabine." - Cro.ss-resistance exists between thioguanine and mercaptopurine. The chief toxic effect is delayed bone marrow depre.ssion. resulting in leukopenia and eventually thrombocytopenia and bleeding. 

Mercaptopurine is used in the treatment of acute leukemias. However, resistant tumor cells develop rapidly, probably because of altered specificity or lack of phospho-ribosyltransferases, so that thio-IMP (the active inhibitor) is not formed. In support of this mechanism, resistant cells respond to 6-methylmercaptopurine ribonu-cleoside, which is phosphorylated to the corresponding nucleotide. Other mechanisms may include altered cell permeability and increased rate of destruction of 6-mercaptopurine. 

Starch-gel electrophoresis of the alkaline phosphatase in the butanol extracts of leukocytes revealed three variants of the enzyme. Peacock et al. (PI) have devised a method for leukocyte alkaline phosphatase assay. An additional variant was detected in blood leukocytes of leukemia patients treated with 6-mercaptopurine (RIO). Robinson and Pierce (R7) indicated that there might be a fundamental difference in molecular structure of the human serum alkaline phosphatase proteins because serum alkaline phosphatase, when incubated with neuraminidase prior to electrophoresis, demonstrated reduced anodal migration of those isoenzymes that are not L-phenylalanine-sensitive. L-Phenylalanine-sensi-tive enzyme of intestinal origin was found to be neuraminidase-resistant. 

Purine and Pyrimidine Analogues - The following comments will center upon work published in 1966 Heidelberger has included earlier work in this area in his recent review. Modifications of 6-mercaptopurine (I) are of continuing interest because of mereaptopurine s activity in human leukemia, and the ultimate development of resistance by the leukemic cells to this drug. 6-methylmercaptopurine riboside (II) is converted to 6-methyImercaptopurine ribonucleotide (III) by a nucleoside kinase. Cells... 

Answer C. The purine antimetabolite 6-mercaptopurine is bioactivated in cancer cells by the purine salvage enzyme hypoxanthine guanine phosphoribosyltransferase (HGPRT). The most common form of resistance to 6-MP is a decrease in activity of this enzyme. Azathioprine, a drug used as an immunosuppressant, is closely related to 6-MP and also requires bioactivation to exert cytotoxic actions. 

It has subsequently been shown that mercaptopurine is first metabolised by the enzyme hypoxanthine phosphoribosyl transferase into thioinosine monophosphate and this then acts as an inhibitor of de novo purine biosynthesis. Mercaptopurine worked even in patients whose leukaemia cells had become resistant to the effects of methotrexate, suggesting that combination chemotherapy with the two drugs might extend the period of remission from disease. The idea of combination chemotherapy to delay induction of resistant strains of microorganisms had, it will be recalled, been suggested by Ehrlich in around 1912. With leukaemia, it was the major breakthrough that was needed to achieve long-term survival in the disease. 

The reverse phenomenon, decreased enzyme synthesis, can also be the mechanism of drug resistance. The antimetabolite pro-drug 6-mercaptopurine (6MP) is activated to its nucleotide by inosine-5 -phosphate pyrophosphorylase. The enzyme is deleted in resistant neoplastic cells. Resistance to 5-fluorouracil similarly develops by deletion of the enzyme converting this pro-drug to its active nucleotide. A mechanism of resistance by which a drug is excluded from its site of action can also be operative. This has been established for tetracycline antibiotics. Here the permeability of the cellular membrane to the drug is altered so that it cannot penetrate and accumulate within the target cell. Similarly, it has been demonstrated with such a membrane modification in MTX-resistant leukemia cells in mice. 

Fig. 4-14). Both 6-mercaptopurine and 6-thioguanine are not active purine inhibitors until they are converted to their respective nucleotides by hypoxanthine-guanine phosphoribo-syl transferase enzymes in the cell. Since this is an intrinsically destructive process to the cell, it is referred to as a lethal synthesis or salvage pathway. Tumor cells or normal cells that have very low levels of this enzyme are resistant to these two agents. Absence of this enzymatic function is not lethal to the cell since the synthesis of purines in the cell can occur by different pathways. 

Methotrexate generally is reserved for patients whose IBD is either steroid-resistant or steroid-dependent. In Crohn s disease, it both induces and maintains remission, generally with a more rapid response than that seen with mercaptopurine or azathioprine. Only limited studies have examined the role of methotrexate in ulcerative colitis. 

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