6-Mercaptopurine drug resistance

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. 

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]. 

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. 

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. 

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. 

Mechanisms of action and resistance Mercaptopurine and thioguanine are purine antimetabolites. Both drugs are activated by hypoxanthine-guanine phosphoribosyltrans-ferases (HGPRTases) to toxic nucleotides that inhibit several enzymes involved in purine... 

Other examples follow of resistance achieved by increased production of a destructive enzyme. The treatment of acute leukaemia with cytosine arabino-side 4.13) fails in proportion as malignant cells with a higher concentration of cytosine deaminase appear (Steuart and Burke, 1971). Acquisition of insensitivity to 6-mercaptopurine by acute lymphocytic leukaemia cells in Man, and by murine sarcoma 180/TG, was shown to be due to an increase in alkaline phosphatase which causes degradation of the tumour-inhibiting nucleotide to which the cell converts this pro-drug (Rosman etal., 1974). 

Human neoplasm cells in culture show a different type of resistance to 6-mercaptopurine from that cited above, in that they delete the enzyme responsible for converting this pro-drug to the therapeutic nucleotide (6-thioinosine 5 -phosphate) (Brockman, 1963). The deleted enzyme is inosine 5 -phosphate pyrophosphorylase. Similarly, resistance to 8-azaguanine is accompanied by loss of the enzyme guanosine 5 -phosphate pyrophosphorylase in human epidermoid carcinoma cells (Brockman et al., 1961). Resistance to 5-fluoro-uracil also depends on the uneconomic cell s ceasing to convert this pro-drug to the nucleotide. 

In experimental tumours, resistance to 6-mercaptopurine is usually caused by deletion of an enzyme required to convert this pro-drug into the active molecule. The enzyme is hypoxanthine—guanine phosphoribosyltransferase (Harrap, 1976). Resistance to this drug in leukaemic patients seems to follow the same course (Rosman and Williams, 1973). 

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