In the absence of successful new therapies for testing cancer understanding the mechanism of action of existing anticancer drugs holds the greatest promise for improving therapy and developing more selective anticancer drugs. Quinones form the largest and most active class of currently approved anticancer agents. Despite extensive study the mechanism of action of the antitumor quinones has not been elucidated. Two features are, however, clearly required for antitumor quinone activity, reductive metabolism and the formation of an alkylating species. The group of enzymes responsible for reductive metabolism of the antitumor quinones is the flavoenzymes. Our hypothesis is that key flavoenzymes both determine the extent of antitumor quinone bioactivation and can themselves be critical targets for quinone cytotoxicity. The Two mechanisms represent different aspects of the same process involving the formation of a reactive quinone species at the active site of the flavoenzyme. If the reactive species diffuses away it will affect other sensitive sites in the cell, but if it is trapped at the active site it will inhibit the enzyme. Our studies are focused on thioredoxin reductase (TR), a key flavoenzyme for ribonucleotide reductase, the first unique step in DNA synthesis and NAD(P)H:(quinone acceptor)oxidoreductase (DT- diaphorase, DT) a flavoenzyme that uniquely catalyzes the two electron reduction of quinones. We have evidence that antitumor quinones are suicide substrate inhibitors of TR and that inhibition of TR is related to quinone cytotoxicity. We also report that DT is present at very high levels in human tumors compared to normal tissue, but these high tumor levels are completely suppressed in smokers. This may affect the response of smokers to chemotherapy. The objectives of the study are: a) to measure DT an TR levels in human tissues and in cell culture models for smoking, and to relate the levels to response to cytotoxic quinones, and b) to conduct mechanistic studies of quinone bioactivation and flavoenzyme inhibition as a mechanism for the action of antitumor quinones. Our goal is to improve the treatment of cancer by understanding how antitumor quinone drugs work and to identify new molecular targets for the development of new drugs.

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National Cancer Institute (NCI)
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Experimental Therapeutics Subcommittee 1 (ET)
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Mayo Clinic, Rochester
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