Enzymology and Structure-Activity Relationships of Quinoxaline Bioreductive Cytotoxins

Tumour hypoxia, a phenomenon described within many solid tumours, arises primarily from the development of a deficient vasculative system. The failure of conventional radiotherapy and chemotherapy in the treatment of solid tumours has been related to the presence of hypoxia within such tumours. For this reason, several methods have been introduced in an attempt to overcome these problems, and hence, lead to effective killing of the whole tumour. The development of bioreductive anticancer agents as a means of exploiting hypoxic cells within solid tumours has been based upon their ability to be preferentially cytotoxic towards hypoxic cells. Furthermore, these agents express a requirement for enzyme-mediated reductive activation in order to exert their cytotoxic effects. The implementation of an enzyme-directed bioreductive drug development programme, therefore has been based not only upon understanding the mechanism of action of particular bioreductive anticancer agents, but also upon determining the level of expression of reductases within both normal and tumour tissues. The National Cancer Institute (NCI possesses a panel of over 60 human tumour cell lines from at least nine different tumour origins. This cell line panel was used, therefore, as a means of determining the expression of several important reductase enzymes (DT-diaphorase: NADPH : cytochrome P450 reductase and NADH cvtochrome b5 reductase) within cell lines from different tumour origins. Importantly, patterns of expression were found for each of the reductase enzymes studied. Subsequent incorporation of this data into the NCI database demonstrated the potential benefits of such a system by indicating possible mechanisms of action of various anticancer agents. For example, a large number of quinone compounds were among those showing positive correlations between DT-diaphorase activity and cell sensitivity under oxic conditions. Of particular importance, were the correlations between DT-diaphorase activity and cell sensitivity towards both the prototype bioreductive, mitomycin C and the investigative indoloquinone agent E09. This thesis has determined the expression and activity of several important reductases within human tumour cells. The potential use of this information in determining the mechanism of action of anticancer agents has been demonstrated with respect to both the NCI database and also to the novel compounds investigated within this thesis. In this regard, the quinoxaline di-N-oxide ZM 81853 was shown to possess the ability to act as a hypoxia-selective bioreductive agent. Importantly, ZM 81853 exhibited similar cytotoxic and metabolic characteristics as the benzotriazine di-N-oxide, tirapazamine. The work of this thesis, therefore, will aid selection of potential bioreductive anticancer agents for further in vivo studies within ZENECA Pharmaceuticals. (Abstract shortened by ProQuest.)