Abstract
XR5944 and XR11576 are two potent DNA interactive agents, previously shown to be capable of stabilising topoisomerase I and II cleavable complexes in vitro. This thesis investigated the possibility that the mechanism(s) of action of these compounds may be unrelated to that of topo inhibition. These studies made use of a wide range of drug-sensitive and resistant cancer cell lines. It was demonstrated that both XR5944 and XR11576 retain potent cytotoxicity in cancer cell lines presenting with atypical drug resistance to single topoisomerase poisons. XR5944 showed less potency in cell lines expressing ABC-transporter proteins, but this may not be sufficient to compromise the activity of this potent anti-tumour agent in the clinical setting. The mechanism of XR11576 induced cytotoxicity was not affected by any of these transporters and from this and other observations it may possess a different mechanism of action from XR5944. An apoptotic response was observed in XR5944 and XR11576 treated cells. Factors such as Bax and t-Bid were expressed in increasing amounts in response to treatment and implicated the mitochondrial route of apoptosis in these compounds' mechanism of cell kill. Both XR5944 and XR11576 induced appreciable levels of DNA-protein crosslink formation and induced a p53 DNA damage response in drag treated cells. Differences in the timing of onset and extent of DNA damage and p53 induction were noticed between the two compounds. Overall, XR5944 was slower at causing these DNA interactive effects in line with the onset of cytotoxicity. However, this did not appear to pose any particular problems, such as scheduling of dosing, with regard to its in vivo activity, as shown by the work of others. These findings suggest that both XR5944 and XR11576 promote potent cytotoxicity in cancer cell lines showing multiple mechanisms of drug resistance and, therefore, should be of use in the clinic for the treatment of drug resistant tumours. The data presented in this thesis suggest that neither agent exerts its cytotoxicity via a topo directed effect. Furthermore, some of the data generated suggests that these two compounds may differ from each other in their mechanisms of action.