The role of nitric oxide as a hypoxic cell radiosensitizer

Abstract

Many tumours contain regions of hypoxia which are difficult to treat by conventional radiotherapy. There is much interest in the ability of nitric oxide (•NO) to radiosensitize hypoxic mammalian cells as a possible adjunct to radiotherapy but mechanisms for its action are unclear. It has been proposed that •NO may radiosensitize cells by ‘fixing’ radiation-induced DNA free radicals, and elevated radiation response by •NO in cells has been partly attributed to increased formation of DNA double strand breaks. In the work carried out for this thesis it is shown that reaction of •NO with radiation-induced nucleobase radicals produces some novel products. New pathways for the reactions of radiation-induced hydroxyl radicals with purine radicals are proposed. In addition, the effects of •NO on the yields of radiation-induced single strand breaks in anoxic plasmid DNA, and on anoxic mammalian cell radiosensitivity are investigated. Kinetics of formation and repair of radiation-induced double strand breaks indicate different effects of •NO on radiation-induced clustered and non-clustered DNA damage involving replication-induced DNA breaks. As •NO is an inhibitor of ribonucleotide reductase, some of the radiosensitizing properties of •NO may be due to reduction in the availability of 2-deoxyribonucleotides. Through studying reactions of •NO with tyrosine radicals, essential components of ribonucleotide reductase, this work has enhanced understanding into how •NO may inhibit the enzyme, which may offer new insights into the development of •NO-releasing anti-cancer agents. The potential for delivery of •NO to hypoxic tissue for radiotherapy has also been investigated in this work, through the development of bioreductively-activated pro-drugs. These novel agents are stable until reduced by one-electron reductants, when a •NO-releasing pro-drug is rapidly evolved, only in those regions which are sufficiently hypoxic. By increasing our understanding into the mechanisms involved in the ability of •NO to radiosensitize hypoxic cells, especially the reactivity of •NO with DNA radicals, knowledge has been gained into the identification, development and repair of radiation-induced DNA damage in cells, including clustered damage, in the presence of •NO. These studies contribute to further development of novel anti-cancer therapies based upon the release of •NO in hypoxic cells.This thesis is not currently available in OR