Discovery and Optimisation of Bacterial Nitroreductases for Use in Anti-Cancer Gene Therapy
Nitroaromatic prodrugs are biologically inert compounds that are attractive candidates for anti-cancer therapy by virtue of their ability to be converted to potent DNA alkylating agents by nitroreductase (NTR) enzymes. In gene-directed enzyme-prodrug therapy (GDEPT), NTR-encoding therapeutic transgenes are delivered specifically to tumour cells, whereupon their expression confers host cell sensitivity to subsequent systemic administration of a nitroaromatic prodrug. The most well studied NTR-GDEPT system involves reduction of the aziridinyl dinitrobenzamide prodrug CB1954 by the Escherichia coli NTR NfsB. However, low affinity of this enzyme for CB1954 has so far limited the clinical efficacy of this GDEPT combination. The research described in this thesis has primarily sought to address this limitation through identification and optimisation of novel NTR enzymes with improved nitroaromatic prodrug reductase activity. Efficient assessment of NTR activity from large libraries of candidate enzymes requires a rapid and reliable screening system. An E. coli-based assay was developed to permit indirect assessment of relative rates of prodrug reduction by over-expressed NTRs via measurement of SOS response induction resulting from reduced prodrug-induced DNA damage. Using this assay in concert with other in vitro and in vivo tests, more than 50 native bacterial NTRs of diverse sequence and origin were assessed for their ability to reduce a panel of clinically attractive nitroaromatic prodrugs. Significantly, a number of NTRs were identified, particularly in the family of enzymes homologous to the native E. coli NTR NfsA, which displayed substantially improved activity over NfsB with CB1954 and other nitroaromatic prodrugs as substrates. This work also examined the roles of E. coli DNA damage repair pathways in processing of adducts induced by various nitroaromatic prodrugs. Of particular interest, nucleotide excision repair was found to be important in the processing of DNA lesions caused by 4-, but not 2-nitro group reduction products of CB1954, which suggests that there are some parallels in the mechanisms of CB1954 adduct repair in E. coli and mammalian cells. Finally, a lead NTR candidate, YcnD from Bacillus subtilis, was selected for further activity improvement through site-directed mutagenesis of active site residues. Using SOS screening, a double-site mutant was identified with 2.5-fold improved activity over the wildtype enzyme in metabolism of the novel dinitrobenzamide mustard prodrug PR-104A. In conclusion, novel NTRs with substantially improved nitroaromatic prodrug reducing activity over previously documented enzymes were identified and characterised. These results hold significance not only for the field of NTR-GDEPT, but also for other biotechnological applications in which NTRs are becoming increasingly significant, including developmental studies, antibiotic discovery and bioremediation. Furthermore, the in vitro assays developed in this study have potential utility in the discovery and evolution of other GDEPT-relevant enzymes whose prodrug metabolism is associated with genotoxicity.