The characterisation and application of bacterial nitroreductase enzymes

Abstract

Cancer is an increasing global concern, with the number of people diagnosed growing rapidly each year. Gene directed enzyme prodrug therapy (GDEPT) is emerging as a front-runner of new technologies that seek to combat the growing number of cases. One developing approach to GDEPT involves the use of bacterial nitroreductase enzymes to reduce prodrug substrates, which, upon reduction to their active form, are toxic to cancer cells through DNA crosslinking.

Nitroreductases have the ability to activate a variety of nitro-quenched compounds, not only anti-cancer prodrugs, but also nil bystander antibiotics and masked fluorophores, through the reduction of strongly electron-withdrawing nitro substituents on aromatic rings. My research initially sought to exploit this capability by partnering nitroreductases with nil bystander antibiotics for targeted cell ablation, as a component of a larger gene directed enzyme prodrug therapy project. This has potential to provide important safety features for removal of viral and bacterial vectors following anti-cancer gene therapy.

From this, the main focus evolved into utilising nitroreductase enzymes for targeted cell ablation for applications in developmental and regenerative biology. This exploited the ability of nitroreductases to activate nil bystander antibiotics in partnership with masked fluorophores for imaging purposes. It has previously been shown that antibiotics can be applied to a nitroreductase under control of a tissue-specific promoter in a transgenic model organism, enabling controlled ablation of that tissue at precise stages of development. However, direct imaging of the nitroreductase location and activity, by application of masked fluorophore probes prior to ablation, has not previously been explored.

During the course of this work, several promising combinations of nitroreductases that exhibit opposing specificities for certain combinations of masked fluorophores and nil-bystander antibiotics were identified through screening in bacterial systems. In general, these results were found to translate effectively into eukaryotic cell lines. Pairs of nitroreductases that have opposite specificities for two different antibiotic substrates offer potential for the multiplexed ablation of either (or both) of two different labelled tissues in the same transgenic model organism, according to the substrate(s) administered to that organism.

Throughout this screening process, a nitroaromatic substrate (niclosamide) was identified that is, uniquely, initially toxic to Escherichia coli but becomes non-toxic upon reduction of the nitro substituent. Using niclosamide, a novel strategy with potential for identification of new nitroreductases, as well as selection-based directed evolution to improve desired activities, was explored.