Abstract B89: Molecular imaging using bacterial nitroreductase reporter genes by repurposing the clinical stage hypoxia PET probe EF5.
Authors: Patterson Adam V. , Copp Janine , Syddall Sophie , Guise Chris , Mowday Alexanda , Abbattista Maria , Li Dan , Williams Elsie , Prosser Gareth , Ashoorzadeh Amir , Denny William , Smaill Jeff , Ackerley David F. 
Published: June 2012

Oncolytic viruses and tumor-tropic bacteria offer promise as cancer therapeutics of the future. There is a need to develop technologies to monitor the spatio-temporal distribution of these live vectors in a manner that is predictive of normal tissue toxicity and antitumor efficacy. Positron emission tomography (PET) is the preferred non-invasive imaging modality (biomarker) but suitable advanced reporter gene/PET probe combinations are lacking. A range of 2-nitroimidazole (2-NI) probes are currently in clinical use for the detection of hypoxia (EF5, FMISO, HX4, FAZA) and thus have already attained a high level of clinical validation. We hypothesized that (2-NI) PET agents might be repurposed to monitor the biodistribution of replicating biological agents, thereby leveraging two decades of research efforts to optimise hypoxia PET probe pharmacology.

Bacterial nitroreductases (NTRs; type I) are efficient O2-independent enzymes that provide the necessary catalytic flexibility to achieve this goal. Historically E. coli NfsB has been the focus of gene therapy applications. We cloned eleven candidate NTRs from E. coli namely; AzoR, KefF, MdaB, NemA, NfsA, NfsB, WrbA, YcaK, YcdI, YdjA and Yief. Using HCT116 cells engineered to express each NTR, we showed NfsA alone was able to metabolise a range of 2-NI probe molecules, including EF5 (pentafluoroetanidazole), leading to efficient cellular retention. We confirmed catalytic efficiency of EF5 reduction by recombinant NfsA (kcat/Km 97 s−1/mM). In contrast NfsB only weakly metabolised EF5 (kcat/Km 0.24 s−1/mM). We compared EF5 adduct retention in HCT116 cells, detected by mAb using flow cytometry, either under anoxia (&lt; 10ppm O2) or engineered to express NfsA or NfsB. Following exposure to EF5 in monolayer, median fluorescence was &gt;100-fold greater in NfsA expressing cells than anoxic cells, whereas NfsB cells were negative. Mixed Wt : NfsA multicellular layers containing various ratios of cells demonstrated the capacity to detect, with precision, single NfsA-positive cells in mixed cell populations. HCT116 xenografts composed of increasing proportions of NfsA cells (0% to 25%) were established in nude mice and analysed by immunohistochemistry and flow cytometry. Ex-vivo pimonidazole-binding confirmed all NfsA positive cells were detected in vivo following administration of EF5.

NfsA, like NfsB, can bioactivate the clinical-stage prodrug PR-104. We determined the relationship between EF5 retention and PR-104 cytotoxicity in HCT116 xenografts harbouring variable proportions of NfsA cells (0%-25%). A correlation was observed between total EF5 retention and global clonogenic cell kill (r2 = 0.828; p &lt;0.001) indicating EF5 binding by NfsA cells predicts for whole tumor sensitivity to PR-104. Tumor growth delay (TGD) studies confirmed the excellent efficacy of PR-104 (1.0 mmol/kg, ip) against tumors containing a minority (14.7%) of NfsA expressing cells (TGD = 386%; p&lt;0.001). This marked tumor control achieved by single dose PR-104 confirmed the presence of a robust bystander cell killing effect able to control the majority (85%) of WT cells. In summary, use of E. coli NfsA enables the repurposing of hypoxia PET probes for reporter gene applications which has significant utility in the development of vector platforms and shows promise in combination with the prodrug PR-104.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B89.