Escherichia coli Nissle 1917 Facilitates Tumor Detection by Positron Emission Tomography and Optical Imaging

^{1 1}Department of Radiology, ²Department of Medical Physics, ³Cyclotron and Radiochemistry Core Facility, ⁴Nuclear Pharmacy, and ⁵Department of Surgery, Memorial Sloan-Kettering Cancer Center, New York, New York; ⁶Genelux Corporation, San Diego Science Center, San Diego, California; and ⁷Institute for Biochemistry, Biocenter; Institute for Molecular Infectious Biology; and ⁸Virchow Center for Biomedical Research, School of Medicine, University of Wuerzburg, Wuerzburg, Germany

^* To whom correspondence should be addressed. E-mail: blasberg{at}neuro1.mskcc.org.

	Abstract

Purpose: Bacteria-based tumor-targeted therapy is a modality of growing interest in anticancer strategies. Imaging bacteria specifically targeting and replicating within tumors using radiotracer techniques and optical imaging can provide confirmation of successful colonization of malignant tissue.

Experimental Design: The uptake of radiolabeled pyrimidine nucleoside analogues and [¹⁸F]FDG by Escherichia coli Nissle 1917 (EcN) was assessed both in vitro and in vivo. The targeting of EcN to 4T1 breast tumors was monitored by positron emission tomography (PET) and optical imaging. The accumulation of radiotracer in the tumors was correlated with the number of bacteria. Optical imaging based on bioluminescence was done using EcN bacteria that encode luciferase genes under the control of an l-arabinose–inducible P_BAD promoter system.

Results: We showed that EcN can be detected using radiolabeled pyrimidine nucleoside analogues, [¹⁸F]FDG and PET. Importantly, this imaging paradigm does not require transformation of the bacterium with a reporter gene. Imaging with [¹⁸F]FDG provided lower contrast than [¹⁸F]FEAU due to high FDG accumulation in control (nontreated) tumors and surrounding tissues. A linear correlation was shown between the number of viable bacteria in tumors and the accumulation of [¹⁸F]FEAU, but not [¹⁸F]FDG. The presence of EcN was also confirmed by bioluminescence imaging.

Conclusion: EcN can be imaged by PET, based on the expression of endogenous E. coli thymidine kinase, and this imaging paradigm could be translated to patient studies for the detection of solid tumors. Bioluminescence imaging provides a low-cost alternative to PET imaging in small animals.

Key Words: Positron emission tomography (PET), bioluminescence imaging, E. coli Nissle 1917 (EcN), bacterial therapy, molecular imaging, tumor targeting, thymidine kinase, [¹⁸F]FEAU, [¹²⁴I]FIAU, [¹⁸F]FDG