ORIGINAL ARTICLE Determination of tumour hypoxia with the PET tracer [ 18 F]EF3: improvement of the tumour-to-background ratio in a mouse tumour model Nicolas Christian & Anne Bol & Marc De Bast & Daniel Labar & John Lee & Pierre Mahy & Vincent Grégoire Received: 3 July 2006 / Accepted: 24 January 2007 / Published online: 3 March 2007 # Springer-Verlag 2007 Abstract Purpose The 2-(2-nitroimidazol-1-yl)-N-(3,3,3-trifluoro- propyl)acetamide (EF3) is a 2-nitroimidazole derivative which undergoes bioreductive activation under hypoxic conditions. Using the PET tracer [ 18 F]EF3 in mice, tumour- to-muscle ratios ranging from 1.3 to 3.5 were observed. This study investigated the impact of various interventions aimed at increasing [ 18 F]EF3 elimination, thus potentially increasing the tumour-to-noise ratio in mice, by increasing the renal filtration rate (spironolactone, furosemide), de- creasing tubular re-absorption (metronidazole, ornidazole, amino acid solution) or stimulating gastro-intestinal elim- ination (phenobarbital). Methods C3H mice were injected i.v. with an average of 12.95 MBq of [ 18 F]EF3. Drugs were injected i.v. 15 min before the tracer or daily 4 days prior to the experiment (phenobarbital). Anaesthetised mice were imaged from 30 to 300 min with a dedicated animal PET (Mosaic, Philips). Regions of interest were delineated around the tumour, bladder, heart, liver and leg muscle. Radioactivity was expressed as a percentage of injected activity per gram of tissue. Results Ornidazole decreased the urinary excretion and increased the liver uptake of [ 18 F]EF3, but without causing any changes in the other organs. Phenobarbital significantly increased the liver concentration and decreased radioactiv- ity in blood and muscle without affecting the tracer uptake in tumour. Consequently, a small but non-significant increase in tumour-to-noise ratio was observed. Although some effects were observed with other drugs, they did not modify the tumour-to-noise ratio. Conclusion Only phenobarbital induced a trend toward an increased tumour-to-noise ratio that could possibly be tested in the clinical situation. Keywords Hypoxia . Small animal PET . EF3 . Mouse Introduction Tumour hypoxia has long been known to be an important prognostic factor in human oncology [1]. Hypoxia leads to a more aggressive phenotype and increases tumour resis- tance to both radiation and chemotherapy [2]. The level of hypoxia within a tumour is correlated with local recurrence and overall survival in several tumour types [3–5]. Therefore, identifying hypoxic fractions in tumours may lead to modification of treatment strategies, such as concomitant use of hypoxic sensitisers or bioreductive agents, or an increase in the radiation dose in hypoxic regions in order to overcome radiation resistance and improve tumour curability. The use of the 2-nitroimidazole compounds is an attractive method to detect tumour hypoxia [6]. These molecules undergo intracellular reduction by nitroreductase enzymes, and under hypoxic conditions their reduced moieties covalently bind to intracellular macromolecules, mainly to thiol-containing proteins. These adducts, trapped in the hypoxic cells, can be detected using specific antibodies by immunohistochemistry or immunofluores- cence on tissue sections, by flow cytometry on cell Eur J Nucl Med Mol Imaging (2007) 34:1348–1354 DOI 10.1007/s00259-007-0376-7 N. Christian (*) : A. Bol : M. De Bast : D. Labar : J. Lee : P. Mahy : V. Grégoire Center for Molecular Imaging and Experimental Radiotherapy, Université Catholique de Louvain, Av Hippocrate, 10, Brussels, 1200 Brussels, Belgium e-mail: nicolas.christian@imre.ucl.ac.be