European Journal of Nuclear Medicine and Molecular Imaging Vol. 31, No. 4, April 2004 Abstract. The first aim of this study was to compare the hypoxia imaging ability of fluorine-18 fluoroerythroni- troimidazole ([ 18 F]FETNIM) with that of fluorine-18 flu- oromisonimidazole ([ 18 F]FMISO) in murine tumours of different sizes under two different oxygenation condi- tions. Secondly, we wanted to assess the biodistribution of the markers in normal tissues under similar condi- tions. Female CDF1 mice with a C3H mammary carcino- ma grown on their backs were used. Tumours were size matched and animals breathed either normal air (21% O 2 ) or carbogen gas (95% O 2 + 5% CO 2 ). The gassing procedure was begun 5 min before the intravenous injec- tion of either [ 18 F]FETNIM or [ 18 F]FMISO and contin- ued until the mice were sacrificed at 120 min. Blood, tumour, muscle, heart, lung, liver, kidney and fat were removed, counted for radioactivity and weighed. The tumour and muscle were frozen and cut with a cryomi- crotome into sections. The spatial distribution of radio- activity from the tissue sections was determined with digital autoradiography. Estimation of the necrotic frac- tion was made on sections from formalin-fixed tumours. Digital autoradiography showed that the whole tumour- to-muscle radioactivity uptake ratios were significantly higher in normal air-breathing mice than in carbogen- treated mice for both [ 18 F]FETNIM (4.9±2.6 vs 1.8±0.5; P<0.01) and [ 18 F]FMISO (4.4±1.0 vs 1.5±0.4; P<0.01). The carbogen treatment had only slight effects on the biodistribution of either marker in normal tissues. The necrotic fraction determined in tumours did not correlate with the tumour volume or with the tumour-to-muscle radioactivity uptake ratio. This study shows that the uptake of both [ 18 F]FETNIM and [ 18 F]FMISO correlates with the oxygenation status in tumours. In addition, our data show no significant difference in the intratumoral uptake between the two markers. However, significantly higher radioactivity uptake values were measured for [ 18 F]FMISO than for [ 18 F]FETNIM in normal tissues. Keywords: [ 18 F]Fluoromisonimidazole – [ 18 F]Fluoroery- thronitroimidazole – Hypoxia – Murine tumours – Digital autoradiography Eur J Nucl Med Mol Imaging (2004) 31:513–520 DOI 10.1007/s00259-003-1404-x Introduction The growth of solid tumours depends on the formation of new blood vessels. Reduced levels of oxygen (hypoxia) are thus a common feature of fast-growing solid tumours, which outgrow the oxygen- and nutrient-carrying capaci- ty of the local vasculature. It is well known that hypoxia is an important determinant for the outcome of many forms of tumour therapy [1, 2, 3]. Furthermore, it has been demonstrated that there is an association between tumour hypoxia and malignant progression in cervical cancer [4] and between oxygenation status and loco- regional control in head and neck squamous cell carcino- mas [5]. Therefore, a variety of techniques, both invasive and non-invasive, have been developed to measure hyp- oxia in tissues [6]. During the last two decades, much attention has been paid to non-invasive imaging techniques using radiola- belled markers for the detection of hypoxic cells in solid tumours [7]. Of these markers, the 2-nitroimidazoles are the most widely studied compounds for imaging of hyp- Tove Grönroos ( ✉ ) Medicity Research Laboratory, Turku PET Centre, Tykistökatu 6 A, 20520 Turku, Finland e-mail: tove.gronroos@utu.fi Tel.: +358-2-3337020, Fax: +358-2-3337000 Original article Comparison of the biodistribution of two hypoxia markers [ 18 F]FETNIM and [ 18 F]FMISO in an experimental mammary carcinoma Tove Grönroos 1 , Lise Bentzen 2 , Päivi Marjamäki 1 , Rumi Murata 2 , Michael R. Horsman 2 , Susanne Keiding 3 , Olli Eskola 1 , Merja Haaparanta 1 , Heikki Minn 1 , Olof Solin 1 1 Medicity Research Laboratory, Turku PET Centre, Turku, Finland 2 Department of Experimental Clinical Oncology, Aarhus University Hospital, Aarhus, Denmark 3 PET Centre and Department of Medicine V, Aarhus University Hospital, Aarhus, Denmark Received: 9 June 2003 / Accepted: 30 October 2003 / Published online: 14 January 2004 © Springer-Verlag 2004