[CANCER RESEARCH 61, 3853–3857, May 15, 2001] Advances in Brief In Vivo Evaluation of 5-[ 18 F]Fluoro-2-deoxyuridine as Tracer for Positron Emission Tomography in a Murine Pancreatic Cancer Model Ulrike Seitz, 1, 2 Martin Wagner, 1 Andreas T. Vogg, Gerhard Glatting, Bernd Neumaier, Florian R. Greten, Roland M. Schmid, and Sven N. Reske Departments of Nuclear Medicine [U. S., A. T. V., G. G., B. N., S. N. R.] and Internal Medicine I [M. W., F. R. G., R. M. S.], University of Ulm, 89081 Ulm, Germany Abstract We used a murine tumor progression model for the evaluation of potential proliferation markers using positron emission tomography (PET). 5-[ 18 F]-2-deoxyuridine ([ 18 F]FdUrd) was synthesized with >98% radiochemical purity and investigated in a pancreatic cancer model, trans- forming growth factor  transgenic mice crossbred to p53 deficient mice. Thymidylate synthase was increased already in premalignant lesions, whereas thymidine kinase 1 mRNA levels were up-regulated 4-fold in the pancreatic cancer specimen of these mice. PET imaging was performed after injection of 1 MBq of [ 18 F]FdUrd and 1 MBq of [ 18 F]fluoro-deoxy- glucose. Animals with pancreatic cancer displayed focal uptake of both tracers. The [ 18 F]FdUrd uptake ratio closely correlated with the prolifer- ation index as evaluated in morphometric and fluorescence-activated cell sorter analysis. These results indicate the potential of our tumor model for the evaluation of PET tracers and suggest [ 18 F]FdUrd as a tracer for the assessment of proliferation in vivo. Introduction Definitive animal models are of paramount importance for our understanding of biological processes and for the development of novel diagnostic and therapeutic strategies of human diseases. The biological relevance of currently used transplantation models in nude mice is limited (1). s.c. transplantation of malignant cells initiates tumors far distant from the natural side of the neoplasm. Furthermore, the lack of an intact immune system could alter pathophysiological characteristics of the neoplasm of even orthotopic implanted tumors. Finally, tumors developed from long-term cultured cancer cell lines might not represent the paternal tumor characteristics. In contrast, genetically engineered mouse models have proven to be a powerful tool to elucidate biological processes and to understand pathophysi- ological alterations of human diseases (1). We have described recently (2) a murine tumor progression model for ductal pancreatic cancer that recapitulates the cellular differentiation, the growth characteris- tics, and the genetic alterations of the human disease. In this model, pancreatic cancer develops from premalignant lesions in TGF- 3 transgenic mice (3, 4). Crossbreeding these TGF- transgenic mice to p53-deficient mice accelerates tumor development and results in pan- creatic cancer development in a close time frame (2). Pancreatic cancer in humans has the worst prognosis of all of the gastrointestinal cancers because of late diagnosis and lack of effective treatment. The risk of pancreatic cancer increases steadily in patients with a long standing history of chronic pancreatitis (5). Current anatomically based imaging procedures like CT and endoscopic pancreaticography detect only indirect signs of invasive tumor growth such as a pancre- atic mass or ductal abnormalities. [ 18 F]FDG has been shown to be more accurate than CT scanning in the differentiation of pancreatic adenocarcinoma from chronic pancreatitis (6, 7). It is currently ac- cepted that an increased cellular influx of the glucose analogue [ 18 F]FDG, a higher rate of intracellular phosphorylation, and a neg- ligible rate of dephosphorylation underlie the high uptake and seques- tration of [ 18 F]FDG in cancer cells (8, 9). Inflammatory processes such as pancreatitis and abscesses take up [ 18 F]FDG avidly, and chronic pancreatitis is recognized as the most common reason for false positive [ 18 F]FDG-PET findings (10). Furthermore, [ 18 F]FDG uptake relates to the number of viable cells rather than to the tumor differentiation or the proliferative activity (11). In contrast to glucose, nucleosides are rapidly incorporated in DNA. [ 11 C]thymidine has been proposed as a radiotracer for imaging tumor proliferation with PET by several groups (12, 13). The clinical applications of [ 11 C]thy- midine are limited because of its the rapid metabolism, the appearance of radiolabeled metabolites of thymidine, and the short half-life of [ 11 C]. The antineoplastic agent FdUrd is predominantly taken up by proliferating cells and phosphorylated to 5-fluoro-2'-deoxyUMP through the S-phase specific enzyme TK-1. Therefore, intracellular FdUrd uptake provides an indirect measurement of cellular TK-1 activity. 5-fluoro-2'-deoxyUMP binds irreversibly to the TS, resulting in an intracellular trapping of the thymidine analogue. High levels of TK-1 and TS have been reported in human breast cancer (14) and are associated with the S-phase fraction (15). In this study, we describe the automated synthesis of [ 18 F]FdUrd with high radiochemical purity and yield. The in vivo evaluation of [ 18 F]FdUrd as tracer for PET indicates a close correlation of tracer uptake and proliferative activity in our murine pancreatic tumor model. Materials and Methods Animals. The generation, genotyping, and phenotypic characterization of TGF- transgenic mice (line #2261-3; Ref. 4) and p53-deficient mice (16) have been described. A detailed morphological and genetic analysis of the tumor progression in crossbred TGF- transgenic and p53-deficient mice was published recently (2, 3). All of the experiments were performed according to the guidelines of the local Animal Care Committee. Preparation of [ 18 F]FdUrd by Electrophile Fluorination. Carrier added [ 18 F]F 2 was produced by irradiating neon gas [Ne with 0.5% F 2 (40 mol)] with 9-MeV deuterons by the 20 Ne(d, ) 18 F nuclear reaction. The irradiated gas was allowed to expand into a solution of 22 mg (70 mol) of DiAcdUrd in 5 ml of 10% acetic acid in CH 2 Cl 2 . The solvent was evaporated, and the sealed reactor was heated to 130°C after adding 0.4 M phosphate buffer (pH 13). The reaction was neutralized with 0.5 M H 3 PO 4 , injected onto a Econo- sphere C18 column (Alltech, Deerfield, IL), and eluted with 0.15 M phosphate buffer (pH 6) at a flow rate of 20 ml/min. The elution time of [ 18 F]FdUrd was 12 min and 30 s, whereas the retention time was 4 min and 30 s for Received 1/19/01; accepted 3/21/01. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 U. S. and M. W. contributed equally to this work. 2 To whom requests for reprints should be addressed, at Department of Nuclear Medicine, University of Ulm, Robert-Koch-Strasse 8, D-89081 Ulm, Germany. Phone: 49-731-500-24509; Fax: 49-731-500-24979; E-mail: ulrike.seitz@medizin.uni-ulm.de. 3 The abbreviations used are: TGF, transforming growth factor; CT, computerized X-ray tomography; [ 18 F]FDG, [ 18 F]fluoro-deoxyglucose; PET, positron emission tomog- raphy; FdUrd, 5-fluoro-2'-deoxyuridine; TK-1, thymidine kinase 1; TS, thymidylate synthase; DiAcdUrd, 3',5'-di-O-acetyl-2'-deoxyuridine; HPLC, high-performance liquid chromatography; PCNA, proliferating cell nuclear antigen; BrdUrd, bromodeoxyuridine; FP, forward primer; RP, reverse primer. 3853