Longitudinal observation of [11C]4DST uptake in turpentine-induced inflammatory tissue
Introduction
Differentiation between residual tumor and inflammatory response is the major premise for accurate therapeutic monitoring of tumor tissues with positron emission tomography (PET). Although fluorine-18 fluorodeoxyglucose ([18F]FDG) is currently the most widely used radiopharmaceutical for this purpose, various forms of inflammatory lesion take up large amounts of [18F]FDG [1], [2], [3], [4]. Several lines of evidence suggest that the accumulation of [18F]FDG is based on enhanced glycolysis, which has often been associated with the growth rate and malignancy of tumor cells. However, all living cells require glucose, and high levels of [18F]FDG uptake are observed in highly metabolic inflammatory cells.
More specific information on the in vivo status of tumors may be acquired by the use of a radiopharmaceutical that is a proliferation marker, as proliferative activity is one of the key factors of malignant disease. For this reason, we proposed carbon-11-labeled 4′-thiothymidine ([11C]4DST, originally designated as [11C]S-dThd) as a tumor imaging agent based on the mechanisms of incorporation into DNA [5], [6], [7], [8]. Recently, we evaluated the tissue kinetics and biodistribution of [11C]4DST in a rodent tumor and acute sterile inflammation model [9]. In our animal model, [11C]4DST produced promising results. [11C]4DST showed the highest level of tumor uptake with high tumor selectivity. Furthermore, [11C]4DST showed completely different kinetics in tumor and inflammatory tissues.
Although the usefulness of [11C]4DST for differentiation between malignant tumor and inflammation was indicated in our experimental model, this previous study had some limitations in that we used the inflammation model in the acute phase without any contribution of proliferative inflammatory cells. In contrast, chronic inflammatory granulomatous lesions include a Ki-67-positive (6.3%) lymphocyte fraction [10]. Inflammatory lung diseases are accompanied by lymphocyte infiltration and involve growth factors that enhance the proliferation of lymphocytes [11]. If [11C]4DST is a real proliferation marker, it may accumulate in chronic granulomatous lesions with proliferative inflammation. To confirm [11C]4DST selectivity, further experimental studies with chronic inflammation models are needed. Therefore, in this study, we evaluated the longitudinal changes of [11C]4DST uptake and cell proliferation status indicated by Ki-67 immunostaining in turpentine-induced acute, subacute, and chronic phases of inflammatory tissues. A tumor selectivity index was also calculated using the previously published biodistribution data in the C6 glioma-bearing rat model [9].
Section snippets
Tracer
[11C]4DST was produced by palladium-mediated Stile cross-coupling reaction of 5-tributylstannyl-4′-thio-2′-deoxyuridine with [11C]methyl iodide [7]. The specific activities and radiochemical purities were 103.3 ± 64.9 GBq/μmol and 99.4% ± 0.2%, respectively, at the time of injection.
Animals and inflammation model
Male Wistar rats weighing 80–100 g were obtained from Japan SLC (Hamamatsu, Japan). Animals were housed under constant environmental conditions with a 12–12-h light–dark cycle. Food and water were provided ad libitum. To
Longitudinal study of [11C]4DST
Fig. 1 shows the time-dependent changes of [11C]4DST uptake measured on several days after turpentine injection. The uptake of [11C]4DST in inflammatory tissue was significantly increased on days 2–4 after turpentine injection, and then decreased thereafter. On day 14, tracer uptake levels returned to those on day 1. The maximum SUV level of inflamed muscle was 0.6, and was approximately 3 times higher than that of healthy muscle on days 2–4 after turpentine injection. The radioactivity levels
Discussion
This study demonstrated the longitudinal changes of [11C]4DST uptake and cell proliferation status in inflammatory tissues in turpentine-induced acute, subacute, and chronic phases. In addition, the tumor selectivity index was evaluated using the previously published biodistribution data in C6 glioma-bearing rats.
Although [11C]4DST uptake in subacute inflammation was 3 times higher than that of healthy muscle, tumor selectivity index remained very high (> 10), due to the low uptake (SUV 0.6) in
Conclusion
In our experimental animal models, [11C]4DST uptake was low but increased significantly in subacute inflammation and then decreased in the chronic phase. This time-dependent changes of [11C]4DST uptake corresponded well to the Ki-67 L.I. Therefore, [11C]4DST uptake reflects the cell proliferation status of inflammatory tissues. These data will aid in the interpretation of [11C]4DST PET data for analysis of tumor treatment response.
Acknowledgments
This work was supported by a Grant-in Aid for Scientific Research (B) No. 22390241 from the Japan Society for the Promotion of Science (to Jun Toyohara) and a Grant from the National Center for Global Health and Medicine (to Jun Toyohara and Kiichi Ishiwata). The authors thank Mr. Kunpei Hayashi and Mr. Takashi Yamaguchi for technical assistance.
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