VII. 1. Synthesis and Biological Evaluation of a New Fluorine-18 Labeled MMP-2 Inhibitor for Cancer Imaging by PET

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1 CYRIC Annual Report 2009 VII. 1. Synthesis and Biological Evaluation of a New Fluorine-18 Labeled MMP-2 Inhibitor for Cancer Imaging by PET Furumoto S. 1,2, Sakai E. 2, Ishikawa Y. 2, and Iwata R. 2 1 Tohoku University School of Medicine 2 Cyclotron and Radioisotope Center Tohoku University Introduction Matrix metalloproteinases (MMPs) consist of a large family of zinc-dependent endpeptidases that degrade specific components of the extracellular matrix. MMP-2, one of a member of the family, is deeply involved in degradation of type IV collagen (a major component of the basement membrane). Enzymatic activity of MMP-2 is generally low or undetectable in normal tissues but enhanced in pathological processes associated with tissue destruction, such as tumor growth and invasion. Therefore, in vivo evaluation of MMP-2 activity would be helpful for assessing such disease malignancy if we can do that non-invasively. For that purpose, we have been working on the development of PET probes for MMP-2 imaging 1,2). In this study we designed and synthesized a new F-18 labeled MMP-2 inhibitor and biologically examined the potential for imaging active MMP-2 by PET. Methods The newly designed MMP-2 inhibitor, SAV49, was synthesized according to the scheme indicated in Fig. 1. Inhibitory activity of SAV49 was examined by in vitro MMP-2 activity assay. The IC 50 value was determined from the dose-response curve. In addition to SAV49, its methyl ester derivative was also synthesized because such ester derivative can be a prodrug in vivo. As a precursor for synthesis of 18 F-labeled SAV49M, a corresponding nitro pyridine derivative was also prepared according to the scheme indicated in Fig. 2. Radiosynthesis of [ 18 F]SAV49M was carried out by a conventional nucleophilic substitution with [ 18 F]KF/Kryptfix in DMSO at 150 for 10 min (Fig. 3). The radiolabeled product was isolated by semi-preparative reverse-phase HPLC. 133

2 For in vivo evaluation of the utility of [ 18 F]SAV49M, HT-1080 cell line was used for preparing subcutaneous tumor xenograft model mice. HT-1080 cells form tumor mass subcutaneously within a short period, and MMP-2 expression is well enhanced in the tumor mass. Ten days after implantation of HT-1080 the mice were used for biodistribution and metabolite analysis of [ 18 F]SAV49M. Radioactivity uptakes (%ID/g) of major organs and tumor and Radioactive metabolites of were investigated. Results and Discussion MMP-2 inhibition assay clearly indicated that SAV49 inhibited MMP-2 activity in a dose-dependent manner (Fig. 4). The IC 50 value calculated from the inhibition curve was 0.61 μm. Decay corrected radiochemical yield of [ 18 F]SAV49M was about 60% on average with a radiochemical purity over 99% after HLC purification. Biodistribution study of [ 18 F]SAV49M exhibited no increase of radioactivity uptake in bone over time (Fig. 5). This result suggest that [ 18 F]SAV49M has resistance to in vivo defluorination. Radioactivity uptakes of tumor and muscle showed similar change over time, suggesting non-specific uptake in tumor of [ 18 F]SAV49. Metabolites analysis of plasma samples revealed that 60 to 80 % of radioactive metabolite was identified as a form of SAV49 (Fig. 6). Meanwhile, examination of radioactivity distribution between erythrocytes and plasma clarified that nearly 100 % of radioactivity existed in the fraction of erythrocytes (Fig. 7). These results suggest that the probe in erythrocyte fraction could not move to the tumor tissue through plasma fraction, resulting in low tumor uptake of [ 18 F]SAV49. However, such unsymmetrical distribution of [ 18 F]SAV49 in the blood of rabbit was not observed. In the case of rabbit, unlike mice, radioactivity of plasma was greater than that of erythrocytes throughout 120 min. These results imply that tumor bearing mouse could be an unsuitable animal model for evaluation of [ 18 F]SAV49 in vivo and require further study to draw a conclusion of the utility as a probe for MMP-2 imaging. Conclusions In this study we newly designed and synthesized fluorine-18 labeled MMP-2 inhibitor [ 18 F]SAV49 for MMP-2 imaging by PET. Although the prodrug [ 18 F]SAV49M showd biodistribution study using tumor bearing mice indicated that the purpose was satisfactorily accomplished, tumor specific uptake was not observed in the tumor model. Radioactivity analysis of the blood suggested that specific retention of SAV49 in the 134

3 erythrocyte fraction caused the low tumor uptake. Such specific retention was not observed in the same study using a rabbit, suggesting that other species of animal would be necessary for evaluation of the utility of SAV49 as a MMP-2 imaging probe. References 1) Furumoto S., Takashima K., Kubota K., Ido T., Iwata R., Fukuda H., Nucl. Med. Bio. 30 (2003) ) Furumoto S., Iwata R., Ido T., J. Lab. Comp. Radiopharm. 45 (2002) 975. Figure 1. Synthesis of SAV49 and SAV49M. Figure 2. Synthesis of a precursor for [ 18 F]SAV49M. Figure 3. Radiosynthesis of [ 18 F]SAV49M. 135

4 Figure 4. Inhibition curve of MMP-2 with SAV49. Figure 5. Biodistribution of [ 18 F]SAV49M. Figure 6. Metabolites of [ 18 F]SAV49M in mouse plasma. 136

5 Figure 7. Distribution of [ 18 F]SAV49M in mouse blood. 137