A Novel Strategy for Preparing Dual-modality optical/PET Imaging Tracers via the Photo-click Chemistry

554 Objectives Dual-modality optical/PET imaging tracers, the targeting capability of which against a disease related biomarker could be confirmed prior to their in vivo administration via in vitro fluorescent staining, are considered as more efficient imaging tools compared with mono-modality tracers. However, wide applications of small molecule based optical/PET tracers were hindered by the complex synthesis work. To simplify the preparation of optical/PET imaging tracers, we herein developed a novel strategy based on the alkene tetrazole photo-click chemistry, by which the linker formed after the click reaction served not only as a bridge between the chelator and the targeting molecule but also as a fluorescent core for the optical imaging. Methods AE105 (a peptide targeting uPAR) and NOTA was functionalized with the alkene group and the tetrazole group respectively. Irradiated by UV 254nm, the photo click reaction was conducted in PBS:ACN = 1:1 at the concentration of 2mM for functionalized AE105 and 400uM for functionalized NOTA. The absorption and emission spectra of the product were recorded. Fluorescent staining of the uPAR over-expressed U87MG cell line using the prepared tracer was subsequently conducted followed by PET/CT scans on nude mice bearing U87MG xenografts. Results The photo click reaction finished within 5min with a yield of 50%. The prepared AE105 based dual-modality tracer exhibited a maximum absorption at 360nm and a maximum emission at 570nm. Results observed in the subsequent fluorescent staining of U87MG (Figure 1) revealed its specific binding against the uPAR, indicating its potential in vivo application. This tracer was then labeled with Ga-68 (specific activity 1mCi/nmole) for imaging nude mice bearing U87MG xenografts. At different time points examined, the tumor could be clearly visualized on PET images with a high signal to background ratio (Figure 2). Conclusions Based on the photo-click chemistry, a novel versatile strategy for preparing dual modality optical/PET tracers has been developed, which significantly simplified the synthetic procedure by automatically forming a fluorescent core after the click reaction.