Toward Independent Nuclear Medicine, Molecular Imaging, and Theranostic Programs.

1037 Objectives: As we know, multiple enzymes are usually involved in a given biological process to regulate a variety of diseases including malignancy. Therefore, direct and accurate monitoring of enzyme activities is extremely important for the early diagnosis of diseases. γ-Glutamyltranspeptidase (GGT) is a cell membrane-bound enzyme, which has been reported to participate in tumorigenesis and development of several human tumors such as liver, colon and breast cancers. Thus, developing a simple method to accurately detect the GGT activity would be highly desirable to improve the diagnosis efficacy. Herein, a GGT-responsive 18F-labeled probe 18F-1-P was rationally designed and prepared for positron emission tomography (PET) imaging of tumors to improve the early diagnosis of diseases. Methods: The labeling precursor 1-P was synthesized by combining a GGT-recognizable substrate, γ-glutamate (γ-Glu), with 1,2-aminothiol group of cysteine (Cys) and the cyano group of 2-cyanobenzothiazole (CBT), where the latter two groups could undergo a click condensation reaction. The cold probe 1-P was characterized and confirmed with HPLC, mass spectra (MS) and nuclear magnetic resonance (NMR), and then it was radiolabeled with 18F in one step. The radiolabelling yield (RLY), radiochemical purity (RCP) and stability in PBS or serum of the probe 18F-1-P were evaluated by HPLC. The selective uptake and cytotoxicity of the probe in GGT overexpressed human colon cancer cells HCT-116 were studied using the traditional MTT assay. In vivo distribution of 18F-1-P in the HCT-116 tumor bearing mice was studied by injecting the probe into the mice through the tail vein. Then dynamic and static microPET imaging were carried out. Regions of interest were drawn on decay-corrected whole-body coronal images using vendor software (ASI Pro 6.7.1.1). Another probe 1 without GGT-responsive group γ-Glu was used as the negative control probe. Results: A satisfactory RLY and RCP was achieved when the probe 18F-1-P was incubated at 80℃°C for 30 min. Stability test proved that it was stable enough in PBS and serum. MTT assay showed that more than 90% of the cells survived up to 24 h at the concentration up to 100 μM of 1-P. The partition coefficient revealed that the probe 18F-1-P was weakly hydrophilic (log P = -0.0038 ± 0.0001). Cell uptake studies suggested that treatment of 18F-1-P with the cold probe 1-P could accelerate the uptake in HCT116 cells, which gradually reached its plateau at 1 h (2.71 % of total radioactivity) and reached its maximum of 2.86% at 2 h. This was about 1.4 times higher than that of 18F-1-P alone as well as 3-fold higher uptake than the negative control probe 1. microPET imaging of HCT116 xenografts clearly indicated 18F-1-P could rapidly accumulate in the tumor when it was co-injected with the cold probe 1-P and the tumor uptake reached 4.41 ± 0.65% ID/g at 15 min post injection, whereas mice only injected with 18F-1-P had the tumor uptake of 1.97 ± 0.17% ID/g at the same time point. Conclusions: A novel GGT-activatable probe 18F-1-P was designed and developed for both in vitro and in vivo imaging of endogenous GGT activity. It showed specific tumor uptake in the tumor bearing mice and the tumor-to-muscle uptake ratio remarkably enhanced when it was co-injected with the cold probe 1-P. Financial supports: Natural Science Foundation of Jiangsu Province (BK20181128), 333 Project of Jiangsu Province (BRA2016518, LGY2018086), and Precision Medical Project of Wuxi Commission of Health and Family Planning (J201806).