Preparation and imaging of 18F-VUIIS1008 targeting TSPO in rheumatoid arthritis model

Abstract

Objective To synthesize a novel 18F labeled probe targeting translocator protein (TSPO) ligand 2-(5, 7-diethyl-2-(4-(2-fluoroethoxy)phenyl)pyrazolo[1, 5-a]pyrimidin-3-yl)-N, N-diethylacetamide (VUIIS1008), and evaluate its biodistribution and imaging in rheumatoid arthritis (RA) model. Methods The tosylate substrate was labeled with 18F using a tosyloxy for fluorine nucleophilic aliphatic substitution to obtain 18F-VUIIS1008. The labeling efficiency, radiochemical purity, and stability in vitro were determined. In vitro cellular uptake and competitive binding assay were performed on RAW264.7 macrophage cells. Biodistribution and microPET/CT imaging were investigated on RA mice established by Complete Freund′s Adjuvant. Two-sample t test was used to analyze the data. Results 18F-VUIIS1008 was synthesized with the labeling yield up to (41.00±5.00)%, the radiochemical purity >98.00%, and the specific radioactivity >1.52×108 MBq/mmol. 18F-VUIIS1008 was highly stable with the radiochemical purity >98.00% at 4 h after incubation in mouse serum. In vitro, it also exhibited high specific TSPO binding in RAW264.7 macrophage cells. The uptake ratio was (14.00±0.30)% at 1 h after incubation, and decreased significantly ((4.00±0.70)%; t=12.894, P<0.05) after adding excessive unlabeled VUIIS1008. The half maximal inhibitory concentration (IC50) of 18F-VUIIS1008 binding to TSPO was 0.05 nmol/L in RAW264.7 macrophage cells. In vivo distribution results showed that the uptake of 18F-VUIIS1008 in the left arthritic ankles reached the peak value of (1.33±0.02) percentage activity of injection dose per gram of tissue (%ID/g) at 1 h after injection. The radioactivity ratio of left ankle arthritic tissue to blood (A/B) and to normal muscle (A/M) was 4.40±0.22 and 1.65±0.07 respectively. MicroPET/CT imaging demonstrated that 18F-VUIIS1008 could specifically target and retained in the inflammation site. Conclusion 18F-VUIIS1008 can be easily synthesized with high radiochemical purity and can clearly visualized in RA imaging with low background, suggesting its potential as a novel promising molecular probe targeting TSPO for RA PET imaging. Key words: Membrane transport proteins; Pyrazoles; Pyrimidines; Fluorine radioisotopes; Arthritis, rheumatoid; Positron-emission tomography; Tomography, X-ray computed; Mice