Preparation and imaging of arthritis of ⁹⁹Tc^m-DTPA-CB86 for TSPO targeted imaging

Objective: To develop a novel radiolabeled translocator protein (TSPO) ligand CB86 targeting agent ⁹⁹Tc^m-diethylene-triamine pentaacetic acid (DTPA)-CB86 and evaluate its biological properties. Methods: DPTA-CB86 was prepared by coupling with a bifunctional chelating agent, and then labeled with ⁹⁹Tc^m to obtain ⁹⁹Tc^m-DTPA-CB86. The labeling efficiency, radiochemical purity, and stability were determined in vitro. In vitro cellular uptake and efflux were performed using RAW264.7 macrophage cells. Biodistribution and micro-SPECT/CT images were investigated on Freund's adjuvant-induced left arthritis in mice. SPSS 18.0 analysis software (t-test) was used to fit the normal distribution and homogeneity of variance. Results: The labeling yields and radiochemical purity of ⁹⁹Tc^m-DTPA-CB86 were (95.86 ± 2.45)% and (97.45 ± 0.69)%, respectively. ⁹⁹Tc^m-DTPA-CB86 displayed good stability, with a radiochemical purity of more than 90%, in phosphate-buffered solution (PBS) at 4 h. It also exhibited high specific TSPO binging in RAW264.7 macrophage cells in vitro. The highest uptake ratio was (36.45 ± 2.18)% at 3 h after incubation, which then decreased significantly [(10.43 ± 2.01)%; t = 6.217, P < 0.05)] after adding excessive unlabeled DTPA-CB86. The difference was significant. Cell efflux analysis showed that ⁹⁹Tc^m-DTPA-CB86 had good cell retention by RAW264.7 cells, with only about 13.99% [decreased from (33.31 ± 2.34)% to (19.32 ± 2.01)% of total input radioactivity] of ⁹⁹Tc^m-DTPA-CB86 efflux observed during 4.5 h to 8 h of incubation. Biodistribution and SPECT/CT imaging demonstrated that the uptake of ⁹⁹Tc^m-DTPA-CB86 in the left arthritic ankles was significantly increased compared with that in contralateral normal ankles. Uptake in the arthritic ankles could be largely blocked by an excess of DTPA-CB86. Conclusion: ⁹⁹Tc^m-DTPA-CB86 can be readily synthesized and clearly visualize arthritis with low background, thus demonstrating its potential as a promising molecular probe targeting TSPO for arthritic SPECT imaging. (authors)