1. Department of Nuclear Medicine, Peking Union Medical College Hospital (PUMCH), Chinese Academy of Medical Sciences & Peking Union Medical College (CAMS & PUMC), Beijing, China.
2. Laboratory of Molecular Imaging and Nanomedicine, National Institute of Biomedical Imaging and Bioengineering (NIBIB), National Institutes of Health (NIH), Bethesda, USA
Aim: 18F-DPA-714 is a PET tracer that recognizes macrophage translocator protein (TSPO), and 18F-Alfatide II (18F-AlF-NOTA-E[PEG4-c(RGDfk)]2) is specific for integrin αvβ3. This study aims to apply these two tracers for longitudinal PET imaging of muscular inflammation, and evaluate the value of 18F-DPA-714 in differentiating inflammation from tumor.
Methods: RAW264.7 mouse macrophage cells were used for cell uptake analysis of 18F-DPA-714. A mouse hind limb muscular inflammation model was established by intramuscular injection of turpentine oil. For the inflammation model, PET imaging was performed at different days using 18F-DPA-714 and 18F-Alfatide II. The specificity of the imaging probes was tested by co- or pre-injection of PK11195 or unlabeled RGD (Arg-Gly-Asp) peptide. PET imaging using 18F-DPA-714 was performed in A549, HT29, U87MG, INS-1, and 4T1 xenograft models. Immunofluorescence staining was performed to evaluate infiltrated macrophages and angiogenesis in inflammation and/or tumors.
Results: Uptake of 18F-DPA-714 in RAW264.7 cells was 45.5% at 1 h after incubation, and could be blocked by PK11195. PET imaging showed increased 18F-DPA-714 and 18F-Alfatide II uptake at inflammatory muscles. Peak uptake of 18F-DPA-714 was seen on day 6 (4.02 ± 0.64 %ID/g), and peak uptake of 18F-Alfatide II was shown on day 12 (1.87 ± 0.35 %ID/g) at 1 h p.i.. Tracer uptakes could be inhibited by PK11195 for 18F-DPA-714 or cold RGD for 18F-Alfatide II. Moreover, macrophage depletion with liposomal clodronate also reduced the local accumulation of both tracers. A549, HT29, U87MG, INS-1, and 4T1 tumor uptakes of 18F-DPA-714 (0.46 ± 0.28, 0.91 ± 0.08, 1.69 ± 0.67, 1.13 ± 0.33, 1.22 ± 0.55 %ID/g at 1 h p.i., respectively) were significantly lower than inflammation uptake (All P < 0.05).
Conclusion: PET imaging using 18F-DPA-714 as a TSPO targeting tracer could evaluate the dynamics of macrophage activation and infiltration in different stages of inflammatory diseases. The concomitant longitudinal PET imaging with both 18F-DPA-714 and 18F-Alfatide II matched the causal relationship between macrophage infiltration and angiogenesis. Moreover, we found 18F-DPA-714 uptake in several types of tumors is significantly lower than that in inflammatory muscles, suggesting 18F-DPA-714 PET has the potential for better differentiation of tumor and non-tumor inflammation.
Keywords: 18F-DPA-714, 18F-Alfatide II, positron emission tomography, inflammation, tumor, TSPO