PET imaging of acute and chronic inflammation in living mice

Purpose In this study, we evaluated the 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced acute and chronic inflammation in living mice by PET imaging of TNF-α and integrin α_vβ₃ expression. Methods TPA was topically applied to the right ear of BALB/c mice every other day to create the inflammation model. ⁶⁴Cu-DOTA-etanercept and ⁶⁴Cu-DOTA-E{Ec(RGDyK)]₂}₂ were used for PET imaging of TNF-α and integrin α_vβ₃ expression in both acute and chronic inflammation. Hematoxylin and eosin staining, ex vivo autoradiography, direct tissue sampling, and immunofluorescence staining were also performed to confirm the non-invasive PET imaging results. Results The ear thickness increased significantly and the TNF-α level more than tripled after a single TPA challenge. MicroPET imaging using ⁶⁴Cu-DOTA-etanercept revealed high activity accumulation in the inflamed ear, reaching 11.1 ± 1.3, 13.0 ± 2.0, 10.9 ± 1.4, 10.2 ± 2.2%ID/g at 1, 4, 16, and 24 h post injection, respectively (n = 3). Repeated TPA challenges caused TPA-specific chronic inflammation and reduced ⁶⁴Cu-DOTA-etanercept uptake due to lowered TNF-α expression. ⁶⁴Cu-DOTA-E{Ec(RGDyK)]₂}₂ uptake in the chronically inflamed ears (after four and eight TPA challenges) was significantly higher than in the control ears and those after one TPA challenge. Immunofluorescence staining revealed increased integrin β₃ expression, consistent with the non-invasive PET imaging results using ⁶⁴Cu-DOTA-E{Ec(RGDyK)]₂}₂ as an integrin α_vβ₃-specific radiotracer. Biodistribution and autoradiography studies further confirmed the quantification capability of microPET imaging. Conclusion Successful PET imaging of TNF-α expression in acute inflammation and integrin α_vβ₃ expression in chronic inflammation provides the rationale for multiple target evaluation over time to fully understand the inflammation processes. Qizhen Cao and Weibo Cai contributed equally to this work.