Inflammation imaging using a ^99mTc-labeled neutrophil-targeting peptide

Objectives Polymorphonuclear neutrophils (PMNs) are the first-line immune cells in acute inflammatory responses. PMNs are also known to mediate the onset and destabilization of atherosclerosis. Formyl-peptide receptor (FPR), which belongs to the family of seven transmembrane domain G-protein-coupled receptors, highly expresses on activated PMNs. This study was designed to evaluate a newly developed FPR-targeting probe, ^99mTc-cFLFLF-PEG3.4-HYNIC (^99mTc-cFP) for inflammation imaging using two animal models: mouse ear edema and Apo-E-deficient (ApoE^-/^-) mouse with atherosclerosis.

Methods .Ear inflammation was induced by topically applying 12-O-tetradecanoyl-phorbol-13-acetate (TPA, 2 μg/ear) to the right ear of ICR mouse (n=5). The left ear received vehicle as a control. After TPA treatment at 24-hr, SPECT images with ^99mTc-cFP were acquired at 2-hr after IV tracer injection. ApoE^-/^- mice (51-wk-old, n=4) fed a Western diet and 3 wild-type mice were imaged with ^99mTc-cFP up to 24 hrs followed by ex vivo autoradiography (AR) of the aorta. Inflammatory responses in the ears and aortas were analyzed by histological and immunohistochemical examination.

Results Higher tracer uptake was clearly detected by SPECT imaging in TPA-treated ears. The activities (%ID/g) in the inflamed and control ears were 1.59±0.48 vs. 0.57±0.11 (P<0.01). Focal radioactive uptake in the aorta areas were barely visualized on SPECT images in ApoE^-/^- mice but not in controls. Biodistribution and AR data showed that aortic radioactivities in the ApoE^-/^- mice were 7.5-fold higher than that in controls. Postmortem analysis showed a significant correlation between the radioactivity and inflammatory response related to PMN infiltration at the ear and aortic inflammatory sites.

Conclusions Imaging with ^99mTc-cFP may provide a useful tool for detection of inflammation by targeting neutrophil accumulation. The uptake level of ^99mTc-cFP in atherosclerotic plaques may be dependent on the intensity of PMN adherence and FPR expression.

Research Support NIH/NIBIB P41-EB002035