Imaging of atherosclerotic evolution with ^99mTc-labeled duramycin

Objectives Phosphatidylethanolamine (PE) is abundantly located in the inner leaflet of the cell plasma membrane. In advanced atherosclerosis, apoptosis of macrophages and smooth muscle cells is linked to development of the necrotic core and thinning of the fibrous cap. The apoptotic cascade leads to exposure of PE on the outer leaflet of the cell membrane, thereby making apoptosis detectable using probes targeting PE. This study was designed to explore capabilities of a PE-specific probe, ^99mTc-duramycin (^99mTc-DM), which binds to PE with high affinity/specificity, in assessing atherosclerotic plaque evolution.

Methods Two hrs after i.v. injection of ^99mTc-DM (2 mCi), SPECT images were acquired from 11 apolipoprotein-E knockout (ApoE^-/-) mice with atherosclerosis. Five mice were imaged once at age 20 wk (20wk ApoE^-/-). Six mice were imaged twice at 20 and 40 wk (40wk ApoE^-/-). ApoE wild-type ApoE^+/+ mice (n=5) were used as controls. Aortas and carotid arteries were harvested for autoradiography and histological analyses.

Results Focal ^99mTc-DM uptake in the aorta and carotid artery were visualized in the 20 wk and 40 wk ApoE^-/-, but not in ApoE^+/+ mice. ^99mTc-DM uptake increased 2.2-fold in aortic arch lesions in 40 wk compared to the sites when the mice were 20 wk old. Autoradiography showed that there was significantly increased focal ^99mTc-DM uptake in the aortic arch, carotid arteries, and abdominal descending aorta, but not in the thoracic descending aorta. The radioactive ratio of aortic arch to averaged thoracic aorta in 40 wk ApoE^-/- mice was 2.3 times higher than that in 20 wk (5.1±0.86 vs. 2.2±0.23, P<0.01). The radioacties were correlated with the severity of plaque inflammation and cell apoptosis/necrosis. There was barely detectable uptake of ^99mTc-DM in the aortic arch of ApoE^+/+mice.

Conclusions ^99mTc-DM is promising for specific detection of atherosclerotic plaque evolution related to PE externalization of macrophages and SMC accompanied by inflammation.