UC San Diego

Background

Oxidation-specific epitopes (OSEs) are proinflammatory, and elevated levels in plasma predict cardiovascular events.

Objectives

The purpose of this study was to develop novel positron emission tomography (PET) probes to noninvasively image OSE-rich lesions.

Methods

An antigen-binding fragment (Fab) antibody library was constructed from human fetal cord blood. After multiple rounds of screening against malondialdehyde-acetaldehyde (MAA) epitopes, the Fab LA25 containing minimal nontemplated insertions in the CDR3 region was identified and characterized. In mice, pharmacokinetics, biodistribution, and plaque specificity studies were performed with Zirconium-89 (⁸⁹Zr)-labeled LA25. In rabbits, ⁸⁹Zr-LA25 was used in combination with an integrated clinical PET/magnetic resonance (MR) system. ¹⁸F-fluorodeoxyglucose PET and dynamic contrast-enhanced MR imaging were used to evaluate vessel wall inflammation and plaque neovascularization, respectively. Extensive ex vivo validation was carried out through a combination of gamma counting, near infrared fluorescence, autoradiography, immunohistochemistry, and immunofluorescence.

Results

LA25 bound specifically to MAA epitopes in advanced and ruptured human atherosclerotic plaques with accompanying thrombi and in debris from distal protection devices. PET/MR imaging 24 h after injection of ⁸⁹Zr-LA25 showed increased uptake in the abdominal aorta of atherosclerotic rabbits compared with nonatherosclerotic control rabbits, confirmed by ex vivo gamma counting and autoradiography. ¹⁸F-fluorodeoxyglucose PET, dynamic contrast-enhanced MR imaging, and near-infrared fluorescence signals were also significantly higher in atherosclerotic rabbit aortas compared with control aortas. Enhanced liver uptake was also noted in atherosclerotic animals, confirmed by the presence of MAA epitopes by immunostaining.

Conclusions

⁸⁹Zr-LA25 is a novel PET radiotracer that may allow noninvasive phenotyping of high-risk OSE-rich lesions.