Development of fibrin-specific PET probes for detection of thrombus

Objectives Thrombosis is implicated in a number of cardiovascular diseases and there remains a need to better detect thrombus to determine treatment options and response to therapy. We hypothesize that simultaneous imaging of thrombus using MRI and PET could lead to more accurate thrombus detection. In this study, we compared 3 fibrin-targeted peptides derivatized with 64Cu-DOTA in a rat model of arterial thrombosis.

Methods Peptides were synthesized using standard solid phase techniques and DOTA was coupled to the N- and C-termini using the PFP ester. Probes were prepared by labeling the conjugate using standard 64Cu radiochemistry. In vitro fibrin affinity was assayed in the presence and absence of plasma. A crush injury model was used to induce thrombosis in the right carotid artery. Imaging was performed with a clinical 3.0-T MR imager with a MR-compatible human PET imager. A blood pool agent, GdDTPA-BSA, was used to visualize the vascular tree on MRI. Serial blood draws were used to determine plasma clearance and metabolic stability. The biodistribution of 64Cu was determined ex vivo, 2 hours after injection by well counting.

Results All probes maintained fibrin affinity and selectivity after derivatized with DOTA. MR angiography showed an occlusion in the right carotid caudal to the bifurcation, at the site of crush injury, and this corresponded to a region of high activity on the PET image (Figure). High thrombus uptake was observed with all 3 PET probes despite rapid metabolism. 2 hour biodistribution showed the highest 64Cu concentration, after excretory organs (kidney and liver), to be in the thrombus.

Conclusions Hybrid PET/MR imaging with a fibrin-specific PET probe enables detection of arterial thrombus in a rat model.

Research Support NIH: 5T32CA009502 and R01EB009738