Structure activity relationship among fibrin-targeted PET probes

240 Objectives Thrombosis is implicated in a number of cardiovascular diseases and there remains a need to better detect thrombosis. In this study, we describe the optimization of a fibrin-targeted PET probe with respect to thrombus uptake, biodistribution, and metabolic stability. Methods A series of fibrin-specific peptides conjugated to various Cu-64-chelates as PET reporters was prepared. Peptides were synthesized using standard solid phase techniques, chelates coupled to the N- and C-termini using various linkers, and probes were prepared by labeling using standard Cu-64 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 of Wistar rats. Imaging was performed with a clinical 3.0-T MR imager with a MR-compatible human PET imager. Serial blood draws were used to determine plasma clearance and metabolic stability. The biodistribution of Cu-64 was determined ex vivo, 2 hours after injection by well counting. Results Probes maintained in vitro fibrin affinity after being derivatized with chelates, however in vitro affinity did not always correlate with in vivo efficacy. These differences were traced to metabolic stability of the probes with respect to peptide degradation and/or copper release. The thrombus was visible on PET imaging as a region of high activity and simultaneous MR angiography localized this area of high activity to the right carotid artery at the site of the crush injury. For the best probes, the second highest Cu-64 concentration, after the kidney, was in the thrombus. Conclusions Hybrid PET/MR imaging with fibrin-specific PET probes enables detection of arterial thrombus in a rat model. Judicious modification of the charge, linker and Cu-64-chelate of fibrin-specific probes enables enhanced in vivo thrombus uptake and metabolic stability. Research Support NIH: 5T32CA009502 and R01HL10944