Abstract 348: Fibrin Targeted in vivo Imaging of Tissue Plasminogen Activator-induced Thrombolysis of Mouse Deep Vein Thrombosis

Introduction Tissue plasminogen activator (tPA) is increasingly used to treat deep vein thrombosis (DVT) to prevent the post-thrombotic syndrome. However, while aging venous thrombi are likely to become resistant to fibrinolysis, the age-dependent response to fibrinolysis has not been systematically assessed. Here we used confocal intravital microscopy (IVM) to dynamically study fibrinolysis of murine thigh DVT at high-resolution using thrombus angiography and a fibrin-specific sensor, FTP11-CyAm7 (FTP11). We hypothesized that fibrinolysis could be tracked and quantified in vivo using serial IVM and molecular imaging of fibrin. Methods C57Bl/6 mice underwent topical 7.5% ferric chloride injury to induce femoral DVT. At day 1 (n=6) or day 4 (n=5) post DVT, IVM was performed during tPA infusion. Mice were i.v. injected with a bolus of tPA, heparin and plasminogen, followed by a continuous 1 hour infusion of tPA+heparin. FTP11, injected 45 min prior to lysis, enabled molecular imaging of fibrin. DVT area and length were quantified using FITC-dextran venography before and after lysis. IVM image stacks were analyzed by NIH ImageJ. After sacrifice, fluorescence microscopy (FM) and Carstair’s staining were performed. Results Compared to day 4, day 1 DVT fibrinolysis showed a greater net decrease in thrombus area (32±11 vs. 6±4%, p=0.004) and FTP11 signal (54±17 vs. 11±8%, p=0.004) after 60 min. Of note, day 1 more lyseable DVT had significantly higher baseline FTP11 signal (target-to-background ratio 1.91±0.86 vs. 1.10±0.06, p=0.004), consistent with greater thrombus permeability at day 1 vs. day 4. The net lysis correlated well with both the baseline FTP11 thrombus signal (r=0.62, p=0.048) and change in FTP11 signal (r=0.79, p=0.006). IVM angiography revealed a trend of increased lysis in the distal thrombus edge, compared to the middle or proximal edge (day 1: 44±11% vs. 26±16% and 29±26%, p>0.05). FTP11 signal colocalized with fibrin-rich thrombi on FM and Carstair’s staining. Conclusion Chemically induced murine DVT lyse more effectively at day 1 than day 4, as tracked by serial IVM molecular imaging. The FTP11-CyAm7 fibrin signal differentiates DVT of different ages and informs the magnitude of triggered fibrinolysis.