Fibrin imaging
Fibrin is another crucial target that is frequently used for molecular imaging of thrombosis. Botnaret al. developed a novel fibrin-binding gadolinium-labelled peptide (EP-1873) for MRI of thrombosis and tested this in an experimental rabbit model of plaque rupture (Botnar et al., 2004). This probe was further optimised (EP-2104R) and successfully validated for MRI-based diagnosis of coronary thrombosis and PE (Spuentrup et al., 2005a, 2005b; Overoye-Chan et al., 2008). In another study using a murine model of venous thrombosis in the inferior vena cava, Andia et al. demonstrated the maximal increased of signal at day 7 of thrombosis and gradual reduction from day 10 onwards (Andia et al., 2014). After radiolabelling 64Cu with EP-2104R, the construct was injected into a rat model of crush injury-induced thrombosis. A clear increase in uptake of radioactivity was observed in the location of the thrombi via PET imaging (Ay et al., 2014). Furthermore, the administration of fibrinolytic therapy resulted in a loss of radioactivity signal, indicating that the thrombi have been thrombolysed, indicating the suitability of this imaging approach for the monitoring of success or failure of thombolytic therapy (Ay et al., 2014). Bimodal imaging performed with 64Cu radiolabeled EP-2104R provided direct visualisation of thrombi via PET and MRI (Uppal et al., 2011). This method also allowed the detection of multisite thrombi located in the carotid artery and the femoral vein in a rat model via a whole-body PET scan (Blasi et al., 2015). In another study, EP-2104R was radiolabelled with 68Ga or111In, whereas a non-binding control was radiolabeled with 64Cu, for multimodal SPECT/PET/CT imaging (Oliveira et al., 2015). 125I-fibrinogen was injected prior to ferric chloride induced thrombosis of the common carotid artery. Multimodal imaging showed a hot spot corresponding to the125I-fibrinogen labelled thrombi with fibrin-targeted68Ga or 111In isotopes, but not the non-binding radioisotopes (Oliveira et al., 2015).
Using EP-2104R, Hara et al. synthesised a fibrin-binding peptide (FTP11) conjugated to a near-infrared dye and demonstrated successful in vivo optical imaging of thrombi in a jugular DVT murine model (Figure 18) (Hara et al., 2012). Using the same construct, the group further demonstrated successful imaging of fibrin deposition on stents that were implanted in rabbits using invasive optical coherence tomography (Figure 19) (Hara et al., 2015). EP-2104R has also been evaluated in Phase II trials, where molecular MRI was performed on patients who were previously diagnosed with thrombosis in the arteries, veins and/or the heart (Spuentrup et al., 2008; Vymazal et al., 2009). Nevertheless, none of these imaging products has reached approval for clinical use (Lanza et al., 2019).