A triple-isotope SPECT/PET-CT study with two novel fibrin-specific probes for non-invasive imaging of thrombus

Objectives Currently, thrombus imaging relies on different modalities depending on the vascular territory (e.g. ultrasonography, CT, etc). Considering the limitations with respect to thrombus location a molecular imaging technique to visualize thrombus in the context of vascular anatomy using PET-CT or SPECT-CT and whole body imaging would be a valuable tool. We recently characterized 4 probes labeled with ⁶⁴Cu and Al¹⁸F for thrombus imaging.(1) Here we extend our findings to ⁶⁸Ga PET and ¹¹¹In SPECT, and use multimodal imaging to demonstrate the in vivo specificity of these probes for fibrin and thrombus.

Methods Probes were synthesized by conjugation of a fibrin-binding peptide to NODAGA or DOTA followed by labeling with ⁶⁸Ga (FBP14) or ¹¹¹In (FBP15), respectively. SPECT/PET imaging properties, biodistribution, pharmacokinetics and stability were assessed in two rat models of carotid thrombosis (crush- and FeCl₃-injury). Target specificity was further evaluated with SPECT/PET-CT imaging by comparing targeted FBP14 and FBP15 to an isomeric untargeted ⁶⁴Cu-D-Cys-FBP8 and by co-localization of the ⁶⁸Ga PET or ¹¹¹In SPECT signal to the ¹²⁵I SPECT signal of thrombus formed by incorporation of ¹²⁵I-fibrinogen.

Results FBP14 and FBP15 showed high in vitro affinity for fibrin (530 and 600 nM), and high thrombus uptake (~0.9% ID/g), favorable biodistribution and fast blood clearance (t_1/2 = 8-15 min) in rats. Thrombus uptake was the second highest, just after the kidneys (2-4% ID/g). Target specificity studies by multimodal SPECT/PET-CT imaging showed that focal signal of ⁶⁸Ga-FBP14 and ¹¹¹In-FBP15 co-localizes with the preformed ¹²⁵I-labeled clot, however the non-binding derivative ⁶⁴Cu-D-Cys-FBP8 does not co-localize with the hot clot (Figure).

Conclusions FBP14 and FBP15 have high target affinity and specificity for fibrin but distinctive physical and imaging properties that make them suitable for different medical applications for thrombus detection.

Research Support HL109448 from the National Heart, Lung, and Blood Institute (Dr. Caravan).