Abstract

We recently showed the high target specificity and favorable imaging properties of (64)Cu and Al(18)F positron emission tomography (PET) probes for non-invasive imaging of thrombosis. Here, our aim was to evaluate new derivatives labeled with either with (68)Ga, (111)In, or (99m)Tc as thrombus imaging agents for PET and single-photon emission computed tomography (SPECT). In this study, the feasibility and potential of these probes for thrombus imaging was assessed in detail in two animal models of arterial thrombosis. The specificity of the probes was further evaluated using a triple-isotope approach with multimodal SPECT/PET/CT imaging.
METHODS: Radiotracers were synthesized using a known fibrin-binding peptide conjugated to NODAGA, DOTA-MA, or a diethylenetriamine ligand (DETA-PA), followed by labeling with (68)Ga (FBP14, (68)Ga-NODAGA), (111)In (FBP15, (111)In-DOTA-MA) or (99m)Tc (FBP16, (99m)Tc(CO)3-DETA-PA), respectively. PET or SPECT imaging, biodistribution, pharmacokinetics and metabolic stability were evaluated in rat models of mural and occlusive carotid artery thrombosis. In vivo target specificity was evaluated by comparing the distribution of the SPECT and PET probes with preformed (125)I-labeled thrombi and with a non-binding control probe using SPECT/PET/CT imaging.
RESULTS: All three radiotracers showed similar affinity to soluble fibrin fragment DD(E) (Ki = 0.53-0.83 µM). After the kidneys, the highest uptake of (68)Ga-FBP14 and (111)In-FBP15 was in the thrombus (1.0 ± 0.2% ID/g) with low off-target accumulation. Both radiotracers underwent fast systemic elimination (t1/2 = 8-15 min) through the kidneys, which led to highly conspicuous thrombi on PET and SPECT images. (99m)Tc-FBP16 displayed low target uptake and distribution consistent with aggregation and/or degradation. Triple isotope imaging experiments showed that both (68)Ga-FBP14 and (111)In-FBP15, but not the nonbinding derivative (64)Cu-D-Cys-FBP8, detected the location of the (125)I-labeled thrombus, confirming high target specificity.
CONCLUSION: (68)Ga-FBP14 and (111)In-FBP15 have high fibrin affinity and thrombus specificity, and represent useful PET and SPECT probes for thrombus detection.