Abstract 804: Noninvasive Integrated SPECT/CT Molecular Imaging of Activated Factor XIII Activity in Thrombosis

Abstract

Background : Activated factor XIII (FXIIIa) is a blood transglutaminase that mediates fibrinolytic resistance and is a hallmark of acute thrombi. Noninvasive molecular imaging of FXIIIa may offer a novel approach to identify acute thrombi and to gauge fibrinolytic resistance in vivo. Here we developed and validated a FXIIIa thrombosis imaging strategy using noninvasive integrated SPECT/CT. Methods: A FXIIIa-targeted peptide agent (F13) was synthesized using NQEQVSPLTLLK chelated to DOTA and then labeled with 111 InCl 3 . A control agent (C13, 111 In-NAEQVSPLTLLK) was analogously synthesized. In vitro validation of the F13 agent was performed in human plasma clots. Next, the in vivo blood-half life of F13 was determined in mice (n=4). In vivo thrombosis studies (n=15 mice) were then performed using 10% ferric chloride jugular venous thrombi aged 1 hour or 16 hours. Mice were intravenously injected with 200 μCi of F13 or C13. After 4 hours, mice underwent integrated CT angiography (72 μm isotropic resolution) and SPECT imaging (32 minute acquisition). In situ thrombi were then resected for radioactivity and weight measurements. Results : Human plasma clots incubated with F13 showed 280–740% greater counts per minute (CPM) than controls (p<0.01). F13 binding was dose-dependent and >90% inhibited by pretreatment with iodoacetamide, an alkylating agent. The blood half-life of F13 was calculated to be 16 minutes. In one hour thrombi, in vivo SPECT/CT imaging revealed strong focal F13 SPECT signal in the co-registered ipsilateral venous thrombi but not the contralateral normal jugular vein. One hour thrombi in the F13 group had 15-fold greater radioactivity than the C13 group (4.6±3.6% vs. 0.3±0.2% injected dose per gram tissue, IDGT, p<0.01). Compared to 1 hour thrombi, 16 hour old thrombi had 4-fold less F13 radioactivity (1.1%±0.1% IDGT, p<0.05). Conclusions : Blood transglutaminase FXIIIa can be noninvasively detected using a FXIIIa-sensitive and specific imaging agent for integrated SPECT/CT. The current in vivo results further validate that activated factor XIII is a hallmark of acute thrombi and declines in activity over time. This clinically translatable imaging strategy could permit visualization of FXIIIa in patients with thrombotic syndromes.