Assessment of Aptamer-Targeted Contrast Agents for Monitoring of Blood Clots in Computed Tomography and Fluoroscopy Imaging

Objective: Random formation of thrombi is classified as a pathological process that may result in partial or complete obstruction of blood flow and limited perfusion. Further complications include pulmonary embolism, thrombosis-induced myocardial infraction, ischemic stroke, and others. Location and full delineation of the blood clot are considered to be two clinically relevant aspects that could streamline proper diagnosis and treatment follow-up. In this work, we prepared two types of X-ray attenuating contrast formulations, using fibrinogen aptamer as the clot-seeking moiety. Methods: Two novel aptamer-targeted formulations were designed. Iodine-modified bases were directly incorporated into a fibrinogen aptamer (iodo-FA). Isothermal titration calorimetry was used to confirm that these modifications did not negatively impact target binding. Iodo-FA was tested for its ability to produce concentration-dependent contrast enhancement in a phantom CT. It was subsequently tested in vitro with clotted human and swine blood. This allowed for translation into ex vivo testing, using fluoroscopy. FA was also used to functionalize gold nanoparticles (FA-AuNPs), and contrast capabilities were confirmed. This formulation was tested in vitro using clotted human blood in a CT scan. Results: Unmodified FA and iodo-FA demonstrated a nearly identical affinity toward fibrin, confirming that base modifications did not impact target binding. Iodo-FA and FA-AuNPs both demonstrated excellent concentration-dependent contrast enhancement capabilities (40.5 HU mM^-1 and 563.6 HU μM^-1, respectively), which were superior to the clinically available agent, iopamidol. In vitro CT testing revealed that iodo-FA is able to penetrate into the blood clots, producing contrast enhancement throughout, while FA-AuNPs only accumulated on the surface of the clot. Iodo-FA was thereby translated to ex vivo testing, confirming target-binding associated accumulation of the contrast material at the location of the clot within the dilation of the external carotid artery. This resulted in a 34% enhancement of the clot. Conclusions: Both iodo-FA and FA-AuNPs were confirmed to be effective contrast formulations in CT. Targeting of fibrin, a major structural constituent of thrombi, with these novel contrast agents would allow for higher contrast enhancement and better clot delineation in CT and fluoroscopy.