Exploring the Unique Contrast Properties of Aptamer–Gadolinium Conjugates in Magnetic Resonance Imaging for Targeted Imaging of Thrombi

Objective: An important clinical question in the determination of the extent of thrombosis-related vascular conditions is the identification of blood clot location. Fibrin is a major molecular constituent of blood clots and can, therefore, be utilized in molecular imaging. In this proof-of-concept study, we sought to prepare a fibrin-targeting magnetic resonance imaging contrast agent, using a Gd­(III)-loaded fibrinogen aptamer (FA) chelate conjugate (Gd­(III)-NOTA-FA) (NOTA = 1,4,7-triazacyclononane-1,4,7-triacetic acid), to endow the ability to specifically accumulate at the location of blood clots, thereby enhancing contrast capabilities. Methods: The binding affinity of FA for fibrin was confirmed by fluorescence microscopy and microscale thermophoresis. The preparation and effective loading of the chelate-aptamer conjugates were confirmed by mass spectrometry and a xylenol orange colorimetric test. Longitudinal and transverse relaxivities and the effects of target binding were assessed using T1- and T2-map sequences at 7 T. T1- and T2-weighted images were acquired after blood clots were treated with Gd­(III)-NOTA-FA. Collagen was used as the protein control, while an unrelated aptamer sequence, FB139, was used as the aptamer control. Results: FA demonstrated a high affinity and selectivity toward the polymeric protein, with a K d of 16.6 nM, confirming an avidity over fibrinogen. The longitudinal (r1) and transverse (r2) relaxivities of Gd­(III)-NOTA-FA demonstrated that conjugation to the long aptamer strand shortened T1 relaxation times and increased T2 relaxation times (3.04 and 38.7 mM–1 s–1, respectively). These effects were amplified by binding to the fibrin target (1.73 and 46.5 mM–1 s–1, respectively). In vitro studies with thrombin-polymerized human blood (clots) in whole blood showed an unexpected enhancement of signal intensity (hyperintense) produced exclusively at the location of the clot during the T2-weighted scan, while the presence of fibrinogen within a whole blood pool resulted in T1 signal intensity enhancement throughout the pool. This is advantageous, as simply reversing the type of a scan from a typical T1-weighted to a T2-weighted would allow to selectively highlight the location of blood clots. Conclusions: Gd­(III)-NOTA-FA can be used for molecular imaging of thrombi, through fibrin-targeted delivery of contrast to the location of blood clots in T2-weighted scans.