SELECTIVE UPTAKE OF RADIOLABELED ANNEXIN-V ON ACUTE PORCINE LEFT ATRIAL THROMBI

Background Annexin V is a human phospholipid binding protein that bind s to activated platelets in vitro. We sought to determine the potentia l of this agent for imaging intracardiac thrombi in swine. Methods and Results Left atrial thrombi were formed by crush injury. In initial n onimaging experiments using intravenous I-125-labeled human annexin V, the mean thrombus/whole blood ratio was 13.4+/-4.8 for the entire thr ombus using well counting of resected specimens (n=8). Using intraveno usly injected Tc-99m-labeled human annexin V, the left atrial thrombus /blood ratio by well counting was similar (14.2+/-10.6 for the entire thrombus and 26.2+/-14.9 for the peak section) (n=12). The ratio for a control protein, I-125-ovalbumin, was only 1.0+/-0.2. Tc-99m tomograp hic imaging was positive (n=10) or equivocal (n=2) in all experiments with but negative in 10 controls without left atrial thrombi. By regio n-of-interest analysis of the tomographic images, the mean left atrial appendage/ blood ratio at 2 hours in animals with a thrombus was 3.90 +/-1.12 compared with 0.84+/-0.10 in closed chest controls and 1.01+/- 0.23 in open chest controls (P<.001). Conclusions We conclude that Tc- 99m-labeled human annexin V detects acute left atrial thrombi in vivo in swine. The combination of a new thrombus detection agent, annexin V , with a Tc-99m label may allow in vivo imaging of thrombi in humans.