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

Background Annexin V is a human phospholipid binding protein that binds to activated platelets in vitro. We sought to determine the potential of this agent for imaging intracardiac thrombi in swine. Methods and Results Left atrial thrombi were formed by crush injury. In initial nonimaging experiments using intravenous I-125-labeled human annexin V, the mean thrombus/whole blood ratio was 13.4+/-4.8 for the entire thrombus using well counting of resected specimens (n=8). Using intravenously 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 tomographic imaging was positive (n=10) or equivocal (n=2) in all experiments with but negative in 10 controls without left atrial thrombi. By region-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.