Technetium 99m-HL-91: A potential new marker of myocardial viability assessed by nuclear imaging early after reperfusion

Objective  99mTc-HL-91 (Prognox) is a potential new hypoxia-avid myocardial imaging agent. The purpose of this study was to determine whether this tracer would demonstrate increased activity in nonviable myocardium in vivo. Methods and Results  A 3-hour left circumflex artery (LCx) occlusion was followed by 1 hour of reperfusion, injection of 99mTc-HL-91 (185 MBq), and 2 hours of gamma camera imaging in 6 open-chest canine experiments. Microspheres were injected during baseline, at occlusion, at the time of tracer injection, and at the end of the experiment. After the animals were killed, heart slices were imaged. Blood flow and tracer activity were determined by well counting. Mean infarct size was 19.2%±2.2% (SEM). All six dogs demonstrated no increased 99mTc-HL-91 myocardial activity other than small foci on in vivo and ex vivo gamma camera images. The mean large region of interest (ROI)-determined LCx/LAD (left anterior descending) ratio was 1.10±0.03 in vivo, and 1.0±0.02 ex vivo. Mean clearance curves from LCx and LAD ROI were not significantly different, and 2-hour retention was 15.2%±2.1% for the LCx and 18.6%±2.7% for the LAD (p=NS). ROI clearance curves demonstrated biexponential clearance over the first hour and linear clearance over the second hour. Myocardial blood flow (microspheres) versus well-counted tracer uptake curves were linear with near-zero slopes for viable tissue, non-viable tissue, and mosaic tissue. Blood clearance was triexponential with a 2-hour retention of 7.8%±1.1%. Conclusions  In contrast to viable ischemic tissue, normal and nonviable myocardium demonstrate similar 99mTc-HL-91 uptake and retention kinetics. This agent warrants further clinical studies in situations where there is a need to differentiate ischemic viable from nonviable myocardium. Supported in part by Nycomed Amersham p/c, Buckinghamshire, UK