Kinetic properties of 99mTc-Q12 in canine myocardium

Abstract

Background: 99mTc-Q12 is a new Tc(III) perfusion imaging agent that permits prompt myocardial visualization in humans. We postulated that 99mTc-Q12 myocardial activity is related to actual myocardial blood flow during conditions of myocardial ischemia and pharmacological coronary artery vasodilation and that 99mTc-Q12 shows little or no myocardial redistribution as long as 4 hours after intravenous injection. Methods and Results: In seven anesthetized, open chest dogs, the left circumflex coronary artery was occluded, and dipyridamole (0.32 or 0.56 mg/kg) was infused into the right atrium, followed by 10 mCi of 99mTc-Q12. Myocardial blood flow was measured by radiolabeled microspheres. The animals were euthanized, and a total of 315 myocardial samples were assayed in a well counter for 99mTc activity. One week later, radiolabeled microsphere activity was determined, and myocardial blood flow was calculated. 99mTc activity (y) was related to myocardial blood flow (x) from O to 2 mL · g-1 · min-1 by the relation y=0.64x+0.35 (r=.88, P=.0001). In 14 additional anesthetized, open chest dogs, an occluder was placed around the left circumflex coronary artery, and an ischemic level of circumflex blood flow was maintained constant over 4 hours as measured by an ultrasonic flowmeter. Dipyridamole (0.56 mg/kg) was infused intravenously beginning 15 minutes after coronary occlusion and then followed by 10 mCi of 99mTc-Q12. Gamma camera images, hemodynamics, microsphere blood flow, and endocardial biopsies (latter in six dogs) were performed at 30, 60, 120, and 240 minutes after 99mTc-Q12 injection. Myocardial blood flow in the distribution of the left anterior descending artery decreased by 29.6% from 30 to 240 minutes (P