Kinetic properties of 99mTc-Q12 in canine myocardium.
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.


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 0 to 2 mL.g-1 x 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 < .05), whereas left circumflex blood flow increased by 40.4% from 30 to 120 minutes (P < .05) as the dipyridamole hemodynamic effects dissipated. Nevertheless, the ratio of myocardial perfusion defect zone counts to normal myocardial zone counts remained constant over 4 hours, as did the technetium counts from the needle biopsy endocardial samples.


CONCLUSIONS
Over a limited range of myocardial blood flows from 0 to approximately 2 mL.g-1 x min-1, 99mTc-Q12 myocardial activity is related to actual myocardial flow at the time of tracer injection. 99mTc-Q12 shows no evidence of myocardial redistribution.
Gerson
M C
MC
Division of Cardiology, University of Cincinnati, Ohio.
Millard
R W
RW
Roszell
N J
NJ
McGoron
A J
AJ
Gabel
M
M
Washburn
L C
LC
Biniakiewicz
D
D
Blankenship
D
D
Mallin
W H
WH
Elder
R C
RC
D000818:Animals; D003326:Coronary Circulation / Q000187:drug effects; D004176:Dipyridamole; D004285:Dogs; D005663:Furans / Q000493:pharmacokinetics; D006321:Heart / Q000000981:diagnostic imaging*; D008297:Male; D008863:Microspheres; D017202:Myocardial Ischemia / Q000000981:diagnostic imaging*; D009206:Myocardium / Q000378:metabolism; D015609:Organotechnetium Compounds / Q000493:pharmacokinetics; D011877:Radionuclide Imaging; D013997:Time Factors
D005663:Furans; D015609:Organotechnetium Compounds; C078496:technetium Tc 99m Q12; D004176:Dipyridamole