INTERACTION OF TECHNETIUM 99M-LABELED TEBOROXIME WITH RED-BLOOD-CELLS REDUCES THE COMPOUNDS EXTRACTION AND INCREASES APPARENT CARDIAC WASHOUT

Background. Tc-99m-labeled teboroxime shows high myocardial extraction in both in vivo animal and in vitro cell culture and isolated heart studies. Whereas in vivo studies show rapid myocardial clearance of teboroxime, in vitro cell culture and isolated heart studies show slower washout comparable to that of Tl-201. Binding of teboroxime to blood components may contribute to these conflicting results. Methods and Results. We measured teboroxime extraction in the isolated blood-perfused rabbit heart after injection in saline solution, brief incubation in red blood cell perfusate, or 4-hour incubation with human red blood cells. Teboroxime in saline solution showed high extraction (E(max) = 0.89 +/- 0.02; E(net) = 0.69 +/- 0.02), whereas brief incubation in perfusate (E(max) = 0.60 +/- 0.06; E(net) = 0.48 +/- 0.05) or prolonged incubation with human red blood cells (E(max) = 0.43 +/- 0.09; E(net) = 0.38 +/- 0.07) resulted in reduced extraction. Teboroxime clearance was similar for all groups and was slower than Tl-201 clearance. Analysis of-total residual cardiac teboroxime (comparable to external imaging) showed that teboroxime clearance was biexponential. Reduced extraction of teboroxime in red blood cells resulted in an increased size of the rapidly clearing (unextracted) fraction, giving the appearance of rapid myocardial washout. Conclusions. Teboroxime has a high myocardial extraction. Binding to blood components reduces teboroxime extraction and increases the rate of cardiac teboroxime clearance.