Background: 99mTc-RP517 is a new leukotriene B4 (LTB4) receptor antagonist developed for imaging acute inflammation or infection. A unique property of 99mTc-RP517 is its ability to label white blood cells in vivo after intravenous injection. The goals of this study were to determine relative 99mTc-RP517 binding to human leukocyte subtypes and the 99mTc-RP517 uptake pattern in canine myocardium where inflammation was induced by either coronary occlusion and reperfusion or tumor necrosis factor alpha (TNFalpha) injection.
Methods and results: Fluorescence-activated cell sorter analysis was performed on whole human blood (n=2) and isolated neutrophils (n= 4) with a fluorescent analog of 99mTc-RP517, [F]-RP517. In whole blood, [F]-RP517 (500 nmol/L) preferentially labeled neutrophils. On isolated neutrophils, [F]-RP517 (10 nmol/L) binding was inhibited by 44% when LTB4 (400 nmol/L) was added. 99mTc-RP517 was injected intravenously in anesthetized, open-chest dogs before coronary occlusion (90 minutes) and reperfusion (120 minutes) (n=9) or before intramyocardial TNFalpha injection (n=3). Ex vivo images of heart slices were acquired. The left ventricle was divided into 72 segments for flow and 99mTc-RP517 uptake analysis. There was an inverse exponential relationship between 99mTc-RP517 uptake and occlusion flow (r=0.73). In the same 15 segments, 99mTc-RP517 uptake was highly correlated with the neutrophil enzyme myeloperoxidase (r=0.91). Ex vivo images revealed tracer uptake in the reperfused area (ischemic to normal count ratio=2.7+/-0.2).
Conclusions: RP517 binds to the neutrophil LTB4 receptor after intravenous injection. After reperfusion, 99mTc-RP517 uptake correlated with myeloperoxidase and was observed on ex vivo images, indicating that this tracer may have potential as an inflammation-imaging agent.