Kinetics of 99mTc-labeled interleukin-8 in experimental inflammation and infection

The cytokine interleukin-8 (IL-8) binds with high affinity to the CXCR1 and CXCR2 receptors on neutrophils. In previous studies, we showed that (99m)Tc-IL-8 could rapidly and effectively delineate foci of infection and inflammation in rabbit models of intramuscular infection, colitis, and osteomyelitis. Here, the in vivo kinetics and pharmacodynamics of (99m)Tc-IL-8 are studied in detail. A derivative of hydrazinonicotinamide (HYNIC) was used as a bifunctional coupling agent to label the protein with (99m)Tc.

To address specificity of uptake of (99m)Tc-IL-8 in the abscess, uptake in turpentine-induced abscesses in neutropenic rabbits was compared with uptake in turpentine-induced abscesses in normal rabbits. The pharmacokinetics of (99m)Tc-IL-8 were studied in neutropenic rabbits and compared with those in normal rabbits. To investigate the interaction of (99m)Tc-IL-8 with blood cells in circulation in normal rabbits, the distribution of the radiolabel over circulating white and red blood cells and plasma was determined. The in vivo kinetics of (99m)Tc-IL-8 were studied by quantitative analysis of whole-body images acquired between 0 and 6 h after injection. The results of this analysis (in vivo biodistribution) were validated by ex vivo counting of radioactivity in dissected tissues.

The abscess uptake (percentage of injected dose per gram of tissue [%ID/g] +/- SEM) in immunocompetent rabbits (0.41 +/- 0.05) was 10 times higher than that in neutropenic rabbits (0.038 +/- 0.014), demonstrating specificity of the target uptake of (99m)Tc-IL-8. Abscess-to-muscle ratios +/- SEM were also 10 times higher (110 +/- 10 vs. 10 +/- 5). Lung and spleen uptake in normal rabbits was 3 times higher than that in neutropenic rabbits. The blood clearance of the radiolabel in neutropenic rabbits was similar to that in normal rabbits. In circulation, most of (99m)Tc-IL-8 (70%) was found in the plasma fraction. Less than one third was associated with red blood cells, and only a very low percentage (<2.5%) was associated with white blood cells. Image analysis revealed a gradually increasing abscess uptake over time up to >15%ID, which was confirmed by ex vivo gamma-counting of the infected muscle. The highest increase in uptake in the abscess was observed after 2 h following injection, when most of (99m)Tc-IL-8 was cleared from the blood, suggesting specific neutrophil-mediated accumulation of (99m)Tc-IL-8 in the abscess. Furthermore, region-of-interest analysis revealed that gradual accumulation of (99m)Tc-IL-8 in the abscess was accompanied by a simultaneous clearance of activity from the lungs, suggesting that neutrophil-associated (99m)Tc-IL-8 that was initially trapped in the lungs migrates to the abscess at later time points, favoring neutrophil-bound transportation from the lungs to the abscess.

Substantial support is given for the hypothesis that (99m)Tc-IL-8 localizes in the abscess, mainly bound to peripheral neutrophils. Accumulation in the abscess is a highly specific, neutrophil-driven process. As assessed by in vivo and ex vivo analysis, the total fraction that accumulates in the inflamed tissue is extremely high (up to >15 %ID) compared with that of other agents used for imaging infection and inflammation.