Specific and rapid scintigraphic detection of infection with 99mTc-labeled interleukin-8

Interleukin-8 (IL-8) is a chemotactic cytokine involved in activation and recruitment of neutrophils to areas of infection. In our previous studies in rabbits we tested 123I-labeled IL-8 for its potential to image infections and showed that IL-8 rapidly and efficiently accumulated in infectious foci. However, labeling of IL-8 with 123I is costly and laborious and the specific activity of the preparation was low. In this study IL-8 was labeled with 99mTc through the hydrazinonicotinamide (HYNIC) chelator.

The leukocyte receptor-binding capacity of the preparation was determined in vitro. Rabbits with Escherichia coli abscesses were injected intravenously with 7 MBq 99mTc-HYNIC-IL-8. Biodistribution of the radiolabel was determined by gamma camera imaging and tissue counting at 8 h after injection. 99mTc-HYNIC-lysozyme was used as a size-matched control.

The leukocyte receptor-binding capacity of the 99mTc-HYNIC-IL-8 preparation was preserved as determined in vitro, but labeling efficiency was modest with a specific activity of 3 MBq/microg. 99mTc-HYNIC-IL-8 accumulated rapidly in the abscess up to 0.33 +/- 0.06 percentage injected dose per gram (%ID/g) at 8 h after injection (vs. 0.025 +/- 0.003 %lD/g for 99mTc-HYNIC-lysozyme). Total uptake in the abscess was 4.9 +/- 0.7 %ID (vs. 0.44 +/- 0.05 %ID for 99mTc-HYNIC-lysozyme). Abscess-to-contralateral muscle ratios increased up to 127 +/- 23 (compared with 6.7 +/- 1.1 for 99mTc-HYNIC-lysozyme) and abscess-to-blood ratios increased to 11.9 +/- 2.2 (0.24 +/- 0.03 for 99mTc-HYNIC-lysozyme). The radiolabel was excreted renally, with a retention in the kidneys of 28 %ID. Gamma camera imaging rapidly visualized the abscess from 1 h after injection onward, with abscess-to-background ratios improving with time up to 22 at 8 h after injection (vs. 2.7 for 99mTc-HYNIC-lysozyme), as determined by quantitative analysis of the images. Most important, only a transient (30 min) moderate drop of leukocyte counts and no leukocytosis were observed after injection of an imaging dose of 99mTc-HYNIC-IL-8.

IL-8 can be labeled with 99mTc using HYNIC as a chelator. By this method the leukocyte receptor-binding capacity is preserved. The preparation allows rapid visualization of infection in a rabbit model with high target-to-background ratios. The mild transient drop of leukocyte counts and the absence of leukocytosis suggest that 99mTc-HYNIC-IL-8 may be used as an imaging agent with only mild and transient side effects.