Relationship between neutrophil-binding affinity and suitability for infection imaging: comparison of (99m)Tc-labeled NAP-2 (CXCL-7) and 3 C-terminally truncated isoforms.
The CXC chemokines are a family of closely related chemoattractant cytokines that bind to, attract, and activate neutrophils to variable degrees. In this study, the relationship between neutrophil-binding affinity and suitability for infection imaging was investigated in a selected group of CXC chemokines. Neutrophil-activating peptide-2 (NAP-2, 70 residues; also called CXCL7) binds with high affinity to the CXCR2 receptor on neutrophils. Recently, C-terminally truncated NAP-2-variants have been described that have enhanced neutrophil-binding affinity and neutrophil-stimulating capacity. Here, NAP-2 and its C-terminal shortened variants NAP-2(1-68), NAP-2(1-66), and NAP-2(1-63) were labeled with (99m)Tc via the hydrazinonicotinamide (HYNIC) chelator and their potential for imaging of infection was investigated in a rabbit model of infection. The CXC chemokine interleukin-8 (IL-8) was used for comparison. In addition, a series of (99m)Tc-labeled CXC chemokines were screened for their potential to image infection, including CTAP-III, GCP-2, ENA-78, PF-4, and IP-10.


METHODS
The receptor-binding affinity of HYNIC-conjugated NAP-2 and its analogs was compared in competitive binding assays on Jurkat cells transfected with the CXCR2 receptor gene. Biodistribution of labeled NAP-2 (analogs) and other CXC chemokines in rabbits with intramuscular Escherichia coli infections was determined both by gamma-camera imaging and by counting dissected tissues at 6 h after injection.


RESULTS
The CXCR2-binding affinity of the HYNIC-conjugated NAP-2 analogs relative to NAP-2 was as follows: NAP-2(1-68), 2.5-fold; NAP-2(1-66), 10-fold; and NAP-2(1-63), 3-fold. In the rabbit model, uptake in the abscess (in percentage injected dose per gram +/- SEM) was 0.084 +/- 0.015 for NAP-2, 0.098 +/- 0.010 for NAP-2(1-68), 0.189 +/- 0.044 for NAP-2(1-66), and 0.114 +/- 0.017 for NAP-2(1-63) at 6 h after injection. In comparison, higher uptake in the abscess was found for labeled IL-8, a modest uptake was found for GCP-2 and ENA-78, and a low uptake was found for CTAP-III, PF-4, and IP-10.


CONCLUSIONS
This study showed a clear relationship between affinity to receptors on neutrophils and suitability for infection imaging. Of the NAP-2 variants, NAP-2(1-66) combined highest affinity to CXCR2 with the best characteristics for imaging. IL-8 binds to both CXCR1 and CXCR2 with high affinity and showed a superior imaging quality. The other CXC chemokines tested bind to neutrophils with lower affinity and were shown to be less suitable for infection imaging in this study.
Rennen
Huub J J M
HJ
Department of Nuclear Medicine, University Medical Center Nijmegen, Nijmegen, The Netherlands. h.rennen@nucmed.umcn.nl
Frielink
Cathelijne
C
Brandt
Ernst
E
Zaat
Sebastian A J
SA
Boerman
Otto C
OC
Oyen
Wim J G
WJ
Corstens
Frans H M
FH
D000818:Animals; D004927:Escherichia coli Infections / Q000000981:diagnostic imaging* / Q000378:metabolism*; D005260:Female; D007553:Isotope Labeling / Q000379:methods; D008657:Metabolic Clearance Rate; D009220:Myositis / Q000000981:diagnostic imaging / Q000378:metabolism; D009504:Neutrophils / Q000000981:diagnostic imaging* / Q000378:metabolism*; D009928:Organ Specificity; D010455:Peptides / Q000493:pharmacokinetics*; D011817:Rabbits; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D023063:Receptors, Interleukin-8B / Q000378:metabolism*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013667:Technetium / Q000493:pharmacokinetics; D014018:Tissue Distribution; D001620:beta-Thromboglobulin
C061615:PPBP protein, human; D010455:Peptides; D019275:Radiopharmaceuticals; D023063:Receptors, Interleukin-8B; D001620:beta-Thromboglobulin; D013667:Technetium