Abstract
1252
Objectives: CD44 is a cell-surface glycoprotein involved in cell-cell interaction, adhesion, and migration. CD44 is found on colon cancer cells and is also highly expressed on immune cells. This study investigated how leukocyte activation affects splenic uptake of 89Zr-anti-CD44 and further explored how regulating the magnitude of splenic uptake by increasing total Ab dose can improve tumor CD44 PET imaging.
Methods: We used IM7, an antibody that reacts with all CD44 isoforms of both human and mouse origin. The antibody was site-specifically labeled with 89Zr-deferoxamine on cysteine residues. Pharmacokinetics, biodistribution, and PET imaging studies were performed in colon cancer tumor-bearing mice.
Results: Human colon cancer cells and monocytic cells showed CD44 expression and highly-specific 89Zr-anti-CD44 binding. When 89Zr-anti-CD44 was administered to Balb/C nude mice, there was remarkably high splenic uptake but low SNU-C5 tumor uptake (1.2 ± 0.7 %ID/g). Among cells isolated from Balb/C mouse spleen, there was greater CD44 expression on CD11b positive myeloid cells than lymphocytes. In cultured monocytic and macrophage cells, lipopolysaccharide (LPS) stimulation upregulated CD44 expression and increased 89Zr-anti-CD44 binding. Similarly, normal Balb/C mice that underwent LPS stimulation showed a significant upregulation of CD44 expression on splenic myeloid cells. Furthermore, LPS treatment stimulated a 2.4-fold increase of 89Zr-anti-CD44 accumulation in the spleen, which was attributable to splenic myeloid cells. Finally, in Balb/C nude mice bearing HT29 tumors, we injected 89Zr-anti-CD44 with greater antibody doses to reduce binding to splenic cells. The results showed lower spleen uptake and improved tumor uptake (2.9 ± 1.3 %ID/g) with a total of 300 μg of antibody dose, and further reduction of spleen uptake and greater tumor uptake (5.7 ± 0.0 %ID/g) with 900 μg antibody dose.
Conclusions: 89Zr-anti-CD44 PET has the capacity to monitor CD44 regulation on splenic myeloid cells and may also be useful for imaging CD44-positive tumors by increasing antibody dose.