Vaso-occlusive crises are a major clinical feature of sickle cell disease (SCD), and the adhesion of sickle erythrocytes (SS-RBCs) to vascular endothelium is crucial to the generation of vaso-occlusion. SS-RBCs express many adhesion molecules. Adhesive SS-RBCs bind to endothelial cell P-selectin and other adhesion molecules, as well as extracellular matrix proteins. Thrombin causes endothelial retraction with exposure of proadhesive extracellular matrix components as well as upregulation of endothelial expression of P-selectin. Other investigators have shown that SS-RBCs can bind to P-selectin, thus suggesting that P-selectin is a suitable target against which to develop reagents to prevent or treat vaso-occlusive crises in SCD. Aptamers are oligonucleotides that bind to molecular targets in a manner conceptually similar to antibodies and have been identified against a wide range of therapeutic targets. RNA aptamers that bind to human P-selectin, inhibit P-selectin binding to its ligand sialyl Lewis X, and inhibit neutrophil-platelet adhesion have previously been identified (
Antisense Nucleic Acid Drug Dev,8:265,1998
). We have now tested one of these high-affinity P-selectin aptamers (clone PF377: ACGCUCAACGAGCCAGGAACAUCGACGUCAGCAAACGCGAGCGCAACCAGUAACACC) for its ability to prevent cellular adhesion of SS-RBCs to endothelial cells. Aptamer clone PF377 bound P-selectin with a Kd of 6nM and a Bmax of 87%. To measure its anti-adhesion activity, an in vitro flow chamber assay was adopted. The anti-adhesion activity of aptamer clone PF377 was confirmed using primary human umbilical vein endothelial cells (HUVECs) treated with IL-13 (50ng/mL) for 48hrs, followed by stimulation with histamine (25μM for 12mins at RT) immediately prior to a flow chamber assay. Aptamer clone PF377 at 60nM had anti-adhesion activity similar to heparin (a previously identified inhibitor of SS-RBC adhesion to P-selectin) and 9E1 (an inhibitory antibody to P-selectin), whereas Bi 9.3t (a non-functional aptamer to thrombin, used as a negative control, 60nM) and AC1.2 (a non-inhibitory antibody to P-selectin), did not prevent adhesion (Figure). Normalized % inhibition by aptamer clone (PF377) at 60nM at 1 dynes/cm2 was 99% (Figure). In conclusion, we observed significant anti-adhesion activity of P-selectin aptamer clone PF377, and this aptamer is now suitable for modification to enhance stability and bioavailability. Since we previously reported strong anti-adhesion activity of integrin αvβ3 aptamer clone 17.16 (Blood,108:206a,2006
), in vivo experiments in mice are currently employing intravital microscopy to measure anti-adhesion activity of aptamer clone 17.16, PF377, and both in combination. The development of combinatorial blocking aptamers against various adhesion molecules, including αvβ3 and P-selectin, represents a novel potential therapeutic option for patients with SCD.
