Anti–P-selectin aptamer ARC5690 reduces sickle RBC adhesion and increases microvascular flow velocities. (A) ARC5690 inhibits sickle RBC adhesion. Adhesion of labeled RBCs was defined as the number of events (adhesion time more than 1.0 seconds/min in a 100-μm length of vessels by frame–by–frame analysis of video replay. Treatment with ARC5690 before hypoxia/normoxia stress reduced the number of adherent RBCs by 90% compared with saline-treated mice (P < .001). No difference in sickle-RBC adhesion was observed in the ARC5694-treated group compared with saline group (P = .427). (B) ARC5690 improves sickle RBC velocities. Sickle RBC velocities were determined off-line by frame–by–frame analysis of video-recorded microscopic images using Image Pro-Plus 5.0 software. Five to 8 RBC velocity measurements were performed along the centerline of the vessel and used to define the mean centerline VRBC. Mean VRBC (Vmean) was calculated using a conversion factor of 1.6 (VRBC/Vmean = 1.6). Compared with saline-treated mice, RBC velocities were increased in ARC5690 treated group (P < .01), but not in ARC5694-treated mice (P = .227). (C-D) Frame-captured images from videotaped intravital microscopy of bone marrow venules in SCD mice injected with saline (C) and ARC5690 (D). Saline-treated mice show more adherent sickle RBCs (white arrows). Pretreatment with ARC5690 reduced the number of adherent sickle RBCs. Open arrowheads identify RBCs moving slowly in saline-treated mice (C) and rapidly in the aptamer-treated mice (D). Values were mean ± SE obtained from 4 to 5 mice in each group. The number of venules in experimental groups ranged from 18 to 21. P values for statistical analyses are shown on top of the figures.