The purpose of the present study was to compare inhibitory effects between AMP579 (a new adenosine analog) and adenosine (Ado) in attenuating an interaction between human neutrophils (PMNs) and cultured human umbilical vein endothelial cells (HUVECs). PMN activation was determined by superoxide anion (O 2 -) production and degranulation (myeloperoxidase release). Cell-cell interaction was quantitated by adherence of fluorescent labeled PMNs to HUVECs. AMP579 inhibited O 2 -(nM/5 × 10 6 PMN) from fMLPactivated human PMNs (55.3 ± 3.1) in a concentration-dependent manner ranging from 31.1 ± 2.9 at 10 nM to 11.7 ± 0.9 at 10 ± µM, all P < 0.01 vs. fMLP group. In the same dose range, however, Ado showed significant inhibition only at 1 µM (30.3 ± 4.1) and 10 ± µM (27.5 ± 4.3) vs. the fMLP group. The calculated IC 50 value (0.11 ± 0.05 µM) in AMP579 group was significantly less than that in the Ado group (4.1 ± 1.2 µM). Although there was no group difference on PMN myeloperoxidase release (percent inhibition from fMLP) between AMP579 and Ado at concentrations greater than 1 µM (52.9 ± 5.2 vs. 46.4 ± 5.9), AMP579 showed significant attenuation of degranulation compared with Ado at 10 nM (31.7 ± 2.5 vs. 11.6 ± 1.9) and 100 nM (48.2 ± 4.6 vs. 25.6 ± 3.8), respectively, suggesting that AMP579 is more potent in inhibiting PMN activation. AMP579 reduced PMN adherence to TNFα-stimulated HUVEC (fluorescent units/well) in a concentration-dependent manner from 472 ± 32 at 10 nM to 214 ± 15 at 10 µM vs. 675 ± 54 in the TNFα group. At 10 nM and 100 nM, adenosine did not attenuate PMN adherence, while it showed significant inhibition at 1 (504 ± 45) and 10 µM (435 ± 50), respectively. The IC 50 value (2.8 ± 1 µM) for AMP579 was significantly lower than that (41 ± 8 µM) in the Ado group. The results from the present study suggest that 1) AMP579 directly inhibits adherenceindependent superoxide radical generation and degranulation from activated PMNs and attenuates cell-cell interaction between PMNs and vascular endothelial cells by preventing damage on endothelial cells. 2) AMP579 exerts more potent protective effect compared with adenosine at a lower dose range, indicating its prospect for clinical application.