Abnormal adhesion of sickle red blood cells (SS RBCs) to vascular endothelium may play an important role in vasoocclusion in sickle cell disease. Accruing evidence shows that endothelial alpha V beta 3-integrin has an important role in SS RBC adhesion because of its ability to bind several adhesive proteins implicated in this interaction. In the present studies, we tested therapeutic efficacy of small-molecule cyclic pentapeptides for their ability to block alpha V beta 3-mediated SS RBC adhesion by using two well-established assay systems, i.e., cultured human umbilical vein endothelial cells (HUVEC) and artificially perfused mesocecum vasculature of the rat under flow conditions. We tested the efficacy of two RGD-containing cyclic pentapeptides, i.e., cRGDFV (EMD 66203) and cRGDF-ACHA (alpha-amino cyclohexyl carboxylic acid) (EMD 270179), based on their known ability to bind alpha V beta 3. An inactive peptide, EMD 135981 (cR beta-ADFV) was used as control. Cyclization and the introduction of D-Phe (F) results in a marked increase in the ability of cyclic peptides to selectively bind alpha V beta 3 receptors. In the mesocecum vasculature, both EMD 66203 and EMD 270179 ameliorated platelet-activating factor-induced enhanced SS RBC adhesion, postcapillary blockage, and significantly improved hemodynamic behavior. Infusion of a fluorescent derivative of EMD 66203 resulted in colocalization of the antagonist with vascular endothelium. Also, pretreatment of HUVEC with either alpha V beta 3 antagonist resulted in a significant decrease in SS RBC adhesion. Because of their metabolic stability, the use of these cyclic alpha V beta 3 antagonists may constitute a novel therapeutic strategy to block SS RBC adhesion and associated vasoocclusion under flow conditions.