Recent evidence highlighted the role of alpha(5)beta(1) integrin in angiogenesis and in regulating alpha(v)beta(3) integrin function. As a consequence, selective alpha(5)beta(1) integrin inhibitors or dual alpha(5)beta(1)/alpha(v)beta(3) integrin inhibitors are considered promising candidates for the development of cancer therapeutic agents. In this paper, we describe the synthesis and pharmacological characterization of a minilibrary of cyclotetrapeptide mimetics containing a PMRI Arg-Gly-Asp sequence. In particular, c[(R)-betaPhepsi(NHCO)Asppsi(NHCO)Gly-Arg] (3) displayed a good activity in inhibiting the alpha(v)beta(3) integrin-mediated cell adhesion of fibronectin or vitronectin, as well as the adhesion of fibronectin to the alpha(5)beta(1) integrin. Interestingly, the diastereomeric compound c[(S)-betaPhepsi(NHCO)Asppsi(NHCO)Gly-Arg] (2) maintained a good efficacy in inhibiting alpha(5)beta(1) integrin while gaining a certain selectivity over alpha(v)beta(3) integrin. These two integrin antagonists significantly inhibited bFGF-induced human endothelial cell tube formation at submicromolar concentrations. Conformational analysis and Molecular Docking calculations suggest that the different alpha(5)beta(1) versus alpha(v)beta(3) selectivity of 2 and 3 can be rationalized on the basis of the alternative display of the aromatic side chain adjacent to Asp.