Structural and Functional Aspects of RGD-Containing Cyclic Pentapeptides as Highly Potent and Selective Integrin α_Vβ₃ Antagonists

The α_Vβ₃ integrin is implicated in human tumor metastasis and in angiogenesis. The design of low-molecular-mass α_Vβ₃ antagonists by “spatial screening” led to the highly active peptides c(RGDFV) and c(RGDFV). Here the influence of the amino acids in positions 4 and 5 flanking the RGD-sequence on the inhibition of vitronectin and fibrinogen binding to the isolated α_Vβ₃ and α_IIbβ₃ receptors was investigated. The influence of the side chain and the backbone conformation on activity and selectivity was studied. The compounds were divided into conformational classes. For each class at least one representative peptide was subjected to detailed structure determination in solution. The peptides of classes 1, 2, and 3 show a βII‘/γ-turn arrangement with the d-amino acid in the i + 1 position of the βII‘-turn. By contrast, the peptides of class 4 reveal a modified βII‘/γ-turn pattern with glycine in the i + 1 position of the βII‘-turn and the d-amino acid in the i + 1 position of the γ-turn. Class 1 is divided into two subclasses:  besides the βII‘/γ-turn arrangement a γ/γ-turn motif is found for two members of this class. Structure−activity relationship (SAR) investigations show that the amino acid in position 4 and the proton of the amide bond between residues 3 and 4 are essential for high biological activities toward α_Vβ₃. By contrast, the amino acid in position 5 has no influence on the activity. A bent conformation of the RGD-sequence, as observed for the peptides of classes 1 and 2, fits the α_Vβ₃ better than the α_IIbβ₃ receptor and so increases the selectivity of these peptides.