Aza-amino acid scanning was performed on the cyclic RGD-peptide Cilengitide, cyclo[R-G-D-f-N(Me)V] 1, and its parent peptide cyclo(R-G-D-f-V) 2, potent antagonists of the αvβ3, αvβ5, and α5β1 integrin receptors, which play important roles in human tumor metastasis and tumor-induced angiogenesis.