Synthesis and Structure−Activity Relationships of Linear and Conformationally Constrained Peptide Analogues of CIYKYY as Src Tyrosine Kinase Inhibitors

A series of peptide analogues of Ac-CIYKYY (1) were synthesized by functional group modifications in peptide side chains or by introducing conformational constraints, to improve the inhibitory potency against active Src kinase. Ac-CIYKF(4-NO2)Y (2, IC50 = 0.53 microM) and conformationally constrained peptide 31 (IC50 = 0.28 microM) exhibited 750- and 1400-fold higher inhibitory activities, respectively, versus that of 1 (IC50 = 400 microM). Compound 2 exhibited a partial competitive inhibition pattern against ATP.