Compound of formula (I) are inhibitors of Polo-like kinases (PLKs), and are useful in treatment of cell proliferative diseases: wherein R1 and R2 are hydrogen, or an optionally substituted (C1-C6)alkyl, (C2- C6)alkenyl, (C2-C6)alkynyl or (C3-C6)cycloalkyl group; R3 and R3' are independently selected from hydrogen, -CN, hydroxyl, halogen, optionally substituted (C1-C6) alkyl, (C2- C6)alkenyl, (C2-C6)alkynyl or (C3-C6)cycloalkyl, -NR5R6 or C1-C4 alkoxy, wherein R5 and R6 are independently hydrogen or optionally substituted (C1-C6)alkyl; ring A is an optionally substituted mono- or bi-cyclic carbocyclic or heterocyclic ring or a ring system having up to 12 ring atoms; T is a radical of formula R-L1 -Y1- wherein R is an alpha amino acid or alpha amino acid ester motif, linked to ring A by linker R-L1-Y1- as defined in the claims.