Abstract: Compounds of formula (I) are inhibitors of Polo-like kinases (PLKs), and are useful, inter alia, in the treatment of proliferative diseases: wherein R1 is hydrogen, or a (C1-C6)alkyl, (C2-C6)alkenyl, (C2-C6)alkynyl or (C3-C6)cycloalkyl group; R2 is 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, —NR6R7 or C1-C4 alkoxy, wherein R6 and R7 are independently hydrogen or optionally substituted (C1-C6)alkyl; ring A is an optionally substituted mono- or bi-cyclic carbocyclic or heterocyclic ring or ring system having up to 12 ring atoms; T is a radical of formula (II) R4R5CH—NH—Y-L1-X1—??(II) Wherein R4 is a carboxylic acid group (—COOH), or an ester group which is hydrolysable by one or more intracellular esterase enzymes to a carboxylic acid group; R5 is the side chain of a natura