A compound of formula I: and isomers, salts, solvates, chemically protected forms, and prodrugs thereof, wherein: R1 and R2 together form, along with the nitrogen atom to which they are attached, an optionally substituted nitrogen containing heterocyclic ring having from 4 to 8 ring atoms; and R3 is selected from hydroxy and —NRN1RN2, where RN1 and RN2 are independently selected from hydrogen, optionally substituted C1-7 alkyl groups, optionally substituted C3-20 heterocyclyl groups and optionally substituted C5-20 aryl groups, or together form, along with the nitrogen atom to which they are attached, an optionally substituted nitrogen containing heterocyclic ring having from 4 to 8 ring atoms, and its use in inhibiting ATM.