A first aspect of the invention relates to a compound of formula (I), or a pharmaceutically acceptable salt or ester thereof, • B is a saturated or partially unsaturated monocyclic or bicyclic, heterocyclic group optionally substituted by one or more R 10 groups; *X is selected from SO2, CO2, CO, CONR11 and (CR12R13) p. Said compounds are capable of inhibiting PAICS and are useful in the treatment of proliferative disorders. Further aspects relate to pharmaceutical compositions, therapeutic uses and process for preparing compounds of formula (I).