Abstract: Piperidine derivatives and pharmaceutically acceptable derivatives thereof useful in methods of treatment of bacterial infections in mammals, particularly in man. The piperidine derivatives include compounds of formula (I): wherein RA, AB, n, R3, R2 and R4 are defined herein. For example, RA is an optionally substituted bicyclic carbocyclic or heterocyclic ring system, and R4 is a group U-R5 in which R5 is an optionally substituted bicyclic heterocyclic ring system as defined herein (e.g. optionally substituted pyrido [3,2, -b][1,4] thiazin-yl).