Compound of Formula I or a salt thereof for treating cancer, wherein X represents CH or N; R1 represents hydrogen or —CH2COR5; R5 represents hydroxy, optionally hydroxylated alkoxy, amino or alkylamido; R2 represents ZYR6; Z represents a bond, or a C1-3 alkylene or oxoalkylene group optionally substituted by R7; Y represents a bond, an oxygen atom or NR6; R6 is a hydrogen atom, COOR8, an alkyl, alkenyl, cycloalkyl, aryl or aralkyl group optionally substituted by one or more selected from COOR8, CONR82, NR82, OR8, ═NR8, ═O, OP(O)(OR8)R7 and OSO3M; R7 is hydroxy, an optionally hydroxylated, optionally alkoxylated alkyl or aminoalkyl group; R8 is a hydrogen atom or an optionally hydroxylated, optionally alkoxylated alkyl group; M is a hydrogen atom or one equivalent of a physiologically tolerable cation; R3 represents C1-8 alkylene, 1,2-cykloalkylene, or 1,2-arylene, optionally substituted with R7; and R4 represents hydrogen or C1-3 alkyl.