FIELD: pharmacology.SUBSTANCE: invention relates to a new compound of formula X or a pharmaceutically acceptable salt thereof which have the properties of a histone deacetylase (HDAC) inhibitor. The compounds can be used as active agents for pharmaceutical compositions suitable for use in the treatment of a tumour disease associated with impaired HDAC activity. Such a disease can be pancreatic carcinoma, hepatocellular carcinoma, colon tumours, mammary tumours, prostate tumours, lymphomas and skin tumours. In particular, tumour diseases can be melanoma and basal carcinomas, large-cell lung cancer, lung adenocarcinoma, small cell lung cancer, stomach cancer, ovarian adenocarcinoma, promyelocytic leukemia, chronic myelocytic leukemia or acute lymphocytic leukemia. In the formula(I) R1 is hydrogen, (C1-C6)alkyl or (C3-C6)cycloalkyl； R2 is (C6)aryl, (C6)aryl(C1-C6)alkyl, heteroaryl(C1-C6)alkyl, wherein heteroaryl is a 5-membered monocyclic ring system containing one heteroatom in the ring selected from nitrogen, wherein the said system is fused to a benzene ring, halo(C6)aryl(C1-C6)alkyl or (C1-C6)alkoxy(C6)aryl(C1-C6)alkyl； R3 is N-hydroxyamino-oxo(C2-C6)alkenyl； R4 is hydrogen, N-hydroxyamino-oxo(C2-C6)alkenyl or N-hydroxyaminocarbonyl(C6)aryl(C1-C6)alkylene； R5 is hydrogen, halogen, N-hydroxyamino-oxo(C2-C6)alkenyl, N-hydroxyaminocarbonyl(C6)aryl(C1-C6)alkylene or (2E)-3-N-(2-aminophenyl)-3-oxo-propenyl； and R6 is hydrogen, N-hydroxyamino-oxo(C2-C6)alkenyl, N-hydroxyaminocarbonylphenyl or N-hydroxyaminocarbonyl(C6)aryl(C1-C6)alkylene； or (II) each of R1, R2, R5 and R6 is as defined in (I) above； R3 is hydrogen or N-hydroxyamino-oxo(C2-C6)alkenyl； and R4 is N-hydroxyamino-oxo(C2-C6)alkenyl or N-hydroxyaminocarbonyl(C6)aryl(C1-C6)alkylene； or (III) each of R1, R2, R4 and R6 is as defined in (I) above； R3 is as defined in (II) above； and R5 is N-hydroxyamino-oxo(C2-C6)alkenyl, N-hydroxyaminocarbonyl(C6)aryl(C1-C6)alkylene or (2E)-3-N-(2-aminophenyl)-3-oxo-propenyl； or (
