Thiazole and oxazole derivatives

A compound of the formula (I):

• 185
• wherein R1 is NHOH, hydroxy, or lower alkyloxy;
• R2 is hydrogen atom, optionally substituted lower alkyl, optionally substituted aryl, optionally substituted aralkyl, optionally substituted heteroaryl, or optionally substituted heteroarylalkyl;
• R3 is hydrogen atom, optionally substituted lower alkyl, optionally substituted aryl, optionally substituted aralkyl, optionally substituted heteroaryl, or optionally substituted heteroarylalkyl;
• R4 is optionally substituted arylene, or optionally substituted heteroarylene;
• R5 is a group represented by the formula:
• 186
• R6 is optionally substituted aryl, optionally substituted heteroaryl or optionally substituted non-aromatic heterocyclic group;
• provided that R5 is thiazole-2,5-diyl when R4 is thiophene-2,5-diyl and R6 is optionally substituted phenyl,
• its optically active substance, its prodrug, their pharmaceutically acceptable salt, or solvate thereof.