The present invention provides a compound represented by formula (I): [wherein X—Y-Z represents R5N—C═O (wherein R5 represents a hydrogen atom, substituted or unsubstituted lower alkyl, substituted or unsubstituted aralkyl, substituted or unsubstituted lower alkenyl, or substituted or unsubstituted lower alkynyl), etc; R1 and R2 may be the same or different and each represents a hydrogen atom, substituted or unsubstituted lower alkyl, etc; and R3 and R4 may be the same or different and each represents a hydrogen atom, substituted or unsubstituted lower alkyl, etc], or a pharmaceutically acceptable salt thereof.