Substituted pyridine or piperidine compounds

A compound selected from those of formula (I):

• wherein:
• A represents pyridine, pyridinium, or piperidine,
• R2 represents hydrogen and R3 represents hydroxy, or R2 and R3 together form oxo,
• R1 and R4 together with the two carbon atoms carrying them, form a ring containing 6 carbon atoms,
• R5 represents a group of formula (II):
• wherein R′1, R′2, R′3 and R′4 have the same meaning as R1, R2, R3 and R4 respectively,
• R6 represents hydrogen or linear or branched (C1-C6)alkyl, the group R6 being present or absent depending on the nature of the ring A,
• its enantiomers and diastereoisomers, and addition salts thereof with a pharmaceutically-acceptable acid or base.