Novel alkyl substituted piperidine derivatives and their use as monoamine neurotransmitter re-uptake inhibitors

A piperidine derivative of the Formula I:

• any of its stereoisomers or any mixture of its stereoisomers, or a pharmaceutically acceptable salt thereof, wherein
• Ra represents hydrogen;
• X represents —NRc—;
• wherein Rc represents hydrogen or alkyl;
• Rb represents a naphthyl group,
• which naphthyl group is optionally substituted with one or more substituents independently selected from the group consisting of halo, trifluoromethyl, trifluoromethoxy, cyano, and alkoxy;
• each of R2, R2′, R6 and R6′ represent alkyl;
• and
• each of R3, R3′, R5 and R5′ represent hydrogen.