Discovery and Biological Characterization of (2 R ,4 S )-1′-Acetyl- N -{(1 R )-1-[3,5-bis(trifluoromethyl)phenyl]ethyl}-2-(4-fluoro-2-methylphenyl)- N -methyl-4,4′-bipiperidine-1-carboxamide as a New Potent and Selective Neurokinin 1 (NK 1 ) Receptor Antagonist Clinical Candidate