Structure-activity relationship studies of 4-[2-(diphenylmethoxy)ethyl]-1-benzylpiperidine derivatives and their N-analogues: Evaluation of behavioral activity of O- and N-analogues and their binding to monoamine transporters

In our effort to develop a pharmacotherapy for the treatment of cocaine addiction, we embarked on synthesizing novel molecules targeting the dopamine transporter (DAT) molecule in the brain as DAT has been implicated strongly in the reinforcing effect of cocaine. Our previously developed DAT-selective piperidine analogue, 4-[2-(diphenylmethoxy)ethyl]-1-benzylpiperidine, was the basis for our current structure-activity relationship (SAR) studies exploring the significance of the contribution of the benzhydryl O- and N-atoms in these molecules in interacting with the DAT. Thus, we replaced the benzhydryl O-atom with an N-atom, altered the location of the benzhydryl N-atom to an adjacent position, and in one other occasion converted the benzhydryl O-ether linkage into an oxime-type derivative. Furthermore, we also evaluated the important contribution of the piperidine N-atom to binding by altering its pK(a) value chemically. Novel analogues were tested for potency in inhibiting [H-3]WIN 35,428, [H-3]- citalopram, and [H-3]nisoxetine binding at the DAT, serotonin transporter (SERT), and norepinepherine transporter (NET). [H-3]DA was used to measure DA reuptake inhibition. The results indicated that the benzhydryl O- and N-atoms are exchangeable for the most part. On the other hand, an enhanced interaction with the SEPT was observed when the benzhydryl N-atom moved to an adjacent position (21a; DAT (IC50) = 19.7, SEPT (IC50) = 137 nM, NET (IC50) = 1111 nM). In either cases, further alkylation of the N-atom reduced the activity for the transporter. The presence of a powerful electron-withdrawing cyano group in compound 5d expectedly produced the most potent and selective ligand for the DAT (DAT (IC50) = 3.7 nM, DAT/SERT = 615). Selected compounds were further analyzed in the dopamine reuptake inhibition assay. Preliminary behavioral assessment of some of the selected compounds in mice indicated that these compounds are much less stimulating when compared with cocaine at comparable doses. In drug-discrimination studies these selected compounds incompletely generalized from the cocaine stimulus in mice trained to discriminate 10 mg/kg cocaine from vehicle.