ISOTHIOCYANATE DERIVATIVES OF -(CIS-3,5-DIMETHYL-1-PIPERAZINYL)PROPYL]-CARBAZOLE (RIMCAZOLE) - IRREVERSIBLE LIGANDS FOR THE DOPAMINE TRANSPORTER

Cocaine has been reported to bind to the dopamine transporter in a bip hasic fashion, and it has been hypothesized that the low-affinity comp onent may play a modulatory role in cocaine's psychomotor stimulant ef fects. In an effort to gain further insight into the roles of the two sites, we have prepared a series of irreversible ligands based on rimc azole -(cis-3,5-dimethyl-1-piperazinyl)propyl]carbazole, 2), a compoun d that has been postulated to bind only to the low-affinity site. The alkylating moiety (isothiocyanate) is attached to the distal nitrogen of the piperazine ring via alkyl chains of varying lengths or directly attached to one of the aromatic groups. It was found that substitutio n on the piperazine nitrogen caused an initial decrease in affinity th at was recovered as the alkyl chain length increased. Importantly, the analogue 16, with the highest affinity for the dopamine transporter ( DAT), binds in a monophasic and irreversible manner, as evidenced by t he greatly diminished binding of [H-3]WIN 35,428 in tissue that had be en preincubated with the Ligand and then thoroughly washed using centr ifugation. The dose-dependent reduction in B-max was accompanied by a concentration-related decrease in K-D values. This shift in K-D to a l ower value suggests that the preincubation with 16 produced a preferen tial irreversible binding to the low-affinity [H-3]WIN 35,428 site on the dopamine transporter. These Ligands may prove to be important tool s with which to study the significance of the low-affinity site on the DAT. Since rimcazole does not share the behavioral profile of cocaine , and in fact appears to play a modulatory role, these compounds may p rovide leads for a novel cocaine-abuse treatment.