Isothiocyanate Derivatives of 9-[3-(cis-3,5-Dimethyl-1-piperazinyl)propyl]- carbazole (Rimcazole):  Irreversible Ligands for the Dopamine Transporter

Abstract

Cocaine has been reported to bind to the dopamine transporter in a biphasic fashion, and it has been hypothesized that the low-affinity component may play a modulatory role in cocaine's psychomotor stimulant effects. In an effort to gain further insight into the roles of the two sites, we have prepared a series of irreversible ligands based on rimcazole (9-[3-(cis-3,5-dimethyl-1-piperazinyl)propyl]carbazole, 2), a compound 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 substitution on the piperazine nitrogen caused an initial decrease in affinity that 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 the greatly diminished binding of [3H]WIN 35,428 in tissue that had been preincubated with the ligand and then thoroughly washed using centrifugation. The dose-dependent reduction in Bmax was accompanied by a concentration-related decrease in KD values. This shift in KD to a lower value suggests that the preincubation with 16 produced a preferential irreversible binding to the low-affinity [3H]WIN 35,428 site on the dopamine transporter. These ligands may prove to be important tools 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 provide leads for a novel cocaine-abuse treatment.

Irreversible binding of a novel phenylisothiocyanate tropane analog to monoamine transporters in rat brain

“…The addition of the phenylisothiocyanate to the parent structure WF-23 significantly reduced affinities at all three transporters. The reduction in affinity by adding phenylisothiocyanate is not unusual in structure activity relationship studies [31,32]. Fortunately, the direct IC 50 values of 4 nM and 14 nM at DAT and SERT, respectively, showed that HD-205 retained relatively high potency at DAT and SERT despite the loss of potency compared to WF-23.…”

“…Irreversible ligands including photoaffinity ligands [28][29][30] and alkylating reagents [31][32][33][34] [30]. These data suggest the distinctive pattern of binding exhibited by a wide variety of cocaine related compounds and the complexity involved in understanding the cocaine pharmacophore.…”

“…Both photoaffinity ligands [28][29][30] and alkylating reagents [31][32][33][34], including the tropane analog [ 125 I]RTI-82 [35], have been used to identify the ligand binding sites of cocaine and related tropane analogs on DAT. In addition, alkylating agents including isothiocyanate and bromoacetamide tagged to cocaine, rimcazole and benztropine analogs have been successful as irreversible DAT analogs [31,32,36].…”

Review of the Pharmacological and Clinical Profile of Rimcazole

“…The apparent selectivity of rimcazole for ó receptors initially generated much interest. However, more recent studies reveal that rimcazole has a 6 to 25 fold higher binding affinity for dopamine transporters than for ó receptors (52,53,55,150) and has a similar binding affinity for serotonin transporters (17,52). Compared with other dopamine transporter ligands, rimcazole exhibits a higher affinity for dopamine transporters, but a significantly lower potency for inhibiting dopamine uptake (52,150).…”

In Vivo Characterization of a Novel Phenylisothiocyanate Tropane Analog at Monoamine Transporters in Rat Brain

“…These compounds include azido-based photoaffinity ligands such as [ 125 (Vaughan et al, 2001;Parnas et al, 2008); and isothiocyanate and bromoacetamide analogs such as rimcazole and tropane derivatives (Boja et al, 1991b;Husbands et al, 1997;Lever et al, 2005). These compounds provide an opportunity to precisely map the location of the binding site of cocaine on DAT.…”