Bivalent biogenic amine reuptake inhibitors

Interrogating the Molecular Basis for Substrate Recognition in Serotonin and Dopamine Transporters with High-Affinity Substrate-Based Bivalent Ligands

The transporters for the neurotransmitters serotonin and dopamine (SERT and DAT, respectively) are targets for drugs used in the treatment of mental disorders and widely used drugs of abuse. Studies of prokaryotic homologues have advanced our structural understanding of SERT and DAT, but it still remains enigmatic whether the human transporters contain one or two high-affinity substrate binding sites. We have designed and employed 24 bivalent ligands possessing a highly systematic combination of substrate moieties (serotonin and/or dopamine) and aliphatic or poly(ethylene glycol) spacers to reveal insight into substrate recognition in SERT and DAT. An optimized bivalent ligand comprising two serotonin moieties binds SERT with 3,800-fold increased affinity compared to that of serotonin, suggesting that the human transporters have two distinct substrate binding sites. We show that the bivalent ligands are inhibitors of SERT and an experimentally validated docking model suggests that the bivalent compounds bind with one substrate moiety in the central binding site (the S1 site), whereas the other substrate moiety binds in a distinct binding site (the S2 site). A systematic study of nonconserved SERT/DAT residues surrounding the proposed binding region showed that nonconserved binding site residues do not contribute to selective recognition of substrates in SERT or DAT. This study provides novel insight into the molecular basis for substrate recognition in human transporters and provides an improved foundation for the development of new drugs targeting SERT and DAT.

Bivalent phenethylamines as novel dopamine transporter inhibitors: evidence for multiple substrate-binding sites in a single transporter

Bivalent ligands--compounds incorporating two receptor-interacting moieties linked by a flexible chain--often exhibit profoundly enhanced binding affinity compared with their monovalent components, implying concurrent binding to multiple sites on the target protein. It is generally assumed that neurotransmitter sodium symporter (NSS) proteins, such as the dopamine transporter (DAT), contain a single domain responsible for recognition of substrate molecules. In this report, we show that molecules possessing two substrate-like phenylalkylamine moieties linked by a progressively longer aliphatic spacer act as progressively more potent DAT inhibitors (rather than substrates). One compound bearing two dopamine (DA)-like pharmacophoric 'heads' separated by an 8-carbon linker achieved an 82-fold gain in inhibition of [(3)H] 2beta-carbomethoxy-3beta-(4-fluorophenyl)-tropane (CFT) binding compared with DA itself; bivalent compounds with a 6-carbon linker and heterologous combinations of DA-, amphetamine- and beta-phenethylamine-like heads all resulted in considerable and comparable gains in DAT affinity. A series of short-chain bivalent-like compounds with a single N-linkage was also identified, the most potent of which displayed a 74-fold gain in binding affinity. Computational modelling of the DAT protein and docking of the two most potent bivalent (-like) ligands suggested simultaneous occupancy of two discrete substrate-binding domains. Assays with the DAT mutants W84L and D313N--previously employed by our laboratory to probe conformation-specific binding of different structural classes of DAT inhibitors--indicated a bias of the bivalent ligands for inward-facing transporters. Our results strongly indicate the existence of multiple DAT substrate-interaction sites, implying that it is possible to design novel types of DAT inhibitors based upon the 'multivalent ligand' strategy.

The synthesis of bivalent 2beta-carbomethoxy-3beta-(3,4-dichlorophenyl)-8-heterobicyclo[3.2.1]octanes as probes for proximal binding sites on the dopamine and serotonin transporters

3-Aryltropanes have been widely explored for potential medications for remediation of cocaine abuse. Research has focused predominantly on 8-azatropanes and it is now well recognized that these compounds can be designed to manifest varied selectivity and potency for inhibition of the dopamine, serotonin, and norepinephrine uptake systems. We had reported that the 8-nitrogen atom present in the 3-aryltropanes is not essential for tropanes to bind to monoamine uptake systems. We demonstrated that compounds in which the amine had been exchanged for an ether or a thioether retained binding potency and selectivity. We have now designed bivalent compounds in which two tropane moieties are linked by an intervening chain. These 8-homo- and 8-heterotropane bivalent compounds allowed a search for adjacent tropane binding sites on the DAT as well as a further exploration of whether the binding sites for 8-azatropanes are the same as those for other 8-heterotropanes. A comparison of these compounds with their progenitor tropanes cast into doubt the existence of proximal binding sites on the DAT, and offered support for the existence of different binding sites for the 8-azatropanes compared with 8-oxa- and 8-thiatropanes. Indeed, 8-aza bivalent tropanes inhibited DAT with potency about 10-fold lower (DAT: IC50 = 31 nM) than their monovalent counterparts. Furthermore, bivalent ligands in which one or both of the tropanes was devoid of an amine suffered a further loss of inhibitory potency. We conclude that it is unlikely that there exist two tropane binding sites in close proximity to one another on either the DAT or SERT.