Monoamine transporters (MATs) are proteins that function as integral plasma-membrane transporters to regulate concentrations of extracellular monoamine neurotransmitters. The three major classes are serotonin transporters (SERTs), dopamine transporters (DATs), and norepinephrine transporters (NETs) and are responsible for the reuptake of their associated amine neurotransmitters. MATs are located just outside the synaptic cleft (peri-synaptically), transporting monoamine transmitter overflow from the synaptic cleft back to the cytoplasm of the pre-synaptic neuron. MAT regulation generally occurs through protein phosphorylation and post-translational modification. Due to their significance in neuronal signaling, MATs are commonly associated with drugs used to treat mental disorders as well as recreational drugs. Compounds targeting MATs range from medications such as the wide variety of tricyclic antidepressants, selective serotonin reuptake inhibitors such as fluoxetine (Prozac) to stimulant medications such as methylphenidate (Ritalin) and amphetamine in its many forms and derivatives methamphetamine (Desoxyn) and lisdexamfetamine (Vyvanse). Furthermore, drugs such as MDMA and natural alkaloids such as cocaine exert their effects in part by their interaction with MATs, by blocking the transporters from mopping up dopamine, serotonin, and other neurotransmitters from the synapse.
Phenyltropanes (PTs) were originally developed to reduce cocaine addiction and dependency. In general these compounds act as inhibitors of the plasmalemmal monoamine reuptake transporters. This research has spanned beyond the last couple decades, and has picked up its pace in recent times, creating numerous phenyltropanes as research into cocaine analogues garners interest to treat addiction.
(+)-CPCA is a stimulant drug similar in structure to pethidine and to RTI-31, but nocaine lacks the two-carbon bridge of RTI-31's tropane skeleton. This compound was first developed as a substitute agent for cocaine.
A serotonin–norepinephrine–dopamine reuptake inhibitor (SNDRI), also known as a triple reuptake inhibitor (TRI), is a type of drug that acts as a combined reuptake inhibitor of the monoamine neurotransmitters serotonin, norepinephrine, and dopamine. It does this by concomitantly inhibiting the serotonin transporter (SERT), norepinephrine transporter (NET), and dopamine transporter (DAT), respectively. Inhibition of the reuptake of these neurotransmitters increases their extracellular concentrations and, therefore, results in an increase in serotonergic, adrenergic, and dopaminergic neurotransmission. The naturally-occurring and potent SNDRI cocaine is widely used recreationally and often illegally for the euphoric effects it produces.
Lobeline is a piperidine alkaloid found in a variety of plants, particularly those in the genus Lobelia, including Indian tobacco, Devil's tobacco, great lobelia, Lobelia chinensis, and Hippobroma longiflora. In its pure form, it is a white amorphous powder which is freely soluble in water.
Troparil is a stimulant drug used in scientific research. Troparil is a phenyltropane-based dopamine reuptake inhibitor (DRI) that is derived from methylecgonidine. Troparil is a few times more potent than cocaine as a dopamine reuptake inhibitor, but is less potent as a serotonin reuptake inhibitor, and has a duration spanning a few times longer, since the phenyl ring is directly connected to the tropane ring through a non-hydrolyzable carbon-carbon bond. The lack of an ester linkage removes the local anesthetic action from the drug, so troparil is a pure stimulant. This change in activity also makes troparil slightly less cardiotoxic than cocaine. The most commonly used form of troparil is the tartrate salt, but the hydrochloride and naphthalenedisulfonate salts are also available, as well as the free base.
2β-Propanoyl-3β-(2-naphthyl)-tropane or WF-23 is a cocaine analogue. It is several hundred times more potent than cocaine at being a serotonin-norepinephrine-dopamine reuptake inhibitor.
Reuptake inhibitors (RIs) are a type of reuptake modulators. It is a drug that inhibits the plasmalemmal transporter-mediated reuptake of a neurotransmitter from the synapse into the pre-synaptic neuron. This leads to an increase in extracellular concentrations of the neurotransmitter and an increase in neurotransmission. Various drugs exert their psychological and physiological effects through reuptake inhibition, including many antidepressants and psychostimulants.
Difluoropine (O-620) is a stimulant drug synthesised from tropinone, which acts as a potent and selective dopamine reuptake inhibitor. Difluoropine is unique among the tropane-derived dopamine reuptake inhibitors in that the active stereoisomer is the (S) enantiomer rather than the (R) enantiomer, the opposite way round compared to natural cocaine. It is structurally related to benztropine and has similar anticholinergic and antihistamine effects in addition to its dopamine reuptake inhibitory action.
HDMP-28 or methylnaphthidate is a piperidine based stimulant drug, closely related to methylphenidate, but with the benzene ring replaced by naphthalene. It is a potent dopamine reuptake inhibitor, with several times the potency of methylphenidate and a short duration of action, and is a structural isomer of another potent dopamine reuptake inhibitor, N,O-Dimethyl-4-(2-naphthyl)piperidine-3-carboxylate. It has been sold as a designer drug since around 2015.
RTI-126 is a phenyltropane derivative which acts as a potent monoamine reuptake inhibitor and stimulant drug, and has been sold as a designer drug. It is around 5 times more potent than cocaine at inhibiting monoamine reuptake in vitro, but is relatively unselective. It binds to all three monoamine transporters, although still with some selectivity for the dopamine transporter. RTI-126 has a fast onset of effects and short duration of action, and its pharmacological profile in animals is among the closest to cocaine itself out of all the drugs in the RTI series. Its main application in scientific research has been in studies investigating the influence of pharmacokinetics on the abuse potential of stimulant drugs, with its rapid entry into the brain thought to be a key factor in producing its high propensity for development of dependence in animals.
RTI(-4229)-336, is a phenyltropane derivative which acts as a potent and selective dopamine reuptake inhibitor and stimulant drug. It binds to the dopamine transporter with around 20x the affinity of cocaine, however it produces relatively mild stimulant effects, with a slow onset and long duration of action. These characteristics make it a potential candidate for treatment of cocaine addiction, as a possible substitute drug analogous to how methadone is used for treating heroin abuse. RTI-336 fully substitutes for cocaine in addicted monkeys and supports self-administration, and significantly reduces rates of cocaine use, especially when combined with SSRIs, and research is ongoing to determine whether it could be a viable substitute drug in human cocaine addicts.
O-2172 is a drug developed by Organix Inc, which acts as a stimulant and potent dopamine reuptake inhibitor. It is an analogue of methylphenidate where the phenyl ring has had a 3,4-dichloro substitution added, and the piperidine ring has been replaced by cyclopentane. It is around 1/3 the potency of methylphenidate, demonstrating that even with the important binding group of the nitrogen lone pair removed entirely, selective DAT binding and reuptake inhibition is still possible.
A monoamine releasing agent (MRA), or simply monoamine releaser, is a drug that induces the release of a monoamine neurotransmitter from the presynaptic neuron into the synapse, leading to an increase in the extracellular concentrations of the neurotransmitter. Many drugs induce their effects in the body and/or brain via the release of monoamine neurotransmitters, e.g., trace amines, many substituted amphetamines, and related compounds.
A serotonin–dopamine reuptake inhibitor (SDRI) is a type of drug which acts as a reuptake inhibitor of the monoamine neurotransmitters serotonin and dopamine by blocking the actions of the serotonin transporter (SERT) and dopamine transporter (DAT), respectively. This in turn leads to increased extracellular concentrations of serotonin and dopamine, and, therefore, an increase in serotonergic and dopaminergic neurotransmission.
O-4210 is a drug developed by Organix Inc which acts as a selective dopamine reuptake inhibitor, with good selectivity over the serotonin transporter but its activity at the noradrenaline transporter is not known. It is a thiatropane derivative, related in chemical structure to phenyltropane derivatives such as RTI-126 and RTI-171, but with the amine nitrogen replaced by sulfur, demonstrating that this nitrogen only plays a minor contribution to receptor binding, in a similar manner to the related oxatropane tropoxane.
1-Methyl-3-propyl-4-(p-chlorophenyl)piperidine is a drug developed by a team led by Alan Kozikowski, which acts as a potent dopamine reuptake inhibitor, and was developed as a potential therapeutic agent for the treatment of cocaine addiction. As with related compounds such as nocaine, it is a structurally simplified derivative of related phenyltropane compounds. Its activity at the serotonin and noradrenaline transporters has not been published, though most related 4-phenylpiperidine derivatives are relatively selective for inhibiting dopamine reuptake over the other monoamine neurotransmitters. While several of its isomers are active, the (3S,4S)-enantiomer is by far the most potent. The rearranged structural isomer 2-[1-(4-chlorophenyl)butyl]piperidine is also a potent inhibitor of dopamine reuptake.
O-2390 is a recreational designer drug from the substituted cathinone family, which acts as a potent inhibitor of dopamine and noradrenaline reuptake in vitro, with weaker but still significant inhibition of serotonin reuptake.
