Detection and assay of cis- and trans-isomers of 4-methylaminorex in urine, plasma and tissue samples

DARK Classics in Chemical Neuroscience: Aminorex Analogues

Aminorex (5-phenyl-4,5-dihydro-1,3-oxazol-2-amine) and 4-methylaminorex (4-methyl-5-phenyl-4,5-dihydro-1,3-oxazol-2-amine) are psychostimulants that have long been listed in Schedules IV and I of the UN Convention on Psychotropic Substances of 1971. However, a range of psychoactive analogues exist that are not internationally controlled and therefore often classified as new psychoactive substances (NPS). Aminorex analogues encompass failed pharmaceuticals that reemerged as drugs of abuse, and newly synthesized substances that were solely designed for recreational use by clandestine chemists. NPS, sometimes also referred to as "designer drugs" in alignment with a phenomenon arising in the early 1980s, serve as alternatives to controlled drugs. Aminorex and its derivatives interact with monoaminergic neurotransmission by interfering with the function of monoamine transporters. Hence, these compounds share pharmacological and neurochemical similarities with amphetamines and cocaine. The consumption of aminorex, 4-methylaminorex and 4,4'-dimethylaminorex (4-methyl-5-(4-methylphenyl)-4,5-dihydro-1,3-oxazol-2-amine) has been associated with adverse events including death, bestowing an inglorious fame on aminorex-derived drugs. In this Review, a historical background is presented, as well as an account of the pharmacodynamic and pharmacokinetic properties of aminorex and various analogues. Light is shed on their misuse as drug adulterants of well-established drugs on the market. This Review not only provides a detailed overview of an abused substance-class, but also emphasizes the darkest aspect of the NPS market, i.e., deleterious side effects that arise from the ingestion of certain NPS, as knowledge of the pharmacology, the potency, or the identity of the active ingredients remains obscure to NPS users.

The psychostimulant (±)-cis-4,4'-dimethylaminorex (4,4'-DMAR) interacts with human plasmalemmal and vesicular monoamine transporters

(±)-cis-4,4'-Dimethylaminorex (4,4'-DMAR) is a new psychoactive substance (NPS) that has been associated with 31 fatalities and other adverse events in Europe between June 2013 and February 2014. We used in vitro uptake inhibition and transporter release assays to determine the effects of 4,4'-DMAR on human high-affinity transporters for dopamine (DAT), norepinephrine (NET) and serotonin (SERT). In addition, we assessed its binding affinities to monoamine receptors and transporters. Furthermore, we investigated the interaction of 4,4'-DMAR with the vesicular monoamine transporter 2 (VMAT2) in rat phaeochromocytoma (PC12) cells and synaptic vesicles prepared from human striatum. 4,4'-DMAR inhibited uptake mediated by human DAT, NET or SERT, respectively in the low micromolar range (IC₅₀ values < 2 μM). Release assays identified 4,4'-DMAR as a substrate type releaser, capable of inducing transporter-mediated reverse transport via DAT, NET and SERT. Furthermore, 4,4'-DMAR inhibited both the rat and human isoforms of VMAT2 at a potency similar to 3,4-methylenedioxymethylamphetamine (MDMA). This study identified 4,4'-DMAR as a potent non-selective monoamine releasing agent. In contrast to the known effects of aminorex and 4-methylaminorex, 4,4'-DMAR exerts profound effects on human SERT. The latter finding is consistent with the idea that fatalities associated with its abuse may be linked to monoaminergic toxicity including serotonin syndrome. The activity at VMAT2 suggests that chronic abuse of 4,4'-DMAR may result in long-term neurotoxicity.

Brain Disposition of cis-para-Methyl-4-Methylaminorex (cis-4,4'-DMAR) and Its Potential Metabolites after Acute and Chronic Treatment in Rats: Correlation with Central Behavioral Effects

para-Methyl-4-methylaminorex (4,4'-DMAR) is a phenethylamine derivative with psychostimulant activity whose abuse has been associated with several deaths and a wide range of adverse effects. We recently validated a high-performance liquid chromatography-tandem mass spectrometry method to measure the compound's concentrations in plasma, and we applied it to describe the pharmacokinetic properties of 4,4'-DMAR after a single dose in rats. In this study, we investigated the brain disposition and metabolism of cis-4,4'-DMAR after intraperitoneal injection as well as its central behavioral effects. Locomotor activity increased after a single injection of 10 mg/kg, peaking at 2 hours and disappearing at 5 hours; in these conditions, brain absorption was very rapid, (t_max = 30-60 minutes) and large (brain-to-plasma ratio = 24); the half-life was approximately 50 minutes. After 14 daily doses, the compound's effect on locomotor activity was greater (approximately 20% compared with the effect after the first dose), but not for pharmacokinetic reasons. Using high-resolution mass spectrometry, we also identified four metabolites of cis-4,4'-DMAR in the plasma and brain of treated rats. Semiquantitative analysis indicated low brain permeability and very low brain concentrations, suggesting that these metabolites do not contribute to central behavioral effects; however, the metabolite originating from oxidation of the para-methyl group (M2) persisted in the plasma longer and at higher concentrations than the parent molecule and could be used to evaluate drug intake in human consumers. Finally, we describe the rewarding effect of cis-4,4'-DMAR in the conditioning place preference test, suggesting a high risk of addiction in humans.

Characterization of a novel and potentially lethal designer drug (±)-cis-para-methyl-4-methylaminorex (4,4'-DMAR, or 'Serotoni')

During the second half of 2013, a total of 26 deaths involving para-methyl-4-methylaminorex (4,4'-DMAR) were reported to the European Monitoring Centre for Drugs and Drug Addiction. While aminorex and 4-methylaminorex (4-MAR) are known psychostimulants, nothing is known about the comparatively new para-methyl analog. Analytical characterization of two independent samples obtained from online vendors confirmed the presence of the (±)-cis isomer that also appeared to be associated with at least 18 of the 26 deaths. Extensive characterizations included crystal structure analysis, single, tandem, and high-resolution mass spectrometry, liquid and gas chromatography, and nuclear magnetic resonance spectroscopy. For the work described here, both the (±)-cis and (±)-trans racemates were also synthesized, confirming that the differentiation between these two forms was straight-forward. Monoamine transporter activity was studied using rat brain synaptosomes which included the comparison with d-amphetamine, aminorex and (±)-cis-4-MAR. (±)-cis-4,4'-DMAR was a potent, efficacious substrate-type releaser at transporters for dopamine, norepinephrine and serotonin with EC50 values of 8.6 ± 1.1 nM (DAT), 26.9 ± 5.9 nM (NET) and 18.5 ± 2.8 nM (SERT), respectively. The potency of (±)-cis-4,4'-DMAR at DAT and NET rivalled that of other psychomotor stimulant drugs like d-amphetamine and aminorex. However, (±)-cis-4,4'-DMAR had much more potent actions at SERT and activity at SERT varied more than 100-fold across the four drugs. The potent releasing activity of (±)-cis-4,4'-DMAR at all three monoamine transporters predicts a potential for serious side-effects such as psychotic symptoms, agitation, hyperthermia and cardiovascular stimulation, especially after high-dose exposure or following combination with other psychostimulants.

Pharmacokinetics and Tissue Distribution of the Stereoisomers of 4-Methylaminorex in the Rat

4-Methylaminorex, a potential psychostimulant drug of abuse, exists as four stereoisomers: cis-4R,5S, cis-4S,5R, trans-4S,5S, and trans-4R,5R, which were shown previously to possess stereospecific effects. This study characterized their pharmacokinetic and tissue distribution profiles, and metabolic turnover to norephedrine and norpseudoephedrine, in male Wistar rats. The rats received each isomer intravenously, intraperitoneally, or orally, followed by blood sample collection via cannula (pharmacokinetic study), or tissue sample collection at predetermined time points (tissue distribution study). The samples were analyzed for cis- and trans-isomers, and when appropriate for norephedrine and norpseudoephedrine, with gas chromatography/mass spectrometry. Trans-4S,5S-, cis-4R,5S-, and cis-4S,5R-isomers behaved comparably kinetically (volume of distribution 1.7-2.3 l/kg, distribution half-life 3.8-7.0 min, elimination half-life 35-42 min, and bioavailability 32-57% intraperitoneally or 4-16% orally), whereas trans-4R,5R-isomer differed from the others, with a longer elimination half-life (118-169 min) and higher bioavailability (100% intraperitoneally or 83% orally). The highest isomer concentrations were observed in the kidney followed most frequently by the liver, brain, muscle, and last by fat and blood. The elimination half-lives of the stereoisomers from the tissues were generally similar to those in blood. No pharmacologically significant amounts of norephedrine or norpseudoephedrine were detected in blood or the brain. In conclusion, differences between the stereoisomers of 4-methylaminorex in the pharmacokinetics and tissue distribution are described. However, these differences are not compatible with, and thus may not account for, the distinct behavioral and neurochemical effects of the stereoisomers demonstrated previously. Furthermore, metabolic turnover to norephedrine and norpseudoephedrine does not seem to contribute significantly to 4-methylaminorex pharmacology.

Acute neurochemical and behavioral effects of stereoisomers of 4-methylaminorex in relation to brain drug concentrations

4-Methylaminorex is a stimulant drug of abuse that exists as four stereoisomers: cis-4R,5S, cis-4S,5R, trans-4S,5S, and trans-4R,5R. These isomers have previously been shown to differ markedly in various respects. In the present study we assessed the effects of the isomers of 4-methylaminorex (2.5, 5.0, and 10 mg/kg i.p.) on extracellular dopamine and 5-hydroxytryptamine (5-HT) levels in the nucleus accumbens, as well as behavior in the rats simultaneously. The relative concentrations of the isomers in the brain were also measured. The samples were collected by in vivo microdialysis and then analyzed for neurotransmitters with high-performance liquid chromatography/electrochemical detection and for cis- and trans-4-methylaminorex with gas chromatography/mass spectrometry. The behavioral effects of the isomers were assessed from videotapes recorded during the microdialysis experiments. All isomers elevated the extracellular levels of both dopamine and 5-HT, with the exception of trans-4R,5R. The rank order of potency for elevating dopamine was trans-4S,5S > cis-4S,5R approximately cis-4R,5S > trans-4R,5R, and for elevating 5-HT cis-4S,5R > trans-4S,5S approximately cis-4R,5S > trans-4R,5R. Analysis of the behavioral data, together with the neurochemical data, suggests that behavioral effects of the isomers of 4-methylaminorex are related to drug-induced dopamine release and, in the case of higher doses of the most efficacious isomers, to 5-HT as well. The brain concentrations of the isomers did not reflect their neurochemical efficacy, which implies that their differences are pharmacodynamic rather than pharmacokinetic.