Stimulus properties of a new designer drug: 4-methylaminorex ("U4Euh")

Characterization of Three Novel 4-Methylaminorex Derivatives Applied as Designer Drugs

The ongoing development of more and more new psychoactive substances continues to be a huge problem in 2022 affecting the European and international drug market. Through slight alterations in the structure of illicit drugs, a way to circumvent the law is created, as the created derivatives serve as legal alternatives with similar effects. A common way of structure modification is the induction of a halogen residue. Recently, halogenated derivatives of the well-known designer drug 4-methylaminorex appeared on the market and are available in various online shops. In this study, three novel halogenated 4-methylaminorex derivatives, namely 4′-fluoro-4-methylaminorex, 4′-chloro-4-methylaminorex, and 4′-bromo-4-methylaminorex, were purchased online and characterized using nuclear magnetic resonance (NMR) spectroscopy, liquid chromatography-high-resolution mass spectrometry (LC-HRMS), and chiral high-performance liquid chromatography with ultraviolet detection (HPLC-UV). These derivatives possess two stereogenic centers, and analyses revealed that all of them were present as a racemic mixture of the trans diastereomeric form.

Characterization of a recently detected halogenated aminorex derivative: para-fluoro-4-methylaminorex (4'F-4-MAR)

Despite the fact that 33% of the new psychoactive substances seized in 2015 were synthetic cathinones, the number of these derivatives has been decreasing in the last years, probably as a consequence of the unfavourable effects reported by users. Thus, the list of possible cathinone analogues is expected to get shorter, and it is likely that the same moiety changes applied for the preparation of synthetic cathinones will be applied in the near future to other stimulants in the search for favourable alternatives to controlled substances. This is evidenced by the increase in newly reported substances belonging to stimulant classes other than cathinones. One of the possible candidates for a new backbone from which to base new stimulants is aminorex, which is classified as a Schedule I substance by the Drug Enforcement Administration. Three derivatives have been reported until now: 4-methylaminorex or 4-MAR (also categorized as a Schedule I substance), para-methyl-4-methylaminorex (4,4′-DMAR) and 3′,4′-methylenedioxy-4-methylaminorex (MDMAR). Recently, the new halogenated 4-MAR derivative, para-fluoro-4-methylaminorex, characterised in this work (and abbreviated as pF-4-methylaminorex or 4′F-4-MAR) was detected by the Slovenian police. In the present work, 4′F-4-MAR has been characterized by high resolution mass spectrometry and nuclear magnetic resonance in a sample obtained from an anonymous consumer. This research shows that the same modifications applied for the preparation of synthetic cathinones are being used to prepare new stimulants based on the aminorex backbone.

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.

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 effects of aminorex in horses

OBJECTIVE

To investigate the pharmacokinetics and behavioral effects of aminorex administered IV and PO in horses.

ANIMALS

7 Thoroughbreds.

PROCEDURES

In a cross-over design, aminorex (0.03 mg/kg) was administered IV or PO. Plasma and urinary aminorex concentrations were determined via liquid chromatography- mass spectrometry.

RESULTS

Decrease of aminorex from plasma following IV administration was described by a 3-compartment pharmacokinetic model. Median (range) values of alpha, beta, and gamma half-lives were 0.04 (0.01 to 0.28), 2.30 (1.23 to 3.09), and 18.82 (8.13 to 46.64) hours, respectively. Total body and renal clearance, the area under the plasma time curve, and initial volume of distribution were 37.26 (28.61 to 56.24) mL x min/kg, 1.25 (0.85 to 2.05) mL x min/kg, 13.39 (8.82 to 17.37) ng x h/mL, and 1.44 (0.10 to 3.64) L/kg, respectively. Oral administration was described by a 2-compartment model with first-order absorption, elimination from the central compartment, and distribution into peripheral compartments. The absorption half-life was 0.29 (0.12 to 1.07) hours, whereas the beta and gamma elimination phases were 1.93 (1.01 to 3.17) and 23.57 (15.16 to 47.45) hours, respectively. The area under the curve for PO administration was 10.38 (4.85 to 13.40) ng.h/mL and the fractional absorption was 81.8% (33.8% to 86.9%).

CONCLUSIONS AND CLINICAL RELEVANCE

Aminorex administered IV had a large volume of distribution, initial rapid decrease, and an extended terminal elimination. Following PO administration, there was rapid absorption, rapid initial decrease, and an extended terminal elimination. At a dose of 0.03 mg/kg, the only effects detected were transient and central in origin and were observed only following IV administration.

Rewarding properties of the stereoisomers of 4-methylaminorex: Involvement of the dopamine system

4-Methylaminorex is a potential psychostimulant drug of abuse that exists as four stereoisomers: cis-4R,5S, cis-4S,5R, trans-4S,5S, and trans-4R,5R. The racemic mixture of the cis-isomers has been encountered in illicit samples, but previous animal studies suggest that also the trans-isomers could have similar stimulant-like properties. We tested whether the stereoisomers possess rewarding properties and compared their potency using the conditioned place preference method in rats. Furthermore, the involvement of the brain dopaminergic system in the 4-methylaminorex reward was tested with the dopamine D1- and D2-receptor antagonists SCH 23390 and raclopride administered systemically, or with the neurotoxin 6-hydroxydopamine injected into the nucleus accumbens. All the four isomers induced place preference, with no apparent differences in their potency. SCH 23990 and raclopride attenuated 4-methylaminorex-induced increase in place preference, and 6-hydroxydopamine also tended to be efficacious. These findings indicate that all the four stereoisomers of 4-methylaminorex possess rewarding properties and thus abuse potential; the trans-isomers are at least as potent as the cis-isomers. Furthermore, the brain dopaminergic system appears to be involved in the 4-methylaminorex-reward.

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.

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

The 4-methylaminorex (4-MAX) is an amphetamine-related psychostimulant drug that has appeared on the clandestine market with a street name of "U4Euh". This compound exists as four stereoisomers, trans-4R,5R, trans-4S,5S, cis-4R,5S and cis-4S,5R, of which the cis forms have been classified as Schedule I substances in the US. The increasing variety of designer drugs has highlighted the importance of detection, identification, and quantitative measurement of these drugs, including 4-MAX, in biological samples. In the present study, the isomers of 4-MAX were detected in urine of rats treated with the drugs by some but not all of the on-site immunoassays tested, mainly as amphetamine or methamphetamine. To facilitate identification of 4-MAX by laboratories specialized in drug analysis, the electron-ionization mass spectrum and TLC data for underivatized 4-MAX using a routine laboratory drug-screening procedure is provided. In addition, a GC/MS method is described for the quantitative determination of cis- and trans-4-MAX as tert-butyldimethylsilyl-derivatives in plasma, urine and tissue.

Recreational use of aminorex and pulmonary hypertension

Pulmonary hypertension has been associated with ingestion of the appetite suppressant aminorex. A similar compound, 4-methyl-aminorex (street names, "U-4-E-uh" [pronounced euphoria] or "ice"), is a "designer" drug with central stimulant activity. This drug was discovered on the property of three individuals with diagnoses of pulmonary hypertension. The association between "recreational" aminorex manufacture and ingestion and the development of pulmonary hypertension is described.

Comparison of the action of the stereoisomers of the psychostimulant 4-methylaminorex (4-MAX) on midbrain dopamine cells in the rat: an extracellular single unit study

In this study, we examined and characterized the action of the stereoisomers of 2-amino-4-methyl-delta 2-5-phenyl-oxazoline (4-methylaminorex, 4-MAX) on spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC or A9) and ventral tegmental area (VTA or A10) in anesthetized male rats. This was accomplished using the technique of extracellular single unit recording. The intravenous (i.v.) administration of the stereoisomers of 4-MAX (0.1-6.4 mg/kg) produced a dose-dependent suppression of the basal firing rate of A10 DA cells with the following rank order of potency: trans 4S,5S > cis 4R,5S approximately cis 4S,5R >> trans 4S,5S 4-MAX. The rank order of potency of the isomers of 4-MAX to suppress the firing of A9 DA cells was trans 4S,5S = cis 4R,5S = cis 4S,5R >> trans 4R,5R. The trans 4S,5S isomer was 5-fold more potent in suppressing DA cell firing in the A10 compared to the A9 area. The suppressant action of the isomers on A9 and A10 DA cells was reversed by the i.v. administration of haloperidol and the D2/D3 receptor antagonists (-)-sulpiride and (-)-eticlopride but not by the D1 receptor antagonists SCH 23390 and SCH 39166. In addition, the suppressant action of the trans 4S,5S isomer on A10 DA cells was not antagonized or reversed by the i.v. administration of the receptor antagonists granisetron (5-HT3), ritanserin (5-HT2A,C), idazoxan (alpha 2), phentolamine (peripheral alpha 1), (+/-)-pindolol (5-HT1A,B beta) or prazosin (alpha 1). The pretreatment of animals with either alpha-methyl-p-tyrosine (AMPT) or reserpine, but not p-chlorophenylalanine (PCPA), (+/-)-fluoxetine or tomoxetine, significantly attenuated the suppression of A10 DA cell firing produced by trans 4S,5S 4-MAX. Overall, our results suggest that the suppressant action of 4-MAX on midbrain DA cell firing may be mediated by the release of DA, which subsequently interacts with D2/D3 receptors.

Stimulus properties of some analogues of 4-methylaminorex

The stimulus properties of aminorex and analogues of 4-methylaminorex, namely (4S,5S)-4-methylaminorex, N-methyl-(4S,5S)-4-methylaminorex, and the regioisomeric (R)- and (S)-2-amino-4-phenyl-2-oxazoline (rexamino) were compared in rats trained to distinguish (S)-amphetamine (1 mg/kg) from saline. The first three compounds, aminorex, (4S,5S)-4-methylaminorex, and N-methyl-(4S,5S)-4-methylaminorex shared discriminative stimulus effects with amphetamine, although the stimulus properties for racemic aminorex were less than those of the other two compounds. The two regioisomers, (R)- and (S)-rexamino, produced only partial generalisation to the amphetamine.

Evaluation of the abuse liability of aminorex

Aminorex is a cyclic phenylisopropylamine that has been marketed as an anorectic. Despite obvious pharmacological similarities to the amphetamines, little is known about its liability for abuse. In the present study, one group of rhesus monkeys (n = 3) was prepared with intravenous catheters and allowed to self-administer either methohexital or saline in daily experimental sessions. When methohexital and saline self-administration were stable and clearly different, various doses of aminorex (0.001-0.1 mg/kg/injection) were made available for self-administration. Aminorex maintained self-administration above that maintained by saline and slightly lower than that maintained by methohexital in all monkeys. The discriminative stimulus effects of aminorex were evaluated in rhesus monkeys trained to discriminate d-amphetamine (n = 3) or pentobarbital (n = 4) from saline. Aminorex substituted completely for d-amphetamine as a discriminative stimulus but engendered little or no pentobarbital-appropriate responding. Aminorex stimulated locomotor activity in mice and exacerbated the withdrawal syndrome in rats that were dependent upon pentobarbital. These findings indicate that aminorex is a psychomotor stimulant that would be predicted to have significant d-amphetamine-like abuse liability in humans.

Cocaine-stimulus generalization to two new designer drugs: methcathinone and 4-methylaminorex

Rats were trained to discriminate 8 mg/kg cocaine from saline vehicle for the purpose of examining the stimulus properties of two novel and structurally related drugs of abuse recently confiscated on the illicit market: (+/-)methcathinone and cis(+/-)4-methylaminorex. The stimulus properties of these controlled substance analogs were compared with those of their parent compounds (+/-)cathinone and aminorex, respectively. All agents resulted in cocaine-stimulus generalization with the following rank order of potency: aminorex (ED50 value = 0.8 microM/kg) > methcathinone (1.9 microM/kg) > cathinone (3.7 microM/kg) > 4-methylaminorex (5.2 microM/kg) > cocaine (7.6 microM/kg).

Aminorex produces stimulus effects similar to amphetamine and unlike those of fenfluramine

A 4-methyl derivative of aminorex has recently appeared on the clandestine market as a designer drug. In the present study, the stimulus effects of aminorex itself were evaluated in rats trained to discriminate either 0.75 mg/kg S(+)-amphetamine or 1.5 mg/kg fenfluramine from saline. The amphetamine stimulus (ED50 = 0.14 mg/kg) generalized to aminorex (ED50 = 0.23 mg/kg), which was found to be slightly less potent than (+)-amphetamine. Fenfluramine stimulus generalization did not occur to aminorex. Thus, the stimulus effects of aminorex are qualitatively similar to those of amphetamine and unlike those of fenfluramine.

Drug abuse potential evaluation in animals

Animal laboratory studies can provide useful information concerning the potential of drugs for abuse. Over the past 50 years, methods have been developed for use with animal subjects which allow the evaluation of pharmacological properties of drugs which are particularly relevant to their abuse. The methods for preclinical drug abuse liability testing are reviewed under six heading: (1) establishment of the degree of pharmacological equivalence to known drugs of abuse, (2) drug discrimination studies, (3) tests of tolerance and cross-tolerance, (4) tests of physical dependence capacity, (5) drug self-administration tests of reinforcing effects, and (6) evaluation of toxicity and performance impairment at self-administered doses. Preclinical studies can be helpful early in drug development to select lead compounds with low abuse potential for further study. In the case of new or already marketed medications, animal testing can often compliment and extend abuse liability evaluation in human subjects. The results of abuse potential evaluation studies can be useful in making decisions about the possible need for regulation under national and international drug laws, and thus play an important role in drug abuse prevention.

Stimulus properties of ring-methyl amphetamine analogs

There are three possible ring-substituted methyl amphetamines (or tolylaminopropanes; TAPs): oTAP, mTAP and pTAP. These agents are positional isomers of methamphetamine. Although all three isomers have been previously reported to possess amphetamine-like character, few studies have examined all three agents in comparison with (+)amphetamine. Using rats trained to discriminate 1 mg/kg of (+)amphetamine from saline under a variable-interval 15-sec schedule of reinforcement, tests of stimulus generalization were conducted with the three positional isomers. Only oTAP (ED50 dose = 4.1 mg/kg) completely substituted for (+)amphetamine. mTAP and pTAP resulted only in partial (ca. 50% amphetamine-appropriate responding) generalization. It is concluded that oTAP is capable of producing amphetamine-like stimulus effects and that it is approximately one-tenth as potent as (+)amphetamine; however, because the partial generalization produced by mTAP and pTAP was followed by disruption of behavior at slightly higher doses, it cannot be reliably stated that these latter two isomers lack amphetamine-like character.

(-)PPAP: a new and selective ligand for sigma binding sites

Most agents employed for the investigation of sigma (sigma) binding sites display relatively low affinity for these sites, bind both at sigma sites and at either phencyclidine (PCP) sites or dopamine receptors with similar affinity, and/or produce some dopaminergic activity in vivo. We describe a new agent, (-)PPAP or R(-)-N-(3-phenyl-n-propyl)-1-phenyl-2-aminopropane hydrochloride, that binds with high affinity and selectivity at sigma (IC50 = 24 nM) versus either PCP sites (IC50 greater than 75,000 nM) or D1 and D2 dopamine receptors (IC50 greater than 5,000 nM). The sigma affinity of this agent is comparable to that of the standard ligands (+)-3-PPP and DTG. Furthermore, although (-)PPAP is structurally related to amphetamine, it neither produces nor antagonizes amphetamine-like stimulus effect in rats trained to discriminate 1 mg/kg of S(+)amphetamine from saline.

Intravenous self-administration of 4-methylaminorex in primates

The reinforcing effects of (+/-)-cis-2-Amino-4-methyl-5-phenyl-2-oxazoline (4-methylaminorex) were determined in two models of intravenous drug self-administration in primates. In baboons, lever pressing was maintained under a fixed-ratio (FR) 80- or 160-schedule of intravenous cocaine delivery (0.32 mg/kg per injection). Each drug injection was followed by a 3-h time-out allowing a maximum of 8 injections per day. Vehicle or 4-methylaminorex doses were substituted for cocaine for a period of 15 or more days. One of the two 4-methylaminorex doses evaluated (0.32 mg/kg per injection) maintained self-administration behavior above vehicle control levels in all four animals. This dose of 4-methylaminorex maintained cyclic patterns of self-injection behavior across days and produced signs of psychomotor stimulant toxicity. In rhesus monkeys, 4-methylaminorex (0.0003-0.1 mg/kg per injection) was made available to animals trained to self-administer cocaine (0.01 or 0.033 mg/kg per injection) under an FR 10 schedule of reinforcement during daily 1-h sessions. Each of the three monkeys self-administered at least two doses of 4-methylaminorex at rates exceeding those maintained by vehicle injections. Taken together with reports of recreational abuse of 4-methylaminorex, the present results indicate that this drug has a potential for abuse similar to that of other psychomotor stimulants.