Structure–Activity Relationship of Novel Second-Generation Synthetic Cathinones: Mechanism of Action, Locomotion, Reward, and Immediate-Early Genes

Abstract

Several new synthetic cathinones, which mimic the effect of classical psychostimulants such as cocaine or MDMA, have appeared in the global illicit drug market in the last decades. In fact, the illicit drug market is continually evolving by constantly adding small modifications to the common chemical structure of synthetic cathinones. Thus, the aim of this study was to investigate the in vitro and in vivo structure–activity relationship (SAR) of six novel synthetic cathinones currently popular as recreational drugs, pentedrone, pentylone, N-ethyl-pentedrone (NEPD), N-ethyl-pentylone (NEP), 4-methyl-pentedrone (4-MPD), and 4-methyl-ethylaminopentedrone (4-MeAP), which structurally differ in the absence or presence of different aromatic substituents and in their amino terminal group. Human embryonic kidney (HEK293) cells expressing the human isoforms of SERT and DAT were used for the uptake inhibition and release assays. Moreover, Swiss CD-1 mice were used to investigate the psychostimulant effect, rewarding properties (3, 10, and 30 mg/kg, i.p.), and the induction of immediate-early genes (IEGs), such as Arc and c-fos in the dorsal striatum (DS) and ventral striatum (VS) as well as bdnf in the medial prefrontal cortex (mPFC), of the test compounds. Our results demonstrated that all tested synthetic cathinones are potent dopamine (DA) uptake inhibitors, especially the N-ethyl analogs, while the ring-substituted cathinones tested showed higher potency as SERT inhibitors than their no ring-substituted analogs. Moreover, unlike NEP, the remaining test compounds showed clear “hybrid” properties, acting as DAT blockers but SERT substrates. Regarding the locomotion, NEP and NEPD were more efficacious (10 mg/kg) than their N-methyl analogs, which correlates with their higher potency inhibiting the DAT and an overexpression of Arc levels in the DS and VS. Furthermore, all compounds tested induced an increase in c-fos expression in the DS, except for 4-MPD, the least effective compound in inducing hyperlocomotion. Moreover, NEP induced an up-regulation of bdnf in the mPFC that correlates with its 5-HTergic properties. Finally, the present study demonstrated for the first time that NEP, 4-MPD, and 4-MeAP induce reward in mice. Altogether, this study provides valuable information about the mechanism of action and psychostimulant and rewarding properties as well as changes in the expression of IEGs related to addiction induced by novel second-generation synthetic cathinones.

A review of synthetic cathinones emerging in recent years (2019–2022)

“…SAR studies suggest that a more bulky and lipophilic substitution on the amino group promotes increased potency of dopamine reuptake inhibition which translates into strong stimulant effects and large rewarding properties. However, this is not a linear relationship: while the change from an N -methyl to an N -ethyl chain increases potency, substituting the pyrrolidine ring with a larger piperidine ring or with an N,N -diethyl substituent decreases potency [ 140 , 180 , 181 ]. Hence, it is likely that N -butyl analogs have fewer stimulatory properties than their N -ethyl counterparts, but this needs to be confirmed in further studies.…”

“…In addition, repeated administration of ephylone caused anxiolytic-like and depressive-like effects and aggressiveness in rats [138,139]. Studies in rodents suggest that ephylone has a high addictive potential [137,[139][140][141]. Additionally, studies in zebrafish showed its neurotoxicity, cardiotoxicity and developmental toxicity [142].…”

Neuropsychopharmacology of Emerging Drugs of Abuse: meta- and para-Halogen-Ring-Substituted α-PVP (“flakka”) Derivatives

“…The uptake inhibition assay was performed as described by [14,61], with minor modifications. Prior to starting the uptake inhibition experiment, the media was aspirated from the cell culture in the 96-well plates and immediately replaced with 200 µL of Krebs-HEPES-Buffer (KHB; 10 mM HEPES, 120 mM NaCl, 3 mM KCl, 2 mM CaCl 2 •2H 2 O, 2 mM MgCl 2 •6H 2 O supplemented with 20 mM D-glucose; pH 7.3).…”

“…The apparatus that is used for this test consists of two compartments, with differences in tactile and visual cues, communicated by a central corridor. Conditioned place preference (CPP) was performed as described by [61]. Briefly, CPP was performed in three phases: preconditioning test and conditioning and post-conditioning test.…”

3,4-Methylenedioxy methamphetamine, synthetic cathinones and psychedelics: From recreational to novel psychotherapeutic drugs

“…In this line, specific changes in the cathinone structure have been reported to change the mode of action on the monoaminergic targets to obtain drugs with the properties cited above. It is known that the addition of a 3,4-methylenedioxy group or substitutions on the phenyl ring, especially in para-position, of methcathinone generally shifts selectivity toward SERT and even can behave as partial 5-HT releasers (127,128,(140)(141)(142)(143)(144)(145). For example, it has been reported that trifluoromethyl-substitution of methcathinone in the paraposition dramatically shifts the selectivity of methcathinone toward SERT (146), becoming a SERT-selective partial releaser.…”