Synthetic Cathinones and Neurotoxicity Risks: A Systematic Review

According to the EU Early Warning System (EWS), synthetic cathinones (SCs) are the second largest new psychoactive substances (NPS) class, with 162 synthetic cathinones monitored by the EU EWS. They have a similar structure to cathinone, principally found in Catha Edulis; they have a phenethylamine related structure but also exhibit amphetamine-like stimulant effects. Illegal laboratories regularly develop new substances and place them on the market. For this reason, during the last decade this class of substances has presented a great challenge for public health and forensic toxicologists. Acting on different systems and with various mechanisms of action, the spectrum of side effects caused by the intake of these drugs of abuse is very broad. To date, most studies have focused on the substances’ cardiac effects, and very few on their associated neurotoxicity. Specifically, synthetic cathinones appear to be involved in different neurological events, including increased alertness, mild agitation, severe psychosis, hyperthermia and death. A systematic literature search in PubMed and Scopus databases according to PRISMA guidelines was performed. A total of 515 studies published from 2005 to 2022 (350 articles from PubMed and 165 from Scopus) were initially screened for eligibility. The papers excluded, according to the criteria described in the Method Section (n = 401) and after full text analyses (n = 82), were 483 in total. The remaining 76 were included in the present review, as they met fully the inclusion criteria. The present work provides a comprehensive review on neurotoxic mechanisms of synthetic cathinones highlighting intoxication cases and fatalities in humans, as well as the toxic effects on animals (in particular rats, mice and zebrafish larvae). The reviewed studies showed brain-related adverse effects, including encephalopathy, coma and convulsions, and sympathomimetic and hallucinogenic toxidromes, together with the risk of developing excited/agitated delirium syndrome and serotonin syndrome.


Introduction
The illicit market and related misuse of drugs has expanded to cover alternative substances [1], such as New Psychoactive Substances (NPSs). NPSs comprise a heterogenous group of substances [2], such as prescription drugs and research chemicals, which are not controlled under the 1961 Single Convention on Narcotic Drugs or the 1971 Convention on Psychotropic Substances, and are synthetized to mimic the psychoactive effects of common drugs of abuse. Consumption of these drugs of abuse [3][4][5][6][7][8][9][10] can cause several adverse effects such as acute psychosis, bradypnea, angina pectoris, migraine, headaches, and life-threatening cardiovascular problems that range from mild tachycardia to arrhythmias, myocardial infarction and even death [11][12][13].
At the end of 2021, the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) was monitoring around 880 new psychoactive substances, and in 2020 alone, cathinone powders represented 65% of the materials seized. A quarter of this was 3-methylmethcathinone (3-MMC) and 3-chloromethcathinone , while Nethylexedrone represented a third of such seizures [2].
Synthetic cathinones (SCs) are also known as "Bath salts"; they are "legal" replacements of controlled stimulants derived from Catha Edulis (khat plant). The khat plant is indigenous to eastern Africa and the Arabian Peninsula. SCs are identified as designer drugs because their chemical structures are easily modified in order to circumvent legal controls and they are designed to mimic the effects of more traditional psychostimulants. They are derived from phenylalkylamines and are analogues of beta-ketone amphetamine (AMPH); indeed, they act in a similar way to AMPH by releasing psychostimulants [2,[14][15][16][17].
Mechanisms of action for synthetic cathinones involve interactions with dopamine, serotonin and norepinephrine transporters with varying affinities and selectivities. This has been shown in in vitro studies, including human cell lines and preclinical models of ring substituted cathinones, such as methylone, which act as transporter substrates that increase the release of dopamine, serotonin, and norepinephrine. Substances possessing a pyrrolidine ring, as in α-PVP (α-pyrrolidinopentiophenone), act as transport blockers (reuptake inhibitors) at the dopamine transporter. Increasing the length of the α-carbon chain increases the affinity and potency at the dopamine transporter. Compounds with a higher potency at the dopamine transporter, includincludeding α-pyrrolidinophenones and 4-fluoroamphetamine (4-FA), exhibit stimulant properties similar to methamphetamine while cathinones that have similar potencies at dopamine and serotonin transporters, or higher potency at the serotonin transporter, may have more empathogenic activity (e.g., ethylone) [11,17,21]. Some of the direct neurotoxicity effects are hyperthermia and neuroinflammation [21]. "Bath salts" are usually sold as brown or white crystal-like powder [22] and are consumed through insufflation, oral or intravenous routes [23]. They are often labelled as "legal highs", "plant food", "not for human consumption", "research chemicals" and are not expensive [21]. Synthetic cathinones that have been found in these products include butylone, dimethylcathinone, ethcathinone, ethylone, 3-and 4-fluoromethcathinone (3-FMC, 4-FMC), mephedrone, methedrone, methylenedioxypyrovalerone (MDPV), methylone, and pyrovalerone (see Figure 1) [24][25][26]. After consumption of SCs, the desired effects appear in 30-45 min and last from 1 to 3 h [27].
A total of 515 scientific articles (350 from PubMed and 165 from Scopus) published from 2005 to 2022 were initially screened for eligibility. Two scientists individually evaluated each entry recovered from a database; titles and abstracts mentioning SC studies and neurotoxic effects on humans and rats were further considered for full-text reading.
The screening process excluded studies according to the following criteria: (1) Articles not written in English; Different studies have confirmed the ability of SCs to stimulate the central nervous system (CNS) with an increase in the subject's need to move [14], associated in some cases with aggressive, uncontrollable behavior or loss of consciousness [22].
The aim of this review is to identify the neurotoxic effects caused by exposure to synthetic cathinones, by evaluating studies conducted on animal models and with humans.

Results and Discussion
A total of 515 studies were initially screened. Of these, 330 articles from PubMed and Scopus were excluded, according to the criteria described in the Methods section. The remaining 240 records were further assessed for eligibility and articles without sufficient data were excluded. Finally, 76 studies were analyzed and classified as "finding on animal models" (n = 56) or "finding on humans" (n = 20) according to the samples investigated. All results are summarized in Tables 1-3. Figure 3 summarizes current understanding of SCs mechanism of action and effects on rat and human brain. A total of 515 scientific articles (350 from PubMed and 165 from Scopus) published from 2005 to 2022 were initially screened for eligibility. Two scientists individually evaluated each entry recovered from a database; titles and abstracts mentioning SC studies and neurotoxic effects on humans and rats were further considered for full-text reading.
The screening process excluded studies according to the following criteria: (1) Articles not written in English; (2) Commentary, editorial letters, surveys; (3) Removal of duplicates; (4) Irrelevant studies.
Finally, articles were excluded for insufficient data.

Results and Discussion
A total of 515 studies were initially screened. Of these, 330 articles from PubMed and Scopus were excluded, according to the criteria described in the Methods section. The remaining 240 records were further assessed for eligibility and articles without sufficient data were excluded. Finally, 76 studies were analyzed and classified as "finding on animal models" (n = 56) or "finding on humans" (n = 20) according to the samples investigated. All results are summarized in Tables 1-3. Figure 3 summarizes current understanding of SCs mechanism of action and effects on rat and human brain. . SCs' mechanism of action and effects on rat and human brain. Synthetic cathinones act as blockers or substrates of DAT, SERT and NET. In this way they increase DA (dopamine), 5-HT (serotonon) or NA (norepinephrine) neurotransmission.

Findings on Animal Models In Vitro and In Vivo
Different types of toxicity can occur following exposure to SCs, notably, neurotoxicity, cardiotoxicity, nephrotoxicity and pulmonary toxicity. In this section, we focus attention on the neurotoxic potential of SCs, since the main mechanism of action of these substances is dysregulation of the monoamine systems. Indeed, the neuro-clinical manifestations (e.g., toxidromes) reported in SC-induced human intoxications tally with the monoamine dysfunction observed in animal studies [29].
The results are summarized in Table 1.
Regarding the studies carried out on animal models, 56 articles that employed in vivo or in vitro experiments on cell cultures were considered ( Table 1). The most studied SCs were mephedrone, methylone and α-pyrrolidinopenthiophenone (α-PVP). The studies evaluated effects on the locomotor system, the ability of SCs to cross the blood-brain barrier (BBB), and neurotoxicity at different targets and interaction pathways.
To date, researchers have conducted various types of experiment to understand how SCs interact with the central nervous system, especially their effects on locomotory activity.
Studies on animal models, in particular on rats, show that treatment with SCs, such as mephedrone, cause ambulatory hyperactivity. Shortall et al. described a sensitization to intermittent treatment with intraperitoneal injections (i.p.) of mephedrone (1, 4 or 10 mg/kg) and they noticed a significant ambulatory hyperactivity after the sixth dose [30]. This is due to dopaminergic neuronal activity and dopamine release in the nucleus accumbens of the rats.
This result was confirmed by other research, including the study by Javadi-Peydaret al., which showed that intravenous administration of pentylone, pentedrone and methylone in rats increased locomotor activity as a result of their psychostimulant effects. In particular, the stimulating effects caused by the intake of pentedrone lasted longer than those caused by the other two substances (at the same dose); the effects were equivalent for male and female rodents [33]. This stimulant capability of pentylone was confirmed . SCs' mechanism of action and effects on rat and human brain. Synthetic cathinones act as blockers or substrates of DAT, SERT and NET. In this way they increase DA (dopamine), 5-HT (serotonon) or NA (norepinephrine) neurotransmission.

Findings on Animal Models In Vitro and In Vivo
Different types of toxicity can occur following exposure to SCs, notably, neurotoxicity, cardiotoxicity, nephrotoxicity and pulmonary toxicity. In this section, we focus attention on the neurotoxic potential of SCs, since the main mechanism of action of these substances is dysregulation of the monoamine systems. Indeed, the neuro-clinical manifestations (e.g., toxidromes) reported in SC-induced human intoxications tally with the monoamine dysfunction observed in animal studies [29].
The results are summarized in Table 1.
Regarding the studies carried out on animal models, 56 articles that employed in vivo or in vitro experiments on cell cultures were considered ( Table 1). The most studied SCs were mephedrone, methylone and α-pyrrolidinopenthiophenone (α-PVP). The studies evaluated effects on the locomotor system, the ability of SCs to cross the blood-brain barrier (BBB), and neurotoxicity at different targets and interaction pathways.
To date, researchers have conducted various types of experiment to understand how SCs interact with the central nervous system, especially their effects on locomotory activity.
Studies on animal models, in particular on rats, show that treatment with SCs, such as mephedrone, cause ambulatory hyperactivity. Shortall et al. described a sensitization to intermittent treatment with intraperitoneal injections (i.p.) of mephedrone (1, 4 or 10 mg/kg) and they noticed a significant ambulatory hyperactivity after the sixth dose [30]. This is due to dopaminergic neuronal activity and dopamine release in the nucleus accumbens of the rats.
This result was confirmed by other research, including the study by Javadi-Peydaret al., which showed that intravenous administration of pentylone, pentedrone and methylone in rats increased locomotor activity as a result of their psychostimulant effects. In particular, the stimulating effects caused by the intake of pentedrone lasted longer than those caused by the other two substances (at the same dose); the effects were equivalent for male and female rodents [33]. This stimulant capability of pentylone was confirmed by Saha et al. who showed that the pentylone locomotor stimulant effect was higher than the butylone effect in mice [34]. Moreover, pentylone and eutylone were stronger locomotor stimulants than butylone in treated mice [35]. Increase of locomotor activity in mice after exposure to SCs seems to be common; indeed, other studies have demonstrated that exposure to 3-chloromethcathinone, 4-chloromethcathinone, 4-fluoro-α-pyrrolidinopentiophenone and 4-methoxy-α-pyrrolidinopentiophenone also produced an increase of horizontal spontaneous locomotor activity with a dose-dependent effect, while α-pyrrolidinophenones increased vertical spontaneous locomotor activity. Furthermore, these substances seemed to increase forced locomotor activity [36]. In the same way, 3-FMC and 4-FMC increased mice locomotor activity, as shown by Marusich et al. [37] and confirmed by Wojcieszak et al., who described effects on locomotor activity due to 3-fluoromethcathinone and methcathinone consumption [38]. In addition, Nadal-Gratacòs et al. confirmed that ring-substituted PVP derivates stimulated locomotor activity in i.p.-injected mice [39]. In the same way, i.p. injections of N-ethyl-pentedrone in mice caused an increase in spontaneous horizontal activity [40]. Ray et al. studied α-pyrrolidinopropiophenone (α-PPP) effects on male mice, by administering 4 unit doses of 80 mg/kg of α-PPP every 2 h, for an 8 h period. After treatment they noted a decrease in mice exploratory activity (through the NOR test); they also found that this cathinone had a locomotor stimulant effect similar to cocaine on mice [41]. In a different study conducted by Centazzo et al., treatment with subcutaneous methylone (6, 12, or 24 mg/kg) significantly altered male mice's locomotor behavior which was positively correlated with the brain concentration of methylone [42]; also i.p. methylone injections in rats caused an increase of locomotor activity [43].
Several studies have focused on the effects caused by methylenedioxypyrovalerone and these indicated that methylenedioxypyrovalerone enhanced mice locomotor activity, even at low concentrations [37,44,45], acting on horizontal or vertical activity [46]; moreover, methylenedioxypyrovalerone i.p. injections can cause interoceptive dose-time dependent effects [47] and increase mice motility 30 min after the first injection [48]. Methylenedioxypyrovalerone induced self administration in rats [49] and compromised discrimination between familiar and unfamiliar objects [50]; it compromised NOR [50,51] and its ingestion caused social play behavior repression in rats [52]. Treatment with α-PVT can induce self administration in rats [53] and stimulate locomotion in mice [54]. N-ethylpentylone injected in rats caused hyperlocomotion and acted in a similar way to methamphetamine [55,56].
Souders et al. investigated the effects of pyrovalerone in Danio Rerio larvae. Pyrovalerone was directly administrated to zebrafish larvae using a special 96-well plate precoated with pyrovalerone. This study reported changes in locomotor activities, showing hyperactivity and higher movement after pyrovalerone administration [57]. This result was observed also in adult zebrafish after α-pyrrolidinopentiophenone exposure [58].
It is known that SCs can cross the blood-brain barrier. In this regard, Fabegrat-Safont et al. investigated the capacity of 13 different cathinone-derived compounds to cross the BBB. Their paper showed that an increased polarity and the presence of fluorine atoms enhanced cathinones' ability to cross the BBB, as compared to cathinones with less polar N functionalization, long alkyl chain or non-polar aromatic ring, which crossed the BBB with difficulty-probably because crossing the BBB is a carrier-mediated process [63].
Several studies investigated how SCs interacted with neurotransmitters [41,42,[64][65][66][67][68][69][70][71][72] and in particular their effects on levels of dopamine (DA) and serotonin (5-HT) in different regions of rat brain. Martìnez-Clemente et al. reported that mephedrone showed affinity for dopamine transporters and could block dopamine and serotonin uptake in the brain [64]. Other studies confirmed that repeated treatment with mephedrone in adolescent rats caused changes in the basal neurotransmitter levels, especially in striatum, nucleus accumbens and frontal cortex. After i.p. injections of methylenedioxypyrovalerone, mephedrone, and methylone in mice, Allen et al. detected an increase of dopamine levels in the substantia nigra and ventral tegumental areas [73]. Kamińska et al. found an increase of extracellular serotonin levels in nucleus accumbens and frontal cortex and that the ingestion of repeated doses of mephedrone in adolescent mice caused single and double-stranded DNA breaks in the frontal cortex in adulthood [65,74]. Other studies, carried out on mephedrone-treated mice, upheld a loss in the dopamine reuptake in striatum [66,67] and in frontal cortex caused by a decrease in the density of dopamine transporters in these tissues [68] and a decrease in serotonin transporter function in striatal and hippocampal synaptosomes [66,68], amygdala and prefrontal cortex. Studies on mephedrone enantiomers showed that R-mephedrone was more selective for dopamine transporters but was less efficient in serotonin release than S-mephedrone [75]. On the other hand, α-pyrrolidinopentiophenone (α-PVP) increased serotonin levels only in the hypothalamus and increased 3,4-dihydroxyphenylacetic acid (DOPAC, metabolite of dopamine) levels in the amygdala [67]. Other research confirmed that α-PVP was a DAT and NET inhibitor [76][77][78] and caused alteration of dopamine levels in hypothalamus, thalamus and striatum [79]. Ray et al. demonstrated that treatment with α-PPP reduced serotonin levels in striatum in male mice, by examining brain sections of treated mice [41]. Despite data about the effect of mephedrone on serotonin levels, Angoa-Perez et al. demonstrated that it did not cause damage to serotonin nerve endings in female mice and hypothesized that mice were not subject to serotonin nerve ending damage [69].
Comparable studies have been undertaken regarding the effects of methylone, methyle nedioxypyrovalerone and α-pyrrolidinopentiophenone.
In particular, it is known that methylone accumulates in the brain of mice in a nonlinear manner [42]. Lopez-Arnaw et al. identified a loss of dopamine transporters in the frontal cortex after treatment of mice with methylone, causing a dose dependent neurotoxicity; damage to serotonin and dopamine nerve terminals in the frontal cortex and hippocampus of mice was found after administration of 4 doses of methylone in one day [70]. In addition, methylone or mephedrone swallowed with methamphetamine seemed to induce damage to dopamine (DA) nerve terminals, while MDPV seemed to protect DA nerve endings from combined treatment with amphetamine [71]. Cameron et al. studied methylenedioxypyrovalerone and mephedrone effects on Xenopus oocytes and they found that these substances had different behavior; indeed, methylenedioxypyrovalerone acted as a dopaminergic reuptake inhibitor as opposed to mephedrone, which was a dopamine releasing agent [80]. However, it emerged that methylone or MDPV did not affect levels of 5-HT in the striatum in male mice [72]. Methylenedioxypyrovalerone caused neurodegeneration in different brain areas [51]; in particular, Colon-Perez et al. demonstrated a decrease in brain connectivity in the prelimbic area and nucleus accumbens [81].
Studies on different cell lines investigated the cytotoxic effect of various SCs. Some studies demonstrated that 3-fluoromethcathinone treatment of immortalized mouse hippocampal cell line HT22 caused a dose dependent decrease in the distribution of cells [82]. Rosas-Hernandez et al. identified effects of MDPV on bovine brain microvascular endothelial cells (bBMVECs). In particular, the MDPV treatment caused a decrease of cellular proliferation and an increase in ROS production, resulting in the disruption of the endothelial cell monolayer and loss of the BBB properties of the cells [83].
In conclusion, the reviewed synthetic cathinones can cross the brain-blood barrier and act like psychostimulants in terms of their effects on neurotransmitter levels in different regions of the brain. Methylenedioxypyrovalerone induced cytotoxicity at low concentration, and it inhibited cellular proliferation, so cells lost their blood-brain barrier properties. The acquisition of self administration was faster for MDPV than methylone; methylone is less powerful than MDPV and did not show a dose-dependent effect.
(2016) [49]  An increase in dopamine levels in nucleus accumbens, striatum, substantia nigra, and ventral tegmental area, is detected after consumption of high doses of 3,4-methylenedioxypyrovalerone and mephedrone, and methylone, an increase of HVA was also found. A combination of drugs induced higher levels of dopamine. The highest dose of combined drugs caused hypolocomotion and immobility that was not caused by the drug administration alone. Pentylone and butylone had psychostimulant effects. Their administration increased extracellular dopamine levels, but butylone had more effect on serotonin levels. Pentylone stimulated hyperactivity more effectively than butylone and had better locomotor activity stimulant effects and increased stereotypy (butylone did not have this effect). Both substances were inhibitors of DAT and SERT but pentylone was more selective for DAT.   After prolonged methylenedioxypyrovalerone exposure, rats' ability to discriminate between familiar and unfamiliar objects was compromised. Acute exposure to methylenedioxypyrovalerone had effects on the general excitability of the brain, acting on neurotransmitter receptors. It was found that methylenedioxypyrovalerone exposure compromised NOR.
(2020) [50]  Synthetic cathinones crossed blood-brain barrier and their permeation was related to their polarity.
The process used to penetrate the blood-brain barrier was carrier-mediated.  Methylone and MDC penetrated the central nervous system and crossed the blood-brain barrier. After methylone exposure there was an increase in forward locomotion, rearing and patterned sniffing. Administration of methylone caused an acute depletion of brain serotonin due to the release of monoamine transmitters.
(2021) [42]  α-pyrrolidinopentiophenone (0.1 mg/kg/inf) or 4-methylmethcathinone (0.5 mg/kg/inf) Regarding self administration, there was no sex-linked difference in response between the drugs, LgA and ShA. Rats showed escalation of drug intake over time. The escalation was greater for the LgA group. ShA self-administration escalation was higher for 4MMC. There were some sex-linked differences in the neurochemistry. ShA groups showed differences for NE levels in amygdala, hippocampus, pre frontal cortex, and striatum. LgA groups showed differences in 5-HIAA levels (5-HT metabolite). LgA self administration altered DA levels in hypothalamus, thalamus, and striatum. Consumption of both drugs affected 5-HT and 5-HIAA levels.
(2021) [43]   All the tested substances presented selectivity for DAT; meta-halogen-PVP derivatives increased DA uptake inhibition potency and DAT binding affinity more than their para-analogs. They decreased 5-HT uptake inhibition potency and SERT binding affinity in vitro. α-PVP halogen-ring-derivates caused an acute anxiogenic state. All tested compounds stimulated locomotor activity.
Unpleasant or adverse effects are linked to the dopaminergic system stimulation. They include agitation, anxiety, cognitive disorders, delusions, visual and auditory hallucinations, aggressive and erratic behavior, paranoia, psychosis, and seizures, which are the most reported neurological and psychiatric symptoms regarding SC-related intoxications or overdoses. Brain-related adverse effects include stroke, encephalopathy, coma, and convulsions [98,99].
In the case of a synthetic cathinone-related lethal intoxication, the abusers develop excited delirium syndrome [103].
There are several studies regarding SC-induced intoxication but only a few papers dealing with fatalities linked to the related brain neurotoxic effects. Regarding this issue, we collected a series of in vitro and in vivo papers about humans and human cell cultures (Tables 2  and 3 Table 2. Leong et al. studied the effects of butylone, pentylone and methylenedioxypyrovalerone treatment on SH-SY5Y cells (human neuroblastoma cell line). They proved that SCs administration caused cell death and an increase in ROS production. This condition is reflected in reduced mitochondrial respiration and in an alteration of Ca 2+ levels in cells. After a molecular analysis, they showed that these substances also promoted apoptosis because they supported caspase 3 and 7 activation [21]. Several other studies on SH-SY5Y confirmed that SCs could reduce cell viability and increase ROS production [104], also interacting with mitochondria and their functionality [105].
Alteration of the blood-brain barrier is a common effect of SCs consumption reflected in in vitro human models. The experiments conducted by Buzhdygan et al. on the effects of mephedrone on human brain microvascular endothelial cells (hBMVEC) showed that the treatment with mephedrone caused a charge-dependent disruption of the endothelial barrier and an increase of barrier permeability [106], demonstrating the ability of mephedrone to regulate the tight junction complex.
In particular, studies on HEK-293 cells showed that SCs acted like DAT inhibitors with different activity levels depending on the type of SC [31,34].

Lethality-Human Toxicology
The results are summarized in Table 3

Toxicology in Human SCs Intoxication
Twenty-seven papers dealt with case reports. The results are reported i and 6.
In 10 out of the 27 cases, the mean age reported at the moment of hospita was 28.8 years.
In 6 out of the 27 cases, gender was not reported; the remaining 21 co males and 2 females. In particular, one woman was pregnant during the event.
In 19 out of the 27 cases, the principal SC was identified and detected. In ing cases (8) the main SCs responsible for the intoxication were suspected bec circumstances (participation in a rave party, or personal admission of the pati friend or parent) without being detected in biological samples.
The cathinone found at the highest blood concentration was α-PVP with while the cathinone found at the lowest blood concentration was MDPHP (m tration = 30.3).
In 6 out of 24 cases, SCs were the only drug detected and were considere agent responsible for the exogenous intoxication. In fact, 18 out of the 27 pa poly-drug abusers; among them, intoxication with benzodiazepines tied w (+metabolites) as the substances most represented in addition to CSs. Ten papers dealt with SC fatalities associated with previous neurological symptoms such as disorientation, hyperthermia, aggressive behavior, rage or fury, psychomotor agitation (shouting, jumping, running, removing clothes), seizures and convulsions. One case dealt with death in utero in a pregnant woman exposed to MDPHP [107]. Stillbirths secondary to drug exposure are rarely described in the literature; however, some case reports described death in utero after exposure to methadone [108] or to cocaine and methamphetamine [109].
The average age in the present case reports was 31 yrs (n = 9) because fetal death was not considered among these. Eight out of ten were male and two were female.
In the cases reported here, SCs were detected and quantified in post-mortem biological samples (n = 8), except for two cases. In the first of these, tests for SC metabolites and for 25INBOMe (5-HTA2 receptor agonist) were negative at hospital admission, while postmortem GC and MS analyses revealed N-ethylpentylone in the urine [110]; in the second case, routine drug screening was performed post-mortem showing N-Ethylpentylone [111].
The highest SC blood concentration was 4200 ng/mL detected in a fatal Eutylone intoxication from a single dose [112], while the lowest serum concentration was 82 ng/mL [113]. It is noteworthy that a fetal case was associated with an even lower concentration equal to 76 ng/mL [107].
In 8 out of 10 SC fatalities a single cathinone was detected in the biological samples, while in the remaining two fatalities two SCs were detected as main drugs of abuse such as MDPV and methylone [114] and 3,4-MDPHP and α-PHP [107].
In 2 out of the 10 fatalities, no other drugs were detected except for SCs [107,115]; in the remaining fatalities, biological samples revealed more drugs or psychotropic substances (poly drug fatalities).
The prevalence of other drugs associated with the main SCs is summarized in Figure 1. The most represented "accompanying drugs" were EtOH and psychotropic drugs (such as Aripiprazole, Tricyclic antidepressants, Lamotrigine). The evidence of psychotropic drug use in the case of multidrug fatalities underlines the great diffusion of SCs among patients already affected by psychiatric disorders and in therapy.

Toxicology in Human SCs Intoxication
Twenty-seven papers dealt with case reports. The results are reported in Figures 5 and 6.      In 10 out of the 27 cases, the mean age reported at the moment of hospital admission was 28.8 years.
In 6 out of the 27 cases, gender was not reported; the remaining 21 comprised 19 males and 2 females. In particular, one woman was pregnant during the intoxication event.
In 19 out of the 27 cases, the principal SC was identified and detected. In the remaining cases (8) the main SCs responsible for the intoxication were suspected because of the circumstances (participation in a rave party, or personal admission of the patient or of his friend or parent) without being detected in biological samples.
The cathinone found at the highest blood concentration was α-PVP with 890 ng/mL, while the cathinone found at the lowest blood concentration was MDPHP (mean concentration = 30.3).
In 6 out of 24 cases, SCs were the only drug detected and were considered the main agent responsible for the exogenous intoxication. In fact, 18 out of the 27 patients were poly-drug abusers; among them, intoxication with benzodiazepines tied with opioids (+metabolites) as the substances most represented in addition to CSs.

Conclusions
The studies demonstrated that the SCs cause severe intoxication and fatalities. Synthetic cathinones, more commonly known as "bath salts", are human-made stimulants, chemically related to cathinone, a substance found in the khat plant. Khat is a shrub grown in East Africa and southern Arabia, where some people chew its leaves for their mild stimulant effects. Synthetic cathinones are part of a group of drugs called new psychoactive substances (NPS). These are designed to mimic the effects of controlled substances and have no legitimate medical use. New substances are introduced into the market in quick succession to evade or hinder law enforcement efforts to address their manufacture and sale.
Not much is known yet about how synthetic cathinones affect the human brain especially neurotoxic mechanisms that lead to fatalities. In both humans and animals, the commonly encountered neuro-clinical toxic syndromes (also known as toxidromes) are sympathomimetic and hallucinogenic, increasing the risk of excited/agitated delirium syndrome and serotonin syndrome. Sympathomimetic toxidromes are characterized by neurological and psychiatric symptoms, such as agitation, anxiety, delusions, hyperactivity, paranoia and seizures, alongside diaphoresis, hyperthermia, and mydriasis. The hallucinogenic toxidrome includes disorientation, hallucinations, memory disruption, psychotic episodes [125,126].
The observed concentrations of synthetic cathinones vary widely and often have overlapping ranges for fatal and nonfatal cases. To draw conclusions about the cause of death it is necessary to know the drug concentrations and also the clinical situation of each case [116,127]. Furthermore, currently, there are difficulties related to the few studies that specifically explain the neurotoxic mechanism underlying death.
Based upon the presented data, it is not possible to establish the exact lethal concentrations of cathinones. Where there is clinical suspicion of death due to synthetic cathinones intoxication, postmortem blood concentrations >1 lg/mL (in persons without developed tolerance) can be cautiously considered as lethal concentrations. This compilation of cathinones concentrations can be a guide to find papers on a specific compound; however, they cannot be taken as absolute values to determine the cause of death.
In conclusion, in the light of the increasing prevalence of these drugs and the continued introduction of new molecules to the drug market, studies on the neurotoxicity mechanism caused by SCs are still inadequate. Despite many studies in recent years, there is still a great deal to clarify about the action of these substances of abuse on the brain. Funding: The review paper was funded from the Project "Implementation of the identification and study of the effects of NPS: Development of a multicenter research to enhance the database of the National drug Addiction Observatory and the Early Warning System" and by the project: "National Early Warning System on Drugs (SNAP)" by the Italian Department of Antidrug policies.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest.