Predicting the Abuse Liability of Entactogen-Class, New and Emerging Psychoactive Substances via Preclinical Models of Drug Self-administration

Rather than reporting new experimental data, the paper is a narrative review focused on preclinical drug self-administration as the principal method for assessing abuse liability. The authors explain the logic and common variants of IVSA procedures: acquisition tests (how readily an animal begins responding for drug), fixed-ratio (FR) dose-substitution curves to estimate potency, progressive-ratio (PR) schedules to estimate reinforcer efficacy (breakpoint), and within-session thresholding procedures. Extended-access paradigms (short versus long daily sessions) are described as means to assess escalation of intake, and additional model extensions that probe DSM5/ICD-10-relevant phenomena are outlined, including punishment (drug-taking despite adverse consequences), reinstatement tests for relapse, and choice paradigms where natural reinforcers compete with drug. The review emphasises that IVSA in rodents is the most predictive preclinical tool for human abuse liability but is not in itself a full model of human addiction. Important methodological variables that shape outcomes are highlighted: per-infusion dose, access duration, ambient temperature, infusion speed, rat strain, training/inclusion criteria and sex. The authors synthesise studies that used different IVSA protocols and doses for MDMA, mephedrone and methylone, and integrate these behavioural data with available in vitro transporter assays, in vivo microdialysis work, and limited pharmacokinetic data to interpret relative abuse potential.

Transporter and neurochemical data indicate that the entactogens differ from classic stimulants in their DAT versus SERT activity. Using human transporter assays, the review reports DAT/SERT inhibition ratios of about 0.08 for MDMA, 1.4 for mephedrone and 3.3 for methylone, versus >10 for METH. Reported DAT inhibition IC50 values were ~17 μM for MDMA, 4.82 μM for methylone, 3.31 μM for mephedrone and 1.05 μM for METH. DAT-mediated release EC50s were 3.75 μM for mephedrone, 22 μM for MDMA, >100 μM for methylone and 1.56 μM for METH. In rat synaptosome assays the DAT/SERT ratios were lower for the entactogens (mephedrone 2.41; methylone 1.82; MDMA 0.97) compared with METH (152.0). From these data the authors infer greater relative serotonergic influence for MDMA and related entactogens compared with METH, and predict that higher serotonergic effects should reduce self-administration in preclinical models. Behavioural IVSA findings temper those pharmacological predictions. MDMA supports IVSA in rodents and nonhuman primates but shows greater inter-individual variability and lower consistency compared with classic stimulants. Across rat studies about 50-60% of subjects reach acquisition criteria under many protocols, suggesting a bimodal distribution of preference. Factors that increase MDMA IVSA include serotonergic dysfunction (SERT deletion or 5-HT neurotoxin), elevated ambient temperature in some acute settings, and possibly extended daily access in some reports. Data on sex differences are sparse; only a single published study examined female MDMA IVSA and reported only slightly more consistent intake than males under 2-h access. By contrast, the cathinone entactogens generally show more robust and consistent IVSA. Early mephedrone studies indicated readily established IVSA, although some initial reports used long access (4 h) and high ambient temperature which may have inflated intake. Aarde and colleagues report that mephedrone at 0.5 mg/kg/infusion produced numbers of reinforcer deliveries similar to METH at 0.05 mg/kg/infusion. Adolescent rats given 0.3 mg/kg/infusion obtained a mean of ~60 infusions after 10 sessions in one study, and direct comparisons found mephedrone to be more efficacious than MDMA. Methylone also supports IVSA: the first report found all rats reached acquisition at 0.5 mg/kg/inf and PR breakpoints comparable to METH-trained animals in 2-h sessions, though later work suggested a somewhat less-efficacious profile. Head-to-head within-study comparisons typically place mephedrone as the most efficacious reinforcer, MDMA as the least, and methylone intermediate. Quantitatively, rats in some studies reached means of ~12–15 infusions of methylone after 10 acquisition sessions, ~7–10 infusions for MDMA (0.5 mg/kg/inf) and ~20–25 infusions for mephedrone (0.5 mg/kg/inf). Pharmacokinetic observations that may help explain behavioural differences include faster brain entry and shorter elimination half-life for mephedrone relative to MDMA: a human study reported MDMA elimination was 3.6-fold slower than mephedrone after oral dosing, and a rat study found methylone had a ~2.5-fold slower half-life than mephedrone after intravenous dosing. Microdialysis work indicates more rapid accumulation of extracellular dopamine relative to serotonin for mephedrone than for MDMA in some experiments, though temporal resolution in that work was limited. The review also notes isolated reports of non-monoamine interactions, such as sigma-1 affinity for mephedrone, which could augment reinforcing properties.

Aarde and colleagues conclude that relative DAT/SERT ratios alone do not fully predict abuse liability within the entactogen class. Although greater serotonergic action has historically been associated with lower IVSA, the cathinone entactogens (mephedrone, methylone) manifest higher and more consistent self-administration than MDMA in multiple rodent studies, indicating enhanced abuse liability compared with MDMA. The authors therefore emphasise that IVSA remains the most precise preclinical predictor for this class despite some other paradigms (e.g. intracranial self-stimulation, locomotor assays, drug discrimination) producing mixed or apparently conflicting signals. The review highlights substantial methodological heterogeneity and relatively sparse data as key limitations: studies vary in access duration, per-infusion dose, ambient temperature, infusion speed, rat strain, inclusion criteria and sex, which complicates direct comparisons. Notably, there is very limited information from female subjects, no published IVSA data for cathinone entactogens in nonhuman primates or mice, and few systematic pharmacokinetic or high-temporal-resolution neurochemical studies. The authors identify several priorities for future work: systematic investigation of sex differences, rapid screening approaches to cope with the large diversity of novel cathinones, extension of IVSA studies to mice and nonhuman primates, detailed pharmacokinetic comparisons (including route-of-administration effects), time-resolved measurements of dopamine versus serotonin responses, and exploration of non-monoamine pharmacology (for example sigma-1 interactions). In sum, the authors interpret the available preclinical evidence as predicting that the NPS entactogens mephedrone and methylone possess greater abuse liability than MDMA, while emphasising that more standardised, broader preclinical investigation is needed to refine these conclusions and to understand mechanisms underlying the differences.