Substitutions to the phenethylamine structure give rise to numerous amphetamines and cathinones, contributing to an ever-growing number of abused novel psychoactive substances. Understanding how various substitutions affect the pharmacology of phenethylamines may help lawmakers and scientists predict the effects of newly emerging drugs. Here, we established structure-activity relationships for locomotor stimulant and monoamine transporter effects of 12 phenethylamines with combinations of para-chloro, β-keto, N-methyl, or N-ethyl additions. Automated photobeam analysis was used to evaluate effects of drugs on ambulatory activity in mice, whereas in vitro assays were used to determine activities at transporters for dopamine (DAT), norepinephrine (NET), and 5-HT (SERT) in rat brain synaptosomes. In mouse studies, all compounds stimulated locomotion, except for 4-chloro-N-ethylcathinone. Amphetamines were more potent stimulants than their β-keto counterparts, while para-chloro amphetamines tended to be more efficacious than unsubstituted amphetamines. Para-chloro compounds also produced lethality at doses on the ascending limbs of their locomotor dose-effect functions. The in vitro assays showed that all compounds inhibited uptake at DAT, NET, and SERT, with most compounds also acting as substrates (i.e., releasers) at these sites. Unsubstituted compounds displayed better potency at DAT and NET relative to SERT. Para-chloro substitution or increased N-alkyl chain length augmented relative potency at SERT, while combined para-chloro and N-ethyl substitutions reduced releasing effects at NET and DAT. These results demonstrate orderly SAR for locomotor stimulant effects, monoamine transporter activities, and lethality induced by phenethylamines. Importantly, 4-chloro compounds produce toxicity in mice that suggests serious risk to humans using these drugs in recreational contexts.

Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: William Fantegrossi reports financial support and equipment, drugs, or supplies were provided by US Department of Justice. Lauren Fitzgerald reports financial support was provided by National Institutes of Health. Michael Baumann reports financial support was provided by National Institute on Drug Abuse Intramural Research Program. Bruce Blough reports financial support was provided by National Institute on Drug Abuse.

• K23 DA000523/DA/NIDA NIH HHS/United States
• R01 DA012970/DA/NIDA NIH HHS/United States
• Z01 DA000523/ImNIH/Intramural NIH HHS/United States
• ZIA DA000523/ImNIH/Intramural NIH HHS/United States
• T32 DA022981/DA/NIDA NIH HHS/United States