Functional characterization of N-octyl 4-methylamphetamine variants and related bivalent compounds at the dopamine and serotonin transporters using Ca2+ channels as sensors

Structural and functional perspectives on interactions between synthetic cathinones and monoamine transporters

Synthetic cathinone derivatives comprise a family of psychoactive compounds structurally related to amphetamine. Over the last decade, clandestine chemists have synthesized a consistent stream of innovative cathinone derivatives to outpace governmental regulatory restrictions. Many of these unregulated substances are produced and distributed as designer drugs. Two of the principal chemical scaffolds exploited to expand the synthetic cathinone family are methcathinone and α-pyrrolidinopentiophenone (or α-pyrrolidinovalerophenone, α-PVP). These compounds' main physiological targets are monoamine transporters, where they promote addiction by potentiating dopaminergic neurotransmission. This chapter describes techniques used to study the pharmacodynamic properties of cathinones at monoamine transporters in vitro. Biochemical techniques described include uptake inhibition and release assays in rat brain synaptosomes and in mammalian expression systems. Electrophysiological techniques include current measurements using the voltage clamp technique. We describe a Ca2+ mobilization assay wherein voltage-gated Ca2+ channels function as reporters to study the action of synthetic cathinones at monoamine transporters. We discuss results from systematic structure-activity relationship studies on simple and complex cathinones at monoamine transporters with an emphasis on identifying structural moieties that modulate potency and selectivity at these transporters. Moreover, different profiles of selectivity at monoamine transporters directly predict compounds associated with behavioral and subjective effects within animals and humans. In conclusion, clarification of the structural aspects of compounds which modulate potency and selectivity at monoamine transporters is critical to identify and predict potential addictive drugs. This knowledge may allow prompt allocation of resources toward drugs that represent the greatest threats after drugs are identified by forensic laboratories.

Structure-Activity Relationships for a Recently Controlled Synthetic Cathinone Dopamine Transporter Reuptake Inhibitor: α-Pyrrolidinohexiophenone (α-PHP)

α-Pyrrolidinohexiophenone (α-PHP) is the one-carbon unit α-extended homolog of the better-known and widely abused synthetic cathinone central stimulant α-PVP ("flakka"); both are now U.S. Schedule I controlled substances. Structurally, α-PVP and α-PHP possess a common terminal N-pyrrolidine moiety and differ only with respect to the length of their α-alkyl chain. Using a synaptosomal assay, we previously reported that α-PHP is at least as potent as α-PVP as a dopamine transporter (DAT) reuptake inhibitor. A systematic structure-activity study of synthetic cathinones (e.g., α-PHP) as DAT reuptake inhibitors (i.e., transport blockers), a mechanism thought responsible for their abuse liability, has yet to be conducted. Here, we examined a series of 4-substituted α-PHP analogues and found that, with one exception, all behaved as relatively (28- to >300-fold) selective DAT versus serotonin transporter (SERT) reuptake inhibitors with DAT inhibition potencies of most falling within a very narrow (i.e., <3-fold) range. The 4-CF3 analogue of α-PHP was a confirmed "outlier" in that it was at least 80-fold less potent than the other analogues and displayed reduced (i.e., no) DAT vs SERT selectivity. Consideration of various physicochemical properties of the CF3 group, relative to that of the other substituents involved here, provided relatively little insight. Unlike with DAT-releasing agents, as previously reported by us, a QSAR study was precluded because of the limited range of empirical results (with the exception of the 4-CF3 analogue) for DAT reuptake inhibition.

Do 2-(Benzoyl)piperidines Represent a Novel Class of hDAT Reuptake Inhibitors?

2-(Benzoyl)piperidines (analogues of 1a), structural hybrids of the clinically employed ADHD medication methylphenidate (2) and the abused synthetic cathinone pentedrone (3), have been previously reported to act as novel and selective reuptake inhibitors of the human dopamine transporter (hDAT). One of the more potent benzoylpiperidines, as is the case with methylphenidate analogues, is its 3,4-dichloroaryl counterpart. Here, we demonstrate using homology models that these compounds (i.e., benzoylpiperidines and methylphenidate analogues) likely bind in a comparable manner at hDAT. In addition, it is shown here that the 3,4-dichlorobenzoylpiperidine analogue of 1a is more potent than its 3,4-dimethyl counterpart, suggesting that the electronic character of the substituents might play a role in the potency of these hybrids. Furthermore, the 3,4-benz-fused (i.e., naphthyl) benzoylpiperidine analogue acts in the same manner as its corresponding methylphenidate counterpart at hDAT. As with its methylphenidate counterpart, the naphthyl compound also acts, rather uniquely (although with lower potency) relative to other members of the two series, at the human serotonin transporter (hSERT). In conclusion, the benzoylpiperidines represent a novel structural class of hDAT reuptake inhibitors that function in a manner similar to their methylphenidate counterparts.

Binding and functional structure-activity similarities of 4-substituted 2,5-dimethoxyphenyl isopropylamine analogues at 5-HT2A and 5-HT2B serotonin receptors

Certain 4-substituted analogs of 1-(2,5-dimethoxyphenyl)isopropylamine (2,5-DMA) are psychoactive classical hallucinogens or serotonergic psychedelic agents that function as human 5-HT_2A (h5-HT_2A) serotonin receptor agonists. Activation of a related receptor population, h5-HT_2B receptors, has been demonstrated to result in adverse effects including cardiac valvulopathy. We previously published on the binding of several such agents at the two receptor subtypes. We hypothesized that, due to their structural similarity, the 5-HT_2A and 5-HT_2B receptor affinities of these agents might be related, and that QSAR studies might aid future studies. For a series of 13 compounds, it is demonstrated here that i) their published rat brain 5-HT₂ receptor affinities are significantly correlated with their h5-HT_2A (r = 0.942) and h5-HT_2B (r = 0.916) affinities, ii) as with r5-HT₂ receptor affinity, h5-HT_2A affinity is correlated with the lipophilicity of the 4-position substituent (r = 0.798), iii) that eight of the ten compounds examined in functional (Ca⁺² mobilization in stable cell lines generated expressing the human 5-HT_2B receptor using the Flp-In T-REx system) assays acted as h5-HT_2B agonists (4-substituent = H, F, Br, I, OCH₂CH₃, NO₂, nC₃H₇, tC₄H₉) and two (n-hexyl and benzyl) as antagonists, iv) h5-HT_2B affinity but not action was correlated with the lipophilicity of the 4-position substituent (r = 0.750; n = 10). The findings suggest that h5-HT_2B receptor affinity, and its relationship to substituent lipophilicity, might be approximated by rat and h5-HT_2A affinity but cannot be used as a predictor of h5-HT_2B agonist action of 2,5-DMA analogs. Furthermore, given that certain 2,5-DMA analogs are on the clandestine market, their potential to produce cardiac side effects following persistent or chronic use via activation of h5-HT_2B receptors should be considered.

Host–guest complexation of APP+ with cucurbit[7]uril. Theoretical and experimental studies on the supramolecular inhibition of its cytotoxicity on SERT

The supramolecular binding behavior of APP+ and CB[7] in aqueous solution was studied by different techniques. APP+ showed cytotoxicity towards HEK293 cells expressing hSERT. This cytotoxicity was inhibited by the treatment of the cells with CB[7].

Non-conserved residues dictate dopamine transporter selectivity for the potent synthetic cathinone and psychostimulant MDPV

Clandestine chemists are currently exploiting the pyrrolidinophenone scaffold to develop new designer drugs that carry the risk of abuse and overdose. These drugs promote addiction through the rewarding effects of increased dopaminergic neurotransmission. 3,4-Methylenedioxypyrovalerone (MDPV) and its analogs are illicit psychostimulants of this class that are ∼50-fold more potent than cocaine at inhibiting the human dopamine transporter (hDAT). In contrast, MDPV is a weak inhibitor at both the human serotonin transporter (hSERT) and, as it is shown here, the Drosophila melanogaster DAT (dDAT). We studied three conserved residues between hSERT and dDAT that are unique in hDAT (A117, F318, and P323 in dDAT), and one residue that is different in all three transporters (D121 in dDAT). hDAT residues were replaced in the dDAT sequence at these positions using site-directed mutagenesis and stable cell lines were generated expressing these mutant transporters. The potencies of MDPV and two of its analogs were determined using a Ca²⁺-mobilization assay. In this assay, voltage-gated Ca²⁺ channels are expressed to sense the membrane electrical depolarization evoked when dopamine is transported through DAT. Each individual mutant slightly improved MDPV's potency, but the combination of all four increased its potency ∼100-fold (2 log units) in inhibiting dDAT activity. Molecular modeling and docking studies were conducted to explore the possible mode of interaction between MDPV and DAT in silico. Two of the studied residues (F318 and P323) are at the entrance of the S1 binding site, whereas the other two (A117 and D121) face the aryl moiety of MDPV when bound to this site. Therefore, these four non-conserved residues can influence MDPV selectivity not only by stabilizing binding, but also by controlling access to its binding site at DAT.