Using Ca2+-channel biosensors to profile amphetamines and cathinones at monoamine transporters: electro-engineering cells to detect potential new psychoactive substances

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.

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.

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.

Methcathinone decreases dopamine transporter function: Role of protein kinase C

Methcathinone (MCAT) is a psychostimulant of abuse that can cause both persistent striatal dopaminergic and serotonergic, as well as hippocampal serotonergic, deficits. Evidence suggests that the rapid effects of stimulants that are structurally and mechanistically similar to MCAT on monoamine transporter function may contribute to the abuse liability and/or persistent monoaminergic deficits caused by these agents. Thus, effects of MCAT on 1) striatal dopamine (DA) transporter (DAT); and 2) striatal and hippocampal serotonin transporter (SERT) function, as determined in tissues from adult male rats, were assessed. As reported previously, a single administration of MCAT rapidly (within 1 hr) decreases striatal [³ H]DA uptake. Similarly, incubation of rat synaptosomes with MCAT at 37℃ (but not 4˚C) decreased striatal [³ H]DA uptake. Incubation with MCAT likewise decreased [³ H]5HT but not vesicular [³ H]DA uptake. MCAT incubation in vitro was without effect on [³ H]DA uptake in striatal synaptosomes prepared from MCAT-treated rats. The decrease in [³ H]DA uptake caused by MCAT incubation: (a) reflected a decrease in V_max , with minimal change in K_m , and (b) was attenuated by co-incubation with the cell-permeable calcium chelator, N,N'-[1,2-ethanediylbis(oxy-2,1-phenylene)]bis[N-[2-[(acetyloxy)methoxy]-2-oxoethyl]-1,1'-bis[(acetyloxy)methyl] ester-glycine (BAPTA-AM), as well as the non-selective protein kinase-C (PKC) inhibitors bisindolylmaleimide-1 (BIM-1) and 2-[1-3(Aminopropyl)indol-3-yl]-3(1-methyl-1H-indol-3-yl)maleimide (or Bisindolylmaleimide VIII; Ro-31-7549). Taken together, these results suggest that in vitro MCAT incubation may model important aspects of MCAT administration in vivo, and that calcium and PKC contribute to the in vitro effects of MCAT on DAT.

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

The early characterization of ligands at the dopamine and serotonin transporters, DAT and SERT, respectively, is important for drug discovery, forensic sciences, and drug abuse research. 4-Methyl amphetamine (4-MA) is a good example of an abused drug whose overdose can be fatal. It is a potent substrate at DAT and SERT where its simplest secondary amine (N-methyl 4-MA) retains substrate activity at them. In contrast, N-n-butyl 4-MA is very weak, therefore it was categorized as inactive at these transporters. Here, N-octyl 4-MA and other related compounds were synthesized, and their activities were evaluated at DAT and SERT. To expedite this endeavor, cells expressing DAT or SERT were co-transfected with a voltage-gated Ca2+ channel and, the genetically-encoded Ca2+ sensor, GCaMP6s. Control compounds and the newly synthesized molecules were tested on these cells using an automated multi-well fluorescence plate reader; substrates and inhibitors were identified successfully at DAT and SERT. N-Octyl 4-MA and three bivalent compounds were inhibitors at these transporters. These findings were validated by measuring Ca2+-mobilization using quantitative fluorescence microscopy. The bivalent molecules were the most potent of the series and were further characterized in an uptake-inhibition assay. Compared to cocaine, they showed comparable potency inhibiting uptake at DAT and higher potency at SERT. These observations support a previous hypothesis that amphetamine-related (and, here, N-extended alkyl and) bivalent arylalkylamine molecules are active at monoamine transporters, showing potent activity as reuptake inhibitors, and implicate the involvement of a distant auxiliary binding feature to account for their actions at DAT and SERT.

A study of the dopamine transporter using the TRACT assay, a novel in vitro tool for solute carrier drug discovery

Members of the solute carrier (SLC) transporter protein family are increasingly recognized as therapeutic drug targets. The majority of drug screening assays for SLCs are based on the uptake of radiolabeled or fluorescent substrates. Thus, these approaches often have limitations that compromise on throughput or the physiological environment of the SLC. In this study, we report a novel application of an impedance-based biosensor, xCELLigence, to investigate dopamine transporter (DAT) activity via substrate-induced activation of G protein-coupled receptors (GPCRs). The resulting assay, which is coined the 'transporter activity through receptor activation' (TRACT) assay, is based on the hypothesis that DAT-mediated removal of extracellular dopamine directly affects the ability of dopamine to activate cognate membrane-bound GPCRs. In two human cell lines with heterologous DAT expression, dopamine-induced GPCR signaling was attenuated. Pharmacological inhibition or the absence of DAT restored the apparent potency of dopamine for GPCR activation. The inhibitory potencies for DAT inhibitors GBR12909 (pIC50 = 6.2, 6.6) and cocaine (pIC50 = 6.3) were in line with values from reported orthogonal transport assays. Conclusively, this study demonstrates the novel use of label-free whole-cell biosensors to investigate DAT activity using GPCR activation as a readout. This holds promise for other SLCs that share their substrate with a GPCR.

Investigation of the Optical Isomers of Methcathinone, and Two Achiral Analogs, at Monoamine Transporters and in Intracranial Self-Stimulation Studies in Rats

Methcathinone (MCAT; 1), the progenitor of numerous and widely abused "synthetic cathinone" central stimulants, exists as a pair of optical isomers. Although S(-)MCAT is several-fold more potent than R(+)MCAT in rodent locomotor stimulation and in stimulus generalization studies in rat drug discrimination assays, the individual optical isomers of MCAT have never been directly compared for their actions at monoamine transporters that seem to underlie their actions and have never been examined for their relative abuse potential. Here, we found that the isomers of MCAT are nearly equieffective at dopamine and norepinephrine transporters (DAT and NET, respectively) as transporter substrates (i.e., as releasing agents) and are ≥63-fold less potent at the serotonin transporter (SERT). In intracranial self-stimulation (ICSS) studies to evaluate abuse-related drug effects in rats, S(-)MCAT was approximately twice as potent as its R-enantiomer. Achiral analogs, α-methyl MCAT (3) and α-des-methyl MCAT (4), also were DAT/NET substrates and also produced abuse-related ICSS effects, indicating that they retain abuse potential and that they might be useful for the further study of the stereochemistry of synthetic cathinone analogs with chiral β- (or other) substituents.

Synthetic Cathinone Analogues Structurally Related to the Central Stimulant Methylphenidate as Dopamine Reuptake Inhibitors

Synthetic cathinones are, primarily, stimulant drugs of abuse that act at monoamine transporters (e.g., the dopamine transporter or DAT) as releasing agents or as reuptake inhibitors. In the past few years, the emergence of >150 new synthetic cathinones has attracted considerable attention from medical and law enforcement communities. threo-Methylphenidate (tMP), used clinically for the treatment of ADHD and narcolepsy, is also a DAT reuptake inhibitor. tMP is somewhat structurally similar to abused cathinone stimulants, and the structure-activity relationships (SAR) of tMP have been well-defined. Hence, available tMP literature might assist in understanding the SAR of synthetic cathinones, about which less is known. In the present study, we synthesized and examined eight 2-benzoylpiperidine analogues (4, 6-12) to determine if tMP SAR might be applicable to cathinone SAR. The benzoylpiperidine analogues were evaluated in a competition assay using live-cell imaging against APP+ in HEK293 cells stably expressing hDAT and in cells coexpressing DAT and voltage-gated Ca2+ channels. All compounds were found to be DAT reuptake inhibitors, and a significant correlation was obtained between the potency of the benzoylpiperidines and tMP binding data (r = 0.91), suggesting that the SAR of tMP analogues might be directly applicable to certain synthetic cathinones as DAT reuptake inhibitors.